Adapting High-rate Anaerobic Treatment to Middle East Conditions

Abstract

High-rate anaerobic technologies offer cost-effective solutions for sewage treatment in the Middle East and Palestine in particular. The sewage characteristics in Palestine are quite different from the values elsewhere and show solids contents of more than 1000 mg chemical oxygen demand (COD)ss/L and total COD values exceeding 2000 mg/L. While summer temperatures exceed 25 °C, temperatures may drop to below 15 °C in winter. Simple model calculations indicate that conventional upflow anaerobic sludge bed (UASB) reactor should be dimensioned on hydraulic retention times (HRTs) approaching 1 day to ensure methanogenic conditions in all seasons. Consequently, reactor volumes are three times the size of similar reactors in the tropics while the feasibility of the hydraulic flow patterns at such reduced rate are questionable. In an alternative approach, the UASB reactor was amended by incorporating a parallel digester unit for enhanced sludge stabilization and generation of active methanogenic sludge to be recirculated to the UASB reactor. The digester operational conditions were assessed by operating eight completely stirred tank reactors (CSTRs) fed with primary sludge. The results showed a high degree of sludge stabilization in the parallel digesters at a solids retention time of ?10 and 15 days at process temperatures of 35 and 25 °C, respectively. The technical feasibility of the UASB-Digester combination was demonstrated by continuous flow pilot-scale experiments. A pilot UASB reactor was operated for 81 days at six hours HRT and 15 °C and was fed with raw domestic sewage. This period was subsequently followed by an 83 days operation period incorporating a parallel digester unit, which was operated at 35 °C. The UASB-Digester combination achieved removal efficiencies of total, suspended, colloidal and dissolved CODs of 66, 87, 44 and 30%, respectively. Preliminary model calculations indicated that a total reactor volume (UASB-Digester) corresponding to 8.6 hours hydraulic retention time (HRT) might suffice for sewage treatment in Palestine