Sewage sludge quality from small wastewater treatment plants

The authors evaluated the quality of sewage sludges of 10 typical small-scale wastewater treatment plants receiving mainly domestic or urban wastewaters from an area characterised by a prevalently rural economy, with only small contributions from industrial effluents. In view of their direct distribution on agricultural land and/or utilisation to produce good-quality compost, in addition to macronutrients (carbon, nitrogen, phosphorus, potassium), micronutrients (calcium, magnesium, iron, boron, molybdenum) and toxic metals, sludges were analysed for extractable organic halogens, polychlorinated biphenyls, dibenzo-p-dioxins and furans, polycyclic aromatic hydrocarbon, di-2-(ethyl-hexyl) phthalate, nonylphenol, nonylphenolethoxylates and linear alkylbenzene sulfonates. Contaminant loads were generally low and did not represent a potential limitation for use in agriculture at the currently allowed rates. Results were examined by principal component analysis of the whole dataset, which did not show any significant grouping according to treatment plant origin, water source and type of treatment, and confirmed the substantial homogeneity of the sludges with respect to both toxic metals and organic contaminants.