Secondary treatments consisting of an anaerobic trickle filter and an activated sludge plant with sludge recirculation allow almost 100 % removal of the organic load and suspended solids with a very high reduction in the nitrogen load and phosphorous load found in domestic sewage. In this way, the effluent that is discharged respects the limits set in table 4 of attachment 5 to Law Decree 152/2006 for discharge to land. The trickle filter is a biological reactor in which the micro-organisms, under anaerobic conditions, make use of the biodegradable substance contained in the effluent. These micro-organisms develop on the surface of special bulk fill material in polypropylene, specifically designed to maximise the contact surface area between the micro-organisms and the effluent. Activated sludge plants are systems in which the bacterial flora develops in colonies that remain in suspension in the effluent and consume the remaining biodegradable material. The process is fully aerobic and the oxygen necessary for the development of the bacteria is supplied by submersed diffusers that emit a flow of fine bubbles of air from the bottom of the tank. This also guarantees continuous effluent mixing. The sludge recirculation system is used to create optimum conditions to develop and maintain the bacteria responsible for removal of the various pollutants present in the effluent.

USE: high level secondary treatment of domestic sewage.

REFERENCE REGULATIONS: Inter-Ministerial Committee for the Protection of Waters Resolution n°48/77, Legislative Decree n°152/06.