Proceedings of the 2004 Water Institute of Southern Africa (WISA) Biennial Conference 2 6 May 2004 ISBN: 1-920-01728-3 Cape Town, South Africa Produced by: Document Transformation Technologies Organised by Event Dynamics CONSIDERATIONS FOR THE USE OF PRIMARY SEWAGE SLUDGE AND SULFATE-REDUCING BACTERIA FOR THE TREATMENT OF SULFATE-RICH WASTES N.E. Ristow, S.W. Stemann, M.C. Wentzel * , G.A. Ekama and R.E. Loewenthal Water Research Laboratory, Department of Civil Engineering, University of Cape Town, Private bag, Rondebosch, 7701. Tel: (021) 650 2583. *E-mail: markw@eng.uct.ac.za ABSTRACT The use of primary sewage sludge (PSS) as the carbon source/ electron for biological sulfate reduction is an attractive low-cost alternative for the treatment of acid mine drainage. Sulfate-reducing bacteria cannot utilize the particulate organic matter in PSS directly, and these particulates need to be solubilised first. A series of experiments have been conducted to determine the effects of sulfate reduction, compared to methane production, on the rate of hydrolysis and solubilisation of PSS. The results show that under sulfate-reducing conditions with low aqueous sulfide concentrations, the rate of hydrolysis was increased by more than 50% over the methanogenic system, while the rates of solubilisation were near identical. The sulfate-reducing systems were able to use all available soluble organic COD, including acetate, when there was sufficient sulfate in the system. The COD: SO 4 utilization ratio was calculated at 0.8, compared to the theoretical value of 0.67. INTRODUCTION Anaerobic digestion of primary sewage sludge (PSS) is accepted as the most cost effective technology for sludge stabilization due to the low energy requirements and the resultant high sludge conversions due to the low mass yields. The products of methanogenic digestion are a stable sludge that can be used as a soil conditioner under certain conditions, and a methane-rich gas stream that can be used as an energy source. However, the PSS can be viewed as a valuable substrate for the support of various other biological systems requiring an external organic carbon source/ electron donor; the processes in methanogenic digestion can be viewed in this way. If the conditions of the anaerobic digester are altered by the addition of other external electron acceptors (O 2 , NO 3 - , SO 4 2- rather than HCO 3 - ), biological populations other than the methanogenic consortium can utilize these electron acceptors, and hence be supported. The PSS also contains all nutrients (N, P, K and trace elements) to support a healthy biological population. Specifically, for the treatment of sulfate-rich waste streams such as acid mine drainage (AMD), the use of sulfate-reducing bacteria (SRB) is becoming an accepted technology and the use of PSS as the carbon source/ electron donor is an attractive low cost alternative. The Environmental Biotechnology Group, Rhodes University, have studied extensively the use of PSS for sulfate reduction [5]. The result of this research has been the development of the BioSURE Process fi for the treatment of AMD using PSS as the carbon source/ electron donor. Further studies have focused on the effects of pH and sulfide on the hydrolysis of PSS, and their results have shown that the presence of sulfide enhances the activity of the hydrolytic enzymes, while pH variations also have an effect on this activity [6,7]. To investigate this further, a research project has been conducted at the Water Research Laboratory, UCT. The main thrust of this research project is to determine the kinetics of PSS solubilisation under methanogenic and sulfate reducing conditions, and to quantify the differences in these rates under the different operating conditions. This paper discusses some of the results (three experiments) obtained from laboratory-scale studies using PSS and sulfate in anaerobic 1533