Metal concentrations in lime stabilised, thermally dried and anaerobically digested sewage sludges.

Abstract

Cognisant of the negative debate and public sentiment about the land application of treated sewage sludges ( biosolids ), it is important to characterise such wastes beyond current regulated parameters. Concerns may be warranted, as many priority metal pollutants may be present in biosolids. This study represents the first time that extensive use was made of a handheld X-ray fluorescence (XRF) analyzer to characterise metals in sludges, having undergone treatment by thermal drying, lime stabilisation, or anaerobic digestion, in 16 wastewater treatment plants (WWTPs) in Ireland. The concentrations of metals, expressed as mg kg-1 dry solids (DS), which are currently regulated in the European Union, ranged from 11 (cadmium, anaerobically digested (AD) biosolids) to 1273 mg kg-1 (zinc, AD biosolids), and with the exception of lead in one WWTP (which had a concentration of 3,696 mg kg-1), all metals were within EU regulatory limits. Two potentially hazardous metals, antimony (Sb) and tin (Sn), for which no legislation currently exists, were much higher than their baseline concentrations in soils (17 to 20 mg Sb kg-1 and 23 to 55 mg Sn kg-1), meaning that potentially large amounts of these elements may be applied to the soil without regulation. This study recommends that the regulations governing the values for metal concentrations in sludges for reuse in agriculture are extended to include Sb and Sn.