This analysis suggests that a holistic or life-cycle approach be taken when assessing the long-term sustainability of sludge reuse options. Issues such as compliance with world best practice environmental/health standards, energyusage, greenhouse gas emissions, odour control and volume reduction must now be taken into account when assessing the long-term sustainability and public acceptability of the various management options. Four major criteria for the assessment of sludge reuse options are proposed and five management options are assessed using these criteria. The results indicate that traditional land application reuse options must be evaluated in more detail to confirm whether they offer a long-term sustainable management option. Newer thermal based management options may however provide long-term sustainable management options.

Management of sewage sludge via "publicly acceptable" methods is becoming increasingly difficult, primarily due to health and environmental concerns with respect to reuse of the product in agriculture. Consequently thermal processes are gaining popularity with significantly increased interest being shown in pyrolysis and gasification processes, due to their "non-incineration status". One such process is the ENERSLUDGE technology which has been developed and commercialised by Environmental Solutions International Ltd (ESI). The world's first commercial ENERSLUDGE plant is located at the Subiaco Wastewater Treatment Plant (WWTP) that was handed over to the client, the Water Corporation of Western Australia in June 2001. Extensive design knowledge and operational experience has now been accumulated from this commercial pyrolysis facility and future applications of the technology will benefit immensely from the lessons learned and experience gained from this facility.

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The Enersludge™ process converts sewage sludge, biomass and other carbonaceous materials to re-usable fuels. Currently the low grade fuels namely char, reaction water and non-condensed gas are used to drive the process with the oil being classified as surplus fuel. Consequently oil quality is of paramount importance with regard to marketability of the Enersludge™ process. The new process comprises a single reactor followed by a catalyst cartridge to refine oil vapours. Oil quality can be controlled by type of catalyst, temperature and WHSV. Oil viscosity was used as a global oil quality indicator to benchmark the enhanced Enersludge™ process against its reactor predecessors. Oil viscosities as low as 3 cSt can be achieved which is equivalent to a tenfold reductioncompared to previous reactor designs. At the same time energy recovery in the oil has only dropped slightly.

The first commercial-scale plant, to convert sewage sludge to oil, using the ENERSLUDGE™ technology, has recently been constructed at the Subiaco Wastewater Treatment Plant (WWTP) in Perth, Western Australia. The plant, designedto process up to 25 dry tonnes per day (tpd) of mixed raw primary and thickened excess activated sludge, comprises numerous unit operations which allows the facility to operate in four distinct modes, namely as a chemical stabilisation plant, an LPG-fired dryer, an autogenous sludge dryer and ultimately as a sludge conversion plant. As of June 1999, the plant is operable in the first three modes of operation. Commissioning of Mode 4, namelysludge conversion to commence in August 1999 and initial results provide confidence in the technology, with the major problems being mechanical in nature. Operating results have indicated sludge quality has a major impact on plant performance. To improve operability, a strainpress has been installed to remove much of the hair and fibre present in the sludge. The plant can now successfully process sludge with between 20 and 50% by weight of thickened excess activated sludge.