Metal Variability in Sewage Sludge (Biosolids) to Agriculture Land

Sewage sludge (biosolids) disposed to agricultural land in Ireland need only be analysed once a year if disposed to agricultural land for plants of <100,000 PE according to the Code of Good Practice For the Use of Biosolids in Agriculture published by the Department of the Environment and Local Government in 2008. Metals in Irish sludges have decreased over the past 20 years resulting in their widespread use in agriculture with 76% of the 60,000 t DS produced each year now going to farmland. In a recent study done by Elizabeth Whitney of the Water Technology Research Group, the variability of sludge metal concentrations from 12 wastewater treatments plants was examined every two weeks for up to six months. The sludges showed wide variability as measured by the coefficient of variation (CV) over time with each plant with at least one metal where the CV was >50% over that period. Mean plant CV (and range) of the metals studied were Cr 29.5% (11-91%), Cd 48.0% (3-118%), Cu 73.4% (4-168%), Pb 26.5% (6-115%), Ni 46.5% (6-267%) and Zn 26.7% (6-100%). High CV values were generally indicative of an increasing or decreasing trend. However, on some occasions periods of high metal concentration were recorded indicating an intermittent source. The results show that significant variation in sludge metal concentrations occur over time and that in order to ensure safe disposal to agricultural land, the required certificates issued to farmers containing details of metals in the sludge should be either the actual metal concentrations in sludge applied or at least the upper (90 percentile) limit values based on recent monthly samples in order that correct and safe loading rates can be calculated.