Improving contaminated sewage sludge : an experiment based assessment of selected treatment options for the SASOL sewage works in Sasolburg (South Africa)

Abstract

Sasol Chemical Industries (SCI) located in Sasolburg, South Africa (SA), have since the early fifties produced fuels and waxes commercially, and recently diversified to produce a wider range of other chemicals. The Sasol One processes as with all industrial processes generate various waste streams. One such waste stream in the case of the Sasol One Site, which was the main theme of this study is the so-called poor quality sewage sludge generated during the treatment of domestic and industrial effluent in biological oxidation ponds at the sewage works. The poor quality of the sludge is related to the metals and pathogenic organisms present in the sludge. Furthermore, the stockpiling area where the sludge is stored is running out of space thus creating an ongoing environmental and operational challenge to the management of the sewage works. The primary objective of the study was therefore to identify suitable sludge treatment options by means of comparing three sludge treatment techniques, viz; Composting, ASP (Activated Sludge Pasteurisation) and SLASH (treatment of sludge with ash and / or lime). It was anticipated that one or a combination of these three techniques would improve the quality of the sludge in terms of its metal and pathogenic content and furthermore, as a more beneficial sludge, possibly support the humus requirements for the revegetation efforts during the rehabilitation activities on the Sasol One waste site. In evaluating these possible treatment options the sludge was subjected to laboratory bench experiments and field plant trials. To arrive at an answer as to which treatment option was the best, a decision matrix was developed that compared and scored the treatment options using various weighted criteria. The criteria used considered (i) the present legal sludge treatment requirements in terms of the Water Research Commission guidelines;(ii) the sustainability of the option that would be the most likely one to succeed in the long term; (iii) the economic viability defined as a capital and operational expenditure required that would give an indication of the financial viability of the preferred option and; (iv) the technical feasibility being defined as the potential for the preferred option to achieve full scale operation and a measure of confidence to implement the option or not. The experiments conducted and results achieved indicated that all three treatment options significantly improved the quality of the sludge in terms of metal and pathogenic content. However the composted sludge scored the highest points followed by SLASH and lastly ASP. Based on these experiments and the decision matrix used, Composting performed best by achieving a satisfactory score based on the WRC classification guidelines including cost and technical feasibility. With respect to this outcome the Composting was recommended as the preferred treatment option.