Anaerobic treatment of synthetic medium-strength wastewater using a multistage biofilm reactor

Bioresource Technology 100 (2009) 1740–1745

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Anaerobic treatment of synthetic medium-strength wastewater using a multistage biofilm reactor S. Ghaniyari-Benis a, R. Borja b,*, S. Ali Monemian c, V. Goodarzi c a

Department of Chemical and Petroleum Engineering, Sharif University of Technology (SUT), P.O. Box 11365-8639, Tehran, Iran Instituto de la Grasa (C.S.I.C.), Avda. Padre García Tejero, 4, 41012-Sevilla, Spain c School of Chemical Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran b

a r t i c l e

i n f o

Article history: Received 15 July 2008 Received in revised form 19 September 2008 Accepted 22 September 2008 Available online 11 November 2008 Keywords: Multistage biofilm reactor Anaerobic treatment COD removal Volatile fatty acids (VFA) Medium-strength wastewater

a b s t r a c t A laboratory-scale multistage anaerobic biofilm reactor of three compartments with a working volume of 54-L was used for treating a synthetic medium-strength wastewater containing molasses as a carbon source at different influent conditions. The start-up period, stability and performance of this reactor were assessed at mesophilic temperature (35 °C). During the start-up period, pH fluctuations were observed because there was no microbial selection or zoning, but as the experiment progressed, results showed that phase separation had occurred inside the reactor. COD removal percentages of 91.6, 91.6, 90.0 and 88.3 were achieved at organic loading rates of 3.0, 4.5, 6.75 and 9.0 kg COD/m3 day, respectively. A decrease in HRT from 24 to 16 h had no effect on COD removal efficiency. When HRT decreased to 8 h, COD removal efficiency was still 84.9%. Recirculation ratios of 0.5 and 1.0 had no effect on COD removal but other factors such as the volatile fatty acid (VFA) content were affected. The effect of toxic shock was also investigated and results showed that the main advantage of using this bioreactor lies in its compartmentalized structure. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction In recent decades, the role of biological processes in wastewater treatment has increased significantly (Reyes et al., 1999). The efficiency of biological processes depends on the configuration of the reactor, the operating conditions and the wastewater characteristics (Mohan et al., 2007). Recently, anaerobic treatment of industrial wastewater over a wide spectrum of wastewaters has improved (Lettinga et al., 1983; Iza et al., 1991; Milán et al., 2001; Borja et al., 2001; Rovirosa et al., 2004; Sánchez et al., 2005; Cresson et al., 2006; Umaña et al., 2008). One of the most interesting biological processes studied extensively is anaerobic biofilm reactors (Farhan et al., 1997; Parawira et al., 2005). In these systems, biocatalysts such as microorganisms, particulate matters and extracellular polymers exist on the surface of an inert media or adhere to other microorganisms (Hope and Wilson, 2003; Saravanan and Sreekrishnan, 2006). In general, after the adaptation of microorganisms such as sludge or biomass seeding on the media surface in biofilm treatment processes, organic content in wastewater such as COD and VFA will be consumed by biofilm when passing through the reactor (Rodgers et al., 2006). The use of biofilm reactors as a suitable option for wastewater treatment processes has more advantages in comparison with * Corresponding author. Tel.: +34 95 4689 654; fax: +34 95 4691 262. E-mail address: [email protected] (R. Borja). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.09.046

the suspended growth system for various reasons: (1) less sludge production; (2) optimal usage of sludge as a biofilm in the reactor; (3) high solid retention times (SRT) and low hydraulic retention times (HRT); (4) simple and low cost operation; (5) methane production as an energy source and low energy demand; (6) high removal efficiency in organic removal for refractory substances (Mohan et al., 2007; Cresson et al., 2006; Rodgers et al., 2006). The multistage biofilm reactor is a combination of the anaerobic baffled reactor (ABR) and upflow anaerobic fixed bed (UAFB) treatment systems which include the advantages of baffled reactor systems and anaerobic filters. Their properties are: better resilience to hydraulic and organic shock loadings, longer biomass retention times, lower sludge yields, and the ability to partially separate between the various phases of anaerobic catabolism (Barber and Stuckey, 1999). The latter causes a shift in bacterial populations allowing increased protection against toxic materials and higher resistance to changes in environmental parameters such as pH and temperature. The greatest advantage of this type of reactors is, probably, its ability to separate acidogenesis and methanogenesis longitudinally down the reactor, allowing the reactor to behave as a two-phase system without the associated control problems and high costs. Two-phase operation can increase acidogenic and methanogenic activity by a factor of up to four, as acidogenic bacteria accumulate within the first stage and different bacterial groups can develop under more favorable conditions (Barber and Stuckey, 1999).

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The aim of this study was to evaluate the performance and practicability of a multistage anaerobic biofilm reactor composed of three sequential compartments treating synthetic wastewaters containing molasses as a carbon source to evaluate COD removal for different influent conditions. 2. Methods 2.1. Experimental set-up The multistage biofilm reactor was composed of three sequential compartments, which were fabricated from Plexiglas. The reactor dimensions were: 58 cm long, 24 cm wide and 44 cm high, with a total working volume of 54 L. The wastewater had an upflow mode inside each stage. The baffle spacing was determined by keeping the compartments the same size, the ratio between the up-comer and down-comer being 4:1. The baffles inside the reactor were used to direct the flow of wastewater in an upflow mode through a series of compartments where each one formed a packed bed using Pall Rings as a media for supporting the biofilm formation. The main characteristics of Pall Rings as a microorganism support medium were: material, PVC; nominal size, 25 mm; height, 25 mm; thickness, 1 mm; surface area, 206 m2/m3; and porosity, 90%. This kind of packing resulted in increased process efficiency and a decrease in the clogging as reported in previous works (Rajeshwari et al., 2000). The initial porosity of the beds was 77% and after the immobilization of anaerobic cells they had the same porosity (65%). Each compartment of the reactor was filled up to 64% of its active volume with the pall packing. Each compartment was equipped with sampling ports that allowed liquid samples to be withdrawn. A peristaltic pump was used to feed the bioreactor. The reactor was covered with a water jacket keeping the operational temperature at 35.0 ± 0.5 °C. 2.2. Synthetic wastewater The reactor was fed with synthetic wastewater containing molasses as a carbon source. It was made up freshly every day by diluting molasses with tap water to achieve the COD concentration required for each loading rate. The characteristics of the molasses used were: pH, 7.6; COD, 1124 mg/L; BOD5: 411 mg/L; Kjeldahl nitrogen, 16.64 mg/L; total phosphate, 0 mg/L; Ca2+, 59.2 mg/L; K+, 3.1 mg/L; alkalinity, 196 mg/L; total sugars, 47.4%; free sugars, 18.7%; non-fermentable sugars, 6%; total dissolved solids (TDS), 38%. These values summarize the main features of the molasses obtained by diluting 1 g of raw molasses into 1 L of distilled water. The COD:N ratio of the wastewater used was 67:1. During the start-up period, urea and ammonium phosphate were used as sources of nitrogen and phosphorus, respectively. A total dose of 925 mL of a micronutrient and trace metal solution was only added at the beginning of the start-up period of the reactor. The composition of this micronutrient and trace metal solution was: CoCl2
6H2O, 0.25 mg/L; H3BO3, 0.05 mg/L; FeCl2
2H2O, 2 mg/L; MnCl2
4H2O, 0.5 mg/L; ZnCl2, 0.05 mg/L; CuCl2, 0.15 mg/ L; Na2MoO4
2H2O, 0.01 mg/L; NiCl2
6H2O, 0.01 mg/L; Na2SeO3, 0.01 mg/L; AlCl3
6H2O, 0.05 mg/L; MgCl2, 1 mg/L; MgSO4
7H2O, 0.3 mg/L; CaCl2
2H2O, 0.18 mg/L. These nutritious substances were used to favor the growth of the biofilm on the surface media. During the start-up period, COD:N:P ratio was 100:5:1. When a steady-state condition was achieved, the COD:N:P ratio changed to 350:5:1. In order to prevent the build-up of localized acid zone in the reactor, sodium bicarbonate was used for supplementing the alkalinity. NaHCO3 is the only chemical which gently shifts the equilibrium to the desired value without disturbing the physical

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and chemical balance of the sensitive microbial population (Metcalf and Eddy, inc., 2003). 2.3. Seeding and experimental procedure The microorganisms used as inoculum in the reactor were originated from the sludge of the ABR system treating non-alcoholic beer wastewater of the Berinuscher Company, located in Shiraz, Iran. The basic characteristics of the anaerobic inoculum used were: total acidity, 178 g acetic acid/m3; total solid content, 69.5 kg/m3; volatile solid content, 28.3 kg/m3; bicarbonate alkalinity, 1374 g CaCO3/m3; and pH, 7.3. A total volume of 19 L of the above-mentioned inoculum was added to the reactor and distributed among compartments before starting the experiments. 2.4. Analytical methods The COD concentration was measured by using a semi-micro method (Soto et al., 1989). Alkalinity was determined in accordance with standard methods (APHA, 1998). The concentration of VFA was determined by using HPLC according to Björnsson et al. (2000). 3. Results and discussion 3.1. Start-up period At the beginning of the start-up, the reactor was run in a batch mode. During this time, sludge was acclimated to the synthetic wastewater by using influent COD concentrations in the range of 0.5 to 1.5 g/L. This initial period lasted 45 days. The continuous operation of the system was started using an initial COD concentration of 3000 mg/L at a HRT of 2 days, which was equivalent to an organic loading rate (OLR) of 1.5 kg COD/m3 day. A COD removal efficiency of 70% was achieved at this level. When there was no fluctuation in different parameters such as COD and VFA in each compartment, then the OLR increased to 3 kg COD/m3 day (HRT = 1 day) as the input flow-rate increased. The reactor was operated at this OLR for 45 days. A COD removal efficiency of 91.6% was achieved at this OLR. An alkalinity value of 900 mg/L in the form of CaCO3 was added at this stage. COD removal profile and pH variations trend were monitored during this period. It was observed that the COD decreased from 980 to 540 mg/L, from 710 to 340 mg/L and from 460 to 250 mg/ L in the compartments 1, 2 and 3, respectively. As could also be observed, there were some irregularities in the pH value variations during the first few days, but as time went by microbial selection and zoning were encouraged inside the reactor, with the acidogenesis in compartments closer to the inlet. Specifically, pH values ranged between 6.5–6.8, 6.4–7.3 and 6.5–7.6 in compartments 1, 2 and 3, respectively, during this start-up period (45 days). 3.2. Effect of decreasing HRT This part of the research studies the reduction effect of HRT on the system performance. The reactor was fed with diluted molasses containing 3000 mg COD/L at two different HRTs of 16 h and 8 h coinciding with organic loading rates of 4.5 and 9 kg COD/m3 day, respectively. The COD and VFA concentration changes in the reactor were monitored through each load. Fig. 1 shows the COD and VFA concentration variations profiles in all compartments for the HRTs of 16 and 8 h, respectively. No matter how much the HRT was reduced, the COD concentration in each compartment increased. The same pattern occurred for VFA values as shown in Fig. 1. For each HRT studied, steady-state

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1000

HRT=16h

900

HRT=8h

1400

800

1200

700 600

1000

500

800

400

600

300

VFA (mg/L)

COD (mg/L)

1600

400

200

200

100 0

0 0

5

10

15

20

Time (day)

COD,com1 COD,com2 COD,com3

25

30

VFA,com 1 VFA,com 2 VFA,com 3

Fig. 1. COD and VFA concentration variations profiles at COD = 3000 mg/L and HRTs of 16 and 8 h.

performance was marked by near constant effluent VFA values and COD values with less than 5% fluctuation. From these figures it can be seen that as time progressed, the response of the reactor to a decrease in HRT became quite rapid. This decrease in the response time was presumably due to the biomass acclimatization to the higher loads as the fraction of active biomass in the reactor increased. The average values of the above parameters (COD and VFA) obtained in the five samples were regarded as the steady-state performance for the HRT under consideration. Table 1 shows the COD removal (%) and steady-state VFA contents for three HRTs of 24, 16 and 8 h. COD removal efficiency of 91.6% was achieved at HRTs of 24 and 16 h, decreasing for a HRT of 8 h which gave a COD removal efficiency of 84.9%. The best performance was observed at a HRT of 16 h (or a loading of 4.5 kg COD/m3 day). When the HRT decreased to 8 h, the performance of the reactor dropped, but the removal rates were still comparatively good. The relatively poor performance observed at a HRT of 8 h was attributed principally to the instability created by the sudden doubling of the influent loading rate. COD removal efficiency decreased from 79% to 69% when the HRT diminished from 48 h to 18 h in a carried anaerobic baffled reactor (ABR) treating sewage at 28 ± 1 °C (Feng et al., in press). This reactor was rectangular and contained six chambers of equal volume, the effective reactor volume being 17 L. The six upcomer regions of this reactor were filled with hollow-sphere carriers made of bamboo (diameter 15 cm approximatly) in settled form. Another modified ABR consisted of three 3.6 L chambers (the first one was a UASB without a gas–solid–liquid separator. The second one was a down-flow fixed film reactor with plastic

media, while the third one was a hybrid UASB-AF with plastic Pall Ring media) was used for treating pre-settled municipal wastewater at ambient temperature (18–28 °C) (Yu and Anderson, 1996). In this study, COD removal decreased from 67.8% to 52.3% when the HRT decreased from 4 h to 2 h. Some other studies have demonstrated that the removal efficiency of this type of reactor was determined by HRT but not by liquid upflow velocity like UASB or EGSB, which may have been caused by differences in their structures (Barber and Stuckey, 1999). Table 1 also shows the VFA production in the different compartments. For all HRTs the VFA production in the first compartment was significantly greater than that in other compartments and it decreased from input to output. High level concentration of VFA at the shorter HRT can be attributed to high organic loading rate. In addition, short HRT promoted the accumulation of intermediate products such as VFA. The higher concentrations of VFA at the shorter HRT are also represented in lower COD removal as shown in Table 1. The effluent VFA concentration at the longest HRT (24 h) was 223 mg/L as acetic acid, which increased to 458 mg/L at the lowest HRT (8 h). VFA concentrations in effluents of an anaerobic migrating blanket reactor (AMBR) increased from 25 to 182 mg/L as the HRT decreased from 10.3 days to 1 day treating synthetic wastewater containing glucose as a carbon source (Kuscu and Sponza, in press). This AMBR reactor consisted of a rectangular tank with an active volume of 13.5 L, which was divided into three compartments, which were mixed equally every 15 min at 60 rpm to ensure gentle mixing. In the present work, effluent VFA concentrations at retention times of 16 h and 24 h were very similar, especially in compartments 2 and 3. This similarity is also reflected in equal effluent COD concentrations as shown in Table 1. It was found that most of the influent COD content was removed in compartment 1 (73.7–81.5%), with a lower reduction occurring in compartment 2 (5.5–8.6%) and in compartment 3, the remaining fraction of influent COD (3.3–5.7%) being removed as shown in Table 1. Compartment-wise profiles in an eight chambered ABR treating low-strength soluble wastewater (COD = 500 mg/L) also indicated that most of the COD and BOD5 were reduced in the initial compartments only (Gopala-Krishna et al., in press). The present system was also found to be very efficient at retaining biomass. Effluent suspended solids (SS) were found to increase with a decrease in HRT. In any case, the effluent suspended solids concentration was relatively low for all HRT applied (Fig. 2). Similar SS concentrations were also observed in the effluents of the above-mentioned modified ABR treating pre-settled municipal wastewater at HRTs higher than 4 h and ambient temperature (18–28 °C) (Yu and Anderson, 1996). Throughout the experiment, pH values increased from input to output. The pH decrease in compartments 1 and 2 can be attributed to the fact that high concentrations of VFA were present in these compartments. pH values at different HRTs remained reason-

Table 1 Variations of the COD removal (%), VFA concentration and total percentage of COD removed in the three compartments with HRT Compartments

HRT (h)

Parameters COD removal (%)

VFA concentration (mg/L)

Total percentage of COD removed (%)

1

24 16 8 24 16 8 24 16 8

81.5 78.6 73.7 88.3 87.2 79.2 91.6 91.6 84.9

913 1154 1258 371 458 959 223 228 458

81.5 78.6 73.7 6.8 8.6 5.5 3.3 4.3 5.7

2

3

1743

HRT=16h

3000

HRT=8h

2500

10

15

20

25

30

COD=4500

COD=3000

350 HRT=24 300 h 250 200 150 100 50 0 5 0

VFA (mg/L)

SS(mg/L)

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2000 1500 1000 500

35

Time(day)

0

0

5

10

Fig. 2. Influence of HRT on effluent suspended solids (SS).

ably stable in the range of 6.4–7.7 within the reactor. The required alkaline level in the form of CaCO3 was 900 mg/L for the three HRTs considered. However, it is thought that the pH values in these compartments were maintained at a constant level due to the effective consumption of VFA. All these observations suggest that the present system promoted a systematic selection and concentration of specific microorganisms in the different compartments in such a manner as to bring about phase separation. Microscopic observations carried out in an anaerobic rotating biological contactor (AnRBC) consisted of four compartments, each one containing 15 12 cm diameter acrylic plastic disks, showing that the acetogenic microorganisms were predominant in the first two compartments, while the methanogenic microorganisms were predominant in the last two (Yeh et al., 1997). The volatile fatty acid profile observed in an eight chambered ABR treating complex wastewater made of cellulose and sucrose (40:60) also demonstrated that hydrolysis and acidogenesis are the main biochemical activities in the first few compartments (Gopala-Krishna et al., 2007). Based on the observed low concentrations of remaining VFA and high reactor performance, it can be concluded that the multistage biofilm reactor used in the present work is a system that can maintain active methanogens by keeping the VFA concentration low, especially at longer HRTs. This allows for high conversion of organic matter to the final end product, methane, without significant accumulation of intermediate products. 3.3. Effect of increasing of COD concentration of influent In this phase of the experiments, the effect of different OLRs was studied by varying the COD of the influent substrate at a constant retention time. The reactor was fed with diluted molasses containing 3000, 4500 and 6000 mg/L at a constant HRT of 16 h. The amount of COD reduction and VFA concentration changing profiles were shown in Figs. 3 and 4, respectively. The results showed that

2500 COD=3000

1500

COD=6000

25

30

35

com2

com3

an increase in OLR brought about an accumulation of VFA in the reactor. However, the reactor showed stable operation at OLRs of 6.75 and 9 kg COD/m3 day. COD values fluctuated transiently following each step increase in OLR but stabilized soon after the first two or three days. The highest VFA concentration was always found in the first compartment. A transient increase in VFA concentration was also observed in response to each step increase in OLR, but this declined after a few days when the reactor was acclimatized to the new feeding regime. An increase in influent COD resulted in a slight decrease in COD removal efficiency but variations in the total COD removal were insignificant. For example, COD removal efficiencies of 91.6%, 90.0% and 88.3% were achieved at OLRs of 4.5, 6.75 and 9.0 kg COD/m3 day, respectively, corresponding to influent COD concentrations of 3000, 4500 and 6000 mg/L, respectively. COD removal efficiencies of around 88% were also observed in the abovementioned eight chambered ABR treating complex wastewater when the OLR was increased from 0.6 to 2.0 kg COD/m3 day (Gopala-Krishna et al., 2007), with OLR values considerably lower than those applied in the present work. During this step, the alkalinity requirement increased as influent COD increased. To maintain the pH at a suitable range, alkalinity requirements were 1350 and 1800 mg CaCO3/L for COD values of 4500 and 6000 mg/L, respectively. The low requirements of alkalinity in this study showed that high bicarbonate alkalinity was generated in the reactor. Fig. 5 shows that the alkalinity increased in the first compartment and throughout the length of the reactor in all conditions. The primary compartments of the reactor generally produced more biomass than the last compartments. This may be attributed to the fact that the growth rate of acidogenesis bacteria is greater than that of methanogens and that acidogenesis reactions predominated in compartments 1 and 2. In ABRs, various profiles of microbial communities may develop within each compartment (Barber and Stuckey, 1999). The microbial ecology within each chamber of the reactor will depend on the type and amount of substrate

3000

1000 500 0 0

20

Fig. 4. VFA profile at COD = 3000, 4500 and 6000 mg/L (at HRT of 16 h).

5

10

15

20

25

30

35

Time (day) comp1

comp2

Alkalinity (mg/L)

COD (mg/L)

COD=4500

15

Time (day) com1

2000

COD=6000

2500

HRT=24h HRT=16h HRT=8h COD=4500mg/l COD=6000mg/l

2000 1500 1000 500 0

com3

Fig. 3. COD profile at COD = 3000, 4500 and 6000 mg/L (at HRT of 16 h).

compartment 1 compartment 2 compartment 3 Fig. 5. Alkalinity values in all compartments.

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present, as well as external parameters such as temperature and pH. In the acidification zone of the ABR (first compartment(s) of reactor), fast growing bacteria capable of growth at high substrate levels and reduced pH will dominate. A shift to slower growing scavenging bacteria that grow better at higher pH will occur towards the end of the reactor (Barber and Stuckey, 1999). 3.4. Effect of recycling effluent

700

R=0

COD (mg/L)

R=0.5

R=1

500 400 300 200 100 0

0

5

10

15

20

Time (day) com1

900 800 700 600

R=0 0

500

R=0.5

R=1

400 300 200

In this phase of the study, the effect of recycling on the reactor performance was investigated. At a loading rate of 3 kg COD/m3 day (HRT = 24h, COD = 3000 mg/L), two recycle ratios (effluent recirculation: fresh feed) of 1:2 (R = 0.5) and 1:1 (R = 1) were introduced into the reactor input. Figs. 6 and 7 show the variation trend of COD and VFA throughout this step, respectively. As can be seen in Fig. 6, COD removal values are almost equal but Fig. 7 shows different values of VFA for two different recycle ratios and one without recycle. VFA contents were less with recycle and VFA decreased as the recycle ratio increased. Therefore, it can be concluded that the COD contents in the effluent of the reactors with recycle flow was not only VFA, but also residual degradable matter. In fact, the remaining COD in the effluent contained less VFA and more residual substrate because recycles bring about high flow rates that probably cause more channeling through the biomass bed, resulting in poor substrate-biomass contact and less degradation of the incoming COD. In addition, part of the COD exiting the reactor could also be explained by the suspended solids (biomass) escaping. Sometimes recycling the effluent stream in ABRs reduces removal efficiency because the reactor approaches a completely mixed system, and therefore the mass transfer driving force for substrate removal decreases despite a small increase in the loading rate (Barber and Stuckey, 1999). According to theory, recycling should have a negative effect on reactor hydrodynamics because it causes increased mixing (which encourages the loss of solids, and disrupts microstructures of bacteria living in symbiotic relationships (Barber and Stuckey, 1999). On the other hand, both recycle ratios were high enough to maintain the system pH in a suitable range without alkalinity supplementation. The need for a recycle was suggested by Sam-Soon et al. (1991). By introducing a recycle from the effluent to the influent, the alkalinity from effluent was recovered; accordingly it reduced the effective influent COD concentration as well as the alkalinity requirement per influent COD. Without imposing a recycle, maintaining the suitable pH in the system would need some alkalinity requirements. Other benefits of recycle are the dilution of toxicants and the reduction of substrate inhibition in the influent. Moreover, higher loading rates and better substrate/biomass contact can be achieved using the appropriate recycle ratios

600

1000

VFA (mg/L)

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com2

com3

Fig. 6. Influence of recycle ratio (R) on COD profile in all compartments.

100 0 0

5

10

15

20

Time (day) com1

com 2

com 3

Fig. 7. Influence of recycle ratio (R) on VFA profile in all compartments.

(Barber and Stuckey, 1999). A modified ABR divided into five compartments, operating with a recycle ratio of 30, was found to be ideal for treating palm oil mill effluent (COD = 16000 mg/L). Under steady-state conditions at HRTs from 3 to 10 days, organic removal efficiencies in the range of 77.3–95.3% were achieved on a COD basis and 72.1–95.9% on TOC basis (Faisal and Hajime Unno, 2001). 3.5. Effect of toxic shock At this stage of the experiments, a toxic shock was applied to the influent of the reactor. The toxic matter was furfural. Furfural and other aldehydes are common constituents of many industrial effluents resulting from chemical operations. Furfural is known as an inhibitor of biological reactions influencing the operation of wastewater treatment plants. Other relevant works had reported anaerobic treatment of a furfural-base wastewater in a standard upflow filter and an upflow blanket filter. Both reactors were able to remove 90% of the influent COD up to OLR of 23 kg COD/m3 day (Randall et al., 1993). For the present study, the reactor was at steady-state operation at feed concentration of 3000 mg/L and HRT of 16 h. To apply the toxic shock to the system performance, reactor was fed with a wastewater containing molasses and furfural. The COD concentration was kept constant at 3000 mg/L. The furfural concentration corresponded to 640 mg/L. The shock load was kept constant for 6 h and then the reactor was returned to initial conditions. The process performance was monitored until the prior COD removal was achieved. COD removal decreased during the initial 12 h after applying toxic shock in all compartments. Steady-state COD removal in compartments 2 and 3 and total COD removal were recovered after 70 h while the COD removal at first compartment had not been recovered after 80 h. This suggested that the main advantage of using the system comes from its compartmentalized structure. The first compartment of the reactor may act as a buffer zone to all toxic and inhibitory material in the feed thus allowing the latter compartments to be loaded with a relatively harmless, balanced and mostly acidified influent. In this respect, the latter compartments would be more likely to support active populations of the relatively sensitive methanogenesis. VFA variation was also monitored during this period. As expected, VFA decreased when toxic shock occurred because of the inhibitory effect of furfural on acidogenic bacteria. Specifically, VFA decreased from 1150 to 901 mg/L, from 470 to 285 mg/L and from 219 to 113 mg/L during the first 4, 8 and 12 h in the compartments 1, 2 and 3, respectively. It then increased up to its initial steady-state value. The biodegradation of 3-monochlorophenol (3-MCP), a toxic compound present in polluted waters, was investigated by ABRs (Farrokhi and Mesdaghinia, 2007). A mixture of 3-MCP and glucose

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as synthetic wastewater was treated in a 9-L laboratory-scale ABR at a HRT of 2 days. When the concentration of 3-MCP was 10 mg/L or less, 3-MCP was able to be metabolized to mineral end products and the COD removal was about 90% or more. By increasing the 3MCP concentration from 10 to 30 mg/L, the degradation of 3-MCP and COD removal decreased significantly. An increase of COD to 2000 mg/L, improved the reactor performance and reduced the toxicity of this compound (Farrokhi and Mesdaghinia, 2007). 4. Conclusions The following conclusions can be drawn from this study: 1. The multistage biofilm reactor proved to be an efficient reactor configuration for the treatment of medium-strength synthetic wastewater. For an OLR of 9 kg COD/m3 day, the molassesbased wastewater was treated with 88.3% COD removal efficiency. 2. VFA values decreased and pH values increased from input to output throughout the length of the reactor. This suggested that phase separation, microbial selection and zoning were encouraged inside the reactor by compartmentalization. 3. Decreasing HRT from 24 h to 16 h had no effect on COD removal efficiency. However, by decreasing it to 8 h, COD removal efficiency decreased up to a value still comparatively good (84.9%). 4. Recycle had no effect on COD removal efficiency, but VFA were lower than when there was recycle and VFA decreased as the recycle ratio increased. Rapid acidification of molasses-based wastewater in the first compartment of the reactor brought about the need for a close control on alkalinity. Introducing recycle reduced the effective influent COD concentration and also alkalinity requirement per influent COD. 5. This reactor showed high resistance and good recovery when toxic shock was applied. Acknowledgements The authors gratefully acknowledge the financial support of the Water Research Center of Greentech (Co. Ltd.), Shiraz, Iran. The authors also thank Dr. Anahita Parsnejad for her help. References APHA, 1998. Standard Methods for the Examination of Water and Wastewater (20th ed.). American Public Health Association/American Water Works Association/ Water Environment Federation, Washington, DC, USA. Barber, W.P., Stuckey, D.C., 1999. The use of the anaerobic baffled reactor (ABR) for wastewater treatment: a review. Water Res. 33 (7), 1559–1578. Björnsson, L., Murto, M., Mattiasson, B., 2000. Evaluation of parameters for monitoring an anaerobic co-digestion process. Appl. Microbiol. Biotechnol. 54 (6), 844–849. Borja, R., González, E., Raposo, F., Millán, F., Martín, A., 2001. Performance evaluation of a mesophilic anaerobic fluidized-bed reactor treating wastewater derived from the production of proteins from extracted sunflower flour. Bioresour. Technol. 76 (1), 45–52. Cresson, R., Carrère, H., Delgenès, J.P., Bernet, N., 2006. Biofilm formation during the start-up period of an anaerobic biofilm reactor—Impact of nutrient complementation. Biochem. Eng. J. 30 (1), 55–62. Faisal, M., Hajime Unno, H., 2001. Kinetic analysis of palm oil mill wastewater treatment by a modified anaerobic baffled reactor. Biochem. Eng. J. 9 (1), 25–31.

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