Mixing in upflow anaerobic filters and its influence on performance and scale-up

Tracer studies were carried out in laboratory-scale and pilot-scale upflow 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 studies at laboratory scale, gas production was shown to have a significantly greater effect on mixing than liquid upflow velocity. A reduction in the quantity of media also resulted in greater mixing due to the greater void space in which synthetic gas bubbles could cause turbulence. In the presence of sludge during reactor operation, at pilot and laboratory-scale, gas production had a significant influence on mixing. However, liquid velocity played an important role in solids distribution in the filter, in conjunction with media depth. At pilot-scale, at a low solids concentration, a high liquid velocity lifted the sludge “bed”, raising the source of gas production. The absence of gas below the sludge bed resulted in a plug flow regime which the incoming substrate entered. A reduction in the quantity of media increased the degree of mixing for a given liquid velocity and gas surface load. Lower liquid upflow velocities are required at a reduced media depth to prevent excessive biomass loss. Shear rates increase at high liquid and gas velocities, resulting in detachment of solids from the media and biomass washout. A close correlation was established between mixing and process performance which led to the development of a programme for start-up and operation of the filter to maintain optimum biomass/substrate contact. A strategy for scale-up was proposed through the development of correlations obtained from laboratory-scale filter studies which were used to predict pilot-scale mixing characteristics. This research highlighted the important factors influencing mixing patterns and scale-up in anaerobic upflow filters.