Do digestates produced by co-digestion of biowaste and biodegradable plastic affect soil and aquatic organisms?

The fast development of the biogas sector requires a better understanding of potential ecotoxicological effects of digestates on both terrestrial (through digestate spreading) and aquatic organisms (through digestate soil leachate). This study is the first one to assess through a multi-species approach that include aquatic organisms short-term ecotoxicological effect in microcosm experiments of digestates (and their respective leachates) derived from biowastes co-digested or not with biodegradable plastics (PLA or PHB). Ecotoxicological tests were performed on six terrestrial/aquatic bioindicators: (1) wheat, (2) tomato, (3) soil bacteria, (4) earthworm, (5) duckweed and (6) water flea. Digestate fertilization treatments were compared to a soil not fertilized and to a mineral fertilization ensuring the same level of N (170 kg N/ha) and P (17 kg P/ha) for all fertilized treatments. No ecotoxicities of all digestates and their soil leachates were observed on wheat, soil bacteria, earthworm, duckweed and water flea following laboratory standardized procedures. Attention should be given to possible ecotoxicity of biowaste digestates (with or without biodegradable plastics) on tomato growth where a lower dry aerial biomass (23 % less in average) was observed compared to the soil chemically fertilized. For all bioindicators, no differences were observed when comparing fertilization with biowaste digestate to its equivalent co-digested with biodegradable plastics. Therefore, no apparent ecotoxicity related to the potential presence of residual biodegradable plastics in digestates was highlighted. Long-term ecotoxicological effects and repeated exposure to biodegradable plastics from digestates as well as comparison with conventional plastics are beyond the scope of this article and will require further research.