MODELLING OF UASB REACTOR

Abstract

In the last two and half decades, the upflow anaerobic sludge blanket (UASB) reactor has been recognized as an efficient anaerobic wastewater treatment system with considerable amount of energy recovery. The behaviour of a UASB reactor is too complex to be described by a single model reported in the literature. The start-up of the reactor is extremely difficult to model. The steady- state operation of the reactor is also complex, although much simpler than the start-up phase. The complexity arises from the fact that the reactor bottom where the feed is mixed with the reactor contents, is a very vibrant pool, with rapid conversion of the organic into acids and the growth of microbial species and the portion above this zone is well mixed with the formation of gas which rises up, bubbling and the interchange and return of the microbial mass from above due to settling and buoyancy. In the present study the UASB reactor is considered as a simple CSTR without by-pass flow. The importance of the biomass concentration which agglomerates and remains within the reactor with very little washout is significant. The model has been used to test the experimental data reported by Hwang et al.(1992) and Kwong and Fang (1996). From the experimental data, kinetic parameters were determined and then used to predict the effluent biomass and COD concentrations. The predicted results were found to satisfactorily represent iii the actual performance data. The model as developed by Singh (1998) was also used for comparison. It was found that the model proposed by Singh does not incorporate the effect of biomass concentration within the reactor.