Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
3~L
--1
This invention relates to a method for
~reating wastewaters with an adsorberlt and a rotat-
ing biological contactor.
A variety of methods are available for the
biological treatmant of sewage and industrial waste-
waters. Commonly used methods are the activated
Yludg~ proaess using air and pure oxygen and the
trickling ilter. Mor~ recently the rotat'ng bio-
logical contactor, e~g., as described in U.S. Patent
3,557,954, has found application in which a series
of dlscs on a common shaft are rotated in the body
o~ the wastewater alternately immersing a biological
film formed on the disc in the liquid and exposlng
the film to the air above the liquid ko provide oxy-
gen for maintaining a biological film on the surfaceof the disc to effect biological oxidati~n of th~
wastewater. Reduced energy consumption f~r oxygen
tran fer i9 claimed in such a system which can be
designe~ for a variety of applications ranging from
~0 ordinary carbonaceous BOD5 removal to biological
nitrification
The broad concept of passing wastewater in
countercurrent flo~ with continuously moving aativat-
ed carbon to remove organic content of the water is
disclosed in U.S. Patent 3,763,040.
More recently, it has been suggested that
powdered aativated carbon be added to the activated
sludge process to effect ~mprovements in performance
of ~he activated sludg~ ~ystem as taught in U.S.
Patent 3,904,518. Benefits claimed are improved
stability, improved liquid solid~ separati.on, and a
- hiyher degree of treatmsnt. The method taught in
: .
: :
.
.
- ~
.
83~
--2~
U.S. Patent 3,904,518 is a sinyle stage suspended
slurry contact system in which the carbon is con-
tacted only with the equilibrium concentration of
adsorbable organics ln the wastewater in the presence
of activated sludgeO For more efflcient contact of
the carbon with adsorbable material, it is desirable
to effect a c3untercurrent contact with water to
expose the carbon incrementally or continuously to
a higher concentration of adsorbable material in the0 wastewater.
Japanese Patent Publicakion No. 139169~76
discloses a method of biotreatment of wastewater
comprising contacting the wastewater with activated
carbon fixed on the surface of a fixed bed or disc.
As more stringent treatment requirements
for wastewater are developed, it has become necessary
to consume greater quantities of energy~ For example,
where oxidation of reduced nitrogenous material is
desired, oxygen requirements are increased markedly
requixing greater energy consumption. Further, the
longer cell residence tim~s result ln greater con-
version of cell mass to C02 and H20 requiring still
greater oxygen transfer and energy consumption.
Further, the more stringent treatment requirements
may make necessary the additlon of tertiary treatment
to remove residual organic materials as well as any
toxic organic materials present in the wastewater.
The invention relates ~o a process for
treating wastewater comprising the following steps:
(a) contacting wastewater in a stream in
a treatment ~one with a multiplicity
of surfaces alternately immers~d in
the liquid and the gas, usually air,
above the liquid for a period of tlme
of at least one-half hour;
~b) adding to the contactor an adsorbent
capable of adsorbing lmpurities from
the liquid; and
(c) removing the ~uspended 301ids from the
contacting zone at a rate equivalent
33~
--3--
to the rate at which solids accumulate
within the contactorO
The preferred adsorbent is powdered
activa~ed carbon, although the use of granular acti-
vated carbon or other adsorbents such as Fuller'searth~ fly ash and the like is contemplated. A pre-
~erred range of concentration of adsorbent is from
25 to 5000 milligrams per liter.
The invention i5 now described with
reference to the accompanylng drawings, wherein:
Figures l and 2 show the basic process; and
Figures 3, 4 and 5 illustrate detailed
embodiments to effect countercurrent contact of the
biomass and carbon slurry with the liquid phase.
Referring to Figure l, raw wastewater
entering at lO receives a preliminary treatment con~
sisting of screening and grit removal in a device 12
for this purpose and passes to an optional primary
treatment step consisting normally of plain sedimenta-
tion at 14 wh~re suspended floating, and settling
solids may be removed as by line 16 and disposed of
by conventlonal means 18c The settled wastewater
is passed then through a line 20 to a basin 22 in
which a series of discs 24 are mounted on a common
shaft 25 and rotated by convenient means to lie
alternately in the liquid in the basin and in the gas
space above the liquid Normally flow through the
basin is in a direction parallel to the shaft and
perpendicular to the planes of the rotating discs.
In some instances, flow is perpendicular to the
shaft and parallel to the rotating discs.
Powdered activated carbon is added to the
mixturP in the basin as at 26, e.g., to provide
adsorption of biodegradable and non-biodegradable
material contained in the wastewater. The rate of
carbon addition varies depending upon the trea~ment
level required as well as the compo~itlon and char-
acteristics of adsorbable material contained in the
wastewater. Normally from 10 to 500 mg/l of treated
wastewater are required, but or some industrial
' ' :
. ~,.. .- '' :
.
` .
~9~3~L
-4-
wastes, quantities as great as 5000 mg/l of waste-
water tr~atment may be required.
~ he film formed on the surface of the disc
consists of a mixture of biomass and adsorbent (acti-
vated carbon). Surprisingly, it has been obs~rvedthat the film has the unusual property of accumulat-
ing the adsorbent into its structure, thereby carry-
ing the adsorbent on the disc surface along with the
biomassn The ratio of the mass of biological
oryanisms to activated carbon will depend essentially
upon the solids residence time of the system and the
composition of the waste being fed to the system.
It is apparent from the Examples which follow that
the adsorbent not only aids in removing adsorbable
material from the wastewater, but also has the effect
of substantially increasing the residence time of
adsorbable and slowly biodegradable substances, and
also has the effect of greatly increasing the over-
all solids residence time of the system.
This effect is beneficial when nitrifica-
tion ~biological oxidation of ammonia nitrogen) is
desired since it permits the accumulation of slow
growing ni*rifying bacteria.
A further unexpected benefit derived from
the addition of activated carbon to the contacting
æone of the rotating biological contactor results
from the carbon's tendency to reduce foaming. In
many applications, the rotational speed of the con-
tactor is limited by, am~ng other things, the
tendency for foaming to occur in the contactor.
Foaming may occur due to the presence in the waste-
water of surface active material or due to surface
active exo~enous organic materials produced by the
organisms themselves. Since most, if not all of
these materials are adsorbed onto the ac-tivated
carbon, foaming is reduced or eliminated, permitting
higher rotational velocities and concomitant oxyyen
transfer.
A still further surprising result of the
addition of activated carbon to the rotating bio-
, . ' ' ''
~ . .
~(39~3~
--5--logical contactor ls the effect of the carbon addi-
tion upon the overall mass transfer rate of oxyg~n.
Measurements o~ the overall oxygen mass transfer
rate (KLa) in the contactor is as much as twenty-
five per cent better than in the ordinary biologicalcontactor. The improved mass transfer rate has a
dual impact upon the performance of the contactor.
First, a higher transfer rate permits slower rota-
tional speeds and hence lower power costs; and
second, the oxygen profile across the film on the
disc is improved providing deeper penetration
through the film with a greater resultant active
aerobic biological mass. This effect is particular-
ly important in nitrifying systems where the nitrify-
ing organisms must be exposed to dissolved oxygenconcentrations in excess of .5 mg/l to be ef~ec~ive.
While the carbon can be added at various
points within the basin, advantages can be derived
by adding the carbon at various points through the
longi~udinal dimension of the basin~ For example,
as shown in Figure 3, if the carbon is added uniform-
ly along the longitudinal ~enath of the basin and
the solids are removed as at 28, 28-1, 28-2 and 28-N
along the longitudinal length, carbon contact con
sists of essentially a series of si~gle stag~ con-
tactors. The rate of addition at each point can be
varied to control the loading on the carbon to match
the treatment requirements of each successive stagen
As is well-known, in adsorption systems,
it is desirable to more efficiently load the adsorbent
with adsorbate by mGving the adsorbent in counter-
current relationship with the adsorbate. In suspend-
ed slurry systems, this is usually done by providing
a separation step following each contacting stag~.
S~lids sepaxated at ~ach stage are moved upstream,
countercurrently to the liquid stream. Figure 4
illustrate~ how this can be achieved in a rotating
biological contactor by introducing the adsorbent
(activated carbon) at the downstream end and provid-
ing a means of moving the carbon upstream, such that
: . . : . . .: . . . . , , ~ , ,
.
- . - . . .:
-: : ' ... .
. . . . . .
.. - . .. .
:, , ' .
--6~
particles separating out at the bottom of the con-
tactor tend to move in countercurrent manner along
the bottom of the basin providing any number of
equivalent countercurrent stages, thus providing
maximum adsorption efficiency of the carbon. A
variety of means can be utilized to effect counter-
current motion of the solids. For example, the
discs can be mounted in a chamber with a sloping
bottom such that the suspended parkicles tend to
move in an upstream direction as they are disturbed
at the bottom of the chamber.
For systems in which flow is introduced
perpendicular to the shaft and parallel to the
discs, a series of parallel shafts rotated such
that the direction of rotation of the disc in ~he
liquid is counter to the flow will produce counter-
current motion of the æolids in the system.
Alternatively, the periphery of each disc
24 can be fitted with plows or blades 30p directing
the net solids flow in a countercurrent direction
as illustrated in Figure 5. An optional relation-
ship exists between the angle of attack of the plows,
the effective rate of countercurrent movement, and
power consumption to rotate the disc.
In another embodiment, the disc may be
~esigned so that its periphery takes the shape of a
screw with a small angle of slope such that when
the disc rotates, the solids are pushed in an up-
stream direction. The downstream surface could re
main flat or preferably takes the corresponding
shape of the upstream face.
Other methods of achieving countercurrent
flow may be apparent after study of the above
examples. The fu~damental principle involved in
the above is that the adsorbent added is moved in
a countercurrent relationship with the bulk of the
liquid medium thereby effecting higher adsorptive
loadings.
In practice, it is never possible to
effect complete suspended solids separation within
:
' . - ~ ,.....
. ~ ,,' ' - :
-- : , : . .
, .
--7--
the chamber ln whlch the discs axe rotated because
of turbulence created by the rotating disc. To
effect improved separation, a quiescent chamber
optionally is provided at the downstream end of
the contactor as at 32, Figure 2 The quiescent
chamber has a steeply sloped bottom 34 to direct
the solids upstream into the region where the disc
can direct it further upstream. Alternatively, a
conventional collector mechanism can be provided to
positively convey the solids to the region of the
disc for transfer countercurrently upstream.
Similarly, a secondary clarifier 36 can
be provided to further clarify the liquid stream.
Material settled in this clarifier as at 38 can be
directed to the inlet end of the contacting chamber
as by line 39, Figure 1, or it may be mixed with
the underflow from the contactor at 42 for further
processing, for example, regeneration of the carbon
adsorbent or to solids disposal.
Virgin carbon can be added to the system
to provide effluent polishing in situations where
the spent carbon is regenerated.
Spent adsorbent at 42 along with the
associated bicmass can be optionally thickened and
disposed of by conventional means 44, or it could
be regenerated as at 46 and returned to the contactor
for reintroduction into the treatment step at
various points in the contactor, see line 48O
The treated wastewater 40 can be further
proc~ssed through an optional sand filter 50 and
disinfected and dischar~e~ to the receiving stream.
Suspended solids captured in the sand filter can
be backwashed and returned upstreàm to any of a
number of upstream points in the system shown in
Figure 1. Other conigurations are possible. For
example, biological nitrification and denitrification
can be achieved by operating a first stage under
aerobic conditions to achieve oxidation of ammonia
and a nitrogen removal stage in anaerobic conditions
to achieve denitrification o nitrate and nitrite
.
,
83~
--8--
nitrogen to elemental nitrogen.
The following examples are intended to
illustrate the process.
EXAMPLE 1
The laboratory rotating biological con-
tactor system consisted of fifty-two 10" diameter
1/4" thick polyethylene discs suspended from a
horizontal shaft and placed in a hemispherical-
shaped 20 liter tank equipped with a cone settler
near the outlet end. As the discs rota~ed, approx-
imately 40 per cent of the surface area thereof was
immersed in liquid. Domestic sewage primary
effluent was metered into one end of the tank. The
mixed liquid overflowed from the opposite end into
a clarifier equipped with a slow moving paddle.
Two recycle lines were used to return thickened
settlings from the clarifier and cone settler to the
inlet end of the tank at rates of 40 ml/min. and 12
ml/min., respectively,
Initially, the system was operated on
primary effluent for 19 days in order to build up
layers of biomass on the discs. During the next 8
days, data on samples taken continuously were obtain-
ed on the system. On the 28th day of operation, 300
grams of activated carbon were added near the tank
inlet. Daily, for the following 38 ~ays, 1 liter of
mixed liquor was withdrawn from the cone and 15.0 g.
dry activated carbon were added to the tank. The
mixed liquor containing biomass and carbon was filter-
ed. Dry filter cake weights rangin~ from 0.3 to 26.4~. were obtained. A feed rate of about 100 ml/min.
was maintained throughout the experiment. Rotation
speeds o~ 6 and 10 RPM were used to maintain the
dissolved oxygen range of 0.5 to 5.0 and 0.6 to 6.0
mg/l in the systems without and with carbon. The
average characteri~tics of the streams are listed in
the following ~able.
Note that the system operated with carbon
showed better COD and BOD5 reduetlon as well as nitri~
fication than the system operated without carbon.
.' ' ' ~ .
.
. ,
. . .
?3~1L
. 9.
EXAMPLE I
__
Without Carbon Carbon Added
Feed Effluent Feed Effluent
~ _
COD, mg/l 160 72 186 36
5 ~ COD Reduction - 54 - 81
BOD, mg/l 64 16 34 2
% BOD Reduction - 7S - 94
Total Kjeldahl
Nitrogen, mg/l 43.3 31.9 50.5 14.4
10 % Total Kjeldahl
Nitrogen Reduction - 26 71
Suspended Solids,
mg/l 32 24 38 9
Suspended Ash,
15 mg/l 9 3 17 2
Ammonia Nitrogen,
mg/l - - 39.8 10.5
Nitrites as Nitro-
gen, mg/l - 1 0.1 1.5
Nitrates as Nitro-
gen, mg/l - 1 1~5 27.8
- Total Phosphorus,
m~/l 9.4 8.712.5 11.5
pH 7O3 7.37.2 6.6
EXAMPLE II
The apparatus described in Example I was
employed with a feed rate of about 200 ml/min. for
30 days. No comparative data of the system without
carbon were obtained. The values for chemical oxygen
deman~, biochemical oxygen demand and Xjeldahl nitro-
gen reductions of this system without carbon at the
faster feed rate would not exceed the flgures for the
100 ml/min. rate listed in Example I. Daily filt~r
cake dry weights varied from 4.08 to 26.14 g. and
averaged 11.50 g. per day. The average data of
the 200 ml./min. experiment are listed in the follow-
ing TableO
3~
--10--
EXAMPLE II
Carbon Added
Feed Effluent
COD, mg/l 215 49
5 % COD Reduction 77
BOD~, mg/l 60 7
% BOD Reduction - 88
Total Kjeldahl Nitrogen, mg/l 48.9 lZ.7
% Total Kjeldahl Nitrogen Reduction - 74
10 Suspended Solids, mg/l 45 7
Suspended Ash, mg/l 7 2
Ammonia Nitrogen, mg/l 40.2 5.2
Nitrites as Nitrogen, mg/l 0.1 1.1
Nitrates as Nitrogen, mg/l 1 23.2
Total Phosphorus, mg/l 14.6 13.1
pH 6.9 6.9
Note that in spite of the doubling of the
loading rate to the system, comparable BOD5 and COD
reductions and nitrification of the ammonia n;trogen
were obtained when compared to the results of
Example I.
EXAMPLE III
~ .
The apparatus described in Example I was
modified to achieve a countercurrent flow of activat-
ed carbon and primary sewage effluent. The conesettler was moved to the inlet of the trough. Clarifier
and cone r~cycle slurries plus the virgin carbon were
introduced near the trough outlet. Spent carbon slurry
was withdrawn daily from the cone settler. Average
feed rates of 111 ml. primary sewage effluent per
minute and 94 mg activated carbon per liter of efflu~
ent were used. An average of 16.1 g. dry solids were
removed daily. The average analytical data are pre-
sented in the following Table.
83~
EXAMPLE III
Feed Effluent
COD, mgjl 227 43
% COD Reduction - 81
5 BOD5, mg/l 60 7
~ BOD Reduction - 88
Total Kjeldahl Nitrogen 48.9 12.7
% Total K~eldahl Reduction - 74
Suspended Solids, mg/l28
10 Suspended Ash, mg/l 2
Ammonia Nitrogen, mg/l47.411.8
Nitrites as Nitrogen, mg/l 0.1 0.5
Nitrates as Nitrogen, mg~l 1.5 17.0
Total Phosphorus, mg/l16.014.7
15 pH 6.5 6.3
EXAMPLE IV
To illustrate the effect of the addition
of activate~ carbon to the contactor upon the over-
all mass transfer rate of oxygen, tesks were con-
ducted in accordance with the methods described byEckenfelder and Ford* to determine Alpha, the ratio
of the mass transfer rate of oxygen in ~he carbon-
biomass slurry and biomass aloneO Tabulated below
are the Alpha values observed:
K a Wastewater
25 Alpha (~) = ~ ~
Biological Biological Plus
Wastewater SysternActivated Carbon
.
Pharmaceutical Waste - 2.43
Domestic Waste 0.821.27
Municipal/Industrial
Waste 0.811.05
The carbon addition has the effect of
greatly increasing the overall oxygen mass transfer
rate (KLa) when compared to the biological system and
permits equivalent oxygen transfer at greatly reduced
peripheral speeds.
* "Experimental Procedures for Process Design"
Water Pollution Control, Pemberton Press,
Jenk~ Publishing Co., Austln, Tex. (1970),
pp. 103-112.
