A pilot scale trickling filter with pebble gravel as media and its performance to remove chemical oxygen demand from synthetic brewery wastewater

A pilot scale trickling filter with pebble gravel as media and its performance to remove chemical oxygen demand from synthetic brewery wastewater first-author †E-mail:haimanot.lemji@uni-rostock.de †E-mail:haimanot.lemji@uni-rostock.de haimanot.lemji@uni-rostock.de Faculty of Agricultural and Environmental Sciences, Institute of Hydromechanics and Water Management, University of Rostock, 18055 Rostock, Germany Faculty of Agricultural and Environmental Sciences, Institute of Hydromechanics and Water Management, University of Rostock, 18055 Rostock, Germany Evaluating the performance of a biotrickling filter for the treatment of wastewaters produced by a company manufacturing beer was the aim of this study. A pilot scale trickling filter filled with gravel was used as the experimental biofilter. Pilot scale plant experiments were made to evaluate the performance of the trickling filter aerobic and anaerobic biofilm systems for removal of chemical oxygen demand (COD) and nutrients from synthetic brewery wastewater. Performance evaluation data of the trickling filter were generated under different experimental conditions. The trickling filter had an average efficiency of (86.81±6.95)% as the hydraulic loading rate increased from 4.0 to 6.4 m 3 /(m 2 ∙d). Various COD concentrations were used to adjust organic loading rates from 1.5 to 4.5 kg COD/(m 3 ∙d). An average COD removal efficiency of (85.10±6.40)% was achieved in all wastewater concentrations at a hydraulic loading of 6.4 m 3 /(m 2 ∙d). The results lead to a design organic load of 1.5 kg COD/(m 3 ∙d) to reach an effluent COD in the range of 50–120 mg/L. As can be concluded from the results of this study, organic substances in brewery wastewater can be handled in a cost-effective and environmentally friendly manner using the gravel-filled trickling filter. Biodegradation Pilot scale trickling filter Aerobic treatment Brewery wastewater Chemical oxygen demand (COD) Trickling filter performance Introduction 1 Industrial discharge into rivers is one cause of irreversible degradation occurring in surface water systems (Rajaram and Ashutosh, 2008 2008 ). Due to their role in carrying off industrial wastewater, rivers are among the most vulnerable water bodies to pollution. There have been significant impairments to rivers from pollutants, rendering the water unsuitable for beneficial purposes, such as domestic use, irrigating agricultural lands, recreation, drinking, wildlife propagation, and food processing purposes in industries; all of these uses are on the rise, particularly in developing urban areas. With increasing scarcity of a treated public water supply, fresh river water has become an alternative source for these purposes (van der Bruggen and Braeken, 2006 2006 ). “Untreated brewing effluent poses a significant treat to surface and ground water qualities.” The main brewery effluent sources include losses during bottle filling, cleaning (of returned bottles, fermentation and conditioning tank, vat and floors) and draining tank bottoms (Yu and Gu, 1996 1996 ; Driessen and Vereijken, 2003 2003 ). Untreated brewery effluent typically contains total suspended solids (TSS) (200–1 000 mg/L), biochemical oxygen demand (BOD) (1 200–3 600 mg/L), chemical oxygen demand (COD) (2 000–6 000 mg/L), nitrogen (N) (25–80 mg/L), and phosphorus (P) (10–50 mg/L), with temperature in the range of 18–40 °C and a pH between 3–12 (Driessen and Vereijken, 2003 2003 ; IFC, 2007 2007 ). The high organic content of brewery effluent classifies it as a very high-strength waste, meaning that the brewery effluent cannot simply be discharged into sewers or water courses (Huei, 2005 2005 ). Direct discharge can bring about a rapid deterioration of the physical, chemical, and biological qualities of the receiving water bodies (The Breweries of Europe, 2002 2002 ; Parawira et al., 2005 2005 ; Al-Rekabi et al., 2007 2007 ). The decomposition of organic matter depletes the amount of dissolved oxygen in the water that is vital for aquatic life. Release of nitrogen and phosphorous compounds in the wastewater also stimulates aquatic plant growth contributing to eutrophication of water bodies. Furthermore, turbidity and color reduce the penetration of light, which, in turn, affects photosynthesis, thereby affecting the primary link in the food chain. The removal of organic compounds from the wastewater is important to avoid anaerobic conditions in the receiving waters. Nutrients like N and P should also be removed to avoid algal blooms that disturb the ecosystem of the receiving waters (Driessen and Vereijken, 2003 2003 ). Sequence batch reactor (SBR), upflow anaerobic sludge blanket (UASB) or conventional methane-oriented anaerobic digestion, activated sludge process, membrane process, and stabilization ponds have been extensively used for the treatment of different wastewaters. However, it has been found that these processes are affected by the presence of ammonium nitrogen (NH 4 + -N). NH 4 + -N in the wastewater can be removed by raising the pH value and then air-stripping the wastewater; however, this method is sometimes quite ineffective, depending on the operational parameters used and water quality, and the cost is usually prohibitively high, especially in developing countries (Zhao, 1999 1999 ; 2001 2001 ; Henry and Prasad, 2000 2000 ; Zhao et al., 2000 2000 ; Youcai et al., 2002 2002 ). It is also well reported in the literature that in a complex wastewater treatment system, activated sludge normally exhibits poor settleability; as such, fixed film systems that would involve trickling filters, rotating biological contactors, et cetera, are recommended (Zurchin et al., 1986 1986 ). In this study, a detailed investigation aimed at analyzing the performance of gravel-filled, naturally aerated trickling filter on brewing industry wastewater. Additionally, we aimed to demonstrate the use of a trickling filter filled with gravel as an alternative biological process over conventional activated sludge process, especially for low income countries. The present study is the first report on COD biological reduction in a pilot scale trickling filter using a mixed culture of microorganisms, originating from industrial wastewater sludge. Materials and methods 1 Wastewater characterization 2 The wastewater was first subjected to pH adjustment using NaOH and HNO 3 , roughing screens, and defoaming by the brewing company. The wastewater samples were then analyzed for COD, 5-d BOD (BOD 5 ), nitrate nitrogen (NO 3 -N), nitrite nitrogen (NO 2 -N), ammonium nitrogen (NH 4 + -N), total nitrogen (TN), phosphate (PO 4 3− ), and total phosphorus (TP) calorimetrically. Prior to analysis, the soluble fraction of the wastewater samples was obtained by filtering with a syringe filter of 25 mm diameter (W/0.45 μm cellulose) for analyses of NO 3 -N, NO 2 -N, NH 4 + -N, and PO 4 3− . An unfiltered wastewater sample was used for analyses of COD, TN, and TP. Pilot scale trickling filter 2 The schematic diagram of the pilot scale trickling filter is shown in Fig. 1 1 . The pilot scale trickling filter consisted of a Plexiglas tube with an inner diameter of 40 cm and a total height of 180 cm. Sampling ports are located at fixed intervals of 260 mm along the height of the biofilter. At the top of the filter, a fixed flow distributor was installed to facilitate a uniform distribution of the wastewater fed to the filter’s free surface. The wastewater was fed to the reactor after being stirred in a wastewater reservoir. Moreover, a secondary clarifier was installed to collect and settle the effluent from the filter’s draining system. The supernatant from the secondary clarifier was collected in a 10-L container for recirculation via a recirculation pump. 1: wastewater reservoir; 2: fed pump; 3: trickling filter; 4: sampling ports; 5: draining pump; 6: secondary clarifier; 7: clarified water; 8: recirculation pump; 9: recirculation The supporting material for bacterial growth was pebble gravel purchased from a gravel producer. The diameter of the filter stones, as obtained from the sieve analysis, was in the range of 4–6 cm. At the surface of the biofilter, a smaller sized stone (about 1.6–3.2 cm) that amounts to 5% of the total filter media was placed to get a larger surface area for bacterial attachment. The filter had a specific surface area and void ratio of 71.83 m 2 /m 3 and 45%, respectively. The total depth of the filter media was 160 cm including the support stones at the base of the reactor. Since the placement of the filter media is of paramount importance to the efficiency of the percolating filter, it was carried out with particular care and under proper control. Filter media were placed without prolonged intermediate storage. After the trickling filter was filled with the filter media, the packed filter media were washed with a sufficiently large volume of tap water and checked for flooding, flow rates, and hydraulic loading rates of the operation (flow rates up to 556 ml/min or hydraulic loadings up to 637 ml/(cm 2 ·d)). The other purpose of washing was to remove sands. Ventilation of the trickling filter 2 The trickling filter was aerated naturally. Passive devices for ventilation of the trickling filter were presented in the form of vent stacks on the trickling filter periphery, extensions of underdrains through trickling filter sidewalls (the underdrain is angled to admit air). The other techniques of aeration in the trickling filter were ventilating manholes, louvers on the sidewall of the tower near the underdrain, and discharge of trickling effluent to outside in an open channel or partially filled pipes. Also the underdrain was kept always half filled. Sludge control mechanism 2 As backwashing was not suitable for this type of bioreactor to control the sludge, the trickling filter was washed by pumping 10 L of 0.1 mol/L NaOH solution repeatedly for the first time after the trickling filter operated for 22 d, and then it was washed every two weeks. To bring the microbial environment near neutral, the reactor was subsequently washed with tap water. The other operation done on the trickling filter to reduce excess biomass was trickling filter starvation for a few days by turning off both the influent flow and recirculation flow for 3 to 4 d. Instrumentation and analytical methods 2 Biomass concentrations were determined by weighing dried (24 h, 105 °C) 100 ml samples of the liquid phase. A spectrophotometer (Hach Lange Xion 500 LPG385) was employed for the measurement of COD and nutrients. Thermostatically controlled incubators with standard/glass door were employed for the measurement of BOD 5 . Microprocessor-controlled standard-pH-ion-meter pMX 3000/pH was used to measure the pH values and temperatures of the influent and effluent. The influent and effluent samples collected and kept in a refrigerator were analyzed for the selected parameters using the Dr. Lange cuvette test system. During all experiments, measurements of pH, concentrations of COD, NH 4 + -N, PO 4 3− -P, and TN, and temperature were made three times per week unless otherwise indicated. COD removal efficiencies of the trickling filter can be calculated based on the reduction of COD concentration between the influent and effluent streams as shown in Eq. ( 1 1 ): $${\rm{COD}} {\text{ }} {\rm{removal}} = \left( {{c_{{\rm{in}}}} - {c_{{\rm{out}}}}} \right)/{c_{{\rm{in}}}} \times {\rm{1}}00\%$$ , where c in is the influent COD concentration (mg/L) and c out is the effluent COD concentration (mg/L). Trickling filter operation 2 The synthetic wastewater simulated a medium-loaded, physically pretreated brewery wastewater. A concentrated substrate was used and later diluted with tap water to prepare the synthetic wastewater. Continuous operation of the trickling filter was conducted during all trickling filter operations. The first operation was the seeding of the trickling filter with real wastewater. The trickling filter was then acclimated firstly with real wastewater, then a mixture of both real and synthetic wastewater, and finally only with synthetic water until the system reached its steady state. COD reduction was monitored to evaluate the growth of the microbial population. Confirmation of the acclimation period was realized when the COD measurements for three consecutive days on the final effluent were approximately the same. The next operation was the investigation of the effects of hydraulic and organic loading rates on the performance of the system. During this operation of the trickling filter, the performance was evaluated at five different hydraulic and organic loading rates. At each hydraulic and organic loading rate, the trickling filter was operated for about five consecutive days to ensure repetitiveness of the result. Investigation of the effect of different daily influent flow and inflow concentrations was also carried out. Initial influent sample analysis was done at the beginning of each run in all cases and final effluent sample analysis was done three times a week to investigate the effects of hydraulic and organic loading rates unless otherwise indicated and on a daily basis to investigate the effect of daily influent variation. The performance of the trickling filter for the temperature range of 20–40 °C and pH range of 3–12 was also investigated. Analyses of effluent COD taken from different heights of the trickling filter bed were also made. Results and discussion 1 Real wastewater characteristics and preparation of synthetic water 2 The detailed characterization that was carried out on the Rostock brewery wastewater is shown in Table 1 1 . The analysis characterized the brewery wastewater from the Rostock brewery as having BOD 5 , COD, NH 4 + -N, NO 2 -N, TN, PO 4 3− -P, and TP concentration ranges of 1.412–3.980, 1.651–9.306, 0.003–0.011, 0.229–0.440, 0.010–0.101, 0.008–0.010, and 0.017–0.050 g/L, respectively. All PO 4 3− -P, NO 3 -N, NO 2 -N, and NH 4 + -N concentrations stated throughout are soluble values only and were determined from microfiltered samples (0.45 μm). Constituent Mean Standard deviation Maximum Minimum COD (mg/L) 5 351 3 228.2 9 303 1 651 BOD 5 (mg/L) 2 101.2 1 015.10 3 980 1 342 NH 4 + -N (mg/L) 4.96 2.94 10.50 2.47 NO 3 -N (mg/L) 15.0 14.3 25.1 4.9 NO 2 -N (mg/L) 0.25 0.14 0.44 0.09 TN (mg/L) 47.4 40.10 102.0 7.2 PO 4 3− -P (mg/L) 6.34 4.20 10.12 0.36 TP (mg/L) 25.42 13.69 43.40 13.00 Temp. (°C) 25 6.4 34 17 pH 9.42 1.60 12.40 8.00 As the brewery and laboratory were far apart, it was difficult to bring real brewery wastewater; therefore, as influent to the trickling filter plant, a synthetic sewage (Boeije, 1996 1996 ) was used with a COD:N:P ratio of 102:2.1:1 (w/w/w). The type and concentration of each ingredient are based on typical concentrations of BOD 5 , COD, pH, and nutrients found in brewery wastewater (Driessen and Vereijken, 2003 2003 ) as well as on the analysis of samples obtained from Rostock brewery, Germany. Synthetic brewery wastewater was composed of 1 g/L malt extract, 0.5 g/L yeast extract, 0.15 g/L peptone, 0.86 g/L maltose, 0.10–2.20 g/L (NH 4 ) 2 SO 4 , and 2.80 ml/L ethanol. Buffering salts of 0.08 g/L NaH 2 PO 4 and 0.14 g/L of Na 2 HPO 4 are added to maintain the pH at 6.7. The final concentrations of COD, N, and P of the synthetic wastewater were also determined. The theoretical COD of the ingredients was calculated from their oxidation equation. For yeast extract, malt extract, maltose, and peptone, it is assumed that 1 mg/L of product equals 1 mg/L of COD. This was verified by comparing the calculated and the measured total COD of the influent (Boeije, 1996 1996 ). According to one chemical supplier (Jeevan Chemicals and Pharmaceuticals, India), the measured value of N in peptone and yeast extract is 10.0% and 10.5%, respectively. And as per another supplier (Murphy & Son Limited, UK), typical malt has an analysis for TN within 1.45%–1.75%. The calculated COD:N:P ratio of the synthetic influent was 102:2.1:1 (w/w/w). The theoretical BOD 5 , assuming a COD to BOD 5 conversion factor of 0.65, was 3 451 mg/L. The composition of the synthetic influent based on this fact is presented in Table 2 2 . Name Formula Concentration Solution  Ammonium sulphate (NH 4 ) 2 SO 4 100–2 200 mg/L Nitrogen source  Disodium hydrogen phosphate Na 2 HPO 4 140 mg/L Phosphorus source & buffer  Ethanol CH 3 CH 2 OH 2.8 ml/L C-source  Malt extract 1000 mg/L C-source  Maltose 860 mg/L C-source  Peptone 150 mg/L C-source  Sodium hydrogen phosphate NaH 2 PO 4 80 mg/L Phosphorus source & buffer  Yeast extract 500 mg/L C-source Trickling filter operation and performance 2 The pilot scale trickling filter was first seeded with real brewery wastewater. Near neutral brewery wastewater sludge (pH 6.7) was collected from the equalizing chamber of Rostock brewery. The wastewater was collected from the bottom of the chamber and taken with a cleaned plastic Jeri Can having a volume of 10 L. The reason to seed the trickling filter with this effluent is due to the fact that there is a high probability of getting microbial populations that are native to the wastewater (Rittman and Whiteman, 1994 1994 ; Leta, 2004 2004 ). After the trickling filter seeding with the real wastewater and before developing performance data, the acclimation of the trickling filter was carried out. Firstly the trickling filter was operated in batch mode for seven consecutive days with a mixture of real and synthetic water by feeding new synthetic water on the 1st, 3rd, 5th, and 7th days. In all cases, the total COD concentration of the solution was 2 000 mg/L. COD concentration was monitored during these operations. COD removal efficiencies of 81.7%, 71.5%, 79.8%, and 86.8% were achieved as monitored on the 1st, 3rd, 5th, and 7th days, respectively. Then the acclimation of the trickling filter continued only with synthetic wastewater from the 7th day onwards. This time the influent COD was varied from about 1 000 to 4 800 mg/L. A COD removal efficiency ranging from 74.0% to 95.4% was achieved during this last acclimation phase of the trickling filter. At this point, we considered the start up period completed and the filter ready for full operation. The trend of COD degradation during acclimation is presented in Fig. 2 2 . a: 75% real and 25% synthetic; b: 75% real and 25% synthetic; c: 50% synthetic and 50% real; d: 25% real and 75% synthetic; e: 100% synthetic Effect of hydraulic loading rate 2 The performance of the trickling filter at different hydraulic loading rates was also investigated. Four different flow rates were used in the feed, namely 800, 700, 600, and 500 L/d including recirculation flow at constant influent COD concentrations. The system efficiency ranged up to 77.70%–93.10%, 79.40%–96.70%, 88.30%–91.20%, and 84.10%–88.30% COD removal for the hydraulic loading of 6.3, 5.6, 4.8, and 4.0 m 3 /(m 2 ∙d), respectively. An important conclusion that can be drawn here is that there is no significant decrease in the efficiency with increase of the hydraulic loading rate in the given range. According to the relationship between mass removal loading rate and mass loading rate shown in Fig. 3 3 , there is a direct relationship between the two parameters with 91% confidence limit. The slope of 0.83 means that when the mass loading rate increased by 1 g/(m 2 ∙d), the mass removal rate increased accordingly by 0.83 g/(m 2 ∙d). Effect of organic loading rate 2 The response of the trickling filter for different organic loading rates was studied by varying the initial influent COD concentration (COD in the feed was increased to 1 000, 1 500, 2 000, and 3 000 mg/L), for a constant hydraulic loading of 6.3 m 3 /(m 2 ∙d) and organic volumetric loading of 1.5, 2.5, 3.0, and 4.5 kg COD/(m 3 ∙d), respectively. The experiment was run for five consecutive days at each organic loading rate. The range of efficiencies at each organic loading rate was 84.20%–92.40%, 76.10%–87.10%, 70.58%–93.50%, and 76.50%–83.10%, respectively. Since real situation wastewater that comes to the reactor varies in flow and concentration from day to day, the performance of the trickling filter at four different flows each with different initial COD concentration was also investigated. Four different flow rates were chosen: 300, 250, 200, and 150 L/d. The trend of the trickling filter effluent during each run was monitored. As can be seen from Fig. 4 4 , the trickling filter efficiency was above 80% for all influent properties. This confirms that there is no significant decrease in trickling filter efficiency with increase in flow rate and COD concentration. Effect of fed water pH 2 The effect of pH on COD degradation capacity of the trickling filter was examined over a pH range of 5.00–9.00. The pH was adjusted manually every 30 min to the desired value by the addition of 0.1 mol/L NaOH or 0.1 mol/L HCl as necessary. The trickling liquid at each specific pH was only kept for two consecutive days unless otherwise indicated. As can be observed in Fig. 5 5 , the removal efficiency was maintained at a high value, (86.67±9.90)% between pH 6.00 and 8.00, and around 94.6% at pH 6.30. The COD removal capacity increased from 66.13 to 81.53 g COD/(m 3 ∙h) when decreasing the pH from 8.00 to 6.30, and dropped to the values of 46.63, 54.90, and 55.92 g COD/(m 3 ∙h) for pH values of 5.10, 8.30, and 9.00, respectively. The optimal degradation capability was obtained when the pH was regulated at a value of 6.3. This may be due to two reasons: on the one hand, it has been shown that different autotrophic and heterotrophic microbial groups and activities dominate at different pH values; on the other hand, the degree of availability of the different substrates is different at different pH values in the wet biofilm where the biodegradation takes place. To conclude, although it performs best in the pH range of 6.30–7.00, there was no significant decrease in the efficiency in the other typical pH values of brewery wastewater, indicating that the trickling filter system was able to effectively cope with the imposed change in pH. Inflow: 300 L/d; Feed COD: (1 102.59±68.93) mg/L Effect of fed water temperature 2 Temperature is a very important parameter during the assessment of the overall efficiency of a biological treatment process. Temperature influences the metabolic activities of the microbial population and has also a profound effect on such factors as gas-transfer rates and the settling characteristics of the biological solids (Metcalf & Eddy, 1991a 1991a ; Crites and Tchobanoglous, 1998 1998 ). Temperatures below the optimum typically have a more significant effect on growth rates than that above the optimum. It has been observed that growth rates double with approximately every 10 °C increase in temperature until the optimum temperature is reached (Metcalf & Eddy, 1991b 1991b ). To investigate the effect of temperature on the trickling filter performance, the system was operated at five different temperatures: 20, 25, 30, 35, and 40 °C (Fig. 6 6 ). This temperature range was chosen to test the performance of the trickling filter, because it is the typical range for the wastewater of most brewing companies (Driessen and Vereijken, 2003 2003 ). The result of this investigation showed that the trickling filter performs above 80% in all cases and the values are close to each other, indicating that temperature only exhibits a minor effect on the performance of the trickling filter for the given temperature range. Influent COD: 1 182 mg/L; Inflow: 300 L/d; Fed water pH: 6.30–6.81, nearly constant COD removal profiles of the trickling filter 2 Fig. 7 7 depicts COD removal profiles of the trickling filter. A normal trend of effluent COD along the trickling filter height (downward decrease in effluent COD) was obtained during analysis of samples taken from the top, middle, and bottom sections of the trickling filter. However, the change in COD was not significant, which implies there was a homogeneous mixture in the reactor. Inflow: 300 L/d; Feed COD: (1 102.59±68.93) mg/L Sludge reduction 2 Sodium hydroxide washing 3 The operation of the trickling filter with an average COD removal efficiency of (92±2.7)% was maintained until Day 17 (Fig. 8 8 ). However, after Day 18, due to excessive biomass accumulation, the removal efficiency was decreased. The higher rate of biomass accumulation was due to the operation of the trickling filter at high hydraulic and organic loading rates. The removal efficiency was below 62% on Days 18–21, and 50% at Day 19. The major cause for the decrease in trickling filter efficiency is believed to be the reduction of the biofilm-specific surface area with increases of biomass contents (Alonso et al., 1997 1997 ). To remove the excess sludge, the trickling filter was washed by repeatedly pumping 10 L of 0.1 mol/L NaOH solution (Weber and Hartmans, 1995 1995 ). There was no significant decrease in the efficiency of trickling filter due to change in pH as a result of NaOH washing. The higher COD removal efficiency of the trickling filter was regained after washing of the trickling filter, because of the higher specific surface area of the trickling filter for the attachment of new biofilm growth and increase in porosity after the excess sludge is reduced. Starvation 3 Decreased efficiency as a result of excess biomass accumulation was also managed by starving the trickling filter for a few days. The trickling filter was starved for a total of 4 d as described in Table 3 3 . After starvation, there was a significant decrease in the biomass which may be the result of microorganism death, endogenous respiration, or secondary processes such as the predation of higher organisms (Zhang et al., 2009 2009 ). The recovery processes commenced after a 4-d starvation period, at which time normal operation of the biotrickling filter resumed. The biotrickling filter obtained high removal efficiency within 2 d of normal operation. Fig. 9 9 shows the efficiency of the trickling filter before and after starvation. The fast recovery of the biotrickling filter for COD removal suggests that there is no need for the buildup of significant amounts of new degrading biomass to resume the normal operation of the trickling filter. Operation day Influent supply Recirculation flow supply Atmospheric air supply Possible condition 1st & 2nd On Off On Defect of recirculation pump, equipment malfunction, discontinuity of electricity 3rd & 4th Off Off On Time of no beer manufacturing (week end or holiday) Conclusions 1 The seeding of the trickling filter can be achieved just by pumping the brewery wastewater itself repeatedly to the trickling filter, which means there is no need to use special inoculation to seed the trickling filter. The trickling filter seeded in such a way has a short acclimation phase before being ready for full operation. Increasing the hydraulic or organic loading rate has no significant effect on the efficiency. An average value of (84.42±6.5)% COD removal efficiency was achieved when the loading rate increased from 1.5 to 4.5 kg COD/(m 3 ∙d). The trickling filter can have sufficient air circulation naturally and the only energy demand is for pumping of the fed water. Handling of the excess sludge is not a problem because the amount of sludge produced is small and the dense nature of the sludge makes it easily settleable inside the secondary clarifier. Therefore, the proposed biological treatment process appears to be a promising wastewater treatment method for coping with beer manufacturing wastewaters with respect to the removal of COD. References C Alonso , MT Suidan , GA Sorial , FL Smith , P Biswas , PJ Smith , RC Brenner . Gas treatment in trickle-bed biofilters: biomass, how much is enough? . Biotechnol Bioeng , 1997 . 54 ( 6 ): 583 - 594 . 18636414 . 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Compliance with ethics guidelines Haimanot HABTE LEMJI and Hartmut ECKSTÄDT declare that they have no conflict of interest. This article does not contain any studies with human or animal subjects performed by any of the authors. 10.1631/jzus.B1300057 Project (No. 7117130553459) supported by the Landeszentralkasse M-V Stipendium, Germany 2013-02-28 2013-09-06 2013-06-19 2022-01-05 Copyright © Zhejiang University and Springer-Verlag Berlin Heidelberg 2013 2013 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 10 14