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Benefits and problems of applying waste water sludge to land

Photograph of a lorry delivering sludge to the field site
Raw sewage cannot be applied directly to the land. Otherwise any pathogens present in the sewage may contaminate fruit, crops and grazing animals. Sewage is therefore treated to remove any pathogens.

Waste water sludge is a useful source of nitrogen, phosphorus and organic matter, and after further processing can be used as a liming material. However, there are limits to what can be applied. For example applications of waste water sludge should not exceed 250 kg nitrogen per hectare per annum particularly within Nitrate Vulnerable Zones.

Problems of applying waste water sludge to land

Raw sewage cannot be applied directly to the land. Otherwise any pathogens present in the sewage may contaminate fruit, crops and grazing animals. Sewage is therefore treated to remove any pathogens.

Waste water sludge may also contains larger concentrations of heavy metals than that present in most soils. Once applied, the heavy metals will accumulate in the topsoil and can remain there for some considerable time. Many heavy metals such as zinc and cadmium are toxic and so can harm the biological processes which are vital for a healthy soil. Soil pH is important in determining which metal is toxic and at what concentration. For example, in pH neutral or near-neutral soils, metals such as chromium lead and mercury are largely insoluble in sludge treated soil and hence are less likely to be bioavailable in soils of this pH. At the moment it is difficult to say for definite which of the heavy metals found in sewage sludge are harmful to the soil and the highest level which can be considered safe for the microbial processes in soil.

Effect of soil pH on the levels of heavy metals which can be applied to soil

 
Metal Soil pH range Maximum advisory limit Maximum permissible limit
Zinc >7   450 mg/kg
Zinc 6.0-7.0   300 mg/kg
Zinc 5.5-6.0   250 mg/kg
Zinc 5.0-5.5   200 mg/mg
Zinc 5-7 200 mg/kg  
Zinc >7 (containing >5% Calcium carbonate) 300 mg/kg  
Cadmium >5.0   3 mg/kg
Copper >7.0 (containing >5% Calcium carbonate)   200 mg/kg
Copper 6.0-7.0   135 mg/kg
Copper 5.5-6.0   100 mg/kg
Copper 5.0-5.5   80  mg/kg

The maximum permitted levels contained in the guidelines were developed using the worst case scenario of coarse-textured soils and sensitive crop species but no account was taken of potential impacts of metals on soil micro-organisms.

Further information on waste water sludge


Printed from /about/facilities/hartwood/LTSE/background/benefits-problems on 11/12/18 02:21:18 PM

The James Hutton Research Institute is the result of the merger in April 2011 of MLURI and SCRI. This merger formed a new powerhouse for research into food, land use, and climate change.