Pretreatment of coking wastewater using anaerobic sequencing batch reactor (ASBR)

Pretreatment of coking wastewater using anaerobic sequencing batch reactor (ASBR) 1 first-author †E-mail:Lilibing188@sina.com †E-mail:Lilibing188@sina.com Lilibing188@sina.com 1 2 Institute of Environmental Science and Engineering, Ocean University of China, Qingdao 266003, China Institute of Environmental Science and Engineering, Ocean University of China, Qingdao 266003, China Department of Chemical and Environmental Engineering, Wuyi University, Jiangmen 529020, China Department of Chemical and Environmental Engineering, Wuyi University, Jiangmen 529020, China A laboratory-scale anaerobic sequencing batch reactor (ASBR) was used to pretreat coking wastewater. Inoculated anaerobic granular biomass was acclimated for 225 d to the coking wastewater, and then the biochemical methane potential (BMP) of the coking wastewater in the acclimated granular biomass was measured. At the same time, some fundamental technological factors, such as the filling time and the reacting time ratio ( t f / t r ), the mixing intensity and the intermittent mixing mode, that affect anaerobic pretreatment of coking wastewater with ASBR, were evaluated through orthogonal tests. The COD removal efficiency reached 38%~50% in the stable operation period with the organic loading rate of 0.37~0.54 kg COD/(m 3 ·d) at the optimum conditions of t f / t r , the mixing intensity and the intermittent mixing mode. In addition, the biodegradability of coking wastewater distinctly increased after the pretreatment using ASBR. At the end of the experiment, the microorganism forms on the granulated sludge in the ASBR were observed using SEM (scanning electron microscope) and fluoroscope. The results showed that the dominant microorganism on the granular sludge was Methanosaeta instead of Methanosarcina dominated on the inoculated sludge. Anaerobic sequencing batch reactor (ASBR) Coking wastewater Anaerobic pre-treatment INTRODUCTION 1 Coking wastewater is generated in the production of coke, coal gas, tar and other coke by-products. The coking wastewater includes inorganic pollutants such as ammonia, cyanogen, sulfocyan, heterocycle compounds and polycyclic aromatic compounds such as phenol, oils, naphthalene, pyridine, quinoline and anthracites, which are difficult to biodegradation under aerobic conditions (Ganczarczyk, 1972 1972 ). Discharge of coking wastewater to the environment may cause severe contamination to it and also threaten the normal life of human beings, so it is necessary to treat the coking wastewater in order to reduce its harm to the environment. The conventional biological treatment of coking wastewater is not efficient enough in providing the required quality criteria (Sotton et al., 1999 1999 ; Yu et al., 1997 1997 ). Considerable success in wastewater treatment was achieved by addition of special microorganisms and immobilized microorganisms in laboratory-scale treatment of coking wastewater (Huang et al., 1995 1995 ; Wang et al., 2002 2002 ). However, the maintenance of the dominant species’ bioactivity and regular supplementation of immobilized microorganisms in practical application is a problem, and their feasibility need to be confirmed. Pretreatment was often used to make coking wastewater more amenable to biological treatment (Ganczarczyk, 1980 1980 ) and the common pretreatment approach included equalization and storage, ammonia stripping, chlorination and air flotation. And recently, for treatment of hard to biodegrade wastewater, combinations of anaerobic and aerobic treatments were effective and less expensive, in which A/O (anaerobic-aerobic) and A 2 /O (anaerobic-anoxic-aerobic) were major methods. In these systems, the anaerobic processes serve as pretreatment for partial degradation of some refractory and inhibitory organic compounds to lighten the load of subsequent anoxic and aerobic processes. So far, A/O and A 2 /O are getting popular for treatment of coking wastewater (Zhang et al., 1998 1998 ; Wen et al., 1991 1991 ; Li et al., 2001 2001 ; 2003 2003 ). Studies showed that the anaerobic treatment could greatly improve the biodegradability of the coking wastewater, and created good condition for subsequent aerobic treatment. ASBR is a high rate anaerobic process (US Pat. No. 5 185 079) developed by Dague and co-workers at Lowa State University (Dague et al., 1992 1992 ; Sung and Dague, 1992 1992 ). The promising feature of the ASBR process is that granular biomass can be achieved, and in this way higher biomass can be maintained in the reactor with efficient biomass setting and a long solids retention time (SRT) (Dague et al., 1966 1966 ; 1970 1970 ). There are many reports on studies of ASBR (Herum and Dague, 1993 1993 ; Wirtz and Dague, 1996 1996 ; Sung and Dague, 1995 1995 ) but few of them were on the treatment of actual wastewater (Timur and Özturk, 1997 1997 ; 1999 1999 ). The main purpose of this paper was to study the pretreatment of coking wastewater using ASBR, and the optimization of the ASBR process operating parameters such as the ratio of filling time to reacting time ( t f / t r ), the mixing intensity (MI) and the intermittent mixing mode (IMM) by orthogonal test. For elaborating the experiments plan, we use the method of orthogonal test for three factors at three levels in this research. MATERIALS AND METHODS 1 Coking wastewater 2 The coking wastewater used in this study was collected from the wastewater treatment plant at Taiyuan Coke Company, China. Wastewater characteristics are summarized in Table 1 1 . Samples were stored at 4 °C in sealed containers with minimum headspace to prevent degradation by reaction with oxygen. Parameter Values COD (mg/L) 400~1300 Total phenols (mg/L) 170~232 Cyanide (mg/L) 30~35 TKN (mg/L) 253~714 NH 3 -N (mg/L) 230~668 Sulfides (mg/L) 65~78 pH 7.2~8.6 Experiment procedure 2 1. The ASBR experimental system 3 In this research, tests were conducted using a laboratory-scale ASBR with total volume of 14 L, which provided an effective 12 L liquid volume and 2 L headspace. The cylindrical reactor has inside diameter of 14.8 cm and liquid depth of 69.8 cm. Several 12 mm diameter ports were located along the side of the reactor to enable input and withdrawal of effluent and water samples. The ASBR reactor was installed in an incubator capable of maintaining 35 °C temperature, and had a gas scrubber. A schematic of the ASBR system used in this research is shown in Fig. 1 1 . Mixing was accomplished by recirculating biogas through a diffuser at the bottom of the ASBR. The biogas was recirculated from the top of the reactor through a 12 mm diameter tygon tubing with a homemade gas pump. In addition, an 8-L gasbag was attached to the ASBR headspace to ensure that gas was available during decanting. A foam separation bottle on the effluent gas stream to prevent diffuser clogging as a result of solids carried out of the reactors by the gas stream. The ASBR operating principles were relatively simple as described in a previous publication (Sung and Dague, 1992 1992 ). The reactor sequence included four steps: filling, reacting, settling and decanting. During the filling and reacting steps, the reactor was intermittently mixed through biogas circulation. During the settling step, mixing was shut off to allow biomass solids separation. The decanting step was implemented after sufficient solids separation had occurred. The decant volume was equal to the volume fed during the filling step. In the initial test, 5 L anaerobic granular biomass was seeded; 7 L sucrose solution at concentration of 7 200 mg/L and appropriate quantity of N and P nutrients with the COD: N: P ratio of 300:7:1 introduced into the ASBR reactor, which resulted in biomass concentration of 21.5 g mlvss/L. The system was operated for 20 d under the following conditions: pH of 7.0~8.0, fill influent of 5 L sucrose solution at concentration of 7 200 mg/L and cycling period of 24 h (4 h filling period, 19 h reacting period, 0.5 settling period, 0.5 decant period). 2. The acclimation of granular biomass seed 3 As the anaerobic granular biomass seed was cultivated with high concentration sucrose synthetic wastewater, acclimation was necessary to induce the seeded biomass microorganisms to adapt to the coking wastewater contain not easily degradable compounds. During the acclimation, the ratio of the coking wastewater to the sucrose in the influent was increased correspondingly with the alternative decrease of the total influent concentration, until the influent contained coking wastewater without any sucrose. During this period, the settling effect was so poor that a little small sludge flowed out of the reactor. And the biomass lost by flotation not replaced by new biomass so that in this case, the settling time was extended from 0.5 h to 2.0 h. The acclimation ceased when there was just very little sludge in the effluent while there was still some biomass in the reactor; the sludge had good sedimentation property and the treatment efficiency stabilized. Total acclimation lasted for 225 d. 3. BMP (biochemical methane potential) measurement 3 At the end of coking wastewater acclimation, the biochemical methane potential (BMP) was measured. BMP reflects the amount of organic contaminant in the wastewater that can potentially be converted into methane by anaerobic process. Therefore, BMP can be used to evaluate the efficiency of anaerobic treatment process. BMP was determined at 35 °C. Seeded culture of 100 ml acclimated anaerobic granular biomass and 350 ml coking wastewater (with the COD of 800 mg/L) were introduced into a 500 ml serum bottle anaerobically purged with carbon dioxide and nitrogen gases. Some background inorganic nutrients were introduced into airproofed (with rubber cap) serum bottles (Speece and McCarty, 1964 1964 ) (Table 2 2 ). To eliminate error introduced by the gasses generated by the self-decomposition of anaerobic biomass, control test was needed to correct the error. The control test was prepared according to the same procedure prescribed above, but it was done without any addition of coking wastewater and acclimated anaerobic granular biomass. Fractions Values in reactor (mg/L) NH 4 Cl 400 MgSO 4 ·7H 2 O 400 KCl 400 Na 2 S·9H 2 O 300 CaCl 2 ·2H 2 O 50 (NH 4 ) 2 HPO 4 80 FeCl 2 ·4H 2 O 40 CoCl 2 ·6H 2 O 10 KI 10 (NaPO 3 ) 6 10 MnCl 2 ·4H 2 O 0.5 NH 4 VO 3 0.5 CuCl 2 ·2H 2 O 0.5 ZnCl 2 0.5 AlCl 3 ·6H 2 O 0.5 NaMoO 4 ·2H 2 O 0.5 H 3 BO 3 0.5 NiCl 2 ·6H 2 O 0.5 NaWO 4 ·2H 2 O 0.5 Na 2 SeO 3 0.5 Cysteine 10 NaHCO 3 6000 The gas produced in the anaerobic reactions includes CO 2 and CH 4 . Because CO 2 production does not represent COD reduction under anaerobic conditions, it is necessary to eliminate CO 2 . In this study, 0.1 mol/L sodium hydroxide solution was used as a CO 2 absorbent, so the gas produced in the anaerobic reactions contains CH 4 only. Gas production from all bottles was measured periodically at incubation temperature (35 °C) by the liquid displacement method. 4. Orthogonal test 3 Empirical data obtained during the acclimation procedure revealed that three key factors affect COD removal efficiency were: the ratio of filling time to reacting time ( t f / t r ), the mixing intensity (MI) and the intermittent mixing mode (IMM). In order to select the optimal ASBR operating parameters, orthogonal test was conducted after the acclimation. In the orthogonal test, the operating cycle period, settling period and decant period were 24, 2.0, and 0.5 h respectively, and influent pH was adjusted to 7.0~8.0. The typical operation of ASBR with fast filling time (Dague et al., 1992 1992 ; Sung and Dague, 1992 1992 ) leads to a low ratio of fill time to reaction time ( t f / t r ). But this operating strategy may cause the acid formation problem observed by Suthaker et al.( 1991 1991 ). Considering the toxicity of the coking wastewater and the accumulation of acid resulting from the fast filling, the filling time was extended properly and three levels of the t f / t r was selected as 0.3, 0.5 and 1.0 in this study. As to the mixing intensity, sufficient intensity should be selected to ensure uniform conditions throughout the reactor and effective contact of the wastewater with the granulated biomass to improve the mass transfer flux. While excessive intensity of mixing may shear the anaerobic granular biomass and result in poor solids separation. Usually the mixing was supplied by biogas recirculation or mechanical agitation (when biogas cannot insufficiently provide suitable mixing) (Sung and Dague, 1995 1995 ; Zaiat et al., 2001 2001 ; Rodrigues et al., 2003 2003 ). It was reported that intermittent mixing rather than continuous mixing resulted in superior performance (Dague et al., 1970 1970 ). Therefore, intermittent mixing supplied by biogas was applied in this study. In the experiments plan we use the method of orthogonal test for three factors at three levels. By levels we mean the values taken by the factors. Table 3 3 lists the factors to be studied and the assignment of the corresponding levels. The orthogonal array L 9 (3 4 ) shown in Table 4 4 was chosen with 9 rows corresponding to the number of tests and 4 columns at three levels. t f / t r : The fill time to reaction time ratio IMM: Intermittent mixing mode; MI: Mixing intensity (L biogas/(L reactor volume·min)) 2 Levels 3 Factors t f / t r MI IMM 1 0.3 0.017 30 s/20 min 2 0.5 0.025 30 s/30 min 3 1.0 0.033 100 s/45 min t f / t r : The fill time to reaction time ratio IMM: Intermittent mixing mode; MI: Mixing intensity (L biogas/(L reactor volume·min)) L 9 (3 4 ) test t f / t r MI IMM Blank 1 0.3 0.017 30 s/20 min 1 2 0.3 0.025 30 s/30 min 2 3 0.3 0.033 100 s/45 min 3 4 0.5 0.017 30 s/30 min 3 5 0.5 0.025 100 s/45 min 1 6 0.5 0.033 30 s/20 min 2 7 1.0 0.017 100 s/45 min 2 8 1.0 0.025 30 s/20 min 3 9 1.0 0.033 30 s/30 min 1 5. Stable operation of ASBR 3 The optimal conditions selected through the orthogonal test were applied for stable ASBR operation. And the biomass in the ASBR reactor, the BOD 5 /COD ratio of the influent and the effluent were determined during the stable operation. Analytical methods 2 Soluble COD, BOD 5 were analyzed following the procedures in the standard methods (American Public Health Association, 1992 1992 ), and samples contained no SS (suspended solid). The biomass in ASBR was measured by weighing method. Size classification was achieved by wet sieving in perspex cylinders (Ø 65 mm×100 mm) equipped with a sieve plate. After filling the sieve column with mineral medium and placing the sample on top, it was flushed with nitrogen gas. During gentle intermittent swirling the granules settle throng the sieve plates according to their size. RESULTS AND DISCUSSION 1 BMP assay 2 When there was no biogas generated in both bottles of the sample test and the control test, the BMP test ended. The results of the BMP study are given in Fig. 2 2 showing that the net gas production of 46.3 ml was equal to the sample test gas production volume minus the control test gas production volume. BMP was referenced either to sample volume (m 3 CH 4 /m 3 sample), sample mass (m 3 CH 4 /kg sample) or sample organic content (m 3 CH 4 /kg COD) (Oven et al., 1979 1979 ). The latter method permits direct transfer of results into percent organic matter converted to methane. Since 395 ml methane at 35 °C is equivalent to 1 g of COD removed from the wastewater during the anaerobic process, there is a stoichiometric relationship which allows calculation of the COD reduction in the liquid phase. In the sample test of the BMP assay, 350 ml coking wastewater was assayed with 100 ml of anaerobic granular biomass inoculums. After 130 d of incubation, 248.6 ml CH 4 was produced in all. As to the control test, there were 100 ml anaerobic granular biomass inoculums without the addition of coking wastewater. After only 58 d of incubation, the gas production stopped, and 202.3 ml CH 4 was produced in all. Therefore, the net CH 4 production was 46.3 ml, and the BMP result was 0.165 m 3 CH 4 /kg COD. In theory, 350 ml coking wastewater with COD of 800 mg/L used in the BMP assay should produce 110.6 ml CH 4 at 35 °C under anaerobic conditions based on the stoichiometric relationship mentioned above. Therefore, the percent coking wastewater converted to methane was 41.9%. The BMP gas production curve in Fig. 2 2 showed that in the first 58 d, the gas production rate of the control test was higher than that in the coking wastewater sample test. This showed that the coking wastewater inhibiting effect on anaerobic biomass inoculums, but that the anaerobic microorganisms adapted to coking wastewater gradually after long time acclimation. Acclimation of 225 d was conducted before the BMP assay, which showed that the gas production rate was still low. The gas production lasted for 130 d, which was far more than the 30~60 d suggested for BMP assay. From the result of the BMP assay it could be concluded that the biodegradability of the coking wastewater was rather low. Therefore, long time acclimation of the inoculated anaerobic granular biomass to the coking wastewater is essential for satisfactory treatment. Orthogonal test 2 The test results of the 9 tests arranged by orthogonal array L 9 (3 4 ) are given in Table 5 5 presenting the effect of the various factor levels on the COD removal efficiency. The tests under each testing condition were conducted for 15 d. The average of the last ten days’ COD removal efficiency is given in the last row of Table 5 5 . In the orthogonal test, the error in the response variable (COD removal efficiency) resulted from the fluctuating influent coking wastewater concentration. There are three levels of each factor in Table 5 5 . With every level occurring three times, so the total number of the test was 9. t f / t r : The fill time to reaction time ratio; MI: Mixing intensity (L biogas/(L reactor volume·min)); IMM: Intermittent mixing mode; K jl : The sum of the three results according to the specific level in the row j , here l represents the level, j represents the row. In this paper, for example, K 11 = Y 1 + Y 2 + Y 3 , K 23 = Y 3 + Y 6 + Y 9 , and so on; $$K{\text{ = }}\sum\limits_{l = 1}^3 {K_{jl} } ,{\text{ }}P{\text{ = }}\frac{{\text{1}}}{{\text{9}}}K^2 ,{\text{ }}Q_j {\text{ = }}\frac{{\text{1}}}{{\text{3}}}\sum\limits_{i = 1}^3 {K_{jl}^2 } ,{\text{ }}S_j = Q_j - P,{\text{ }}Q{\text{ = }}\sum\limits_{i = 1}^9 {Y_j^2 } ,{\text{ }}S_{\text{T}} = Q - P$$ , R j =max{ K jl , l =1,2,3}−min{ K jl , l =1, 2, 3} 2 Test 1 2 3 4 Y j (Mean value) t f / t r MI IMM Blank COD removal efficiency 1 0.3 0.017 30 s/20 min 1 16.6 2 0.3 0.025 30 s/30 min 2 15.7 3 0.3 0.033 100 s/45 min 3 15.9 4 0.5 0.017 30 s/30 min 3 25.2 5 0.5 0.025 100 s/45 min 1 35.5 6 0.5 0.033 30 s/20 min 2 27.0 7 1.0 0.017 100 s/45 min 2 20.4 8 1.0 0.025 30 s/20 min 3 19.3 9 1.0 0.033 30 s/30 min 1 25.0 K j 1 48.2 62.2 62.9 77.1 K =200.6 K j 2 87.7 70.5 65.9 63.1 P =4471.2 K j 3 64.7 67.9 71.8 60.4 R j 39.5 8.3 8.9 16.7 Q j 4733.5 4483.3 4484.8 4524.7 Q =4812.7 S j 262.3 12.1 13.6 53.5 S T =341.5 Table 5 5 can be used to analyze the optimal level for every factor. For the factor t f / t r , K 12 > K 13 > K 11 . It shows that level 2 ( t f / t r was 0.5) was the optimal level for the factor of t f / t r . For the factor MI, K 22 > K 23 > K 21 . It indicates that level 2 (mixing intensity was 0.025 L biogas/(L reactor volume·min)) was the optimal level for the factor of mixing intensity. For the factor IMM, K 33 > K 32 > K 31 . It shows that level 3 (intermittent mixing mode was 100 s/45 min) was the optimal level for the factor intermittent mixing mode. In Table 5 5 , S j is the quadratic sum of dispersion factor j . From S j we can determine the importance of different factor. Therefore, the relation S 1 > S 3 > S 2 indicates that the factor t f / t r is the most important parameter affecting treatment efficiency. In addition, R j the range of K j l , and the value of R j reflect the importance of the factor j . Here R 1 > R 3 > R 2 , which indicates that the factor of t f / t r was the most important parameter affecting treatment efficiency. The same result of analysis was obtained by using S j (the quadratic sum of dispersion), so the result from this orthogonal test was credible. As for the intermittent mixing mode, the best value of 100 s/45 min was the biggest of three values (30 s/20 min, 30 s/30 min and 100 s/45 min). So higher IMM values were used for conducting further study which revealing poor effect of sedimentation on biomass in the reactor. Therefore, t f / t r of 0.5, mixing intensity of 0.025 L biogas/(L reactor volume·min) and intermittent mixing mode of 100 s/45 min, were the optimal conditions for the orthogonal test. Stable operation of ASBR 2 According to the orthogonal test, the COD removal efficiency increased as intermittent mixing time increased from 30 s/30 min to 100 s/45 min. So it was estimated that the COD removal efficiency would increase as the intermittent mixing time because larger than 100 s/45 min. However, a 20 d’s experiment operated under condition of larger intermittent mixing time (120 s/45 min) showed that there was a decline of biomass in the ASBR and that the increase in suspended sludge in the effluent was due to the long mixing time. So 100 s/45 min was selected as the optimal intermittent mixing mode. A stable operation of three months was carried out under the optimal conditions stated above. In ASBR, when the influent coking wastewater COD concentration was 796~1304 mg/L, influent ammonia nitrogen concentration was 230~668 mg/L, and influent pH was 7.2~8.6, COD removal rate can steadily reach 38%~50%, which agrees with the BMP determination result. Furthermore, the BOD 5 and COD of the effluent and influent of the coking wastewater to the ASBR were measured, and the sample measured for BOD 5 and COD contained no SS. The ratio of BOD 5 /COD, COD removal efficiency and biomass in the stable operation are given in Table 6 6 . The results indicated that the average ratio of BOD 5 /COD to the influent was 0.27, while the average ratio of BOD 5 /COD to the effluent was up to 0.58, which showed that the biodegradability of the coking wastewater increased greatly after the pretreatment using ASBR. During the stable operation, there was higher biomass to be maintained in the reactor and solids retention time (SRT) was more than 150 d, so there was very little biomass overflowing out of the reactor (Table 6 6 ). Time (d) COD removal efficiency (%) BOD 5 /COD (Influent) BOD 5 /COD (Effluent) Biomass (g/L) Effluent SS (mg/L) SRT (d) 7 41.3 0.28 0.54 23.5 152 154.6 14 42.5 0.29 0.60 23.8 145 164.1 21 39.6 0.32 0.54 23.2 136 169.1 28 41.0 0.31 0.59 23.4 143 163.6 35 44.6 0.31 0.61 23.8 139 171.2 42 50.3 0.29 0.61 23.6 143 165.0 49 45.7 0.23 0.57 23.7 147 161.2 56 44.6 0.23 0.58 24 151 158.9 63 45.1 0.26 0.62 24.1 160 150.6 70 43.5 0.27 0.60 23.9 144 166.0 77 38.6 0.29 0.55 23.7 145 163.4 84 42.9 0.28 0.56 23.6 138 171.0 91 45.8 0.27 0.57 23.8 143 166.4 During the stable ASBR operation, there was a steady-going run and the COD removal efficiency was as high as 38%~50% even with the fluctuating influent of coking wastewater. According to Table 6 6 , the BOD 5 /COD ratio increased greatly. All these results were due to the higher granular biomass maintained in the ASBR, and the granular biomass had good sedimentation property which resulted in the long SRT inductive to treatment efficiency. The different bacteria on the granular biomass are spaced farther a part than the bacteria of the suspended biomass, and can slightly metabolize the intermediate products of the reaction. Granular biomass microorganism 2 The seeded anaerobic granular biomass used in the ASBR had relatively larger particle size, most of which were 1.0~3.0 mm, among which 9.6% were larger than 3 mm. After the granular biomass had acclimated to coking wastewater, the shape of the granular biomass became approximately spheroid, the particle size decreased mostly to about 0.5~2.0 mm, with no particle being larger than 3 mm. Some large particles were bonded together by several small particles. The granular biomass was characterized by softness, tenacity, and viscosity. The predominant bacteria attached to the seeded granular biomass were Methanosarcina both on the surface of the particle and in the middle of the sludge (Tian, 2001 2001 ). After ASBR anaerobic pretreatment of coking wastewater, the microorganisms on the granular biomass in the ASBR was observed using SEM (scanning electron microscope) and fluoroscope. The surface of the granular biomass in the ASBR was densely occupied by many small cavities (Fig. 3a 3a ), which were the passages formed due to the release of gas generated by internal bacteria and to inlet’s substrate entering into the biomass. The observed result showed that the main body of granular biomass was Methanosaeta in the middle part of the granular sludge (Fig. 3b 3b ). A small mass of Methanosarcina was found in the outer hole of the granular biomass (Fig. 3c 3c ). The top parts of Fig. 3b 3b and Fig. 3c 3c are amplification of the bottom parts of the pictures. Methanosaeta and Methanosarcina have different metabolization characteristics ( μ max and k s ), and Methanosaeta has lower substrate affinity than Methanosarcina as mentioned elsewhere (Sekiguchi et al., 1999 1999 ; Wu et al., 1996 1996 ). Many substances in coking wastewater in the influent of the ASBR resist biodegradation and the concentration of influent was low, which were favorable for Methanosaeta instead of Methanosarcina to be the dominant bacteria in the ASBR as observed in this study. CONCLUSION 1 The following conclusions were derived from the results of the tests described above: In this study, the acclimation lasted for 225 d. The long time acclimation of the inoculated anaerobic granular biomass was essential for inducing the microorganisms to adapt to the coking wastewater without disrupting granular biomass activity. The BMP assay indicated that 0.165 m 3 CH 4 /kg COD had been converted into methane in coking wastewater under anaerobic condition. The result of the BMP assay showed that the maximum mechanization of the coking wastewater under anaerobic condition was 41.9%. Analysis of the orthogonal test results showed that when the coking wastewater was pretreated using ASBR, the optimal t f / t r , the mixing intensity and the intermittent mixing mode were 0.5, 0.025 L biogas/(L reactor volume·min) and 100 s/45 min respectively. The cycle of ASBR operation was 24 h, of which the filling time was 7.17 h, the reacting time was 14.33 h, the settling time was 2.0 h and the decant time was 0.5 h. When the OLR ranged from 0.37 to 0.54 g COD/(L·d), the removal efficiency of COD reached 38~50%. The SRT was more than 150 d. There was a great increase of the biodegradability of coking wastewater after its pretreatment with ASBR, and the ratio of BOD 5 /COD also increased from 0.27 in influent to 0.58 in effluent. 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Coke plant wastewater treatment by fixed biofilm system for COD and NH3-N removal 10.1631/jzus.2005.B1115 Project (No. 40272108) supported by the National Natural Science Foundation of China 2005-05-19 2005-08-09 2005-10-28 2022-01-05 Copyright © 2005, Journal of Zhejiang University Science 2005 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 11 6B