Sewage sludge and biomass incineration in South Africa using a fluidized-bed reactor

The responsible disposal of wastewater sludge is a problem facing municipalities in South Africa. Coal and Waste Utilisation (Pty) Ltd (CWU) has been operating the Fluidized Bed Reactor (FBR) incinerator plant at the KwaMashu Wastewater Treatment Works (KWTW) in Durban, KwaZulu-Natal, which treats both raw and digested de-watered sludge. The facility operates on a 24/7 basis and incinerates sludge at temperatures in excess of 850°C, producing a stable ash product. As sewage sludge is classified as hazardous waste material, CWU has the required permits to ensure the strict environmental management requirements are met. This includes gas emissions, ash disposal, correct equipment operation and an ongoing local monitoring committee. This publication focuses on the selection and combustion performance of renewable supplementary biomass fuels, including chipped wood from pallets, pine bark, pine sawdust and bagasse. The results are compared to previous operation with coal as a reference scenario. The study forms part of the project's broader aim to utilise renewable energy sources for power generation, in conjunction with the sludge destruction process. It is significant research, as the technology can be applied to many municipalities in South Africa as a sludge disposal solution. The selection criteria formulated for a suitable biomass fuel were in terms of fuel calorific value, physical properties, fluidisation/combustion characteristics and cost. It was found that chipped wood and pine bark were best suited for the KWTW facility, with sawdust a possibility as very large amounts would be required. For chipped wood and pine bark, the cost of production was lower compared to coal (at equivalent production rates), and decreased CO emissions compared to coal. This supports the FBR technology as a sustainable solution for sludge disposal.