Introduction

Sludge to Land - the Current Position

Sludge (biosolids) recycling to land is expected to double by 2006 but the security of this route is threatened by environmental concerns and health scares. Strategic investment is needed to ensure sustainable and secure sludge recycling outlets.

The environmental risk linked to soil phosphorus (P) saturation is recognized, in the UK, by the 1998 MAFF Water Code and may limit nutrient inputs, including those derived from biosolids, on fields at ADAS Soil P Index 3 or above. Index 3 areas cover 56% arable and 30% grassland soils in the UK.

Such restrictions are based on limited information and may not optimize the available capacity of the agricultural system when viewed in terms of the integrated river basin management approach enshrined in the E.U. Water Framework Directive.

The SEAL Project

We believe that not all land has an equal risk of contributing nutrients to receiving waters. This risk is controlled by location and not the P Index. The SEAL Project is investigating whether it is possible to minimize nutrient loss by applying sludge to land outside critical source areas regardless of P Index status.

Research is underway to develop a predictive and spatially-sensitive semi-distributed model of critical thresholds for sludge application that goes beyond traditional 'end-of-pipe' or 'edge-of-field' modelling, to include delivery to receiving waters from non-point sources at the river basin scale.

Our research output will be synthesized in an advice matrix (the NERM.) for end-users such as our Water Utility and Environment Agency collaborators, to determine the most appropriate form and frequency of spatially-sensitive sludge application to land to achieve sustainable sludge management without detriment to the environment and receiving water quality.

The specific project objectives are:

1. To develop an advice matrix for end-users to promote environmentally sensitive sludge recycling to land.
2. To produce a transferable, semi-distributed model predicting the environmental impact of nutrient export from non-point sources at the catchment scale.
3. To evaluate whether inclusion of critical source areas (CSAs) in predictive models can improve mapping of risk areas for biosolids-amended land.
4. To derive spatially-sensitive nutrient export coefficients to validate modelling of surface and subsurface flowpaths of nutrient loss from biosolids-amended land.
5. To advance understanding of the form and fate of nutrients derived from biosolids applied to land.

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