REALISING THE BIOCHEMICAL POTENTIAL OF FEED STOCKS IN THE CIRCULAR ECONOMY

Transcription

1 REALISING THE BIOCHEMICAL POTENTIAL OF FEED STOCKS IN THE CIRCULAR ECONOMY Smyth, M. 1, Horan, N. 2 1 Aqua Enviro, 2 The University of Leeds Corresponding author matthewsmyth@aquaenviro.co.uk Abstract Anaerobic digestion is an integral part of The Circular Economy and the vast majority of AD plants recycle organic and nutrient rich digestate to land. Very few, if any, AD plants have considered the biochemical potential of the feedstock and how manipulation of the available pathways may add value (or valorise) and deliver alternative end products. There are a number of technologies at various stages of development and based around the manipulation of digester biochemistry and microbiology, that are able to convert organic material in both the feedstock and the digestate into high value materials. These include for instance: short-chain fatty acids, alcohols and ketones that are of high value to the chemical industry, and polyhydroxyalkanoates - of value to the biodegradable plastics industry. This paper considers: The current situation for AD plants applying digestate to land in the UK now and in a future landbank constrained environment. Potential alternative end products to methane and digestate. How an AD facility under the concept of the biorefinery may look in the future. Enablers required to deliver a step change in the way industry approaches organics wastes, including knowledge gaps, legislative change requirement and current/future funding mechanisms. Keywords The Circular Economy, biorefinery, VFA factory, biochemical feedstock Introduction A liner model of resource consumption follows a pattern of take, make and then dispose. This approach leaves companies, even whole industries exposed to higher resource prices and supply disruptions. The Circular Economy, by contrast (figure 1), is restorative and regenerative by design with a focus on re-use rather than end-of-life (Ellen MacArthur Foundation, 2013). Anaerobic digestion is a key stage of The Circular Economy, producing renewable energy and recycling nutrient rich, organic material to land (the Biosphere).

2 Figure 1: The Circular Economy (Ellen Macarthur Foundation 2014) The amount of organic waste treated by anaerobic digestion has doubled since 2009 when the UK Government published Shared Goals, which summarised its aspirations for uptake of AD in the UK (Defra, 2009). There are now ~150 plants in the non-sewage sector treating around 11 million wet tonnes of organic waste annually. Whilst there is current concern about feedstock availability with the food waste AD sector, the rate of increase since 2010 has been 25 to 30 new plants each year (Eunomia, 2014). The capacity at the end of 2014 is estimated to reach 16 million wet tonnes of waste processed and of this around 43% derives from the food waste sector. Although the digestion process destroys between 60 and 90% of the volatile solids fraction of the feedstock, the water and ash are conserved and so there is very little reduction in the actual volume of the digestate produced. Indeed in some cases the volume may increase if the feedstock requires dilution before feeding the digester (Smyth & Horan, 2014). At present as much as 99% of the digestate produced in the UK is used in agriculture and horticulture and although it plays a valuable role in closing nutrient cycles, its financial value to the operator is low. In addition, reliance on a single route leaves the AD sector, indeed The Circular Economy, exposed to significant risk, in particular for those operators that lack their own secure land bank; typically digesters processing food waste. Recent WRAP-funded reviews of alternative options for digestate enhancement suggest that the scale and throughput of commercial AD facilities are too small to warrant application of many capital-intensive enhancement and recovery technologies that can deliver digestatederived fertiliser products; so called first generation enhancement technologies. There are, however, a number of second-generation technologies at various stages of development that may offer greater application. These are generally based around the manipulation of digester biochemistry and microbiology to convert organic material in the waste stream into a wide range of high-value bioproducts, sometimes referred to as the Biorefinery Concept, the VFA Factory or as in The Circular Economy, Extraction of biochemical feedstock.

3 In order for the optimum the value of feedstocks to be fully valorised a step change in the way industry treats them is required and for this to occur a number of enablers are required, for example legislative change that permits users to simultaneously manage food waste and sewage sludges would lead to the generation of much larger quantities of alternative products, which is attractive to the end users (biorefineries). Digestate Types Digestate produced from predominantly food waste is rich in organic material and nutrients. However, when compared to sewage sludge cake, especially from sites incorporating thermal hydrolysis it struggles to compete. Table 1: Digestate characteristics (*WRAP, 2011) It is more odorous due to higher residual levels of ammonia-nitrogen and volatile fatty acids and it is largely in liquid form. In United Utilities (figure 2) for example, as a result of investment in the 120,000 tonnes dry solids Sludge Balanced Asset Programme (SBAP) UU are able to offer the end user a consistent dry solids, stackable, friable final product (free of charge). Any food waste digester operator in this region without an existing long term agreement to apply their digestate to land will struggle to compete with this attractive offer and superior product. Figure 2: SBAP has expanded the area that United Utilities can apply sludge cake to land (Hart, M., 2014) AD or Intervention at the feedstock to convert a proportion of the organics into alternative, higher value products? Organic mass in AD systems is a complex mixture of fats, oils, carbohydrates (including sugars) and proteins along with smaller organic material and non-digestible material. The

4 process is mediated by several groups of microorganisms. In microbiological terms the organic mass is subject to four major interrelated and sequential steps; hydrolysis, acidogenesis, acetogenesis and methanogenesis (Geradi, 2003, Bitton, 2005). A conventional anaerobic digester is operated to optimise these stages and convert organic material in the feedstock to bioenergy, in the form of methane (figure 3). Hydrolysis and acidogenesis describe a range of different processes which hydrolyse and oxidise the complex organic matter into simple, digestible substrates. Proteins are converted into amino acids and via the Stickland reaction (Nisman, 1954) into fatty acids. Carbohydrates and sugars are hydrolysed to glucose monomers which can be further hydrolysed (Nokie, 2004 ). Fats and oils are hydrolysed into long chain fatty acids which undergo stepwise degradation via a β oxidation pathway (Jerris & McCarthy, 1965; Kim et al. 2004). Anaerobic processes are low energy yielding relative to aerobic systems, and the range of substrates used by many of the key organisms in AD is much more limited. For this reason the systems are very sensitive to small changes in conditions and each intermediate step is governed by strict requirements. The most critical are the acetogenesis and methanogenesis (Schink & Stams, 2005), which are rate limiting (Amani et al. 2011). Volatile fatty acids (VFA), as sources of acetate (Ac-), propionate (Pr-) and Butyrate (Bu-), are key substrates in these stages (Schmidt, 1995). However it is these intermediate products that offer the opportunity to increase (or valorise) the value of the feedstock as they are, in pure form, the basis of many chemicals and products. The reactions of acidogenesis and acetogenesis are both rapid and in order to ensure that VFA end-products do not accumulate in the digester, it is operated at a long hydraulic retention time and low organic loading rate. In this way the rate at which VFAs are produced from the organic fraction of the feedstock can be controlled at the rate VFAs are reduced to methane and thus stable operating conditions ensue. This process of methanogenic AD has a number of advantages. It employs a mixed microbial community that is selected or acclimatised for a given feedstock and thus it does not require the use of pure cultures of microorganisms thus necessitating a sterile feedstock. In addition the methane produced in the process is easily separated and can be converted into several useful forms of energy.

5 Figure 3: Anaerobic digestion metabolic pathways of organic wastes (adapted from Shanmugan & Horan, 2009) Anaerobic digestion does however have two major disadvantages since the products of methane and digestate are both of low value. However by manipulating the operating conditions (figure 4) within the digester away from methane production such that the reactions of acidogenesis predominate and methanogenesis is inhibited, a wide range of high-value chemicals can be produced from the feedstock.

6 Figure 4: Manipulation of HRT, ph and OLR to achieve different end products. Traditionally this approach has been used with pure cultures of bacteria growing on defined media and the best-known example is probably the acetone, butanol, ethanol (ABE) fermentation that is driven by Clostridium acetobutylicum. This bacterium can produce up to 20g/l butanol with acetone and ethanol as minor products. The economic viability of this process is a balance between the cost of the feedstock and the value of the product (figure 5) which is closely linked to the price of crude oil, both of which fluctuate widely making long-term capital investment a risky proposition. Availability of a waste substrate with a guaranteed long-term supply at a fixed price would favourably alter the economics of the ABE process. Figure 5: The value of alternative (to methane) products (Smyth, M., 2014) Waste feedstocks offer the potential to become inexpensive raw materials for a number of such integrated fermentation processes and the composition of the feedstock would drive the choice of process. High-carbohydrate organic wastes in particular are appropriate for conversion to a wide range of valuable products through fermentative processes. Even the traditional end products of VFAs can have value added, for instance by their bioconversion to other products, such as polyhydroxyalkanoates, that form the backbone of many biodegradable plastics. PHAs have very similar physical properties to synthetic polymers such as polypropolene, they can be altered by either changing the bacteria used as the source of polymer or by changing the growth conditions of the microorganism. Unlike acetate and butanol, which are soluble, PHAs are particulate and contained within the microorganism, thus the yield is very dependent upon the concentration of PHA in these organisms. Like

7 acetone and butanol the price of PHAs is related to oil and until this link is broken large scale PHA production is likely to be limited. VFA production is potentially attractive on large sites with multiple digesters where it may be possible to convert a single digester into a short retention time. In this example, however, it is unlikely to be economic to fully process a VFA rich soup (on site) into a 'clean' product. The two factors that are known to influence process economics are the conversion yield and the separation and purification of the products from the bulk liquid. Consequently it is these two areas where most research is being undertaken and which are defined by the Biorefinery Concept, namely: A facility that integrates biomass conversion processes and equipment to produce fuels, power, and chemicals from biomass. The concept of the biorefinery brings many new possibilities for innovations and market opportunities, with numerous benefits because of the diversity of available feedstocks and products. This potential solution therefore becomes more viable where an existing (or new) refinery is located in close proximity to the digestion plant and able (and willing) to separate and purify products from the bulk liquid (figure 6). In addition to intervention at the feedstock thermal options (gasification, pyrolysis, hydrothermal carbonisation, supercriticial wet oxidation) could be employed on the digestate and unlike VFA production they offer the opportunity to reduce the volume of digestate applied to land in a future landbank constrained environment. Figure 6: An AD plant of the future incorporating multiple processes and linked to a biorefinery (WRAP and Zero Waste Scotland, 2014) However the investigation of conversion yields, separation processes and the end products of thermal processes on organic waste feedstocks require widely differing skills and are generally being undertaken by different research groups, not necessarily based at the same

8 Institution. For example improving bioconversion rates has focused on process microbiology, for instance by the use of immobilized cultures or multi-phase reactors operated to select specific reaction processes. By contrast product separation and purification is based around chemical engineering techniques such as supercritical fluid technology, anionic fluidized bed columns and product crystallisation. Research, Funding & Investment Research groups at several UK Universities (Cranfield, South Wales, Southampton, Imperial College and Leeds) together with commercial organisations such as the CPI and the Institute of Food Research, all undertake some research into novel products from wastes that includes bioplastics, hydrogen and carboxylates. This situation has been recognised by the UK research councils such as BBSRC, NERC and EPSRC that are attempting to coordinate such research through the formation of networks of academic and commercial partners, organised along broad themes. The BBSRC has recently funded 13 networks in the area of Industrial Biotechnology and Bioenergy. These include a Food Processing Waste and By- Products Utilisation Network coordinated by the University of Reading and the Institute of Food Research, and an AD Network coordinated through the University of Southampton which includes groups such as The VFA factory: extraction, downstream processing and conversion. The situation is similar in Europe with a number of existing EU-Funded projects and a number of large consortiums bidding for EU Horizon 2020 funding in this area. The US has a more focused research effort with a number of Institutions (in particular Cornell which has a large teams investigating waste to bioproduct conversions) looking to convert wastes into liquid fuels as oil replacements. The Japanese are following similar lines with a high technology approach using sub- and super-critical water as well as super critical CO2 to achieve hydrothermal liquefaction of digestate and other organic sources, to generate fuel oils. WRAP and Zero Waste Scotland are also looking to fund multiple feasibility studies of those technologies which show potential to convert biowastes to novel products and to develop a technology hub which shall act as a funding and development point for bringing new technologies to market through full scale demonstration. The hub is expected to draw on a partnership approach with bodies including NERC, BBSRC, EPSRC, TSB and International funding and knowledge partners (WRAP and Zero Waste Scotland, 2014). Conclusions In order to transform organic waste feedstocks and/or digestate from a low value material to a more valuable commodity, then the whole of the digestion process must be considered, from feedstock though digester operation to the digestate itself. There are a range of technology options that could be applied which include thermal processes or manipulation of carboxylate platform. Although the use of e.g. fermentation to produce a range of short chain acids and alcohols using pure cultures growing on defined substrates is a mature technology with an extensive literature, the research and expertise in the application of this technology, for instance through the Biorefinery Concept using organic wastes as a feedstock, is fragmentary, and often just a small component of a large team s research objectives. There appears to be a concerted, if not consolidated effort, to advance research in this area and to bring to realisation those technologies that are near to market through a range of

9 funding mechanisms. With any process that looks to replace a market leading product, albeit fossil fuel derived, buy-in from the end user is critical. The feedstock to a biorefinery will need to be consistent both in terms of its characteristics and also its volume. Three possible routes that could be developed to increase the volume of feedstock materials to biorefineries are: 1) centering a biorefinery around a cluster of digesters; 2) ensuring that multiple digestion plants located 'close to' existing biorefineries simultaneously adopt a strategy to provide feedstock products; and 3) simultaneous processing of food waste and sewage sludges. Each of these options in the current economic and regulatory environment has significant hurdles to be cleared. References Amani T., Nosrati M., Mousavi S.& Kermanshahi, R. (2011) Study of Syntophic Anaerobic Digestion of Volatile Fatty Acids Using Enriched Cultures at Mesophilic Conditions. International Journal of Environmenal Science and Technology, 8(1), Bitton G. (2005)Wastewater microbiology. New Jersey, Hoboken: John Wiley & Sons. Defra (2009) Anaerobic Digestion Shared Goals, February Defra. Ellen Macarthur Foundation (2013). Towards The Circular Economy - Economic and Business Rationale for an Accelerated Transition. Ellen Macarthur Foundation. Ellen Macarthur Foundation (2014). Towards The Circular Economy - Accelerating the scaleup across global supply chains. Ellen Macarthur Foundation. Eunomia (2014) Anaerobic Digestion Market Update - Addressing the Feedstock Famine, July Eunomia. Geradi M. (2003) Wastewater microbiology series : The microbiology of anaerobic digesters. New York : John Wiley and Sons. Hart, M (2014). Davyhulme Wastewater Treatment - A Works in the Making. In ed. Horan, NJ, Activated Sludge: Past, Present & Future, April. Aqua Enviro, Wakefield. Jerris J., & McCarthy P. (1965, Feb) The Biochemistry of methane fermentation using C14 tracers. Journal (Water Pollution Control Federation), 37(2), Kim S., Han H. & Shin H. (2004) Two phase anaerobic treatment system for fat containing wastewater. J Chem Technol. Biotechnol,, 79, Nisman B. (1954) The Stickland Reaction. Bacteriol Reviews, Nokie T. E. (2004 ) Characteristics of carbohydrate degredation and the rate-limiting step in anaerobic digestion. Biotechnology and Bioengineering, Schink B. & Stams A. (2005) The prokaryotes: an evolving electronic resource for the microbiological community. (M. Dworkin, Ed.) New York: Springer. Schmidt J. A. (1995) Interspecies electron transfer during propionate and butyrate digradation in mesophilic granular sludge. Appl. Environ. Microbiol., 7, Shanmugam, P & Horan, N.J., (2009). Optimising the biogas production from leather fleshing waste by by co-digestion with MSW. Bioresource Technology, 100,

10 Smyth, M (2014). New Digestate Products. Conference presentation at UK ADBA & Biogas Event, 3 rd July ADBA Smyth, M., and Horan, N. (2014). Operating Experiences from Organic Waste Digestion Plants. In ed. Horan, NJ, 19 th European Biosolids and Organic Resources Conference, November. Aqua Enviro, Wakefield. WRAP, Digestate & Compost in Agriculture, Bulletin 2 November WRAP WRAP and Zero Waste Scotland (2014). Converting biowastes to novel products. [Online] Available from: [Accessed: 1 st October 2014].