Ecotoxicological assessment of waste-derived organic fertilizers and long-term monitoring of fertilized soils using a multi-matrix and multi-species approach

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.

Toxicological and genotoxicological assessment of water extracts of sewage sludge and other biogenic wastes: A piece of the SLURP jigsaw puzzle

Given the considerable quantities of biogenic matrices employed in agricultural applications, there is growing concern about the potential negative effects resulting from the presence of harmful contaminants. The project "SLURP - SLUdge Recovery in Agriculture: Environment and Health Protection" planned a multi-stage approach in which the application of a wide battery of bioassays was proposed as an effective tool to measure the direct interaction of matrices with the different components of the ecosystem, from the molecular to the whole organism level. The aim of the present study, which is a part of the "SLURP" project, was to characterise the toxicological and genotoxicological properties of water extracts from biogenic wastes using several assays based on plant, bacterial and human cells. The aqueous extracts of four sewage sludges, a liming material, two manure slurries of swine and bovine origin, a digestate from bovine manure and a compost were chemically characterised for inorganic ions and heavy metals. Then the extracts were analysed using tests on A.cepa, C.sativus, L.sativum, S.typhimurium and human hepatoma cell line (HepG2) to assess toxicity (seed germination, root elongation, proliferation), mutagenicity and genotoxicity (primary DNA damage, chromosomal aberrations). The extracts exhibited chemical heterogeneity. Ammonia nitrogen, Ca2+, Fe and Zn were the most abundant elements. Toxic effects were caused on A.cepa and L.sativum by all extracts, while there were non-toxic effects on human cells. Genotoxic effects on A.cepa and L.sativum were instead caused by almost all the extracts, at least at the highest dose tested, while only four samples from one sewage sludge, liming material, digestate, and compost, caused DNA damage on human cells. None of the extracts induced mutagenic effects in S.typhimurium. A comprehensive interpretation of these results can only be achieved through the integrated evaluation of all eco-toxicological and chemical data obtained throughout the entire project.

Terrestrial bioassays for assessing the biochemical and toxicological impact of biosolids application derived from wastewater treatment plants

Wastewater treatment plants (WWTP) are facilities where municipal wastewater undergoes treatment so that its organic load and its pathogenic potential are minimized. Sewage sludge is a by-product of this process and when properly treated is preferentially called "biosolids". These treatments may include some or most of the following: thickening, dewatering, drying, digestion, composting, liming. Nowadays it is almost impossible to landfill biosolids, which however can well be used as crop fertilizers. Continuous or superfluous biosolids fertilization may negatively affect non-target organisms such as soil macro-organisms or even plants. These effects can be depicted through bioassays on terrestrial animals and plants. It has been shown that earthworms have been affected to various degrees on the following endpoints: pollutants' bioaccumulation, viability, reproduction, avoidance behavior, burrowing behavior. Collembola have been affected on viability, reproduction, avoidance behavior. Other terrestrial organisms such as nematodes and diplopods have also shown adverse health effects. Phytotoxicity have been caused by some biosolids regimes as measured through the following endpoints: seed germination, root length, shoot length, shoot biomass, root biomass, chlorophyll content, antioxidant enzyme activity. Very limited statistical correlations between pollutant concentrations and toxicity endpoints have been established such as between juvenile mortality (earthworms) and As or Ba concentration in the biosolids, between juvenile mortality (collembola) and Cd or S concentration in the biosolids, or between phytotoxicity and some extractable metals in leachates or aquatic extracts from the biosolids; more correlations between physicochemical characteristics and toxicity endpoints have been found such as between phytotoxicity and ammonium N in biosolids or their liquid extracts, or between phytotoxicity and salinity. An inverse correlation between earthworm/collembola mortality and stable organic matter has also been found. Basing the appropriateness of biosolids only on chemical analyses for pollutants is not cost-effective. To enable risk characterization and subsequent risk mitigation it is important to apply a battery of bioassays on soil macro-organisms and on plants, utilizing a combination of endpoints and established protocols. Through combined analytical quantification and toxicity testing, safe use of biosolids in agriculture can be achieved.

To spread or not to spread? Assessing the suitability of sewage sludge and other biogenic wastes for agriculture reuse

Sewage sludge (biosolids) management represents a worldwide issue. Due to its valuable properties, approximately one half of the EU production is recovered in agriculture. Nevertheless, growing attention is given to potential negative effects deriving from the presence of harmful pollutants. It is recognized that a (even very detailed) chemical characterization is not able to predict ecotoxicity of a mixture. However, this can be directly measured by bioassays. Actually, the choice of the most suitable tests is still under debate. This paper presents a multilevel characterization protocol of sewage sludge and other organic residues, based on bioassays and chemical-physical-microbiological analyses. The detailed description of the experimental procedure includes all the involved steps: the criteria for selecting the organic matrices to be tested and compared; the sample pre-treatment required before the analyses execution; the chemical, physical and microbiological characterisation; the bioassays, grouped in three classes (baseline toxicity; specific mode of action; reactive mode of action); data processing. The novelty of this paper lies in the integrated use of advanced tools, and is based on three pillars:•the direct ecosafety assessment of the matrices to be reused.•the adoption of innovative bioassays and analytical procedures.•the original criteria for data normalization and processing.

Sewage sludge amendment of rice as a potential alternative to mineral fertilizer: Analyses of physiological, biochemical and molecular mechanisms of plant response

Sewage sludge (SS) disposal poses environmental concerns, yet its organic matter, macro- and micronutrients, make it potentially beneficial for enhancing soil quality and crop yield. This study focuses on three types of SS: "R10" (SS1), which is commonly used in agricultural practices, and two environmentally friendlier options (SS2 and SS3), as alternatives to mineral fertilizer (urea) for rice cultivation. A pot experiment was conducted to investigate the ecophysiological, biochemical, and molecular responses of rice at three different growth stages. SS application led to a significant increase in biomass production (particularly SS3), along with increased nitrogen (N) levels. Enhanced chlorophyll content was observed in SS-treated plants, especially during inflorescence emergence (with the highest content in SS3 plants). At the ecophysiological and biochemical levels, SS treatments did not adversely affect plant health, as evidenced by unchanged values of maximal PSII photochemical efficiency and malondialdehyde by-products. At biochemical and gene expression levels, antioxidant enzyme activities showed transient variations, likely related to physiological adjustments rather than oxidative stress. Ascorbic acid and glutathione did not significantly vary. This study concludes that the use of SS in soil can be a viable alternative fertilizer for rice plants, with positive effects on biomass, chlorophyll content, and no adverse effects on plant health. Among the tested SSs, SS3 showed the most positive effect, even compared to commercial fertilizer. These results suggest that SS application could improve rice yield while addressing environmental concerns surrounding SS disposal.