Combining Mesophilic Anaerobic Digestion with Post-Aerobic Digestion to Enhance Volatile Solids Reduction and Reduce Sidestream Ammonia Adrienne Menniti1*, Bruce Johnson1, Bill Leaf1, Nitin Kumar1, Dave Oerke1, Tracy Crane2, Clint Dolsby2 1
CH2M HILL, 2City of Meridian, Idaho *Corresponding author:
[email protected] CH2M HILL, 2020 SW 4th Avenue, Portland, Oregon, USA, 97201 ABSTACT Anaerobic-aerobic phased digestion, or post-aerobic digestion, is a promising advanced digestion process applying the sequential operation of anaerobic digestion followed by aerobic digestion. Post-aerobic digestion provides total nitrogen removal without the need for chemical addition in addition to enhanced volatile solids reduction. Since nitrification occurs in the post aerobic digester, digester biomass may also represent an external source of nitrifiers for bioaugmentation. The City of Meridian, ID pilot tested bioaugmentation using post-aerobic digestion. Two 106-L activated sludge reactors with settling for biomass retention and one 16.5 L post-aerobic digester were constructed at the Meridian wastewater treatment plant. The activated sludge reactors were operated at a solids retention time (SRT) resulting in unstable nitrification. One reactor was bioaugmented with post-aerobic digester biomass. Bioaugmentation resulted in stable nitrification for ten days of reactor operation. However, the slow recovery of stable nitrification after an upset of the bioaugmented reactor suggests further optimization of the system is required to ensure stable bioaugmentation is maintained. No detrimental effects due to recycling of digested biosolids were observed in the bioaugmented reactor and the results of the current study suggest that further investigation of this new wastewater treatment concept is warranted. The performance of post-aerobic digestion was also demonstrated here. The post-aerobic digester achieved 80% ammonia removal and complete denitrification without the need for chemical addition. Thirty percent volatile solids reduction was achieved at a digester SRT of 9 days. The observed volatile solids reduction is twice as high as previous studies have reported at the same SRT and represents a significant decrease in the mass of biosolids for disposal if implemented full scale. KEYWORDS Anaerobic-aerobic phased digestion, post-aerobic digestion, sidestream nitrogen removal, bioaugmentation INTRODUCTION Advanced digestion processes have gained recent interest because they provide improved volatile solids reduction and reduced biosolids for disposal. Furthermore, increasingly stringent effluent nutrient limits have focused industry attention on the management of ammonia-rich recycle streams typically associated with anaerobic digesters. Post-aerobic digestion is a promising advanced digestion process where anaerobic digestion and aerobic digestion are operated sequentially. Operation of the post aerobic digester using either cyclic aeration or simultaneous nitritation/denitritation at low dissolved oxygen provides total nitrogen removal
without the need for external carbon source addition or alkalinity addition. Greater than 90 percent ammonia removal has been observed without the need for chemical addition (Novak et al., 2009; Parravicini et al., 2008) and near complete total nitrogen removal can be achieved if nitritation/denitritation at low dissolved oxygen concentrations can be optimized (Parravicini et al., 2008). Since nitrification occurs in the post aerobic digester, this biomass may also represent an external source of nitrifiers if a fraction of the digested sludge is returned to the aeration basin. Figure 1 shows a process flow diagram of bioaugmentation with post-aerobic digester biomass.
Figure 1. Process flow diagram of bioaugmentation using post-aerobic digestion.
In anticipation of possible future effluent limits for nitrogen along with the removal of nitrogen required for water reuse applications, the City of Meridian, Idaho, evaluated bioaugmentation alternatives to allow the wastewater treatment plant (WWTP) to maintain nitrification at its current 5-day solids retention time during the coldest winter conditions. Bioaugmentation using post-aerobic digester biomass was chosen for further evaluation because it can provide the additional benefits of total nitrogen removal without chemical addition and reduced biosolids for disposal or land application. The City of Meridian and CH2M HILL pilot tested bioaugmentation using post-aerobic digester biomass to the determine the feasibility of the process. The pilot testing was also performed to verify the overall performance for enhanced biosolids reductions and digester total nitrogen removal. The results of this pilot testing effort are presented here. MATERIALS AND METHODS Experimental Overview. Two 106-L activated sludge reactors (designated the test reactor and the control reactor) with settling for biomass retention and a 16.5-L post-aerobic digester were constructed at the Meridian WWTP. A picture of the pilot system is shown in Figure 2. The design parameters of the pilot system are shown in Figure 3. The activated sludge reactor flow rates and volumes were chosen such that the hydraulic retention times of the anaerobic, anoxic and aerobic zones were the same as the full scale Meridian WWTP at the current average annual
flow (7.0 million gallons per day). Both the test and control reactors were operated identically for 22 days. On day 22 (December 14, 2009), bioaugmentation of the test reactor with postaerobic digester biomass began. This bioaugmentation was accomplished by manually transferring 500 mL from the digester to the anoxic zone of the test reactor once per day. This bioaugmentation rate was chosen to limit the estimated aeration basin mixed liquor suspended solids increase to 25 percent. The target solids retention time in the activated sludge reactors was 4 days because nitrification was expected to be unstable at this value but would not result complete nitrifier washout.
Control Reactor
Test Reactor
PostAerobic Digester
Figure 2. The pilot system constructed at the Meridian WWTP. The test and control activated sludge reactors and the post-aerobic digester are labeled. MLR = 2Q = 25 L/hr
Control Reactor
Anaerobic 8.3 L
Anoxic 15.9 L
Q = 12.5 L/hr
Aerobic 27.3 L
Aerobic 27.3 L
WAS Aerobic 27.3 L
RAS = 0.5Q = 6.25 L/hr
PE Tank 1,900 L
MLR = 2Q = 25 L/hr
Test Reactor
Clarifier 113.5 L
Anaerobic 8.3 L
Anoxic 15.9 L
Aerobic 27.3 L
Aerobic 27.3 L
WAS Aerobic 27.3 L
Clarifier 113.4 L
RAS = 0.5Q = 6.25 L/hr Batch nitrifier augmentation 500 mL daily Solids Holding Tank 20 L
Batch feed 150 mL every 2 hours
Post Aerobic Digester 16.5 L
Gravity overflow
= peristaltic pump, arrow indicated flow direction
Figure 3. Pilot system design parameters.
Activated Sludge Reactor Operation. The activated sludge reactors were fed continuously with primary effluent (PE). The primary effluent tank was refilled three times per week and mixed continuously. The average influent ammonia concentration throughout the experiment was 38 ± 4.5 mg/L as N (average ± standard deviation), the average influent total and soluble COD concentrations were 381 ± 45 mg/L COD and 75 ± 16 mg/L COD respectively. The dissolved oxygen in the aerobic zones was maintained above 2 mg/L using aquarium air pumps and diffusers. All zones were mixed using aquarium recirculation pumps. The clarifiers were constructed from 113.5 L conical fermenters. Even with a one revolution per minute motor and various scrapper designs, sludge would accumulate in the bottom of the clarifier due to short circuiting of clear supernatant through the sludge blanket. To resolve this problem, the clarifier was operated in a semi-continuous sequencing batch mode where mixed liquor was fed continuously but effluent was withdrawn only every 3 hours. Every 45 minutes, the entire contents of the clarifier were fully mixed for 1 minute and allowed to resettle for 44 minutes. Operation in this mode allowed a relatively continuous return of sludge and avoided short-circuiting of clear supernatant. Every fourth mixing cycle, the effluent pump would turn on 24 minutes after the mixer turned off and withdraw effluent for 19 minutes. The clarifier mixing and effluent withdraw were controlled using a ChronTrol CD unit (ChronTrol Corporation, San Diego, California). Operation of the clarifier in the mode described above successfully maintained a solids inventory in the activated sludge reactors. However, the short settling time resulted in high effluent total suspended solids (TSS) concentrations throughout the experiment. For the first 22 days of the experiment, the only biomass wasting from both activated sludge reactors was due to suspended solids leaving in the effluent. When bioaugmentation began in the test reactor on December 14, 2009, the increased solids concentration required intentional wasting from the test reactor to maintain the target SRT of 4 days. Therefore, four gallons of mixed liquor per day were removed from the test reactor after bioaugmentation began. The test reactor experienced a starvation condition for three days from December 25 – December 28, 2009 due to a broken influent pump. This starvation condition resulted in a nitrification upset. Intentional wasting was stopped on January 5, 2010 in an attempt to stabilize the reactor. Intentional wasting from the control reactor began on December 21, 2009 to maintain the SRT in the control reactor at 4 days similar to the test reactor. One gallon per day of mixed liquor was removed from the control reactor after this time. Post-Aerobic Digester Operation. The post-aerobic digester was fed with anaerobically digested biosolids once every two hours. The average influent ammonia concentration throughout the experiment was 701 ± 108 mg/L as N. The SRT was equal to the hydraulic residence time of the reactor and the SRT was adjusted by changing the volume of anaerobic biosolids fed. Aeration was cycled on and off in the digester to achieve total nitrogen removal and alkalinity recovery through a denitrification cycle. The time intervals for aeration on/off periods were optimized throughout the study as was the digester SRT. The optimum intervals were determined to be an air on interval of 4 minutes and an air off internal of 16 minutes. The digester feed pump and digester air pump were controlled using a ChronTrol CD unit.
Reactor Performance Monitoring. For all reactors, influent and effluent ammonia was measured 3-5 times per week and effluent nitrate and nitrite were measured twice per week. The sample preservation method converted nitrite to nitrate. Therefore, the values are reported as the sum of the two compounds and denoted as NOx. The total and soluble COD of the influent and the soluble COD of the activated reactor effluent was measured twice per week. The digester effluent was tested for nitrite separately on December 14, 2009. The mixed liquor and effluent TSS of the control reactor were measured twice per week. Before bioaugmentation of the test reactor began, the mixed liquor and effluent TSS of the test reactor were measured twice per week. After bioaugmentation began, the mixed liquor and effluent TSS in the test reactor were measured five times per week. The volatile suspended solids concentrations of the mixed liquor in both reactors were measured twice per week throughout the experiment. The alkalinity of the digester effluent was measured once per week and the pH of the digester effluent was measured three times per week. All reactor performance monitoring measurements were performed according to the Standard Methods for the Examination of Water and Wastewater Treatment (1998). RESULTS AND DISCUSSION Post Aerobic Digester Performance. The Figure 4(a) shows the post-aerobic digester ammonia removal performance. For the first 28 days of operation, fouling of the aquarium diffusers used for oxygen transfer was an ongoing problem in the pilot-scale digester. This diffuser fouling resulted in dissolved oxygen concentrations generally below 1 mg/L during this time. The low dissolved oxygen concentrations disrupted nitrification in the digester and resulted in an ammonia build up. On December 2, 2009, the number and arrangement of diffusers was altered such that successful oxygen transfer was achieved. The feed to the digester was turned off for a day on December 2, 2009 to remove residual ammonia build up. After reliable oxygen transfer was achieved, the aeration intervals were then optimized. The optimum aeration interval was determined to be 20 minutes in length with 4 minutes of aeration and 16 minutes without. The aerated interval was selected to ensure the dissolved oxygen was between 1 and 3 mg/L during aeration. Digester foaming was observed throughout the experiment, and increasing the aeration time exacerbated digester foaming. The air off interval was optimized based on reactor pH. The interval was selected to maintain the reactor pH above 6.0 units. After digester operation was optimized, the pH in the reactor was 6.2 ± 0.2 units. The SRT was also adjusted during digester optimization to adjust the loading rate of ammonia to the digester. Once digester operation was optimized, an average ammonia removal of 81 ± 2.3 percent was achieved as was complete denitrification. Thus, the total nitrogen removal of the reactor was also 81 percent. The average digester effluent alkalinity was 133 ± 46 mg/L as CaCO3 during that time. Testing for nitrite on December 13, 2009 showed that more than half (58 percent) of the digester effluent NOx compounds consisted of nitrite. This suggests that the low dissolved oxygen conditions prior to this time may have resulted in partial washout of nitrite oxidizing organisms. The reduced carbon requirement for denitrification over nitrite rather than nitrate may have been increased the endogenous denitrification capacity of the digester. Figure 4(b) shows the volatile solids reduction achieved through the post-aerobic digester throughout the study. The VSR is related to solids retention time in the digester, which is
expected from previous studies (Kumar et al., 2006). The average VSR observed in this study at a 9-day digester SRT is 32 ± 5.8 percent. Novak et al. (2009) showed a 12 percent VSR through their post-aerobic digester as an SRT of 5 days, which agrees with the VSR of 16 percent observed by Parravicini et al. (2008) at the same digester SRT. At digester SRT of 9 days, Kumar et al. (2006) found a 15 percent VSR. Thus, the digester VSR in this study is twice as high as previously reported values.
Nitrogen Conc (mg/L N)
1,000
Influent NH4 SRT = 14 days
Effluent NH4
SRT = 7 days
SRT = 14 days
Effluent NOx SRT = 9 days
800
600 Turned off digester feed for 1 day
400
200
(a)
0 10/24/09
11/13/09
12/3/09
12/23/09
1/12/10
Digester Volatile Solids Reduction
100
SRT = 14 days
SRT = 7 days
SRT = 14 days
SRT = 9 days
VSR (%)
80
60
40
20
(b)
0 10/24/09
11/13/09
12/3/09
12/23/09
1/12/10
Date Figure 4. Post aerobic digester performance for (a) nitrogen removal and (b) VSR. The digester SRT was decreased from 14 to 7 days on November 15, 2009. The feed to the digester was turned off on December 2, 2009. On December 3, 2009, the digester feed was turned back on and the SRT was increased to 14 days. One December 13, 2009, the digester SRT was decreased to nine days and the effluent was tested for nitrite.
Activated Sludge Reactors and Bioaugmentation. Initially, both the test and control reactors were operated without bioaugmentation. High effluent solids in the test and control reactors resulted in a variable SRT as shown in Figures 5(a) and (b). For the first 22 days of test reactor operation and the first 26 days of control reactor operation, the only biomass wasting was due to suspended solids leaving in the effluent. Wasting only of effluent solids resulted in an average SRT of 5.7 ± 3.7 days for the control reactor and an average SRT of 4.5 ± 3.2 days for the test reactor. Figures 5(e) and (f) shows that this operational mode resulted in unstable nitrification in
both reactors and the mixed liquor concentration in both reactors was less than 1,000 mg/L TSS (Figures 5e and f).
Effluent wasting only SRT = 5.7 ± 3.7 days
Control Reactor SRT
Test Reactor Effluent TSS
Intentional wasting SRT = 3.7 ± 1.6 days
8
40
4
12/23/09
80
40
0 11/13/09
0 1/12/10
12/3/09
Control Reactor MLSS
TSS Conc (mg/L)
2,000
1,500
1,000
500
12/3/09
12/23/09
1,500
1,000
500
0 11/13/09
1/12/10
Ammonia Conc. (mg/L N)
Ammonia Conc. (mg/L N)
10
5 1 mg/L N
12/23/09
Date
Influent pump broken
12/3/09
12/23/09
1/12/10
20
(f)
1/12/10
15
10
5
0 11/13/09
Influent pump broken
Bioaugmentation Begins
15
12/3/09
0 1/12/10
12/23/09
Test Reactor Effluent Ammonia
(e)
0 11/13/09
4
(d)
Control Reactor Effluent Ammonia 20
8
Test Reactor MLSS
(c)
0 11/13/09
12
Bioaugmentation Begins
TSS Conc (mg/L)
2,000
16
SRT (days)
80
120
Test Reactor SRT Intentional wasting SRT = 4.2 ± 0.8 days
Bioaugmentation Begins
12
12/3/09
(b) Effluent wasting only
SRT = 4.5 ± 3.2 days
120
0 11/13/09
160
16
SRT (days)
TSS Conc (mg/L)
(a)
TSS Conc (mg/L)
Control Reactor Effluent TSS 160
1 mg/L N
12/3/09
12/23/09
1/12/10
Date
Figure 5. Effluent suspended solids and SRT in (a) the control reactor and (b) the test reactor. Mixed liquor concentration in (c) the control reactor and (d) the test reactor. Effluent ammonia in (e) the control reactor and (d) the test reactor. Note that the influent ammonia concentration was 38 ± 4.5 mg/L N on average throughout the experiment. Bioaugmentation and intentional wasting in the test reactor began on December 14, 2009. Intentional wasting in the control reactor began on December 21, 2009. The influent pump broke on December 25, 2009 resulting in starvation of the test reactor. The influent pump was fixed on December 28, 2009. Intentional wasting of the test reactor was stopped on January 5, 2010 in an attempt to stabilize nitrification in the reactor.
When bioaugmentation began in the test reactor on December 14, 2009, the increased solids concentration required intentional wasting from the test reactor to maintain an SRT of 4 days. The SRT was maintained at 4.2 ± 0.8 days in the test reactor. Intentional wasting of the control reactor began on December 21, 2009 in an effort maintain uniform operation between the two reactors. The control reactor had an average SRT of 3.7 ± 1.6 days after this time. Based on
previous test reactor ammonia removal results and on control reactor ammonia removal results, nitrification in the test reactor should have been unstable at an SRT of 4 days. Nitrification in the test reactor remained stable for 10 days after bioaugmentation began, which is more than 2 SRTs of stable operation. The influent pump broke on December 25, 2009 resulting in starvation of the test reactor for three days until it was repaired on December 28, 2009. The MLSS concentration in the test reactor decreased to 360 mg/L TSS over those three days (Figure 5d), despite ongoing bioaugmentation of the reactor. Three days of continuous aeration and no influent resulted in a nitrification upset in the test reactor. Regular wasting from the reactor continued until January 5, 2010 when wasting was stopped in an effort to stabilize nitrification in the reactor. Time constraints did not allow complete recovery so it is unknown if stable nitrification could have again been achieved in the test reactor. Therefore, the feasibility of bioaugmentation of with post-aerobic digester biomass remains uncertain. No detrimental effects were observed in the test reactor during bioaugmentation. The effluent soluble COD remained stable as shown in Figure 6 and the settling in the reactor remained good. The SVI of the test and control reactors was measured on January 8, 2010. The control reactor has an SVI of 100 mL/g while the test reactor has an SVI of 138 mL/g. Test Reactor Effluent
Control Reactor Effluent
Soluble COD (mg/L)
50
40
30
20
10
0 10/24/09
11/13/09
12/3/09
12/23/09
1/12/10
Date
Figure 6. Effluent soluble COD in the control and test reactors.
PRACTICAL IMPLICATIONS AND FUTURE RESEARCH Though the feasibility of bioaugmentation using post-aerobic digester biomass remains uncertain, the preliminary results presented here suggest further research on this new bioaugmentation concept is warranted. A number of variables could be optimized to determine the conditions that maximize bioaugmentation stability. For instance, previous studies have shown good nitrogen removal performance with a digester SRT of 5 days but time spent optimizing digester aeration eliminated opportunities to optimize digester SRT. Bioaugmentation relies on input of active biomass (Rittmann, 1995) and biological activity decreases with increased digester SRT because decay is enhanced. Shorter digester SRTs may therefore increase the efficiency of bioaugmentation through increased nitrifier activity. An increase in the volume of digester biomass recycled to the activated sludge reactor would
increase the number of external nitrifiers added to the reactor, further increasing bioaugmentation efficiency. However, there is a limit to the mass of biosolids that can be added to the secondary process because the overall mixed liquor concentration increases with increased recycling. This increased mixed liquor concentration limits the treatment plant capacity that may be gained by achieving reliable nitrification at a low SRT. Further research to understand the degree to which the mixed liquor may increase with the rate of biosolids recycling while help clarify the potential capacity increase associated with this bioaugmentation concept. Finally, whole plant simulations indicate that recycling biosolids back to the secondary process decreases the overall mass of biosolids for disposal by an additional 10 percent beyond just the increased VSR through the post-aerobic digester. Further research to understand the degree to which the mass of biosolids for disposal can be reduced through recycling of digester sludge back to the secondary process will help clarify viability of this potential benefit. CONCLUSIONS
Once optimized, the post-aerobic digester achieved 81 percent ammonia removal and 81 percent total nitrogen removal without the need for chemical addition.
At a digester solids retention time of 9 days, post aerobic digestion provided 31 percent volatile solids reduction beyond that achieved with anaerobic digestion alone.
The bioaugmented activated sludge reactor has stable nitrification for more than 2 SRTs of operation. . However, the slow recovery of stable nitrification after an upset of the bioaugmented reactor suggests further optimization of the system is required to ensure stable bioaugmentation is maintained.
Recycling of biosolids from the post-aerobic digester to the bioaugmented activated sludge reactor caused no observed detrimental effects in the reactor. Further research to understand the potential benefits of this new concept in wastewater treatment is therefore warranted.
REFERENCES APHA, AWWA, WEF. (1998) Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington, DC.
Kumar, N., Novak, J. T, and Murthy, S. (2006) Effect of Secondary Aerobic Digestion on Properties of Anaerobic Digested Biosolids. Proceedings of the Water Environment Federation 79th Annual Technical Exhibition and Conference, Dallas, TX, USA. Novak, J., Wilson, C., Banjade, S., Tanneru, C., Murthy, S., Bailey, W. (2009) A New Solids Technology Train: Can 70% Volatile Solids Reduction and Class A Biosolids be Achieved from Digestion? Proceedings of the Water Environment Federation 82nd Annual Technical Exhibition and Conference, Orlando, FL, USA. Parravicini, V., Svardal, K., Hornek, R., and Kroiss, H. (2008) Aeration of Anaerobically Digested Sewage Sludge for COD and Nitrogen Removal; Optimization at Large-Scale. Water Science and Technology 57(2), 257-264.
Rittmann, B. E. (1996) How Input Active Biomass Affects Sludge Age and Process Stability. Journal of Environmental Engineering 122(1), 4-8.
