Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage. It includes physical, chemical, and biological processes to remove these contaminants and produce environmentally safe treated wastewater (or treated effluent). A by-product of sewage treatment is usually a semi-solid waste or slurry, called sewage sludge, that has to undergo further treatment before being suitable for disposal or land application.
Since the liquid sewage sludge disposal to sea had been outruled in Europe there is a large pressure for recycling sewage sludge to land, mainly to farm land. The viscous, black cake adds free organic matter and fertilizer to poor soils, making them productive and profitable. However, careful use of sewage sludge, also called biosolids, is necessary to ensure that pathogens, nutrients and heavy metals would not contaminate the groundwater.
The following processes are documented in detail by the attached slide shows.
1) As the first step DIRK operated up to 20 sewage sludge centrifuges and chamber presses to reduce the sludge volume dramatically down to 35% dry substance.
2) Dewatered sludge can be used as fuel supplement for co-combustion in coal fired power stations.
3) Dewatered sludge can undergo a Class B alkaline sludge stabilization processes to fulfil the legal requirements for safe disposal or safe recycling to arable land.
4) A special process provides for the immobilisation of heavy metals in toxic sludge for final disposal in to landfill.