Page 36 - Energize February 2021
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TECHNICAL


















                                                               Figure 4: Anaerobic digestion phases. 6


                                                               digestion tanks a series of biological processes are harnessed in
                                                               order to produce biogas, as shown in Figure 4. The process of
                                                               biodigestion depends on the temperature of the digestate.
        Figure 2: Three-week average municipal food waste sample  Anaerobic digestion consists broadly of three phases,
        composition (Valorgas 2011).                           namely, enzymatic hydrolysis, acid formation, and gas
                                                               production. Hydrolysis is the process where the organic material
                                                               is solubilised into the digestion liquid. It then undergoes the
           Some cities provide a separately marked refuse bin for food  intermediate steps of acidogenesis and acetogenesis which
        waste (including garden plant waste) which is collected separately  create the precursor molecules for methanogenesis.
        from other waste. This separates food waste from the balance of  Methanogens feed off these precursors and produce methane
        other non-recyclable waste and allows separate treatment.   and a cellular waste product. The end-product is a mixture of
           Even where pre-separation of waste is not carried out it is  methane and carbon dioxide. 6
        possible to efficiently separate the organic portion of municipal  In large plants these two gases can be separated and used
        solid waste (OFMSW) from the rest of the waste at the waste  individually. In smaller plant the combination is used directly. The
        centre. Mechanical waste separation systems which allow food  biogas is then converted into renewable power in the form of
        or organic waste to be separated from other waste are used in  electricity and heat via cogeneration or CHP with gas engines.
        waste treatment sites.                                 The residue from the digestion process is further processed into
                                                               liquid and solid fertiliser.
        Food waste treatment
        Biodigestion process                                   Two common applications exist for biodigestion of food waste:
        Figure 3 shows a general layout of a food waste biodigestion  •  On-site biodigestion. This applies to shopping malls, food
        plant.                                                    production facilities, hospitals and other facilities using small
           Pre-treatment is an important consideration when using food  to medium sized biodigestion plants. The plant is installed at
        waste anaerobic digestors. The first stage involves reduction of  the site where the food waste is generated, and there is on–
        the substrate to a size suitable for biodigestion. This may be  site use of the gas produced. At the bottom end of the range
        achieved by shredding or grinding the waste which has been  are household and remote eco-lodge sized units that rely on
        separated, or conversion to a pulp in the waste separation plant.   food and other organic waste to produce gas which is used
           Pre-treatment of the pulped waste may be applied depending  mainly for cooking. At the upper end are food processing
        on the process. Heat treatment has been found to increase the  plants, such as wineries or fruit juice producers. Small
        gas yield, and the addition of minerals and enzymes also impacts  transportable modular units have been developed, making it
        the process. From here it is loaded into the digestor by various  possible to process food waste from shopping malls, hospitals,
        means depending upon the constitution of waste materials. In the  universities, military camps, holiday resorts and other sites
                                                                  where food is sold or distributed in fresh or prepared form.
                                                               •  Centralised food waste biodigestion at waste disposal sites
                                                                  using large, mechanised plant, and sorted or mixed solid
                                                                  waste. The gas is used onsite for CHP generation with
                                                                  surplus power fed to the grid, or the gas may be sold via a
                                                                  distribution network.

                                                               Large scale food waste biodigestion projects
                                                               Large scale centralised biodigestors tend to be more efficient
                                                               than on site units and allow more control over the digestion
                                                               process and such features as pre-treatment. Biodigestion of food
                                                               waste at waste disposal sites has been applied extensively. In the
                                                               UK in 2019 there were 94 food waste biodigestors in operation,
                                                               handling approximately 8% of British food waste, and producing
        Figure 3: Food waste biodigestion plant                an estimated 220 MWe of electricity. 2


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