Sewage sludge must be periodically wasted, or in other words, removed, from a treatment works so as to not cause an excessive biomass content in the system, or result in a pass through of sewage sludge to rivers or other surface waters.


Regulations have been developed in all the developed countries to ensure that the public health and the environment are protected when sewage sludge is disposed of by each of the following accepted methods:-


(1) application to the land as soil conditioner or fertilizer, 
(2) disposal on land by placing it in a surface disposal site (not permitted in the EU), 
(3) placing it in a municipal solid waste landfill unit (no longer permitted in the EU) 
(4) incineration 
(5) anaerobic digestion.


Sewage sludge is produced from our wastewater plants in huge quantities day after day, year after year. Municipalities find themselves under never-ending pressure to get rid of the quantities produced.


Spreading the sludge to land is the cheapest solution, however, any wash-off during rain into watercourses is highly polluting and there are health concerns. Even if those are dealt with by, for example, injecting the sludge into the soil below the surface, there remains the danger of a slow and dangerous build-up of certain heavy metals in the injected soils.


Surface disposal in drying beds needs a dry hot climate, and incurs health risks and potential for flies and odors. The resulting material is then still very often spread on land, with similar heavy-mtelas build-up worries as in (1.). Also, before the sludge dries and solidifies it may cause pollution, however, this is a reasonably economic method.

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The sludge may of course be simply placed in landfill, and this is again, not particularly expensive, or damaging as long as only small quantities are placed compared with much larger volumes of solid (usually domestic solid waste). However, this is now not permitted at all in many countries.


In the US sewage sludge is typically treated and then either spread on land or is disposed in an approved landfill. It, as we have already indicated, typically has high levels of heavy metals and pathogens.