The fertilisers may be harmful for organisms due to harmful organic substances, microplastics, and even genotoxic compounds. The harmful substances can accumulate in the field, plants, and soil organisms.
─ In Finland, up to 90 per cent of the sludge from sewage treatment plants is utilised as a fertiliser or soil improver either in agriculture or in green area development, explains Katri Senilä, who is researching the subject for her dissertation. Senilä works as a junior researcher at the University of Eastern Finland, and a doctoral researcher for Finnish Environment Institute.
─ Recycled fertilisers are more topical now than ever before. Russia’s invasion of Ukraine is increasingly accelerating Finland’s transition towards a circular economy. Fertilisers are not imported to Finland due to sanctions against Russia. The fertiliser legislation is also currently being reformed at EU and national level.
The harmful substances contained in the sewage sludge end up in the treatment plant with wastewater. In continuous use, some of these harmful substances are accumulated in the field, and also in the field organisms.
According to Senilä, the best way to improve the quality of sewage sludge is to prevent the harmful substances from entering the treatment plant altogether.
─For example, this would mean using chemicals and medical substances that are more environmentally friendly, as well as restrictions on the use of the harmful substances.
Katri Senilä
Early Stage Researcher, UEF and Doctoral Researcher, Finnish Environment Institute
Senilä’s research is a part of the BIOLTA project coordinated by SYKE. The aim of the BIOLTA project is to produce up-to-date and objective information on the environmental impacts of sewage sludge fertilisation.
The accumulation and transport of harmful substances are examined with extensive testing
Among other subjects, Senilä has previously researched the parasites in fish and leaching of the pesticide glyphosate from field to water.
—I am especially interested in the environmental protection issues of agriculture. SYKE proposed bioassays of sewage sludge as the topic of my dissertation, and I also found a partner from a composting company that mainly processes sewage sludge, Senilä explains.
—The research examined the sewage sludge compost and pelleted sewage sludge. I studied fertiliser products and water extracts from them through a number of different bioassays, with the purpose of finding the most suitable bioassay methods. Additionally, the samples have been tested for heavy metals and a large variety of harmful organic substances. Based on the results, we are able to recommend which harmful substances above all should be monitored in the fertilisers.
The research subjects also include fields treated with sludge-based fertilisers, where the sewage sludge-based material has been used as the sole fertiliser for decades. The control samples have been taken from adjacent fields fertilised with broiler manure, where sewage sludge has never been used.
—These samples were analysed for the same harmful substances as the fertiliser products, and I have compared the harmful substance concentrations in fields fertilised in different ways. This will help detect which substances have accumulated in the field as a result of decades of fertilisation. Earthworms were also collected from the fields, and the worm tissues were examined for the same harmful substances as in the soil.
Transport modelling is also an essential part of the research.
Katri Senilä
Early Stage Researcher, UEF and Doctoral Researcher, Finnish Environment Institute
─The breakdown and migration of certain harmful substances is currently being by computer simulations, Senilä explains.
—The modelling was supported with field samples, which were analysed for triclosan (TCS) used as an antibacterial substance and its degradation product methyl triclosan (MTCS), as well as the perfluorinated alkyl compounds (PFAS) before fertilisation, after fertilisation, during the growing season, after the growing season, and the following spring.
—The samples were used to monitor changes in the concentrations of substances over time. We also draw up guidelines for the routine examination of sewage sludge-based fertilisers.
—It is already clear from the results of the research that the current fertiliser legislation is not sufficient to ensure the safety of sewage sludge fertilisation. Simply examining heavy metals from fertiliser is not enough, we need to monitor the harmful organic substances also.
—In particular, strict limits should be set for the quantities of PFAS in fertilisers in order to safeguard the environment and the purity of food. In addition, the genotoxicity of sewage sludge-based fertilisers should be investigated by means of bioassays, Senilä says.
