Note: Descriptions are shown in the official language in which they were submitted.
1~543Z 1123~-~
BACXGROUND OF THE INVENTION
-
Field of the Invention
This invention relates generally to a process for
warm digestion of sludge, carried out under aerobic
and anaerobic conditions.
Description of the Prior Art
With continued growth of industry and population, ~he
probls~s associated with wastewater di3posal are correspond-
ingly increased. Although physical, che~?cal and biological
treatment systems have been developed wl-ich can efficiently
treat polluted waters to produce an effluent suitable for
release to natural receiving waters, almost all of the
basic wastewater treatment systems currently in use, in-
cluding clarification, chemical precipitation, biologicai
filtration and activated sludge, convert the water pollu-
tants into a concentrated form called sludge. Particularly
in the activated sludge process, which is among the most
popular of conventionally employed wastewater treatment
systems, there is usually a significant net positive
production of volatile suspended solids (MLVSS), i.e., the
rate of cell synthesis exceeds the rate of cell destruction.
Therefore, an increasing inventory of sludge builds up and
the excess activated sludge must be discarded from the
process continuously or periodically.
~ 3';-2
As the overall volumes of wastewater reauiring
treatment increases, particularly under the impetus of
increasingly stringent pollution control legislation,
the quantity of waste sludge produced by the above-
mentioned wastewater treatment processes is corresponding-
ly increased. Accordingly, it is highly desirable to
process this waste sludge in such manner that it can be
readily and economically disposed of without creating
further pollution of the ecosphere. While much effort has
been spent in development of improvements in sludge treat-
ment technology as well as in refinement of existing sludge
treatment processes, there still exists a great need for
better and more efficient sludge treatment sys~ems.
The basic aim of all sludge treatment processes is
to economically and efficiently reduce and stabilize
sludge solids. In addition, the sludge treatment system
should desirably also produce an end product which is
fully suitable for final disposal without further physical
or chemical treatment. In conventional practice sludge
disposal is commonly carried out by either ocean dumping,
combustion, land filling or land spreading. In many
instances, land disposal is employed and is partic-
ularly attractive due to minimal long-term environmen-
tal effects. In fact, land spreading of sludge may be highly
advantageous in promoting reconditioning of the soil. How-
ever, the use of land spreading as a final sludge disposal
method requires a well-pasteurized end product, so that the
concentration of pathogenic organisms in the sludge is
sufficiently low to avoid a potential health hazard in
disposition of the sludge.
Z
1~ ~3
Traditionally, three distinct processes have been
widely utilized for treating waste sludge: ox~dation ponds,
anaerobic digestion and aerobic digestion.
Oxidation ponds are generally employed in the form of
comparatively shallow excavated basins in the earth which
extend over an area of land and retain wastewater prior ~o
its final disposal. Such ponds permit the biological
oxidation of organic material by natural or artifically
accelerated transfer of oxygen to the water from the ambient
air. During the bio-oxidation process, the solids in the
wastewater are biologically degraded to some extent and
ultimately settle to the bottom of the pond, where they may
become anaerobic and be further stabilized. Periodically
the pond may be drained and the settled sludge dretged
out to renew the volumetric capacity of the pond for fur-
ther wastewater treatment,and the withdrawn sludge is util-
ized for example for landfill. Oxidation ponds thus repre-
sent a functionally simple system for wastewater and sludge
treatment. The use of oxidation ponds, however, has limit-
ed utility, since their operation requires sizable land
areas. Moreover, no significant reduction of the level of
pathogens in the sludge is accomplished by this treatment
and disposal method.
11~3' -~
Anaerobic digestion has generally been the mcse
e~tensively used digestisn process for stabilizing con-
centrated organic solids, such as are removed from settling
tanks, biological filters and activated sludge plants. In
common practice, the e~cess sludge is accumulated in large
domed digesters where the sludge is fermented anaerobical-
ly for 20-30 days. The major reasons for commercial
acceptance of anaerobic sludge digestion are that this
method is capable of stabilizing large volumes of dilute
organic slurries, results in low biological solids (biomass)
production, produces a relatively easily dewaterable sludge
and is a producer of methane gas. Additionally, it
has been variously alleged that anaerobic digestion
produces a pasteurized sludge. Even though this pasteur-
izing capability of anaerobic digestion is questionable,
anaerobic digestion is widely used in practice because it
reduces the solid residue to a reasonably stable form
which can be discarded as land fill without creating a sub-
stantial nuisance. The anaerobic digestion is characteristi-
cally carried out in large scale tanks which are more or lesc
thoroughly mi~ed, either by mechanical means or by the
recycling of compressed digester gas Such mixing rapidly
increases the sludge stabilization reactions, by creating
a large zone of active decomposition.
As indicated above,anaerobic digestion has commonly
been practiced with long retention times on the order of
20 - 30 days, without any heat imput to the system. It has
4~
1 ~3'_2
been found by the prior art that elevated te~oeratures in
the mesophilic range of 30 to 40C ~acilitat e reduction
of the retention time requirement, to about 12 - 20 days.
Such reduction in treatment time is a consequence of the
fact that the rate of activity of the organisms responsible
for digestion is greatly influenced by temperature, and tha~
in the 30 to 40C temperature range highly active
mesophilic microorganisms are the dominant microbial strain
in the sludge undergoing digestion. The best temDeratures
for mesophilic digestion are in ~he range of about 35 to
38C, with minimum retention times on the order of 12 - 15
days. Temperatures up to 35C increase the rate of digestior
and may allow shorter retention times, but at the expense
of system operating stability while temperatures below 35C
require longer retention times.
Methane gas is produced during anaerobic digestion
and is characteristically used in combustion heaters to
offset heat losses of the anaerobic digestion system oper-
ating at elevated temperature. However, seasonal tempera-
ture variations and fluctuations in the suspended solids
level of the influent sludge have a significant effect on
both the methane gas production and the amount of heating
which is necessary to maintain the digestion zone at the
~esired elevated temperature operating level. As a re-
sult, if elevated temperature conditions are eO be main-
tained year round in the anaerobic digestion zone, an
auxiliary heat source is generally an essential apparatus
1~ 1.5~;~2 1 ~ 3 - 2
element of the sludge digestion system.
Since the rates of anaerobic digestion and resultant
methane gas formation are strongly influenced by the
suspended solids content of the sludge undergoing treat-
ment and by the temperature level in the diges-
tion zone, it is in general desirable to feed as concentrat-
ed a sludge as possible to the digester, thereby minimiz-
ing heat losses in the effluent stabilized sludge stream
discharged from the anaerobic digester while maximizing
methane production in the digester. However, even with
such provisions elevated temperatures are difficult to
maintain economically in the anaerobic digestion zone,
especially during winter months. Furthermore, even compar-
atively small temperature fluctuations in the anaerobic
digestion zone may result in disproportionately severe
process upset and souring of the digester contents, as
is well known.
In the anaerobic digestion process, the sludge solids
being treated undergo essentially three distinct sequential
treatment phases: first, a period of solubilization,
secondly, a period of intensive acid production (acidifica-
tion), and finally, a period of intensive digestion and
stabilization (gasification). Each of these steps is char-
acterized by the production of various intermediate and
end products in the digestion zone. Under nor~al opera-
ting conditions, all three phases occur simultaneously.
The primary gases produced during the final gasification
~ 4~2 l1~3' -~
phase are methane and carbon dioxide, ~hich normally form
more than 95% of the gas evolved, with 65-70% comprising
methane. Production of methane g2S in anaerobic digestion
results from the breakdo~n of many compounds by numerous
interdependent biochemical reactions which take place in
an orderly and integrated fashion. The comple~ organic
species in the sludge are converted by a variety of common
bacteria called acid-formers to volatile acids and alcohols,
without production of methane. These products from the
acid-forming phase are then converted to methane gas by
another variety of bacteria known as methane-formers.
The facultative acid-forming bacteria utilized in
anaerobic digestion are hardy and highly resistant to pro-
cess changes in their environment. Methane-forming bac-
teria, on the other hand, require anaerobic ^onditions
and are extremely sensitive to process changes in their
environment. For such reasons, oxygen should not be
present in the anaerobic digestion zone. The inadvertent
introduction of air to the digester will adversely affect
methane fermentation, as well as creating a potentially
hazardous situation due to combination of the combustible
methane gas with oxygen. In addition, methane-forming
bacteria are sensitive to such process conditions as p~
variations and presence of detergents,
ammonia and sulfides. In this respect, temperature stabil-
ity of the anaerobic digestion zone is particularly import-
ant. The methane-formers necessary in the digestion
-- 8 --
~154~2 l 1 ~3 2
process are highly susceptible to temperature fluctua-
tions, which decrease their activity and viability, re-
sùlting in e~cessive relative growth of acid-formers.
This in turn results in inadequately stabilized sludge and
a sludge product which is unsuitable, without further
treatment, for landfill or similar disposal. Further,
these methane-formers have a relatively low rate of
growth and such factor necessitates the long retention
times employed for anaerobic digestion even at mesophilic
temperatures~ Due to this low growth rate, there is danger
of washing the methane-forming organis m 9 out of the diges-
ter if the sludge solids retention time therein is reduced
beyond the previously described retention time lower
limits. Inasmuch as the anaerobic digester thus requires
long retention times to insure the presence of adequate
methane-formers and the influent sludge flow rate to the
digestion zone is in general quite low, the tankage re-
quirements for the digester are very large. Operation at
elevated temperature is thus difficult, requirlng large
imputs of heat to the digester together with close con-
trol of the digester temperature level. As previously dis-
cussed, the prior art,faced with these considerations, has
utilized the methane produced by the anaerobic digestion
process as heating fuel for the digester, to maintain
constant elevated temperature even under e~treme ambient
temperature fluctuations. Such use of methane has proven
effective in minimizing the large heating energy
9 -
1 _ 3 _ 7
11~54~Z
requirements of the process.
As an alternative to the foregoing methods, biode-
gradable sludge can be digested aerobically. Air has
commonly been employed in practice as the oxidant for this
purpose. It is known that aerobic digestion proceeds ~ore
rapidly at elevated temperatures. As temperature rises
from 35C, the population of mesophilic microorganisms
decline and thermophilic forms increase. The temperature
range of 45C to 75C is often referred to as the thermo-
philic range where thermophils predominate and where most
mesophils are extinct. Above this range, the thermophils
decline, and at 90C, the syste~ becomes essentially sterile.
Because of the more rapid oxidation of sludge, thermophilic
digestion achieves more complete removal of biodegradable
volatile suspended so~ids than the same period of diges-
tion at ambient temperature. A more stable residue is
obtained which can be disposed of without nuisance. It
is also established that -hermophilic digestion can ef-
fectively reduce or eliminate pathogenic bacteria in the
sludge, thereby avoiding the potential health hazard
associated with its disposal.
When diffused air systems are used to supply oxygen
for digestion, with the air being passed through the body
of sludge in a digestion tank and freely vented to the
atmosphere, the loss of heat from the s:ludge to the air
- 10 -
11234-2
~ 4 ~2
being passed through ~he digester tends to become sub-
stantial in magnitude. As a result, aerobic digestion
using air has heretofor typically involved digestion
withmesophilic microorganisms. Air systems in general
are not employed to carry out thermophilic digestion,
unless a substantial level of heating energy is readily
available for maintaining temperature of sludge in the
digester in the thermophilic range. Such situation may
for example exist if the digestion system is located in
close physical proximity to a power generating plant
which produces a large quantity of waste heat, so that
such heat energy is in essence "free" for use in the
digestion facility, Air contains only 2110 oxygen and
only about 5-10% of the oxygen content thereof is dissolved.
As a result, a very large quantity of air must be used to
supply the oxygen requirements, and the sensible heat
of the "spent" air and the latent heat required to
saturate the spent air with water vapor are substantial.
As a result of these heat losses in air digestion,
autothermal heat effects are generally minor, and very
large quantities of external heat are needed to sustain
temperatures at beneficial levels.
It is known that the heat losses in aerobic
digestion can be greatly reduced by using oxygen-enriched
gas rather than air. If the oxygen is utilized effectively.
the amount of gas which must be fed to and vented from
the digester is considerably smaller compared to air,
because much or all of the nitrogen has been preliminarily
removed. Heat losses due to sensible warmup of the gas and
-11-
11234-2
1~ 154;~Z
to water evaporation into the gas are decreased. These
reductions in heat losses are sufficient for autothermal
heat alone to sustain temperature at levels appreciably
higher than ambient, so that the digestion zone is able
to operate efficiently in the thermophilic temperature
~ 4 ~ ~ 11234-2
regime with little or no input of external heat to the procesc
Since thermophilic stabilization is much more rapid than
mesophilic stabilization,the necessary residence ti~e in
the aerobic digestion zone is greatlv reduced in the ther~o-
philic mode. This in turn permits the use of smaller
basins which further reduces heat losses to the surround-
ings. Because of the faster rate of oxidation of sludge,
thermophilic aerobic digestion can achieve suitably high
biodegradable volatile solids reduction, as for example,
80-90% reduction levels, in comparatively short sludge
retention periods on the order of 3 to lO days.
Despite its substantial attractiveness, thermo- r
philic aerobic digestion has several associated dis-
advantages relative to anaerobic digestion. First,
since the thermophilic aerobic digestion process is
oxidative in character, the process produces a bio-
oxidation reaction product gas containing carbon dioxide
and water vapor which have no end use utility but rather
are desirably vented to the atmosphere. By contrast,
anaerobic digestion produces methane gas as a reaction
by-prctuct which may be exported from the treatment racil-
ity and is also useful as a fuel gas for satisfying the
heating energy requirements associated with digestion at
elevated temperatures. In addition, the aerobic diges-
tion zone requires a much greater energy expenditure,
for mixing and gas-sludge contacting, than is re-
quired in the anaerobic digestion system for mixing of
1 ~ ~<~ ~ ~ 2 L'~3'-~
the digester contents.
Accordingly, it is an object of the present inven-
tion to provide an improved process for digestion of
sludge.
It is another object of the invention to provide a .,
sludge digestion process employing aerobic digestion and
anaerobic digestion at elevated temperature, in a manner
which utilizes the advantages of each while minimizing
their attendant disadvantages.
Other objects and advantages of this invention will
be apparent from the ensuing disclosure and appended
claims.
- 14 -
34-2
S~IARY OF T~IE _rNVE~TION
Thi3 inventlon relates to a process for warm
digestion of sludge, utilizlng aerobic and anaerobic
digestion.
~ riefly, the sludge dlgestion process of this
invention comprises introducing the 9 ludge and aeration
feed gas comprising at least 20 percent oxygen (by volume)
to a first digestion zone and mlxing same therein in
sufficient quantity ar.d rate for aerobic digestion of the
sludge while maintaining total suspended solids content
(MLSS) of the sludge at least at 20,000 mg/l and temper-
ature of the sludge in the range of from 35 to 75~ in
the first digestion zone.
The foregoing aerobic digestion is continued for
Yludge retention time (dur~tion) of from 4 to 48 hours
to partially reduce the biodegradable volatile suspended
solids content of the sludge introduced to the first
digection zone, with partially stabilized sludge being
discharged from t~e first digestion zone~ This dis-
charged partially stabilized sludge is then anaerobically
di~ested in a covered second digestion zone w~ile
maintaining temperature of the sludge therein in the range
of from 25 to 60C for sufficient sludge retention time
~duration)to further reduce the biodegradable volatile suspend-
ed solids content of the sludge, to less than about ~l0/O of
l~ lX~Z 11234-2
the biodegradable volatile suspended solids content of
the sludge introduced to the first digestion zone, and
form methane gas. Further stabilized sludge and the
methane gas are discharged from the second digestion
zone.
As used herein, the term "sludg~'means a solids-
llquid m~ e eh~racterized ~y a solids phase ~nd
an associated l'.quid phase, in which
the solids are at least partially biodegradable, i.e.,
capable of being broken down by the action of living
microorganisms. Biodegradable sludges are generally
characterized according to their biodegradable vol-
atile suspended solids content (BVSS) and also by
their volatile suspended solids content (VSS), with
the latter parameter including both biodegradable
and non-biodegradable volatile suspended solids. As
used herein "biodegradable volatile suspended solids
content" is essentially tke maximum reduction in solids
achievable by aerobic digestion of the sludge, as car-
ried out by aerating the sludge with oxygen-containing
gas at ambient temperature, e.g., 20C. Maximum
reduction of solids is assumed to be reached after 30 day~
aeration. Specifications for such determination are
contained in "Water Pollution Control", Eckenfelder,
W. W. and Ford, D. L., The Pemberton Press, 1970, page
152. By determining VSS levels for the fresh sludge and
again after 30 days aeration, the biodegradable fraction
of the total VSS may be calculated as:
11234-2
~ a ~
VSS(Fresh) - VSS(30 days)
VSS(Fresh)
As used herein "volatile suspended solids content"
of a sludge means the volatile content of the sludge
solids as determined in accordance with tests 224A and
224B in "Standard Methods for the Examination o$ Water
and ~astewater", Thirteenth Edition (1971), published
jointly by American Public Health Association, American
Water Works Association, and Water Pollution Control
Federation, pages 535-536. The term "stabilized
sludge" refers to sludge having a reduced biodegrad-
able volatile suspended solids content subsequent to
and as a result of digestion treatment. "Sludge re-
tention time" as used herein means the average duration
of time in which the sludge is contained in a given
digestion zone, as calculated by the following for-
1~ mula:
~ , Vd
where
- sludge retention time (days, or hours);
Vd ~ volume of sludge in the digestion zone
undergoing treatment, (ft3); and
Qs = volumetric flow rate of sludge fed to
the digestion zone, tft3/day, or ft3/hr)
As used herein, the term "aerobic digestion"
means the biodegradation of sludge solids as carried out
under the action of aerobic microorganisms. Such mode of
digestion requires that oxygen be dissolved in the liquid
phace of the sludge, so as to be accessible to microo-;gan-
- 17 -
11~34-~
1JI ~54~
r
isms in the sludge, in sufficient quantity and rate so th2t
the oxygen requirements for biodegradation are met. As used
herein, the phrase "mixing sludge and aeration feed gas in
sufficient quantity and rate for aerobic digestion of the
sludge" means either oxygenation of the sludge at a rate
which is equal to at least 10% of the empirically determined
specific oxygen uptake rate tSOUR) value, as determined by
the procedure set forth hereinafter, or sludge oxygenation
with the relative quantities of aeration feed gas and sludge
and rate of aeration being sufficient to obtain utilization
of at least 0.03 lbs. of oxygen per lb. of volatile sus-
pended solids in the sludge introduced to the first digestion
zone, or any suitable alternative quantification of the
sludge/aeration feed gas contacting step which is sufficient
to insure the existence of aerobic digestion in the first
digestion zone, in accordance with the definition of aerobic
digestion set forth hereinabove.
In terms of the specific oxygen uptake rate, the sludge
oxygenation requirements necessary for aerobic digestion may
suitably be fixed in accordance with the following procedure,
which is readily adaptable as a bench-scale method of ident-
ifying the sludge oxygen demand. The sludge to be treated is
flowed through a small-scale test vessel at sufficient
volumetric flow rate to obtain the predetermined sludgeret~
time selected for the aerobic digestion operation, which for
the aerobic digestion step of the instant invention is in the
range of 4 to 48 hours, while contacting the sludge with an
aeration gas containing at least 50 percent oxygen (by vol-
11234-2
ume). The aeration is carried out so as to maintain a dissolved
oxygen concentration (DØ) of at least 2 mg/l in the sludge,
as measured by any suitable DØ probe of conventional type.
During the aeration, sludge in the test vessel is maintained at
the predetermined temperature selected for the aerobic dig-
estion operation, which for the aerobic digestion step of the
instant invention is in the range of 35 to 75C. The fore-
going test treatment of sludge, which may require dilution
of the influent sludge to the test vessel with tap water in
order to obtain the required DØ level of at least 2 mg/l,
is conducted until steady-state operation is achieved, which
may require an extended period of operation of the test
system as for example on the order of 5 - 7 days.
Upon the achievement of steady-state operation
in the test system, a measured sample volume of sludge is
withdrawn from the test vessel and, while maintained at the
same temperature as previously existing in the test vessel,
is rapidly aerated, as for example by intense agitation
contacting of the sludge with aeration gas containing at
least 50 percent oxygen (by volume), so as to raise the DØ
level of the aerated sludge to about 7.0 mg/l. At the
point at which the DØ level of approximately 7.0 mgtl
is reached, aeration of the sample volume of sludge is
terminated. Thereafter, during the subsequent decay of DØ
level in the sludge from the value o~ approximately 7.0 mg/l
existing at the termination of aeration down to substantially
negligible DØ level, the time which is required for the
- 19 -
~ 34_2
D.O. to drop from a value of 6.0 mg/l down to 1.5 mg/l is
measured. The oxygen uptake rate (OUR) of the sample
volume of sludge is then computed by dividing the change
in D.O. level during the period of measurement , i.e..
4.5 mg/l (a 6.0 mg/l - 1.5 mg/l), by the time which was
required for the D.O. level to decline from 6.0 mg/l to
1.5 mg/l. From the resulting OUR value,
the specific oxygen uptake rate (SOUR) is calculated by
dividing the OUR value, having units of mg/l/time, by the
solids concentration of the sample volume of sludge, in
mg/10 The ~OUR value as thus calculated has units of mg
oxygen/time/mg solids.
Based on the foregoing calculation of the SOUR
parameter for the sludge to be treated, the quantity and
rate of oxygen transfer from the oxygen-containing aeration
gas to the sludge in the aerobic digestion step of the
instant process can be established. In order to satisfy the
respiration (oxygen consumption) requirements of the sludge
for aerobic digestion, as based on considerations of
obtaining adequate stabilization of sludge in the aerobic
digestion step prior to the subsequent anaerobic digestion
step, the oxygenation of sludge in the first digestion
zone in the instant process should be carried out at a
rate which is equal to at least 10% of the empirically
calculated SOUR value. In preferred practice, such
oxygenation of sludge should be carried out at a rate
which is equal to at least 50V/o of the empirically calculated
SOUR value.
- 20 -
.
~ 4 ~ 234-2
Alternatively, based on considerations of t'ne quantity
of oxygen which is required to biodegrade a unit quantity
of volatile suspended solids in a given sludge, as deter- -
mined for sludges of various characteristics, the mixing
of sludge and oxygen-containing aeration feed gas to carry
out aerobic digestion in the first digestion zone in the
instant process should be conducted with the relative
quantities of aeration feed gas and sludge and rate of
aeration being sufficient to obtain utilization, i.e.,
uptake by the sludge, of at least 0.03 lbs. oxygen per lb.
of volatile suspended solids in the sludge introduced to
the aerobic digestion zone. The minimum value of such con-
tacting ratio is associated with a threshold level of aerobic
digestion which is necessary to insure adequate stabiliz-
ation of sludge in the first digestion step of the instant
process prior to the subsequent anaerobic digestion step
thereof. As a practical matter, it is generally desirable
to conduct the sludge oxygenation with the relative propor-
tions of sludge and aeration feed gas and rate of aeration
being sufficient to obtain utilization by the sludge of
from 0.1 to 0.35 lbs. oxygen per lb. of volatile suspended
solids in the sludge introduced to the aerobic digestion
zone. Such gas-to-sludge contacting ratios generally per-
mit the volatile suspended solids content of the sludge in-
troduced to the first digestion zone in the instant process
to be aerobically reduced by from about 5 to 20% in the
first digestion zone. In general, the volatile suspended
solids content of the sludge entering the aerobic digestion
,.one is desirably reduced by at least 5% therein in order
- 21 -
~ 11234 ~
to produce a sufficiently partially stabilized sludge for
passage to the subsequent anaerobic digestion step. Such
minimum level of partial stabilization is particularly des-
irable so that the anaerobic digestion step is adequately
"buffered" by the aerobic digestion step against process
upsets, deriving from the changes in the character of the
sludge entering the overallprocess system. On the other
hand, the reduction of the volatile suspended solids
content of the feed sludge introduced to the aerobic
digestion zone, in the course of treatment in such
zone, is desirably maintained at a level of about 20%
or less in order to fully realize the synergistic advan-
tages of this invention. Such advantages are discussed
more fully hereinafter and include an unexpectedly high
net production of methane gas from the anaerobic digestion
second step relative to a conventional anaerobic digestion
process system. As a balance of the forego ng consider-
ations, the aeration feed gas and sludge mixing operation
in the first digestion zone should most preferably be
conducted with the relative quantities of aeration feed
gas and sludge and rate of aeration being sufficient to
obtain utilization by the sludge of from 0.15 to 0.25 lbs.
oxygen for each lb. of volatile suspended solids in the
sludge introduced to the first digestion zone.
As used herein, the term "anaerobic digestion" means
the biodegradation of sludge solids as carried out in the
absence of free oxygen.
- 22 -
X~Z ll~3~l-2
The present invention is based on the surprising dis-
covery that an aerobic digestion zone operating in the thermo-
philic or near-thermophilic temperature regime may advan-
tageousl~ be integrated with a downstream anaerobic digestion
zone to provide partial digestion of sludge in each of the
sequential zones, and that such integration provides sub-
stantial process improvement beyond that which would be
expected based on consideration of the respective digestion
steps in the treatment process taken separately, as shown
more fully hereinafter.
The prior art has not sought to combine elevated temp-
erature aerobic and anaerobic digestion of sludge in the
manner contemplated by the present invention for
numerous reasons. First, the tankage associated with
the anaerobic digestion process, as discussed earlier
herein, is extremely large and it has been found necessary
to produce large amounts of methane for heating fuel to
insure econo~ic operation of the hugç digester tanks. The
combination of an anaerobic digester with an aerobic di-
gestion step would thus appear undesirable due to con-
siderations of overall tankage requirements for the com-
bined process, which one would expect to be larger than
the tankage associated with either digestion process alone.
Such combination thus appears on its face to merelv dupli-
cate the functions normally associated with each of the
aerobic and anaerobic digestion processes, at an increas-
ed cost of equipment wiehout expected benefit in treat-
ment efficiency.
_ 23 -
~ 4~z 11234-2
Furthermore, the combination of an anaerobic di-
gester with an aerobic digestion step would appear un-
desirable due to the expected inability of the anaerobic
digestion step to provide only partial digestive treatment
of sludge in the combined system,at sludge retention time
levels less than the long retention times characteristic
of conventional anaerobic digesters operating alone. As
discussed earlier herein, long retention times are
necessary in the anaerobic digestion step to obtain
efficient methane production and sludge stabilization. If
anaerobic retention time were reduced below its normal
full-treatment level in a combined aerobic/anaerobic di-
gestion process, so as to secure only partial digestion
in the anaerobic step, one would expect an excessive
depletion of the methane-formers in the short retention
time anaerobic step,by loss of these slow-growing species
in the effluent from the digester, with resulting in-
adequacy of sludge stabilization in the combined process.
In addition to the foregoing reasons, the combined
aerobic/anaerobic digestion system would appear disadvanta-
geous from the standpoint of operating stability, since
each of the aerobic and anaerobic digestion steps alone
requires close operating temperature control when opera-
ting at elevated temperature levels, so that coupling of
the two respective processes would appear to require still
tighter temperature control with a potential for increas-
ed adverse effect from temperature instability and
fluctuation.
Finally, a combined aerobic/anaerobic digestion
system would appear to be disadvantageous based on a
- 24 -
1123'1 - ~
4~
consideration of potential carryover of residual dissolved
oxygen from the upstream aerobic step to the downstream
anaerobic segment of the process. ~s indicated herein-
above, methane-forming bacteria present in the anaerobic
digestion zone are strictly anaerobic in character
and are extremely sensitive to changes in their environ-
ment. It is well established that any significant intro-
duction of oxygen into the anaerobic digestion zone willadversely affect sludge stabilization by methane formation
and create the danger of evolution of oxygen from the
liquor to the methane-containing gas phase and formation
of a combustible gas mixture in the digester.
In contrast to the foregoing anticipated behavior, it
has unexpectedly been found that the teployment of a
thermophilic or near-thermophilic aerobic digestion ~one
upstream of a mesophilic or thermophilic temperature anaerobic
digestion zone and operation of these respective
zones in accordance with the process of the present
invention not only provides an operable and eoo~-
omic sludge treatment system but results in a digestion
system with unique overall process improvements relative
to prior art processes, due to the synergism which is
achieved between the aerobic and anaerobic digestion seg-
ments in the instant process. For example, the process
of the instant invention is able to provide a thermal
operating stability in the overall sludge digestion system
which it is not possible to achieve in either constituent
- 25 -
11234-2
4~2
digestion step operating alone. Furthermore, ehe integrated
digestion process according to the preseot invention
produces a highly stabilized sludge, despite a marked re-
duction in sludge retent.ion time for the o~erall process
beyond ~hat which would be expected based on the anticipated
additive retention time requirements for the constituent
partial digestion steps. Particularly surprising in this
respect is the finding that the anaerobic digestion zone
in this process is able to operate at sludge retention
time levels substantially less than are required
'- - 26 -
1123'-2
for full stabilization treatment of sludge in conventional
anaerobic digesters operating alone, and that such opera-
tion is achieved without loss of utility or treatment
efficiency such as would be expected. As an example of
retention times suitably employed under the i~vention, a
pilot plant system emb~dying the instant process has been sat-
isfactorily operated with a slutge retention time in the aer-
obic first step of 24-48 hours and a retention eime in the
anaerobic second step as low as ' - S days. The foregoing
advantages are realized in the present invention along
with a substantial reduction in system tankage require-
ments relative to a conventional anaerobic digester
system, but with retention of an unexpectedly large
portion of the methane production capacity of the con-
ventional anaerobic digester taken alone, as will be shown
in greater detail hereinbelow. However, by way of example,
the system of the present invention may employ about 60%
of the tankage required by a prior art anaerobic diges-
tion system, yet retain approxim ately 75 percent of the
methane production capacity of the latter. The instant
process yields substantially greater production of methane
than is required for process heating fuel requirements,
with the result that a significantly larger amount of high
methane-content off gas is available for export from the
sludge digestion facility relative to the prior art
anaerobic digestion system. Finally, no significant
- 27 -
11~34-2
1~ 154~Z
carryover of oxygen from the first digestion zone to the gas
phase in the second digestion zone has been found to occur
in the instant process.
The reasons for the unexpected advantages of this in-
vention, as described above, are not fully understood. It
is probable, however, that the absence of significant carry-
over of oxygen from the first digestion zone to the second
is due to the unexpectedly high oxygen uptake rate of the
sludge in the first digestion zone which serves to rapidly
and thoroughly deplete any dissolved oxygen content in the
sludge passed from the first to the second digestion
zone, before any appreciable evolution of dissolved
oxygen to the gas phase in the second digestion zone
can occur. The striking low sludge retention times in the
instant process, particularly in the anaerobic digestion
step, together with the thermal stability characteristic of
the process and the unexpectedly high methane production
capacity of the anaerobic step, may be a consequence of
a chemical or biological acclimatization of the sludge
and microorganisms in the aerobic digestion zone which
provides enhancement of efficiency of the subsequent an-
aerobic treatment step. Nonetheless, we do not wish to be
bound by any particular theory by way of explanation of such
performance behavior and, accordingly, the foregoing should
not be construed in any limiting manner as regards the
present invention, subject only to the essential steps
and features disclosed and claimed herein.
- 28 -
1123~_2
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic flowsheet of a digestion
process according to one embodiment of the instant inven-
tion, wherein heat is recovered from the effluent streams
from each of the first and second digestion zones.
Fig. 2 is a schematic flowsheet according to another
embodiment of the invention, wherein oxygen-depleted
digestion gas discharged from a covered first digestion
zone is utilized in secondary treatment oxygenation of
BOD-containing water.
Fig. 3 is a schematic flowsheet according to yet
another embodiment of the invention, wherein sludges from
primary and secondary wastewater treatment steps are
passed to the sludge digestion zones.
Fig. 4 is a graph of the temperature of the influent
sludge to the first digestion zone which is necessary to
maintain a 50C operating temperature in the first diges-
tion zone, plotted as a function of the total suspended
; solids content (MlSS) of the influent sludge to the first
digestion zone.
Fig. 5 is a schematic flowsheet of still another
embodiment of the invention, wherein a minor portion of
the influent sludge to the process system is diverted to
the second digestion zone.
- 2~ -
11234-2
DESCRIPTIO~ OF THE PREFERRED EMBODIMENTS
Referring now to Fig. 1, a schematic flowsheet
of a process according to one embodiment of the instant
invention is shown such as is suitable for sludge treatment
with a thermoPhilic or near-thermo~hilic aerobic first di-
gestion step followed by mesophilic anaerobic digestion.
Sludge, which may derive from a source such as a primary
sedimentation tank, the clarifier in an activated sludge
~0 wastewater treatment plant, or a trickling filter, or from
som~ other sludge-producing system, enters the process in
line 8 and is sequentially heated in heat ex^hangers 22
and 15, as for example to a temperature of 30 - 35C,
prior to introduction to the first digestion zone 10, to
maintain the temperature in the zone in the range of from
35 to 75C, and preferably in the thermophilic range of
from 45 to 75. The ambient temperature sludge in line
8 is first heated in heat exchanger 22 by passage of the
sludge in indirect heat exchange counter-current flow re-
lationship with the further stabilized sludge ~ischarged
from covered second digestion zone 20 in line 24. In
this manner heat is recovered from the further stabilized
sludge and the resulting cooled stabilized sludge is dis-
charged from the heat exchanger 22 and passed out of the
system in line 25 to final disposal or other end use.
The further stabilized sludge entering the heat exchanger
22 in line 24 may suitably be at 2 temperature of 35 -
40C so that the influent sludge exiting the hea~ exchanger
- 30 -
~ 2 11~3~-2
in line 9 is warmed to temperature of 28 - 30C. From
line 9 the partially warmed influent sludge is further
heated in heat exchanger 15 to a temperature of 30 to 35C
by indirect countercurrent flow heat exchange with the
partially stabilized sludge discharged from the first
digestion zone 10 in line 14 and passed from the heat
exchanger in line 16 to the second digestion zone 20.
As an alternative to the above-described heae ex-
change with sludge product streams from the respec-
tive di~estion zones, the influent sludge mav he
heated prior to introduction to the first digestion zone
by indirect heat exchange with a suitable externally
supplied heating medium such as steam or hot water, al-
though heat recovery from the warm digestion zone product
streams is preferred since it efficiently serves to con-
serve heat within the process and minimizes heating energy
requirements for the proc~ss. Although heating of the
influent sludge prior to its introduction to the first
digestion zone is not essential in the broad practice of
the present invention, it may be desirable in practice to
maximize the therm~l efficiency of the elevated tempera-
ture process. The desirability of such heating of the
sludge, as will be discussed more fully hereinafter, de-
pends on the influent sludge solids content,sludge re-
tention time in the aerobic digestion zone,and other
process parameters.
- 31 ~
11234-2
Z
The further heated sludge discharged from the
heat exchan~er 15 in line 11 is introduced to first diges-
tion zone 10 along with aeration feed gas from line 17 as
the process fluids for the first digestion step. The
aeration feed gas in line 17 minimally comprises at least
,'0 percent oxygen (by volume), with at least 50 percent
and desirably at least 80 percent oxygen content (by volume)
aeration feed gas being preferred in order to provide suit-
ably high mass transfer driving force and rate of oxygen
dissolution in the sludge at the high sludge temper-
atures in the first digestion zone contemplated under the
present invention. Line 17 is connected to a source of
oxygen-containing aeration feed gas (not shown) which may for
example comprise compressed air supply means or, if the
aeration feed gas is, as preferred, of high oxygen content,
the source for same may suitably comprise a cryogenic
oxygen plant or supply vessel or an adiabatic pressure
swing adsorption air separation unit, as conventionally
a~-ailable as supply source means for enriched oxygen-
containing gas. As shown, the oxygen-containing aeration
feed gas in line 17 may also be heated by heater 19 to
assist in maintaining the temperature in the digestion
zone 10 at the desired process level. In the general
practice of the present invention, air or other aeration
feed gas of low oxygen content, i.e., 20-50 percent
- 32 -
234-2
oxygen by volume, may suitably be employed when auto-
thermal heating of the sludge in the aerobic digestion
zone is not required to maintain t~le temperature of
sludge therein in the required range of from 35 to 75C,
such as where a large source of externally supplied
heat energy is available for sludge heating to maintain the
requisite high temperature in the aerobic digestion zone.
As mentioned earlier herein, heat losses with air (or
other low oxygen content aeration feed gas) tend to be
very large, so that aeration feed gas of at least 50 per-
cent and desirably at least 80 percent oxygen content (by
volu~e) is preferred in order to promote autothermal sludge
heating in the aerobic digestion zone while minimizing the
quantity of oxygen-depleted digestion gas which is wasted
from such digestion zone and which otherwise carries
heat energy out of the process system. In addition, high
oxygen content aeration feed gas, i.e., containing at least
50 percent oxygen (by volume), is preferred in order to
increase the extent of oxygen mass transfer from the
aeration feed gas to the sludge during aerobic digestion
and thereby facilitate a more intense aerobic digestive
action than is achievable with low oxygen content aeration
feed gas. Regardless of whether high oxygen content or low
oxygen content aeration feed gas (as defined above) is
employed in the aerobic digestion zone of the instant
process, it is generally preferred to provide the aerobic
digestion zone with a cover to form a gas space overlying
,
11234-2
~ 4~ Z
the sludge therein from which waste oxygen-depleted
digestion gas can be vented. Such ~rrangement permits a
controlled venting of waste gas,as for example by a small
vent conduit passing through the cover and join~ng the gas
space with the external gas environment, and thereby pro-
motes heat retention in the aerobic digestion zone relative
to an uncovered zone wherein oxygen-depleted aeration gas
is allowed to pass freely in bulk from the sludge volume
being treated into the external gas environment, i.e., the
ambient atmosphere. In addition, where high oxygen con-
tent aeration feed gas is employed in the aerobic digestion
zone, it may be desirable to provide a cover for the
digestion zone to form a gas space from which the oxygen-
containing aeration gas can be recirculated against the sludge
as for example by recirculation of gas from the overhead
gas space to a submerged sparger device, or in which the
sludge can be recirculated against the aeration gas, as
for example by means of a surface aeration device.
Such aeration gas or sludge recirculation arrangements
permit the aerobic digestion step to realize high utilization
of the oxygen content in the aeration feed gas introduced
to the aerobic digestion zone.
In the aerobic digestion zone 10, the sludge and
aeration feed gas fluids are mixed. If the digestion
zone 10 is provided with a cover and high oxygen con-
tent aeration feed gas is employed, one of the sludge
and aeration feed gas fluids may desirably, as indicated
above,and simultaneously
-34-
L1234-2
~5 ~r,
.~4
with the mixing,be recirculated agains~ the other fluid
in the digestion zone in sufficient quantity and rate for
aerobic digestion of the sludge while maintaining the
total suspended solids content (MLSS) of the sludge at
least at 20,000 mg/l. Such mixing and fluid recirculation
is suitably effected by the contacting means 12 which may
in practice comprise a submerged turbine sparger and
a gas compressor, with the latter coupled to the gas head
space in the digestion zone and to the gas sparger, for
recirculation of the oxygen-containing aeration gas against
the sludge, or, alternatively, the contacting means may
comprise a surface aeration device for recirculating
sludge against aeration gas in the gas head space of
digestion zone 10. Recirculation of one of the sludge
and aeration gas fluids against the other fluid in the
aerobic digestion zone may, as indicated above, be des-
irable in practice where high oxygen content aeration feed
gas is employed in order to obtain high levels of oxygen
dissolution in the sludge and high utilization of the
oxygen contained in the aeration feed gas. Nonetheless,
such recirculation is not essential in the broad practice
of the present invention and in some instances it may be
possible to obtain adequate dissolution of oxygen in the
sludge and high utilization of oxygen in the aeration feed
gas with a once-through flow of aeration feed gas through
the aerobic digestion zone. The relative proportions of
aeration feed gas and sludge to be contacted in the first
digestion zGne for aerobic digestion nerein may suitably
- 35 -
11234-2
1~111 .~4~Z
be established in the manner disclosed in the preceding
Summary section herein, as based for example on an em-
pirical determination of the specific oxygen uptake
rate (SOUR) of the sludge to be treated, or on the basis
of the amount of oxygen which is required to biodegrade
a unit quantity of volatile suspended solids in such
sludge. In some systems, it may be desirable to insure
the existence of intense aerobic digestive action in the
first digestion zone by the maintenance of high dissolved
oxygen (DØ) levels in the sludge therein, as for example
at least 2 mg/l, but in general, the oxygen uptake rate of
sludge in the first digestion zone of the instant process
is sufficiently high so that maintenance of a substantial
D,0. level in the sludge being oxygenated is not necessary
for efficient aerobic digestion.
In the aerobic digestion zone, the total suspended
solids content (MLSS) of the sludge is maintained at
least at 20,000 mg/l so as to facilitate the maintenance
of high sludge temperature in the first digestion zane,
- 36 -
11234-~
Z
as necessary to obtain a satisfactory degree of partial
sludge stabilization in the aerobic digestion zone at
short retention times.
Under the foregoing process conditions, sludge is
maintained in the first digestion zone for digestion at
a temperature in the range of from 35 to 75 C, and pre-
ferably in the thermophilic range of from 45 to 75C,
for rapid biodegradation of the sludge volatile suspended
solids content. In this respect, it is to be appreciated
that aerobic digestion in the near-thermophilic temper-
ature range of 35 to 45C may suitably be employed in
the broad practice of the present invention to achieve
solids degradation rates which, while not as rapid as the
rates characteristic of thermophilic operation, are suf-
ficiently high to achieve adequate sludge stabilization
at the low sludge retention time values characteristic of
the first digestion step in the instant process.
The aerobic digestion step is continued in the
first digestion zone for sludge retention time of from r
4 to 48 hours, to partially reduce the biodegradable vol-
atile suspended solids content of the sludge introduced to
the first digestion zone. As indicated earlier herein,
the aerobic digestion step is préferably conducted so as
to reduce the volatile suspended solids content
~ 11234-2
of the sludge introduced to the first digestion zone ~y
from 5 to 20 percent, for the reasons previously set forth.
In the aerobic digestion step, the sludge retention time
should be at least 4 hours in order to obtain
a sufficient extent of partial stabilization in
the first digestion zone; at retention times below 4 hours,
the extent of sludge stabilization required in the subse-
quent anserobic treatment step becomes disproportionately
large relative to the stabilization level in the aerobic
first step and the overall sys~em retention time and
tankage requirements begin to approach those of the
conventional anaerobic digestion system, with increasing
loss of the unexpected improvement in these process var-
iables (i.e., overall system retention time and tankage)
characteristic of operation at retention times in the
aerobic digestion step of from 4 to 48 hours. For
correspondingly similar reasons, the sludge retention
time in the aerobic digestion zone should not exceed ~8
hours. Above such value, the extent of sludge stabiliza-
tion in the aerobic digestion zone becomes unduly large
with regard to the residual stabilization in the down-
stream anaerobic step, so that methane production in the
latter step tends to be seriously adversely reduced, and
again there is increasing loss of the unexpected improve-
ment of the overall system retention time and tankage
requirements achievable in connection with the aerobic
digestion sludge retention time range of from 4 to 48
- 38 -
~ 4~ 34-2
hours. Preferably, the retention time is in the range
of from 12 to 30 hours, and suitzbly from 12 to 24 hours,
based on the foregoing considerations.
Following the above-described aerobic digestion treat-
ment, partially stabilized sludge is discharged from the
aerobic zone in line 14 and oxygen-depleted digestion gas
is separately discharged from the aerobic zone in line 18.
In the case where aeration feed gas containing at least 50
percent oxygen (by volume) is introduced to the first
diges~ion zone, the oxygen-depleted digestion gas dis-
charged therefrom desirably contains at least 21 percent
oxygen (by volume) in order to obtain suitably high util- -
ization of oxygen contained in the aeration feed gas
while maintaining the expenditure of energy for aeration
gas and sludge contacting at a suitably low level for
economic operation. In order to insure high oxygen util-
ization, particularly when using high oxygen content
aeration feed gas, the oxygen purity level of the digestion
zone vent gas in line 18 may readily be maintained at an
appropriate level by suitable regulation of the relative
rate~ of aeration gas introduction via line 17 and venting
in line 18, as for example by gas flow control valves in
either of the inlet or vent gas lines, coupled in controlled
relationship with an oxygen purity analyzer (not shown)
disposed in the vent line 18, in a manner well known to
those in the art.
- 39 -
11234-2
~ 4 ~ Z
It has been found that by maintaining the sludge in
the aerobic digestion zone of the present invention at
a ~hermophilic temperature of at least about 50-52C,
substantially complete pasteurization of the sludge is
achievPd. In the broad practice of the present invention,
partially stabilized sludge is discharged from the
aerobic zone 10 in line 14 at temperature in the range
of between 35C and 75C. Inasmuch as this specific
embodiment of the invention employs mesophilic anaerobic
digestion in the covered second digestion zone 20, heat may
desirably be removed from the partially stabilized sludge
in line 14 to ensure efficient operation of the anaerobic
sludge treatment step at a lower temperature than that
employed in the first digestion zone 10. Accordingly, the
sludge in line 14 is flowed through the heat exchanger 15
in indirect heat exchange relationship with the partially
warmed influent sludge entering heat exchanger 15 in line
9. The cooled partially stabilized aerobically treated
sludge then flows through line 16 for introduction to
the covered second digestion zone 200 Alternatively, the
partially stabilized sludge in line 14 could be cooled by
an externally supplied cooling medium such as the clarified
effluent of a wastewater treatment plant. Additionally,
in winter operation, there may be no need to utilize a
heat exchange step such as carried out by heat exchanger 15
for cooling of the partially stabilized sludge stream, since
heat losses to the environment from the second digestion
zone and the sludge stream flowing from the first to the
second digestion zone may satisfactorily compensate for
- 40 -
. ;, . ::
11234-2
the absellce o~ such heat exchanger. ~
The partially stabilized sludge introduced to the
second digestion zone from ine 16 is maintained therein
under anaerobic conditions at temperature of from 25
to 45C for sufficient sludge retention time (duration) to
further reduce the biodegradable volatile suspended solids
content of the sludge, to less than about 40%, and prefer-
ably less than 20%, of the biodegradable volatile suspen-
ded solids content of the sludge introduced to the first
digestion zone, and form methane gas.
In the broad practice of the present invention the
temperature of the sludge in the covered second digestion
zone is maintained in the range of 25 to 60C, which in-
cludes both operation in the mesophilic range of 25 to r
45C and operation in the thermophilic range of 45 to 60C.
For highly efficient operation, the anaerobic zone in
mesophilic operation is maintained at a sludge treatment
temperature of between 35C and 40C, and preferably bet-
ween 37C and 38Co A preferred operating temperature range
for anaerobic themophilic digestion is from 45 to 50CO
Operation in the foregoing preferred temperature ranges
provides particularly rapid degradative action of bio-
degradable volatile solids by the microbial strains involved.
In the operation of the anaerobic digestion zone 20,
the digestion zone contentsare advantageouslycontinuously
mixed byagitation means 21, there~ creating a large zone of
active decompositionand significantly increasing the rate
of the stabilization reactions. Retention time of the
sludge in the second digestion zone may suitably lie in the
range of from 4 to 12 days and preferably in the range of
from 5 to 9 days. Sludge retention times in the second
digestion zone of less than 4 days may be undesirable because
below such value, the retention time tends tobecome increas-
ingly inadequate to support a large viable population of
~ 41 -
11234-2
methane formers in the anaerobic step, with consequent
adverse effect on the overall sludge stabilization per-
formance of the digestion system. On the other hand, at
sludge retention times in the anaerobic digestion step of
greater than 12 days, the retention time for the second
digestion zone becomessuperfluously long, and the syner-
gistic retention time and tankage requirement benefits
realized by the integrated process o~ the invention in the
broad retention time range of 4 - 12 days become increas-
ingly difficult to achieve.
After anaerobic treatment of the sludge in the
; second digestion zone 20 is complete, the further stabiliz-
ed sludge produced thereby is discharged from the second
digestion zone in line 24 and heat exchanged for recovery
of heat content against the influent sludge feed in heat
exchanger 22 prior to final discharge from the process in
line 25 The methane gas formed in the second digestion
zone 20 as a product of the biochemical reactions con-
ducted therein is discharged from the anaerobic treatment
step in line 23 having flow control valve 26 disposed
therein.
As discussed earlier herein, continuous operation of
high ràte anaerobic digester at optimum above-ambient
temperatures has been inherent~ difficult to maintain in
conventional practice. Ambient temperature fluctuations
typically cause variation in both the temperature of the
influent sludge and the heat leak of the digester tank,
- 42 -
~ 4~ ~ 11234-2
which in turn results in undesirable temperature fluctua-
tions within the digestion tank. Such variations in te~pera-
ture, as discussed, influence the relative growth rates
of the acid-forming and methane-forming bacteria. The
acid-forming bacteria are typically very hardy and moder-
ate temperature fluctuations do not alter their metabolic
activity to any significant degree. Methane-forming
bacteria, on the other hand,are extremely sensitive to en-
vironmental conditions. If the maintenance of constant
temperature in the anaerobic digestion zone is upset by
even minor temperature fluctuations, instability in the
activity and growth of the methane-formers will be likely
to result. In consequence, the activity of the acid-
formers will dominate with an attendant accumulation of
the acidic intermediate products of decomposition and
lowering of the pH level in the digestion zoneO As the pH
d-ops, the activity of the methane-formers is further
reduced and a severe upset to the process is brought about.
- 43 -
1123~-2
~ 4~
The solution attempted for the above-described pro-
cess upset condition in the conventional anaerobic digestion
zone generally involves the addition of large quantities of
lime to the digester in order to increase the buffering,
and thereby raise the pH level in the digester. By in-
reasing the pH and decreasing the influent feed rate, it
is sometimes possible to bring the digester experiencing
such upset back into operation. This corrective measure,
however, is in general only suitable in the case of short-
term fluctuations or process upsets and is not usually
advantageous in the case of long-term fluctuations or
upset conditions
In the process of the present invention, control and
maintenance of elevated digestion zone operating tempera-
tures with minimal temperature fluctuations regardless of
climatic conditions is achieved through the integration of
a thermophilic or near-thermophilic aerobic digestion
step with a subsequent anaerobic sludge treatment step.
- 4~ -
~ 4~ ~ 11234-2
In the process of this invention, the thermophilic or
near-thermophilic aerobic digestion step is
generally capable of furnishing more than enough heat to
thermally stabilize the anaerobic step, by virtue of the
heat content of the partially stabilized sludge stream
which is flowed from the aerobic digestion zone to the
anaerobic step. As a result, temperature upsets in the
anaerobic zone of the instant process can be virtually
eliminated by varying such process parameters as sludge
retention time in the aerobic zone, the solids content of
the sludge fed to the aerobic zone, and the amount of heat
exchange warming of the feed sludge prior to its intro-
duction to the aerobic zone.
.__ _ .. ., . ,. . .. _ ., .. .. .. _ _ ,, . .. _ . ~.. _ . _ _ _ . _ . . . .... . _ ..
_, _._. _ .. _ ~ _. .. _ . __ .. . _ __. ... .. .. _ .,
- 45 -
.
11234-2
<~
Another substantial benefit provided by the inte-
grated process of this invention, beyond that attributable
to operating temperature stability, is its ability to ac-
commodate a sporadic upset, such as a shock load, without
loss of process efficiency. In a conventional anaerobic
digestion zone not only does the initial solubilization
phase of the digestion process occur rapidly, but micro-
bial action by faculative acid-forming bacteria also
occurs at a high rate. Upon the incidence of a sudden high
solids loading on the conventional anaerobic digestion
system, solubilization and acidification occur at a
faster rate than the methane-forming bacter~a can use
the acidic intermediate products. As a result, accum-
ulation of acidic constituents occurs in the digestion
zone, the pH in the digestion zone falls and a souring
of the digester contents is prone to occur. In the in-
stant process, however, the upstream thermophilic or
near thermophilic aerobic step promotes rapid solubiliz-
ation of biodegradable species in the sludge so that upon
the incidence of a shock loading to the aerobic zone a
resulting rapid solubilization occurs in the aerobic di-
~tion as well as stabilization of the most volatile portion
of the sludge, thereby smoothing out the shock and greatly
diminishing its effect on the downstream anaerobic zone.
In the further treatment step the anaerobic zone receives
a partially stabilized sludge on which the acid-forming
bacteria and methane-forming bacteria can grow in balance
- 46
~ 11234-~
Inasmuch as the aerobic digestion step in the
sludge treatment process of the present invention
preferably employs a thermophilic digestion zone, the
process disclosed and claimed in U.S. Patent No.
3,926,794 issued December lS, 1975 to N.P. Vahldieck
may advantageously be employed in conjunction with
the process of the instant invention for treatment of
wastewater for BOD removal therefrom by the activated :
sludge process and treatment oi the resultant waste
activated sludge by the process of this invention.
B
. - 47 -
~ 11234-2
By way of background, activated sludge secondary
treatment of wastewater is conventionally carried out in the
following manner. BOD-containing wastewater, as for ex-
ample municipal sewage, may first be su~jected to treat-
ment steps such as degritting and primary sedimentation
to separate a primary sludge comprising biodegradable
suspended solits from the wastewater and to thereby form
solids - depleted primary effluent which then enters the
secondary treatment system. In the secondary treatmen~,
the primary effluent and recycle sludge are mixed and aer-
ated at sufficient rate and for sufficient time to form
mixed liquor of reduced 80D content. Thereafter, the
mixed liquor is separated in~o purified liquid and activa-
ted sludge and at least a major portion of the activated
sludge is returned for mixing with the primary effluent as the
aforementioned recycle sludge. This wastewater treatment
system may suitably be employed in conjunction with the
process of the invention, wherein the primary sludge and
unreturned activated sludge are introduced to the first
digestion zone of the instant process as the sludge feed
therefor.
As taught in the aforementioned Vahldiec~ patent,
oxygen gas is used for thermophilic digestion of activated
sludge in a warm covered digestion zone and the vent gas
from the digestion zone is used at least as the major part
of the aeration gas in a cooler covered zone for activated
- 48 -
ll~.S~ 11234-2
sludge secondary treatment of wastewater. This patent
discloses that in order to obtain the high mass transfer
driving force necessary for efficient oxygen dissolution
in elevated temperature aerobic digestion, relatively
high purity oxygen gas should be used as aeration gas for
the digestion step. At elevated temperatures, the increas-
ed rate of the biochemical degradative action on the
sludge in the aerobic digestion zone produces substantial
amounts of carbon dioxide as gaseous reaction product.
Since the solubility of carbon dioxide is comparatively
low at the high temperatures characteristic of aerobic ther-
phil$c digestion, a significant amount of carbon dioxide
evolves into the gas phase in the aerobic digester, there-
by reducing the effective concentration of oxygen in the
aeration gas therein. Moreover, at high thermophilic
temperatures the oxygen gas phase concentration in the di-
t gestion zone is reduced further by water vapor present in
the aeration gas, due to the relztively high vapor
pressure of water at such temperature levels.
The foregoing effectssubstantially reduce the driving
force for oxygen mass transfer from the gas phase to the
sludge in the thermophilic digestion zone. The driving
force for oxygen mass transfer to the liquid sludge phase
is also reduced as a result of lower oxygen solubility at
elevated thermophilic temperature levels. For these
reasons, the Vahldieck patent teaches that the oxygen-
- 49 -
~ 4 ~ Z 11234-2
containing gas introduced to the thermophilic digestion
zone desirably has an oxygen concentration of at least
80% by volume. Under such operating constraint, the spent
aeration gas from the thermophilic digestion zone can
advantageously be used as the oxidant gas in an activated
sludge secondary treatment system. Fig. 2 herein is a
schematic flowsheet according to another embodiment of the
invention, illustratively showing a manner in which the
teachings of the Vahldieck patent may advantageously be
employed in conjunction with the practice of the present
invention.
Referring now to Fig. 2, BOD-containing water as for
example Qewage enters aeration zone 102 through conduit
101. First gas comprising at least 4Q% oxygen (by volume)
enters zone 102 through conduit 118 (dotted-line) and re-
cycled activated sludge also enters zone 102 through con-
duit 108 having pump 109 therein. Supernatant liquid from
sludge thickener 151 is also introduced to the covered
aeration zone 102 in line 150. In this Figuse, liquid ant
sludge flow conduits are showm by solid lines whereas
gas flow conduits are shown by dotted lines. For pur-
poses of simplicity, valves are not illustrated, but the
appropriate use of same in practicing the invention will be
well understood by those skilled in the art. The afore-
mentioned streams are intimately mixed in aeration zone
102 by mechanical agitation means 103. The latter may
comprise motor-driven impellers located near the liquid
- 50 -
4:~Z 11234- 2
surface or submerged below the surface, and the oxygen gas
may be introduced through conduit 118 either above or below
the liquid. Such apparatus is well known to those skilled
in the art and should be selected to achieve high contact
area between the fluids with minimal work expenditure. If
the oxygen gas is sparged or diffused into the liquid,
the bubbles should be small so that their total surface
area is large and their buoyancy is low. Dissolution of
oxygen is also aided by submerging the gas dispersion means
to a depth in the liquid where the hydrostatic effect is
signlficant.
In preferred practice, means are suitably provided
for continuously recirculating one fluid against the other
fluids in aeration zone 102. For example a compressor (not
shown) may be joined to the gas space in the aeration zone by
suitable conduit means for recirculating aeration gas to the
lower portion of the zone for release as small gas bubbles
through a conventional type sparger device. Alternatively,
the aforementioned mixing means may also be employed for
fluid recirculation as in the case of surface aeration im-
pellers. Aerating devices are commonly rated by the so-
ca~ed "air standard transfer eff~'ciency" which identifies
the capability of the device to dissolve oxygen from air
into zero - D0 tap water at one atmosphere pressure and
20C. Suitable devices are those which have an air standard
transfer efficiency of at least 1.5 lb 2 per HP-hr and
preferably at least 3Ø For these purposes the power used
in rating the device is
- 51 -
., . . , .; . . .
.. ....
4~2~
11234-2
the total power consumed both for agitating the liquor
and for gas-liquor contaceing~
The aforementioned oxygen is introduced and contacted
with the mixed liquor in sufficient quantity and rate to
maintain the dissolved oxygen content (D0) of the mixed
liquor at least at 0.5 mg/l. Also, the liquor temperatureiS
preferably maintained at least at 15C, so that ~neans may .
be needed in cold weather to prevent lower temperature in
ae~tion zone 102, as for example means for heating the in-
~ming wastewater in line 101. Design and operation of
wastewater aeration zone 102 may be as described in any
of U.S. Pat. Nos. 3,547,811: 3,547,812; or 3,547,815.
The oxygenated mixed liquor is discharged ~rom~cDver-
ed aeration zone 102 and passed through conduit 104 for
separation into purified supernatant liquid and activated
sludge in clarifier 105. Unconsumed oxygen-containing gas
is discharged from aeration zone 102 through conduit 119
and may for example be vented to the atmosphere. This gas
is discharged from the aeration zone at rate controlled
so that its oxygen content is no more than 40% of the
total oxygen introduced to the covered aerobic digestion
zone (discussed hereinafter). Returning now to clarifier
105, supernatant purified liquid is discharged through
conduit 106 and activated sludge drawn off through conduit
107 containing concentrated E&icroorganisms in the concen-
tration of about 10,000 to 40,000 mg/l total suspended
11234-2
solids content (MLSS). The major part of the activated
sludge,e.g., at least 8~o~ ~ returned through conduit 108
and pump 109 to the aeration zoneS preferably at a flow
rate relative to the BOD-containing wastewater such that
the recirculating sludge/BOD containing wastewater volume
ratio is 0.1 to 0.5. The flow rates into the covered aer-
ation zone 102 are preferably such that the total suspended
solid concentration (MLSS) therein is 4,000 - 12,000 mg/l
and the volatile suspended solid content (MLVSS) is 3,000 -
10,000 mg/l. The liquid-solid contact time in aeration
zone 102 for organic food absorption-assimilation is be-
tween 30 minutes and 24 hours. This time varies depend-
ing upon the strength (BOD-content) of the wastewater,
the type of pollutant, solids level in aeration and
temperature, all of which is understood by those skilled
in the art.
Not all the sludge separated in clarifier 105 is
returned to the aeration zone 102 for two reasons.
First, the activated sludge process protuces a net yield
of microorganisms because the mass of new cells synthe-
sized from i~purities in the wastewater is greater than
the mass of cells autooxidized during treatment. Seconl,
the wastewater normally contains non-biodegradable solids
which settle and accumulate with the biomass. Therefore,
a small fraction of the activated sludge must be discarded
in order to balance the microorganism population and the
food (BOD) supply and in order to suppress the
- 53 -
~ 4~ 11234 2
accumulation of inert solids in the sys~em. Sludge
wasting will usually comprise less than 3% of the total
separated sludge and rarely re thaD 15%.
While the waste sludge is a small fraction of the
total solids separated in the clarifier, it nevertheless
is often a large absolute quantity of material. Regard-
less of quantity, its disposition represents a significant
part of the cost of wastewater treatment, and in addition,
poses a serious ecological problem. The sludge is putres-
cible and is highly active biologically, and often contains
pathogenic bacteria. Potentially, the sludge is useful as
fertilizer and/or land fill, but before such use, it must
be well stabilized to avoid nuisance and health hazards,
and its high water content (e.g. 96-98%) must be reduced.
The waste sludge from the clarifier 105 is withdrawn
from the sludge recirculation loop in conduit 111, contain-
ing 10,000 to 40,000 mg/l MLSS, and initially at about the
ssme tem~erature as the wastewater in aeration zone 102,
e.g., 15 to 25 C, and is passed to a thickening tank 151.
The thickening tank 151 concentrates the sludge to between
20,000 and 60,000 mg/l ~LSS and passes the thickened
sludge underflow via conduit 152 to the sludge digestion
s~stem.
In some instances, such as high a~bie~t tem~erature
wastewater treatment operation, and high soLids concentration
levels in the underflo~- withdr~wn from thP clArifier~ thick-
ening of the w~ste sludgQ fro~ thP cl~rifier m3~ not ~e nec-
- 54 -
~ 4 ~ 2 11234-2
essary and t~ sludge in conduit 111 may suitably by-pass
tank 151 through conduit 153 and subsequently enter conduit
152 for passage to the sludge digestion system. The
thickener overflow (supernatan~ liquid) is passed via
conduit 150 to aeration zone 102, as previously described.
The thickened sludge in line 152 may be heated if
necessary before introduction into aerobic digestion zone
110, by methane boiler 130. Alternatively, the sludge
could be he3t exchanged with the stabilized sludge effluent
from the anaerobic digestion zone 120b, in a manner similar
to that illustratively described earlier herein in connec-
tion with the embodiment shown in Fig. 1. Waste sludge
is introduced to a covered first digestion ~one 110
either continuously or intermittently from line 152. The
aerobic digestion zone ~0 is maintained at a temperature
level in the range of from 35 to 75C and preferably in the
thermophilic regime of from 45 to 75C. A preferred range
for autothermal operation in the aerobic digestion zone is
from 50 to 65C. The elevated temperature in the covered
first digestion zone 110 can also be maintained by supplying
external heat, as for example by a sui~able heated fluid
circulating in a heat exchange means (not shown) disposed
internally in the digestion zone. Because of the coating
and plugging tendency of the solids, heat transfer sur-
faces disposed within the digester should not be intricate
or closely
~ 4~ 11234-2
spaced aDd may advantageously be embedded in or bonded to
the wall of the tank.
Second oxygen gas comprising at least 80Z oxygen
(by volume) is introduced to covered first diges-
tion zone 110 through conduit 117. As discussed herein-
af~er, this gas is sufficient ~n quantity to provide part
of the first oxygen gas introduced to aeration zo~e 102
through conduit 118.
Preferably, the-elevated temperature in covered first
digestion zone 110 is obtained autothermally without need
for heat exchangers such as 130. The concentrated sludge which is
characteristically obtained in the oxygen aeration process
of U.S. Pat. 3,547,813 is very favorable to autothermal
operation because of its re~uced ~ter content relative to its
biodegradable "fuel" content. ~oreover, high solids
concentrations reduce digester size and hence reduce con-
ductive heat losses through the walls of the digester tank.
As previously indicated, the total suspended solids content
(MISS) of the sludge in the digestion zone should be at
least 20,000 mg/l, based on such considerations.
Upper limits on aerobic digester solids concentration
are generally imposed by two factors. Broadly, the maxi-
mum concentration depends upon capability of conventional
sedimentation and thickening devices to reduce water content.
Flotation devices, centrifugal separators, and gravity
thic~eners often pr~duce 60,000 mg/l total suspended solids
coDcentrations. Solids levels can be further increased by
- 5~ -
~ 11234-2
admixture of primary sludge or concentrated waste from a
source other than the wastewater. The second factor which
limits solids concentrations is the increasing difficulty in
dissolving oxygen and mixing solids in the digester. A
preferred upper limit is 80,000 mg/l, and most preferably
60,000 mg/l, to insure that adequate oxygenation of the
sludge can be carried out without excessive power ex-
penditure in the aeration gas and sludge mixing operation.
Digester tank construction also affects the maintenance
of elevated temperature levels and concrete walls are
preferred over metal because of the lower conductive heat
loss through concrete. Heat loss can be further reduced by
embedding the tank below grade and mounding earth against any
exposed vertical wall of the tank. Thermal insulation such
as low-density concrete or foamed plastic can be applied over
a metal cover if required.
It is also preferable to practice the invention in
aerobic and anaerobic digesters having a surface-to-volume
ratio less than 0.8 ft2/ft3 (2.62 m2/m3). For these
purposes, "surface" refers to the entire wall surface area
of the covered digester including top, bottom and side
walls. Surface-to-volume ratios larger than 0.8 result
- 57 -
~ ~ 5 ~ ~ z 11234-2
in large heat conduction losses through the walls in
relation to the quantity of heat necessary to be main-
tained in the digester. Such heat loss is likely to
necessitate thermal insulation on walls exposed to ambient
atmosphere.
Retention time of the sludge in the aerobic digester
also affects the elevated temperature levels, particularly
autothermal temperature levels, which can be maintained.
It will be appreciated that numerous factors enter the
relationship between sludge retention time and temperature,
such as degradability of the sludge and strength (solids
level) of the sludge. In the broad practice of the present
invention, the sludge retention time ln the first digestion
zone is from 4 to 48 hours. Preferably, the sludge
retention time in the first digestion zone is in the
range of from 12 to 30 hours, and suitably from 12 to
24 hours.
First digestion zone llO is provided with mechanical
agitation means 112 which may be the same type employed as
means 103 in aeration zone 102, together with means for
continuously recirculating one of the second gas and acti-
vated sludge fluids against the other fluids in the diges-
tion zone.
The second gas comprising at least 80% oxygen is
introduced to the covered aerobic digestion zone 110 and
mixed with the sludge therein in sufficient quantity and
rate for aerobic digestion of the sludge while maintaining
total suspended solids content of the sludge at least at
20,000 mg/l.
- 58 -
~ Z 11234-2
Oxygen-depleted digestion gas of at least 40% oxygen
purity is discharged from the covered digestion zone 110
through conduit 118 at rate such that its oxygen conten~
is at least 35~ of the oxygen content of the oxygen feed
gas entering through conduit 117. The gas in conduit 118
is introduced to covered aeration zone 102, as at least a
major part of the aforementioned first gas supplying the
oxygen requirement for biochemical oxygenation of waste-
water. If needed, a supple~entary external source of oxygen-
containing gas may be supplied to augment the oxygen-
containing gas stream in line 118.
After the desired level of aerobic digestive treat-
ment is completed in zon~ 110, partially stabilized sludge
is discharged from the first covered digestion zone 110
in line 114 and passed to the anaeroblc treatment portion
of the integrated sy tem. In this embodiment the second
anaerobic digestion zone comprises an acidification sub-
zone 120a and a methane fermentation sub-zone 120b. The
partially stabilized sludge in line 114 from the first
digestion zone 110 is introducéd to the acidification
sub-zone 120a and maintained therein for sludge retention
time of from 24 to 60 hours as required for sludge
acidification. The contents of sub-zone 120a are continuously
mixed by agitation means 121a to maintain a uniform rate
of degradation of carbohydrates, fats and proteins to
lower fatty acids therein. After completion of the
- 59 -
~ 11234-2
necessary retention time in sub-zone 120a, the acidified
sludge is discharged therefrom in line 126 If the temp-
erature of the sludge exciting zone 120a is at elevated
temperature above optimum methane-forming levels, the
temperature of the acidified sludge is desirably lowered
to ensure satisfactory operation of methane-forming sub-
zone 120b. Accordingly, the sludge in conduit 126 is passed
through heat exchanger 115 against a coolant stream flowed
through the heat exchanger in conduit 160. The resulting
partially cooled, partially stabilized sludge discharged
from heat exchanger 115 then flows through conduit 127
to methane fermentation sub-zone 120b. The coolant heat
exchange medium in line 160 may suitably comprise a cooling
water stream, as for example a portion of the effluent from
the secondary clarifier from line 106 or, as in the pre-
viously described embodiment, the influent sludge feed
stream to the digestion system.
The anaerobic digestion sub-zone 120b comprises the
methane-forming digestion step of the process. For
optimal operation, the sludge in the methane fermentation
sub-zone is maintained at a temperature of from 35C to
40C, and preferably from 37C to 38 C. The contents
of zone 120b are continuously mixed by agitation means
:.21b, thereby creating a large zone of active decomposition
and significantly increasing the rate of the stabilization
- 60 -
~ 4~ 11234-2
reactions therein. Sludge retention time in the methane
fermentation sub-zone is preferably between 4 days and 8
tays under the previously discussed considerations
governing the anaerobic second digestion zone sludge
retention duration. Methane gas produced by the bio-
chemical reactions occurring in sub-zone 120b is discharged
therefrom in conduit 128 having flow control valve 129
disposed therein. A portion of this discharged methane
gas may be passed to the boiler 130 in con~uit 132, while
the remaining portion is withdrawn from the process in
conduit 131 to further treatment and/or other end use
steps. The further stabilized sludge, containing no re
than 40% of the original biodegradable volatile suspended solids
content of the influent sludge and preferably no more than
20% thereof, is discharged from the process in conduit 133.
Fig. 3 is a schematic flowsheet of another embodiment
of the invention wherein sludge from primary and secondary
wastewater treatment steps are passed to the sludge
digestion system. This embodiment illustrates a process
sequence under the instant invention in which a the D -
philic aerobic first digestion step is integrated with a
thermophilic anaerobic second digestion step. Heretofore,
thermophilic anaerobic digestion has not been widely employed in
commercial practice. The reason for such limited usage is that
the problems attendant conventional mesophilic anaerobic op-
eration of inherent thermal inst~ilit~ ~d extreme sensitivity
to change in process conditions, as discussed earlier herein,
are ~resent in
- 61 -
~ 4 ~ ~ 11234-2
ther philic anaerobic digestion to an even re critical
extent. In fact, it is because of the erratic operating
stability of thermophilic anaerobic digestion that this
sludge treatment process has received little attention
to date in commercial sludge digestion applications.
These problems of operating instability and undue sensi-
tivity to process fluctuations are overcome in ~he the -
philic aerobic/anaerobic embodiment of the invention in
the same manner as described earlier herein in connection
with the embodiments of the instant invention employing
an anaerobic mesophilic second digestion step.
ID the Fig. 3 system, raw wastewater composed,
for example, of municipal sewage, industrial wastewater,
ant storm water flows through conduit 240 into the
primary sedimentation zone 241. Sedimentation zone 241
may suitably consist of a gravity clarifier of a conventional
type well-known in the art. In the sedimentation zone the
influent wastewater is separated into a reduced BOD-
containing primary effluent, which flows by conduit 201
into aeration zone 202, and a settled sludge underflow,
removed from zone 241 via conduit 242. The aeration
zone 202 also receives oxygen-containing aeration gas
in conduit 218, sludge thickener supernatant liquid in
conduit 250 and return activa~ed sludge in conduit 208.
A fluid mixing and recirculation means 203 is disposed
in aeration zone 202 for mixing of the various fluids
introducet to the aeration zone to form mixed liquor
- 62 -
~ Z 11234-2
and simultaneous continuous recirculation of one of the
mixed liquor and oxygen-containing aeration ~a;- fluids
against the other fluids therein. As discussed earlier
herein, the fluid mixing and recirculation means may
suitably comprise a submerged gas sparger in co~b;nation
with a sub-surface mixing impeller, or a surface aeration
impeller device. After the requisite aeration period,
e.g. 2-6 hours, a BOD-depleted mixed liquor and an oxygen-
depleted aeration gas of at least 21% oxygen (by volu~e)
are discharged from the aeration zone 202 in conduits
204 and 219, respectively.
The BOD-depleted oxygenated mixed liquor in conduit
204 is passed to the secondary sedimentation zone 205
wherein activated sludge is separated from the purified
liquid, with the latter being discharged from the process
in line 206. The settled activated sludge is withdrawn
from the secondary sedimentation zone in line 207. A
ma~or p~rtion of this withdrawn sludge is recirculated as
the recycle sludge to the aeration zone 202 in line 208
having recycle pump 209 disposed therein. The remaining
unreturned portion, which may comprise between 3% and 10%
of the sludge in conduit 207, is flowed in conduit 252
to the sludge thickener 251.
Sludge thickener 251 comprises a further sludge
settling thickening zone which concentrates the sludge
to between 2% and 6Z solids, i.e., an MISS level of between
20,000 and 60,000 mg/l. The thickened sludge underflow
- 6~ -
11234-2
~ 4 ~
is flowed in line 245 and joined with primary sludge in
line 24Z from the primary sedimentation zone 241 to form
the combined sludge stream in line 211. The supernatent
liquid from the sludge thickener 251 is passed in line
250 to the aeration zone 202, as previously described.
The combined sludge stream in line 211 may be par-
tially warmed, if desired, by indirect heat exchange with
the warm stabilized sludge discharged from the second
digestion zone 220, as described more fully hereinar'ter,
and is then flowed to the first digestion zone 210 in
conduit 248. Prior to introduction to the first digestion
zone 210, the sludge in conduit 248 m~y be further heated
by the methane-fired heater 231, which receives methane
gas for combusion from conduit 227.
If ambient temperature conditions are sufficient to
eiiminate the necessity of heating the influent sludge
to the digestion system, it may be by-passed around heat
exchanger 244 and heater 231 by the by-pass conduits
261 and 263, respectively.
In the covered first digestion zone 210, thermophilic
aerobic digestion of the influent waste sludge i5
carried out. Aeration gas containing at least 507. oxygen
(by volume) and preferably at least 80% is delivered to
the digestion zone 210 in conduit 217, and mechanical agitation
means 212 mix and simultaneously continuously recirculate
ehe influent sludge mixture against the oxygen-containing
gas. The aeration gas feed rate and the energy input to
- 64 -
11234-2
mechanical agitation means 212 are such that oxygen is
dissolved in the sludge in first digestion zone 210 in
sufficient quantity and rate to satisfy the aerobic diges~ion
respiration requirements of the sludge therein.
Sludge is retained in the first digestion zone 2~0 at
thermophilic temperature of 45 to 75C for duration of from
4 to 48 hours to partially reduce the biodegradable
volatile suspended solids contentOf the sludge.
Partially stabilized sludge is discharged from the ae.obic
tigestion zone 210 in line 216 and the oxygen-depl~ted
digestion gas is separately discharged from the digestion
zone in line 218.
From line 216 the partially stabilized sludge from
the first digestion zone is introduced to the covered
second digestion zone 220,
The second digestion zone 220 comprises a thermophilic
anaerobic digester. For optimal operation the sludge in
this digestion zone is maintained at temperature iD the
anaerobic thermophilic temperature range of between 40 C
and 60C, and preferably between 45C and 50C. As a
result of thermophilic operation in both the first and the
second digestion zones in this embodiment of the invention,
the partially stabilized sludge from the first digestion
zone may be passed directly to the second digestion zone
as shown without heating or cooling heat exchange between
the zones if the thermophilic eemperatures in the
respective zones are sufficiently closely aligned.
Alternatively, it may be desirable in some cases to
- 65 -
11234-2
operate the second digestion zone at sufficiently higher
or lower temperatures relative to the first aerobic di-
gestion zone so that interzone heating or cooling of the
partially stabilized sludge from the aerobic digestive
step is advantageous. Heating may be carried out by a
methane-fired heater similar to heater 231; cooling may be
carried out by heat exchange of the partially stabilized
sludge from the first digestion zone with the influent
sludge flowed to the digestion system, as described
hereinabove in connection with the embodiments of the
invention shown in Figs. 1 and 2. Additionally, since
it is even re critical in thermophilic anaerobic diges-
tion than in mesophilic anaerobic digestion to ensure that
temperature fluctuations do not occur in the digestion
zone, it may be desirable to employ a well-insulated
tank as the sludge treatment vessel for the thermophilic
anaerobic digestion step, thereby providing a safeguard
against severe climatic variations.
In the second digestion zone 220, the sludge is con-
tinuously mixed by mechanical agitation means 221 to
~aintain a high rate of stabilization. Methane gas pro-
duced as a result of the biochemical reactions occurring
in the anaerobic digestion is discharged from the second
digestion zone in conduit 223. This methane gas may be
mixed with oxygen-containing gas such as air or tbe oxygen-
depleted digestion gas from the aerobic digestion zone
and combusted as fuel to provide heat for maintaining
- 66 -
sludge in one or both of the digestion zones at elevated
temperature. In the process as shown a portion of the
methane gas from conduit 223 is passed by conduit 227
to the methane boiler 231 and combusted to provide
heat for maintaining sludge in the first digestion zone at
thermophilic temperature of from 45 to 75C. The remaining
portion is discharged from the process system in conduit
228. The further stabilized sludge from the anaerobic
digestion zone, containing no re than 40% of the bio-
tegradable volatile suspended solids content of the influent
sludge to the digestion system in line 248 and preferably
no more than 20Z thereof, is discharged from the second digestion
zone through conduit 225, passed throu~h heat exchanger 244
for recovery of heat from the discharged sludge and finally
discharged from the process system in conduit 243.
The nature of the biological activity in the aerobic
digestion zone in the Figure 3 embodiment just described
is significantly different than the biological activity in
the aerobic zone of the earlier described Figure 2
embodiment of the invention, by virtue of the difference
in sources of the sludge. In the Fig. 2 embodiment, the
sludge passed to the digestion system as the influent
feed therefor is solely activated sludge from the second-
ary wastewater treatment system, whereas in the Fig~ 3
embodiment the influent sludge comprises both the second-
ary sludge from the activated sludge treatment step and also
the primary sludge from the raw wastewater sedimentation
-67 -
~ 2 11234-2
step. Since the organic material of secondary sludge
is primarily viable microorganisms, aerobic digestion of
this sludge comprises the various biochemical reaction
steps of cell lysis, assimilation of the lysis products
for synthesis of new viable material, and respiration.
Primary sludge, on the other hand,is primarily composed
of non-viable organic material, which the micro-organisms
present in the sludge are able to use as food. Accordingly,
during the aerobic digestion of a primary sludge the micro-
bial population of the sludge experiences a substantial
cell synthesis phase in addition to cell lysis,
assimilation of lysis products and respiration. As a
result, aerobic digestion of primary sludge takes place
wieh a greater level of both cell synthesis and cell
respiration than is present in aerobic digestioD of
secondary sludge. Furthermore, aerobic digestion of
pri~ary sludge results in a smaller net reduction of
biodegradable volatile solids than does aerobic digestion
of secondary sludge based on a comparable sludge retention
time for digestion. The net reduction of biodegradable
volati~e solids in the sludge during digestion represents
a difference in the competing digestive processes of cell
synthesis and cell respiration.
Cellular respiration in the sludge digestion process
is exothermic in character and, for the reasons discussed
above, primary sludge exhibits a higher heat generating
capacity per unit weight o~ biodegradable volatile
- 68 -
~ ~ 11234_2
suspended solids re ved in digestion than does secondary
sludge. Accordingly, a lower net reduction in volatilé
suspended solids is required to achieve and maintain
a given temperature level in the aerobic digestion step
with primary sludge than with secondary sludge. Thus,
the Fig. 3 embodiment of the invention, wherein the
sludge to the digestion system comprises both primary
ant secoDdary sludge,may be operated at a given t~mperature with a
lower level of volatile suspended solids reduction in the aer-
obic digestion zone than the aerobic zone in the Fig. 2
embodiment processing only secondary sludge. A lower
biodegradable volatile solids reduction in the aerobic
zone of the digestio~ system in turn requires that the
sludge retention time in the anaerobic digestion zone be
correspondingly increased to obtain a given overall level
of volatile suspended solids re val. Since an increased
portioD of the overall volatile suspended solids reval
is effected in the a~aerobic digestion zone in such a
case, the system processing primary sludge can therefore
obtain an increased level of methane generation in the
anaerobic digestion zone relative to the digestion system
processing only secondary slutge. Thus, the Fig 3
embodiment is inherently capable of providing greater
quantities of methane than the Fig. 2 system, but at a
cost of increased sludge retention time in the anaerobic
digestion zone in the former case.
- 6 -
11234-2
~ 3 ~
With respect to the foregoing discussion, the
capacity for heating the influent sludge to the digestion
system prîor to introduction of the slutge to the aerobic
digestion zone is provided in each of the previously des-
cribed illustrative embodiments of the invention. Such
heating may or may not be necessary in a given applica-
tion depending upon various factors such as sludge solids
content, ambient temperature, aerobic digestion zone
sludge retention time and the type of sludge involved.
Figure 4 is a graph of the temperature or the influent
sludge to the first digestion zone which is necessary to
autothermally maintain a 50 C operating temperature in the
first digestion zone for a ~4 hour sludge retention time,
plotted as a function of the total suspended solids con-
tent (MLSS) of the influent sludge to the first digestion
zcne. This graph represents a biological secondary sludge
having a volatile suspended solids/total suspended solids
(VSS/TSS) ratio of 0.79 and a biological heat content of
14,000 BTU/lb. volatile suspended solids removed.
- 70 -
11234-2
The graph of Fig. 4 indicates that thermophilic
operation may be achiéved without the need for heating
of the influent sludge to the digestion system prior to
its introduction to the aerobic digestion zone when the
influent sludge has a sufficient solids concentration.
For example, if a sludge with a 3% total solids concen-
tration is to be digested, the temperature of the sludge
introduced to ~he thermophilic aerobic zone need only be
about 16 C to maintain auto-thermal operation.
All of the previously described embodiments of the
invention are able to produce a completely pasteurized
sludge product, since in each of these cases all of the
influent sludge to the digestion system is passed through
an aerobic digestion zone in which high temperatures on
the order of at least 50-52C may be employed to provide
complete sludge pasteurization. However, there may be
applications in which the final disposition of the sludge
does not require a completely pasteurized product, or
where the sludge itself does not require pasteurization
because of the absence of any appreciable concentration
of pathogens therein Fig. 5 is a schematic flowsheet of
another embodiment which is within the broad scope of
the present invention, wherein a minor portion of the
influent sludge to the process system is diverted to the
second digestion zone, and which is suitable for the afore-
mentioned sludge digestion applications in which complete
sludge pasteurization is not required. In the Fig. 5
embodiment, a major portion of the influent sludge
- 71 _
11234-2
~ 4 ~ ~
entering the process system in line 311 is fed to the
covered first digestion zone 310 by line 331. Prior to
introduction to first digestion zone 310, the sludge in
line 331 can be heated, if desirable, by methane-fired
boiler 330.
Oxygen-containing aeration gas, comprising at least ;.
50% and preferably at least 80% oxygen (by volume), is
introduced to the aerobic digestion zone 310 through con-
duit 317. The sludge flowing into this zone is suitably
mixed and continuously recirculated against the oxygen
containing aeration gas therein by the agitation mearAs
312, with the sludge and aeration gas mixing being car-
ried out in sufficient quantity and rate for aerobic di-
gestion of sludge in zone 310. Sludge is maintained in
the aerobic digestion zone at thermophilic temperature of
between 45 and 75C for a retention time of between 4
and 48 hours. Oxygen-depleted digestion gas is discharged
from the first digestion zone in line 318 and sludge par-
tially depleted in biodegradable sus~ended solids content
is separately discharged from the digestion zone in line
316.
The partially stabilized sludge in line ~16 is then
introduced to the covered second digestion zone 320
operating in the mesophilic temperature range. Since
- 72 -
~ 11234-2
the temperature of the sludge discharged from the first
digestion zone is between 45C and 75C, its temperature
is suitably lowered prior to introduction to the second digestion
zone so that preferred mesophilic temperature conditions for
the mesophilic anaerobic digestion process in the second digestior
zone can be maintained. In the illustrative embodiment,
the ~inor portion of the influent sludge to the process
by-passes the methane boiler 330 and aerobic digestion
zone 310 in conduit 329 and is ~'xed directly with the
warm sludge in conduit 316. The flow rate of the
influent sludge bypass stream is adjusted so that the
temperature of the combined sludge stream introduced to
the anaerobic digestion zone 320 is sufficient to
m iDtain an operating temperature in zone 320 of between
35C and 40C.
In the second digestion zone the sludge is mixed
by recirculation of methane gas against the sludge there-
in in order to actively maintain the stabilization rate
in the second zone at high levels. Methane gas produced
as a result of the biochemical reactions occurring in
the second tigestion zone 320 is discharged therefrom in
conduit 323. A side strea~ of this gas is diverted into
flow loop 340 having compressor 326 disposed therein and
the resultant compressed methane gas is introduced into
the sludge in the second digestion zone, as for example
by sparging means (not shown), to effect the aforementioned
sludge mixing and recirculation. From line 323, a
~ 11234-2
portion of the methane gas may be passed in line 327
to the methane-fired boiler 330 and the remainder is
discharged from the process system in line 328. The
further stabilized sludge, containing less than 40Z
of the original biodegradable volatile suspended solids
content of the influent sl~dge to the process system in
line 331, is discharged from the second digestion zone
in conduit 325, to further treatment (e.g., dewatering)
and/or final disposal.
The advantages of this invention are illustrated by
the following examples:
Example I
This example compares the performance `of the instant
invention operated according to the Figure 2 embodiment
with a conventional high rate anaerobic system. The
further description will be based on treatment of ~aste
sludge from a 10 million gallon per day (MGD) wastewater
treatmRnt plant, and referenced to the Fig. 2 schematic
flowsheet.
A combined 50-50 primary and secondary sludge
initially at 18C is fed to the digestion system of the
Fig. 2 process in conduit 111. The sludge, having a
total suspended solids content of 39,400 mg/l and a
Yolatile suspended solids/total suspended solids fraction
of 72Z, is fed to the system at a flow rate of 0.09 MGD.
To maintain the sludge in aerobic digestion zone 110
at a 50C operating temperature with 24 hour sludge
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~ 11234-2
retention time, the influent sludge is heated to about
23C by the me~hane boiler 130. Based OD a 50Z conversion
efficiency of the methane gas fuel value to heat, appro-
ximately 25,000 cubic feet per tay of the methane gas
protuced in the anaerobic digestion zone is needed to
supply the boiler 130.
Approximately 8Z volatile suspended solids (16%
biodegradable volatile suspended solids; the biode-
gradable volatile suspended solids are approximately 50%
of total volatile suspended solids) reduction is obtained
in the aerobic digestion so that a partially digested sludge
with a volatile suspented solids content of 26,100 mg/l
is fed by conduit 114 to acidification sub-zone 120a.
This sub-zone is operated at ther philic temperature
with a 24 hour sludge retention time. A 1~% reduction
of the influent ~olatile suspended solids fraction is
effected in this stage. A sludge with a volatile sus-
pended solids content of 23,500 mg/l is then discharged
to the methane fermentation sub-zone 120b in conduit 126.
Sufficient heat is re ved from the discharRed sludge
in heat exchanger 115 to ensure an operating temperature
in the methane fermentation sub-zone 120b of 38C.
The methane fermentation sub-zone is operated with
a 5 day sludge retention time, resulting iD an overall
volatile sus~ended solids reduction of 40% for the integ-
rated svstem (biodecrr?d~le ~rolatile suspended soli~s -e-
ducrion of ~n,/~. The methane fermentation su~-zone produces
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~ 11234~2
approximately 73,000 cubic feet of methane gas per day,
amouDting to a total fuel value of 43 million BTU per
day. Since 25,000 cubic feee of methane gas per day is
needed to operate methane boiler 130, 48,000 cubic feet
of methane gas per day, amounting to a total fuel value
of 29 million BTU per day, is available for export from
the sludge digestion system.
If the 0.09 MGD of combined sludge on which the
above description is based is instead passed to a conven-
tional high rate anaerobic digestion tank approximately
a 13 day sludge retention time would be necessary to
achieve the same volatile solids reduction. Although
128,000 cubic feet of methane gas per day, a unting to
about 77 million BTU per day, is produced by the conveD-
tional high rate digester tank, approximately 60 million
BTU per tay of heating is needed, at a 50% conversion
of fuel value to heat, to maintain optimum operating temp-
erature conditions in the high rate tank. Thus, the
conv~ntional system, as compared to the above-described
embodiment of the present invention, requires 86% mcre
tankage, based on retention time requirements, and has
available for export approximately 40% less methane
under normal operating conditioDs.
ExamDle II
This example describes a specific operation of the
present invention according to the Figure 5 embodiment.
The influent sludge feed comprises 0.06 MGD of a combined
1123~-2
50-50 primary and secondary sludge from a wastewater
treatment facility. The influent sludge stream in line
311 at 20C and 4% total suspended solids (VSSjTSS=0.75)
is divided, with 0.046 MGD flowing in line 331 directly
to the thermophilic aerobic digestion zone and 0.014 MGD
forming the bypass stream in conduit 329. Inasmuch as
the temperature and suspended solids concentration of
the sludge in line 311 are sufficiently high to promote
autothermal operation in the thermophilic aerobic di-
gestion zone 310, there is no need in this instance to
heat the sludge prior to introduction thereof to the
aerobic zone. The retention time in the first digestion
zone 310 is approximately 24 hours and, as indicated,
thermophilic temperatures therein are reached autother-
mally. A pasteurized sludge at temperature of 50C
is discharged from the aerobic digestion zone in conduit
316 and is mixed with the cool bypass,stream from con-
duit 329. This combined sludge stream then flows to the
anaerobic digestion zone 320 in which the sludge is
maintained in the absence of oxygen for approximately 8
days resulting in approximately 40% overall volatile solids
reduction (80% biodegradable volatile suspended solids re-
duction) The anaerobic digestion zone produces methane
gas at a rate of approximately 72,000 cubic feet per day,
amounting to about 40 million BTU per day. All of this
methane is available for export from the process system.
If the 0.06 MGD of combined influent sludge feed is
instead passed to a conventional high rate anaerobic
digestion tank, approximately a 15-day retention time would
be necessary to achieve the same volatile solids
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11234-2
reduction. Although 90,000 cubic feee per day of meehane
gas,a unting to about 50 million BTU per day, is
produced by the con~entional anaerobic digestion tank,
approximately 45 million BTU per day is needed, at a 50Z
conversion of fuel value to heat, to maintain optimum
anaerobic operating temperature conditions in the
high rate tank. Therefore, in this case, a conventional
anaerobic digestion system requires approximately a 55%
longer sludge retention time but generates only a net gas energy
equivalent of 5 million BTU per day compared to 40 million BTU
per day for the combined system. Therefore, after using the
internally generated methane gas as a source of heat, the conven-
tional system has substantially less methane gas available for ex-
port than does the process of the present invention.
Example III
This example compares the performance of the instant
invention when operated according to the Figure 1
embodiment with a conventional high rate anaerobic
system.
A secondary sludge from an oxygenation wastewater
treatment system, initially at 15C, i~ first heated
in heat exchanger 22 with anaerobic digester effluPnt
and then further heated with thermophilic aerobic digester
effluent in he&t exchanger 15. The first heat exchange
step in heat exchanger 22 raises the temperature of the
iDfluent sludge from 15C to about 25C while lowering the
temperature of the stabilized sludge effluent from the
anaerobic digestion zone 20 from about 35C to 25~C.
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11234-2
The second heae exchange step in heat exchanger 15
increases the influent sludge temperature to about 30C
while the sludge discharged from the aerobic digestion
zone 10 is reduced in tempera~ure from about 50C to
45~C. The influent sludge having a total solids conteDt
(MLSS) of 34,400 mg/l and a volatile suspended solids/
total suspended solits fraction of 78% is introduced to
the first digestion zo~e 10 at a rate of 0.06 MGD. A
50C opera~ing temperature is maintained with a 24 hour
sludge retention time in the aerobic first zone.
Approximately 16Z volatile suspended solits reduc-
tion (32% biodegradable volatile suspended solids
reduction) is achieved in the aerobic seage so that a
partially stabilized sludge with a volatile suspended solids
content of 22,500 2g/l is introduce~, after heat exchange
with the influent sludge in heat exchanger 15 to the
anaerobic digestion zoDe in conduit 16.
The anaerobic digestion zone operates with an
8 day retention time, resulting in an overall volatile
suspendet solids retuction of 42% (biodegradable volatile
suspended solids reduction of 84%) for the integrated
system. The anaerobic digestion zone 20 produces appro-
ximately 51,800 cubic feet of meehane gas per day, amounting
to a total fuel value of about 28 million BTU per day.
All of this methane gas is available for export from
the digestion system.
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~ 2 11234-2
If the 0.06 MGD of influent sludge to the digestion
process described above was instead passed to a conven-
tional high rate anaerobic digestion tank, at least a 14
day sludge retention time would be necessary to achieve
the sa~e volatile suspended solids reduction. Although
84,600 cubic feet of methane gas per day is produced
in such conventional high rate system, amounting to
about 47 million ~TU per day, approximately 45 million
BTU per day is needed, at a 50% conversion of fuel value
to heat, to maintain optimum operating temperature con-
ditions in the high rate tank. The conventional anaerobic
system therefore requires about 55% more tankage, and has
about 26 million BTU of methane gas per day less to ex-
port, than does the corresponding above-described system
of the present invention.
Although preferred embodiments have been described in
detail, it will be appreciated that sther embodiments are
contemplated with modification of the disclosed features,
as being within the scope of the invention. For example,
2~ the aerobic digestion step of the instant process may be
carried out in serial treatment tanks or in a partitioned
basin wherein the constituent tanks in the series or
the separate volumes in the partitioned basin
functio n as sub-zones of the aerobic digestion zone.
In such manner, the aerobic digestion zone may be pro-
vided in the form of a sludge digestion chamber with
multiple compartments for staged cocurrent sludge and
aeration gas flow; such aerobic digestion zone config-
uration is well known to those skilled in the art and is
disclosed for example in the aforementioned U.S. Patent
No. 3,926,794 to N.P. ~1ahldieck
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