Distribution and change of microbial activity in combined UASB and AFB reactors for wastewater treatment

A thermophilic upflow anaerobic sludge blanket (UASB) reactor was combined with a mesophilic aerobic fluidized bed (AFB) reactor for treatment of a me dium strength wastewater with 2,700 mg COD l(-1). The COD removal efficienc y reached 75% with a removal rate of 0.2 g COD l(-1) h(-1) at an overall hy draulic retention time 14 hours. The distribution of microbial activity and its change with hydraulic retention time in the two reactors were investig ated by measuring ATP concentration in the reactors and specific ATP conten t of the biomass. In the UASB reactor, the difference in specific ATP was s ignificant between the sludge bed and blanket solution (0.02 mg ATP g VS-1 versus 0.85 mg ATP g VS-1) even though the ATP concentrations in these two zones were similar. A great pH gradient up to 4 was developed along the UAS B reactor. Since a high ATP or biological activity in the blanket solution could only be maintained in a narrow pH range from 6.5 to 7.5, the sludge g ranules showed a high pH tolerance and buffering capacity up to pH 11. The suspended biomass in AFB reactor had a higher specific ATP than the biomass fixed in polyurethane carriers (1.6 mg ATP g VS-1 versus 1.1 mg ATP g VS-1 ), which implies a starvation status of the immobilized cells due to mass t ransfer limitation. The aerobes had to work under starvation conditions in this polishing reactor. The anaerobic biomass brought into AFB reactor cont ributed to an increase in suspended solids, but not the COD removal because of its fast deactivation under aerobic conditions. A second order kinetic model was proposed for ATP decline of the anaerobes. The results on distrib ution of microbial activity in the two reactors as well as its change with hydraulic retention time lead to further performance improvement of the com bined anaerobic/aerobic reactor system.