Anaerobic Digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste and/or to produce fuels. The end result of the process is mainly methane biogas and an organic nutrient rich fertiliser known as ‘digestate’.
Biogas is a mixture of approximately 55% methane and 45% carbon dioxide with traces of hydrogen sulphide and ammonia – although the exact make-up depends on the type of waste which has been used in the digestion process. Biogas and its derivative, upgraded biomethane, can be used for:
Waste is delivered to a treatment facility. A combustion chamber heats the waste and creates steam. Steam is converted to energy.
Digestate is made up of left over organic material and dead micro-organisms. It contains mineralised, bioavailable nutrients − including 25% more plant available nitrogen. Digestate has a reduced volume of about 90 – 95% of the original feedstock and and is a highly effective fertiliser. Digestate is weed and pathogen-free and eliminates pre-spreading manure storage times. It can be piped directly onto fields, thereby reducing soil compaction. It also significantly reduces and/or eliminates the farmer’s reliance on bought-in fossil fuel fertiliser. This enables farmers to produce environmentally-friendly food at no increased cost to the consumer.
Below is a simple diagram to show the processes involved in Anaerobic Digestion:
Our Springhill anaerobic digester plant was commissioned in August 2013 and was the first gas to grid plant in the UK. The plant size is 1.5mw consisting of 1x500kw CHP engine and Pentair gas to grid technology, exporting up to 7800m³/day of methane into the national grid. This is Pentair’s first upgrading plant in the UK to produce biomethane. Our CHP engine is based on the site, which supplies power and heat for the daily operations of the plant as well as producing heat for our neighbouring glasshouse production.
The plant has 2 digester tanks with the combined capacity of 4200m³. Each digester is designed to produce 300m³ of biogas/hr with a residence time of 25-30days. With sight glasses and maintenance windows installed around the tanks, it has become easy to see what happens inside the digesters, which allows us to respond to any problems promptly and efficiently as well as carrying out maintenance to parts such as the agitators.
We have a secondary digester which extends our retention time to 50days and produces Methane rich Biogas. In addition to this, a combination bag was also installed for both liquid and gas storage. With a total capacity of 5000m³, the bag is designed with temperature control and mixing technology to achieve the full potential of anaerobic digestion.
Our eco bag has a capacity of 5000m³, and is used to store centrate, which is used as a liquid fertilizer to spread on our fields.
Biogas from the Digesters is produced from energy crops consisting mainly of grass, maize, sugar beet and wheat. We feed these tanks up to 70 tonnes per day. All our feedstock is produced in-house as we currently grow on over 2000 acres of land. Alongside this we have a close-knit network of farmers in the area that also grow crops for our plants, ensuring that we have a constantly supply of feed all year round. We always ensure high standards and the right quality of crops are selected.
Below is an aerial picture of the plant and you can also see our glasshouses in the background where local British tomatoes are grown.
Our Rotherdale anaerobic digester plant was commissioned in May 2015 and was designed and built in house by our Vale Green Energy team. The plant size is 4.5mw consisting of 2 x 500kw CHP engines and Pentair gas to grid technology, exporting up to 22000m³/day of methane into the national grid. This is Pentair's largest installation in the UK. Pentair have also installed a through the membrane separation, which allows us to produce food grade CO2 which is tested and approved for European standards. We currently capture 30,000kg of CO2 daily.
One of our CHP engines is based on the site, which supplies power and heat for the daily operations of the plant. The second CHP engine is a satellite unit installed at E.C Drummond which supplies heat & power for their chicken sheds. Any extra power that is generated is exported to the national grid.
The plant has completed with Phase 2 in 2017, by installing the additional 2 digesters and also commissioning the Biogas upgrade to full capacity of 900m³/hr. Currently each digester is designed to produce 600m³ of biogas/hr with a residence time of 25 - 30days. With sight glasses and maintenance windows installed around the tanks, it has become easy to see what happens inside the digesters, which allows us to respond to any problems promptly and efficiently as well as carrying out maintenance to parts such as the agitators. We have a secondary digester which extends our retention time to 50 days and produces Methane rich Biogas. The secondary Digester also provides a gas storage of 8000m³. The Secondary Digester is designed with temperature control and mixing technology to achieve the full potential of anaerobic digestion.
Our eco bag is used to store centrate which is used as a liquid fertilizer to spread on our fields.