CA1117669A - Process for the secondary treatment of wastewater - Google Patents
Process for the secondary treatment of wastewaterInfo
- Publication number
- CA1117669A CA1117669A CA000320880A CA320880A CA1117669A CA 1117669 A CA1117669 A CA 1117669A CA 000320880 A CA000320880 A CA 000320880A CA 320880 A CA320880 A CA 320880A CA 1117669 A CA1117669 A CA 1117669A
- Authority
- CA
- Canada
- Prior art keywords
- wastewater
- bacteria
- aeration
- bod
- shock
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F1/00—Treatment of water, waste water, or sewage
- C02F1/24—Treatment of water, waste water, or sewage by flotation
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F3/00—Biological treatment of water, waste water, or sewage
- C02F3/02—Aerobic processes
- C02F3/12—Activated sludge processes
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
- Y02W10/00—Technologies for wastewater treatment
- Y02W10/10—Biological treatment of water, waste water, or sewage
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- Life Sciences & Earth Sciences (AREA)
- Water Supply & Treatment (AREA)
- Hydrology & Water Resources (AREA)
- Engineering & Computer Science (AREA)
- Environmental & Geological Engineering (AREA)
- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Microbiology (AREA)
- Biodiversity & Conservation Biology (AREA)
- Activated Sludge Processes (AREA)
- Physical Water Treatments (AREA)
- Biological Treatment Of Waste Water (AREA)
- Purification Treatments By Anaerobic Or Anaerobic And Aerobic Bacteria Or Animals (AREA)
Abstract
D.E.Severeid-D.D.Jech 1-1 Abstract of the Disclosure:
A process for the secondary treatment of wastewater comprising aerating wastewater containing bacteria until the BOD level of the wastewater is substantially reduced, the food to microorganism ratio of the wastewater being greater than one, introducing the aerated wastewater under pressure to a flotation cell to separate suspended solids including bacteria from the pressurized wastewater by dissolved air flotation clarification, recycling said suspended solids and bacteria to said aeration step in an amount sufficient to maintain said food to micro-organism ratio and removing and disposing of the remaining reduced BOD level wastewater.
A process for the secondary treatment of wastewater comprising aerating wastewater containing bacteria until the BOD level of the wastewater is substantially reduced, the food to microorganism ratio of the wastewater being greater than one, introducing the aerated wastewater under pressure to a flotation cell to separate suspended solids including bacteria from the pressurized wastewater by dissolved air flotation clarification, recycling said suspended solids and bacteria to said aeration step in an amount sufficient to maintain said food to micro-organism ratio and removing and disposing of the remaining reduced BOD level wastewater.
Description
~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$tant ~ol~ne, in~raases the BOD removal efficiency. It can b~3 seer~ that sati~factory bio~nass settle-ability i~ e~sential to ac:hievinq optimuzn peri~ormance in an AAS
system. T~e AAS proc~ss generates exce~s biomass which mus' be wasted from the sy5tem and handled separately.
It is generally acc:epted that the performance of an A}~
system is ganrerned by ~e organic- loading expresse~ as the ~ood-11~76~
D. E. Severeid et al 1-1
This inven~on relates to a proces~ for the secondary treatment of wastewater and paxticularly to the secondary trea~-m~nt of s~lfite pulp mill ei~1u~nt.
S~condary treatment proce~ses reduce organic tBOD) S conter~t of wa~tewater throu~h biological acti~ity. In general, wastewater is intimat~ly mixed with a bac~erial population wnich utilize~ organic ;naterial as a food source. Nutrient addition, aeration, and ad justment of pEI and temperature are employed to provide ar~ environment suitable for survival ~nd reprodu~ on and thu~ re!~ bacteria growth. Normally the bioma~s produc~d du~ing a~ration is 3eparated ~rom 'che treated water before disch~ge to the receiving water.
At loc:ation~ where land area is not plentiful, biolo~ical treat:m~nt is g~nerally accomplished in high-~ate air activat~d sludge (AAS) sy~tems.,, In ~uch systems, BOD removal i~ achie~red ~ ~ aeration basin. The liquor and biomass from t~e aeration basin are sent to a gravity clarif ier where the biomass is reco~rer-ed, thickened, and recycled to the aeration basin. T~is maintains a higA bis~mass concentra~ion ~ t~e aera~ion basin and, for an aeration ba~in of c~n$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
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 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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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 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
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 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.
~t'''''"'~,'"''''. , ' ' . ' ' ~
; '.' .
' '.'. ' '.'.'.' '.'. ' ' .
'.'.'....'.'.'''. '.
'_. . ".
. .
~' ..' -' . '' ;'.' ''".'-'.'.'. '.
r::::::::::: ::
...........
'..'.'''.'.'''.'.'''''.':``
.:::::::::::::::
.. :::.. : .. -;..-.-:- .::...: .:.
...............
::. :. . :::..
:-.:- :::::
. . ..:: . . . ::
~.::::....:-:..
:. :....
...........
.............
':.:. .'. .'' .::.:. .:. ..
'.:.':. :.
~ .
~1~L7669 a~ O r~ ~D
. ~
CO ~ ~ N r-l O
~ U~
~ ;
O
In C~ ~ u~
~ :~ OP C~ Cl~ ~` In I
U ~ a~
O
u~ ~ ~ ~ e~
0 !~ ~1 a) ~ 4 U~
~ _l ~ ~ ~ ~ I~
_~ ~ o 1` u~
H ¦ O ~ Ct) I` U~ U~ ~ o ~ U~
U~--I O O O O O
~ ~ ~ O~
E~ t~ r~
~ E~ 111 ~) N --i t~
1`
, ~
O O~ ~ O O
. .
a Ln~ o _l o ~
~ r~ O U~
.~ ¢ ~ ~
~J U
~ ~ `
_~ ~ O -a) ~ ~ o ~ o o o o ~ X
h ~I-rl 0 a) ~
o ~0 ~
rl .C
h ~ O O ou~
O S~ '. . . .
:~ a) o t` r`r~
~ E~
-~ C)-~
~~1~ ~ ~rLn ~
X
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 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
Claims (7)
1. A process for the secondary treatment of wastewater by a modified air activated Sludge system comprising aerating wastewater containing bacteria, under conditions in which bacteria growth occurs, by continuously metering dif-fused air into a reactor until the BOD level of the wastewater is reduced by at least 90%, the food to microorganism rationof 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-water by dissolved air flotation clarification, recycling said suspended solids and bacteria to said aeration step in an amount sufficient to maintain said food to microorganism ratio and removing and disposing of the remaining reduced BOD level wastewater.
2. The process of Claim 1 in which the wastewater is aerated in a deep tank aeration system.
3. The process of Claim 1 in which the BOD of the waste-water is greater than 400 mg/l.
4. The process of Claim 3 in which the wastewater is sulfite pulp mill effluent.
D, E. Severeid-D. D. Jech 1-1
D, E. Severeid-D. D. Jech 1-1
5. The process of Claim 1 in which the bacteria are re-cycled to the aeration step in a time insufficient to kill a significant amount of said bacteria.
6. The process of Claim 5 in which the bacteria are re-cycled to the aeration step in less than 30 minutes after completion of aeration.
7. The process of Claim 1 in which bacteria growth in the wastewater is assured by substantially neutralizing the waste-water and adding thereto nitrogen and phosphorus nutrients.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US87532778A | 1978-02-06 | 1978-02-06 | |
| US875,327 | 1978-02-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1117669A true CA1117669A (en) | 1982-02-02 |
Family
ID=25365605
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000320880A Expired CA1117669A (en) | 1978-02-06 | 1979-02-06 | Process for the secondary treatment of wastewater |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS54113962A (en) |
| CA (1) | CA1117669A (en) |
| DE (1) | DE2903548A1 (en) |
| FI (1) | FI790165A (en) |
| FR (1) | FR2416199A1 (en) |
| GB (1) | GB2013646B (en) |
| SE (1) | SE7900866L (en) |
Families Citing this family (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3028336C2 (en) * | 1980-07-25 | 1985-08-01 | Wilhelm Roediger Gmbh + Co, 6450 Hanau | Process for the biological purification of waste water |
| US4452699A (en) * | 1981-05-07 | 1984-06-05 | Shinryo Air Conditioning Co., Ltd. | Method of improving the SVI of mixed liquor in aeration tank |
| SE443930B (en) * | 1981-05-08 | 1986-03-17 | Flaekt Ab | DEVICE FOR PURIFICATION OF POLLUTANEATED AIR FROM A SPRAYBOX WITH A BIOLOGICAL DEGRADATION |
| NL8204829A (en) * | 1982-12-14 | 1984-07-02 | Schneider & Schuurman | METHOD AND APPARATUS FOR PURIFICATION OF WASTE WATER. |
| JPS6078696A (en) * | 1983-10-04 | 1985-05-04 | Shinryo Air Conditioning Co Ltd | Svi improvement of liquid mixture in aeration tank |
| GB8607854D0 (en) * | 1986-03-27 | 1986-04-30 | Cjb Developments Ltd | Gas flotation system |
| FR2700323B1 (en) * | 1993-01-14 | 1995-03-31 | Cee Qua Bio | Method of biological water treatment and device implementing the method. |
| DE19848346C2 (en) * | 1998-10-21 | 2001-09-13 | Mueller Blanke Norbert | Flotation process for the retention of biomass-forming microorganisms suspended in water in a basin |
| WO2010058434A1 (en) * | 2008-11-24 | 2010-05-27 | Hawk Partners Sas | Vertical rector digestor for biological sludges |
| JP7384452B2 (en) * | 2018-04-20 | 2023-11-21 | アドバンスト・イノベーターズ・インコーポレイテッド | Systems and methods for treating wastewater and producing class A sludge |
| CN110468033B (en) | 2019-08-26 | 2020-12-08 | 同济大学 | Anaerobic digestion device for strengthening methane production by utilizing self-sustaining air flotation |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR1478749A (en) * | 1965-04-29 | 1967-04-28 | Purac Ab | Process for purifying water by means of a plurality of treatments applied in series |
| DE2111442A1 (en) * | 1970-06-29 | 1972-01-05 | Aquanox Inc | Effluent treatment - by high pressure aerobic decomposition |
| FR2116662A5 (en) * | 1970-12-02 | 1972-07-21 | Degremont | |
| US3864246A (en) * | 1973-01-24 | 1975-02-04 | Air Prod & Chem | Non-bulking activated sludge process |
| GB1527731A (en) * | 1975-04-07 | 1978-10-11 | Ici Ltd | Sewage treatment-flotation apparatus |
| JPS51139173A (en) * | 1975-05-27 | 1976-12-01 | Nitto Chem Ind Co Ltd | Activated sludge treatment |
-
1979
- 1979-01-18 FI FI790165A patent/FI790165A/en unknown
- 1979-01-30 GB GB7903226A patent/GB2013646B/en not_active Expired
- 1979-01-31 FR FR7902456A patent/FR2416199A1/en active Granted
- 1979-01-31 DE DE19792903548 patent/DE2903548A1/en not_active Withdrawn
- 1979-02-01 SE SE7900866A patent/SE7900866L/en not_active Application Discontinuation
- 1979-02-06 CA CA000320880A patent/CA1117669A/en not_active Expired
- 1979-02-06 JP JP1274379A patent/JPS54113962A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| FI790165A (en) | 1979-08-07 |
| GB2013646B (en) | 1982-07-21 |
| FR2416199B1 (en) | 1984-10-26 |
| JPS54113962A (en) | 1979-09-05 |
| SE7900866L (en) | 1979-08-07 |
| GB2013646A (en) | 1979-08-15 |
| DE2903548A1 (en) | 1979-08-09 |
| FR2416199A1 (en) | 1979-08-31 |
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