to read the full report - Ecolateral by Peter Jones
to read the full report - Ecolateral by Peter Jones
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126<br />
Evaluation of Opportunities for Converting Indigenous UK Wastes <strong>to</strong> Wastes and Energy<br />
AEA/ED45551/Issue 1<br />
9 Biological Processes<br />
The recycling initiatives targeting paper, cardboard, glass, plastic, metal and garden waste have also had<br />
<strong>the</strong> effect of increasing <strong>the</strong> food waste content (on a weight percent basis) of <strong>the</strong> remaining waste,<br />
particularly in <strong>the</strong> household waste stream. The food waste stream is now being increasingly targeted for<br />
separated collection and subsequent treatment <strong>by</strong> ei<strong>the</strong>r composting (using in-vessel composting) or<br />
anaerobic digestion. Ultimately this could result in up <strong>to</strong> 4 million <strong>to</strong>nnes per annum of food waste being<br />
sent <strong>to</strong> anaerobic digestion facilities which could generate bio-gas, potentially used for electricity<br />
generation or transport gases, and a solid digestate. We can also add <strong>to</strong> this <strong>the</strong> waste from food<br />
manufacture, distribution and sale and <strong>the</strong> substantial quantities of manures from agriculture. Bio-solids<br />
are a fur<strong>the</strong>r specialised form of wet waste that is used extensively for energy.<br />
9.1 Anaerobic Digestion (AD)<br />
Wet organic<br />
wastes<br />
Anaerobic<br />
digestion<br />
Process Description<br />
Anaerobic digestion is a natural biological process carried out <strong>by</strong> bacteria in low oxygen conditions.<br />
Organic material is converted in<strong>to</strong> biogas (a mixture of methane, carbon dioxide and o<strong>the</strong>r trace gases)<br />
and a solid residue or digestate.<br />
The biological reactions occur in three sequential steps. The organic waste is first broken down in<strong>to</strong><br />
simple sugars and amino acids <strong>by</strong> enzymes (hydrolysed). These <strong>the</strong>n ferment <strong>to</strong> produce fatty acids that<br />
are converted <strong>to</strong> hydrogen, carbon dioxide and acetate <strong>by</strong> ace<strong>to</strong>genic bacteria. Finally methanogenic<br />
bacteria produce biogas, a mixture of carbon dioxide (40%) and methane (60%) and o<strong>the</strong>r trace<br />
compounds. These biological processes operate in aqueous media, and hence are best suited <strong>to</strong> wet<br />
wastes.<br />
Anaerobic bacteria occur naturally and are commonly found in soils and deep water. They produce<br />
biological cultures that thrive within three temperature ranges, 7 - 24°C (psychrophilic), 35-39°C<br />
(mesophilic) and 55-60°C (<strong>the</strong>rmophilic).<br />
Table 67 Comparison of AD systems<br />
Biogas/<br />
Digestate<br />
Heat, power,<br />
grid gas<br />
Psychrophilic Mesophilic Thermophilic<br />
Optimal Temperature and<br />
Range ( o C)<br />
22 (7 - 25) 35 (25 - 42) 60 (49 - 72)<br />
Concentration V dilute Medium High<br />
Retention time (d) >50 20-40 5-20<br />
Gas generation rate Very slow Medium High<br />
Conversion of solid material Low Med <strong>to</strong> High High<br />
Construction Covered dam Steel or Steel or<br />
or lagoon concrete tank concrete tank