Sludge is the residual semi-solid material left from industrial, water treatment, or wastewater treatment processes.
When fresh sewage or wastewater is added to a settling tank, approximately 50% of the suspended solid matter will settle out in about 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. Once anaerobic bacteria take over, the sludge will become putrescent in a short time and must be removed from the sedimentation tank before this happens.
This is commonly accomplished two ways. In an Imhoff tank, fresh sludge is passed 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. After digesting for an extended period, the result is called "digested" sludge and may be disposed of by drying and then landfilling.
Alternately, the fresh sludge may be continuously extracted from the tank mechanically and passed to separate sludge digestion tanks that operate at higher temperatures than the lower story of the Imhoff tank and, as a result, digest much more rapidly and efficiently.
Excess solids from biological processes such as activated sludge can be referred to as sludge, although more often called "biosolids," a public relations term that is increasingly used by water professionals in the United States. Industrial wastewater solids are also referred to as sludge, whether generated from biological or physical-chemical processes. Surface water plants also generate sludge made up of solids removed from the raw water.
European legislation on dangerous substances has eliminated the production and marketing of some substances that have been of historic concern such as persistent organic micropollutants. The European Commission has said repeatedly that the "Directive on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture" (86/278/EEC) has been very successful in that there have been no cases of adverse effect where it has been applied. The EC encourages the use of sewage sludge in agriculture because it conserves organic matter and completes nutrient cycles. Recycling of phosphate is regarded as especially important because the phosphate industry predicts that at the current rate of extraction the economic reserves will be exhausted in 100 or at most 250 years.
Digested sewage sludge can be used as a soil conditioner, but may contain toxic materials. To be USDA-certified organic, sludge (biosolids) cannot be used. After the 1991 Congressional ban on ocean dumping, the U.S. Environmental Protection Agency (EPA) instituted a policy of digested sludge reuse on agricultural land. The EPA promoted this policy by presenting it as recycling. This practice has been highly controversial. EPA has no system to track and respond to health complaints related to exposure to sewage sludge, over three hundred and fifty people have reported sludge-based health incidents to the Cornell Waste Management Institute alone. Symptoms reported have included: asthma, weight loss, fatigue, eye irritations, flu-like symptoms, gastrointestinal complications, headaches, immunodeficiency problems, lesions, nausea, nosebleeds, rashes, respiratory complications, abscesses, reproductive complications, cysts, and tumors.