Approximately one million tons of solid matter, known as sewage sludge, is separated from wastewater annually in Finnish wastewater treatment plants. 90 % of the nutrient rich sludge is used in agriculture and landscaping. The use of sewage sludge as fertiliser and soil conditioner increases soil organic matter that has often been diminished owing to repeated artificial fartilisation. Sewage sludge also improves soil structure and water holding capacity. Fertilisers produced from sewage sludge are monitored for the concentrations of certain bacteria and heavy metals. However, the agricultural use of sewage sludge-based fertilisers is often hampered by organic contaminants and microplastics, for which there are currently no limit values. The occurrence of organic contaminants and microplastics in sewage sludge is not routinely investigated. This is partly due to lack of research data and high cost of chemical analytics.
As sludges are mixtures of numerous compounds, their safety cannot be reliably assessed by chemical analyses only. In bioassays, the interactions between substances and the importance of bioavailability are examined comprehensively. Bioassays take all contaminants into consideration, including those not detected by the chosen chemical analyses. In previous studies, mainly aquatic species have been used in ecotoxicity estimations of sewage sludge. It is, however, difficult to assess terrestrial effects based on information obtained from aquatic bioassays onely. Routine chemical analyses and bioassays would help determine the potential harmfulness of sludge. The obtained results could then be utilised in the development of sludge treatment methods.
In this project:
- The most suitable bioassays for investigating the adverse effects of sewage sludge-based fertilisers will be identified
- The concentrations, bioavailability, transport and adverse effects of chemical contaminants and microplastics in treated sewage sludge will be investigated by using bioassays, chemical analyses and computer modelling
- Guidelines for assessing the safety of sewage sludge-based fertilisers will be developed