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Influence of recirculation on the performance of anaerobic sequencing batch biofilm reactor (AnSBBR) treating hypersaline composite chemical wastewater
Bioresource Technology 98 (2007) 1373–1379
Influence of recirculation on the performance of anaerobic sequencing batch biofilm reactor (AnSBBR) treating hypersaline composite chemical wastewater S. Venkata Mohan, V. Lalit Babu, Y. Vijaya Bhaskar, P.N. Sarma
*
Bioengineering and Environmental Center, Indian Institute of Chemical Technology, Hyderabad 500 007, India Received 31 March 2006; received in revised form 15 May 2006; accepted 18 May 2006 Available online 7 July 2006
Abstract Influence of recirculation on the performance of anaerobic sequencing batch biofilm reactor (AnSBBR) was studied in the process of treating hypersaline (total dissolved inorganic solids (TDIS) 26 g/l) and low biodegradable (BOD/COD 0.3) composite chemical wastewater. Significant enhancement in the substrate removal efficiency and biogas yield was observed after introducing the recirculation to the system. Maximum efficiency (COD removal efficiency – 51%; SDR – 3.14 kg COD/cum-day) was observed at recirculation to feed (R/F) ratio of 2 (OLR – 6.15 kg COD/cum-day; HLR – 2.30 cum (liquid)/cum day; UFVA – 0.023 m/h). Subsequent increase of R/F to 3 (OLR – 6.15 kg COD/cum-day; HLR – 3.07 cum (liquid)/cum-day; UFVA – 0.035 m/h) resulted in reduction in COD removal efficiency (32%; SDR – 1.97 kg COD/cum-day). The enhanced performance of the system due to the introduction of recirculation was attributed to the improvement in the mass transfer between the substrate present in the bulk liquid and the attached biofilm. The hydrodynamic behavior due to recirculation mode of operation reduced the concentration gradient (substrate inhibition) of substrate and reaction by-products (VFA) resulting in mixed flow conditions. 2006 Elsevier Ltd. All rights reserved. Keywords: Anaerobic sequencing batch biofilm reactor (AnSBBR); Recirculation; Composite chemical wastewater; Hypersalinity; Low-biodegradable; VFA; Kinetic coefficient
1. Introduction Industrial wastewater, originating from chemical based industries contains toxic organic compounds, solvents, inorganic chemicals, salts, etc. and the characteristics of wastewater are highly variable and complex in nature (Venkata Mohan et al, 2001; Venkata Mohan and Sarma 2003, 2006). Shock loads, high carbon load, presence of toxic and inhibitory organic, solvents and inorganic compounds, consistent change in process, variability of the wastewater on both flow and composition (change of manufacturing product, transitory operation of the plant, washing, etc.) and wastewater characteristics normally *
Corresponding author. E-mail address: [email protected] (S.V. Mohan).
0960-8524/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2006.05.036
encountered in chemical process wastewater inherently hamper the treatment efficiency. Excessive usage of inorganic salts in the process results in high salt concentrations (hypersalinity) in wastewater and this affects the biological treatment process due to plasmolysis and/or loss of biological activity (Galinski and Truper, 1994; Venkata Mohan et al., 2001; Uygur and Kargi, 2004). In biological process efficiency depends on the type of reactor configuration used and the associated operating conditions adopted along with the nature and characteristics of wastewater being treated. Among the reactor configurations, biofilm (fixed film) configured systems are considered to be effective over the corresponding suspended growth systems due to the possibility of higher hydraulic loading rates and optimal utilization of the biomass by retention in the form of biofilm in
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the reactor (Jou and Huang, 2003; Bouwer, 1989; Wobus and Roeske, 2000). In biofilm systems, the immobilization capacity promotes and propagates the slow growing microbe, i.e., xenobiotic-degrading bacteria and these bacteria persist independent of the hydraulic retention time (HRT) (Jou and Huang, 2003; Bouwer, 1989; Maekinen et al., 1993). Retention of high biomass concentration within the reactor increase the process stability and resistance to shock loads. These systems are reported to be well suited for the treatment of wastewater containing poorly degradable compounds (Jou and Huang, 2003; Bouwer, 1989; Maekinen et al., 1993; Zhang et al., 1995). The attached form microbial growth as in biofilm/fixed film systems were effective, robust and could survive in extreme environments compared to the non-attached growth (suspended growth) (Bishop, 1997). On the whole biofilm configured systems exhibit higher substrate removal rates, greater system stability, simple to operate, could handle the shock loads, require less power, produce less sludge and the overall efficiency clearly exceeded the traditional methods of wastewater degradation (DeFilippi and Lewandowski, 1998; Jou and Huang, 2003). The recirculation [recirculation (R) to feed (F) ratio] is one of the important parameter, which has considerable influence on the overall process performance of the biological system. It affects the microbial ecology, hydraulic regime and characteristics of the operating system by directly influencing the biochemical system. In biofilm configured reactors, the hydraulic regime exerts significant influence on the overall process efficiency of the treatment irrespective of the applied kinetics. The hydraulic conditions in the biofilm reactors at high recirculation are very much similar to those of a completely mixed system (Samson and Guiot, 1990). Well-mixed reactors provide more uniform conditions for the microbial growth by increasing the active area in filling zone and also absorb the sudden organic overloads by reducing the effects of the acidogenic bacteria. The effects of recirculation in the whole process were difficult to specify and can be attributed to the different growing characteristics of the related species apart from the operational factors (Kennedy and Gorur, 1989). Recirculation of treated wastewater to mix with the influent wastewater affords a method of balancing the hydraulic loading with the carbon loading (Winkler, 1981). Introducing recirculation to the system facilitates increase in the hydraulic loading without a concomitant increase in the carbon loading or the carbon loading could be reduced while the hydraulic loading was maintained. Recirculation was used as an economical option for the treatment of very strong wastewater, which could not be diluted. In anaerobic systems recirculation is generally used to increase the influent alkalinity and to modify the kind of flow (Lomas et al., 2000). The aim of this work was to study biofilm configured anaerobic sequencing batch system treatment of hypersaline composite chemical wastewater under the influence of recirculation. The biochemical process was monitored
by evaluating process parameters viz., pH, ORP, COD, VFA and alkalinity to delineate the process performance with respect to recirculation as function. 2. Methods 2.1. Wastewater The strengths and characteristics of the wastewaters used as feed in this experiment are presented in Table 1. Wastewater was a combined chemical wastewater collected from a common effluent treatment plant (CETP) in Hyderabad, India where wastewater received from about 110 chemical processing industries was being treated. The wastewater was a composite one aggregated from pharmaceutical, drug, textile and dye intermediate manufacturing units, pesticide manufacturing units, bulk drugs, pharmaceuticals, pesticides and various chemical process units (soluble COD – 8100 mg/l; BOD – 2480 mg/l; total dissolved inorganic solids (TDIS) – 26 g/l; pH – 7.9; ORP – 75 mV; suspended solids – 980 mg/l; total alkalinity – 120 mg/l; chlorides – 7100 mg/l). The wastewater was complex in nature as it had low-biodegradability (BOD/COD ratio 0.31), high sulfates concentration (1.75 g/l) and hypersaline (TDIS – 26 ± 2 g/l) in nature. Wastewater was stored at 0 C prior to use in order to maintain uniform characteristics. The wastewater was not corrected for deficiency of trace elements. 2.2. Reactor configuration details AnSBBR was fabricated in the laboratory with ‘perplex’ material with active volume of 1.3 l (L/D ratio of 6) for this study. The operational set-up, flow diagram and reactor details are shown in Fig. 1. The reactor was designed to operate in upflow mode with biofilm configuration. Inert stone chips (2.5 cm · 1.5 cm) were used as fixed bed for support the biofilm formation. The fixed bed was placed up to the height of 0.7 m and the fixed bed had a void ratio of 0.54 after immobilization of anaerobic mixed consortia. The reactor was fabricated using leak proof sealing with proper inlet and outlet arrangements. Biogas was collected through a gas-outlet arrangement provided at the top of the reactor by means of water displacement method. The walls of the reactor were wrapped with aluminum sheet foil to restrict the light passage. Two separate peristaltic pumps (Maclins, India) were used to control influent and recirculation rates. Table 1 Reactor operation conditions with the function of recirculation ratio Feed (F ) (L/day)
Recirculation feed (R) (L/day)
R/F ratio
HLR (cum (liquid)/cum day)
UFVA (m/h)
1.0 1.0 1.0 1.0
0.0 1.0 2.0 3.0
0 1 2 3
0.77 1.53 2.30 3.07
0 0.012 0.023 0.035
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Biogas collector
AnSBBR
Feed Tank
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and the performance of the reactor was evaluated at various recirculation to feed ratios (R/F) from 0 to 3 at an increment of one with a constant cycle period (HRT) of 24 h. The system was operated at each recirculation ratio until steady state performance with respect to COD removal efficiency was achieved. Wastewater pH was not adjusted prior to giving the feed. The reactor was operated at a constant mesophilic temperature of 30 ± 2 C. Due to operation of the reactor at varied recirculation to feed ratios, varying hydraulic loading conditions and upflow velocity existed in the system (Table 1). The hydraulic loading rate (HLR) and actual upflow velocity (UFVA) were calculated using the following equations: HLR ¼ ½ðQF þ QR Þ=V ;
ð1Þ
UFVA ¼ ½QR =ACS =CF;
ð2Þ
Outlet
Peristaltic pumps
Peristaltic pumps
Fig. 1. Schematic details of AnSBBR.
where V represents volume of the reactor (cum), Q represents flow (cum/day) of feed or recirculation as suffixed, ACS represents cross sectional area of the reactor (cum) and CF indicates correction factor (in this case void ratio of reactor fixed bed of 0.52). 2.4. Analytical procedures
2.3. Reactor operation Reactor was operated in sequencing batch mode with a total cycle period of 24 h (HRT) consisting of 15 min of fill phase, 23.00 h of reaction (anaerobic) phase, 30 min of settling phase and 15 min of decant phase. At the beginning of each cycle, immediately after withdrawal (earlier sequence), a pre-defined volume (1.0 l) was fed to the reactor. The total reactor volume was recirculated during the reaction phase only. At the end of the cycle, suspended biomass (if any) settled and the effluent was withdrawn from the reactor. Various sequence phase operations (feed, reaction and decant) during reactor operation were controlled by pre-programmed timers employing peristaltic pumps (Maclins, India). To start the reactor, anaerobic sludge (mixed consortia) acquired from a laboratory scale UASB reactor which was being used for treating chemical wastewater for past three years was used as seed. After seeding (VSS – 12.5 g/l), the reactor was operated at recirculation to feed (R/F) ratio of one employing designed synthetic feed (g/l) (glucose, 0.9; sodium acetate, 0.7; MgSO4 Æ 7H2O, 0.21; FeSO4 Æ 7H2O, 0.5; NiSO4 Æ H2O, 0.02; NaHCO3, 5; NH4Cl, 1.3; KH2PO4, 0.05; CaCl2, 0.058; Na2HPO4, 0.3) to facilitate the initial colonization of biofilm on the supporting medium. After achieving steady state performance with respect to COD removal (±3% variation) the reactor was fed with the designated chemical wastewater. After start up, for the purpose of adaptation of the attached microflora the reactor was initially fed with the designated chemical wastewater at an organic loading rate (OLR) of 3.0 kg COD/cum-day at recirculation ratio of 1. Subsequently after achieving stable performance, the reactor was operated at higher OLR of 6.15 kg COD/cum-day
The performance of the AnSBBR reactor was assessed by monitoring COD removal throughout the operation. In addition, pH, ORP, VFA and total alkalinity were also determined during the sequence phase operation. The analytical procedures for monitoring the above parameters were adopted from standard methods (APHA, 1998). The composition of the biogas was analyzed using gas chromatography (Schimadzu GC 17A) with a thermal conductivity detector (TCD) and Supelco molecular sieve 5A capillary column. Temperatures of the injection block and detector cell were kept at 230 C and 200 C respectively during the analysis. 3. Results and discussion The performance of AnSBBR was evaluated by estimating COD (substrate) removal efficiency (nCOD) calculated using Eq. (3). CSO represented the initial COD concentration (mg/l) in the feed and CS denoted COD concentration (mg/l) in the reactor outlet: nCOD ¼ ðC SO C S Þ=C SO :
ð3Þ
The hypersaline nature of the wastewater associated with the low-biodegradability signified the complex nature of the wastewater. After achieving successful startup and adaptation, the reactor was operated initially in non-recirculation mode (R/F 0) at an OLR of 6.15 kg COD/cumday (HLR of 0.77 cum (liquid)/cum-day; UFAA – 0). The reactor performance was assessed by monitoring the COD removal efficiency during the sequence phase operation (Fig. 2). COD removal efficiency of 29% accounting
COD Removal Efficiency (%)
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a
50
R/F0 R/F1 R/F2 R/F3
40 30 20 10 0
2
0
6
4
8
10
12
14
16
18
20
22
24
Cycle Period (h) COD Removal Efficiency (%)
60
b 50 40 30 20 10 0 0
R/F-0
4
8
12
16
20
R/F-3
R/F-2R
R/F-1
24
28
32
36
40
44
48
52
56
60
Time (days)
Fig. 2. COD removal efficiency during the reactor operator at various recirculation ratios (a) during cycle period and (b) during reactor operation.
for substrate degradation rate (SDR) of 1.72 kg COD/ cum-day was observed during stable performance (after 8 days of feeding the wastewater) in the during non-recirculation mode operation of the reactor. Low substrate removal efficiency was observed during the non-recirculation mode of operation. The resulting poor performance might be attributed to the complex nature of wastewater and the mode of operation. In general, under non-recirculation conditions, anaerobic systems are generally under plug flow regime. Particularly with periodic discontinuous/sequencing batch mode operation un-mixed and nonflow conditions prevail along with plug flow regime. This leads to a concentration gradient in the reactor due to little contact with the biomass and directly influencing the reactor performance. AnSBBR was further operated with recirculation to feed ratios (R/F) of 1, 2 and 3, respectively, to study the role of recirculation on the process performance. COD removal efficiency was found to increase with increase in the recirculation. At R/F ratio of 1 (OLR – 6.15 kg COD/cum-day; HLR – 1.53 cum (liquid)/cum day; UFAA 0.012 m/h), significant improvement in the COD removal efficiency (41%; SDR – 2.52 kg COD/cum-day) was observed. Further increase in R/F ratio to 2 (OLR – 6.15 kg COD/ cum-day; HLR – 2.30 cum (liquid)/cum day; UFAA – 0.023 m/h) resulted in further enhancement in the COD removal efficiency (51%, SDR – 3.14 kg COD/cum-day). However, at R/F of 3 (OLR – 6.15 kg COD/cum day; HLR – 3.07 cum (liquid)/cum day; UFAA – 0.035 m/h) showed reduction in the performance (COD removal efficiency – 32%; SDR – 1.97 kg COD/cum-day). Rapid stabilized period was observed in SBBR for every
change in organic loading with respect to COD removal efficiency. Evidently in the periodic batch mode operation, OLR or SDR varied with the cycle period up to the end of the cycle. The SDR and OLR profiles represented a mirror image for all the studied cases representing the fact that SDR was an integral part of the OLR. Variation of SDR/OLR (calculated with transient OLR) and (SDR/OLR)actual (calculated with initial OLR) during sequence phase operation are shown in Fig. 3. Maximum SDR/OLR ratio was obtained for the reactor operation at R/F ratio of 2 compared with the other cases studied. Improvement in biogas yield was observed with the introduction of the recirculation and maximum biogas yield was noticed at R/F ratio of 2 (data not given). The effective mass transfer conditions and the reduction in the prevailing concentration gradient in the reactor height showed positive influence on the biogas production due to introduction of recirculation. Increase in recirculation to 2 resulted in improved performance. However, exceeding certain ratio (>R/F 2) showed retarded performance. R/F ratio of 2 was found to be optimum under the operated conditions for the effective performance of the AnSBBR with this wastewater. The results indicated that the overall substrate removal rate increased as the recirculation ratio increased and the behavior might be attributed to the improvement in the mass transfer phenomena between the substrate present in the bulk liquid and the self-immobilized anaerobic biofilm in the reactor. Among the studied cases, non-recirculation mode operation registered low SDR value. Interestingly a rapid stabilizing period was noticed for the each shift in the recirculation. At non-recirculation mode of operation, steady state performance was established within 8 days of operation, while at R/F of 1, 2 and 3, stable performance was observed within 14, 7 and 12 days, respectively. Rapid stabilizing period evidenced in AnSBBR operation might be attributed to the operation of system in periodic discontinuous operation. Rapid startup period was reported in the literature pertaining to the SBR operation (Wilderer et al., 2001; Rodrigues et al., 2003; Chandrasekhara Rao et al., 2005; Venkata Mohan et al., 2005a,b,c). To analyze the influence of recirculation on the performance, experimental data was fitted into an empirical equation (Eq. (4)) and the first order kinetic model (Eq. (5)) (Rodrigues et al., 2003): eI ¼ ða1 tÞ=ða2 þ tÞ; Rs ¼ dC I =dt ¼ k 1 ðC I C O Þ;
ð4Þ ð5Þ
where a1 and a2 are the empirical parameters representing the maximum value of eI and the time to attain 50% of the maximum value respectively. In Eq. (5), the parameter k1 represents an empirical kinetic coefficient embodied the yield factor and both intrinsic consumption kinetic constant as well as internal and external mass transfer constants (Rodrigues et al., 2003) and Rs denotes substrate removal rate. The profile of substrate concentrations in the reactor and the substrate removal rate (Rs) for the stud-
0.5
a
0.4 5
OLR SDR SDR/OLR actual SDR/OLR
4
0.3
3
0.2
2 0.1 1 0
0 0
0.5
2
4
6
8
12
15
22
7
0.8
b
6 0.6
5 OLR SDR SDR/OLR SDR/OLR actual
4 3
0.2
1 0
0 0
24
0.5
2
4
4 0.6 3 0.4 2 0.2
1
0
0 0.5
2
4
6
8
12
15
22
24
SDR and OLR (kg COD/cum-day)
0.8
0
8
12
15
22
24
0.6
d
SDR OLR SDR/OLR SDR/OLR (actual)
6 5
0.4 4 3 0.2
2 1 0
SDR/OLR (kg COD/cum-day)
1
SDR/OLR (kg COD/cum-day)
SDR and OLR (kg COD/cum-day)
7
1.2 SDR OLR SDR/OLR SDR/OLR actual
5
6
Cycle Period (h)
7 6
0.4
2
Cylce Period (h) C
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SDR/OLR (kg COD/cum-day)
6
SDR and OLR (kg COD/cum-day)
7
SDR/OLR (kg COD/cum-day)
SDR and OLR (kg COD/cum-day)
S.V. Mohan et al. / Bioresource Technology 98 (2007) 1373–1379
0 0
0.5
2
4
Cycle Period (h)
6
8
12
15
22
24
Cycle Period (h)
Fig. 3. Variation of OLR and SDR with the function of recirculation rates ((a) R/F 0, (b) R/F 1, (c) R/F 2, (d) R/F 3).
8.4 8.2
pH
8 7.8 7.6 R/F 0 R/F 1 R/F 2 R/F 3
7.4 7.2 7 0
0.5
2
4
6
8
12
15
22
24
Cycle Period (h)
24
22
15
12
8
6
4
2
0
0.5
Cycle Period (h) 0
-20
ORP (mV)
ied experimental variations estimated by the first order model Eq. (5) are depicted in Table 2. The values of parameters a1 and a2 obtained from the empirical model (Eq. (4)) and the parameter k1 obtained from the first order model as a function of the recirculation are presented in Table 2. The variation of parameters a2 and k1 as a function of recirculation ratio indicated the stabilization tendency of the reactor at R/F ratio of 2. Variations of outlet pH and ORP during the reactor operation are shown in Fig. 4. In anaerobic process, system pH was considered to be one of the important parameters to decide upon the status of the internal reactor environment (Rittmann and McCarty, 2001; Venkata Mohan et al., 2005b). Extreme variation in pH was not observed during the reactor operation irrespective of the process variation. For all the studied experimental variations pH varied in between 7.65 and 8.20 resulting in slightly alkaline range, which was considered to be effective for anaerobic fermentation. In the case of ORP, the variation was in
-40
R/F 0 R/F 1 R/F 2 R/F 3
-60
-80
Table 2 Values of parameters derived from empirical model (Eq. (4)) and first order model (Eq. (5)) R/F
ei (%)
a1 (dimensionless)
a2 (min)
k1 (h1)
C0 (mg COD/L)
0 1 2 3
28 ± 0.5 41 ± 0.6 51 ± 0.4 32 ± 0.3
0.28 0.41 0.51 0.32
168 252 384 210
1.978 1.108 0.738 1.634
5760 4720 3920 5440
-100
Fig. 4. Variation of pH and ORP during sequence phase operation with the function of recirculation rates.
the range of 40 mV to 90 mV during the reactor operation, which confirmed the existence of the persistent anaerobic environment in the biofilm reactor. A gradual decrease
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in the ORP values was observed with increase in the recirculation ratio. Production of the volatile organic acids (VFA) is one of the important and general phenomena in the acidogenic step of the anaerobic fermentation (Rittmann and McCarty, 2001). Accumulation of VFA in the reactor environment was normally observed in the case of unfavorable operating conditions and this affected the buffering capacity of the system leading to the process inhibition (Rittmann and McCarty, 2001; Venkata Mohan et al., 2005b). In the case of non-recirculation mode operation, VFA concentration varied inconsistently between 2000 and 4000 mg/l (Fig. 5). This phenomenon might be attributed to the prevailing plug flow and non mixed conditions existing in the system leading to concentration gradient which resulted in incoherent VFA concentration. Variation in VFA concentration was observed with the variation in recirculation. Comparatively low concentration of VFA was observed with R/F ratio of 2. At R/F ratio between 1 and 3, the VFA concentration staggered in and around 4000 mg/l. The relatively low variation in VFA concentration during reactor operation in the recirculation mode operation might be attributed to consumption of VFA which was indicative of effective anaerobic buffering environment. Outlet alkalinity concentration varied between 10 and 70 mg/l during the reactor operation. Consistently higher values of alkalinity were observed with R/F ratio of 2. Higher alkalinity concentration in anaerobic system was essential to maintain the buffering capacity. Comparatively good buffering capacity along with effective SDR was observed with R/F ratio of 2. During non-recirculation mode operation, the alkalinity concentration was quite low indicating disturbed buffering conditions in the reactor
10000 R/F 0 R/F 1 R/F 2 R/F 3
VFA (mg/L)
8000 6000 4000 2000 0 0
0.5
2
4
6
8
12
15
22
24
Cycle Period (h)
Alkalinity (mg/L)
60 R/F 0 R/F 1 R/F 2 R/F 3
50 40 30 20 10
due to VFA accumulation and consequence of alkalinity consumption. With every shift in the recirculation rate, a low concentration of alkalinity was observed in the initial stages (for few days). This might be attributed to the consumption of alkalinity due to the shift in substrate loading conditions. This led to disturbance in the anaerobic environment that might result in the accumulation of VFA. Further operation had resulted in rapid recovery of the buffering environment of biofilm reactor. Low VFA concentration during non-recirculation operation mode might be attributed to either the incomplete anaerobic fermentation of wastewater or due to the consumption of the alkalinity (buffering capacity) by the accumulated VFA or both. The reactor also showed considerable utilization of alkalinity to buffer the inconsistent formation and utilization of VFA. With change in the recirculation rate significant variation in reactor operating conditions were observed (Table 1). This was especially evident with the UFVA and HLR. With increase in recirculation rate consistent increase in UFVA was noticed. In the case of HLR also, a consistent increase with the increase in R/F ratio was observed. Due to recirculation, the HLR could be increased without a concomitant increase in the OLR. OLR could be reduced while maintaining the HLR which helps to reduce the substrate concentration in the influent. This is beneficial especially for complex and inhibitory wastewater. Recirculation facilitated linear velocity and this restricted the existence of a concentration gradient and the reactor could be considered as completely mixed. With recirculation operation, complete mixed condition prevailed in the reactor and helped to reduce the induced concentration gradient and increase effective contact with microflora. Recirculation helped to achieve a homogeneous and uniform distribution of substrate along the reactor depth to achieve effective contact with the biomass. This facilitated to reduce the mass transfer limitations between the substrate and the biomass in the reactor. Biodegradation efficiency of the substrate normally relates to the existing mass transfer conditions rates rather than the degradation rates. The hydrodynamic behavior due to recirculation mode of operation facilitated completely mixed flow conditions. This resulted in reduction of substrate inhibition and VFA accumulation through the reactor height facilitating optimum environment (buffering capacity) for proper anaerobic metabolism. The variation in the outlet pH, alkalinity and VFA concentration in combination with SDR was in good agreement with the variation of recirculation rates studied. The study revealed the efficiency of recirculation in biofilm reactor up to certain level of R/F ratio. The recirculation facilitated effective mixing of the reactor volume leading to reduction in substrate concentration gradient and VFA concentration.
0 0
0.5
2
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22
24
4. Conclusions
Cycle Period (h)
Fig. 5. Variation of VFA and alkalinity during sequence phase operation with the function of recirculation rates.
The results obtained from the present studies showed the effectiveness of recirculation as process parameter to
S.V. Mohan et al. / Bioresource Technology 98 (2007) 1373–1379
enhance the process efficiency of anaerobic biofilm reactor (AnSBBR) in treating hypersaline composite complex chemical wastewater. Substantial enhancement in the process efficiency was observed with the introduction of recirculation and threshold limit of recirculation rate for economic purpose was found to be two for the system conditions adopted in this study. Recirculation facilitated an efficient contact between the substrate and the microflora in biofilm at the same time reducing the induced concentration gradient and resulted in the overall enhancement in the process performance due to the improvement of the mass transfer conditions. Also, due to recirculation mode, the hydrodynamic behavior facilitated reduction in the concentration gradient with the height of the reactor due to mixed flow conditions. This study also presented an approach to verify the affect of recirculation on the performance of the biofilm configured reactor operated in periodic discontinuous mode. References APHA (American Public Health Association), 1998. Standard methods for examination of waters and wastewaters. APHA, AWWA. Bishop, P.L., 1997. Biofilm structure and kinetics. Water Science and Technology 36, 287–294. Bouwer, E.J., 1989. Transformation of xenobiotics in biofilms. In: Characklis, W.G., Wilderer, P.A. (Eds.), Structure and Function of Biofilms. John Wiley, New York, pp. 251–267. Chandrasekhara Rao, N., Venkata Mohan, S., Muralikrishna, P., Sarma, P.N., 2005. Treatment of composite chemical wastewater by GACBiofilm configured sequencing batch reactor (SBGR) operated in aerobic environment. Journal of Hazardous Material 124, 59–67. DeFilippi, L.J., Lewandowski, G.A., 1998. Biological Treatment of Hazardous Wastes. John Wiley and Sons, New York. Galinski, E.A., Truper, H.G., 1994. Microbial behavior in salt stressed ecosystems. FEMS Microbiology Reviews 15, 95–108. Jou, C.G., Huang, G., 2003. A pilot study for oil refinery wastewater treatment using a fixed-film bioreactor. Advances in Environmental Research 7, 463–469. Kennedy, K.J., Gorur, S., 1989. Media effects on performance of anaerobic hybrid reactors. Water Research 23, 1397–1405. Lomas, J.M., Urbano, C., Camarero, L.M., 2000. Influence of recirculation flow in a pilot scale down flow stationary fixed film anaerobic reactor treating piggery slurry. Biomass and Bioenergy 18, 421–430. Maekinen, P.M., Theno, T.J., Ferguson, J.F., Ongerth, J.E., Puhakka, J.A., 1993. Chlorophenol toxicity removal and monitoring in aerobic treatment: recovery from process upsets. Environmental Science and Technology 27, 1434–1439.
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Rittmann, B.E., McCarty, P.L., 2001. Environmental Biotechnology – Principle and applications. McGraw-Hill International Edition, Boston. Rodrigues, J.A.D., Ratusznei, S.M., Camargo, E.F.M., Zaiat, M., 2003. Influence of agitation rate on the performance of an anaerobic sequences batch reactor containing granulated biomass treating low strength wastewater. Advances in Environmental Research 7, 405–410. Samson, R., Guiot, S., 1990. Les nouveaux secteurs a fort potential de development en digestion anaerobic Quebec. Centere Quebecois de valorisation de la biomasse, Canada. Uygur, A., Kargi, F., 2004. Salt inhibition on biological nutrient removal from saline wastewater in a sequencing batch reactor. Enzyme and Microbial Technology 34, 313–318. Venkata Mohan, S., Sarma, P.N., 2003. Anaerobic treatment process for pharmaceutical wastewater. PharmaBioMed 2, 101–108. Venkata Mohan, S., Sarma, P.N., 2006. Advanced bioremediation processes in chemical industries – Special focus on effluent treatment plants. Chemical Business 20, 35–44. Venkata Mohan, S., Prakasham, R.S., Satyavathi, B., Annapurna, J., Ramakrishna, S.V., 2001. Biotreatability studies of pharmaceutical wastewaters using an anaerobic suspended film contact reactor. Water Science and Technology 43, 271–276. Venkata Mohan, S., Annapoorna, J., Ramakrishna, S.V., 2001. Hypersalinity and its effect on Biological wastewater treatment – A review. Asian Journal of Microbiology, Biotechnology and Environmental Sciences 3, 249–261. Venkata Mohan, S., Chandrashekar Rao, N., Krishna Prasad, K., Madhavi, B.T.V., Sarma, P.N., 2005a. Treatment of complex chemical effluents by sequencing batch reactor (SBR) adopting aerobic suspended growth configuration. Process Biochemistry 40, 1501–1508. Venkata Mohan, S., Chandrasekhara Rao, N., Prasad, K.K., Krishna, P.M., Sreenivasa Rao, R., Sarma, P.N., 2005b. Anaerobic treatment of complex chemical wastewater in a sequencing batch biofilm reactor: process optimization and evaluation of factors interaction using the Taguchi dynamic DOE methodology. Biotechnology and Bioengineering 90, 732–745. Venkata Mohan, S., Chandrasekhara Rao, N., Prasad, K.K., Sarma, P.N., 2005c. Bioaugmentation of anaerobic sequencing batch biofilm reactor (ASBBR) with immobilized sulphate reducing bacteria (SRB) for treating sulfate bearing chemical wastewater. Process Biochemistry 40 (8), 2849–2857. Wilderer, P.A., Irvine, R.L., Goronszy, M.C., 2001. Sequencing batch reactor technology. Scientific and Technical Report. IWA publishing, No. 10, 2001. Winkler, M.A., 1981. Biological treatment of wastewater. Ellis Horwood Ltd., Chichestel. Wobus, A., Roeske, I., 2000. Reactors with membrane-grown biofilms: Their capacity to cope with fluctuating inflow conditions and with shock loads of xenobiotics. Water Research 34, 279–287. Zhang, T.C., Fu, Y.C., Bishop, P.L., Kupferle, M., Fitzgerald, S., Jiang, H.H., Harmer, C., 1995. Transport and biodegradation of toxic organics in biofilms. Journal of Hazardous Materials 41, 267–285.
Influence of recirculation on the performance of anaerobic sequencing batch biofilm reactor (AnSBBR) treating hypersaline composite chemical wastewater S. Venkata Mohan, V. Lalit Babu, Y. Vijaya Bhaskar, P.N. Sarma
*
Bioengineering and Environmental Center, Indian Institute of Chemical Technology, Hyderabad 500 007, India Received 31 March 2006; received in revised form 15 May 2006; accepted 18 May 2006 Available online 7 July 2006
Abstract Influence of recirculation on the performance of anaerobic sequencing batch biofilm reactor (AnSBBR) was studied in the process of treating hypersaline (total dissolved inorganic solids (TDIS) 26 g/l) and low biodegradable (BOD/COD 0.3) composite chemical wastewater. Significant enhancement in the substrate removal efficiency and biogas yield was observed after introducing the recirculation to the system. Maximum efficiency (COD removal efficiency – 51%; SDR – 3.14 kg COD/cum-day) was observed at recirculation to feed (R/F) ratio of 2 (OLR – 6.15 kg COD/cum-day; HLR – 2.30 cum (liquid)/cum day; UFVA – 0.023 m/h). Subsequent increase of R/F to 3 (OLR – 6.15 kg COD/cum-day; HLR – 3.07 cum (liquid)/cum-day; UFVA – 0.035 m/h) resulted in reduction in COD removal efficiency (32%; SDR – 1.97 kg COD/cum-day). The enhanced performance of the system due to the introduction of recirculation was attributed to the improvement in the mass transfer between the substrate present in the bulk liquid and the attached biofilm. The hydrodynamic behavior due to recirculation mode of operation reduced the concentration gradient (substrate inhibition) of substrate and reaction by-products (VFA) resulting in mixed flow conditions. 2006 Elsevier Ltd. All rights reserved. Keywords: Anaerobic sequencing batch biofilm reactor (AnSBBR); Recirculation; Composite chemical wastewater; Hypersalinity; Low-biodegradable; VFA; Kinetic coefficient
1. Introduction Industrial wastewater, originating from chemical based industries contains toxic organic compounds, solvents, inorganic chemicals, salts, etc. and the characteristics of wastewater are highly variable and complex in nature (Venkata Mohan et al, 2001; Venkata Mohan and Sarma 2003, 2006). Shock loads, high carbon load, presence of toxic and inhibitory organic, solvents and inorganic compounds, consistent change in process, variability of the wastewater on both flow and composition (change of manufacturing product, transitory operation of the plant, washing, etc.) and wastewater characteristics normally *
Corresponding author. E-mail address: [email protected] (S.V. Mohan).
0960-8524/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2006.05.036
encountered in chemical process wastewater inherently hamper the treatment efficiency. Excessive usage of inorganic salts in the process results in high salt concentrations (hypersalinity) in wastewater and this affects the biological treatment process due to plasmolysis and/or loss of biological activity (Galinski and Truper, 1994; Venkata Mohan et al., 2001; Uygur and Kargi, 2004). In biological process efficiency depends on the type of reactor configuration used and the associated operating conditions adopted along with the nature and characteristics of wastewater being treated. Among the reactor configurations, biofilm (fixed film) configured systems are considered to be effective over the corresponding suspended growth systems due to the possibility of higher hydraulic loading rates and optimal utilization of the biomass by retention in the form of biofilm in
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the reactor (Jou and Huang, 2003; Bouwer, 1989; Wobus and Roeske, 2000). In biofilm systems, the immobilization capacity promotes and propagates the slow growing microbe, i.e., xenobiotic-degrading bacteria and these bacteria persist independent of the hydraulic retention time (HRT) (Jou and Huang, 2003; Bouwer, 1989; Maekinen et al., 1993). Retention of high biomass concentration within the reactor increase the process stability and resistance to shock loads. These systems are reported to be well suited for the treatment of wastewater containing poorly degradable compounds (Jou and Huang, 2003; Bouwer, 1989; Maekinen et al., 1993; Zhang et al., 1995). The attached form microbial growth as in biofilm/fixed film systems were effective, robust and could survive in extreme environments compared to the non-attached growth (suspended growth) (Bishop, 1997). On the whole biofilm configured systems exhibit higher substrate removal rates, greater system stability, simple to operate, could handle the shock loads, require less power, produce less sludge and the overall efficiency clearly exceeded the traditional methods of wastewater degradation (DeFilippi and Lewandowski, 1998; Jou and Huang, 2003). The recirculation [recirculation (R) to feed (F) ratio] is one of the important parameter, which has considerable influence on the overall process performance of the biological system. It affects the microbial ecology, hydraulic regime and characteristics of the operating system by directly influencing the biochemical system. In biofilm configured reactors, the hydraulic regime exerts significant influence on the overall process efficiency of the treatment irrespective of the applied kinetics. The hydraulic conditions in the biofilm reactors at high recirculation are very much similar to those of a completely mixed system (Samson and Guiot, 1990). Well-mixed reactors provide more uniform conditions for the microbial growth by increasing the active area in filling zone and also absorb the sudden organic overloads by reducing the effects of the acidogenic bacteria. The effects of recirculation in the whole process were difficult to specify and can be attributed to the different growing characteristics of the related species apart from the operational factors (Kennedy and Gorur, 1989). Recirculation of treated wastewater to mix with the influent wastewater affords a method of balancing the hydraulic loading with the carbon loading (Winkler, 1981). Introducing recirculation to the system facilitates increase in the hydraulic loading without a concomitant increase in the carbon loading or the carbon loading could be reduced while the hydraulic loading was maintained. Recirculation was used as an economical option for the treatment of very strong wastewater, which could not be diluted. In anaerobic systems recirculation is generally used to increase the influent alkalinity and to modify the kind of flow (Lomas et al., 2000). The aim of this work was to study biofilm configured anaerobic sequencing batch system treatment of hypersaline composite chemical wastewater under the influence of recirculation. The biochemical process was monitored
by evaluating process parameters viz., pH, ORP, COD, VFA and alkalinity to delineate the process performance with respect to recirculation as function. 2. Methods 2.1. Wastewater The strengths and characteristics of the wastewaters used as feed in this experiment are presented in Table 1. Wastewater was a combined chemical wastewater collected from a common effluent treatment plant (CETP) in Hyderabad, India where wastewater received from about 110 chemical processing industries was being treated. The wastewater was a composite one aggregated from pharmaceutical, drug, textile and dye intermediate manufacturing units, pesticide manufacturing units, bulk drugs, pharmaceuticals, pesticides and various chemical process units (soluble COD – 8100 mg/l; BOD – 2480 mg/l; total dissolved inorganic solids (TDIS) – 26 g/l; pH – 7.9; ORP – 75 mV; suspended solids – 980 mg/l; total alkalinity – 120 mg/l; chlorides – 7100 mg/l). The wastewater was complex in nature as it had low-biodegradability (BOD/COD ratio 0.31), high sulfates concentration (1.75 g/l) and hypersaline (TDIS – 26 ± 2 g/l) in nature. Wastewater was stored at 0 C prior to use in order to maintain uniform characteristics. The wastewater was not corrected for deficiency of trace elements. 2.2. Reactor configuration details AnSBBR was fabricated in the laboratory with ‘perplex’ material with active volume of 1.3 l (L/D ratio of 6) for this study. The operational set-up, flow diagram and reactor details are shown in Fig. 1. The reactor was designed to operate in upflow mode with biofilm configuration. Inert stone chips (2.5 cm · 1.5 cm) were used as fixed bed for support the biofilm formation. The fixed bed was placed up to the height of 0.7 m and the fixed bed had a void ratio of 0.54 after immobilization of anaerobic mixed consortia. The reactor was fabricated using leak proof sealing with proper inlet and outlet arrangements. Biogas was collected through a gas-outlet arrangement provided at the top of the reactor by means of water displacement method. The walls of the reactor were wrapped with aluminum sheet foil to restrict the light passage. Two separate peristaltic pumps (Maclins, India) were used to control influent and recirculation rates. Table 1 Reactor operation conditions with the function of recirculation ratio Feed (F ) (L/day)
Recirculation feed (R) (L/day)
R/F ratio
HLR (cum (liquid)/cum day)
UFVA (m/h)
1.0 1.0 1.0 1.0
0.0 1.0 2.0 3.0
0 1 2 3
0.77 1.53 2.30 3.07
0 0.012 0.023 0.035
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Biogas collector
AnSBBR
Feed Tank
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and the performance of the reactor was evaluated at various recirculation to feed ratios (R/F) from 0 to 3 at an increment of one with a constant cycle period (HRT) of 24 h. The system was operated at each recirculation ratio until steady state performance with respect to COD removal efficiency was achieved. Wastewater pH was not adjusted prior to giving the feed. The reactor was operated at a constant mesophilic temperature of 30 ± 2 C. Due to operation of the reactor at varied recirculation to feed ratios, varying hydraulic loading conditions and upflow velocity existed in the system (Table 1). The hydraulic loading rate (HLR) and actual upflow velocity (UFVA) were calculated using the following equations: HLR ¼ ½ðQF þ QR Þ=V ;
ð1Þ
UFVA ¼ ½QR =ACS =CF;
ð2Þ
Outlet
Peristaltic pumps
Peristaltic pumps
Fig. 1. Schematic details of AnSBBR.
where V represents volume of the reactor (cum), Q represents flow (cum/day) of feed or recirculation as suffixed, ACS represents cross sectional area of the reactor (cum) and CF indicates correction factor (in this case void ratio of reactor fixed bed of 0.52). 2.4. Analytical procedures
2.3. Reactor operation Reactor was operated in sequencing batch mode with a total cycle period of 24 h (HRT) consisting of 15 min of fill phase, 23.00 h of reaction (anaerobic) phase, 30 min of settling phase and 15 min of decant phase. At the beginning of each cycle, immediately after withdrawal (earlier sequence), a pre-defined volume (1.0 l) was fed to the reactor. The total reactor volume was recirculated during the reaction phase only. At the end of the cycle, suspended biomass (if any) settled and the effluent was withdrawn from the reactor. Various sequence phase operations (feed, reaction and decant) during reactor operation were controlled by pre-programmed timers employing peristaltic pumps (Maclins, India). To start the reactor, anaerobic sludge (mixed consortia) acquired from a laboratory scale UASB reactor which was being used for treating chemical wastewater for past three years was used as seed. After seeding (VSS – 12.5 g/l), the reactor was operated at recirculation to feed (R/F) ratio of one employing designed synthetic feed (g/l) (glucose, 0.9; sodium acetate, 0.7; MgSO4 Æ 7H2O, 0.21; FeSO4 Æ 7H2O, 0.5; NiSO4 Æ H2O, 0.02; NaHCO3, 5; NH4Cl, 1.3; KH2PO4, 0.05; CaCl2, 0.058; Na2HPO4, 0.3) to facilitate the initial colonization of biofilm on the supporting medium. After achieving steady state performance with respect to COD removal (±3% variation) the reactor was fed with the designated chemical wastewater. After start up, for the purpose of adaptation of the attached microflora the reactor was initially fed with the designated chemical wastewater at an organic loading rate (OLR) of 3.0 kg COD/cum-day at recirculation ratio of 1. Subsequently after achieving stable performance, the reactor was operated at higher OLR of 6.15 kg COD/cum-day
The performance of the AnSBBR reactor was assessed by monitoring COD removal throughout the operation. In addition, pH, ORP, VFA and total alkalinity were also determined during the sequence phase operation. The analytical procedures for monitoring the above parameters were adopted from standard methods (APHA, 1998). The composition of the biogas was analyzed using gas chromatography (Schimadzu GC 17A) with a thermal conductivity detector (TCD) and Supelco molecular sieve 5A capillary column. Temperatures of the injection block and detector cell were kept at 230 C and 200 C respectively during the analysis. 3. Results and discussion The performance of AnSBBR was evaluated by estimating COD (substrate) removal efficiency (nCOD) calculated using Eq. (3). CSO represented the initial COD concentration (mg/l) in the feed and CS denoted COD concentration (mg/l) in the reactor outlet: nCOD ¼ ðC SO C S Þ=C SO :
ð3Þ
The hypersaline nature of the wastewater associated with the low-biodegradability signified the complex nature of the wastewater. After achieving successful startup and adaptation, the reactor was operated initially in non-recirculation mode (R/F 0) at an OLR of 6.15 kg COD/cumday (HLR of 0.77 cum (liquid)/cum-day; UFAA – 0). The reactor performance was assessed by monitoring the COD removal efficiency during the sequence phase operation (Fig. 2). COD removal efficiency of 29% accounting
COD Removal Efficiency (%)
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a
50
R/F0 R/F1 R/F2 R/F3
40 30 20 10 0
2
0
6
4
8
10
12
14
16
18
20
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24
Cycle Period (h) COD Removal Efficiency (%)
60
b 50 40 30 20 10 0 0
R/F-0
4
8
12
16
20
R/F-3
R/F-2R
R/F-1
24
28
32
36
40
44
48
52
56
60
Time (days)
Fig. 2. COD removal efficiency during the reactor operator at various recirculation ratios (a) during cycle period and (b) during reactor operation.
for substrate degradation rate (SDR) of 1.72 kg COD/ cum-day was observed during stable performance (after 8 days of feeding the wastewater) in the during non-recirculation mode operation of the reactor. Low substrate removal efficiency was observed during the non-recirculation mode of operation. The resulting poor performance might be attributed to the complex nature of wastewater and the mode of operation. In general, under non-recirculation conditions, anaerobic systems are generally under plug flow regime. Particularly with periodic discontinuous/sequencing batch mode operation un-mixed and nonflow conditions prevail along with plug flow regime. This leads to a concentration gradient in the reactor due to little contact with the biomass and directly influencing the reactor performance. AnSBBR was further operated with recirculation to feed ratios (R/F) of 1, 2 and 3, respectively, to study the role of recirculation on the process performance. COD removal efficiency was found to increase with increase in the recirculation. At R/F ratio of 1 (OLR – 6.15 kg COD/cum-day; HLR – 1.53 cum (liquid)/cum day; UFAA 0.012 m/h), significant improvement in the COD removal efficiency (41%; SDR – 2.52 kg COD/cum-day) was observed. Further increase in R/F ratio to 2 (OLR – 6.15 kg COD/ cum-day; HLR – 2.30 cum (liquid)/cum day; UFAA – 0.023 m/h) resulted in further enhancement in the COD removal efficiency (51%, SDR – 3.14 kg COD/cum-day). However, at R/F of 3 (OLR – 6.15 kg COD/cum day; HLR – 3.07 cum (liquid)/cum day; UFAA – 0.035 m/h) showed reduction in the performance (COD removal efficiency – 32%; SDR – 1.97 kg COD/cum-day). Rapid stabilized period was observed in SBBR for every
change in organic loading with respect to COD removal efficiency. Evidently in the periodic batch mode operation, OLR or SDR varied with the cycle period up to the end of the cycle. The SDR and OLR profiles represented a mirror image for all the studied cases representing the fact that SDR was an integral part of the OLR. Variation of SDR/OLR (calculated with transient OLR) and (SDR/OLR)actual (calculated with initial OLR) during sequence phase operation are shown in Fig. 3. Maximum SDR/OLR ratio was obtained for the reactor operation at R/F ratio of 2 compared with the other cases studied. Improvement in biogas yield was observed with the introduction of the recirculation and maximum biogas yield was noticed at R/F ratio of 2 (data not given). The effective mass transfer conditions and the reduction in the prevailing concentration gradient in the reactor height showed positive influence on the biogas production due to introduction of recirculation. Increase in recirculation to 2 resulted in improved performance. However, exceeding certain ratio (>R/F 2) showed retarded performance. R/F ratio of 2 was found to be optimum under the operated conditions for the effective performance of the AnSBBR with this wastewater. The results indicated that the overall substrate removal rate increased as the recirculation ratio increased and the behavior might be attributed to the improvement in the mass transfer phenomena between the substrate present in the bulk liquid and the self-immobilized anaerobic biofilm in the reactor. Among the studied cases, non-recirculation mode operation registered low SDR value. Interestingly a rapid stabilizing period was noticed for the each shift in the recirculation. At non-recirculation mode of operation, steady state performance was established within 8 days of operation, while at R/F of 1, 2 and 3, stable performance was observed within 14, 7 and 12 days, respectively. Rapid stabilizing period evidenced in AnSBBR operation might be attributed to the operation of system in periodic discontinuous operation. Rapid startup period was reported in the literature pertaining to the SBR operation (Wilderer et al., 2001; Rodrigues et al., 2003; Chandrasekhara Rao et al., 2005; Venkata Mohan et al., 2005a,b,c). To analyze the influence of recirculation on the performance, experimental data was fitted into an empirical equation (Eq. (4)) and the first order kinetic model (Eq. (5)) (Rodrigues et al., 2003): eI ¼ ða1 tÞ=ða2 þ tÞ; Rs ¼ dC I =dt ¼ k 1 ðC I C O Þ;
ð4Þ ð5Þ
where a1 and a2 are the empirical parameters representing the maximum value of eI and the time to attain 50% of the maximum value respectively. In Eq. (5), the parameter k1 represents an empirical kinetic coefficient embodied the yield factor and both intrinsic consumption kinetic constant as well as internal and external mass transfer constants (Rodrigues et al., 2003) and Rs denotes substrate removal rate. The profile of substrate concentrations in the reactor and the substrate removal rate (Rs) for the stud-
0.5
a
0.4 5
OLR SDR SDR/OLR actual SDR/OLR
4
0.3
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2 0.1 1 0
0 0
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5 OLR SDR SDR/OLR SDR/OLR actual
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1
SDR/OLR (kg COD/cum-day)
SDR and OLR (kg COD/cum-day)
7
1.2 SDR OLR SDR/OLR SDR/OLR actual
5
6
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7 6
0.4
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SDR/OLR (kg COD/cum-day)
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SDR and OLR (kg COD/cum-day)
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SDR/OLR (kg COD/cum-day)
SDR and OLR (kg COD/cum-day)
S.V. Mohan et al. / Bioresource Technology 98 (2007) 1373–1379
0 0
0.5
2
4
Cycle Period (h)
6
8
12
15
22
24
Cycle Period (h)
Fig. 3. Variation of OLR and SDR with the function of recirculation rates ((a) R/F 0, (b) R/F 1, (c) R/F 2, (d) R/F 3).
8.4 8.2
pH
8 7.8 7.6 R/F 0 R/F 1 R/F 2 R/F 3
7.4 7.2 7 0
0.5
2
4
6
8
12
15
22
24
Cycle Period (h)
24
22
15
12
8
6
4
2
0
0.5
Cycle Period (h) 0
-20
ORP (mV)
ied experimental variations estimated by the first order model Eq. (5) are depicted in Table 2. The values of parameters a1 and a2 obtained from the empirical model (Eq. (4)) and the parameter k1 obtained from the first order model as a function of the recirculation are presented in Table 2. The variation of parameters a2 and k1 as a function of recirculation ratio indicated the stabilization tendency of the reactor at R/F ratio of 2. Variations of outlet pH and ORP during the reactor operation are shown in Fig. 4. In anaerobic process, system pH was considered to be one of the important parameters to decide upon the status of the internal reactor environment (Rittmann and McCarty, 2001; Venkata Mohan et al., 2005b). Extreme variation in pH was not observed during the reactor operation irrespective of the process variation. For all the studied experimental variations pH varied in between 7.65 and 8.20 resulting in slightly alkaline range, which was considered to be effective for anaerobic fermentation. In the case of ORP, the variation was in
-40
R/F 0 R/F 1 R/F 2 R/F 3
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-80
Table 2 Values of parameters derived from empirical model (Eq. (4)) and first order model (Eq. (5)) R/F
ei (%)
a1 (dimensionless)
a2 (min)
k1 (h1)
C0 (mg COD/L)
0 1 2 3
28 ± 0.5 41 ± 0.6 51 ± 0.4 32 ± 0.3
0.28 0.41 0.51 0.32
168 252 384 210
1.978 1.108 0.738 1.634
5760 4720 3920 5440
-100
Fig. 4. Variation of pH and ORP during sequence phase operation with the function of recirculation rates.
the range of 40 mV to 90 mV during the reactor operation, which confirmed the existence of the persistent anaerobic environment in the biofilm reactor. A gradual decrease
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in the ORP values was observed with increase in the recirculation ratio. Production of the volatile organic acids (VFA) is one of the important and general phenomena in the acidogenic step of the anaerobic fermentation (Rittmann and McCarty, 2001). Accumulation of VFA in the reactor environment was normally observed in the case of unfavorable operating conditions and this affected the buffering capacity of the system leading to the process inhibition (Rittmann and McCarty, 2001; Venkata Mohan et al., 2005b). In the case of non-recirculation mode operation, VFA concentration varied inconsistently between 2000 and 4000 mg/l (Fig. 5). This phenomenon might be attributed to the prevailing plug flow and non mixed conditions existing in the system leading to concentration gradient which resulted in incoherent VFA concentration. Variation in VFA concentration was observed with the variation in recirculation. Comparatively low concentration of VFA was observed with R/F ratio of 2. At R/F ratio between 1 and 3, the VFA concentration staggered in and around 4000 mg/l. The relatively low variation in VFA concentration during reactor operation in the recirculation mode operation might be attributed to consumption of VFA which was indicative of effective anaerobic buffering environment. Outlet alkalinity concentration varied between 10 and 70 mg/l during the reactor operation. Consistently higher values of alkalinity were observed with R/F ratio of 2. Higher alkalinity concentration in anaerobic system was essential to maintain the buffering capacity. Comparatively good buffering capacity along with effective SDR was observed with R/F ratio of 2. During non-recirculation mode operation, the alkalinity concentration was quite low indicating disturbed buffering conditions in the reactor
10000 R/F 0 R/F 1 R/F 2 R/F 3
VFA (mg/L)
8000 6000 4000 2000 0 0
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Cycle Period (h)
Alkalinity (mg/L)
60 R/F 0 R/F 1 R/F 2 R/F 3
50 40 30 20 10
due to VFA accumulation and consequence of alkalinity consumption. With every shift in the recirculation rate, a low concentration of alkalinity was observed in the initial stages (for few days). This might be attributed to the consumption of alkalinity due to the shift in substrate loading conditions. This led to disturbance in the anaerobic environment that might result in the accumulation of VFA. Further operation had resulted in rapid recovery of the buffering environment of biofilm reactor. Low VFA concentration during non-recirculation operation mode might be attributed to either the incomplete anaerobic fermentation of wastewater or due to the consumption of the alkalinity (buffering capacity) by the accumulated VFA or both. The reactor also showed considerable utilization of alkalinity to buffer the inconsistent formation and utilization of VFA. With change in the recirculation rate significant variation in reactor operating conditions were observed (Table 1). This was especially evident with the UFVA and HLR. With increase in recirculation rate consistent increase in UFVA was noticed. In the case of HLR also, a consistent increase with the increase in R/F ratio was observed. Due to recirculation, the HLR could be increased without a concomitant increase in the OLR. OLR could be reduced while maintaining the HLR which helps to reduce the substrate concentration in the influent. This is beneficial especially for complex and inhibitory wastewater. Recirculation facilitated linear velocity and this restricted the existence of a concentration gradient and the reactor could be considered as completely mixed. With recirculation operation, complete mixed condition prevailed in the reactor and helped to reduce the induced concentration gradient and increase effective contact with microflora. Recirculation helped to achieve a homogeneous and uniform distribution of substrate along the reactor depth to achieve effective contact with the biomass. This facilitated to reduce the mass transfer limitations between the substrate and the biomass in the reactor. Biodegradation efficiency of the substrate normally relates to the existing mass transfer conditions rates rather than the degradation rates. The hydrodynamic behavior due to recirculation mode of operation facilitated completely mixed flow conditions. This resulted in reduction of substrate inhibition and VFA accumulation through the reactor height facilitating optimum environment (buffering capacity) for proper anaerobic metabolism. The variation in the outlet pH, alkalinity and VFA concentration in combination with SDR was in good agreement with the variation of recirculation rates studied. The study revealed the efficiency of recirculation in biofilm reactor up to certain level of R/F ratio. The recirculation facilitated effective mixing of the reactor volume leading to reduction in substrate concentration gradient and VFA concentration.
0 0
0.5
2
4
6
8
12
15
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4. Conclusions
Cycle Period (h)
Fig. 5. Variation of VFA and alkalinity during sequence phase operation with the function of recirculation rates.
The results obtained from the present studies showed the effectiveness of recirculation as process parameter to
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