- Email: [email protected]
Implementation of the biological nutrient removal program at Calgary's Bonnybrook wastewater treatment plant
~
Pergamon
Wal. Sci. Tech. Vol. 38, No. I, pp. 47-S4, 1998.
IAWQ. ~
1998 Published by ElsevierScienceLtd
Printedin GreatBritain.
PH: S0273-1223(98)00389-8
0273-1223/98 $19'00 + 0'00
IMPLEMENTATION OF THE BIOLOGICAL NUTRIENT REMOVAL PROGRAM AT CALGARY'S BONNYBROOK WASTEWATER TREATMENT PLANT A. W. Wilson*, P. Do** and W. E. Keller** * Reid Crowther & Partners limited, 340 Midpark Way S.E., Calgary, Alberta, Canada, 1'2XJ P J ** City of Calgary, Bonnybrook WWTP. 4302 J 5th Street S.E.• Calgary, Alberta, Canada, 1203MB
ABSTRACT This paper describes the biological nutrient removal (BNR) program that has been implemented in stages at Calgary's Bonnybrook wastewater treatment plant (WWTP) over the past eight years. Process design parameters and performance data for two retrofit BNR projects and one new BNR expansion project are described. These projects have made the Bonnybrook WWTP the largest cold weather, suspended growth BNR plant in the world. @ 1998 IAWQ. Published by Elsevier Science Ltd. All rights reserved.
KEYWORDS Activated sludge; biological nutrient removal; filamentous bulking. INTRODUCTION Calgary is the largest city in the Province of Alberta, Canada with a 1997 population approaching 800,000. The City owns and operates two wastewater treatment plants (WWTP's), which together provide treatment for 100 percent of Calgary's wastewater prior to discharge to the Bow River. The Bow River is a world-class sport fishery, an important natural and recreational resource, as well as the water supply of several downstream communities. It is one of the many tributaries of the SaskatchewanlNelson River system that rise on the eastern slopes of the Rocky Mountains and flow generally eastward into Hudson Bay. This paper focuses on the staged implementation of a biological nutrient removal (BNR) program at the larger of Calgary's two wastewater treatment plants - the 500,000 m3/d Bonnybrook WWTP. Wastewater treatment facilities have been located on the Bonnybrook site since the early 1930's. In the interim, the plant has been expanded and upgraded several times to meet an increasing population and tighter treated effluent quality requirements. Current flows to the plant average about 380,000 m 3/d from a catchment population approaching 600,000. Wastewater temperatures, as measured in the bioreactors, vary from a high of 18°C in the late summer to 10°C in January and February. Prolonged spells of cold winter weather (ambient temperature <-30°C) can cause the wastewater temperature to dip as low as 8°C for a few 47
48
A. W. WILSON et al.
days. The City is served by a separate system of sanitary and storm sewers with the exception of relatively small parts of the older downtown sectionswheresomecombinedsewers are still in service. The current treated effluent quality requirements for the Bonnybrook plant are listed in Table 1. Figure 1 illustrates a block diagramflowchart of the mainstream and sidestream processing units. In all, there are:
• • • • • •
Six 19 mm mechanically-raked bar screens Four aerated grit chambers Fourteen primaryclarifiers Ten bioreactors distributed among threedistinctsecondary treatment modules Thirty secondaryclarifiers Three gravitythickeners for primarysludge thickening Four dissolvedair flotation tanks for wasteactivated sludge thickening Twelve mesophilic anaerobic digesters A low pressuremercuryvapourlamp ultraviolet disinfection system.
• • •
Digested sludge is pumped to the Shepard Lagoon system. located approximately 12 km southeast of the plant. At Shepard. the sludge is allowedto settle and thicken to about 10 percent solids content, after which it is hauled to agricultural lands for sub-surface injection. Decant water from the lagoon is returned continuously to the headworks of the plant.
....cnoo=Y
.....r
--... --...----Figure 1. Bonnybrook wastewater treatment plantblockprocess flowdiagram.
Table I. Effluentlimitsfor the Bonnybrook WWTP(effective 1996) Parameter 5-day Biochemical OxygenDemand Total Suspended Solids Ammonia Nitrogen(July-Sept.) AmmoniaNitrogen(Oct.-June) Total Phosphorus Fecal Colifonns Total Coliforms
Units mg/L mg/L mg/L mg/L mg/L CFU/IOOmL CFU/IOOmL
EffiuentLimit 20 20 5 10 1.0
200 1000
Averaging Period (Monthly) Arithmetic Mean Arithmetic Mean Arithmetic Mean Arithmetic Mean Arit1unetic Mean Geometric Mean Geometric Mean
CHEMICAL COSTSFOR PHOSPHORUS REMOVAL Since 1982.under directionfrom AlbertaEnvironmental Protection. the City of Calgaryhas been requiredto remove phosphorus from both treatment plants to prevent eutrophication in the Bow River. Initially, liquid
Implent ation of the biolo gical nutrient removal program
49
alum solutio n at an average dosage of 65 mglL as AI2(S04)3 14H20 was metered into the mixed liquor leaving the bioreactors to precipitate phosphorus . By the late 1980's, the an nual chemical expend iture s at Bonn ybrook were over $2 million CON, whic h represented a significant percent age of the total wastew ater treatment costs. Therefore, rap id popul ation growth requiring additional treatment capa city , together with new and more stringent treated efflu ent limits, ca used the Cit y to undertake a review of new techn olog ies for nutrient remov al that had the potential to be more economical than the co nve ntio nal acti vated sludge proc ess using costly pho sphorus-precipitating chemicals such as iron or aluminum salts. Con sequently a full -scale trial of biolog ical phosphorus remov al was initiated in 1989 by retrofitting half of the oldes t seco ndary treatment port ion of the plant (Seco ndary Plant A) to a simple anaerobic/aerob ic process configuration. Thi s repre sented about 20 percent of the plant's nom inal capa city at that time. The trial was succes sful to the extent that the cost of the retrofit mod ifications were recovered in about four months by the sav ings in liquid alum. It was then decided to retrofit the ent ire Secondary Plant A to BNR technology, this time using a Mod ified Bardenpho proces s configur ation to provide for nitrate-oxygen recovery, a degree of total nitrogen remo val, and to minimize the interference that nitrates in the return acti vated sludge (RAS) stream had on biolog ical phosphorus remov al perform ance . A major plant expansion project was co mpleted in 1994 which, among other things, included a green field 100,000 m 3/d Secondary Plant C modul e with full BNR capabilities. A retrofit BNR design for Secondary Plant B, which was initially commissioned as a complete-mix activat ed sludge plant in 1984, has also been co mpleted and has entered the construct ion stage in the summer of 1997 . Th e design feature s of the three BNR modul es are described in the chro nological order in wh ich they were imple mented (A, C and B) under the next three head ings. Plant perfor mance data are presented under the penultimate head ing and the final headin g introduces the concluding rem ark s for the paper. INITIAL BNR RETROFIT - SECONDARY PLANT A Secondary Plant A con sists of four biore actors, each with two passes, and each pass ha ving five 45 kW , fixed-mounted, 47 rpm surface mechanical aerators co mplete with draft tubes. The bioreacto rs have a side water depth of 4 .6 m and the initial installation was capable of operating in plug flow , step feed or compl ete mix modes. Operat ion in the latter mode was effected with a propell er pump located at the end of the seco nd pass which could circulate mixed liquor to the first pass at a rate of six times primary effluent flow . Each pair of biore actor s is linked with six 32.0 m squa re x 4.6 m side water depth sec ondary cla rifiers fitted with circular mech anism s. Settled sludge withdrawal is by means of 'organ pipe ' siphons to a centre well from which return activated slud ge (RAS) is directed to screw -lift pump s for return to the bioreactor s. Waste acti vated sludge is withdrawn from the RAS line. Secondary settling tank configurations such as this were not uncommon when this portion of the plant was designed circa 1970. Figur e 2 illustrates the layout of a typical bioreactor in Secondary Plant A as retrofitted for BNR.
AD. I'
AD. , •
AD. "
,
Fig ure 2. Second ary plant A - BNR retro fit layo ut.
Plll.\WlY U1UI. 1T
50
A. W. WILSON et al.
The operating modes are listed in Table 2. Secondary influent can be introduced into eithercell 1 or cell 2 of the bioreactor. The latter mode of operation is used to providefor endogenous denitrification of the RAS to protect the anaerobic zone from nitrate-oxygen. When operating in a Bardenpho mode, there is also flexibility to direct the nitrified mixed liquor return stream to either cell 2 or cell 3. A more complete description of the early performance of this BNR retrofithas been reportedby Wilson and Do (1994). Table 2. Secondary plant A bioreactor volume(typicalof 4 bioreactors) Cell # 1 2 3
4to 10
Operation Modes RAS Deniteor Anaerobic Anaerobic Anaerobic or Aerobic Aerobic
Cell Volumes 1060 m3 1060 m3 1060 m3 1060 m3 each
The cost of retrofitting all four bioreactors was approximately $1.000.000 CON. The City is currently initiating a project to replace the aging surface mechanical aerators with a fine bubble aeration system to improve oxygentransferefficiency. GREENFIELD BNR PROJECT - SECONDARY PLANTC The secondary treatment portion of the 1994expansion project includes two 1o-eell BNR bioreactors, six secondary settling tanks, and twin two-stage complete-mix/gravity thickener fermenter systems. Flexibility is provided to take various cells out of service for maintenance as well as to operate in Bardenpho, DCT. Modified DCT, and anaerobic/aerobic process modes. Fine bubble. 270 mm diameter, flexible membrane diffusers are installed in cells 5 to 10 of each bioreactor. Slow-speed 'banana-blade' mixers are installed in cells I through 6 of each bioreactor. Cells 5 and 6 can be operated in either aerated or unaerated mode. A dissolved oxygen control system is used to match airflow rates to biomass oxygen demand. Submersible mechanical mixers are installed in cells 9 and 10 of each bioreactor to maintain the mixed liquor in suspension wheneverthe air supply providesinsufficient mixing. Figure 3 illustrates in plan view the layout of a typicalbioreactorin Secondary Plant C.
_-.... ....
---'.----r r.===
r'---"'~~
"AI
10
MC a.wI'.
Figure 3. Secondary plantC • Greenfield BNRlayout,
The operating modes and nominal volumes of each cell in the bioreactor are listed in Table 3. These bioreactors have a nominal side water depth of 6 m. The BioWin"" wastewater treatment process simulator was used as a key tool for specifying overall bioreactor volume, individual cell sizes. and aeration requirements. Additional details on the design and performance of Secondary Plant C are given by Rabinowitz et al: (1997).
Implentationof the biologicalnutrientremoval program
SI
Table 3. Secondary plant C bioreactor volume (typical of 2 bioreactors) Cell Volumes 1390 m J 1390mJ each 1390mJ each 3165 mJ each
Operation Modes Anaerobic Anaerobic or Anoxic Anaerobic, Anoxic or Aerobic Aerobic
Cell # 1
2 to 4 5&6 7 to 10
Waste activated sludge (WAS) is withdrawn from the mixed liquor channel between the bioreactor and the secondary settling tanks. Surplus sludge is wasted 'selectively' from the surface of the channel in order to eliminate foam-causing organisms from the system. Each bioreactor is associated with three 32.0 m diameter x 6.0 m side water depth secondary clarifiers fitted with a flocculating well, twin-arm hydraulic sludge withdrawal tubes, and a full-width scum skimmer. The total cost of the 1994 expansion project was $70 million CDN, which included two new primary clarifiers, the two BNR bioreactors and six secondary clarifiers, the twin fermenter system, the UV disinfection system for the entire plant and a new computerized data acquisition and control system. The cost of the bioreactors, secondary clarifiers and fermenter system was approximately $55 million CON. BNR RETROFIT· SECONDARY PLANT B Secondary Plant B, consisting of four complete-mix bioreactors each 44.0 m wide x 66.0 m long x 6.0 m side water depth. was commissioned in 1984. It has a jet aeration system consisting of twelve jet nozzle clusters in each bioreactor. The bioreactors and aeration system were designed for biochemical oxygen demand (BOD) removal only. Tests on the jet aeration system using higher than design airflow rates to satisfy the additional aeration demand required for nitrification have shown a sharp decrease in aeration efficiency with increasing airflow. Given the favourable performance experienced by the fine bubble. flexible membrane aeration system recently installed in Secondary Plant C, it was decided to replace the jet aeration system in Plant B with a similar fine bubble aeration system. Concurrent with this upgrade, it was also decided to install partitions and unaerated zones in the Secondary Plant B bioreactors to implement biological phosphorus removal as well as to save on liquid alum chemical costs. Each bioreactor is served by three 39.0 m diameter x 4.0 m side water depth circular clarifiers. Senled sludge in these clarifiers is withdrawn by 'organ pipe' siphons similar to Secondary Plant A. Figure 4 illustrates in plan view the layout of a typical bioreactor in Plant B that is currently being retrofitted.
-......... ..... ....
--- ,.. -,
r-
~
........
......
l-
f-.
ll-
f-. f-.
--
II.=.:. -
L..
......
r-'
r-' I-<
....-_.. ......... """,",10
-.u_
Q.AOIJIIM
Figure 4. Secondaryplant 8 • 8NR retrofitlayouL
The operating modes and nominal volumes of each bioreactor cell in Plant B are listed in Table 4. Cell 1 is intended for RAS denitrification; cell 2 is the anaerobic zone; cell 3 is either aerobic or anoxic (discussed in
S2
A. W. WILSON et al,
more detail below), and cell 4 is the aerobic lone. As with Secondary Plant C, the BioWin Tlol wastewater treatment process simulator was a key tool used in the design of the BNR retrofit in Secondary Plant B. Table 4. Secondary plant B bioreactor volume (typical of 4 bioreactors) Cell # 1 2 3 4
Operation Modes RAS Denitrification Anaerobic Anoxic!Aerobic Aerobic
Cell Volumes 878m' 3 2633 m 3 878m 13167mJ
Because they incorporate large unaerated zones, BNR plants are susceptible to biological foam and scum accumulation in the bioreactors. Often, such accumulations lead to sludge bulking problems. BNR plants in Johannesburg, South Africa, have experienced such problems for almost twenty years (Pitman, 1996). After more than six years of BNR operation at Bonnybrook, an excessive foam and scum incident occurred in November 1996 which subsequently led to mixed liquor bulking in all BNR bioreactors. Mixed liquor sludge volume index (SVI) values increased from the normal range of 80 to 120 mUg to 350 to 450 mUg. Subsequent microscopic examination identified Microthrix parvicella as the principal causative organism. To bring bulking under control, chlorine was continuously added to the RAS streams at a dosage of 6 kg CI2/l 000 kg MLVSS·d for over a period of four weeks. Recent technical literature (Casey et al., 1993, 1995) (Still et al., 1996) (Ekama et al., 1996a) (Gabb et al., 1996a, 1996b) (Ekama et al., 1996b) suggests that the denitrification intermediate nitric oxide (NO) is toxic to the floc-forming organisms . As an innovative means of controlling the growth of filamentous organisms, the BNR retrofit design of Secondary Plant B incorporates a relatively small volume 'swing zone' that is inserted between the anaerobic zone and the aerobic zone (Barnard, 1997). This 'swing zone' can be operated in either an aerobic or an anoxic mode. In the aerobic mode, the 'swing zone' is operated at a high dissolved oxygen setpoint to quickly suppress any tendency of the mixed liquor to denitrify. The intent is to minimize the risk of NO toxicity to the floc-formers. On the other hand, if filamentous organisms are proliferating because of a soluble COD breakthrough from the anaerobic zone, then the 'swing zone' can be operated in an anoxic mode to minimize the risk of low F:M bulking. The construction cost of the BNR retrofit in Secondary Plant B is $4.0 million CON. BNR PLANT PERFORMANCE Figures 5, 6 and 7 illustrate the cBOD s, NH 3-N and Total P performance respectively of Secondary Plant C. It is seen that the plant has been operating within Alberta Environmental Protection limits since the commissioning period ended in early 1995. --, 21O~
i
1 ~
1'0 1.0 HD
IIeli ::: II
!
..
2O+---------------------------------t _ _"_ D~."".~
_..~~~~=_"=$:'''*''........_ _$'=:$=*=~II=III=t~_..
Figure S. Secondary plant C cBOD.5 removal 1995-1997.
Implentation of the biologicalnutrientremovalprogram
53
30..-
~
i
-,
2S
~ ~ 20
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15
~ ~ ~ 8 10
c ~
i
+----
5
Figure 6. SecondaryplantC - NH3-Nremoval 1995-1997. ' .0
i
j~
!~ i~ ~
e
5 .0 4.0
3.0 2.0 1.0 0.0
j
i
~
!
I
J
~
___p,l", 8''1 Itiiu.nt
i
~'I".1
~ liII"l"t
Per. Ii liN it
I
Figure7. Secondary plantC total P removal 1995-1997.
Table 5 compares the treated effluent quality of the three secondary plants with raw and primary effluent quality at Bonnybrook. Plant B in Table 5 is the complete mix activated sludge process with chemical phosphorus removal. It is noted that the two BNR secondary plants consistently produce effluent having betterquality than the chemical-feed activated sludgeplant while requiring littleor no chemicals. Table5. Comparison of raw sewage, primary effluent, and treatedeffluents fromBonnybrook's BNR and activated sludge processes cBOD, (mgIL) Average Range 137-182 162 Raw Sewage 72-144 PrimaryEffluent 118 4-6 5 SecondaryPlant A (8NR) 4-8 SecondaryPlant 8 (ACL 51.) 6 4 2-6 Secondary PlantC (8NR) Item
TSS(mglL) Average Range 162 151-179 81 62·92 10 6-14 12 8-16 7 4-10
Total P (mgIL) Average Range 5.4 4.6-6.6 5.0 4.2-6.2 0.7 0.4-1.0 0.7 0.4-1.0 0.4 0.2-0.6
Alum Dosage Average Range
10 65 5
0-20 60-70 0-10
Figure 8 illustrates the dramatic chemical savings achieved for the Bonnybrook plant as more and more BNRbioreactors have beencommissioned over the years. Fromthe first implementation of BNR technology at the plant in late 1989, the population in the Bonnybrook catchment area has increased by over ten percent. By 1996, chemical savings amounted to over $2 million CDNannually.
54
A. W. WILSON et al.
---
.. U .MI ....
,.......
-
....
n ,, ",tot
_ ., t ... ..
<....
.... c.... ,oea
-.-
e §. . ...
....... 47 . ....
• .-
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1 j
I
• M .... .
tI"
.u,
"If
"..
'" '
"
lin
t ttl
"It
1I1t
"II
.
Figure 8. Bonnybrook W.W.T .P. chemical cost savings due to BNR irnplentation .
CONCLUDING REMARKS The Bonnybrook WWTP is the largest BNR plant in Canada and the largest cold weather BNR plant in the world. It has consistently achieved permit compliance with substantial cost savings in phosphorusprecipitating chemicals. Because of the Bonnybrook plant. the City of Calgary is the only Canad ian city to be awarded a grading of A by the Sierra Club for its proactive efforts 10 protect water quality. The City has also been recognized by Chatelaine Magazine with a similar award . REFERENCES Barnard, J. L. (1997). Personal Communication. Casey, T. G., Ekama , G. A., Wentzel , M. C. and Marais , G. v. R (1993). Causes and Control of Low FIM Filamentous Bulking in Nutrient Removal Activated Sludge Systems . Water Sewage and Effluent, 13(4), 10-26. Casey, T. G., Ekama , G. A., Wentzel , M. C. and Marais , G. v, R (1995 ). Filamentous Organ ism Bulking in Nutrient Removal Activated Sludge Systems . Paper 1: A Historical Overview of Causes and Control. Water SA, 21(3), 231-238 . Ekama , G. A., Wentzel, M. C, Casey, T. G. and Marai s, G. v. R. (1996a). Filamentous Organ ism Bulking in Nutrient Removal Activated Sludge Systems . Paper 3: Stimulation of the Selector Effect Under Anoxic Conditi ons. Water SA . 22(2), 119126. Ekama, G. A., Wentzel, M. C.• Case y, T. G. and Marais, G. v. R. ( 1996b). Filamentous Organ ism Bulking in Nutrient Removal Activated Sludge System s. Paper 6: Review , Evaluation and Consol idation of Results . Water SA. 22(2). 147-152 . Gabb, D. M. 0 .. Ekarna, G. A., Jenk ins. D., Wentzel , M. C., Casey , T. G. and Marais, G. v. R. (1996a) . Filamentous Organism Bulking in Nutrient Removal Activated Sludge Systems. Paper 4: System Configur ation and Operating Conditions to Develop Low F/M Filament Bulking Sludges at Laboratory-Scale. Water SA, 22(2), 127-138. Gabb, D. M. D., Ekama, G. A.. Jenk ins, 0 .. Wentzel. M. C; Casey, T. G. and Marais, G. v. R. (1996b). Filament ous Organism Bulking in Nutrient Removal Activated Sludge Systems . Paper 5: Experimental Examination of Aerobi c Selectors in Anoxic-Ae robic Systems. Water SA, 22(2), 139-146. Pitman, A. R. (1996 ). Bulking and Foaming in BNR Plants in Johanne sburg : Problems and Soluti ons. Wat. Sci. Tech., 34(3-4), 291-298 . Rabinowitz, B., Fries, M. K., Dawson, R. N., Keller, W. E. and Do, P. (1997). Biolog ical Nutrient Removal at the Calgary Bonnybrook WWTP Replacing Costly Chemical Phosphorus Removal. Proc. 70th Annual Conference and Exposition of the Water Environment Federation. Chicago , Illinois. October 18-22. Still, D. A.. Ekama , G. A., Wentzel . M. C., Casey , T. G. and Marais , G. v. R. (1996) . Filamentous Organism Bulking in Nutrient Removal Activated Sludge Systems . Paper 2: Stimulation of the Selector Effect Under Aerob ic Cond itions. Water SA, 22(2) . 97-118. Wilson, A. W. and Do, P. (1994). Case History : Retrofitting of the Bonnybrook Wastewater Treatment Plant with Biological Nutrient Removal. Proc. 23rd Annual Con! of the Water Environment Association of Ontario, Windsor, Ontario, April 14-19.
Pergamon
Wal. Sci. Tech. Vol. 38, No. I, pp. 47-S4, 1998.
IAWQ. ~
1998 Published by ElsevierScienceLtd
Printedin GreatBritain.
PH: S0273-1223(98)00389-8
0273-1223/98 $19'00 + 0'00
IMPLEMENTATION OF THE BIOLOGICAL NUTRIENT REMOVAL PROGRAM AT CALGARY'S BONNYBROOK WASTEWATER TREATMENT PLANT A. W. Wilson*, P. Do** and W. E. Keller** * Reid Crowther & Partners limited, 340 Midpark Way S.E., Calgary, Alberta, Canada, 1'2XJ P J ** City of Calgary, Bonnybrook WWTP. 4302 J 5th Street S.E.• Calgary, Alberta, Canada, 1203MB
ABSTRACT This paper describes the biological nutrient removal (BNR) program that has been implemented in stages at Calgary's Bonnybrook wastewater treatment plant (WWTP) over the past eight years. Process design parameters and performance data for two retrofit BNR projects and one new BNR expansion project are described. These projects have made the Bonnybrook WWTP the largest cold weather, suspended growth BNR plant in the world. @ 1998 IAWQ. Published by Elsevier Science Ltd. All rights reserved.
KEYWORDS Activated sludge; biological nutrient removal; filamentous bulking. INTRODUCTION Calgary is the largest city in the Province of Alberta, Canada with a 1997 population approaching 800,000. The City owns and operates two wastewater treatment plants (WWTP's), which together provide treatment for 100 percent of Calgary's wastewater prior to discharge to the Bow River. The Bow River is a world-class sport fishery, an important natural and recreational resource, as well as the water supply of several downstream communities. It is one of the many tributaries of the SaskatchewanlNelson River system that rise on the eastern slopes of the Rocky Mountains and flow generally eastward into Hudson Bay. This paper focuses on the staged implementation of a biological nutrient removal (BNR) program at the larger of Calgary's two wastewater treatment plants - the 500,000 m3/d Bonnybrook WWTP. Wastewater treatment facilities have been located on the Bonnybrook site since the early 1930's. In the interim, the plant has been expanded and upgraded several times to meet an increasing population and tighter treated effluent quality requirements. Current flows to the plant average about 380,000 m 3/d from a catchment population approaching 600,000. Wastewater temperatures, as measured in the bioreactors, vary from a high of 18°C in the late summer to 10°C in January and February. Prolonged spells of cold winter weather (ambient temperature <-30°C) can cause the wastewater temperature to dip as low as 8°C for a few 47
48
A. W. WILSON et al.
days. The City is served by a separate system of sanitary and storm sewers with the exception of relatively small parts of the older downtown sectionswheresomecombinedsewers are still in service. The current treated effluent quality requirements for the Bonnybrook plant are listed in Table 1. Figure 1 illustrates a block diagramflowchart of the mainstream and sidestream processing units. In all, there are:
• • • • • •
Six 19 mm mechanically-raked bar screens Four aerated grit chambers Fourteen primaryclarifiers Ten bioreactors distributed among threedistinctsecondary treatment modules Thirty secondaryclarifiers Three gravitythickeners for primarysludge thickening Four dissolvedair flotation tanks for wasteactivated sludge thickening Twelve mesophilic anaerobic digesters A low pressuremercuryvapourlamp ultraviolet disinfection system.
• • •
Digested sludge is pumped to the Shepard Lagoon system. located approximately 12 km southeast of the plant. At Shepard. the sludge is allowedto settle and thicken to about 10 percent solids content, after which it is hauled to agricultural lands for sub-surface injection. Decant water from the lagoon is returned continuously to the headworks of the plant.
....cnoo=Y
.....r
--... --...----Figure 1. Bonnybrook wastewater treatment plantblockprocess flowdiagram.
Table I. Effluentlimitsfor the Bonnybrook WWTP(effective 1996) Parameter 5-day Biochemical OxygenDemand Total Suspended Solids Ammonia Nitrogen(July-Sept.) AmmoniaNitrogen(Oct.-June) Total Phosphorus Fecal Colifonns Total Coliforms
Units mg/L mg/L mg/L mg/L mg/L CFU/IOOmL CFU/IOOmL
EffiuentLimit 20 20 5 10 1.0
200 1000
Averaging Period (Monthly) Arithmetic Mean Arithmetic Mean Arithmetic Mean Arithmetic Mean Arit1unetic Mean Geometric Mean Geometric Mean
CHEMICAL COSTSFOR PHOSPHORUS REMOVAL Since 1982.under directionfrom AlbertaEnvironmental Protection. the City of Calgaryhas been requiredto remove phosphorus from both treatment plants to prevent eutrophication in the Bow River. Initially, liquid
Implent ation of the biolo gical nutrient removal program
49
alum solutio n at an average dosage of 65 mglL as AI2(S04)3 14H20 was metered into the mixed liquor leaving the bioreactors to precipitate phosphorus . By the late 1980's, the an nual chemical expend iture s at Bonn ybrook were over $2 million CON, whic h represented a significant percent age of the total wastew ater treatment costs. Therefore, rap id popul ation growth requiring additional treatment capa city , together with new and more stringent treated efflu ent limits, ca used the Cit y to undertake a review of new techn olog ies for nutrient remov al that had the potential to be more economical than the co nve ntio nal acti vated sludge proc ess using costly pho sphorus-precipitating chemicals such as iron or aluminum salts. Con sequently a full -scale trial of biolog ical phosphorus remov al was initiated in 1989 by retrofitting half of the oldes t seco ndary treatment port ion of the plant (Seco ndary Plant A) to a simple anaerobic/aerob ic process configuration. Thi s repre sented about 20 percent of the plant's nom inal capa city at that time. The trial was succes sful to the extent that the cost of the retrofit mod ifications were recovered in about four months by the sav ings in liquid alum. It was then decided to retrofit the ent ire Secondary Plant A to BNR technology, this time using a Mod ified Bardenpho proces s configur ation to provide for nitrate-oxygen recovery, a degree of total nitrogen remo val, and to minimize the interference that nitrates in the return acti vated sludge (RAS) stream had on biolog ical phosphorus remov al perform ance . A major plant expansion project was co mpleted in 1994 which, among other things, included a green field 100,000 m 3/d Secondary Plant C modul e with full BNR capabilities. A retrofit BNR design for Secondary Plant B, which was initially commissioned as a complete-mix activat ed sludge plant in 1984, has also been co mpleted and has entered the construct ion stage in the summer of 1997 . Th e design feature s of the three BNR modul es are described in the chro nological order in wh ich they were imple mented (A, C and B) under the next three head ings. Plant perfor mance data are presented under the penultimate head ing and the final headin g introduces the concluding rem ark s for the paper. INITIAL BNR RETROFIT - SECONDARY PLANT A Secondary Plant A con sists of four biore actors, each with two passes, and each pass ha ving five 45 kW , fixed-mounted, 47 rpm surface mechanical aerators co mplete with draft tubes. The bioreacto rs have a side water depth of 4 .6 m and the initial installation was capable of operating in plug flow , step feed or compl ete mix modes. Operat ion in the latter mode was effected with a propell er pump located at the end of the seco nd pass which could circulate mixed liquor to the first pass at a rate of six times primary effluent flow . Each pair of biore actor s is linked with six 32.0 m squa re x 4.6 m side water depth sec ondary cla rifiers fitted with circular mech anism s. Settled sludge withdrawal is by means of 'organ pipe ' siphons to a centre well from which return activated slud ge (RAS) is directed to screw -lift pump s for return to the bioreactor s. Waste acti vated sludge is withdrawn from the RAS line. Secondary settling tank configurations such as this were not uncommon when this portion of the plant was designed circa 1970. Figur e 2 illustrates the layout of a typical bioreactor in Secondary Plant A as retrofitted for BNR.
AD. I'
AD. , •
AD. "
,
Fig ure 2. Second ary plant A - BNR retro fit layo ut.
Plll.\WlY U1UI. 1T
50
A. W. WILSON et al.
The operating modes are listed in Table 2. Secondary influent can be introduced into eithercell 1 or cell 2 of the bioreactor. The latter mode of operation is used to providefor endogenous denitrification of the RAS to protect the anaerobic zone from nitrate-oxygen. When operating in a Bardenpho mode, there is also flexibility to direct the nitrified mixed liquor return stream to either cell 2 or cell 3. A more complete description of the early performance of this BNR retrofithas been reportedby Wilson and Do (1994). Table 2. Secondary plant A bioreactor volume(typicalof 4 bioreactors) Cell # 1 2 3
4to 10
Operation Modes RAS Deniteor Anaerobic Anaerobic Anaerobic or Aerobic Aerobic
Cell Volumes 1060 m3 1060 m3 1060 m3 1060 m3 each
The cost of retrofitting all four bioreactors was approximately $1.000.000 CON. The City is currently initiating a project to replace the aging surface mechanical aerators with a fine bubble aeration system to improve oxygentransferefficiency. GREENFIELD BNR PROJECT - SECONDARY PLANTC The secondary treatment portion of the 1994expansion project includes two 1o-eell BNR bioreactors, six secondary settling tanks, and twin two-stage complete-mix/gravity thickener fermenter systems. Flexibility is provided to take various cells out of service for maintenance as well as to operate in Bardenpho, DCT. Modified DCT, and anaerobic/aerobic process modes. Fine bubble. 270 mm diameter, flexible membrane diffusers are installed in cells 5 to 10 of each bioreactor. Slow-speed 'banana-blade' mixers are installed in cells I through 6 of each bioreactor. Cells 5 and 6 can be operated in either aerated or unaerated mode. A dissolved oxygen control system is used to match airflow rates to biomass oxygen demand. Submersible mechanical mixers are installed in cells 9 and 10 of each bioreactor to maintain the mixed liquor in suspension wheneverthe air supply providesinsufficient mixing. Figure 3 illustrates in plan view the layout of a typicalbioreactorin Secondary Plant C.
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Figure 3. Secondary plantC • Greenfield BNRlayout,
The operating modes and nominal volumes of each cell in the bioreactor are listed in Table 3. These bioreactors have a nominal side water depth of 6 m. The BioWin"" wastewater treatment process simulator was used as a key tool for specifying overall bioreactor volume, individual cell sizes. and aeration requirements. Additional details on the design and performance of Secondary Plant C are given by Rabinowitz et al: (1997).
Implentationof the biologicalnutrientremoval program
SI
Table 3. Secondary plant C bioreactor volume (typical of 2 bioreactors) Cell Volumes 1390 m J 1390mJ each 1390mJ each 3165 mJ each
Operation Modes Anaerobic Anaerobic or Anoxic Anaerobic, Anoxic or Aerobic Aerobic
Cell # 1
2 to 4 5&6 7 to 10
Waste activated sludge (WAS) is withdrawn from the mixed liquor channel between the bioreactor and the secondary settling tanks. Surplus sludge is wasted 'selectively' from the surface of the channel in order to eliminate foam-causing organisms from the system. Each bioreactor is associated with three 32.0 m diameter x 6.0 m side water depth secondary clarifiers fitted with a flocculating well, twin-arm hydraulic sludge withdrawal tubes, and a full-width scum skimmer. The total cost of the 1994 expansion project was $70 million CDN, which included two new primary clarifiers, the two BNR bioreactors and six secondary clarifiers, the twin fermenter system, the UV disinfection system for the entire plant and a new computerized data acquisition and control system. The cost of the bioreactors, secondary clarifiers and fermenter system was approximately $55 million CON. BNR RETROFIT· SECONDARY PLANT B Secondary Plant B, consisting of four complete-mix bioreactors each 44.0 m wide x 66.0 m long x 6.0 m side water depth. was commissioned in 1984. It has a jet aeration system consisting of twelve jet nozzle clusters in each bioreactor. The bioreactors and aeration system were designed for biochemical oxygen demand (BOD) removal only. Tests on the jet aeration system using higher than design airflow rates to satisfy the additional aeration demand required for nitrification have shown a sharp decrease in aeration efficiency with increasing airflow. Given the favourable performance experienced by the fine bubble. flexible membrane aeration system recently installed in Secondary Plant C, it was decided to replace the jet aeration system in Plant B with a similar fine bubble aeration system. Concurrent with this upgrade, it was also decided to install partitions and unaerated zones in the Secondary Plant B bioreactors to implement biological phosphorus removal as well as to save on liquid alum chemical costs. Each bioreactor is served by three 39.0 m diameter x 4.0 m side water depth circular clarifiers. Senled sludge in these clarifiers is withdrawn by 'organ pipe' siphons similar to Secondary Plant A. Figure 4 illustrates in plan view the layout of a typical bioreactor in Plant B that is currently being retrofitted.
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Figure 4. Secondaryplant 8 • 8NR retrofitlayouL
The operating modes and nominal volumes of each bioreactor cell in Plant B are listed in Table 4. Cell 1 is intended for RAS denitrification; cell 2 is the anaerobic zone; cell 3 is either aerobic or anoxic (discussed in
S2
A. W. WILSON et al,
more detail below), and cell 4 is the aerobic lone. As with Secondary Plant C, the BioWin Tlol wastewater treatment process simulator was a key tool used in the design of the BNR retrofit in Secondary Plant B. Table 4. Secondary plant B bioreactor volume (typical of 4 bioreactors) Cell # 1 2 3 4
Operation Modes RAS Denitrification Anaerobic Anoxic!Aerobic Aerobic
Cell Volumes 878m' 3 2633 m 3 878m 13167mJ
Because they incorporate large unaerated zones, BNR plants are susceptible to biological foam and scum accumulation in the bioreactors. Often, such accumulations lead to sludge bulking problems. BNR plants in Johannesburg, South Africa, have experienced such problems for almost twenty years (Pitman, 1996). After more than six years of BNR operation at Bonnybrook, an excessive foam and scum incident occurred in November 1996 which subsequently led to mixed liquor bulking in all BNR bioreactors. Mixed liquor sludge volume index (SVI) values increased from the normal range of 80 to 120 mUg to 350 to 450 mUg. Subsequent microscopic examination identified Microthrix parvicella as the principal causative organism. To bring bulking under control, chlorine was continuously added to the RAS streams at a dosage of 6 kg CI2/l 000 kg MLVSS·d for over a period of four weeks. Recent technical literature (Casey et al., 1993, 1995) (Still et al., 1996) (Ekama et al., 1996a) (Gabb et al., 1996a, 1996b) (Ekama et al., 1996b) suggests that the denitrification intermediate nitric oxide (NO) is toxic to the floc-forming organisms . As an innovative means of controlling the growth of filamentous organisms, the BNR retrofit design of Secondary Plant B incorporates a relatively small volume 'swing zone' that is inserted between the anaerobic zone and the aerobic zone (Barnard, 1997). This 'swing zone' can be operated in either an aerobic or an anoxic mode. In the aerobic mode, the 'swing zone' is operated at a high dissolved oxygen setpoint to quickly suppress any tendency of the mixed liquor to denitrify. The intent is to minimize the risk of NO toxicity to the floc-formers. On the other hand, if filamentous organisms are proliferating because of a soluble COD breakthrough from the anaerobic zone, then the 'swing zone' can be operated in an anoxic mode to minimize the risk of low F:M bulking. The construction cost of the BNR retrofit in Secondary Plant B is $4.0 million CON. BNR PLANT PERFORMANCE Figures 5, 6 and 7 illustrate the cBOD s, NH 3-N and Total P performance respectively of Secondary Plant C. It is seen that the plant has been operating within Alberta Environmental Protection limits since the commissioning period ended in early 1995. --, 21O~
i
1 ~
1'0 1.0 HD
IIeli ::: II
!
..
2O+---------------------------------t _ _"_ D~."".~
_..~~~~=_"=$:'''*''........_ _$'=:$=*=~II=III=t~_..
Figure S. Secondary plant C cBOD.5 removal 1995-1997.
Implentation of the biologicalnutrientremovalprogram
53
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Figure 6. SecondaryplantC - NH3-Nremoval 1995-1997. ' .0
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e
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Figure7. Secondary plantC total P removal 1995-1997.
Table 5 compares the treated effluent quality of the three secondary plants with raw and primary effluent quality at Bonnybrook. Plant B in Table 5 is the complete mix activated sludge process with chemical phosphorus removal. It is noted that the two BNR secondary plants consistently produce effluent having betterquality than the chemical-feed activated sludgeplant while requiring littleor no chemicals. Table5. Comparison of raw sewage, primary effluent, and treatedeffluents fromBonnybrook's BNR and activated sludge processes cBOD, (mgIL) Average Range 137-182 162 Raw Sewage 72-144 PrimaryEffluent 118 4-6 5 SecondaryPlant A (8NR) 4-8 SecondaryPlant 8 (ACL 51.) 6 4 2-6 Secondary PlantC (8NR) Item
TSS(mglL) Average Range 162 151-179 81 62·92 10 6-14 12 8-16 7 4-10
Total P (mgIL) Average Range 5.4 4.6-6.6 5.0 4.2-6.2 0.7 0.4-1.0 0.7 0.4-1.0 0.4 0.2-0.6
Alum Dosage Average Range
10 65 5
0-20 60-70 0-10
Figure 8 illustrates the dramatic chemical savings achieved for the Bonnybrook plant as more and more BNRbioreactors have beencommissioned over the years. Fromthe first implementation of BNR technology at the plant in late 1989, the population in the Bonnybrook catchment area has increased by over ten percent. By 1996, chemical savings amounted to over $2 million CDNannually.
54
A. W. WILSON et al.
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Figure 8. Bonnybrook W.W.T .P. chemical cost savings due to BNR irnplentation .
CONCLUDING REMARKS The Bonnybrook WWTP is the largest BNR plant in Canada and the largest cold weather BNR plant in the world. It has consistently achieved permit compliance with substantial cost savings in phosphorusprecipitating chemicals. Because of the Bonnybrook plant. the City of Calgary is the only Canad ian city to be awarded a grading of A by the Sierra Club for its proactive efforts 10 protect water quality. The City has also been recognized by Chatelaine Magazine with a similar award . REFERENCES Barnard, J. L. (1997). Personal Communication. Casey, T. G., Ekama , G. A., Wentzel , M. C. and Marais , G. v. R (1993). Causes and Control of Low FIM Filamentous Bulking in Nutrient Removal Activated Sludge Systems . Water Sewage and Effluent, 13(4), 10-26. Casey, T. G., Ekama , G. A., Wentzel , M. C. and Marais , G. v, R (1995 ). Filamentous Organ ism Bulking in Nutrient Removal Activated Sludge Systems . Paper 1: A Historical Overview of Causes and Control. Water SA, 21(3), 231-238 . Ekama , G. A., Wentzel, M. C, Casey, T. G. and Marai s, G. v. R. (1996a). Filamentous Organ ism Bulking in Nutrient Removal Activated Sludge Systems . Paper 3: Stimulation of the Selector Effect Under Anoxic Conditi ons. Water SA . 22(2), 119126. Ekama, G. A., Wentzel, M. C.• Case y, T. G. and Marais, G. v. R. ( 1996b). Filamentous Organ ism Bulking in Nutrient Removal Activated Sludge System s. Paper 6: Review , Evaluation and Consol idation of Results . Water SA. 22(2). 147-152 . Gabb, D. M. 0 .. Ekarna, G. A., Jenk ins. D., Wentzel , M. C., Casey , T. G. and Marais, G. v. R. (1996a) . Filamentous Organism Bulking in Nutrient Removal Activated Sludge Systems. Paper 4: System Configur ation and Operating Conditions to Develop Low F/M Filament Bulking Sludges at Laboratory-Scale. Water SA, 22(2), 127-138. Gabb, D. M. D., Ekama, G. A.. Jenk ins, 0 .. Wentzel. M. C; Casey, T. G. and Marais, G. v. R. (1996b). Filament ous Organism Bulking in Nutrient Removal Activated Sludge Systems . Paper 5: Experimental Examination of Aerobi c Selectors in Anoxic-Ae robic Systems. Water SA, 22(2), 139-146. Pitman, A. R. (1996 ). Bulking and Foaming in BNR Plants in Johanne sburg : Problems and Soluti ons. Wat. Sci. Tech., 34(3-4), 291-298 . Rabinowitz, B., Fries, M. K., Dawson, R. N., Keller, W. E. and Do, P. (1997). Biolog ical Nutrient Removal at the Calgary Bonnybrook WWTP Replacing Costly Chemical Phosphorus Removal. Proc. 70th Annual Conference and Exposition of the Water Environment Federation. Chicago , Illinois. October 18-22. Still, D. A.. Ekama , G. A., Wentzel . M. C., Casey , T. G. and Marais , G. v. R. (1996) . Filamentous Organism Bulking in Nutrient Removal Activated Sludge Systems . Paper 2: Stimulation of the Selector Effect Under Aerob ic Cond itions. Water SA, 22(2) . 97-118. Wilson, A. W. and Do, P. (1994). Case History : Retrofitting of the Bonnybrook Wastewater Treatment Plant with Biological Nutrient Removal. Proc. 23rd Annual Con! of the Water Environment Association of Ontario, Windsor, Ontario, April 14-19.