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126 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 9 Biological Processes The recycling initiatives targeting paper, cardboard, glass, plastic, metal and garden waste have also had the effect of increasing the food waste content (on a weight percent basis) of the remaining waste, particularly in the household waste stream. The food waste stream is now being increasingly targeted for separated collection and subsequent treatment by either composting (using in-vessel composting) or anaerobic digestion. Ultimately this could result in up to 4 million tonnes per annum of food waste being sent to anaerobic digestion facilities which could generate bio-gas, potentially used for electricity generation or transport gases, and a solid digestate. We can also add to this the waste from food manufacture, distribution and sale and the substantial quantities of manures from agriculture. Bio-solids are a further specialised form of wet waste that is used extensively for energy. 9.1 Anaerobic Digestion (AD) Wet organic wastes Anaerobic digestion Process Description Anaerobic digestion is a natural biological process carried out by bacteria in low oxygen conditions. Organic material is converted into biogas (a mixture of methane, carbon dioxide and other trace gases) and a solid residue or digestate. The biological reactions occur in three sequential steps. The organic waste is first broken down into simple sugars and amino acids by enzymes (hydrolysed). These then ferment to produce fatty acids that are converted to hydrogen, carbon dioxide and acetate by acetogenic bacteria. Finally methanogenic bacteria produce biogas, a mixture of carbon dioxide (40%) and methane (60%) and other trace compounds. These biological processes operate in aqueous media, and hence are best suited to wet wastes. Anaerobic bacteria occur naturally and are commonly found in soils and deep water. They produce biological cultures that thrive within three temperature ranges, 7 - 24°C (psychrophilic), 35-39°C (mesophilic) and 55-60°C (thermophilic). Table 67 Comparison of AD systems Biogas/ Digestate Heat, power, grid gas Psychrophilic Mesophilic Thermophilic Optimal Temperature and Range ( o C) 22 (7 - 25) 35 (25 - 42) 60 (49 - 72) Concentration V dilute Medium High Retention time (d) >50 20-40 5-20 Gas generation rate Very slow Medium High Conversion of solid material Low Med to High High Construction Covered dam Steel or Steel or or lagoon concrete tank concrete tank
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Figure 17 Anaerobic Digester schematic 228 In Europe more than 85% of plants operate within the mesophilic temperature range as the greatest number of suitable bacteria species operates at this temperature and are more tolerant of varying environmental conditions. 8% of AD systems in Europe operate in the thermophilic range, mostly in Denmark as the thermophillic temperature range is especially applied in centralised AD plants, as are common in Denmark. The advantage of such systems is the reduced time required and the greater level of sterilisation achieved of the digestate. The drawback of such systems is the greater heat requirement. Less than 5% of European systems operate in the psychrophilic range. AD systems used to treat food and farm wastes have been adapted from well established systems used to treat wastewater solid fractions (bio-solids). The digestion process takes place in sealed tanks (digesters) that are continuously mixed to ensure a uniform reaction rate. The amount of biogas produced using AD will vary depending on the volatile solids component of the waste, the process design, such as reactor residence times and operating temperature. 228 Schematic taken from “AD biological cycle” page on the “Renewable Energy Association”, http://www.r-e-a.net/biofuels/biogas/anaerobic- digestion/ad-biological-cycle 127
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South East 327 - - 55% of MSW by 20
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Appendix 3 MBT Methods Specific in
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Figure 27 Orchid Environmental proc
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