ADLib Glossary (S)

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Sewage Sludge and the Sludge Matrix Agreement

Sewage sludge is the by-product of the treatment of raw sewage. Some of this sludge is used for agricultural land improvements. The remaining portion is disposed of at landfill sites,

incinerated or disposed of at sea. Sewage sludge arising are thought to be between 30 and 40 million tonnes per year in the UK alone. The dry weight is around 1 million tonnes.

The bulk of its content derives from human wastes, but there are significant contributions from discharges to sewer from industrial effluents, animal and vegetable wastes as well as run-off of storm water from roads and paved areas. Consequently, in addition to organic waste the sludge may contain traces of many of the contaminating substances used in our modern society. Some of these substances can by phytotoxic and some toxic to humans or animals at certain concentrations in the soil or in food. It is therefore necessary to monitor and control the concentrations in the soil of these potentially toxic elements and their rate of application to the soil. This is achieved by the Sludge (Use in Agriculture) Regulations 1989 which enforce the EU Directive 86/278/EEC aimed at soil protection. The regulations place strict controls on the limits of certain substances in soil after sewage sludge applications.

A cross-industry consortium (which includes Water UK and the British Retail Consortium), known as the ADAS Matrix or the Safe Sludge Matrix, on the use of biosolids in agriculture has reached agreement on how and on what crops sewage sludge may be applied. The agreement includes guidelines for fruit, salads, horticulture, combinable crops, animal feed crops, grass and silage maize. These stipulate that (as off September 1999):

  • Untreated sewage sludge cannot be applied to any crop except combinable and feed crops. From 31st December 1999 this exception will cease and untreated sludge will not be permitted for use on any crop.
  • Treated sludges such as digested cakes and liquids cannot be used on fruit, salads, vegetables, other horticultural crops (except as part of a rotation), and grass for grazing (unless the sludge is injected or ploughed in).
  • Advanced treated sludges such as thermally dried granules can be applied to the crops addressed by the agreement.

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