Proceedings

As a result of the relatively long persistence of polycyclic aromatic hydrocarbons (PAHs) in the environment, concerns have been expressed that repeated biosolids additions to agricultural soils could lead to the accumulation of elevated PAH levels. Notably, proposed revisions to the EU Sludge Directive have included maximum permissible concentration limits for PAHs in biosolids. In order to assess the potential for long-term accumulation in agricultural soils, PAH concentrations in untreated control treatments were compared with biosolids amended treatments at seven of the “Long-term Sludge Experiment” (LtSE) sites following eleven years of annual biosolids additions.

At the seven LtSE sites, there were no significant increases (P>0.05) in soil PAH concentrations (sum of the 9 EU congeners of concern) above the untreated control. In summary, the added risk from eleven years of annual biosolids applications was un-measurable in the soil.

Keywords: Biosolids, organic materials, polycyclic aromatic hydrocarbons.

Introduction: The recycling of organic materials to land is regarded as the best practicable environmental option in most circumstances, completing both natural nutrient and carbon cycles. Organic materials are valuable sources of major plant nutrients (i.e. nitrogen – N, phosphate – P2O5, potash – K2O and sulphur – SO3), which are essential for plant growth and therefore sustainable crop production. Organic materials also provide a valuable source of organic matter, which improves soil water holding capacity, workability and structural stability etc.

Treated sludges (commonly called biosolids) and other organic materials (e.g. livestock manures, compost, digestate) are widely used by farmers and growers to meet crop nutrient requirements and to maintain soil fertility. Biosolids (e.g. digested cake, lime stabilised cake) and green compost (i.e. composted plant and vegetable material) are the most common non-farm organic materials that are applied to agricultural land. On a fresh weight basis, 3-4 million tonnes of biosolids (Water UK, 2010) and 1.9 million tonnes of compost (WRAP, 2012) are recycled to agricultural land on an annual basis.

The application of organic materials to land, as well as conferring benefits to the receiving soil, must not cause harm to either the receiving land or the wider environment (i.e. soil, water and air quality, or human health). The risks posed by nutrient enrichment have been documented and researched (Shepherd, 1996; Withers, 2011), and as a result, legislation governing the recycling of organic resources to land is aimed at limiting the impact of nutrients (e.g. Defra/EA, 2008) on the wider environment. Similarly, research has focused on the impact of heavy metals in biosolids on soil quality and fertility (Gibbs et al., 2006; McGrath, 1994; McGrath et al., 1995) and as a result legislation (i.e. SI, 1989) and the Code of Practice for the Agricultural Use of Sewage Sludge (DoE, 1996) limit heavy metal loading rates to soils.

In contrast, there are no guideline or limit values controlling Organic Compound Contaminant (OCC) loading rates to agricultural land. However, the 3rd Working Document of the EU Commission on Sludge Management (EU, 2000) and the Second Draft Working Document on the Biological Treatment of Biowaste (EU, 2001) contained proposed limit values for polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins/dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs), amongst others (Table 1).