Ecotoxicity assessment of biodegradable plastics and sewage sludge in compost and in soil: Dissertation

Biodegradable plastics, either natural or synthetic polymers, can be made from renewable or petrochemical raw materials. The most common applications for biodegradable plastics are packaging materials and waste collection bags. Other applications include catering products, wrappings, food containers, laminated paper, golf tees, hygiene products and agricultural applications. The one thing in common for all these biodegradable items is that at the end of their life cycle they should degrade into harmless end products, during a specified time frame. Depending on the target application and excluding medical applications, the degradation may take place in soil, in water, in an anaerobic digestion plant or in compost. In addition to its use as a waste treatment process for biodegradable plastics, composting can be used in the removal of organic contaminants from sewage sludge. Due to mixed contamination present in soil, in compost, or in sewage sludge the environmental impact of biodegradable materials is difficult to assess based only on concentrations of chemical constituents. Therefore, biotests are needed for detecting potential risks derived from the use of biodegradable materials in environmental applications. In this study the biodegradation properties of bioplastics targeted for agricultural and compost applications were investigated. In addition, the potential of biotests were evaluated in ecotoxicity assessment of biodegradable plastics and their components during the biodegradation process in vermiculite, compost and soil. Acute toxicity of polymer components and degradation products was screened with a kinetic luminescent bacteria test (ISO 21338), and possible hazardous compounds could be identified and further studied. During the biodegradation of chain-linked lactic acid polymers and polyurethane-based plastic material in controlled composting conditions the release of toxic degradation products could be demonstrated by biotests. Clear toxic responses in the luminescent bacteria test and/or plant growth test (OECD 208) were observed. The fate of an endocrine disrupting plasticizer, diethyl phthalate (DEP) was also studied in a controlled composting test, in pilot composting scale and in plant growth media. A high concentration of DEP induced changes in the microbial community, gave a clear response in the biotest and its degradation was inhibited. However, in pilot scale composting toxicity was not detected and the degradation of DEP was efficient. The studied starch-based biodegradable mulching films showed good product performance, good crop quality and high yield in protected strawberry cultivation. Furthermore, no negative effects on the soil environment, Enchytraeidae reproduction (ISO 16387) or amoA gene diversity were detected. Biotests were also used to study compost quality during sewage sludge com-posting. If sewage sludge is used as a soil conditioner, many harmful substances can potentially end up in the environment. In our study, composting reduced efficiently the amount of organic contaminants such as DEHP, PAH, LAS, and NPs in sewage sludge. In addition, composting resulted in reduction in acute toxicity, genotoxicity and endocrine-disruption potential of the sewage sludge. The use of biotests is recommended as an indicator of potential risk when sewage sludge-based products are used in agricultural or landscaping applications. Potentials and also limitations were recognized in the performances of different biotests when studying the ecotoxicity of biodegradable materials during biodegradation processes in compost and in soil environment. With biotests it was possible to identify potential hazardous polymer components or degradation products that might be released to the environment during the degradation. In addition, the biotest could be used to monitor the detoxification of sewage sludge during the composting process. However, soil, compost and sludge as testing environments do set limitations for the use of biotests. Colour, amounts of nutrients, additional carbon sources, presence of bark and peat, compost immaturity, and high microbial activity are some of the factors limiting the use of biotests or complicate interpretation of the results.