ARCHIVE: Fertiliser Manual (RB209)

Sewage sludge (Biosolids)

Treated sludges (commonly called biosolids) are valuable fertilisers and soil conditioners, which have undergone processes to create a product suitable for beneficial use in agriculture.

Where sludges are applied to agricultural land the conditions of The Safe Sludge Matrix, The Sludge (Use in Agriculture) Regulations and the Code of Practice for Agricultural Use of Sewage Sludge must be followed. The Safe Sludge Matrix provides the minimum standard for sustainable sludge recycling to agricultural land. Where sludges are used on agricultural land, usage must be recorded and the soil tested by the producer. These operating requirements ensure that sludge applications to farmland are strictly controlled, that contents of heavy metals do not accumulate to elevated levels in soils and crops, and that disease risks to humans and livestock are minimised. Before spreading on the field, users are advised to consult their product purchasers concerning any possible use restrictions imposed by the food supply chain.

Nutrient content of sludge

Biosolids are a valuable source of major plant nutrients and organic matter, which can be used by growers to meet crop nutrient requirements and to maintain soil fertility (see the example on page 78). Solid sludges (e.g. digested cake, lime stabilised cake) are the most common products applied to farmland.

Based on the analysis of a large number of samples, typical nutrient content data for the main biosolids types applied to farmland are summarised in the table below. However, the characteristics of these products can vary depending on the individual source and treatment process. Most biosolids products are now supplied by water companies with specific nutrient content data and other information.

Around 50% of the total phosphate content of biosolids is available to the next crop grown, with the remainder becoming available over future years. However, availability may be lower if the biosolids have been tertiary treated using iron and aluminium salts to enhance the removal of phosphorus from wastewater. The phosphate supplied by a biosolids application should be considered over the whole crop rotation by managing inputs in relation to crop offtake and soil analysis. Biosolids contain only small amounts of potash. Useful quantities of sulphur and magnesium are also applied which will help to meet crop needs and contribute to the maintenance of soil reserves. Lime-stabilised sludges also have value as liming materials (neutralising value typically in the range of 2-6% per tonne fresh weight) that can balance the acidifying effects of atmospheric inputs and of inorganic (urea and ammonium-based) nitrogen fertilisers.

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