An innovative sludge management system based on separation of primary and secondary sludge treatment

An innovative sludge management system based on separation of treatment and disposal of primary and secondary sludge is discussed with reference to a sewage treatment plant of 500,000 equivalent person capacity. Secondary sludge, if treated separately from primary sludge, can be recovered in agriculture considering its relatively high content of nitrogen and phosphorus and negligible presence of pathogens and micropollutants. One typical outlet for primary sludge is still incineration which can be optimised by rendering the process auto thermal and significantly reducing the size of the incineration plant units (dryer, fluidised bed furnace, boiler and units for exhaust gas treatment) in comparison with those required for mixed sludge incineration. Biogas produced in anaerobic digestion is totally available for energy conversion when sludge treatment separation is performed, while in the other case a large proportion may be used as fuel in incineration, thus reducing the net electric energy conversion from 0.85-0.9 to 0.35-0.4 MW for the plant considered.