Ecotoxicological assessment of sewage sludge-derived biochars-amended soil.

The study aimed to evaluate the ecotoxicity of soil (S) amended with biochars (BCKN) produced by the thermal conversion of sewage sludge (SSL) at temperatures of 500 °C, 600 °C, or 700 °C and SSL itself. The ecotoxicological tests were carried out on organisms representing various trophic levels (Lepidium sativum in plant, Folsomia candida in invertebrates, and Aliivibrio fischeri in bacteria). Moreover, the study evaluated the effects of three plants (Lolium perenne, Trifolium repens, and Arabidopsis thaliana) growing on BCKN700-amended soil on its ecotoxicological properties. The experiment was carried out for six months. In most tests, the conversion of sewage sludge into biochar caused a significant decrease in toxicity by adding it to the soil. The pyrolysis temperature directly determined this effect. The soil amended with the biochars produced at higher temperatures (600 °C and 700 °C) generally exhibited lower toxicity to the test organisms than the SSL. Because of aging, all the biochars lost their inhibition properties against the tested organisms in the solid-phase tests and had a stimulating influence on the reproductive ability of F. candida. With time, the fertilizing effect of the BCKN700 amended soil also increased. The aged biochars also did not have an inhibitory effect on A. fischeri luminescence in the leachate tests. The study has also demonstrated that the cultivation of an appropriate plant species may additionally reduce the toxicity of soil fertilized with biochar. The obtained results show that the conversion of sewage sludge to biochar carried out at an appropriate temperature can become a useful method in reducing the toxicity of the waste and while being safe for agricultural purposes.