Sewage sludge is produced from the treatment of wastewater and consists of two basic forms; raw primary sludge (basically faecal material); and secondary activated sludge (a living ‘culture’ of organisms that help remove contaminants from wastewater before it is returned to rivers or the sea). In 2009 waste water sludge production in U.K amounted to around 1.2 megatonnes of dry solids of which 77% was recycled to agricultural land, and for EU as a whole there are about 6.5 megatonnes of dry solids produced annually. The sludge production values has significantly increased, possibly by as much as 50%, as the urban waste water treatment directive was implemented over the period up to 2005 and in the next decade, sludge disposal to all the established outlets could become increasingly difficult. The challenges faced had been how to (a) maintain cost effective and secure methods of sludge disposal and (b) engender public confidence in all disposal and recycling options. Of increased importance is the potential of renewable energy from the anaerobic digestion of sewage sludge. Sludge treatment and disposal may account for 40% of the operating costs of a wastewater treatment facility. Prior to treatment the sludges contain between 1 and 7% solids (they are therefore nearly all water) which are usually highly putrescible and offensive. A wide range of treatment processes and disposal options has been used, although, recently, the cost of energy has reduced the numbers currently employed because of economic considerations.
Most often the sewage sludge is transformed into biosolids using a number of complex
treatments such as digestion, lime stabilisation, thickening, dewatering and drying.