PRE–PASTEURISATION AND ANAEROBIC DIGESTION A Low Cost Route to Class A Biosolids

The development of a combined pasteurisation and stabilisation process to satisfy the criteria for USEPA503 Class A is described. The application of this in the design of a full scale treatment plant is outlined.Treating sewage sludge at higher temperatures demands more from the heating plant which is critical to the success of this process. The use of steam, hot water, and hot gases to achieve pasteurisation temperature is considered and the advantages and limitations of each method are discussed.In the past, equipment design has been empirically rather than fundamentally based and the trend towards processing thicker sludges will require future plants to be based on the fundamental rheology. The effectiveness of heat transfer and mixing are very much influenced by the sludge rheological properties and these are more important in pasteurisation than in anaerobic digestion.Sludge heating equipment is typically designed for digested sludges with 3 to 5% solids concentration whereas pasteurisation involves raw sludges at 6 to 8% solids concentration. The viscosity of the raw feed is typically six times that of the digested sludge and clearly has a major effect upon the design of the equipment.Traditional practice is to use rheological data from the literature but data specific to the sludge being treated allows more precise design of the equipment and improves process security. It is important that the viscosity is determined under both shear and temperature conditions similar to those in the full scale plant.The large number of variables to be considered in designing a pasteurisation process lends itself to a modelling approach. Monsal has developed a computerised mathematical process model to allow optimisation of the design of the heating process and its associated heat recovery and cooling stages. This allows comparison of various flow sheets to be made as well as optimisation of equipment selection.