An innovative anaerobic fluidized bed (AFB) with extreme height (21 metres) was designed by the UCL engineers. Food processing wastewater with relatively low COD concentration less than 1,000 ppm was treated. During the start-up period of seven months, the applied organic loadings (2 to 16 kgCOD/m3day) were increased stepwise with decreasing the HRT from 24 to 1.5 hours. Certain upflow velocities of 25 and 37 metres/hour were maintained to fluidize the ceramic particles up to 10 metres high. Population dynamics of the biofilm attached on particle medium was investigated in terms of increasing organic loading, cumulation of volatile fatty acids, SEM microscopic morphology and biogas production rate of BMP batch tests. About 0.09 kgVSS/kg – medium of biofilm contributed to more than half amount of biomass while the suspended sludge was susceptible to hydraulic washing-out. Filamentous network and thrix-like anaerobic bacteria were grown initially in the deep holes or crevices of the medium. After this initial biofilm network formation, certain types of short rods and small cocci were embedded in the biofilm. High organic loading and high VTA concentration provided rapid growth of discrete rods which were washed out of the fluidized bed. Appropriate upflow velocity, HRT and organic loading were essential to this process stability.

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