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

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 (CO2) and sulphide mixed weak acid/base chemistry part in which the digester pH is calculated from the HCO3 and HSspecies formed. From the stoichiometry, it was found that the PSS is carbon limited in that it does not generate sufficient HCO3 alkalinity for the sulphate reduction, i.e., its COD/C ratio is too high which accounts for the observed zero gas (CO2) generation. The H2S/HS system provides the alkalinity shortfall and establishes the system pH. Once developed and calibrated, the model results were compared with experimental data from 2 laboratory-scale upflow anaerobic sludge bed reactors (operated at 35oC and 20°C respectively) fed PSS and sulphate. The predicted COD and sulphate removals, alkalinity and digester pH correspond very well to the measured data. The model assists in identifying design and operation parameters sensitive to the system and provides a basis for developing an integrated biological, chemical and physical process dynamic model.