POST TREATMENT OF UASB EFFLUENT BY EXTENDED AERATION

Abstract

here are some 33 major STPs in India in which the first treatment- technology is based on the UASB (Up - flow Anaerobic Sludge Blanket) technology. Experience has shown that the UASB with and without the combination of treatment(s) has in the majority of cases been unable to meet acceptable treatment goals (Vinod Tare and AsitNema; unpublished report). The present study summarizes the operation of a bench scale combination of the UASB and a conventional activated sludge process as a simple means of meeting acceptable discharge standards. The results outlined are more of a screening treatability study undertaken on the Roorkee, IIT campus using its treatability laboratory facilities. Domestic sewage was sourced from campus for this project. This study involved the operation of existing bench scale units to generate a UASB effluent as influent for a conventional flow through constant volume activated sludge (AS) reactor (Khan, 2011) fitted with a separate clarifier. The return rate of settled sludge was fixed at 135 L/d as was the volume of the clarifier. Seed activated sludge (MLSS 2890 mg/L, MLVSS 1780 mg/L) was taken from a conventional activated sludge STP (Haridwar, 18 MLD).The USAB was designed to treat a daily flow of 135 L/d and to operate without temperature regulation at ambient temperature which at the beginning of the study was 11 °C. The AS reactor was operated to provide indicative treatability over three phases which included `Extended Aeration (EA).' For consistency the terminology EA is used throughout the study to generally describe the activated sludge process. It is recognized that the initial loading of Phase 1 was significantly higher than in conventional extended aeration as it was necessary to acclimate the system and to obtain an upper reference loading limit. Typical effluent quality .generated by the UASB was initially estimated'at 60 mg/L BOD, 80 mg/L COD, 150 mg/L TSS and 50 mg/L NH3-N. The EA reactor volume was adjusted to provide set hydraulic retention times of 1.7, 4.5 and 6.5 h for Phase 1, Phase 2 and Phase 3, respectivelyof the study. Mean organic loading was 0.45, 0.12 and 0.17 mg.BOD/ mg MLVSS/ d, respectively. Effluent quality ranged from almost zero ammonia maximum ammonia removal during the study as a result of the operating conditions. During higher temperatures and at EA loadings rising sludge was observed in the clarifier. This .would obviously require consideration in the design and operation of a full scale technology.