A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 1991

A relatively new process, the Multi-Stage Reversing Flow Bioreactor (MRB) has been found to be effective for the treatment of weak wastewater utilizing the interrelationship among microorganism related to sulfur metabolism, by introducing a limited oxygen supply to the sludge blanket type bioreactor. It appears that most of the advantages of the conventional aerobic and anaerobic treatment can be utilized with the disadvantages minimized with this process. A laboratory scale MRB reactor employing a five stage unit of total volume 173 litres was operated for 120 days after start up using domestic wastewater from AIT campus. The effect of various hydraulic and organic loadings on process efficiency were measured to estaplish the optimum operating conditions and to simulate granulation in the system. Routine continuous tests, steady state experiments and batch tests were carried out in three phases to study the mechanism associated in organics removal, sludge production and the possibility of nitrification in the system. The stabilization of organic matter was essentially via sulfur metabolism in the BV's and to some extent by aerobic metabolism in the AV's. COD removal due to methanogenesis was negligible. Although no discrete microbiological granules developed, the high effectiveness of the process was due to accumulation of a highly dense floculated sludge especially from BV1 to BV4 and light aerobic sludge in BV5 the later imparting a polishing treatment to the wastewater. After operating the MRB with a HRT of 2.5-4.5 hours (flow rates: 1.15 l(m - 0.64 l/m) and an organic loading ranging from 0.45 Kg COD/m3.d - 5.9 Kg COD/m3.d an optimum condition of HRT = 4.5 hours and an Organic loading of about 2 Kg COD/m3.d were realized. Under these conditions a removal efficiency of 93% and 94% for Total-COD and Filtrate-COD respectevly were achieved. The typical Yield Coefficient value for MRB was found to be 0.133 which supports the postulate of low sludge production in the system It was also possible by introducing plastic tube media in the 4th & 5th AV's to prevent sulfide build-up and enhance nitrification. The extent of denitrification was however negligible.