Sludge

From Wikipedia, the free encyclopedia

For other meanings see: Sludge (disambiguation).

Sludge is a generic term for solids separated from suspension in a liquid by a variety of processes. Most commonly sludge refers to solid waste extracted in the process of sewage treatment; the term sewage sludge is used commonly. 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 by two different ways. In an Imhoff tank, fresh sludge is passed through a slot to the lower story or digestion chamber where decomposition by anaerobic bacteria takes place resulting in liquefaction and a reduction in the volume of the sludge. After digesting for an extended period of time, 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 by mechanical means and passed on to separate sludge digestion tanks which 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 also 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. Digested sewage sludge can be used as an organic soil conditioner, although if proper precautions are not taken, it may contain toxic materials. 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. EPA promoted this policy by presenting it as recycling. This practice has been controversial, with illness being reported after exposure to field applied digested sludge. No firm connection between illness rates and digested sewage sludge has been established.

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.

1 Background
2 The treatment process
3 Benefits of treatment
4 The final product
5 See also
6 External links

[edit] Background

Biosolids, the treated form of sewage sludge, have been in use in UK and European agriculture for over forty years. A safe, sustainable, highly regulated and environmentally sound solution to the increasing challenge of waste disposal, biosolids recycling represents approximately 2% of organic material applied to land in the UK. The practice is recognised as the Best Practicable Environmental Option (BPEO) in most circumstances by the EU and UK government.

[edit] The treatment process

Sewage sludge is produced from the treatment of wastewater and consists of two basic forms – raw primary sludge (basically faecal material) and secondary sludge (a living ‘culture’ of organisms that help remove contaminants from wastewater before it is returned to rivers or the sea). The sludge is transformed into biosolids using a number of complex treatments such as digestion, thickening, dewatering, drying and lime stabilisation.

[edit] Benefits of treatment

The treatment process reduces the water content of the sludge, reduces its ability to produce gas and renders it virtually free from harmful organisms. Treated biosolids are therefore easily transportable, less odorous and almost 100% pathogen free. Some treatments also reduce the amount of waste, as the solids content of treated sludge is lower than that of raw sludge.

[edit] The final product

Treated biosolids can be produced in cake, granular, pellet or liquid form and are spread over land before being incorporated into the soil or injected directly into the soil by specialist contractors.