The first step in wastewater treatment is normally the removal of solids from the wastewater flow by means of screens. The removed screenings contain household waste, faecal matter, toilet paper and mineral solids. The screenings volume depends, not only on the separation size of the screen, but also on the type of sewer system.
The solids content of municipal screenings varies between 10 % and 25 %, depending on the type of screen. Approximately 90 % of the solids are volatile (organic).
Due to their very high water content, their heterogenous composition and unaesthetic appearance screenings must be treated before they can be disposed of.
The best method of screenings treatment is washing and compaction with a wash press. Fecal matter and other organic materials are removed and returned into the wastewater flow. As a result, a good wash press increases the BOD5 load to the biological treatment process by about 6 %.
After washing, the screenings are compacted to reduce the water content and increase the solids concentration. Dewatering is improved by the removal of organic materials during washing. A good wash press can achieve a weight and volume reduction of up to 80 %.
A wash press reduces the mass and volume of the screenings and consequently the disposal costs.