SEWAGE TREATMENT BY AN UPFLOW ANAEROBIC SLUDGE BLANKET REACTOR (UASB) UNDER SUBTROPICAL CONDITIONS

Abstract

The upflow anaerobic sludge blanket reactors seeded with cow dung manure and activated sludge of a dairy wastewater treatment plant, termed as UASBCD and UASBASDIT, were used to treat medium strength raw domestic wastewater. The start-up of UASBCD reactor required a period of 120 days. In case of UASBASDIT reactor, sludge bet was stabilized in a period of 80 days. The performance of both reactors to treat wastewater was enhanced with an increase in the sludge age and temperature. Under psychrophilic temperature (17 OC) and at early sludge age (60 days), COD and BOD removal by the reactors was in the range of 57-62% and 61- 66%, respectively. However, COD and BOD removal efficiency of the reactors elevated to the range of 79-81% and 77-83%, respectively at sludge age of 150 days and temperature of 30 OC. In short, overall performance of both reactors was optimal at sludge age ranging from 120-150 days and temperature varying between 25-30 O C. However, overall performance of UASBCD reactor was always relatively better than UASBASDIT reactor due to better granulation, biomass growth and settling characteristics of sludge with time in the former reactor. Both reactors were re-started after a long shutdown period of 12 months, and operated at HRT of 12 hrs and 30 OC temperature. The reactors restored normal performance in a time period of 30 days. Post-treatment of UASB wastewater was accomplished in order to improve the quality of wastewater in accordance with the permissible limits for irrigation water. The application of ozone, UV irradiation and H2O2 at their optimal exposure time (ozonation time varying between 26-30 min; UV irradiation time of 8-15 min) and dose (H2O2 dose in the range of 280-336 mg/L) showed 99.9% elimination of pathogens (total coliform, fecal coliform and E.coli) in wastewaters of both reactors. The disinfection efficiency of combined systems (O3/UV; H2O2/UV; O3/H2O2) was, however, spectacular. The application of these systems showed 99.9% removal of pathogens at very short contact time and less dose as compared to time and dose required in their individual application. There was no re-growth in wastewater samples disinfected at optimal contact time (ozonation time of 25 min; UV irradiation for 15 min) and maximum dose (H2O2 dose of 336 mg/L) in the presence of nutrients during a time period of 21 days. However, underdozed samples and wastewater samples exposed for inadequate time period showed re-growth, which was a function of nutrient content, temperature and light, and regrowth of pathogens was substantially increased with increasing temperature and adding nutrients.