Sludge

SlamT-arv, Slam (torrsubstans) från avloppsreningsverk

Sludge, also known as digested sludge and sewage sludge, is formed in sewage treatment plants when the wastewater is treated. Sludge consists of organic matter, nutrients and almost everything else that is flushed away with the wastewater.

Use

As sewage sludge contains nutrients, it would be useful if all sludge could be returned to farmland. But the sludge sometimes contains pollutants, in which case it has to be used instead on other land or in some other way.
Since 2005, it has been illegal to place sludge in landfills in Sweden. In future the sludge can be used as a fertiliser on arable land, in forestry and as an ingredient of topsoil. Dried or dewatered sludge can be burnt for energy recovery. If the sludge has undergone certain pre-treatment such as composting it is exempt from the ban on landfilling.

Dispersal routes

The degree of purification in present-day Swedish sewage treatment plants is good, and is steadily improving, but emissions from wastewater are nevertheless a significant source of eutrophicating substances (phosphorus, nitrogen and organic matter) in our waters. Emissions of eutrophicating substances have, however, decreased substantially in recent decades.

Environmental impact

Eutrophication is due to too great a supply of nitrogen and phosphorus in comparison with the natural situation. The principal cause of eutrophication of lakes and watercourses is leaching of phosphorus from arable land and inputs from municipal sewage treatment plants and industrial installations. Stormwater and private sewerage systems in rural areas also account for a significant proportion of phosphorus inputs. The marine environment may be affected by both phosphorus and nitrogen, depending on which substance is in deficit for the production of algae.

Source: Swedish Environmental Protection Agency