Water & Wastewater Treatment

28 | MARCH 2020 | WWT | www.wwtonline.co.uk of biochemical pathways within the digester so that organic material is converted into a range of valuable bio- products. This is known as the "Biorefinery Concept" whereby varying the retention time, pH, and organic loading rate will yield different end products. One specific example of this concept is the "VFA Factory" or "Carboxylate Plat- form" which extracts interme- diate digestion products such as acetate which has a market price of around £1,000 per tonne. Other products which can be derived from this type of biochemical manipulation include alcohols, ketones, hydrogen and carbon dioxide. Hydrogen and carbon diox- ide are useful products in their own right but can also be used in a traditional AD system to increase methane yields. Injecting excess hydrogen from a digester producing VFA's into a digester producing methane will improve yields via the reaction below: CO2 + 4H2 à CH4 + 2H2O. An alternative method is to combine the carbon dioxide and hydrogen in an anaerobic reactor that contains hydrog- enotrophic microbes which will perform the same conver- sion. An extension of the pathway which produces VFA's is the synthesis of polyhydroxyalkanoates (PHA) which are naturally occurring, biodegradable polymers with similar properties to synthetic polymers such as polypropyl- ene. PHA's are used in the pro- duction of bioplastics so, with the continuing trend away from the use of fossil fuels, they are an increasingly valu- able resource. PHA's are re- ported to be produced by over 300 types of microorganisms as a storage compound when there is excess organic mate- rial (carbon) but a deficiency of a nutrient such as nitrogen and phosphorus. It has been found that digestate liquor rich in VFA's is a good environ- ment for the growth of these microorganisms so PHA pro- duction perfectly supplements the production of carboxylates as described previously. Yields can be increased by optimising the process in terms of choice of bacterial strain and carbon substrate, and altering the nu- trient ratios, pH, and dissolved oxygen. The cost-effective extraction of these compounds tends to be the main barrier to com- mercialisation rather than the ability to control and manipu- late the biological system. As well as biological means of producing valu- able bio-products, there are thermal technologies which can be used to add value to digestate. Technologies like pyrolysis and gasification involve thermal decomposi- tion or partial combustion of organic material to produce a flammable mixture of gases called syngas which can itself be burned, and a carbon-rich biochar which can be used as a carbon sink or a soil im- prover. These technologies rely on a fuel with a high dry solids content so are only suitable for dewatered digestate cake or separated fibre. Hydrothermal carbonisa- tion (HTC) is a similar technol- ogy but can be carried out on wastes with a much lower dry solids concentration (as low as 20 per cent) so is potentially more suitable for dewatered digestates or even certain AD feedstocks. The material is subjected to temperatures of 200o o C and pressures of 10-20 bar for a few hours and the resulting product is a coal like substance similar to biochar. It is an effective way of seques- tering carbon because the material used in the process has already taken in carbon dioxide from the atmosphere prior to digestion and subse- quent carbonisation. Resulting liquors are usually returned to the digester to recover these breakdown products. Another similar technology is hydrothermal liquefaction (HTL) which can be used to convert biomass with a low dry solids content into an oily substance called biocrude. The process uses heat and pres- sure, with the water content of the feedstock acting as a cata- lyst to facilitate the reaction. The resulting biocrude can be used as a heavy fuel oil in ships or upgraded into a more refined fuel which can be used as a substitute for diesel in trains or jet fuel in aeroplanes. Although many of the novel technologies in the sector are at any early stage of development and need proof of concept and further investment to get them to a commercially viable stage, there are several which have already emerged as full-scale alternatives to the traditional linear model of production and consumption. With increasing media coverage of global warming, plastic pollu- tion in the seas, and high- profile demonstrations by the likes of Extinction Rebellion, public perceptions are chang- ing. International government is also recognising the need for change, with all 197 UNFCC (United Nations Framework on Climate Change) members having ratified (186) or signed up (11) to the Paris Agree- ment which seeks to hold the increase in global average temperature to below 20 o C above pre-industrial levels. The Circular Economy is key to achieving these goals, so it is vital to have more focus and investment in AD and the new generation of technologies associated with it. Aqua Enviro is an independ- ent consultancy and laboratory supporting the wastewater and AD sectors. Services offered include treatability and pilot trials, feasibility studies, tech- nology reviews, and regulatory guidance. (Source: WRAP 2014) "An emerging risk to AD operators in the commercial and wastewater sectors is the pros- pect of a complete ban on the use of digestates and biosolids on agri- cultural land." The Knowledge: circular economy

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