Effects of Organic Chemicals in Sludge Applied to Soil

5. Conclusions

The following conclusions may be derived from the present study:

1. The biodegradation of sludge-bound contaminants as e.g. DEHP, NP and short-chained NPEs, and PAHs is expected to be slow under normal aerobic soil conditions.
2. Sludge-bound LAS may be rapidly degraded in aerobic soil, and the half-life of 26 days which was estimated in this study, is in accordance with results from field experiments.
3. Biodegradation of organic chemicals in an intact sludge sample reduced the toxicity of the sludge to soil nitrification. However, the sludge was still toxic to the reproduction of F. candida after 14 days of aerobic incubation.
4. NP or LAS that were spiked to sludge samples were not toxic to soil nitrification or to 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.
5. The slow biodegradation of sludge-bound NP and NPEs, and the possible formation of other nonylphenolic degradation products, indicates that negative effects on the soil community may follow the application of sludge containing high levels of NP and NPEs.
6. Sludge-bound LAS may cause adverse effects to soil-living organisms immediately after the application of sludge. However, LAS will normally degrade rapidly under aerobic soil conditions and is not expected to accumulate to toxic levels in soil when sludge with a typical LAS content (e.g. below 6,000 mg/kg) is applied in the amounts prescribed in the regulation (Ministry of Environment and Energy 1996).