Since 2005, biowastes and energy crops from Västerås in Sweden have been treated in an integrated biogas plant. The outcome? Fuel equivalent to 2.3 million litres of petrol each year for all the city’s buses and waste trucks and 500 other vehicles, generation of heat and power, and valuable fertilizers for local farms. Surely this remarkable project could serve as a role model for other towns around the globe.
Sweden has declared its intention of becoming completely independent of oil in the coming 20-30 years. How? More plants like the new one at Västerås offer one route - here a community is replacing oil products with transport fuel made from its own biowaste resources. This is a fine project, but is by no means unusual in Sweden, where many buses and other vehicles bear the label ‘run on biogas’ and toll motorways have signposts saying ‘toll-free for biofuelled vehicles’.
The beauty of this particular system is that it takes separated household biowastes, together with a grass crop grown and harvested by local farmers, and generates biogas for use as a vehicle fuel in the nearby city of Västerås, while surplus gas is used in a combined heat and power plant to provide electricity which feeds into the grid, and heat for the local district heating system.
 Buses and other heavy vehicles are filled up with the biofuel produced at the plant |
Sweden requires all its municipalities to draw up waste management plans with goals, strategies and action plans to improve the handling of waste from an environmental point of view. The municipalities within this region of Sweden, to the west of Stockholm, take an active share in the regional waste management planning, along with the regional waste management company, VafabMiljö. Planning encompasses issues such as waste avoidance and minimization, information and education activities, economic incentives such as differentiated waste handling fees, source separation, collection and transport of waste and waste fractions. Reuse, material recovery and energy utilization of clean, source-separated waste fractions are high priorities. The residual waste is source-separated into two parts. The first is a high-quality fraction containing wet organic waste, mainly food residuals, for biological waste treatment in combination with energy utilization if possible. The other is a residual, high calorific-value residual waste fraction for incineration in waste-to-energy plants - these produce electricity, but in addition the heat is often used within a municipality’s district heating system.
Keeping the waste green
Households in the district covered by the Växtkraft plant are not obliged to separate their waste. They have the option of participating in the source separation scheme, which means that their biowaste is collected for processing at the plant. However, if they prefer, they can compost it themselves, or put it out with other kitchen waste for treatment in a waste-to-energy plant. The voluntary nature of the arrangement minimizes the risk of the biowaste being contaminated by other wastes in unmotivated households.
The households that do sign up to separate their biowaste sign a written agreement with the municipality, committing themselves to separating the waste in accordance with instructions from the municipality (it also gives the municipality the right to do occasional checks if required).
Of the 144,000 households in the region, approximately 90% participate in the scheme, with another 7% carrying out home composting. Only 3% choose not to separate their waste.
In the bag
Households are provided with special brown paper bags for the biowaste, and these are printed with sorting instructions to serve as a reminder. The only permissible contents are food leftovers, garden waste, wilted flowers, pot plants and household paper. Householders place the filled bags in special ventilated plastic bins. In apartment blocks there are special bins in the refuse chamber or separate recycling houses (where newspapers, glass, etc. are also placed for collection). Waste from institutional kitchens is handled in the same way (sludge from grease separators in commercial/institutional kitchens is collected in slurry exhauster vehicles and delivered straight to the plant.)
Benefits on the farm
Seventeen local farmers are co-owners of Svensk Växtkraft AB. Together they account for 20% of the share capital. These farmers are also contracted for cultivation of the ley crops used in biogas production. This is a grass crop with a high percentage of clover - under EU regulations it may be grown on set-aside land. It should, though, be part of the regular crop rotation, usually undersown in a cereal crop of in oil plants for harvest in spring. Svensk Växtkraft provides guidelines for the undersowing, fertilizing and crop management.
 Ley crop being collected from the field |
The crop is cut at the same time that cattle silage is being made. Svensk Växtkraft sends in contractors to do this. The crop is wilted and then chopped finely, then fed into giant plastic bags - or tubes - each up to 90 metres in length, stored adjacent to the biogas plant.
The farmers receive high-quality fertilizer in two forms: first a liquid fertilizer, which can be pumped and applied to the fields using a conventional slurry-spreader; second, a solid fertilizer. Both of these are delivered to the farms by the company. It’s early days yet, but it is anticipated that these digestion residuals will replace the use of mineral fertilizers on approximately 1500 hectares of cereals.
 The crop is then packed and stored in plastic bags, each up to 90 metres long all photos: svensk växtkraft |
Liquid and solid digestates from the biogas plant are accepted as fertilizers in organic farming under EC regulations - and when the decision to build the plant was made, it was a precondition that the digestates should be accepted by the appropriate Swedish bodies for use in conventional cereal production and in organic farming. Regular quality checks are carried out.
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The plant itself
The anaerobic digestion plant is located on the edge of Västerås, at Gryta. The schematic of the plant (see Figure 1) shows the many processes. In brief, waste trucks unload the source-separated organic waste in the receiving hall of the biogas plant. Stored silage is also brought in here from the store. The solid material is then diluted and mixed, in three turbomixers, into a slurry with process water (10% solid matter). The slurry is then screened before passing into one of three sanitizer tanks, where it is kept at 70ºC for one hour (usually one of the tanks is in the process of being filled, one is treating slurry, and the other is in the process of being emptied). After being sanitized, the slurry is pumped into the digester. This has a volume of 4000 m3 and has no moving parts inside - the digestate is mixed by compressed biogas.
 Figure 1. Plant schematic. source: svensk växtkraft |
Gas is produced at a rate of 250-350 Nm3/hour. The gas immediately goes through a compressor, and the bulk of it passes into the gasometer (some is fed back into the digester to mix the next lot of digestate). The gasometer regulates the flow of biogas to the on-site upgrading plant, and acts as storage for up to 800 m3 of pressurized biogas.
The primary uptake of the biogas is in the town as a vehicle fuel (see below), and any excess is used in an on-site gas-engine heat and power plant that supplies the town. Occasionally, further excess gas has to be flared off, as its longer-term storage is impractical.
Scrubbing up
Biogas from the anaerobic digester at Gryta goes through an upgrading process. So does a smaller amount of biogas (150-250 Nm3/hour) from an existing digester at the town’s sewage treatment plant, which is delivered the few kilometres to the Gryta site by pipeline.
 Organic waste is diluted and mixed with process water into a slurry in three turbomixers |
Upgrading of the raw biogas to fuel quality is done with pressurized water in a scrubber column. At a pressure of 10-12 bar, any carbon dioxide and other impurities are absorbed in the water. Purified biogas (methane) is collected at the top of the column. When it leaves the scrubber, the process water is fed into a flash tank, where the pressure is reduced to 2-4 bar in order to recover any methane in the water - recovered methane is fed back to the inlet of the compressor. (The loss of methane in the process is guaranteed to be less than 2%, and is normally expected to be less than 1%.)
The used water is regenerated in a ‘stripper column’, where degassing of carbon dioxide and hydrogen sulphide is carried out at atmospheric pressure with air that is blown through the column. (Before the gas is blown into the atmosphere it is treated to avoid any odour problems.) When working at full capacity, the fresh water demand is calculated at under 1 m3/hour.
 In the digester, mixing of digestate is done by compressed biogas |
Throughout the process, an advanced measurement and surveillance system monitors the system and the quality of the gas.
Biogas on the road
The treated gas is then piped to filling stations in the middle of town.
Local biogas is sufficient to supply at least 40 city buses, 10 refuse-collection vehicles and some 500 cars and light transport vehicles (a back-up supply of LNG is kept, in case the supply of biogas should falter temporarily). The biogas system offers:
• fast refuelling of buses and refuse collection vehicles - less than 5 minutes
• very high availability due to:
- double high-pressure compressors with 1000% redundancy
- few critical components in the fuelling system
- LNG reserve
- large high-pressure storage
- ability to refuel up to 40 buses without use of high-pressure compressors
• a public filling station for cars and other small vehicles.
A little history
It was farmers near Västerås who first came up with the idea of a biogas plant using ley crops, back in 1990. In 1995 the idea came up of doing this in combination with biowaste. Planning work for the biogas plant began in earnest in 1998, and the Svensk Växtkraft AB company was set up in April 2003, its owners being local farmers (20%), the Swedish farmers’ federation (20%), the local energy company (40%) and the local waste management company (40%).
Planning was finished in September 2003, and Svensk Växtkraft decided to go ahead with the project. In November 2003, it became a demonstration project within the EU AGROPTI-gas programme (see boxed text on page 80). The first production of vehicle fuel started in October 2004, using biogas from the sewage treatment plant. A few months later, in July 2005, the biogas plant was taken into operation.
Jackie Jones is Editorial Director of Waste Management World.
e-mail: wmw@jxj.com
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AGROPTI-gas
In 2003, the Växtkraft project became an EU demonstration project within the AGROPTI-gas 5th framework programme, adding national and international partners into the project. These partners co-operate in the demonstration, evaluation and dissemination of the project.
AGROPTI-gas is divided into various parts:
• demonstration - including purchasing, building and start-up of the systems
• analyses of the socioeconomic effects of the project
• analyses of the handling systems for ley crop and digestion residuals
• evaluation of the technical and biological processes
• dissemination of findings.
Project partners are Svensk Växtkraft, JTI (Swedish Institute of Agricultural Engineering), SDU (University of Southern Denmark), FAL (Federal Agricultural Research Centre, Germany), BAI (Bulgarian Association of Investors), LRF (National Federation of Swedish Farmers) and the Municipality of Växjö.
See www.agroptigas.com for further information.
Main contractors and financial support
The biogas plant was supplied and installed by Ros Roca International AS (www.rosroca.de); The upgrading plant and fuel system by YIT Vatten och miljötechnik (www.vit.fi); and the gas pipelines and liquid digestate storage by Lindesberg Grus och Maskin.
The Växtkraft project is carried out with financial support from the Local Investment Programme and the European Union (5th Research and Technological Development Programme). The farmers’ participation in the project is sponsored by Sparbanksstiftelsen Nya and LRF.
