Anaerobic digestion (AD) processes plant materials (biomass) into gas for heating and power. The gas is called methane or biogas. It is produced by bacteria, which digest biomass and produce methane as a by-product.
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Biomass includes anything that is plant-derived: municipal solid waste, manure, crop residues, compost, food waste, paper and waste water. Crops can be grown specifically for use in AD, as a supplementary feedstock or a stabilising material. Biogas has been used in the UK since 1895, when gas from sewage was used in street lamps across the city of Exeter.
The UK produces over 100 million tonnes of organic material that is suitable for treatment by AD. This includes:
The amount of energy produced by AD will vary depending on the material that goes into it and the particular type of digester that is used. Digesting 1 tonne of food waste can generate about 300 kWh of energy; slurry is lower yielding and purpose grown crops higher. According to the Renewable Energy Association, if all the UK's domestic food waste was processed by AD, it would generate enough electricity for 350 000 households.
AD could generate 10-20 TWh of heat and power per year by 2020. To put this in context, the UK's largest power station Drax sold 27.1 TWh of electricity in 2012. AD could represent 3.8-7.5% of the renewable energy we estimate will be required in 2020.
AD has been used for many years in the UK by the water industry. It currently treats 66% of the UK's sewage sludge in AD plants. Beyond the water industry AD in the UK is in its infancy, but growing rapidly. There are currently 650 operational AD facilities (excluding traditional water treatment plants) in the UK. You can see the locations of operational AD plants on the Biogas Map. There are many more digesters that are currently in the 'planning' stage of development.
No. Digestate is not compost, although they have some similar properties. Compost is produced by aerobic (with air) decomposition of biological material and digestate is produced by anaerobic (without air) decomposition of biological material. They can both be used as fertiliser under specific regulations.
There is some odour associated with the organic material that goes into a digester. However, AD can actually reduce nuisance odours as waste is delivered in closed vessels and vehicles, received in a closed reception area, and the digestion process takes place in a sealed tank. The digestion of slurry, for example, is significantly less odorous than the common practice of storing slurry in pits.
This website is a good place to start. There is an AD cost calculator to look at the economics and there are lots of links to useful information and organisations.
The key questions for a potential developer are:
Commercial vendors are promoting aerobic digesters to California businesses and institutions that generate large quantities of food waste, such as restaurants, hotels, grocery stores, universities, and prisons. This webpage provides information that may be helpful when evaluating whether this type of technology is appropriate for your facility. CalRecycle strongly recommends that generators contact the entities indicated below, based on the type of technology and proposed end use for material generated, for assistance in meeting local program requirements.
When considering the purchase of such a machine, a business owner that generates food waste should calculate the initial and ongoing costs of purchasing an aerobic digestor and compare it with the cost of separated organics collection service. Although vendors are promoting aerobic digesters as cost-efficient solutions for managing organic waste on site, this may not always be the case. The true cost of the equipment includes:
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Generators should check with their local wastewater system operator, as operating an aerobic digester may increase the cost of their sewer services. Wastewater treatment rates for commercial and industrial customers are based on the amounts of flow, biological oxygen demand (BOD), total suspended solids (TSS), and other potential constituents of the wastewater stream. Increases in those values are likely to increase the rate that commercial and industrial establishments pay for sewer service.
First, a business owner should consider whether an aerobic digester can be placed in proximity to people handling food waste. Second, a business owner should also consider how to manage airborne emissions, liquid emissions, and solid output.
Local and regional wastewater treatment authorities treat wastewater so that its discharge meets standards established by the Regional Water Quality Control Board. For industrial or commercial establishments, pre-treatment programs impose limitations on what may be sent down the sewer lines, in order to ensure there is no negative impact on the treatment process or the authority’s ability to meet their waste discharge requirements. In addition to flow rate, biological oxygen demand (BOD), and total suspended solids (TSS), nitrogen and pathogen levels in the effluent are other factors that may be measured in the wastewater generated by each commercial customer.
If you are at a facility that treats its own wastewater, check with your water treatment staff before installing an aerobic digester.
Though the volume may be significantly reduced, partially digested food waste is still considered solid waste. Not all aerobic digesters generate a solid output; many send their entire output down the drain. If the unit under consideration does generate a solid output, sometimes called digestate, then the following considerations should be made.
State regulations require the owner of any business remove solid waste at least once a week.
Local governments license solid waste self-haulers. If a generator intends to self-haul the digestate, check with your local (city or county) government to determine how to comply with local rules regarding solid waste handling and transport. If you are hiring someone other than your solid waste hauler to remove that material, check their local government licensing status.
As previously stated, digestate is not compost. However, digestate could be feedstock at a composting facility permitted to accept this material type. Digestate should be blended with other compostable materials, such as greenwaste, and then be composted. Finished compost produced at a permitted composting facility must meet CalRecycle’s environmental health standards before it can be sold.
In some cases, digestate may be a suitable fertilizer. However, fertilizer manufacturers must be licensed and registered with the state if they sell their products and make claims about the nutrient value of the end product or its suitability for use in organic food production. The sale of fertilizers by unlicensed manufacturers is against the law. Fertilizer manufacturers are inspected annually. The California Department of Food and Agriculture’s fertilizer program oversees fertilizer licensing and registration in California.
If the digestate is to be directly applied to land, with no claims about its nutrient value or suitability for organic production, then it must meet CalRecycle regulations for land application of compostable materials, which include digestate (see CCR 14, Section 17852 (a)(24.5). If compostable material that fails to meet these standards is applied to land, the material is considered disposed. The disposal of compostable material requires a permit. Disposal without a permit is illegal and will result in enforcement action.
In addition to CalRecycle regulations, California Water Board regulations may also apply. Anyone considering sending digestate to direct land application should consult with their Local Enforcement Agency and their Regional Water Quality Control Board to determine what is required.
Dehydrators | Liquefiers
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