Assessment of biosolids produced in Northern Greece applied to agricultural soils as phytoprotective agents

In the context of circular economy, biosolids from three WWTPs (WWTP1, 2 and 3) in Northern Greece were tested for their metal content (pseudo-total and leachate content) and their ecotoxicity on D. magna neonates, as part of a risk assessment strategy. Results showed metal concentration much lower than the limits of Greek legislation for use in agriculture. Low but non-negligible toxicity on D. magna was detected, irrespective of biosolids’ origin or seasonality. Biosolids from WWTP1 were applied against Verticillium wilt of pepper plants caused by the fungus Verticillium dahliae. These biosolids had undergone additional processing in relation to previous research in 2018–2019, but this did not alleviate D. magna toxicity whilst it affected metal leachate in the following ways: in the EN-12457-2 leaching test, Ni and Zn content were increased in relation to previous research; Cu leaching was variable, while Pb, Cd (and Hg) remained undetectable. A correlation between D. magna toxicity and Cr concentration in the leachates was observed but it did not reach statistical significance. Regarding their ability to act as phytoprotective agents, biosolids incorporation at 2 % w/w reduced wilt severity caused by V. dahliae on pepper. Similar results have previously been observed for Fusarium root rot of tomato. Plant growth characteristics were not enhanced in healthy plants. However, several growth parameters characteristics were promoted in the presence of the pathogen as also noted in our previous research. The results encourage research into novel agronomic uses of biosolids, including their use to produce novel plant growth boosters.