Sludge is the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term “septage” also refers to sludge from simple wastewater treatment but is connected to simple on-site sanitation systems, such as septic tanks.
When fresh sewage or wastewater enters a primary settling tank, approximately 50% of the suspended solid matter will settle out in an hour and a half. This collection of solids is known as raw sludge or primary solids and is said to be “fresh” before anaerobic processes become active. The sludge will become putrescent in a short time once anaerobic bacteria take over and must be removed from the sedimentation tank before this happens.
This is accomplished in one of two ways. Most commonly the fresh sludge is continuously extracted from the bottom of a hopper shaped tank by mechanical scrapers and passed to separate sludge digestion tanks. In some treatment plants an Imhoff tank is used where sludge settles through a slot to the lower story or digestion chamber where it is decomposed by anaerobic bacteria, resulting in liquefaction and reduced volume of the sludge.