Biological sulphate reduction with primary sewage sludge in an upflow anaerobic sludge bed reactor – Part 5: Steady-state model

Abstract

This paper describes the development of a steady-state anaerobic digestion model for biological sulphate reduction using primary sewage sludge (PSS) as substrate. The model comprises: a chemical oxygen demand (COD) based hydrolysis kinetics part in which the PSS biodegradable COD and sulphate removals are calculated for given hydraulic and sludge retention times; a C, H, O, N, P, S, COD and charge mass balance stoichiometry part in which the alkalinity generated (from both the HCO3-and HS-) is determined from the COD and sulphate removals; and an inorganic carbon (CO 2) and sulphide mixed weak acid/base chemistry part in which the digester pH is calculated from the HCO 3-and HS-species formed. From the stoichiometry, it was found that the PSS is carbon limited in that it does not generate sufficient HCO 3-alkalinity for the sulphate reduction, i.e., its COD/C ratio is too high which accounts for the observed zero gas (CO 2) generation. The H2S/HS-system provides the alkalinity shortfall and establishes the system pH.

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