Pre-processed household waste is transported from the receiving and sorting plant in Gufunes to GAJA. There it is mixed with structure material and placed in specially designed process modules. There the biological nutrients are washed from the mix and pumped to anaerobic digesters where the actual biogas production takes place. The solids that are left in the process modules are then composted.
Controlled air exchange to prevent odour pollution
Pre-processed waste from Gufunes is transported to GAJA in transport. The wagons unload the waste through doors with seals that minimise air exchange while the waste is being unloaded. In addition, the entire area is negatively pressurised and all drive-thru doors are controlled in such a manner that it is not possible to open other drive-thru doors while the receiving door is open. This prevents all non-controlled emission of air from the processing plant. All air emitted from the plant goes through biofilters, of which there are 6.
Gas generation process under anaerobic conditions
A mix of this waste and structure material (shredded tree branches) is prepared in the receiving area. The structure material has an important part to play in the processing, such as opening a path for percolate (from reactor tanks) through the waste, maintaining moisture, ensuring ventilation in the composting process and providing bioprocesses with carbon.
After mixing, the mixed waste is transported to the process modules, of which there are 20 and take a total of around 8,500 cubic metres. The first stage of the gas generation takes place in the process modules, i.e. hydrolysation. At the same time, nutrients are rinsed out with percolate from the anaerobic digesters that is sprayed over the mixed waste in the module. The percolate drains through the mass and is then pumped back into the gas anaerobic digester in a continuous cycle. This process is anaerobic, so the process modules are completely airtight during this process.
Once the gas generation process has been completed, composting takes over
After 3–4 weeks, the first phase of the composting process begins. The pump in the module in question is then stopped and air sucked through the mass and emitted through biofilters that eliminate foul smelling gasses from the exhaust air. An aerobic decomposition process starts, composting, which lasts for another 3–4 weeks. The procedure used in the process means that there are always approximately half the process modules in the biogas generating process and half in the composting process.
When the first phase of the composting process has been completed, a wheel loader is used to transfer the material to the maturation bays. Materials from two process modules fit into each maturation bay, of which there are 10 and take a total of 8,000 cubic metres of half-mature soil improver. The total volume is somewhat smaller than the process modules, as the material shrinks and dries in the process. Only aerobic composting is carried out in the maturation bays. Air is sucked through the mass and blown out through biofilters. This process takes approximately 6–8 weeks until fully mature soil improver is taken from the maturation bays and sifted. The structure material that is screened in the sifting process is recycled and reused in the processing loop. Plastic, metal and minerals that are sifted out are sent for recycling. The entire processing loop, as well as the treatment and storage of soil improver, is carried out indoors, which makes GAJA that little bit more special.
The anaerobic digesters, of which there are two and can hold a total of 6,000 cubic metres, are constantly in operation, and nutrients from the process modules are continuously being added to them. A similar volume of water as comes into the plant with the processing materials evaporates in the process. Water probably needs to be added to the process rather than there being any water excess. The biogas that forms is pumped directly to SORPA’s biogas scrubbers located approximately 600 meters from where it is generated, and the final product is a high-quality fuel, methane.