"Towards a Zero Carbon Community"
Anaerobic Digestion in the Bradford River Valley
Posted Wednesday, September 2, 2009
Introduction to AD/CAD in the Bradford River valley.
Anaerobic digestion (AD) is a natural biological process carried out by bacteria in the absence of air, by which organic material is broken down into fertiliser products and biogas. The anaerobic bacteria involved in the process are an integral component of nature’s waste management and are commonly found in soils and deep waters, as well as in landfill sites.
The organic waste (feedstock) is broken down into sugars and amino acids by enzymes similar to those found in our mouths that help digest our food. These are then fermented to produce volatile fatty acids and then converted by acetogenic bacteria in to hydrogen, carbon dioxide and acetate. Finally methanogenic bacteria produce biogas, a mixture of carbon dioxide (40%) and methane (60%) and other trace elements. While there are many ways of building anaerobic digesters (AD) the basic principle takes a feedstock that is fed into a completely enclosed tank, which is heated and regularly mixed for a period of days, before producing a digestate output (fertiliser) as liquor (for grassland) and fibre (for soil conditioning) forms. AD plants of various sizes can take the form of single, on-farm installations or wastewater/sewage treatment plants, or centralised plants (CAD) serving a group of farms or municipal/commercial waste sources covering an area or region.
FEEDSTOCKS. Types of biodegradable feedstocks vary considerably, almost any (non-woody) biodegradable material can be processed in a digester. Typical feedstock includes farm manures, sewage sludge, catering wastes and food processing wastes and can include uncooked and cooked food, meat and bone. Feedstocks with a high moisture content are more suited to the AD process, but dry materials can also be digested depending on the technology used. Recently grass and crops have been used as a feedstock, but local farmers are not keen to grow crops for feedstock.
PRETREATMENT. If the feedstock contains animal by-products the law generally requires the feedstock to be pasteurised at 70°C for 1 hour to irradiate the pathogens such as Salmonella and E.coli. The particle size would need to made to less than 12mm. Where appropriate pre-treatment can also include removal of unwanted materials such as metals, plastics or stone, and protects downstream treatment processes.
POST-TREATMENT Sometimes pasteurisation takes place after the digestion process, but this increases the risk of re-infection by harmful bacteria. Other post treatments may include separation of the whole digestate into a liquid and fibre portion, and then sometimes further treatment of these materials
ENERGY USE. The biogas can be used in a boiler to produce heat only, or in a combined heat and power unit (CHP) to produce electricity and heat. Normally a biogas engine can gain an electrical conversion efficiency of up to 35% with most of the remainder being available as heat. At larger CAD plants the biogas can be upgraded to Natural Gas quality and either used as a vehicle fuel or injected into the gas grid.LOGISTICS. SY anticipates that the consultants’ Stage 1/2 recommendations for a CAD in our area will suggest the feed in of 70t/day of farm wastes from 10-15 farms, all within a 3 mile radius of the CAD. Many of these farms are already moving manure significant distances from the farmyard to distant fields. Long term storage of the liquid fertiliser will be in tanks, sometimes shared between farms, and located near to spreading areas. A tanker will deliver the liquid fertiliser when it goes to pick up manure near to the storage tank. Therefore the increase in transport movements from farms will be minimal. 15 t/day of off-farm, biodegradable material will be used as feedstock too, most of which will generate a net reduction in transport miles. In the case of Friden 1 site, the traffic flows at the CAD gate on to the Via Gellia (rather than the narrower lane to DSF) will be c.85 tonnes in and 82 tonnes out - amounting to about 7 tankers per day. The routes taken will avoid settlements and will make little if no net addition to traffic patterns, mainly on main roads designed for this purpose The access to Friden 1 was originally designed to cope with a large scale tipping operation. The access to Friden 2 would be the existing main gate to DSF or a new route and increase traffic density on the narrower secondary road. Fuller details are contained in the PPT qv and will available in the Stage1/2 report. ENVIRONMENTAL, SOCIAL AND ECONOMIC BENEFITS
At a well designed and managed AD the environmental impacts of the anaerobic digestion (AD) process are positive and include:
- a benign and safe biodegradable waste management system that produces energy, high grade fertiliser products and has minimal emissions
- Pollution and odour control
- Reduction in the use of artificial fertilisers
- Accurate nutrient management planning with less losses of nutrients to the environment
- Control of pathogenic bacteria and viruses
- Production of renewable energy
- Reduction in greenhouse gas emissions
- Carbon negative processing.
- Create a cleaner environment
- Create local employment
- Provide security of energy supply and costs, particularly important as oil availability reduces
- Provide diversification for rural businesses such as farms
- Reduce production costs at farms
- Helps farms to meet the NVZ requirements
- Achieves the “Proximity Principle” i.e. treating wastes close to their source
- Is sustainable.
- Charging a gate fee for the treatment of wastes
- Production and sale of biogas as both electricity and heat
- Production and sale of fibre and liquid fertilisers
- Improve the efficiency of farming practices by offsetting the use of artificial fertilizer and energy bills.