Development of a kinetic model for biological sulphate reduction with primary sewage sludge as substrate

Abstract

The Rhodes BioSUREÆÊ Process is a low-cost active treatment system for acid mine drainage (AMD) waters. Central to this process is biological sulphate reduction (BSR) using primary sewage sludge (PSS) as the electron donor and organic carbon source, with the concomitant reduction of sulphate to sulphide and production of alkalinity. To optimise the design, operation and control of (and research into) BSR with PSS, a mathematical kinetic model would be an invaluable aid. This study describes the development of such a kinetic model. A two-phase (aqueous/gas) physical, biological and chemical processes kinetic model for the methanogenic anaerobic digestion of sewage sludges has been proposed (UCTADM1). This model incorporates biological processes for sewage sludge hydrolysis/solubilisation (usually the rate-limiting step) and acidification, acetogenesis, and acetotrophic(clastic) and hydrogenotrophic methanogenesis. Additionally, the background weak acid/base chemistry for water, carbonate, acetate, propionate, ammonium and phosphate species have been included, as well as the physical gas exchanges for carbon dioxide and ammonia. The compound H+ is explicitly included in the model as a predictive parameter, with corresponding pH inhibition of the methanogenic bioprocesses. Using this model as a basis, it is extended to incorporate BSR. The stoichiometry and kinetics for the bioprocesses (growth and death) mediated by the propionate degrading, acetotrophic and hydrogenotrophic sulphate-reducing bacteria are formulated, including sulphide and pH inhibition. These bioprocesses produce and consume inter alia sulphate and sulphide acid/base species which are not present in the original UCTADM1 model. Accordingly, following the approach in the UCTADM1 model, chemical processes for these are included. Further, in the BSR model the end-product sulphide has a gaseous equilibrium not in the UCTADM1 model, and hence the physical gas exchange for sulphide is included. The BSR biological, chemical and physical processes are integrated with those of the UCTADM1 model, to give a complete kinetic model for competitive methanogenic and sulphidogenic anaerobic digestion with PSS as substrate. This model currently is being evaluated, by application to a series of experimental systems fed a mixture of PSS and sulphate, operated over a range of retention times and pHs.