Water & Wastewater Treatment

WWT March 2020

Water & Wastewater Treatment Magazine

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26 | MARCH 2020 | WWT | www.wwtonline.co.uk The Knowledge Anaerobic digestion (AD) is a well-established technology which closes the loop on the circular economy by turning waste products into energy and resources. Traditionally the only products considered valuable from anaerobic digestion were methane and digestate. In this article, Paul Pickard from Aqua Enviro considers some of the potential alternative products which could be derived from the process. W e'll start by looking at the traditional digestate paradigm and its limitations. There are reported to be over 650 opera- tional AD sites in the UK, with over 500 of these processing non-sewage feedstocks. The vast majority of the resulting digestate is used in agriculture and horticulture, with a small percentage treated thermally in traditional processes such as incineration, with trials being done on novel processes like pyrolysis and gasification. Depending on the sector in which the site operates, there are a variety of problems as- sociated with the utilisation of digestates. The primary concern for many sites in the commer- cial and industrial sectors is making the economics add up. Whilst digestate has an inherent value as an organic fertiliser, the costs of pro- cessing and transporting the product to the end user o-en outweigh the revenue received by the producer. Many agricultural AD plants process waste gener- ated on site such as crop residues and cattle slurry, and then spread the digestate back onto their own land thus reducing haulage costs as well as potentially avoiding some of the more onerous regulations. Coupled with the savings from a reduction in their fertiliser expenditure, the use of diges- tate in the agricultural sector o-en does make financial sense. However, the market is constrained by availability of suitable landbank. Whilst the number of AD sites continues to increase, the available land for spreading the digestate is decreasing, so operators run the risk of having nowhere to send their product. The sur- plus of digestate entering the market drives the price down, compounding the economic problem further. Another problem for producers of digestate for ag- ricultural use is the temporal variability of demand due to the rules governing the ap- plication of digestates to land. Farmers are required to store their digestate and only apply it at optimum times of the year when the nutrients will be taken up by the crops rather than being allowed to leach away and cause environmental pollution. Even within this window, spreading cannot take place on flooded or frozen ground, so digestate must remain in storage. All of this equates to an unreliable mar- ket and yet another headache for AD operators to deal with. An emerging risk to AD operators in the commercial The hidden value of anaerobic digestion and wastewater sectors is the prospect of a complete ban on the use of digestates and biosolids on agricultural land. Several reasons such as the presence of microplastics in food waste digestates or public perception surrounding the use of biosolids (sewage waste digestate) could be a threat for the practice to cease altogether. For these reasons, the industry is exploring ways of recovering valuable products from digestates or trying to manipulate the biological pathways within the anaerobic digestion process to deliver alternative end products. The focus of this article is on the latter, but a brief overview of some "first-generation" diges- tate enhancement technolo- gies is provided below. Algal bioreactors – a tech- nology being developed which uses algae to produce a range of organic chemicals with the residues being used as animal feed or as an organic fertiliser. The role of digestate in this technology is in using the liquid fraction as a source of nutrients and using CO2 (po- tentially from biogas) to allow the algae to photosynthesise. Struvite recovery – stru- vite (magnesium ammonium phosphate) in digestate can cause fouling of pipework in the downstream processing

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