Note: Descriptions are shown in the official language in which they were submitted.
~C~6Z8Z()
BACKGROUND
I'lle cre.~ enc of contnminat~ wnxte watet involvcs a seq~lcnce of
processing steps for maximizing water purification at minimum costs. Indus-
trial effluents, particularly waste water from oil refineries, include a broad
spectrum of contaminants and, consequently, such waste water is usually
more difficult to decontaminate than waste water from municipal sewage sys-
tems. Four main sequential process treatments are used to decontaminate
such industrial effluents. These are a primary, intermediate, secondary,
and tertiary treatments, The primary treatment calls for removal of gross
amounts of hydrocarbons and solids from the waste water. In the oil industry,
usually separators of American ~etroleum Institute design are employed for
removal of free, separable oil and solids. The intermediate treatment is the
next process and it is designed to adjust water conditions so that the water
entering the secondary treatment zone will not impair the operation of the
secondary treatment processes. In other words, intermediate
treatment is designed to optimize water conditions so that the secondary
treatment process will operate most efficiently. The secondary treatment
calls for biologically degrading dissolved organics and ammonia in the water.
One of the most common biological treatment processes employed is the acti-
vated sludge process discussed below in greater detail. The tertiary treat-
ment calls for removing residual biological solids present in the effluent
from the secondary treatment zone and removing contaminants which contri-
bute to impairing water clarity or adversely affecting water taste and odor.
This is usually a filtration of the water, preferably through beds of sand, or
combinations of sand and coal, followed by treatment with activated carbon.
The acLivated sludge process is a conventional waste water treating
process which produces the highest degree of biological treatment in reason-
ably compact facilities at the present time. The application of this process
to the treatment of industrial waste water has, however, been slow compared
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- 1 - q~
1~6;28ZO
with municipal applications. Industrial applications of this process are
nevertheless increasing rapidly, Currently, the activated sludge process is
capable of achieving about 85% to 93% reduction in the five-day biological
oxygen demand (BOD5). However, the BOD5 contaminants present in indus-
trial waste water are relatively small compared with the total oxygen demand-
ing contaminants present in such waste water, For example, the BOD5 con-
taminants present in the effluent from an activated sludge process typically
ranges from 10 to 20 parts per million parts of water It is not uncommon to
also find present in such effluent 10 to 20 times this amount of other oxygen
demanding contaminants.
The activated sludge process has four stages of treatment, In the
first stage, contaminated water is contacted with the activated sludge. The
sludge includes micro-organisms which feed on the contaminants in the wa~er
and metabolize these contaminants to form cellular structure. This decon-
taminated water flows into a second clarifier stage where suspended sludge
particles are separated from the decontaminated water. A portion of the
sludge is recycled to the first stage and the remainder is forwarded to the
third and fourth stages. This sludge forwarded to the third and fourth stages
includes water. In the.third stage the sludge is thickened to remove excess
water and in the fourth stage the thickened sludge is permitted to digest. That
is, the micro-organisms feed upon their own cellular structure and are
stabilized, Normally, the average age of these micro-organisms in the sludge
is substantially less than ten days.
THE INVENTION
We have now invented an improved process for treating waste water
including high concentrations of BOD5 contaminants, COD contaminants, hydro-
carbons, inert solids, ammonia, phenolics, and other contaminants which are
relatively refractory. Our process is specially adapted to treat waste water
10~2820
from oil refineries and chemical complexes where the waste water
from the refining oil is mixed with waste water from chemical
plants. As conventional, our process calls for primary, inter-
mediate, secondary and tertiary water treatment. We have,
however, made important and novel modifications in the inter-
mediate and secondary treatment steps which result in substantial
improvement in effluent water quality.
In a broad aspect the present invention provides, in
a multi-stage activated sludge process wherein in a first stage
contaminated water is contacted with activated sludge for a
period of time to biologically degrade contaminants in the water
and in a second stage decontaminated water is separated from the
activated sludge, the improvement comprising reducing the oil
and grease, and solids content of the contaminated water to
less than about 20 ppm oil and grease and less than about 20 ppm
solids, prior to the first stage. Preferably, the oil and
grease, and solids content are reduced to less than about 10 ppm
each.
In another aspect, the present invention provides, in
an activated sludge process wherein in a first stage contami-
nated water is contacted with activated sludge for a period of
time to biologically degrade contaminants in the water and in
a second stage decontaminated water is separated from the
activated sludge, the improvement comprising treating the
contaminated water prior to the first stage to reduce oil and
grease, and solids to less than about 20 ppm oil and grease
and less than about 20 ppm solids and maintaining the average
sludge age in the first and second stages in excess of about
ten days.
In another aspect, the present invention provides,
in an activated sludge process wherein in a first stage con-
taminated water is contacted with activated sludge for a period
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of time to degrade contaminants in the water, and in a second
stage decontaminated water is separated from the activated
sludge, and in a third stage a portion of the sludge from the
second stage is thickened, the improvement comprising reducing
the oil and grease, and solids content of the contaminated
water to less than about 20 ppm oil and grease and less than
about 20 ppm solids, prior to the first staye, and recycling
a portion of the thickened sludge from the third stage to the
first or second stages.
In another aspect, the present invention provides
such a process as described in the immediately proceeding para-
graph wherein, prior to recycling the portion of the thickened
sludge from the third stage to the first or second stages, the
portion of the thickened sludge is digested. Preferably, the
reduction of the oil and grease and solid contents of the
contaminated water is carried out by filtration and a coagulant
or flocculant is added to the waste water prior to filtration.
In another aspect, the present invention provides,
in the activated sludge process wherein in a first stage con-
taminated water is contacted with activated sludge for a periodof time sufficient to biologically degrade contaminants in the
water and in a second stage decontaminated water is separated
from the activated sludge, a first portion of said separated
sludge being recycled for recontact with the water in the first
stage and a second portion of said separated sludge being
treated in downstream operations, the improvement comprising:
reducing the oil and grease, and solids content of the con-
taminated water to less than about 20 ppm of oil and grease and
less than about 20 ppm of solids prior to the first stage; and
introducing oxygen into the water and sludge mix entering the
second stage so that the sludge in the second zone is maintained
in an aerobic state and separated decontaminated water from
~ - 3a -
1~6Z8210
said second stage contains at least about three parts of dis-
solved oxygen per million parts of water. Preferably, the
oxygen is introduced into the water and sludge mix entering the
second stage by aspirating air into a stream of the water and
sludge mix flowing between the first and second stages.
In a further aspect, the present invention provides
a method of pretreating waste water including from about 25 to
about 150 parts per million of solids per million parts of
water and/or from about 25 to about 300 parts of oil and grease
per million parts of water upstream of activated sludge treat-
ment including a first stage of contacting the pretreated
waste water with activated sludge for a period of time to
biologically degrade contaminants in the water and a second
stage of separating the decontaminated water from the activated
sludge comprising, passing the water through an equalization
zone including at least two separate water retention compart-
ments in series so that the water is mixed in each compartment
and flows from one compartment to the next compartment and a
given quantity of water is retained for predetermined period
in each of said compartments, introducing air into the water
in at least one of the compartments so that the water in the
compartment is vigorously agitated and the effluent in the
aerated compartment includes at least three parts of dissolved
oxygen per million parts of water, adjusting the pH of the
water in the equalization zone so that the pH of the water
in one of the compartments and in the effluent from said zone
ranges between about 6.5 and about 9.5, destabilizing colloidal
particles suspended in the water, and filtering the effluent
water from the equalization zone so that said filtered water
includes no more than about twenty parts of oil and grease per
million parts of water and no more than about twenty parts of
suspended solids per million parts of water.
~ - 3b -
106Z8Z0
In a still further aspect, the present invention
provides a continuous process for purifying contaminated water
including solids, oil and grease, comprising
(a) passing the water through an equalization zone where-
in the pH of the water is adjusted to a range from about 6.5
to about 9.5 and the contaminated water is distributed in a
larger body of water so that the changes in concentration of
contaminants in the effluent water to the equalization zone
will produce gradual changes in concentration of contaminants
in effluent water from said zone,
(b) aerating the water in the equalization zone so that
the dissolved oxygen in the water is at least about three
parts of dissolved oxygen per million parts of water,
(c) adding a destabilizing agent to the water so that
colloidal particles in the water aggregate,
(d) passing the water from the equalization zone through
a filter so that particles and hydrocarbons will be removed
therefrom and the effluent from the filter will have less than
about ten parts of suspended solids per million parts of water
and less than about ten parts of oil and grease per million
parts of water,
(e) passing effluent from the filter through a multi-
stage biological treating zone having a first stage where the
water flows into a contact zone and contacts activated sludge
which decontaminates the water by biodegradation of contaminants,
a second stage where the water from the first stage is clari-
fied to separate suspended sludge particles from decontaminated
water, a portion of said separated sludge particles being
recycled to the first stage and the bulk of the clarified
decontaminated water being withdrawn from the second stage, a
third stage where that portion of the separated sludge particles
not recycled are concentrated by removing the bulk of the
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1062820
residual water therefrom, and a fourth stage where said con-
centrated sludge particles are digested,
(f) aspirating air into the water and sludge mix as it
flows between the first and second stages, so that the sludge in
the second stage is maintained in an aerobic state and clari-
fied water from said second stage contains at least about five
parts of dissolved oxygen per million parts of water, and
(g) filtering separated water withdrawn from the second
stage to remove minute suspended sludge particles not separated
from this water in said second stage.
Primary Treatment
As conventional, gross amounts of oil and solids are
removed from the oil refinery waste water by means of American
Petroleum Institute separators. The effluent from this primary
treatment typically includes from about 25 to about 150 parts
of suspended solids per million parts of water and from about
25 to about 300 parts of hydrocarbon per million parts of water.
As is not commonly recognized, such waste water containing
relatively large amounts of oil and solids, cannot be fed
directly into an activated sludge process where the sludge aae is
in excess of about ten days without upsetting the activated
sludge process. Based on pilot plant studies and theoretical
calculations, if the water entering the activated sludge pro-
cess contains more than about ten parts of oily solids per
million parts of water and more than about ten parts of hydro-
carbon per million parts of water, gross quantities of oily,
emulsified material collect in the first stage or mix liquor
tank of the activated sludge process. Such oily, emulsified
solids impair or prevent the activated sludge from decontaminat-
ing the water, causing the effectiveness of the activated sludgeprocess to be substantially diminished. In accordance with
an important feature of our invention excessive oil and solids
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1062820
are removed from the waste water by our intermediate treatment.
Intermediate Treatment
Oil refinery waste water and waste water from a
chemical plant are combined and subjected to intermediate
treatment where excessive solids and hydrocarbons are removed
and contaminant concentrations are equalized so
106Z8Z0
that such concentrations of contaminants remains more or less constant even
hollgll ~ contnlllinl)llr eollc~llt~r~ltion in tlle infl~ n~ to th(` etlll.lli~.,l~iOIl II(`~
ment stage sharpIy changcs from time to tlmc. If contaminant concen~ration in
the influent changes and such change is sustained, this will ultimately result
in a change in the contaminant concentration in the effluent from the e~ualiza-
tion section. But because of the design of our equalization section, this changeinitially will occur gradually over a relatively long time interval. This per-
mits the micro-organisms in the downstream activated sludge process to
adapt or acclimate to this change in contaminant concentration.
In our process, intermediate treatment includes equalization and fil-
tration. Equalization is conducted in a basin having two, preferably three or
four compartments. These compartments are mixed and arranged in series
so that water flows from one compartment to the next succeeding compartment.
The total retention time of water in the basin is less than about 10 to 15 hours
preferably 2 to 15 hours maximum. Consequently7 heat loss/minimized.
Normally, the difference in temperature between the influent and effluent
water is 20F or less. Preferably the retention time in each compartment is
30 to 90 minutes.
Waste waters from the various sources are mixed in the first compart-
ment, and the contaminant concentration is monitored. Usually pH, toxic met-
als, COD contaminants, phenolic, and ammonia concentrations are measured
either manually or automatically. Since waste waters from multiple sources
are fed into the relatively confined space in the first compartment, several ad-
vantages occur. First it is easy to monitor contaminant concentration and
readily detect any drastic change in concentration indicating, for example, a
break in a chemical line. The reason is because the first compartment in a
multiple compartment system will more rapidly increase in concentration to
more readily detectable levels than a single CQmpartment system. Also neutral-
ization is achieved. For example, one source of water may be highly acidic and
1062~32~)
another highIy basic. Ncutralization occurs as these streams mix in the
fi rst compa rtment.
It is important to adjust the pTI in the equalization basin in order to
maximize oxidation of certain contaminants, particularly sulfides. pl-I is
adjusted by adding acid or base to the water in the second compartment until
the water has a p~I ranging from about 6. S to about 9, 5, preferably between
7. 5 and 8. 5. Our experiments indicate that at least about three parts of dis-
solved oxygen per million parts of water must be present tO satisfy the
immediate oxygen demand (IOD) of the contaminants in the water at a reason-
able rate of oxidation. Preferably hydroquinone or gallic acid is added to the
water to catalyze the oxidation of IOD contaminants. If this IOD is not satis-
fied, the downstream activated sludge process can be adversely affected.
Consequently, the water in the equalization basin is aerated. Conventional
floating aerators may be used. We have found that aeration is more effective
in a confined zone. About 0.15 or more horsepower per thousand gallons of
water provides excellent aeration. Aeration also thoroughly agitates and
mixes the water with the result that floating solids accumulate on the water
surface. These solids are removed by skimming. In order to ensure that the
water to the activated sludge process includes less than about ten parts of
hydrocarbon per million parts of water and less than about ten parts of solids
per million parts of water, we add a coagulating and/or flocculating agent to
the water in the equalization basin or to the stream of water flowing to the
activated sludge process. The coagulating and/or flocculating agent destabil-
izes colloidal particles which then aggregate. The aggregates are carried
with the effluent stream to a filter and removed prior to introduction to the
activated sludge process. wc ~ preferably intro-
duce air into the stream of water flowing into the downstream activated sludge
process to ensure that the immediate oxygen demand to the water is satisfied.
1062820
Seconcla ry I reatment
-
In .lc~ordancc witll anotller le.ltlll~c of our invcntion w.ltcr from inter-
m~diate treatment flows through a conventional activated sludgc plant which
has been modified in two important ways: (1) the sludge-water mix flowing
between stages of the activated sludge process is aerated, and (2) the sludge
of different ages from different stages is recycled to one or more upstream
stages of the activated sludge process. In our process, oxygen either pure or
most preferable in air is introduced, for example pressurized or most pre-
ferably by aspiration into the stream of sludge and water flowing between the
mix liquor tank of the first stage and the clarifier tank of the second stage.
This stream of sludge, water and air or oxygen is subjected to the increased
pressure created by the hydrostatic heads of water in the mix liquor and clar-
ifier tanks. Consequently, this stream is saturated or supersaturated with
dissolved oxygen. The dissolved oxygen maintains the sludge in the clarifier
tank aerobic and ensures that the effluent water to the subsequent tertiary
treatment section includes at least five parts of dissolved oxygen per million
parts of water. We also inject oxygen either in air or pure form under pres-
sure into the sludge and water streams flowing between the second and third
stages and the third and fourth stages of the activated sludge process. Con-
sequently, the sludge in the thickener and digester can be retained for a longer
period of time. This aged sludge from the thickener and digester is recycled
to the first stage or mix liquor tank either directly or preferably by mixing
with the stream of sludge and water flowing between the first and second stages.
Tertiary Treatment
In our process, the effluent from the clarifier or second stage of the
activated sludgeprocess is filtered to remove biological solids in the effluent
m~y be
and then/contacted with activated carbon to remove odor causing and other
residual trace components by adsorption. Chemical agents may be added to
the clarifier effluent to de~billZecolloidal suspensions and assist filtration.
;Z820
I-lowev-~r, be~ausc of the intcrstagc a~ration, thc water has at least five part~s
of di~soIved oxygen pcr million parts of water ancl cons~ cntly or~anisms
collected in the filter and on the carbon are maintained in an aerobic condition,
avoiding odor and any degradation in quality of the filtered effluent. Further,
the effluent water to the receiving stream has a high level of oxygen in it.
Thus, it does not contribute to deterioration of the water quality of the receiv-
ing stream.
DETAILED DESCRIPTION
A waste water treating facility 10 embodying our improved process is
schematically illustrated in the attached Figure. Typical contaminant
water is the waste water from an oil refinery and waste water from a chemical
plant. Table I below illustrates common characteristics of oil refinery waste
water and Table 11 below illustrates common characteristics of waste water
from a chemical plant.
TABLE I
REFINERY WASTE WATER CHARACTERISTICS AFTER
PRIMARY TREATMENT IN API SEPARATOR
Median Values for Class C Refineries (USA~
. _
Parameters Concentration, mg/liter
Biochemical Oxygen Demand, 5-day 163
Chemical Oxygen Demand 473
Total Organic Carbon 160
Oil and Grease 51
Phenolics 1 1
Suspended Solids 52
Ammonia 48
Sulfide 2
_ _
;Z820
TABLE II
~()MI` CI II MI('AI, I'I,AN I' W/~,S I ~' WA rli R (~I IA I~C"I~ I,'; I`IC,~
lER INPLANT PRET~E~ rML~N T
Parameters Concentration Range, mg~liter
Biochemical Oxygen Demand, 5-day 50 - 5000
Chemical Oxygen Demand 500 - 20, 000
Suspended Solids 30-- 100
Ammonia 50 - 250
As shown in the Figure, the oil refining and chemical plant waste
waters are mixed t~gether in the first compartment 29 of a multiple compart-
ment equalization basin 12. The efflucnt from this basin 12 flows throu~h
valved lines 13 and 14 into a bank of pressure filters/and through a head
tank 18 into a biological treating plant 20. The oil refinery waste water first
flows into a sump 22 and then into a conventional API separator 24 where gross
amounts of oil and solids are removed. Under normal conditions, the treat-
ment facility 10 can handle a maximum design quantity of water per day. For
example, a large facility may have a capacity of 25, 000, 000 gallons of water
per day. Heavy rain storms could, however, overload this facility. Conse-
quently, a compartmented surge basin 26 is provided for holding abnormal]y
large quantities of water, and as will be explained in detail below, for storing
shock loads of contaminants such as acids or alkalis. A pump 28 forwards
any excess water from the sump 22 to this surge basin 26.
In accordance with one feature of our invention, the concentration of
contaminants in the water flowing to the downstream biological treating plant
20 is controlled so that variations in contaminant levels are equalized. The
equalization basin 12 serves to level out or equalize contaminant concentration
by passing the waste water through three separate compartments 29, 30 and 31
in the basin 12. When a sharp increase in the noxious contaminant is exper-
ienced in the influent to the basin 12, the initial effluent concentration from
-- 8 --
1~628ZO
the third compartment 31 is lower or changes less than from a single compart-
ment basin. ~his provides time for acclimation of the micro-organisms in the
biological treating plant 20.
Any sharp increase in contaminant concentration or any drastic change
in the type of contaminants entering basin 12 has the greatest and most imme-
diate impact on water quality conditions in compartment 29. When water from
this first compartment 29 is mixed with the body of water in the second com-
partment 30, contaminant concentration is reduced. When the water from the
second compartment 30 is mixed with the body of water in the third compart-
ment 31, contaminant concentration in the third compartment is substantially
reduced again. Mixing the water in this manner tends to dilute the contamin-
ants so that their initial effluent concentration from the third compartment 31
is lower than if a single basin is used. Thus, if a slug of contaminants flows
into the first compartment 29, this slug would be blended gradually in the
quantities of water in the second and third compartments 30 and 31, be diluted
and therefore initially would not increase or otherwise change the contaminant
concentration or character by any substantial amount in the third compartment
31. As a consequence, the micro-organisms in the downstream biological
treating plan 20 acclimate to the slow exposure of the changes in contaminant
concentration or character and adapt to biologically degrade this higher concen-
tration of contaminants or different character of contaminants.
In accordance with another feature of our invention, we maintain at a
minimum the average time the water is retained in the equalization basin 12.
Thus, the heat in the water is retained at a maximum. High heat in water
fosters increased biodegradation of contaminants in the treating plant 20.
Average water temperature entering the plant 20 preferably ranges between
90F and 100F.
The water in the first compartment 29 is monitored to determine the
presence of especially noxious contaminants, for example, ammonia, phen-
- 1~6Z820
olics, BUlrl~-, acids, callstics, etc., so that their source may b~ traced ancl
col r ~et-iv~ acli~ll taken. In tll~ secoIld compartmell~ 30 pH is contl-oIIc(l by
addition of acids or alkalis so that it is in thc rangc (~f about ( . 5 Lo ~ but prc-
ferably from about 7. 5 to 8. 5 when air oxidation of contaminants is required.
This pH range is optimal for the oxidation reactions to occur and when desired
the reactions are accelerated by adding hydroquinone or gallic acids.
Conventional floating aerators (not shown) float on the surface of the
water in each compartment 29 through 31 and introduce air into the water to
aerate and thoroughly mix the waste water. Such aerators (not shown) in com-
partment 30 mix and aerate to maintain dissolved oxygen levels in the preferred
range of 3 mg 02/liter or greater. The preferred ratio of the aerators is 0. 2
horsepower aeration or more per 1000 gallons compartment volume.
If for any reason the equalization pond 12 is flooded with an extremely
high concentration of contaminant beyond handling capability, for example, if
a line carrying acid broke, a valve 34 in a recycle line 36 is opened and the
valve 38 in the filter inlet line 14 is closed. A pump 40 then pumps this highly
acidic water to the shock load compartment 26a of a surge basin 26 where it
is retained and gradually reintroduced into the first compartment 29 of the
basin 12 through a valved line 42. This protects the downstream biological
treating plant 20 from being poisoned by shock loads of contaminants.
The mixing, aeration, pH control, chemical reactions, etc., taking
place in equalization basin 12 causes coagulation and flotation of considerable
contaminant matter. This matter is skimmed from the surface of the basin 12.
Conventional slotted skim pipe (not shown) at the surface of the water in com-
partment 31 may be used.
The effluent from the final compartment 31 contains colloidal matter to
which coagulants or flocculants such as aluminum or iron salts, and/or high
molecular weight organic polyelectrolytes are added. The coagulants or floc-
culants destabilize, for removal by filtration the colloidal particles which are
- 10 -
1~)62820
carried by the effluent from the basin 12 to the bank of filters 16. The filtered
water passing into the head tank 18 is lifted by thc pump 40. The preferre(I
filtcr mcdium u9cd in thc bank of filtcrs 16 i9 Sall(l 0r Ll COmbill~ l)ll ol` sand
and coal. It is important that the water flowing to the downstream biological
treating plant 20 be filtered to reduce suspended solids and oil to levels which
do not interfere with the process. Under most conditions, the water flowing
into the biological treating plant 20 preferably must contain no more than ten
parts of oil or hydrocarbons per million parts of water and no more than ten
parts of oily suspended solids per million parts of water. Periodically, a fil-
ter unit in the bank of filters 16 must be backwashed. This is achieved by
closing a valve in the feed line to the filter unit being backwashed and opening
a valvc in a backwash waste line (not shown) such that the effluents from the
onstrcam filters are used for backwash water. One function of the head tank
18 is to provide a constant back pressure on the filtered water thereby pro-
viding a constant pressure backwash water source. The backwash water
washes out the solids trapped in the filters, carrying them with the water into
a sludge surge basin (not shown).
The biological treating plant 20 has four process stages. A contact
stage 44 where the contaminated water contacts a biologically active sludge 46.
A clarifier stage 48 where sludge is separated from decontaminated water.
A thickening stage 50 where separated sludge is thickened to remove excess
water. And a digestion stage 52 where thickened sludge is digested. In the
first stage 44 water essentially free of solid and oily matter contacts the acti-
vated sludge mass 46 in a contact tank 54 called a mixed liquor tank. This
sludge 46 includes micro-organisms which feed on the contaminants in the
water. The metabolic processes of the micro-organisms convert the contam-
inants to cellular structure of the organisms, carbon dioxide, and various
intermediate products. In the second stage 48, water and activated sludge
from the mixed liquor tank 54 flows into a clarifier tank 56 via a line 72.
~ iZ8zO
~ s will be explained further below, activated sludge from a second source is
added to line 72 via line 100 and the combined sludges and water flows to clar-
ifier tank 56. The line 72 and an isolated zone 84 of the clarifier tank 56 pro-
vide for contact of the second activated sludge recycle component and the resid-
ual contaminants in the water leaving the mixed liquor tank 54. This results
in furtherpurification of the water. Water is separated from these sludge
particles by allowing the sludge particles 46 to settle on the bottom of the clar-
ifier tank 56. Decontaminated water flows from the top of ~he clarifier through
a second bank of filters 58 into a receiving stream 60, Preferably through a
bed of activated carbon 66 for removal of trace solublc contaminants before
discharge to the receiving stream.
In the third stage S0, the sludge 4 6 withdrawn from the bottom of the
clarifier tank 56 is concentrated and the bulk of any water retained by the sludge
is separated and withdrawn, In the fourth stage 52, thickened sludge is held in
a tank 62 for a period of time sufficient to allow the micro-organisms to metab-
olize stored food material. This digested sludge is then spread over land and
permitted to decompose and serve as a fertilizer. Alternately, the sludge can
be incinerated.
In accordance with our invention, interstage aeration is provided to
aerate the water as it flows into the biological treating plant 20 and between the
four stages of the plant 20. The most important interstage aeration is the
aeration of the streams of water and sludge flowing in lines 72 and 74 between
the first and second stages 44 and 48. Because of this aeration, the water
leaving the clarifier tank 56 and being discharged into a receiving body of water
contains at least about five parts of dissolved oxygen per million parts of water.
This is highly desirable especially when carbon adsorption is employed. The
oxygen in the discharged water from the clarifier tank 56 maintains any
micro-organisms trapped in ~he filter 58 or following carbon bed aerobic. If
there is insufficient air in this discharge water, the micro-organisms trapped
in the filter go anaerobic producing hydrogen sulfide which would contaminate
- 12 -
11~6ZB20
thc dischar~ed water In addition, the dissolved oxy~en in the water in the
clal if iel- tallk ~'i( m.~ t.liIls thc ~lu~ 46 otl tl)~ Ottolll of tl~is l.ulk .I( l ol-ic,
pcrmit~:ing tlIc slll-lgc to be retaincd in lhc tl~ickcnel 5() ~lnd cl.l~ ilicl ~,Y lollgcl
than conventional. This provides more effective thickener and clarifier oper-
ation.
We achieve interstage aeration by aspirating air into water flowing
between tanks or positively injecting pressurized air into the transfer line.
In addition to backwashing the bank of filters 16, the head of the water in the
tank 18 can be utilized advantageously to aspirate air into the water flowing
into the mixed liquor tank 54. The water level in the head tank 18 ls above
the water level of the mixed liquor tank 54. Water thus flows from the top of
the head tank 18 downwardly through a line 64 and along a long generally
horizontal line 66 which turns upwardly into a line 68 leading into the center
of the mixed liquor tank 54. The horizontal line 66 is either at ground level or
preferably below ground level to maximize the hydrostatic pressure. Thus,
the air aspirated into the water is subjected to high pressure due to the water
standing in the head and mixed liquor tanks 18 and 54. The horizontal line 66
can have a larger diameter than the downwardly extending line 64 or purposely
be extended by looping, for example, so that the dwell time of the water and
air mix can be extended. This substantially saturates or even supersaturates
with respect to atmospheric pressure the water entering the mixed liquor
tank 54 with dissolved oxygen. Normally this water flowing into the mixed
liquor tank 54 will contain at least about 6 to 8 parts of dissolved oxygen per
million parts of water and typically can reach levels above saturation of about
12 parts of dissolved oxygen per million parts of water. In a similar manner,
air is aspirated or pressured into the water flowing from the mixed liquor
tank 54 into the clarifier tank 56. The vertical line 72 transfers the water
and suspended sludge particles downwardly to the horizontal line 74 which
turns upward into a line 76 terminating near the surface of the clarifier tank
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1~6Z8ZO
S(. An aspi rator 78 sucks air into thc downward flowing water in the line 72.
I hc water elevation in tl~e tanks 54 and 56 subject the ~ir-water mixture ~o higl
prcssure as it flows through the line 74. This can saturate or supersaturate
the water with dissolved oxygen.
The clarifier tank 56 is designed to receive the water from the upwardly
extending line 76 into a confined mixing region formed by cylindrical baffle 82
concentric with the side walls of the tank. The diameter of cylindrical baffle
82 is preferably about 1/2 the diameter of the clarifier 56 and extending to
about six feet from the bottom. Line 76 upwardly extends to well within the
circular baffle 82 and, as the air-water mix exits line 76, the air lift pumping
action creates a turbulent zone 84 in the center of the clarifier tank 56 that
provides for further activated sludge-water contact, oxygen transfer and floc-
culation, The preferred contact time in line 76 and turbulent zone 8~ is at
least 20 minutes, The clarifier tank 56 includes weirs 80 at the top of the tank
that maintains the water level and provides for discharge of clarified water
from the quiescent zone 86. Activated sludge particles settle to the bottom of
the tank where they are withdrawn by a conveyor and pump 88 system
In our process, air under pressure from sources 90 and 92 is injected
into the sludge flowing between the clarifier tank 56 and thickener 50 and
between the thickener 50 and the digester 52. This high pressure aeration of
~ lld
sludge permits the sludge to be maintained in the clarifier tank 56/ thickener
for periods in excess of what is normally considered feas-
ible in the activated sludge process. For example, the activated sludge-water
mass in the feed to the thickener and clarifier in the normal system contains
1 mg 02/liter or less. As the sludge blanket settles the dissolved oxygen in
the interstitial water is rapidly depleted by the respiration of the micro-organ-
isms and the facultative organisms start to remove oxygen from the nitrogen
and sulfur compounds present in the water. This released hydrogen sulfide
and nitrogen gas upsets the sludge settling process and seriously degrades
1~62820
water quality. In our process about ten times the dissolved oxygen concentra-
tion can be provicled compared to conventional practice. This greatly decreases
the rate at whicll septicity occurs and alleviates substantially the problems
associated with retaining the sludge in the clarifier and thickener until the
excess water is substantially removed,
Another aspect of our invention relates to the use of sludges with differ-
ent properties recycled to different points to achieve different functions, all in
a single activated sludge plant 20. As conventional, sludge wi thdrawn from the
elarifier tank 56 is reeyeled through valved branehed line 94 into the mixed
liquor tank 54 with exeess sludge to the thickener 50. A portion of this recycled
sludge is introduced through braneh 96 into the sludge-water mix flowing
between the mixec3 liquor tank 54 and the elarifier 56. This absorptive sludge
portion entering via line 96 has capacity to absorb and store residual soluble
contaminants and improve the floeculating properties of the total sludge mass
for improved separation in elarifier 56. The interstage aeration and elarifier
design provides for eontaet time, mixing and aeration to optimize the eapacity
of this system. Similarly, the recycle sludge could be routed through the
thiekener 50 and via line 98 into the sludge-water mix flowing between the mixed
liquor tank 54 and the elarifier tank 56 Sludge from the thickener 50 has been
without food longer and therefore has greater absorptive and storage capacity
and is cOntained in a reduced volume because of the c3ewatering aetion of the
thiekener, Maintaining the thiekener sludge aerobie using interstage aeration
is a requisite for satisfactory sludge quality for recycle from the thiekener 50.
Another souree in our proeess of the reeyele sludge is obtained by routing that
sludge through the thiekener 50, the aerobie digester 52, and valved line
100 into the sludge-water mix flowing in lines 72 and 74 between the mixed
liquor tank 54 and the elarifier tank 56. The sludge eomponent from the
aerobie digester 52 has had typically one to four weeks to acclimate to the
residual refractory substrate contaminants. This acelimated sludge is
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1C)62820
especially effective for absorbing and biodegrading the residual ~ub~trate
in thc watcr exitln~ mixcd liquor tank S4. When the combined sllIdgc m~ss
enters clarifier 56, the acclimated sludge combines with sludge in the clar-
ifier tank 56 and seeds the sludge being recycled to the mixed liquor tank
54 via line 94. Seeding the main recycle sllldge mass continuously with
sludge acclimated to residual, refractory materials shifts the equilibrium
to increase removal of these contaminants by the main sludge mass in the
mixed liquor tank 54. After equilibrium is attained there is no longer high
concentrations of refractory substrate in the water leaving the clarifier
tank 54. Introduction of any new refractory materials into the system causes
the rapid development of acclimsted organisms.
As evident to those skilled in the art, modifications can be made in
our process without departing from the principles of our invention claimed
herein. For example, oxygen may be substituted for air in the interstage
aeration system.
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