Anaerobic digesters have been used for many years; on farms to control odour, ammonia, hydrogen sulphide, volatile organic compounds, greenhouse gases and particulate matter (PM), which have a significant effect on air and water quality.
Prevention of soil erosion has recently become an important goal for agriculture in both North America and Europe. Regulations in the US. have begun to include phosphorus and pathogens in water quality goals and PM, odour and NH3 in air quality goals.
Greenhouse gas emissions from industry, including farms, will soon come under new legislation from Europe.
The methane from Anaerobic Digestion directly replaces fossil fuels, because the carbon dioxide from the use of methane in energy generation is absorbed by growing plants, fertilised by digested materials. Anaerobic Digestion is therefore carbon dioxide neutral in global warming terms.
Sustainable living, renewable energy production and recycling of our wastes were given the highest priority by many world leaders at the Johannesburg Conference. This is to implement the Kyoto agreement, to reduce world pollution and greenhouse gases.
Anaerobic Digestion systems are very effective air and water pollution control systems. They treat high strength wastes to reduce bio solids volume and control odours. While initial capital costs may be high (depending upon the size of systems installed), use, sales of energy and composted manure solids and savings on existing waste disposal costs can provide financial returns within a surprisingly short period.
Anaerobic digesters have been installed on several large livestock farms in the US. and have been shown to:
- Reduce odour from land-applied slurry by 75%.
- Enable the sale of electricity and provide a heat source to the farm.
- Maintain the manure's fertilizer value (Pigg and Vetter, 1984).
- Improve handling and solids separating characteristics of manure.
- Stabilise manure by converting up to 70% of organic N into NH4-N.
- Destroy about 60 to 75% of the volatile solids.
- Conserve water and produce marketable digester "fibre".
- Reduce transportation costs by reducing manure solids by 70 to 95%.
- Reduce BOD levels by up to 90% and COD by 60-70% (AgSTAR, 1997).
- Reduce odour and gas emissions.
- Destroy weed seeds and reduce pathogens by more than 99%.
- Reduce attractiveness of the manure to rodents and flies.
In addition, where livestock farms pollute groundwater river systems that have ecological and tourist potential, the use of anaerobic digesters can reduce the pollution and maintain rural employment.
Once the European Landfill Directive finally comes into force in the UK, it is expected that the dates for reduction of Biodegradable waste to landfill in the UK will be as follows:
Reduction of total biodegradable MSW (by weight) produced in 1995 to:
- 75% by 2010
- 50% by 2013
- 35% by 2020
In order to meet the directive the Government will work towards targets for the recycling or composting of MSW, reducing the proportion of controlled waste going to landfill, the date by which 1 million tonnes per annum (tpa) are to be composted and the date by which 40% of the total market requirements for soil conditioners and other growing media in the UK are to be supplied by non-peat materials.
The above targets, when in force, are will be readily met by widespread use of composting and Anaerobic Digestion.
The disposal of wastes in landfill has been a concern of the European Commission since the late 1980s. Concern over how the disposal of waste affected the environment was expressed in a communication (SEC89) 934 final), dated 18.8.1989.
This was followed by a Resolution on the disposal of waste (Official Journal No. C 122, of 18.5.1990) that sought "to minimise landfilling of waste (a last resort in waste management) and promoted waste prevention and recycling".
Anaerobic Digestion seeks to recycle the most polluting components of domestic and food processing wastes and offers an environmental and economically viable alternative to landfill disposal and incineration.
The issue of renewable energy has been a concern of the European Commission for several years, as shown in the Communication COM (97) 196, which saw the need for increasing "the share accounted for by renewable energy sources from 6% to 12% by 2010".
Recycling projects in general are effective in improving employment prospects, as they generate resources from what would otherwise be waste products. Anaerobic Digestion will encourage a more effective use of a range of waste products, in the form of low cost, sterile compost and peat substitutes.
The small-scale recycling of food processing wastes will allow the expansion of local food processing and other market garden produce, without the present negative effects on the environment that this expansion is presently causing.
The improved production and use of organic compost will also improve the productivity of local organic food producers, so increasing their income.
The use of the organic content of general wastes would encourage more recycling of other waste materials, such as metal, plastic and glass, as they would then be less difficult and noxious to recycle.
A large proportion of the raw material for food and paper, and wastes arising from these, is grown on land that is being depleted in organic matter. Anaerobic Digestion could recycle useful material back to this land and employ people to do this at a cost less than either incinerating or landfilling.
By recycling wastes and using the methane produced in generators, this proposed project also meets a second objective stated in this Communication of "limiting greenhouse gases particularly methane from the waste sector".
Food wastes disposed of in unsealed landfill pits and unregulated piles will produce methane that enters the atmosphere.
The Social Impact is that if domestic and trade wastes were to be treated and recycled by the Anaerobic Digestion and Composting, instead of disposal by incineration or landfill, new employment can be created at the rate of 1 person per 5,000 to 10,000 head of population.
Many of the new jobs created will be at the local waste treatment sites and on farms where the digested and screened material can be composted, stored and applied to farmland and forestry. Other jobs can be created in the recovery and recycling of materials, manufacture of new products and sales.
This could reverse rural economy decline and depopulation, which is a serious problem in many remote parts of the UK and overseas.
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