In recent years there has been increasing awareness worldwide regarding the food waste management and the potential use of food waste for energy production. It has been estimated that due to the inefficiency of Food Supply Chain (FSC), nearly one third of edible food is wasted each year (Gustavsson et al., 2011). According to the reports from the European Commission, nearly 42% of the total food produced is wasted during the final consumption stage of the FSC, in many European countries. The total food (excluding agricultural waste) loss in the EU is estimated to be 89 Mt relevant to 179 kg per capita in 2006 and is constituted as: 42 % produced by households, 39 % occurs in the food manufacturing industry, 14% applies to food sector (restaurants, catering) and 5 % lost during distribution (Mirabella et al., 2014). Food waste is often defined as the food wasted/ lost at the end of the FSC, i.e. distribution, retail and final consumption (Gustavsson et al., 2011). According to Parfitt et al. (2010), depending on the source of the food waste, specially focusing on household, two categories of food waste could be made: (a) avoidable and possibly avoidable waste referring to ‘edible’ food waste and (b) unavoidable food waste that are non-edible, those that are derived primarily from food preparations. Recently, several research groups have studied extensively the environmental impact of food waste especially in terms of Greenhouse Gas (GHG) emissions (Venkat, 2011) or wastage of water resources (Lundqvist et al., 2008). The carbon footprint of food waste is estimated to be 3.3 billion tons of CO2 equivalents per year of GHG released into the atmosphere. Similarly, 1.4 billion hectares of land – 28 percent of the world’s agricultural area – is used each year to produce food that is lost or wasted. In addition to the adverse environmental implications, the food wastage also poses several socio-economic impacts. It is estimated that the average amount of food waste generated in developed countries per year (220 million tons) accounts to almost equal to the total net food production of Sub-Saharan African countries (Gustavsson et al., 2011). Given the environmental and socioeconomic implications of the food waste, the present study reports the various methods and strategies for efficient usage of food waste for energy recovery (such as biofuels viz., bioethanol and biogas) and to produce other value added products (such as pectin and other nutraceutical products).
Food waste generation
Global Scenario It is estimated that while about 870 million people are reported to be chronically undernourished, global food waste accounts to approximately 1.3 billion tons per year, i.e. one third of the global food production (Kojima and Ishikawa, 2013). While considering the global food waste generation per year, reports suggest: the United States 61 million tons (GMA, 2012); China 92.4 million tons (Lin et al., 2011); Europe 90 million tons (EC, 2013); Australia 4 million tons (Dee, 2013.); South Korea 6.24 (Hou, 2013) and Japan 21 million tons (Kojima and Ishikawa, 2013). Reports suggest that in industrialized as well as developing countries the magnitude of food waste generated are consistent (Gustavsson et al., 2011). However, there are substantial differences in the source of food waste in both industrialized and developing Countries. In the latter, more than 40% of food losses occur at the postharvest and processing stages, while in the former, about 40% of losses occur at the retail and consumer levels in the FSC. It is estimated that on a per-capita basis, more food is wasted in the industrialized countries compared to the developing nations. In a recent FAO report (Gustavsson et al., 2011) it is explained that the causes of food losses and waste in both industrialized as well developing countries as due toIn Industrialized countries: a. Food gets lost when production exceeds demand. b. ‘Disposing is cheaper than using or re-using’ attitude leads to food waste c. High ‘appearance quality standards’ from supermarkets for fresh products d. Wide range of products/ brands in supply In developing countries: e. Food lost due to premature harvesting f. Poor storage facilities and lack of infrastructure cause post-harvest food losses g. Lack of processing facilities h. Inadequate market systems
Why ‘Resource Recovery’ from Food Waste? Recently, considerable attention is being made on the concept of ‘Biorefineries’ that utilizes organic food waste for production of bio-based products or to generate fuel/ energy. Based on the origin, food waste could be categorized as either from a) agriculture or industrial food processing or b) household food waste. These food wastes generally contains low levels of suspended solids and low concentrations of dissolved materials, which results in production of different moldering gases and offensive odors, further leading to several adverse environmental impacts. These wastes generation often leads to a waste of resources that are used in the food production and distribution process (Pham et al., 2015). Currently, most fraction of the food wastes are recycled, mainly as animal feed and compost (Lin et al., 2013), while the remaining quantities are incinerated and disposed in landfills, causing serious emissions of methane (CH4), a potent greenhouse gas that significantly contributes to the climate change. Historically, landfill was once the primary choice for handling food wastes, but has now been banned in many developed countries because of the exhaustion of existing landfill sites. Moreover, the leachate generated by these materials makes secondary wastewater treatments necessary (Yan et al., 2011), and the incineration of food waste is unsuitable because of its high water content and the likelihood of dioxin emission (Nishijima et al., 2004). The conventional recycling method for food waste, i.e. as animal feed and fertilizer, often creates hygiene problems (Moon et al., 2009). The valorization of food waste could also have positive implications on an environmental point of view, were a reduction in methane gas emissions from the landfills and also the preservation of natural resources such as coal and fossil fuels could be achieved in long run. It also pose a positive stance towards the socio-economic prospects by reducing the food vs. fuel competition and also costs saving linked to surplus food production globally (Girotto et al., 2015). It is therefore imperative to develop a recycling method that can convert food waste into a valuable product /energy, at the same time environmentally friendly.
Energy and value added products from food waste
Processing methods Depending on the source of production, food waste is characterized to possess variable chemical composition, comprising a mixture of carbohydrates, lipids and proteins. Hence the methods for generation of biofuels or other value added products from food waste greatly depends on their source of production (Girotto et al., 2015). Food waste can be converted into biofuels or products by means of biological processes or thermo-chemical processes that mainly includes: a. transesterification of oils and fats to produce biodiesel; b. fermentation of carbohydrates to produce bioethanol or biobutanol; c. anaerobic digestion to produce biogas (methane rich gas); d. dark fermentation to produce biohydrogen; e. pyrolysis and gasification; f. hydrothermal carbonization or g. incineration Energy generation from food waste Food waste is globally considered as an unexploited resource with great potential for energy generation. Several technologies have been developed recently that help to recover valuable fuels from food waste, resulting in reduced environmental burden of its disposal, avoid depletion of natural resources, minimize risk to human health and maintain an overall balance in the ecosystem (Pham et al., 2015). However, utilization of food waste for energy conversion currently possess several challenges due to various reasons such as the heterogeneously variable composition, high moisture contents and low calorific value of food waste. This constitutes an impediment for the development of robust, large scale, and efficient industrial processes of energy conversion. In the following sections, we describe various researches and industrial process developed for an efficient usage of food waste for energy recovery.
Bioethanol production from food waste Food waste is an important source of organic solid waste with high percentage of moisture. In general, food waste is a complex biomass and its major ingredients suitable for ethanol production are components such as starch and/or lignocellulose. The carbohydrates content of food waste has been estimated to be as high as 65% of the total solids, making it a promising substrate for producing ethanol (Kim et al., 2011). The complexity of food waste composition makes its utilization process difficult, especially for ethanol producing microorganisms such as baker’s yeast, Saccharomyces cerevisiae. Hence a pretreatment process, hydrolyzing the food waste and producing fermentable sugars, is required. Kim et al. (2011) stressed the importance of pretreatment with hydrolyzing enzymes (carbohydrase, glucoamylase, cellulase, and protease) for efficient ethanol production from food waste. Food waste to ethanol: Recent research studies Feasibility of food waste for ethanol production has been investigated in many lab scale studies. In their study on the potential of food waste for ethanol production, Zhang and Richard (2011) used compost site samples with a composition of 23.3% w/w total reducing sugars, 34.8% w/w starch, and 1.6% w/w fibers. Similarly, Moon et al. (2009) also studied ethanol production from food waste with high starch (30.1% w/w) and fiber (14.9% w/w) contents, but with a total of 17.6% w/w reducing sugars, making it necessary to use both amylases and cellulases enzymes . High starch content (63.9% w/w) in combination with low cellulose amounts was investigated by Yan et al. (2011) in their experiments on household food waste. Matsakas et al. (2014) recently reported a final ethanol yield of 108 g/kg dry material (64% of the theoretical maximum) from household food waste comprising 12.5% total reducing sugars, 18% cellulose, and 7% hemicellulose. Despite its potential, only scanty information about utilizing food waste for ethanol production exists in the literature as compared to other waste substrates.
Industrial ethanol production from food waste
Etanolix® by St1 The Etanolix concept (www.st1.se/etanolix) is promoted by St1 Biofuels (www.st1biofuels.com) – a joint venture of the energy company St1 and the VTT Technical Research Centre, Finland. Raw material consisting of waste products from the food industry (bakery waste) is the major feedstock in the Etanolix-process. The purity of the ethanol produced in this process is approximately 85%, (CO2 -reduction >90%) and in the subsequent recovery process, stillage remains as a by-product which is processed for animal feed, or for the production of biogas. The ethanol production capacity of the plant is 5,000 m3 per year with an estimate of approximately 52,000 tons stillage during a normal year. St1 refinery is also launching a plant for Etanolix 2.0 (the short name for the LIFE+ project), adjacent to St1’s refinery in Gothenburg, Sweden. This plant claims a capability of processing 15,000 -21,000 tons of waste products from the food industry per year. An assessment suggests a production of 5,000 m3 of ethanol per year, which when used as fuel for transportation, will achieve a 90% reduction in CO2-emissions. Food waste as biogas production feedstock Food wastes are generally the main waste streams of organic solid waste for biogas production in urban areas, while food waste together with livestock waste are preferred in rural areas (Kim and Oh, 2011). The entire food waste-based biogas system consists of different subsystems viz., pretreatment process, followed by the actual power generation with biogas or heat utilization, and the final disposal of biogas residues or slurry (Jin et al., 2015). In general, nearly all kinds of organic solid waste could be the feedstock for anaerobic digestion (AD), producing biogas. EU policies concerning renewable energy have set forward the task of supplying 5% of the European energy demands from AD biogas by year 2020 (Holm-Nielsen et al., 2009). Though AD of food wastes had been extensively studied, there occur several major problems associated with the process. For example, the pH drop owing to the rapid accumulation of organic acids during the digestion process, especially in the high-solid process condition inhibits the methanogenesis process and produce less biogas (Yang et al., 2015). Moreover, the methane generated would also vary depend on the chemical composition of the food waste.
Food waste to biogas
Recent research studies In a recent review on the anaerobic digestion process of food waste for biogas production, Zhang et al. (2014), described that the AD of food waste is a complex process that should simultaneously digest all organic substrates (e.g., carbohydrate and protein) in a single-stage system. It is reported that the AD process is mainly governed by different key parameters such as temperature, volatile fatty acid, pH, ammonia, nutrients, and trace elements in the food waste. Hence it is important to maintain the key parameters within the appropriate range for long term operations. Several researchers around the globe had extensively studied the potential of food waste for biogas production. Recently, few Chinese researchers studied the AD of food waste resulting in final total solids (TS) and volatile solid (VS), reaching the maxima of 52.07%, 63.46% and 90.92%, 95.85% respectively. Their study not only decreases the quantity of wasted food but also produce clean biogas (Yang et al., 2015; Zhang et al., 2014). Several studies on the life-cycle assessment on the energy consumption and environmental impact of an integrated food waste-based biogas plant had been conducted recently (Jin et al., 2015; Xu et al., 2015). This proves the increasing significance and the industrial viability of the process. Sobacken biogas plant: Success model for food waste to energy conversion The biogas plant owned and managed by Boras Energy and Environment at Sobacken in the city of Borås, Sweden, is one of the successful examples for the industrial scale conversion of food waste to energy. The organic food wastes from municipal or food processing industry in Boras are collected and separated into liquid and solid fraction at the plant. The liquid is fed into an anaerobic sequence batch reactor (SBR) for biogas production while the solid phase is fed into the waste incineration plant for combustion. Prior to feeding the reactor, the liquid is collected in a buffer tank for the feed preparation. The substrate is retained for 6 hours in thermophilic conditions, whereby improving the process efficiency (Eriksson, 2009; Jin et al., 2015). It is estimated that around 30,000 tons organic waste or 6000 tons dry matter is being processed in a 2600 m3 anaerobic sequence batch reactor (SBR) every year. The plant produces 3.5 million Nm3 biogas or 25000 MWh energy and in addition, around 2500 tons biofertilizer per year (PURAC, 2015). Value added products from food waste While energy recovery from food waste tends to focus more on mixed/household food waste, the production of value added chemicals are based on specific type of food waste material. In the following sections, the most common methods of valorization of industrial food processing waste are discussed. In a recent study, PURAC (2015) described the use of exotic fruits, olives and tomatoes for the production of antioxidant, fiber, phenols, polyphenols and carotenoids. Similarly, dairy by-products and slaughter house waste serve as potential source for lactic acid and protein extraction (Mirabella et al., 2014). Few elected examples for value added products from food waste are described in the following section. Fruits are rich in fibers, vitamins and various bioactive compounds. Fruit processing produces a large quantity of waste products such as seeds, kernels, flesh and peels; which contain valuable compounds in higher quantities (Mirabella et al., 2014). Recovery of pectin from passion fruit processing waste (Mirabella et al., 2014); bromelain from pineapple stem (Canteri et al., 2010); amino acids and phenolic compounds from mango seeds (Upadhyay et al., 2012); polyphenols, carotenoids, vitamins, enzymes and dietary fibers from mango peels (Abdalla et al., 2007); coconut protein powder from coconut processing industry waste (Ajila et al., 2010) etc. have been studied extensively. Similarly, pectin, a food additive that is mainly used in the food, pharmaceutical and chemical industries are mainly derived from waste materials, such as apple pomace, orange peel (Naik et al., 2012), and sugar beet pulp (Thakur et al., 1997). It is reported that orange, lemon, and grapefruit peels contain 25-30 % pectin of their dried peel mass (Wicker et al., 2014) and apple pomace have a pectin content of about 10-15 % on a dry basis (Bagherian et al., 2011). Globally, meat industry discards large amounts of slaughter house waste (skin, bones, entrails, fatty tissues, feet, skull) that are bound by severe hygiene and health limitations. The studies on protein recovery from beef and pork waste (Bouaziz et al., 2008); functional proteins, collagen and gelatin production from poultry and marine wastes (Selmane et al., 2008) are reported in literature. Cheese-whey, the by-product of cheese manufacturing, is considered to be an environmental pollutant due to its high BOD and COD contents (Gómez-Guillén et al., 2011). It represents about 85-95 % of the utilized milk volume and retains about 55 % of the nutrients (Yadav et al., 2015). Today the 50 % of nascent whey world-wide is turned into value added products such as whey powder, whey protein, whey permeate and other products: bioethanol, biopolymers, hydrogen, methane, electricity, bioprotein (or single cell protein) and probiotics (Siso, 1996; Yadav et al., 2015). Cheese-whey is known to consist of mostly lactose (4.5-5 % w/v), soluble proteins (0.6-0.8 % w/v), lipids (0.4-0.5 % w/v) and mineral salts (8-10 % of dried extract); less quantities of lactic acid, citric acid, non-protein nitrogen compounds and B vitamins are also reported (Prazeres et al., 2012). Since the main constituent of whey is lactose and soluble proteins various application with valorization are used such as biotechnological, physicochemical treatments and direct land applications (Siso, 1996). Conclusions Developing sustainable resource recovery strategies for an efficient food waste management is the need of the hour. Exploring the potential of food waste as an energy source tends to achieve social, economic and environmental benefits. Measures to develop a proper equilibrium between food production and consumption are also equally important. Many nongovernmental organizations and volunteer-based groups worldwide are making considerable initiatives and movements to reduce the food waste generation. ‘Guerilla Kitchen Amsterdam’ is a successful example for collecting, cooking and using food, which could otherwise be categorized as ‘food wastes’.
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