Chemical Monitoring of Sewage Sludge in Pennsylvania: Variability and Application Uncertainty

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Abstract
Application of sewage sludge to farmland in Pennsylvania (PA) is regulated by crop N needs and the cumulative loading of eight trace elements. A survey of 7746 sewage sludges produced at 177 publicly owned (wastewater) treatment works (POTWs) in PA from 1978 to 1997 was conducted to determine what changes have occurred in nutrient and trace element concentrations, how currently produced sewage sludges compare with regulatory trace element standards, inter‐ and intra‐POTW variability in sewage sludge constituents, and uncertainty estimates for application of nutrients and trace elements. Concentration medians and ranges of nutrients changed very little during the 20‐yr period, whereas there were large decreases in Cd, Cr, Cu, Pb, Hg, Ni, and Zn. Rates of decrease for concentrations of these elements and As, Se, and Mo were smaller or not different from zero in the last 5 yr of the survey. Assessment of sewage sludge constituents among a subset of 12 POTWs showed larger inter‐ than intra‐POTW variability. Sewage sludge N content uncertainty showed that the maximum error in determination of plant available N ranged from 0.39 to 1.09 of the intended amount. This uncertainty could be reduced by increased frequency of N analysis prior to sewage sludge application. Application uncertainty for trace elements was large relative to the amount of trace elements added to the soil in a single application, but very small relative to the cumulative loading limit (generally <0.01). Further decreases in trace element variability or increased frequency of analysis would not be expected to decrease error in trace element cumulative loading calculations.

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