PROJECT DESCRIPTION
BACKGROUND
Biomass waste such as sewage sludge and manure can be processed to extract energy and phosphates. The first stage is the production of biogas through anaerobic digestion - the breakdown of organic matter in the absence of oxygen. This leaves a sludge which still contains up to 50% of the available energy. This can be extracted through incineration, to produce both energy and an ash from which phosphates can be recovered. However, before incineration can take place, the sludge needs to be dried to reduce its high water content, typically 80-97%. Unfortunately, given current techniques, more energy needs to be consumed to dry the sludge than would be extracted through its incineration. A more efficient drying method could release the tremendous potential of wet biomass waste for energy recovery and phosphate recycling. Sweden alone produces 214 000 dry tonnes/year of sewage sludge from municipal wastewater treatment, representing some 3 TWh/year of potential energy.
OBJECTIVES
LIFE SludgeisBiofuel aimed to demonstrate an energy-efficient method for drying waste biomass ready for incineration, to recover energy and phosphates. It planned to develop a new application of an existing industrial drying process, to contribute to a greater valorisation of biomass, contribute to the EU’s long-term goals around renewable energy, and improve management of phosphates throughout their lifecycle. The project beneficiary aimed to adapt a process used to dry wood and peat developed by Skellefteå Kraft AB, to create a concept for the industrial drying of biomass sludge. The aim was to test and optimise the drying technology for sewage, manure and digestion residues on a pilot scale at Skellefteå's municipal sewage treatment plant. The goal was to reduce by one quarter the energy required for evaporating water off the residue. This would make the energy needed to dry the biomass less than the energy that can be extracted from the dried biomass through incineration, thus demonstrating the commercial feasibility of full-scale plants for drying biomass residues to recover energy and phosphates.
RESULTS
The LIFE SludgeisBiofuel project demonstrated that biomass waste, in the form of sludge, can be efficiently dried and processed to extract energy and phosphates. The energy-efficient biomass drying method was demonstrated at a pilot scale, and it achieved significant energy and cost savings compared to traditional methods. The project team took an existing technology, developed for drying wood and peat, and adapted it for sludge. It was demonstrated as a separate unit at the Skellefteå municipal sewage treatment plant in northern Sweden (to minimise the impact on existing water treatment plant operations). Drying waste with a high water content is still a global technical challenge, so once fully developed, the tested technology could be transferred to any country in the world. Energy-efficient drying offers long-term benefits, as sustainable use of wet materials results in the production of bio-fuel (reducing greenhouse gas emissions) as well as nutrient recycling (e.g. phosphorous recovery from sewage sludge).
The pilot plant comprises a closed-loop steam dryer, which consumes less energy than conventional dryers. Savings are at least 400 kWh of energy per tonne of water evaporated (when drying sludge), which is a saving of 50% of energy compared to conventional dryers. This energy saving per tonne of evaporated water (due to more efficient sludge drying process) saves approximately €20 at current energy costs, which in context is a very significant energy and financial saving.
The positive energy balance should create the incentives to make this a widespread practice, contributing to wider renewable energy goals. However, building a full-scale plant for drying sewage would be costly (about €20-30 million), so potential clients are most likely to be local and regional authorities. The pilot dryer equipment was modified to optimise performance throughout the project, though it needs further technology development before the product could be sold in the market. In particular, the project worked on engineering solutions to improve the pilot plant outlet area, and to address a technical problem involving sludge sticking in the inlet area.
The project contributes to a greater valorisation of biomass in Europe, and furthers EU policy goals regarding renewable energy, waste recycling and the sustainable management of phosphates. The project beneficiary has received many inquiries expressing interest in the achieved results, particularly regarding phosphates recovery. This may be a consequence of the new nutrient recycling priority in the Circular Economy Package, to which the project outcome contributes.
Cost-efficient drying is the key to further utilise wet materials for energy production and/or nutrient recovery. The project therefore has many potential benefits for the local economy: increased demand for excess manure for drying would benefit farmers economically; additional jobs could be created in farmer cooperatives, SMEs and municipalities processing manure; sanitation of sludge and farmyard slurries would improve food safety; the closed drying loop prevents odours and makes the handling of sludge more socially acceptable; and less eutrophication means that areas, such as beaches, rivers and seas, will be improved for recreational purposes.
The pilot plant continued to be used by the sole beneficiary after the project. It is being used to test other types of biomass, as well as to further develop and improve the pilot plant. The beneficiary is working to quickly solve all technical problems, because their sister company in Germany would like to market the final product as a waste biomass dryer. In addition, the subsequent step of the biomass drying process is incineration and this step has already been accomplished by the German sister company. The technological approach is being replicated, for example, through the beneficiary’s involvement in two subsequent projects (in Sweden and Finland) where the dryer plays an important part.
Further information on the project can be found in the project's layman report (see "Read more" section).
