Operational temperature within the digester is about 131☏ (55☌).Īll inputs are combined in a mixing unit in the reception hall (top). This crossflow paddle mixer slowly turns the input material to facilitate the process and release biogas. The core technology is the latest TTV horizontal thermophilic plug-flow digester that employs a horizontal steel digester equipped with a massive single end-to-end paddle mixer. The compost hall is about 170-feet by 130 feet, and the digester, including input pipework and other support infrastructure, has a footprint of 115 feet by 40 feet (the digester body itself is about 33-feet wide).Īfter a competitive bid process, Thöni Industriebetriebe GmbH of Austria was selected as the technology supplier. The footprint of the existing site - about 5 acres - was adequate to accommodate the addition of the HSAD. The decision was made, therefore, to build a high solids anaerobic digester (HSAD) versus a wet AD system with greater volumes of low solids digestate. It also meant that in operational terms, production of large volumes of low value liquid digestate would not be welcome in an area with limited agricultural production. This meant a guaranteed construction “window” of only six months. Selecting HSADĬlimatically, the Gävleborg region typically receives substantial snow during the cold, dark winters. The new facility had to continue the success it had achieved processing commingled yard trimmings by handling as much food waste as possible while generating quality products for established markets. In 2016, that number rose to 529 lbs/person (240 kg). The quantity of food waste collected in the Gävleborg region was 485 lbs/person (220 kg) in 2015.
To achieve that energy output, a throughput capacity of up to 38,500 tons (35,000 metric tons) was necessary. Designing the plant to an energy target rather than tons per annum of waste was a new approach to facility development and unusual in the organic waste sector, where design capacity tends to principally reflect seasonality and local waste collection issues. The site operator, Gästrike Ekogas AB, a company owned by Gästrike återvinnare and Gävle Energi, set a key objective of generating a target of 25 Gigawatt hours (Gwh) per year of energy as biomethane. In 2014, the municipalities that own the plant decided to restructure the business with a particular focus on energy, increasing the range of waste accepted, improving product quality, enhancing site environmental protection - particularly in terms of allowed greenhouse gas emissions as methane slippage - and integrating the site with other energy interests of the management group. High aeration also meant inclusion of a high proportion of wood such as landscaper hardwoods that could be recirculated through the mix, rather than the more plentiful local softwoods. The composting process relied on very high aeration rates and air circulation, which was proving costly. It was one of only a handful in Sweden approved for treatment of material such as food waste under the European Animal By-Products Regulations.Īround 2013, it became apparent that process modifications were needed, even though operation of the facility was technically good, with compost quality meeting the needs of local markets. Inputs were mostly yard trimmings with some putrescibles. Its state-of-the-art in-vessel composting (IVC) system was supplied by VAPO of Finland.
A recent addition to the region, in the town of Forsbacka, is a high solids anaerobic digestion plant that was integrated into an existing in-vessel composting facility that opened in the 1990s to serve the Gävleborg and a population of around 100,000 residents. Inland from the city of Gävle, Sweden on the Baltic coast, the Gävleborg region is crisscrossed with wetlands, lakes and rivers and surrounded by miles of the famous Swedish timber forests. Municipalities that own a composting plant restructure with a particular focus on energy, increasing the range of waste accepted and improving product quality.
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