Chemical monitoring of sewage sludge in Pennsylvania: Variability and application uncertainty

Application of sewage sludge to farmland in Pennsylvania (PA) is regulated by crop N needs and the cumulative loading of eight trace elements. A surve y of 7746 sewage sludges produced at 177 publicly owned (wastewater) treatm ent works (POTWs) in PA from 1978 to 1997 was conducted to determine what c hanges have occurred in nutrient and trace element concentrations, how curr ently produced sewage sludges compare with regulatory trace element standar ds, inter- and intra-POTW variability in sewage sludge constituents, and un certainty estimates for application of nutrients and trace elements. Concen tration medians and ranges of nutrients changed very little during the 20-y r 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 uncerta inty showed that the maximum error in determination of plant available N ra nged from 0.39 to 1.09 of the intended amount. This uncertainty could be re duced by increased frequency of N analysis prior to sewage sludge applicati on. Application uncertainty for trace elements was large relative to the am ount of trace elements added to the soil in a single application, but very small relative to the cumulative loading limit (generally <0.01). Further d ecreases in trace element variability or increased frequency of analysis wo uld not be expected to decrease error in trace element cumulative loading c alculations.