Toxic potential of different types of sewage sludge as fertiliser in agriculture: ecotoxicological effects on aquatic,sediment and soil indicator species

Purpose

Treated and processed sewage sludges (biosolids) generated during the treatment of wastewater usually contain substantial concentrations of nutrients, especially phosphorus, which is essential for plant growth. Sewage sludge therefore can be used as an alternative fertiliser in agriculture. But since sewage sludge could also contain pollutants, analysis and ecotoxicological tests on affected soil and stream water organisms are necessary in order to guarantee its harmless use.

Materials and methods

Three test species were chosen to cover the environmental compartments, water, sediment and soil. The following test species and parameters were applied to evaluate the acute effects of three sewage sludge samples: Lemna minor (growth inhibition, discolouration and colony breakup), Gammarus fossarum (mortality, behaviour) and Eisenia fetida (avoidance behaviour). Chemical assessment included nutrients, organic pollutants and heavy metals.

Results and discussion

The assessment of a non-dewatered sludge (S1) sample resulted in an inhibition of growth of L. minor starting from 0.6 g total solid (TS)?l^?1 after 7 days (EC₅₀ 1.2 g TS l^?1). G. fossarum displayed significantly decreased movement activity at 0.5 and 1.2 g TS l^?1 sludge concentration during an exposure time of 2 days, leading to decreased survival after 4 days of exposure in 0.5 g TS l^?1 (LC₅₀ 0.5 g TS l^?1). After 2 days, E. fetida exhibited an increased avoidance behaviour of contaminated soil from 0.2 g TS kg^?1 sewage sludge (EC₅₀ 0.4 g TS kg^?1). The dewatered sludge samples (S2 and S3) had a lower toxic effect on the test organisms. G. fossarum was the most sensitive test species in the applied test setups. The realistic application amounts of the tested sewage sludge samples of approximately 6.0 g TS kg^?1 (maximum allowed application amount of sewage sludge) and approximately 3 g TS kg^?1 (maximum agronomical relevant application amount) in worst case studies are higher than the analysed EC₅₀/LC₅₀ values of S1 and of the LC₅₀ (G. fossarum) of S2 and S3.

Conclusions

All three tested sewage sludge samples have to be classified as toxic at high concentration levels under laboratory conditions. Realistic output quantities of S1 will negatively influence soil invertebrates and freshwater organisms (plants and crustacean), whereas the dewatered sludge samples will most likely not have any acute toxic effect on the test organisms in the field. Test with environmental samples should be conducted in order to support this hypothesis.