Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~7669
- 1 - D . E . Severeid-D . D. Jech 1-1
PROCESS }~OR T~E SECONDARY TREATMENT O~ WASTEWATER
This inven~on relates to a proces~ for the secondary
treatment of wastewater and paxticularly to the secondary trea~-
m~nt of s~lfite pulp mill ei~1u~nt.
S~condary treatment proce~ses reduce organic tBOD)
S conter~t of wa~tewater throu~h biological acti~ity. In general,
wastewater is intimat~ly mixed with a bac~erial population wnich
utilize~ organic ;naterial as a food source. Nutrient addition,
aeration, and ad justment of pEI and temperature are employed to
provide ar~ environment suitable for survival ~nd reprodu~ on
and thu~ re!~ bacteria growth. Normally the bioma~s produc~d
du~ing a~ration is 3eparated ~rom 'che treated water before
disch~ge to the receiving water.
At loc:ation~ where land area is not plentiful, biolo~ical
treat:m~nt is g~nerally accomplished in high-~ate air activat~d
sludge (AAS) sy~tems.,, In ~uch systems, BOD removal i~ achie~red
~ ~ aeration basin. The liquor and biomass from t~e aeration
basin are sent to a gravity clarif ier where the biomass is reco~rer-
ed, thickened, and recycled to the aeration basin. T~is maintains
a higA bis~mass concentra~ion ~ t~e aera~ion basin and, for an
aeration ba~in of c~n$tant ~ol~ne, in~raases the BOD removal
efficiency. It can b~3 seer~ that sati~factory bio~nass settle-
ability i~ e~sential to ac:hievinq optimuzn peri~ormance in an AAS
system. T~e AAS proc~ss generates exce~s biomass which mus' be
wasted from the sy5tem and handled separately.
It is generally acc:epted that the performance of an A}~
system is ganrerned by ~e organic- loading expresse~ as the ~ood-
11~76~
D. E. Severeid et al 1-1
- 2 -
,,,, ., -, ",.. .
to-microorganism (F/M) ratio. The F/M ratio is defined as the ~
total wastewater BOD fed ~o the system per day per unit of - -
biomass maintained in the aeration basin. BOD removal effi-
ciency generally worsens rapidly as the F/M ratio is increased.
.. --........................................................................... -.:: .
Biomass settleability is adversely affected by a very low or
._. ......
very high F/M ratio. Therefore, AAS systems are normally de-
signed with an F/~ ratio of 0.2-0.5 to obtain both good treat- =
..... .
ment efficiency and a sa~isfactory biomass settling rate. The
F~M ratio is also important because it is related to the size --
of the treatment plant.
A recent variation of the AAS process involves aeration ,-====
in a deep tank. Deep tank aeration utilizes an above or below
ground tank of at le~st 9 meters, normally greater than 15 ~-
meters, depth. The depth of the tank creates a hydrostatic -
15 pressure of sufficient magnitude to increase the rate of oxygen -
transfer so that the aerobic bacteria are much more efficiently
supplied with the aix they require than is the case with activated
sludge processes. Deep tank aeration processes are disclosed ~==
at a number of places in the literature, as for example, in U.S.
Patent 3,574,331.
While clarification of aerated li~uor is conventionally =
carried out by gravity separation, there have been suggestions =
that deep tank aeration be coupled with dissolved air flotation -:_
clarification. Dissolved air (or gas) flotation is a well-known
solid-liquid separation process. In dissolved air flotation,
the wastewater, saturated with air under pressure, enters a - =
flotation cell maintained at atmospheric pressure. The reduc-
tion in pressure causes the air to be released from solution
~1~7t~69
D. E. Severeid et al 1-1
- 3 -
in the form of fine bubbles which attach to the sludge or other
suspended material and carry them to the surface of the water t~
in the flotation cell. Dissolved air flotation is frequently
used as a method for recovering fibers or thickening biomass -
5 wasted from activated sludge systems. There are, however, few
cases where dissolved air flotation has been used for clarifi- -
cation in secondary treatment systems, i.e., for both solids
removal and-for recovery and recycle of bacteria. The combina-
tion of dissolved air flotation with deep tank aeration has -
10 been suggested as possessing advantages over a conventional ~AS
system. See for example New Civil Engineer, B. Appleton, April
17, 1975.
Regardless of which type of secondary treatment system
is used, the size of the treatment plant and hence capital costs ---
could be reduced by lowering the amount of bac~eria needed to
reduce a given amount of BOD or, stated otherwise, by increasing
the F/M ratio. Organic loading, expressed as the F/M ratio, is - -
genexally accepted as governing the performance of a secondary
treatment system. See, for example, Adams & Eckenfelder, _rocess
Design Techniques for Industrial Waste Treabment, Nashville, Tenn.,
Enviro Press, Inc., 1974 However, the use of F/M ratios of over - -
about 0.5 has generally not been considered feasible with conven~
tional AAS systems. The use of significantly diffexent ratios
for deep tank aeration~dissolved air flotation systems has also
i..-. .-, = . . .
25 been ruled out for fear of unacc~ptable BOD removal~ Moreover,
sudden changes in the BOD level of the wastewater, its flow rate --
temperature of pH create what is known as shock loadings which - ~
depress the efficiency of the system. It has generally been
felt that high F/M ratios in such deep tank systems would be ~ -
30 susceptible to shock loadings because of the short treatmen~ -
~. , , . . . . ' ,~
~i766g
D. E~ 5evereid et al 1-1
-- 4 --
retention time associated with a high F/M.
It is accordingly a primary object of this invention to
provide a process for the secondary treatment of wastewater
which successfully utilizes higher food-to-microorganism ratios
than have heretofore been possible.
It is an additional object of this invention to provide
a secondary treatment process for wastewater which minimizes
the size of treatment facilities and hence, their capital costs.
It is a more specific o~ject of this invention to provide
an economical but efficient process for the secondary treatment
of sulfite pulp mill effluent.
It has now been found that food-to-microorganism ratios
greater than one may be successfully utilized in secondary treat-
ment systems in which the suspended solids are separated by
dissolved gas flotation clarification. F/M ratios up to ten-fold
higher than that of conventionally designed activated sludge
systems have been successfully utilized. More specifically, the
process of the present invention comprises aerating wastewater
containing bacteria, under condi~ions in which bacteria growth
occurs, by continuously metering diffused air into a reactor until
the BOD level of the wastewater is reduced by at least 90%, the food
to microorganism ratio of the wastewater treatment system being
greater than one, introducing the aerated wastewater to a flotation
cell to separate suspended solids including bacteria from the waste-
25 water by dissolved air flotation clarification, recycling saidsuspended solids and bacteria to said aeration step in an amount
sufficient to maintain said food to microorganism ratio and
removing and disposing cf the remaining reduced BOD level wastewater.
,,, ~
~.
~1~'7669
- 5 - D.E.Severeid et al. 1-1
The single figure of the drawing is a schematic flow
diagram of the process of the invention.
The process of the invention has been found, quite
surprisingly, to achieve excellent BOD and toxicity removal
efficiency through a range of F/M ratios of from 1 to 4 and
even higher. It was also found, again surprisingly, that the
process was ~uite immune to shock loadings, even at these higher
F/M ratios. ~uring a severe shock loading trial with spent
sulfite liquor, in whish the F/M ratio was more than doubled and
the biomass suf fered anaerobic conditions for 12 hours, the BOD
removal efficiency was almost unaffected, dropping from 96% to
94% and returning to 96% the day after the shock. In addition,
the BOD treatm~nt efficiency of the system was unaffected by an
abrupt change in temperature tfrom 30 C. to 20 C. in 24 hours3.
Moreover~ studies have shown that the dissolved gas flotation
clarifier consi~tently achieved greater than 90% solids recovery,
even during spent sulfite liquor shock loadings.
Al~hough the reasons for successful operation a~ the
higher F/M ratios useful in the invention are not fully under-
stood, it is believed at least in part to result from the faster~olids separation achie~ed through the use of dissolved air
flotation. Thus, the elapsed time from completion of aeration,
through clarification, back to aeration is believed insufficient
to kill a significant amount of bacteria. In conventional
2S gravity clarification systems, a substantial number of the
aerobic bacteria are believed to be killed during the prolonged
clarification step. In the present process, recycling of the
suspended solids and bacteria occuxs rapidly -- typically within
~17~6g
- 6 - D.E. Severeid et al. 1-1
10-15 minutes, usually in less than thirty minutes and rarely
in more than an hour.
The process of the present in~ention is schematically
shown in the drawing. Except to the extent herein set forth, the
first step of the process, aeration, is carried out in accordance
with well known prior art techniques. In the case of a sulfite
mill, mill wastewater normally includes primary treated waste-
water and clea~ effluent from spent sulfite liquor, hot caustic
extraction and bleach plant effluent. As shown in the flow
diagram, the wastewater is fed to an aeration tank where it i~
neutralized to a pH of a~out seven with caustic or lime and then
nstrogen and phosphorus nutrients in the form of, for example,
ammonium hydroxide and phosphoric acid are added~ The wastewater
is then aerated with air (1) to keep the system turbulent and
(2) the bacteria well distributed and (3) to provide oxygen during
the retention time in the aeration tank. Aeration may be carried
out in a shallow basin with conventional air activated sludge
~AAS~ systems, but it is preferable that aeration be carried out
in a deep tank or column. In deep tank aeration, diffused air
is metered into the bottom of the tank at a rate su~ficient to
provide the desired dissolved oxygen concentration. Non-absorbed
gases leave the top of the column, which is open to the atmos-
phere. In the case of sulfite pulp mill effluent, BOD levels
are very high, generally over 400 mg~l and as high as 1000 to
1200 mg/l during manufacture o~ high purity dissolving pulp,
as contrasted with a BOD level of about 200 mg/l for municipal
wastewater. Retention time in the aeration tank can be much
lower with the deep tank aeration systems as compared with the
conventional AAS systems (i.e., 3-8 hours vs. 20 24 hours).
il~7669
D. E. Severeid et al 1-1
Aeration should reduce the BOD level about 80% and usually over
90%, depending on the F/M ratio and the wastewater treated. - -
In accordance with the present invention, ~he F/M - -
ratio in the aeration tank is maintained at an average level
higher than one. The F/M ratio as used herein means the total
wastewater BOD in grams fed to the aeration basin per day per
gram of biomass (volatile suspended solids) maintained in the ' ~
aeration basin. BOD or biochemical oxygen demand as used herein - -
is defined as ~he amount of oxygen (mg~l) consumed by micro-''
organisms in five days at 20C.
Upon completion of aeration the aerated wastewater is -
transferred to a dissolved air flotation clarifier. Aerated -- -
liquor is introduced into the clarifier with sludge and ---
clarified effluent being removed from the clarifier top and . !===
bottom, respectivel~. A portion of the clarified efluent is
aerated under pressure (by passing it through a pressurizing pump ~''''
and small retention tank) and recycled to the clarifier to
increase the amount of dissolved gas available for flotation. '''''-''
~ecycle through the pump and retention tank is desirable with ~'''''''''
deep tank aeration systems to increase clarifier efficiency.
Such recycle is necessary with systems using shallow aeration
tanks to introduce the dissolved gases under pressure to''-'' '''
effect the flotation clarification action. Dissol~ed air flota- --
.= :::::::
tion systems in which the effluent is partially or wholly~--''=:-:
recycled through a pump and retention tank are known and are dis-
closed, for example, in Metcalf and Eddie,''Inc., Waste Water ''''''-'-'''
Engineerin~: Collection', ~reatmen't and Disposal, New York,
McGraw-Hill, 1972, pages 296 to 301. : -
~' ' . ==.
!
B
~17669
D. E. Severeid et al 1-1
~ 8
As a blanket of float ~solids and bacteria) builds ---
up in the clarifier, the uppermost float is pushed above the
liquid level and allowed to drain~ Thus, the~de~per the float !~
blanket the higher the solids concentration of the skimmings. ---
.-
This is also true for increased air-to-solids ratios which
increase the air available for flotation. The top of the float
blanket is continuously removed in commercial scale facilities
and scraped up on an inclined beach where it drains further. A - -
normal depth for the float blanket of a commercial unit is about
30 cm.
The followi~g examples are illustrative of the practice - -
of the invention. Unless otherwise indicated, all parts are by --
weight.
Examples 1-6
The total effluent from a sulfite pulping mi11 was
neutralized with caustic to a pH of 7.5, heated with indirect
ste ~ to 309C, fortified with phosphorus a d nitrogen nutrients ~--
(P:N:BOD~1:10:100) and then fed to an aeration tank. A deep t~nk
aeration-dissolved air flotation system of t e type shown schemati~
~ally in the drawing was used. The aeration vessel was a 10 centi~
meter (i.d.) column with 4.62 meter working depth, equivalent to
a working volume of 40 liters. A pressure reducing valve at the ~-
top of the col D was adjusted to maintain ~ ~ack pressure of 69 ~ -
K~ag (Kilo pascals gauge) at the top of t e col n to simulate a
deep tank having a working depth of 18.3 meters~ The dissolved
oxygen concentration of the mixea liquor in the aerator was
maintained at 2-3 mg~lO The clarifier was a 5.6 ~ (i.d.) by
46 cm. tall tube. T~e mixed liquor was introduced by a
t:::::::::::::::
~il7669 D. E. S~ !re~t et al l-l
-- 9 ~
variable speed metering pu~p into the clarifier with sludge and ~
:.,., ..:
clarified effluent being removed from the clarifier top and
. :.-.-...-.--. :::
bottom respectively. A portion of the clarified effluent was
re-aerated under pressure and recycled to the clarifier to in-
. -.
crease the amount of dissolved gas available for flotation.
: ....:..:.. ... -.
Table l records the operating conditions and results of six -- -
successive tests (Examples 1-6) at varying F/M ratios. The
.. :.,, ..:
liquor retention time is the residence time of the feed liquor ---
- . . . ~
in the aeration tank. The temperature of the mixed liquor was --
..... -
lowered from 30 to 20 C. in order to simulate a more severe
temperature condition which might be faced at a sulfite pulp mill.
. -. . - .-
The lower the temperature the slower the bacterial action. The -:
.. . ..
F/M ratios given are the average of the F/M ratios during each -
example test period. MLVSS stands for mixed liquor volatile -
suspended solids and is the term used to denote the suspended
F
solids concentration ~Mg/l) in the aeration tank.
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~1~L7669
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669
D~ E. Severeid e~ al 1-1
............
Table I shows that at F/M ratios of from 0.6 to as high as 4.0, -~
. .
high BOD removal rates are achieved. Prior literature has --
t.:.:.:.:.:.:.:.:,::.:,:.::::
indicated that at these high F/M ratios, and particularly at
ratios over 1, high solubl~ sOD removal levels could not be
5 maintained. The column headed "Sludge Age" measures the average :--
age of the bacteria }n the aeration tank. The lower the sludge
age, the smaller the treatment system can be. Ideally, sludge ;:-~
.-::.-::-.... :..
age should-be as low as possible to minimize treatment costs but ---
. . .-. . . :-:. = -
without sacrifice of high soluble BOD removal rates. It will
be seen that at F/M ratios over 1, sludge age is reduced by order
of magnitude with only a slight drop in BOD removalO - -
Example 7 :
In this example, mill effluent was treated as in ---
Examples 1-6 except that the system was subjected to a series
15 of shock loadings of up to 12 hours duration over a two week '~
period. The F/M ratio was targeted at one, the mixed liquor
..... ,.. ,.,.,-,.- ::.
temperature at 20C., the retention time at 7 hours and the ~LVSS .~
- ., , . .-
concentration at about 2500 mg/l. After each shock, the system
:..... ::
was operated for 2-3 days at nominal (target) conditions to allow :
= - .
it to recover from the shock. The magnitude of the shocks was
..: -..- ....
determined by assuming that the full-scale system would have to ---
treat its normal load plus all the spent sulfite liquor (SSL)
. . . - -
solids generated by the mill for periods of 4-12 hours dura~ion.
r ,,~,' ' , ~
This amount of SSL at 10% solids concentration would approxi- ---
mately double the influent BOD concentration yet have a negligible
effect on the hydraulic loading to the system. Prior to each --
--.. =
shock, the nominal air rate was detemined experimetally and -~
;:, .:. - .
the air rate was then res~ricted during the shock. The restric- t:::::::::::::::-
tion on air simulates the finite~compressor capacity in a full~
t - -
.. .........
` ~i7669
D. E. Severeid et al 1-1
- 12 -
scale system. The first shock was limited to 50% extra air
~ .
while the fourth was limited to no extra air. The first
..
simulates a system which normally needs two air compressors
, .....
but a third is available for shock loads. The fourth shock
5 simulates the same system with only the two compressors, no
extras. The results of these tests are set forth in Table II.
.. .:: .- .. .:
TAE~LE II - -
.. ............
Shock Nun~er 1 2 3 4
Duration of Shock, hr 4 6 12. 12
Period After Shock, days2 5/63 3/43 1/2 3 1/2
Air ~ate
Nominal, SLPM* 4.3 4.4 4.1 3.7 --
Avg. Day of Shock, SLPM6 . 28 . 2 5.7 ~.6 - :
Maximum, % of Nominal149 211 180 97 ~- -
Influent Total BoDs(Nominal)~mg/l 848 868 823 732
Applied F/M Ratio :-
During Shock 2.63 2.40 2.012.44
Day of Shock ~Nominal)1.281.07 0.831.01
Avg. Day of Shock 1.51 1.40 1.441.74 -
Day Before Shock 1.27 0.96 0.961.27 ~----- -
Day After Shock 0.99 1.12 0.740.93 r~
Avg. for Entire Period1.151.13 1.171.38 ~: ----
Effluent Soluble BOD, mg/l
Day Before Shock 30 32 25 31
Day of Shock 47 32 28 71
Day After Shock 34 32 24 27
Soluble BOD Remo~red, % E
Day Before Shock 96.0 96.2 96.995.9
Day of Shock 95.1 96.8 97.994.4
2G Day After Shock 95.9 96.5 97.095.7 l- -
..::....:...
*SLPM is: standard liters per minute. ,=~
The stability of soluble BOD removed during shock leads should i - -
be noted. The results indicate an ability to withstand severe r~
35 shock loadings in succession without irrevocable damage to the
. . .'. . .
biomass. It should be noted that in shock 4, in which the F/M - -
ratio was more than doubled and the biomass suffered anaerobic -
. ...
conditions for 12 hours, the BOD removal efficiency was almost - --
.. . .
unaffected, dropping from g6% to 9496 and returning to ~69s the
.:....,.: .
40 the day after the shock.
~ --- - .:
,.............
,. ..
, .. .
i~l7~6g
- 13 - D.E. Severeid et al 1-1
The foregoing results indicate that the present process
is capable of excellent and reliable treatment of wastewater at
F/M ratios as high as ten-fold greater than that of conventional
activated sludge systems.
HJH~rc
January 20, 1978
