Pilot Plant Comparison between the AFBR and the UASB Reactor for Municipal Wastewater Pretreatment

Abstract

Recent investigations have demonstrated that it is feasible to utilize a combined technology, consisting of an upflow anaerobic sludge bed (UASB) reactor for anaerobic pretreatment, followed by aerobic posttreatment, to efficiently treat municipal wastewater. In the proposed system the excess sludge produced in the aerobic stage is recycled to the anaerobic unit for sludge stabilization. The proposed configuration is an attractive alternative for secondary wastewater treatment because the costs associated with sludge digestion are eliminated. The present paper compares two anaerobic pretreatment technologies, namely, the anaerobic fluidized bed reactor (AFBR) and the UASB, and demonstrates that both have similar performances with regard to chemical oxygen demand (COD) removal, suspended solids removal, and gas generation. Of the solids loading applied to the AFBR [1.02 kg suspended solids (SS)/m³ day], 0.16 kg SS/m³ day were consumed, and 0.17 kg SS/m³ day accumulated in the bed and eventually would need to be removed. In comparison, of the solids load of 1.15 kg total suspended solids (TSS)/m³ day applied to the UASB, 0.38 kg/m³ day were consumed, and 0.054 kg/m³ day accumulated in the unit. Therefore, a much more efficient sludge stabilization was achieved in the UASB. In addition, the UASB requires significantly lower energy requirements for effluent recirculation than the AFBR, and thus it would be more economical to operate. As in the case of the conventional wastewater treatment plants with separate sludge digestion, the costs associated with digested sludge dewatering and final disposal must be considered. However, the costs of having a separate sludge digester are eliminated, because the anaerobic reactor, in addition to being a pretreatment unit, plays the role of the sludge stabilization unit. The proposed UASB/SC process is not only effective for providing secondary wastewater treatment, but it also minimizes the surplus sludge production and produces a well-stabilized sludge.