The Environmental Protection Agency definition of Sludge is “any solid, semisolid or liquid waste generated from a municipal, commercial, or industrial wastewater treatment plant, water supply treatment plant, or air pollution control facility exclusive of the treated effluent from a wastewater treatment plant.”
That’s the broad description, but a more targeted and perhaps more common use of the word refers to “a semi-solid slurry” that is the by-product in the modern world of a wide variety of industrial processes, including oil and gas drilling and mining operations. In common use, sludge is further restricted to the leftover semi-solid residue that results from modern wastewater or sewage treatment. Fecal sludge or “septage” is most often used to describe the sludge that accumulates in septic tanks which must be periodically removed and disposed of offsite.
Sewage sludge management, according to an abstract on Industrial and Municipal Sludge published in 2019 is “a huge challenge in the field of environmental engineering.”
The authors, Agnieszka Rorat and Sebasatien Lemiere, state: “Two points of view conflict regarding the recycling of those ‘wastes.’ Primarily, dehydrated sewage sludge is considered a reservoir of nutrients and organic matter that can be used as a fertilizer in agriculture or as an organic amendment in the remediation of contaminated sites or to build ‘anthroposols.’ On the other hand, recycled sewage sludge is seen as a potential source of soil contamination by organic and inorganic pollutants and pathogens; potentially toxic elements . . . “Those potentially toxic elements can include trace metals, biocides and pharmaceuticals, personal care products and residuals, synthetic hormones, microplastics and microorganisms, among others, according to the authors.