By Athanasios Stasinakis
The pilot-scale sewage sludge treatment system is ready for use as of March 2024! The system was installed in the Sewage Treatment Plant of Mytilene (Lesvos Island, Greece) and it combines sludge pre-treatment (thermal or ultrasound), modified anaerobic digestion, hydrothermal carbonization and solar drying. During the next 2 years, it will be tested for the removal of PFAS and other persistent, mobile and toxic and very persistent and very mobile (PMT/vPvM) substances from sewage sludge.
In 2020 more than 8.9 million tonnes of sewage sludge were produced in the EU-27 countries. According to the available Eurostat data, 35% of the sludge was directly used is agriculture, 15% was composted, and 10% was landfilled. Sewage sludge is very often contaminated with persistent, mobile and toxic and very persistent and very mobile (PMT/vPvM) substances such as per- and polyfluoroalkyl substances (PFAS), benzotriazoles (BTRs) and other organic micropollutants. Applying sewage sludge to soil increases the risk of exposure of the terrestrial environment, contaminating groundwater and affecting plant and animal life.
Sludge treatment is typically carried out using mesophilic anaerobic digestion. However, this practice is characterised by a lack of integrated knowledge about the fate of diverse PMT/vPvM sustances. Recently, modifications of the conventional anaerobic digestion system (via sludge pre-treatment, addition of voltage or conductive materials in the bioreactor) have shown increased removal efficiency of conventional pollutants and biogas production. In addition, novel thermal processes such as hydrothermal carbonization have begun attracting attention at lab and pilot scale during the last years for sludge and other biosolids treatment, for potential upscaling. This thermochemical process, that involves the conversion of organic materials into a carbon-rich solid product (hydrochar) under high-temperature and high-pressure conditions in the presence of water, offers specific advantages such as carbon sequestration, energy recovery, nutrient retention and the reduction of pathogens and odours, outcomes that are consistent with a more circular economy.
To date there is no information available about the fate of PMT/vPvM substances during modified anaerobic digestion and hydrothermal carbonization. To address this a pilot-scale sewage sludge treatment system was designed by members of University of the Aegean research team, installed in the Sewage Treatment Plant of Mytilene (Lesvos Island, Greece) and is now ready for use. The pilot system combines sludge pre-treatment (thermal or ultrasound), modified anaerobic digestion, hydrothermal carbonization and solar drying. During the next 2 years, different operational conditions will be tested and performance and energy requirements will be monitored. Samples will be systematically collected from the different stages and will be analyzed for PMT/vPvM substances. To address the circular economy, the hydrochar prodcued will be studied for its potential future uses such as for soil amendment or as a fossil fuel alternative.
Contact: Athanasios Stasinakis, astas@env.aegean.gr, University of the Aegean
More info: Visit our Test Sites page and see more of our Technical Solutions to PFAS and PMT/vPvM substances.