Proceedings

This paper reports on the development of a nutrient management matrix for the application of biosolids to agricultural land. The agronomic benefits (i.e. nitrogen, phosphate and organic matter etc.) and risks to water quality (for nitrogen and phosphorus) of recycling biosolids to agricultural land were assessed, and a Biosolids Nutrient Management Matrix was developed, which provided the ‘best’ agronomic and environmental balance for recycling all types of biosolids (e.g. digested cake/liquid, thermally dried, lime stabilised, composted) to agricultural land. The Biosolids Nutrient Management Matrix should be used to complement, and not replace, Nutrient Management Planning for both nitrogen and phosphate on farms where biosolids are used.

Keywords: Biosolids, nutrient management, phosphorus, nitrogen, cross compliance, recycling organic materials to land.

Introduction Biosolids (i.e. treated sewage sludge) recycling to agricultural land and incineration (with energy recovery) are the only practical management options available in the UK. “The UK Government considers that the recycling of treated sewage sludge to agricultural land is the Best Practicable Environmental Option (BPEO) in most circumstances”. Incineration with energy recovery, although preferable to landfill is a relatively expensive option, but can be BPEO where there is a limited agricultural landbank in close proximity to wastewater treatment plants/sludge treatment centres. Notably, biosolids recycling to land aligns with the principles of the Defra “Waste Strategy” of waste prevention, re-use, recycling/composting, energy recovery or disposal to maximise resource use efficiency (Figure 1). Biosolids provide a valuable source of plant available nutrients (i.e. nitrogen, phosphate, potash and sulphur etc.) which can be beneficially recycled to agricultural land. In particular, biosolids are a source of (slow release) phosphate which is a finite and nonrenewable natural resource (Steen, 1998; Soil Association, 2010). Also, lime-stabilised biosolids have value as a liming material. A typical biosolids application to farmland is worth over £300/ha (in terms of the reduced need to use manufactured fertilisers) to farmers. Moreover, the use of biosolids can help to reduce a farm’s carbon footprint (through replacing the need to apply manufactured fertilisers) by over 20 kg CO2e/tonne of biosolids applied, or around 0.5 tonnes of CO2e/hectare (if applied at the maximum permitted field limit of 250 kg total N/ha in Nitrate Vulnerable Zones). Biosolids also contain stable organic matter which can be a benefit to soil quality and fertility. Notably, repeated biosolids applications have been shown to increase plant available water supply (Chambers et al., 2003), which can increase crop yields and quality; particularly on drought prone sandy soils.