Anaerobic digestion (AD) of sewage sludges and recycling of biosolids has a long and successful history, but the current upsurge of interest in solid wastes digestion is driven primarily by the requirement to reduce the biodegradability of materials going to landfill. While this is an important target, it should be remembered that AD is a sophisticated multi-option process that also offers opportunities for enhanced recycling of organics and nutrients, as well as the potential for reclaiming a substantial proportion of the inherent energy of organic waste.
The Waste Strategy actively promotes AD and the current focus is on source segregated materials. This is an important step forward, as this is likely to improve the quality of the digestate and promote the recycling of nutrients back to productive land. More work is needed on establishing the factors that would add value to this product and make it an attractive alternative to other fertilisers, but it is a first and positive move that the waste industry is moving away from regarding AD as a sort of one-size-fits-all skip.
An AD process will always involve a trade-off between extracting the maximum amount of energy and achieving digestate stability in the minimum size of plant. There is an urgent need for research into process optimisation with the aim of striking a balance between these, sometimes, conflicting interests. More use needs to be made of tools such as energy and carbon flow analysis which allow prediction of gas and energy production based on elemental compositions, proximate analysis, degradation rates and gas compositions. There is also potential for more novel approaches to improve our understanding of the process at a molecular and microbiological level, especially those interactions that can lead to unstable digester conditions through the overproduction of volatile intermediates that impair carbon flow through to the final biogas product.
A huge potential exists for co-digestion of organically derived municipal waste with sewage sludge, animal slurry and a range of food processing and other industrial organic wastes. This may lead to further opportunities in creating value added products which may be more chemically balanced fertilisers or, as the concept of the bio-refinery evolves, the production of bulk industrial chemicals or specific gas mixes.
The potential for AD for resource recovery from waste cannot be ignored, and at last an awaking to this potential seems to be happening!
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