MIXING IN UPFLOW ANAEROBIC FILTERS AND ITS INFLUENCE ON PERFORMANCE AND SCALE-UP

Tracer studies were carried out in laboratory-scale and pilot-scale up flow anaerobic filters to determine the effect of liquid velocity, gas production and media depth on mixing patterns. A computer simulation model was developed to analyse tracer-response curves. In water studie s at laboratory scale, gas production was shown to have a significantl y greater effect on mixing than liquid upflow velocity. A reduction in the quantity of media also resulted in greater mixing due to the grea ter void space in which synthetic gas bubbles could cause turbulence. In the presence of sludge during reactor operation, at pilot and labor atory-scale, gas production had a significant influence on mixing. How ever, liquid velocity played an important role in solids distribution in the filter, in conjunction with media depth. At pilot-scale, at a l ow solids concentration, a high liquid velocity lifted the sludge ''be d'', raising the source of gas production. The absence of gas below th e sludge bed resulted in a plug flow regime which the incoming substra te entered. A reduction in the quantity of media increased the degree of mixing for a given liquid velocity and gas surface load. Lower liqu id upflow velocities are required at a reduced media depth to prevent excessive biomass loss. Shear rates increase at high liquid and gas ve locities, resulting in detachment of solids from the media, and biomas s washout. A close correlation was established between mixing and proc ess performance which led to the development of a programme for start- up and operation of the filter to maintain optimum biomass/substrate c ontact. A strategy for scale-up was proposed through the development o f correlations obtained from laboratory-scale filter studies which wer e used to predict pilot-scale mixing characteristics. This research hi ghlighted the important factors influencing mixing patterns and scale- up in anaerobic upflow filters. Copyright (C) 1996 Elsevier Science Lt d