PROJECT DESCRIPTION
BACKGROUND
Communities, agriculture and industry in the EU produce in total about 56 million tons of organic waste yearly, out of which some 13,6 million tons or 24% are collected and utilized in separate bio waste flows. Considerable parts of these wastes are characterised by a high water content which makes a conventional treatment like combustion and composting ineffective. Moreover, conventional treatments do not take new legal requirements sufficiently into account and lead to the withdrawal of valuable nutrients from the circle of nature. Anaerobic digestion is considered as an appropriate alternative. Due to improved technologies and new legal requirements digesters at waste water treatment plants do not operate at their full capacity. Therefore, co-fermentation of sewage sludge and bio waste would contribute to both a solution both in terms of recycling and the economic use of infrastructure resources. The project contributes to Article 5 of the Landfill Directive 1999/31/EC, which is concerned with strategies for the reduction of biodegradable waste. Furthermore, it is also relevant with respect to the Sewerage Sludge Directive (86/278/EC) for the treatment and disposal of sludge, sand and screenings and the Urban Waste Water Treatment Directive 91/271/EC.
OBJECTIVES
The project would involve the following tasks: • Regional data collection of organic wastes; technical, logistic and economic investigation in terms of suitability for co-fermentation. Based on a survey of the most important regional waste flows, their suitability for co-fermentation together with sewage sludge will be investigated. Contaminants (heavy metals, trace organic components) would be taken into account, as well as potential hygienic problems and disturbing components. • Pilot-scale investigations on co-fermentation for poorly degradable (solid) organic wastes with sewage sludge. • Technical-scale demonstration of co-fermentation with approved wastes and sewage sludge. • Construction of a prototype for bio waste percolation for poorly degradable waste materials, which are not suitable for direct feeding to the sewage sludge digester even after homogenization and contraries removal. • Dissemination activities for scientists, the operators and administrations of Waste Water Treatment Plants.
RESULTS
In the community of Tulln investigations were carried out to determine material flows. All recorded material flows were tested for their importance and applicability for co-fermentation in the digestion tank. Those which were found to be the most suitable were chosen for demonstration trials: • Direct Co-fermentation: flotation sludge, fat trap contents For direct co-fermentation besides removal of disturbing substances (for example removal of metal pieces) no extra pre-treatment is necessary. The organic material can be put directly into the digestion tanks The pilot trial (digestion tank volume 6.2 m³) for the semi-technical demonstration (sewage sludge fermentation with flotation sludge) showed up at which amount of flotation sludge added to the sewage sludge the degradation works at its optimum. Degradation performed at its best with a 10 % flotation share in the total mixture. The WWTP could almost double its energy production and by these means substitute fossil energy sources. A Macro-technical demonstration was implemented at the WWTP in Schwechat with a total capacity of 9000 m³. Monitoring showed wide deviations within the field of digestion gas productivity, which can be explained by varying charges of co-substrates. The results showed clearly that it is necessary to add the co-substrate to the digestion tank continuously to get constant digestion biogas production. By that way the energy management of the plant can be influenced to achieve good results. • Indirect Co-fermentation: bio-container from inner city areas, food waste, wastes from the pharmaceutical industry. Simple pre-treatment of the waste is necessary for indirect co-fermentation. Direct application to the digestion tank is impossible because of composition and consistence of the substrate. It needs to be grinded and diluted with water if necessary. The implemented pilot trial (digestion of bio-container and food waste) showed a gas production which yielded at maximum 50 % above the levels for biogas production from only sewage sludge. The digestion tanks of the municipal WWTP in Tulln (capacity of 620 m³ and 450 m³) served as a demonstration plant for co-treatment of highly protein-containing wastes from the pharmaceutical industry for the macro-technical trails. 4 % of protein wastes were added to the digestion tank, which finally led to a 50 % increase of biogas productivity. With the laboratory tests (demonstrations with food waste and sewage sludge in a micro-technical scale) the emphasis was laid on a comparison at different digester temperatures, mesophilic at 35°C and thermophilic at 55°C. The thermophilic tests showed better degradation of the organic matter than the mesophilic. The gas yield in co-fermentation increased for only 8 % under mesophilic conditions, it increased for 20 % under thermophilic conditions. • Wastes needing further processing: bio-containers with a high content of wood or gardening residues. Bio-wastes with a high content of impurities have to be processed intensively before co-treatment in the digestion tank. For this purpose a prototype (percolation reactor) was developed. The prototype rinses substances from the bio-waste in the percolation line, which can be biologically well utilized. After percolation the material was separated by pressing it into two material flows: - A solid one with a high content of dry matter, suitable for composting and; - A liquid phase which was treated anaerobically together with sewage sludge in a pilot reactor. • Dissemination activities were carried out at different seminars, through publications targeted at the operators and administration personnel. The project’s result will be taken into account for further regional planning in waste management. A technical guideline “ Limits and Merits of Co-digestion at Waste Water Treatment Plants” was compiled and distributed via the European Network on Anaerobic Digestion (AD-nett).
