Effects of Organic Chemicals in Sludge Applied to Soi

Summary

The biodegradability of sludge-bound organic contaminants was examined in mixtures of agricultural soil and wastewater sludge from two wastewater treatment plants. Most of the contaminants were slowly degraded at 15° C in soil. E.g., the concentrations of di(2-ethylhexyl) phthalate decreased to between 80 and 91% of their initial level, while the concentrations of nonylphenol (NP) and short-chained nonylphenol ethoxylates (NPEs) were constant during 28 days. A more extensive biodegradation was seen for linear alkylbenzene sulfonates (LAS) as the concentrations were reduced to below 50% of the initial values during 28 days. The biological half-life, T_½, for sludge-bound LAS in soil was estimated to 26 days. The biodegradation of sludge-bound contaminants in slurries was followed over a 14-day period, and the pre-degradation toxicity and post-degradation toxicity of the sludge were examined with one sludge sample by use of the collembolan Folsomia candida and a soil nitrification inhibition test. Biodegradation did reduce the toxicity of the sludge sample to soil nitrification but the sample was still toxic to the reproduction of F. candida after 14 days of aerobic incubation. The toxicity of sludge-bound NP and LAS was examined in an experiment in which different concentrations of NP or LAS were spiked to sludge samples. Neither NP nor LAS caused observable effects on soil nitrification or the survival of adult F. candida within the examined concentration ranges. The reproduction of F. candida was severely affected by the added chemicals as the EC50 values were between 16 and 30 mg/kg for NP and between 8 and 14 mg/kg for LAS. The Predicted Environmental Concentrations (PECs) and the Predicted No Effect Concentrations (PNECs) in soil were estimated on the basis of the concentrations of NP and LAS in the two specific sludges, an application of 6 tons of sludge per hectare, and the available effect concentrations for NP and LAS. NP and NPEs were slowly degraded in soil, and the PEC/PNEC ratio for NP was above 1 which indicates that negative effects on the soil community may follow the application of sludge containing high levels of NP and NPE. Also LAS showed a PEC/PNEC >1 immediately after sludge application to soil. However, LAS will normally degrade rapidly under aerobic soil conditions and the PEC/PNEC ratios are expected to decrease to a level below 1 after 141 and 174 days for the two sludges, respectively, if the degradation of LAS is assumed to follow first order kinetics with a T_½ of 26 days.