In 2014, 25% of the sludge produced at Swedish municipal wastewater treatment plants was applied to agricultural land. Even though the Swedish heavy metal limits for sludge to be used in agriculture are amongst the most stringent in the EU, more stringent heavy metal limits are proposed. Most sludge applied to agricultural land is recycled within a certification system, Revaq. Revaq has targets for control at source management and improvement of sludge quality. Statistics based on data collected within the Revaq system was used to differentiate between local and general sources of heavy metals and assess the need to improve sludge quality. The analysis indicates that proposed future national limits on the quality of the sludge can be met by most of the sludge. The improvement needed for about 20% of the sludge is feasible through local control at source management. The levels of cadmium, copper and mercury need to be reduced if these metals are not to limit the amount of sludge that may be applied per unit area of arable land. Finally, the long term Revaq targets for cadmium and silver will be difficult to meet.

To provide input to sewage sludge management strategies that address expected new regulations in terms of hygienisation and phosphorus recovery in Sweden, an environmental life cycle assessment (LCA) was made. The LCA identified environmental hot spots for methods that may permit sludge or phosphorus from sludge to be applied on agricultural land. In particular, thermophilic digestion, pasteurisation, thermal hydrolysis, urea treatment and mono-incineration with phosphorus recovery were compared. In addition, a sludge management system involving drying of sludge before use in forestry was investigated. The results showed that some major impacts are related to large uncertainties, such as those related to emissions from sludge storage. It also showed that large gains can be achieved when products from the systems replace other products, in particular when biogas is used to replace natural gas in vehicles, but also when sludge is used in agriculture and forestry. In general, there are small differences between the sludge management methods. Retaining the sludge matrix to allow for its utilisation in agriculture may conflict with keeping emissions to air and water from the sludge matrix low. It is recommended that any sludge management option minimises emissions from sludge to air and water and that resources are recovered and used, in line with the principles of a circular economy.

At the Bromma sewage plant in Stockholm a project is being carried out where the aim is to elaborate different operational strategies for the plant. One of the strategies, the seeding of nitrificating bacteria from one line to other lines in the activated sludge process, was simulated with the assistance of the Danish computer program EFOR. The simulations showed that it was possible to use EFOR for this kind of simulation and that the seeding effect which was achieved in the sewage plant was of relatively small importance for reducing the NH4-N in the effluent water.

No abstract

No abstract