Abstract:
Industrial wastewaters are complex in nature, having soluble, insoluble and/or potentially insoluble compounds, which may be biodegradable or not and which may give rise to foaming or scaling. Industrial wastewaters with Chemical Oxygen Demand (COD) greater than 1500 mg/l are generally categorized as high strength industrial wastewaters.
In the present research, a laboratory scale study on COD removal in a 12.04 liters capacity UASB was investigated for a period of 236 days. The study was carried out at ambient temperature (approximately 280 C) and the reactor was inoculated with the seed sludge obtained from a working anaerobic digester treating brewery wastewater. Two trials were conducted: The first trial (T 1) was continued for 151 days and the second trial (T 2) for 85 days.
T 1 comprised 11 sub experimental runs (R0-R10), R0- R2 were the “Flow Through” acclimatization period in which the flow rate was maintained at 300 ml/hr. From R3-R10 the flow rate of the reactor was increased at 150 ml/hr steps. The reactor was operated continuously with COD removal efficiencies 60%-70%, until the flow rate was increased up to 750 ml/hr in the period R6. During the period of R6 sludge wash out was evident with reactor acidification. However the reactor recovered in 10 days period. Nevertheless, COD removal efficiencies were around 50% and periodic sludge washouts were experienced. Therefore the reactor operation was stopped and recommenced with fresh sludge.
In the T 2 trial, “Batch” acclimatized sludge was used as inoculum. The organic loading rate (OLR) was maintained in the range of 10-15 kg COD/m3.day at a flow rate of 900 ml/hr and influent COD was varied in the range of 5000-9000 mg/l. The reactor showed stable COD removal efficiency of 70-75%. Effluent recirculation was successfully used as an alternative for alkalinity supplementation by chemicals by employing a 0.5:1 recirculation ratio.
The methane yields observed in both trials were low compared with the theoretical value of 0.35 CH4 l/ g COD removed at Standard Temperature and Pressure, but in second trial methane yield showed a trend of increment indicating some growth of methanogens.
The reactor showed a satisfactory COD removal efficiency at OLR s of 10-15 kg COD/ m3.day in this study. However it did not show a satisfactory outcome as an energy recovering technique. Further studies must be carried out on that aspect.
According to the results it can be recommended that by using upflow velocity of 0.129 m/hr and influent pH range of 7-7.2 OLR s up to 15 kg COD/m3.day can be handled successfully in a UASB reactor. For maintaining influent pH in the above range no added chemicals are needed, but an effluent recirculation ratio of 0.5:1 can be employed.
Key words: UASB, COD, High strength industrial wastewater
