A three-phase fluidized bed reactor in the combined anaerobic/aerobic treatment of wastewater

Aerobic degradation or polishing is an essential step in the combined anaer obic/aerobic treatment of wastewater. In this study, a type of porous glass beads was used for immobilization of microbial cells in a three-phase aero bic fluidized bed reactor (AFBR) with an external liquid circulation. The e ffects of superficial gas and liquid velocities on bed expansion, solid and gas hold-ups and specific oxygen mass transfer rate, k(L)a, were investiga ted. A tracer study showed that the mixing and flow pattern in the 8 dm(3) reactor could be simulated by a non-ideal model of two continuous stirred t ank reactors (CSTRs) in series. By treating an effluent from an upflow anae robic sludge blanket (UASB) digester, the distribution of suspended and imm obilized biomass in the reactor as well as the kinetics of COD removal were determined. The specific oxygen mass transfer rate, kLa, at a superficial gas velocity of 0.7 cm s(-1) dropped by about 30% from 32 h(-1) in tap wate r to 22 h(-1) after a carrier load of 15% (v/v) was added. The measured kLa further dropped by about 20% to 18 h(-1) in the wastewater, a typical valu e of the bubbling fermenters with no stirring. Compared with the aerobic he terotrophs under optimum growth conditions, the microbes in this reactor wh ich was fed with anaerobic effluent plus biomass behaved like oligotrophs a nd showed slow specific COD removal rates. This might be attributed to the presence of a significant amount of obligate anaerobes and facultative orga nisms in the aerobic reactor. This was confirmed by a relatively low intrin sic oxygen uptake rate of the microbial population in the reactor, 94mg O-2 dm(-3) h(-1) or 19 mg O(2)g VS-1 h(-1). (C) 1999 Society of Chemical Indus try.