Depending on local waste management arrangements, organic waste can include kitchen waste as well as municipal and commercial green waste. With such waste mixtures, which typically contain high proportions of solids and foreign matter, the advantages of dry anaerobic digestion come into their own. A distinction is made between two types of waste fraction:

• Separately collected organic waste: Organic waste is collected and delivered separately. In addition to biogas this fraction is a source of high-grade fertiliser for farms and gardens.
• Organic fraction of municipal solid waste: Here the organic portion is separated from the rest of the waste in a sorting unit. Recyclable materials are recovered, and the rest of the organic waste is fed into the Kompogas^® digester for further treatment. Digestate from this fraction is reused in landscaping or roads and to revegetate landfill sites.

Both material flows produce a high-grade renewable fuel in the form of biogas.

One tonne of organic waste yields:

• Around 850 kg of compost and liquid fertiliser. Both are used as a valuable source of nutrients and soil conditioner in agriculture in the form of natural fertiliser. In areas where the discharge of liquid fertiliser is prohibited, the digestate is processed to be presswater-free.
• Around 100 Nm³ of biogas containing some 600 kWh_th of energy.

This biogas can be treated in a gas upgrading plant to produce approximately 60 Nm³ of biomethane and converted into compressed or liquid biogas, a carbon-neutral fuel replacing about 60 litres of diesel. The biomethane can also be fed into the existing gas grid, making an important contribution to decarbonisation.

Alternatively the biogas can be converted into around 250 kWh of green electricity, plus at least the same amount of heat, in a combined heat and power unit.

This way, Kompogas^® technology closes both the materials and energy cycles.

Waste is received in either a deep bunker or tipping area with an odour trap. The fully automated crane system transports the waste from the bunker to the shredder where the organic material is shredded and sieved. The fraction in the >60mm range is returned to the bunker, while a conveyor system takes the sieved-out substrate to the digester.

The core module of every Kompogas^® plant is the digester. Material fed into the digester is digested by thermophilic microorganisms at a temperature of 55°C under anaerobic conditions. The substrate moves steadily and horizontally forward through the plug-flow digester as digestion proceeds, producing biogas in the process. Fresh material is constantly fed in, with part of the recirculate from the digestate added to inoculate it and immediately activate the digestion process. Retaining the substrate for around 14 days at this temperature makes sure that it is completely sanitised and its gas potential fully exploited.

Once the retention time has elapsed, the digested substrate is pumped out at the discharge outlet of the digester. The Kanadevia Inova Kom+Press dewatering press separates it into a solid and a liquid fraction. After a short post-treatment phase, the solid digestate is ready for use as a raw organic fertiliser and soil conditioner, or for further processing to produce high-grade compost. Where there are no regulations to the contrary, the liquid digestate is also used in agriculture. If necessary, the dry matter content is reduced even further using the Kanadevia Inova SpeedScreen.

In cases where the use of liquid fertiliser is prohibited, the liquid fraction is either passed into a waste water purification system or used for presswater-free partial stream anaerobic digestion. In the latter case only the energy-rich component of the biowaste is fed into the digester. The highly structured material is shredded, and only mixed into the digestate after the digestion process. This completely eliminates the need for dewatering. Then the material undergoes aerobic composting.

The Kompogas^® plant is designed as an enclosed system to prevent the emission of odours. During the process, the exhaust air is gradually collected and pre-cleaned in an acid scrubber. Then it passes through a biofilter made of torn root wood and tree bark before being released into the atmosphere practically odour-free.

The raw biogas produced in the digestion process is either upgraded to biomethane using Kanadevia Inova’s BioMethan technology and fed into the grid, or further treated (compressed or liquefied) to produce high-grade fuel. Alternatively, the raw biogas can be used to generate carbon-neutral electricity and heat in a combined heat and power (CHP) unit. In this case a small portion of the heat is used to maintain the temperature in the digester, with the rest used in buildings close by or fed into the district heating network.