Many of the treatment processes described give rise to primary or secondary sludges. Typically, these byproducts require disposal and, like many forms of solid waste, a proportion have been consigned to either landfill or incineration. For some treated sludges, especially those derived from domestic sewage or food residuals, agricultural use has been an option, often requiring additional treatments to ensure its freedom from human pathogens, before land spreading or injection beneath the surface. The effectiveness of microbes in metal sequestration means, inevitably, that most treated sludges have a degree of heavy metal contamination, which itself makes possible the accumulation of these con-taminants in soils exposed to these products. In addition, there are increasingly stringent controls on the release of nitrogen to the environment, particularly within escalating European Union legislation regarding nitrogen vulnerable zones. It would seem, then, that the future land use of ‘spent’ sludges is likely to be somewhat more heavily regulated than previously.
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