Environ Sci Pollut Res DOI 10.1007/s11356-015-4235-y
BIOLOGICAL WASTE AS RESOURCE, WITH A FOCUS ON FOOD WASTE
Food waste collection and recycling for value-added products: potential applications and challenges in Hong Kong Irene M. C. Lo & Kok Sin Woon
Received: 22 December 2014 / Accepted: 13 February 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract About 3600 tonnes food waste are discarded in the landfills in Hong Kong daily. It is expected that the three strategic landfills in Hong Kong will be exhausted by 2020. In consideration of the food waste management environment and community needs in Hong Kong, as well as with reference to the food waste management systems in cities such as Linköping in Sweden and Oslo in Norway, a framework of food waste separation, collection, and recycling for food waste valorization is proposed in this paper. Food waste can be packed in an optic bag (i.e., a bag in green color), while the residual municipal solid waste (MSW) can be packed in a common plastic bag. All the wastes are then sent to the refuse transfer stations, in which food waste is separated from the residual MSW using an optic sensor. On the one hand, the sorted food waste can be converted into valuable materials (e.g., compost, swine feed, fish feed). On the other hand, the sorted food waste can be sent to the proposed Organic Waste Treatment Facilities and sewage treatment works for producing biogas. The biogas can be recovered to produce electricity and city gas (i.e., heating fuel for cooking purpose). Due to the challenges faced by the value-added products in Hong Kong, the biogas is recommended to be upgraded as a biogas fuel for vehicle use. Hopefully, the proposed framework will provide a simple and effective approach to food waste separation at source and promote sustainable use of waste to resource in Hong Kong. Keywords Anaerobic digestion . Food waste . Optical sorting . Separation and collection . Waste-to-energy . Waste valorization Responsible editor: Bingcai Pan I. M. C. Lo (*) : K. S. Woon Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China e-mail: [email protected]
Overview of food waste generation and disposal in Hong Kong Waste is a typical problem of affluent societies like Hong Kong. When the public can afford greater convenience and more purchases, they are inclined to discard more waste. Hong Kong is no exception to this (HKEPD 2014a). Hong Kong is a highly urbanized and populated city with limited space available for municipal solid waste (MSW) disposal. At present, Hong Kong relies mainly on landfills for MSW disposal. Approximately 9000 tonnes of MSW are still disposed of at the landfills in 2012 every day. It is expected that the current three strategic landfills in Hong Kong, namely South East New Territories, North East New Territories, and West New Territories, will reach their maximum capacities one by one by 2020 (HKEPD 2013). Among the 9000 tonnes of MSW discarded in the three strategic landfills in Hong Kong daily, about 3600 tonnes is food waste, accounting for 40 % of the waste disposed and is the largest constituent of MSW in Hong Kong. Of the 3600 tonnes of food waste generated daily, about 2500 tonnes (69 %) is contributed from domestic households, and around 1,100 tonnes (31 %) from food-related commercial and industrial (C&I) sources (HKEB 2013). The average per capita per daily disposal quantity of food waste in Hong Kong is about 0.4 kg/person/day −1, which is high when compared with some developed cities in Asia with similar economic status such as Taipei and Seoul (about 0.2 kg/ person/day −1). The food waste recycling rate in Hong Kong is very low, which was a mere 0.6 % of the total food waste disposal in 2012 (HKEPD 2014b). This is far below other urban areas such as South Korea and Taiwan, with recycling rates of 95 and 31 %, respectively (LegCo 2013; EPA 2014a, b).
Environ Sci Pollut Res
Systematic food waste collection and recycling for valorizing food waste to value-added product Albeit prevention and reduction of food waste are always the top priority to combat with food waste management issue in Hong Kong, there is still a substantial amount of food waste being disposed of at the three strategic landfills every day. The current practice of disposing food waste at landfills is not sustainable and is environmentally undesirable as it results in rapid depletion of limited landfill space, creates odor nuisance, and generates greenhouse gases and leachates that require further mitigation, thus imposing severe burdens on the environment. Besides, accelerating growth of human population, increased environmental issues, and growing global demand for energy and materials in our society have fostered intensive research efforts to valorize waste to resource in order to meet such global targets. In this context, food waste that cannot be avoided should be collected systematically and valorized to value-added products as far as possible. Various food waste management techniques have been practiced by the Hong Kong Special Administration Region (HKSAR) Government. The food waste management techniques emphasize prevention and reduction of food waste at source, donation of surplus food for human consumption, recycling of food waste, and treatment and disposal of MSW where food waste has not been separated, collected, and recycled (HKEB 2013). While the HKSAR Government has been adopting a multi-pronged approach to tackle the problem, more action is required and active participation from the community is also needed to alleviate the food waste problem. At present, much of the food waste recovery activities do not take place in individual households. Assuming that the food waste collection rates of both domestic household and C&I sectors in the future are 50 %, the total food waste collection rate in Hong Kong is about Fig. 1 Proposed framework of food waste collection and recycling for food waste valorization in Hong Kong
1800 tonnes per day (tpd). This amount constitutes a huge challenge to the society if collected food waste is not recycled in a cost-effective and environmentally friendly manner. A sustainable food waste management framework should combine a systematic food waste separation and collection system together with low environmental impact food waste valorization technologies. In tandem with this matter, a sustainable framework of food waste collection and recycling for various food waste valorizations is proposed. The proposed framework is developed with reference to the food waste management systems in some cities such as Linköping in Sweden, Oslo in Norway, Lille in France, and Ulsan in South Korea, as well as in consideration of the food waste management environment and community needs in Hong Kong. The overview of the proposed framework is shown in Fig. 1. The proposed framework comprises a food waste collection and separation at source using an optic bag system, various food waste valorization alternatives to value-added products, and other MSW treatment. Food waste separation and collection at source with optic bag system Various kinds of food waste separation and collection systems have been applied in other countries based on their community needs and environmental conditions. For example, in Taipei, Taiwan, the public is required to discard their food waste at designated times and locations. For areas and municipalities with a majority of single houses in Sweden, a multicompartment bins system is largely used. The bins are divided into two or four compartments to be used for apartment buildings or commercial and industrial areas (Al Seadi et al. 2013). Meanwhile, a MiniVac system has been designed by Envac Company, in which food waste produced from commercial kitchens such as canteens in offices and hospitals, restaurants,
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and industrial food factories, is thrown into conveniently located inlets within the kitchen and vacuumed through a pipe network to a refrigerated storage unit located in the basement or outside the building. These storage units are then emptied using a vacuum truck, and the food waste is transferred to respective food waste recycling facilities (Envac 2011). This system has been practiced in few cities such as London, Vancouver, Seoul, and Dubai. For the current practice in Hong Kong, nearly most of the food waste, especially food waste from domestic households, does not sort out from other types of MSW prior to disposing. The separation and collection of food waste is always an important aspect of any food waste recycling system. Effective food waste recycling requires the waste to be first separated from other types of MSW and then collected for delivery to food waste recycling facilities (HKEPD 2014a). Food waste that has been mixed with other types of MSW is contaminated and cannot be recycled. Consequently, the separation and collection of food waste is a crucial part of any food waste recycling system. A complicated sorting process in households, however, can hamper the residents from categorizing the waste generated by them. A less behavioral change of the public is required for the collection and separation of food waste. This is imperative to minimize any changes that might affect the lifestyle of the public and reduce potential impacts on the livelihoods of the public. In addition, the food waste collection system should be reasonably practicably constructed based upon the existing waste management system, including practices, infrastructure, and waste reduction and recovery schemes. By doing so, the public will not be required to travel to a new place or change their daily routine considerably for the sorting of food waste, and thus be more willing to take part in separating food waste at source. Due to this circumstance, a simple sorting process with less behavioral change of residents for sorting process is of utmost importance in order to encourage the residents to sort food waste out from other MSW at source. In the interest of achieving this objective, food waste and other types of MSW can be efficiently separated via an optic bag system. Food waste will be packed using an optic bag (i.e., green bag), while the residual MSW will be packed in a common plastic bag. All the packed wastes will still be discarded in the same designated collection bins or in existing rubbish chutes, eliminating the need for extra storage space for the fractions. All the wastes will then be collected in the conventional way (i.e., through refuse collection vehicles) and transported to the respective refuse transfer stations in Hong Kong. The refuse collection vehicles used for delivering the optic bags in Oslo, Norway, are having similar configurations (i.e., with rear compactor) like those in Hong Kong. The optic bag is not broken when it arrives at the optical sorting plant in Oslo. Besides, the common plastic bag used in Hong Kong is
usually not broken when it is transported to the refuse transfer stations. The optic bag is relatively thicker than the common plastic bag; hence, the optic bag will not be broken when it arrives at the refuse transfer stations. Upon arrival, all the waste bags will be dumped into a receiving pit and transferred to a conveyor belt. At this juncture, no separation of the bags has yet to be taken place. Once on the conveyor belt, the bags will be sorted automatically using camera technology applied with optic reader which recognizes the colors of the bag. When an optic bag with green color (i.e., food waste) is detected, a signal will be sent which pushes the green bag off the main conveyor belt to a second belt and then directed to its designated container. The collected food waste will then be separated from the optic bag via an Enviflex system. In the system, the food waste with optic bag is fed forward to a hydraulically driven roller that opens the optic bag. The food waste is then broken into pieces, and the optic bag is pulled in long strip. After that, the food waste and the optic bag will be screened for separation. The food waste will then be compacted and containerized in purposely built containers for onward transportation to assorted food waste recycling facilities for further treatment. Meanwhile, the other residual MSW such as plastics and textiles, which is packed in common plastic bags, will be delivered to a proposed advanced incineration facility for combustion, turning the waste into electricity. The optic bag system can be executed in line with the MSW charging scheme, which is going to be launched by the HKSAR Government. Based on the quantity-based system, the waste charge could be imposed through the mandatory use of pre-paid designated garbage bags. The optic bag can be assigned as one of the pre-paid designated garbage bags for packaging food waste (the cost of an optic bag is about 0.1 SEK/bag or 0.1 HKD/bag based on Sweden case), while the other pre-paid designated garbage bags can still be used for the disposal of other types of MSW. Hence, the introduction of optic bag system virtually makes the proposed MSW charging scheme unaffected. From the economic perspective, a standard optical sorting plant with an annual capacity of approximately 30,000 tonnes is built for about 30 SEK million or 30.5 HKD million based on a Sweden plant in 2011 (OptiBag 2014). The optical sorting plants have been operated in some European countries. Figure 2 illustrates the example of green bag for packing food waste and the optic sensor being used by the Tekniska verken company, a waste collection company in Linköping, Sweden. According to the performance of the collection and sorting plant operated by the Tekniska verken company, separation efficiency of food waste from other MSW can be achieved as high as 98 % (Tekniska verken 2012). The Haraldrud plant, which located in Oslo, Norway, also applies the optic bag sensor technology since 2009 and handles wastes approximately 150,000 tonnes/year (Envac 2014).
Environ Sci Pollut Res Fig. 2 Photos of (a) green bag (i.e., optic bag) for packing food waste; (b) optic sensor to separate food waste from other MSW; and (c) separated green bag waiting for transferring to food waste recycling center
The Haraldrud plant is currently the world’s largest sorting facility for separating food waste from other hold waste. Experience from this city suggests that tend to sort at source when it is easier for them to separated waste (Envac 2014).
optical housepeople handle
Valorization of food waste to compost, swine feed, and fish feed The valorization of biowaste (e.g., food waste, yard waste, animal manure, etc.) is an increasingly hot topic (Lin et al. 2012). For example, maize silage has been anaerobically fermented with cow manure on a commercial scale to obtain a carbon powder as solid biofuel (Maroušek et al. 2014a). Slag and fly ash generated from the combustion of wheat straw have been extracted to yield solubilized K2O and SiO2, which can be used in formulations for bioderived adhesive utilized in bioboards (Dodson 2011). Grass cuttings have been utilized to produce high-quality biochar using anaerobic fermentation followed by continuous pyrolysis processes (Maroušek et al. 2013, 2014b; Maroušek 2014). Besides, Viriya-empikul et al. (2012) reported that CaO from eggshells and molluse shells can be used to catalyze the production of fatty acid methyl esters from pre-esterified used cooking oils into biodiesel. After efficient separation and collection of food waste in Hong Kong, the collected food waste can be valorized as a source of valuable material or as a source of renewable energy. In this section, three types of valuable materials that can be valorized from food waste in Hong Kong are highlighted. The
three types of materials encompass compost, swine feed, and fish feed. Composting process has been practised by the HKSAR Government through the development of a Kowloon Bay Pilot Composting Plant (KBPCP) with a treatment capacity of 500 tonnes/year of food waste feedstock at the Kowloon Bay Waste Recycling Centre in mid-2008. The composts generated from the KBPCP could be used for landscaping and vegetable and fruit production. It, however, should be noted that the annual demand for compost in Hong Kong is low due to diminishing agricultural activities in Hong Kong. It is estimated that the demand for compost in Hong Kong is about 50, 000 tonnes/year or 137 tpd. If 1 tonne of food waste can produce 0.4 tonnes of compost, only 340 tpd of food waste is used to shelter the compost demand in Hong Kong every day. Subsequently, it is not feasible to valorize all collected food waste to compost in Hong Kong. Besides converting food waste into compost, food waste can also be valorized to swine feed. Currently, there are two companies in Hong Kong for converting food waste into swine feed. The two companies are Green Environmental Kitchen Residue Recycle Limited and Hong Kong Organic Waste Recycling Centre (HKOWRC). The Green Environmental Kitchen Residue Recycle Limited, situated at Ho Sheung Heung, Sheung Shui, treats about 10 tpd of food waste using drying process. Meanwhile, the HKOWRC, which is located at Kam Tsin Tsuen, Sheung Shui, collects food waste from both schools and hotels. The treatment capacity of the HKOWRC is about 12 to 15 tpd of food waste. Food waste
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collected from the schools has lower nutritional value and is sold to pig farms for direct feeding. Meanwhile, food waste collected from the hotels has higher nutritional value and is used to produce dried swine feed. For the production of swine feed from food waste with higher nutritional value, the operational processes consist of grinding, 48-h fermentation using effective microorganisms (EM), followed by drying. For these two companies, it is estimated that 10 tonnes of food waste can produce 1 tonne of dried swine feed. The swine feed is sold at 2500 HKD/tonne to two local pig farms. Nevertheless, there is a conundrum in turning food waste to swine feed in Hong Kong, primarily due to hygiene issue. The Hong Kong Agriculture, Fisheries and Conservation Department does not encourage the use of food waste as swine feed in Hong Kong. This is mainly attributed to the fact that the UK Government has banned its use in 2001 (DEFRA 2012), followed by the European Union in 2003 (EC 2002), in order to avoid diseases transmission such as bovine spongiform encephalopathy (i.e., mad cow disease) and hand, mouth, and foot disease. Besides, the market demand of swine feed in Hong Kong is limited. Currently, there are 43 pig farms in Hong Kong, in which it can only consume about 170 tpd of food waste. In Hong Kong, a plant named Kowloon Biotechnology Limited currently collects 20 tpd of food waste to produce fish feed using drying process. The fish feed is sold to four to five fish pond owners in Hong Kong and made cheaper than the imported fishmeal from mainland China. Meanwhile, another company named South China Reborn Resources (Zhongshan) Company Limited is constructing a food waste processing plant with a treatment capacity of 200 tpd of food waste in Ecopark in order to produce high protein content feed for aquaculture using fermentation and drying processes. The plant is expected to be commissioned in 2015, and the feed product will be exported to the USA, European Union, Japan, Taiwan, and mainland China.1 The valorization of food waste to fish feed in Hong Kong is a promising alternative. There are, however, some challenges for valorizing food waste to fish feed in Hong Kong. On the one hand, the fish feed demand in Hong Kong is not high. It is estimated that the fish feed demand from food waste in Hong Kong is about 10 tpd of food waste. On the other hand, export of fish feed products to the other regions such as mainland China can be difficult, mainly due to the license and regulatory issues. A feed factory license from the HKSAR Government is required to apply for feed import license from the Ministry of Agriculture of the Chinese Central Government (AQSIQ 2014). In addition, the regulation has become stricter as the Quality and Technology Supervision Bureau of the Chinese Central Government 1 Based on a local newspaper, the plant owner of the Kowloon Biotechnology Limited will work with the South China Reborn Resources (Zhongshan) Company Limited once the food waste processing plant in Ecopark is operated (Mingpao 2012).
requires the HKSAR Government to register with the Chinese Central Government before importing any feed products to mainland China. This requires communication between both governments, and thus hinders feed trade between cities. Table 1 provides the summary of food waste recycling facilities, food waste conversion rate, expected consumption of food waste, and challenges for compost, swine feed, and fish feed. In Asian countries such as South Korea, Taiwan, and Japan, more than 40 % of the food waste is recycled into compost, swine feed, and fish feed to partly substitute for chemical fertilizers and conventional feed ingredients. Nevertheless, the recycling of food waste into compost, swine feed, and fish feed in Hong Kong is low mainly due to the diminishing agricultural and fisheries industries. Hence, the current food waste recycling rate in Hong Kong is very low compared with these Asian countries. Renewable biogas generation from proposed organic waste treatment facilities As aforementioned, the local market of compost, pig farms, and fish ponds can only consume 340, 170, and 10 tpd of food waste, respectively. Hence, the total recycling rate of food waste for these three products is about 520 tpd. Based on the assumed food waste collection rate of 1800 tpd and the market demand of compost, fish feed, and swine feed in Hong Kong, there is still about 1280 tpd of collected food waste remained to be recycled. In this light, the feasibility of turning food waste into a source of renewable energy should be explored. Figure 3 summarizes the expected amount of food waste to be converted to compost, swine feed, fish feed, and biogas for renewable energy production in Hong Kong. According to the recently published government report entitled A Food Waste and Yard Waste Plan for Hong Kong 2014–2022, the HKSAR Government envisages that Hong Kong needs to build a network of Organic Waste Treatment Facilities (OWTF) in different locations in Hong Kong between 2014 and 2024. The OWTF aims to considerably reduce the amount of food waste and lessen the burden of landfills. The OWTF will adopt anaerobic digestion technology which stabilizes the food waste via biological decomposition of food waste in the absence of oxygen. Food waste is decomposed into less harmful inorganic products, producing biogas that can be recovered for energy generation. The proposed sites for the first phase and second phase of the OWTFs are located at Siu Ho Wan of North Lantau and Shaling at North District, respectively (HKEPD 2013). For phase 1 OWTF, it is expected that 200 tpd of food waste will be treated for the production of biogas. The biogas produced by the OWTF will be used as renewable energy for electricity generation. Meanwhile, phase 2 OWTF will have a treatment capacity of 300 tpd of food waste. However, it should be emphasized that the proposed phase 1 and phase 2 OWTFs
Environ Sci Pollut Res Table 1 Summary of value-added products (i.e., compost, swine feed, and fish feed) regarding existing or proposed plant, food waste conversion rate, expected consumption of food waste, and challenge faced in Hong Kong Type of Recycling plant value-added product
Plant Food waste to product operational conversion rate capacity (tpd)
Compost
Kowloon Bay Pilot Composting Plant
1.4
Swine feed
Green Environmental Kitchen Residue Recycle Limited Hong Kong Organic Waste Recycling Centre
10
Fish feed
12 to 15
Kowloon Biotechnology Limited 20 South China Reborn Resources 200 (Zhongshan) Company Limited
Expected Challenge faced consumption of food waste in Hong Kong (tpd)
10 tonnes of food waste 340 to produce 4 tonnes of compost 10 tonnes of food waste 170 to produce 1 tonne of dried swine feed
10 tonnes of food waste 10 to produce 2 tonnes of dried fish feed
are only able to recycle 500 tpd food waste. It is, therefore, estimated that three or four more OWTFs, each with a proposed capacity of 200 to 300 tpd, are required to be built in pursuance of accommodating the remaining 780 tpd food waste (i.e., remaining amount of collected food waste after fulfilling the demand of compost, swine feed, fish feed, phase 1 and 2 OWTFs in Hong Kong). The energy recovery system in the OWTF is an added credit to the food waste recycling as it enhances the use of renewable energy in the fuel mix for electricity generation in Hong Kong, thus reducing the environmental pollution and greenhouse gas emissions. Nevertheless, the HKSAR Government is obligated to collaborate with the power companies in Hong Kong (i.e., China Light and Power and Hong Kong
Fig. 3 Assumed food waste collection rate from domestic and C&I sectors and expected amount of food waste to be converted to compost, swine feed, fish feed, and biogas generated from Organic Waste Treatment Facilities (OWTF) in Hong Kong
Low demand of compost due to diminishing agricultural sector in Hong Kong. (1) Limited and declining of swine feed in Hong Kong. (2) Due to hygiene issue, the Hong Kong Agriculture, Fisheries and Conservation Department does not encourage the use of food waste as swine feed in Hong Kong. (1) Low demand of fish feed in Hong Kong. (2) Export of feed products to the other regions such as mainland China can be difficult, mainly due to the license and regulatory issues.
Electric) for employing the electricity generated from the OWTF. Negotiations with the power companies in transmitting the excess electricity to the grid are necessary, and there is a possibility that the power companies might not purchase the electricity generated from the OWTF. In addition, the electricity is generated via a combined heat and power (CHP) in the OWTF. The efficiency of electricity generation via CHP, however, is low while the heat generated is not used in Hong Kong. Besides turning the biogas into electricity, another alternative is to purify and upgrade the biogas to city gas and used as a cooking gas for gas cooker in domestic households. This utilization of biogas demonstrates the beneficial reuse of energy from waste in an environmentally friendly manner. Yet, the conversion of biogas to city gas faces the same caveat as like turning the biogas into electricity. The HKSAR Government also needs to deal with the Hong Kong and China Gas Company Limited (i.e., Towngas Company) for taking on the biogas. Similar to the power companies, there is a likelihood that the Towngas Company might not involve in the acquisition of biogas generated from the OWTF. While food waste can be sent to the proposed OWTF for further treatment, it is also proposed that co-digestion of food waste and sewage sludge in a sewage treatment work can be another approach to recycle food waste. Food waste codigestion with sewage sludge is a well-established and promising valorization technology that has been applied in many countries such as Sweden, Germany, Switzerland, and South Korea (Gallert et al. 2003; Kim et al. 2004; Murto et al. 2004; Appels et al. 2011). Figure 4 shows the process flow diagram of the co-digestion of food waste and sewage sludge for biogas generation. After separating food waste from other MSW
Environ Sci Pollut Res Fig. 4 Process flow diagram of the co-digestion of food waste and sewage sludge in sewage treatment works
in the refuse transfer station, a portion of the collected food waste can be delivered to the sewage treatment works in Hong Kong (e.g., Shatin Sewage Treatment Works and Yuen Long Sewage Treatment Works) for anaerobic co-digestion with sewage sludge. By co-digesting the food waste with the sewage sludge, it accelerates the growth of anaerobic microorganisms and hence improves the digestion performance (Iacovidou et al. 2012). The addition of food waste into sewage sludge enhances the biogas output of the co-digestion process, providing more energy for electricity generation (Sosnowski et al. 2008). This is attributed to the fact that food waste has three times the methane production potential than sewage sludge (USEPA 2013). The high electricity demand in sewage treatment works has always been an issue needed to be resolved as sewage treatment is an energy-intensive process. The current biogas production from sewage sludge treatment in Hong Kong is still not able to meet the overall on-site electricity demand in the sewage treatment work (DSD 2011; 2013). In view of the higher calorific value of the co-digestion process, it is, therefore, expected that more energy can be recovered to fulfill the on-site electricity demand. Taking the example from a co-digestion plant in YongYeon, South Korea, it is reported that 180 tpd food waste is codigested with sewage sludge and around 11 Mm3 biogas (61 % methane by volume) is produced annually (Ejlertsson and Magnusson 2013). With the sewage sludge to food waste mixing ratio of 10:3 (by weight) and hydraulic retention time of 20 days, it is projected that about 350 tpd food waste can be codigested with sewage sludge in Hong Kong (the current weight of sewage sludge in Shatin Sewage Treatment Works and Yuen Long Sewage Treatment Works is about 1160 tpd). Assuming that the lower heating value of methane is 10 kWh/m3 and the CHP efficiency is 60 %, it is estimated that the energy generation from the co-digestion process is around 78 GWh/year, which contributes to about 0.2 % of total electricity consumption in Hong Kong. However, it is worth noting that negotiations with power companies are still needed if there is excess electricity to be transmitted to the electricity grid. This is the hurdle of converting the biogas generated from the OWTF to electricity in Hong Kong.
Food waste as renewable biogas fuel for vehicle use More alternatives should be explored in order to utilize the biogas generated from the OWTF and sewage treatment works. In this light, it is recommended to further purify and upgrade the biogas to a quality suitable for use as a vehicle biogas fuel. With the advent of advanced biogas upgrading technologies (e.g., pressure swing adsorption, water scrubbing, and amine scrubbing), the methane (present at around 50–70 % by volume in raw biogas) and carbon dioxide (25– 45 % in raw biogas) can be separated effectively and efficiently, turning the biogas into vehicle biogas fuel. The upgraded biogas fuel typically has a methane content of around 98 % by volume (Patterson et al. 2011). Table 2 shows the lower heating value (LHV) of biogas fuel compared with other fuels. The upgraded biogas fuel is expected to have higher LHV than the synthetic natural gas (i.e., treated landfill gas) for Towngas and biogas from sewage sludge due to higher methane content in the biogas fuel, as well as conventional fuels such as natural gas, liquefied petroleum gas, and liquefied natural gas. From the environmental perspective, the use of biogas fuel for vehicle use reduces the reliance on foreign fossil fuel supplies, as well as significantly provides lower emissions of greenhouse gases, nitrogen oxides, and particles than petrol and diesel. Papacz (2011) reported that biogas-fuelled vehicles can reduce greenhouse gas emissions by between 75 and 200 % compared with fossil fuels. Table 3 summarizes the average exhaust emissions for passenger car fleet based on different types of fuels. The biogas fuel offers environmental benefits compared with petrol and diesel, as well as releases less CO2 emissions compared with compressed natural gas. In view of economic perspective, Murphya et al. (2004) stated that the use of biogas as a vehicle fuel has an economic advantage over its use in generating heat and electricity. The biogas has the potential to save 0.39 EUR/Nm3 (3.77 HKD/ Nm3) as a petrol substitute, 0.28 EUR/Nm3 (2.71 HKD/Nm3) as a diesel substitute, 0.19 EUR/Nm3 (1.84 HKD/Nm3) in a combined heat and power plant, and 0.143 EUR/Nm3 (1.38 HKD/Nm3) for electricity-only production.
Environ Sci Pollut Res Table 2
Energy content (lower heating value) of biogas fuel compared with other fuels
Type of fuel
Lower heating value (MJ/kg)
Lower heating value (kWh/kg)a
Biogas fuel from food waste for vehicle use (98 % methane by volume) Synthetic natural gas for Towngas (treated landfill gas, 80 % methane by volume) Biogas from sewage sludge (60 % methane by volume) Liquefied petroleum gas (propane) Liquefied petroleum gas (butane) Natural gas Liquefied natural gas
50.1 31.4 18.7 45.6c 45.3c 48.0d 44.2d
13.92b 8.71b 5.19b 12.67 12.58 13.33 12.28
a
1 MJ=0.2778 kWh
b
It is assumed that the lower heating value of methane is 10 kWh/m3 , while carbon dioxide has zero (Swedish Gas Centre 2012). The energy content of biogas is therefore directly related to the methane concentration. In other words, assuming a biogas composition with 80 % methane, the lower heating value is around 8.0 kWh/m3 . The densities of methane and carbon dioxide are 0.680 and 1.871 kg/m3 , respectively. Assuming that the main compositions of biogas comprise CH4 and CO2 only, the densities of biogas fuel from food waste, synthetic natural gas, and biogas from sewage sludge are 0.704, 0.918, and 1.156 kg/m3 , respectively
c
Adapted from Boisen and Lage (2009)
d
Adapted from Carbon Disclosure Project (2012)
The HKSAR Government can import biogas-fuelled cars which are readily manufactured by some major car manufacturers such as Ford, Mercedes-Benz, Volvo, Volkswagen, and etc. The biogas fuel can be firstly used by the government service cars in the government departments, as the HKSAR Government is not required to deal with any private companies for its implementation. For example, the HKSAR Government can introduce biogas refuse collection vehicles for the Food and Environmental Hygiene Department, biogas fire trucks for the Fire Services Department, biogas ambulance for the Department of Health, and biogas police car for the Hong Kong Police Force. The use of biogas fuel for vehicle use can be followed by the private cars used by the Hong Kong people. To encourage the use of biogas-fuelled cars among the Hong Kong people, the HKSAR Government can provide economic incentive and tax reductions to the Hong Kong people if biogas-fuelled cars are bought. Hafeez (2013) stated that the biogas generated from 1 kg of food waste equals to about 0.13 L petrol. This biogas allows a private car, with an engine size of 2501–3500 cm3 and an average consumption of 14 L/100 km (EMSD 2013), to drive Table 3
0.93 km/kg treated food waste. Considering the 1280 tpd food waste treated in the OWTF, it is estimated that an equivalent of 166,000 L petrol will be produced daily. Assuming the private car is travelled 100 km/day, the biogas fuel generated from food waste can be used to fuel approximately 12,000 private cars/day, which is about 2.6 % of total private cars (there are 464,595 licensed private cars as at the end of June 2013) in Hong Kong. Using the CO2 (wellto-wheel) emission factors of petrol and biogas fuel from Table 3, the application of biogas fuel in the 12, 000 private cars will reduce the CO2 emissions by about 96,000 tonne/year, which is equivalent to 1.3 % of the total emitted amount by the transport sector in Hong Kong in 2011. In addition, the switch from petrol to biogas fuel in the 12,000 private cars will reduce the CO, NOx, and SO2 emissions by 4,092 kg/year, 492 kg/year, and 1,200 kg/year, respectively. To further enhance the use of biogas fuel, the HKSAR Government can deal with the Hong Kong city buses (e.g., Kowloon Motor Bus) and taxis companies in favor of promoting the use of biogas fuel in public transports.
Average exhaust emissions for passenger car fleet
Type of fuel
CO (g/km)
CO2 (kg/km)
CO2, well-to-wheela (kg/km)
NOx (g/km)
Particular matter (g/km)
SO2 (g/km)
Petrol Compressed natural gas Biogas fuel
4.4 0.99 0.99b
0.23 0.09 0
0.27 0.13 0.05
0.45 0.04 0.04b
0.0047 0.0047 N.D.c
0.0014 0.0004 0.0004b
The table is adapted from Rydberg et al. (2010). The passenger car fleet is applied in urban area of Sweden. a
Well-to-wheel incorporates the feedstock or fuel production and processing, fuel delivery or energy transmission, and vehicle operation itself.
b
Since there is no data specifically for biogas fuel, the emissions of biogas fuel are assumed to be the data for compressed natural gas due to similar composition. c
N.D. stands for no data.
Environ Sci Pollut Res Fig. 5 Photos of (a) biogasfuelled car; (b) biogas-fuelled public bus; (c) biogas-fuelled car filling station; and (d) biogasfuelled bus filling station in the vicinity of Linköping city, Sweden
Regarding the use of biogas fuel generated from food waste, it has been widely practiced by some countries such as Sweden and France. Sweden represents the most advanced nation in terms of deployment of biogas upgrading technology with 32 biogas upgrading plants (Patterson et al. 2011). Figure 5 shows the photos of the biogas-fuelled cars and biogas-fuelled filling stations located within the vicinity of Linköping city. A biogas plant in Linköping has an annual treatment capacity of 100,000 tonnes food waste, produces 4.7 million m3 of upgraded biogas fuel that is used in 64 buses and a number of heavy and light duty vehicles (IEA Bioenergy 2014). The biogas in Linköping is distributed via well-developed distribution grids. The grids connect the refueling stations with the production plants and minimize road-bound transportation. The food waste produces biogas fuel to about 6 % of the vehicles use, including the fleet of city buses and taxis in Linköping. It is reported that each bus running on biogas fuel contributes to reducing CO2 emissions by 90 tonnes/year and NOx emissions by 1.2 tonnes/year (Francis and Bell 2008). Similarly, a biogas plant named as Organic Recovery Centre (ORC) has been established in Lille, France, to treat 100,000 tonnes/year of food waste using anaerobic digestion since 2007. This biogas plant produces upgraded biogas to refuel 100 urban buses and 10 waste collection trucks (BCEP 2014). To offer high quality of biogas for transport fuel use, national standards have been developed for grid injection of biogas or utilization as a vehicle biogas fuel in some European countries such as Sweden, Switzerland, Germany, and France (Patterson et al. 2011).
Conclusions This paper demonstrates a proposed food waste management framework for separating, collecting, and valorizing food waste in Hong Kong. The proposed framework considers the application of an optic bag system and valorization of food waste to various valuable materials and renewable energy, offering a more holistic approach in addressing the food waste issue in Hong Kong. In this paper, the benefits and ways of implementation of the optic bag system in Hong Kong have been explicitly discussed. It is hoped that the simplicity of the optic bag system can motivate the Hong Kong public to practice food waste separation at source. This paper also highlights the valorization of food waste to different value-added products, such as compost, swine feed, fish feed, and renewable biogas (i.e., electricity and city gas) in Hong Kong. In this paper, it is recommended to upgrade the biogas generated from the OWTF to biogas fuel for vehicle use. Based on this study, it is found that the valorization of food waste to biogas fuel for vehicle use can substantially reduce the emissions of greenhouse gas and air pollutants in Hong Kong. Through the implementation of the proposed framework of food waste collection and recycling for renewable biogas fuel production, the epitome of the waste-towealth concept for sustainable food waste management in Hong Kong can be hopefully accomplished.
Compliance with Ethical Standards The authors declare that they have no conflict of interest.
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BIOLOGICAL WASTE AS RESOURCE, WITH A FOCUS ON FOOD WASTE
Food waste collection and recycling for value-added products: potential applications and challenges in Hong Kong Irene M. C. Lo & Kok Sin Woon
Received: 22 December 2014 / Accepted: 13 February 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract About 3600 tonnes food waste are discarded in the landfills in Hong Kong daily. It is expected that the three strategic landfills in Hong Kong will be exhausted by 2020. In consideration of the food waste management environment and community needs in Hong Kong, as well as with reference to the food waste management systems in cities such as Linköping in Sweden and Oslo in Norway, a framework of food waste separation, collection, and recycling for food waste valorization is proposed in this paper. Food waste can be packed in an optic bag (i.e., a bag in green color), while the residual municipal solid waste (MSW) can be packed in a common plastic bag. All the wastes are then sent to the refuse transfer stations, in which food waste is separated from the residual MSW using an optic sensor. On the one hand, the sorted food waste can be converted into valuable materials (e.g., compost, swine feed, fish feed). On the other hand, the sorted food waste can be sent to the proposed Organic Waste Treatment Facilities and sewage treatment works for producing biogas. The biogas can be recovered to produce electricity and city gas (i.e., heating fuel for cooking purpose). Due to the challenges faced by the value-added products in Hong Kong, the biogas is recommended to be upgraded as a biogas fuel for vehicle use. Hopefully, the proposed framework will provide a simple and effective approach to food waste separation at source and promote sustainable use of waste to resource in Hong Kong. Keywords Anaerobic digestion . Food waste . Optical sorting . Separation and collection . Waste-to-energy . Waste valorization Responsible editor: Bingcai Pan I. M. C. Lo (*) : K. S. Woon Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China e-mail: [email protected]
Overview of food waste generation and disposal in Hong Kong Waste is a typical problem of affluent societies like Hong Kong. When the public can afford greater convenience and more purchases, they are inclined to discard more waste. Hong Kong is no exception to this (HKEPD 2014a). Hong Kong is a highly urbanized and populated city with limited space available for municipal solid waste (MSW) disposal. At present, Hong Kong relies mainly on landfills for MSW disposal. Approximately 9000 tonnes of MSW are still disposed of at the landfills in 2012 every day. It is expected that the current three strategic landfills in Hong Kong, namely South East New Territories, North East New Territories, and West New Territories, will reach their maximum capacities one by one by 2020 (HKEPD 2013). Among the 9000 tonnes of MSW discarded in the three strategic landfills in Hong Kong daily, about 3600 tonnes is food waste, accounting for 40 % of the waste disposed and is the largest constituent of MSW in Hong Kong. Of the 3600 tonnes of food waste generated daily, about 2500 tonnes (69 %) is contributed from domestic households, and around 1,100 tonnes (31 %) from food-related commercial and industrial (C&I) sources (HKEB 2013). The average per capita per daily disposal quantity of food waste in Hong Kong is about 0.4 kg/person/day −1, which is high when compared with some developed cities in Asia with similar economic status such as Taipei and Seoul (about 0.2 kg/ person/day −1). The food waste recycling rate in Hong Kong is very low, which was a mere 0.6 % of the total food waste disposal in 2012 (HKEPD 2014b). This is far below other urban areas such as South Korea and Taiwan, with recycling rates of 95 and 31 %, respectively (LegCo 2013; EPA 2014a, b).
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Systematic food waste collection and recycling for valorizing food waste to value-added product Albeit prevention and reduction of food waste are always the top priority to combat with food waste management issue in Hong Kong, there is still a substantial amount of food waste being disposed of at the three strategic landfills every day. The current practice of disposing food waste at landfills is not sustainable and is environmentally undesirable as it results in rapid depletion of limited landfill space, creates odor nuisance, and generates greenhouse gases and leachates that require further mitigation, thus imposing severe burdens on the environment. Besides, accelerating growth of human population, increased environmental issues, and growing global demand for energy and materials in our society have fostered intensive research efforts to valorize waste to resource in order to meet such global targets. In this context, food waste that cannot be avoided should be collected systematically and valorized to value-added products as far as possible. Various food waste management techniques have been practiced by the Hong Kong Special Administration Region (HKSAR) Government. The food waste management techniques emphasize prevention and reduction of food waste at source, donation of surplus food for human consumption, recycling of food waste, and treatment and disposal of MSW where food waste has not been separated, collected, and recycled (HKEB 2013). While the HKSAR Government has been adopting a multi-pronged approach to tackle the problem, more action is required and active participation from the community is also needed to alleviate the food waste problem. At present, much of the food waste recovery activities do not take place in individual households. Assuming that the food waste collection rates of both domestic household and C&I sectors in the future are 50 %, the total food waste collection rate in Hong Kong is about Fig. 1 Proposed framework of food waste collection and recycling for food waste valorization in Hong Kong
1800 tonnes per day (tpd). This amount constitutes a huge challenge to the society if collected food waste is not recycled in a cost-effective and environmentally friendly manner. A sustainable food waste management framework should combine a systematic food waste separation and collection system together with low environmental impact food waste valorization technologies. In tandem with this matter, a sustainable framework of food waste collection and recycling for various food waste valorizations is proposed. The proposed framework is developed with reference to the food waste management systems in some cities such as Linköping in Sweden, Oslo in Norway, Lille in France, and Ulsan in South Korea, as well as in consideration of the food waste management environment and community needs in Hong Kong. The overview of the proposed framework is shown in Fig. 1. The proposed framework comprises a food waste collection and separation at source using an optic bag system, various food waste valorization alternatives to value-added products, and other MSW treatment. Food waste separation and collection at source with optic bag system Various kinds of food waste separation and collection systems have been applied in other countries based on their community needs and environmental conditions. For example, in Taipei, Taiwan, the public is required to discard their food waste at designated times and locations. For areas and municipalities with a majority of single houses in Sweden, a multicompartment bins system is largely used. The bins are divided into two or four compartments to be used for apartment buildings or commercial and industrial areas (Al Seadi et al. 2013). Meanwhile, a MiniVac system has been designed by Envac Company, in which food waste produced from commercial kitchens such as canteens in offices and hospitals, restaurants,
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and industrial food factories, is thrown into conveniently located inlets within the kitchen and vacuumed through a pipe network to a refrigerated storage unit located in the basement or outside the building. These storage units are then emptied using a vacuum truck, and the food waste is transferred to respective food waste recycling facilities (Envac 2011). This system has been practiced in few cities such as London, Vancouver, Seoul, and Dubai. For the current practice in Hong Kong, nearly most of the food waste, especially food waste from domestic households, does not sort out from other types of MSW prior to disposing. The separation and collection of food waste is always an important aspect of any food waste recycling system. Effective food waste recycling requires the waste to be first separated from other types of MSW and then collected for delivery to food waste recycling facilities (HKEPD 2014a). Food waste that has been mixed with other types of MSW is contaminated and cannot be recycled. Consequently, the separation and collection of food waste is a crucial part of any food waste recycling system. A complicated sorting process in households, however, can hamper the residents from categorizing the waste generated by them. A less behavioral change of the public is required for the collection and separation of food waste. This is imperative to minimize any changes that might affect the lifestyle of the public and reduce potential impacts on the livelihoods of the public. In addition, the food waste collection system should be reasonably practicably constructed based upon the existing waste management system, including practices, infrastructure, and waste reduction and recovery schemes. By doing so, the public will not be required to travel to a new place or change their daily routine considerably for the sorting of food waste, and thus be more willing to take part in separating food waste at source. Due to this circumstance, a simple sorting process with less behavioral change of residents for sorting process is of utmost importance in order to encourage the residents to sort food waste out from other MSW at source. In the interest of achieving this objective, food waste and other types of MSW can be efficiently separated via an optic bag system. Food waste will be packed using an optic bag (i.e., green bag), while the residual MSW will be packed in a common plastic bag. All the packed wastes will still be discarded in the same designated collection bins or in existing rubbish chutes, eliminating the need for extra storage space for the fractions. All the wastes will then be collected in the conventional way (i.e., through refuse collection vehicles) and transported to the respective refuse transfer stations in Hong Kong. The refuse collection vehicles used for delivering the optic bags in Oslo, Norway, are having similar configurations (i.e., with rear compactor) like those in Hong Kong. The optic bag is not broken when it arrives at the optical sorting plant in Oslo. Besides, the common plastic bag used in Hong Kong is
usually not broken when it is transported to the refuse transfer stations. The optic bag is relatively thicker than the common plastic bag; hence, the optic bag will not be broken when it arrives at the refuse transfer stations. Upon arrival, all the waste bags will be dumped into a receiving pit and transferred to a conveyor belt. At this juncture, no separation of the bags has yet to be taken place. Once on the conveyor belt, the bags will be sorted automatically using camera technology applied with optic reader which recognizes the colors of the bag. When an optic bag with green color (i.e., food waste) is detected, a signal will be sent which pushes the green bag off the main conveyor belt to a second belt and then directed to its designated container. The collected food waste will then be separated from the optic bag via an Enviflex system. In the system, the food waste with optic bag is fed forward to a hydraulically driven roller that opens the optic bag. The food waste is then broken into pieces, and the optic bag is pulled in long strip. After that, the food waste and the optic bag will be screened for separation. The food waste will then be compacted and containerized in purposely built containers for onward transportation to assorted food waste recycling facilities for further treatment. Meanwhile, the other residual MSW such as plastics and textiles, which is packed in common plastic bags, will be delivered to a proposed advanced incineration facility for combustion, turning the waste into electricity. The optic bag system can be executed in line with the MSW charging scheme, which is going to be launched by the HKSAR Government. Based on the quantity-based system, the waste charge could be imposed through the mandatory use of pre-paid designated garbage bags. The optic bag can be assigned as one of the pre-paid designated garbage bags for packaging food waste (the cost of an optic bag is about 0.1 SEK/bag or 0.1 HKD/bag based on Sweden case), while the other pre-paid designated garbage bags can still be used for the disposal of other types of MSW. Hence, the introduction of optic bag system virtually makes the proposed MSW charging scheme unaffected. From the economic perspective, a standard optical sorting plant with an annual capacity of approximately 30,000 tonnes is built for about 30 SEK million or 30.5 HKD million based on a Sweden plant in 2011 (OptiBag 2014). The optical sorting plants have been operated in some European countries. Figure 2 illustrates the example of green bag for packing food waste and the optic sensor being used by the Tekniska verken company, a waste collection company in Linköping, Sweden. According to the performance of the collection and sorting plant operated by the Tekniska verken company, separation efficiency of food waste from other MSW can be achieved as high as 98 % (Tekniska verken 2012). The Haraldrud plant, which located in Oslo, Norway, also applies the optic bag sensor technology since 2009 and handles wastes approximately 150,000 tonnes/year (Envac 2014).
Environ Sci Pollut Res Fig. 2 Photos of (a) green bag (i.e., optic bag) for packing food waste; (b) optic sensor to separate food waste from other MSW; and (c) separated green bag waiting for transferring to food waste recycling center
The Haraldrud plant is currently the world’s largest sorting facility for separating food waste from other hold waste. Experience from this city suggests that tend to sort at source when it is easier for them to separated waste (Envac 2014).
optical housepeople handle
Valorization of food waste to compost, swine feed, and fish feed The valorization of biowaste (e.g., food waste, yard waste, animal manure, etc.) is an increasingly hot topic (Lin et al. 2012). For example, maize silage has been anaerobically fermented with cow manure on a commercial scale to obtain a carbon powder as solid biofuel (Maroušek et al. 2014a). Slag and fly ash generated from the combustion of wheat straw have been extracted to yield solubilized K2O and SiO2, which can be used in formulations for bioderived adhesive utilized in bioboards (Dodson 2011). Grass cuttings have been utilized to produce high-quality biochar using anaerobic fermentation followed by continuous pyrolysis processes (Maroušek et al. 2013, 2014b; Maroušek 2014). Besides, Viriya-empikul et al. (2012) reported that CaO from eggshells and molluse shells can be used to catalyze the production of fatty acid methyl esters from pre-esterified used cooking oils into biodiesel. After efficient separation and collection of food waste in Hong Kong, the collected food waste can be valorized as a source of valuable material or as a source of renewable energy. In this section, three types of valuable materials that can be valorized from food waste in Hong Kong are highlighted. The
three types of materials encompass compost, swine feed, and fish feed. Composting process has been practised by the HKSAR Government through the development of a Kowloon Bay Pilot Composting Plant (KBPCP) with a treatment capacity of 500 tonnes/year of food waste feedstock at the Kowloon Bay Waste Recycling Centre in mid-2008. The composts generated from the KBPCP could be used for landscaping and vegetable and fruit production. It, however, should be noted that the annual demand for compost in Hong Kong is low due to diminishing agricultural activities in Hong Kong. It is estimated that the demand for compost in Hong Kong is about 50, 000 tonnes/year or 137 tpd. If 1 tonne of food waste can produce 0.4 tonnes of compost, only 340 tpd of food waste is used to shelter the compost demand in Hong Kong every day. Subsequently, it is not feasible to valorize all collected food waste to compost in Hong Kong. Besides converting food waste into compost, food waste can also be valorized to swine feed. Currently, there are two companies in Hong Kong for converting food waste into swine feed. The two companies are Green Environmental Kitchen Residue Recycle Limited and Hong Kong Organic Waste Recycling Centre (HKOWRC). The Green Environmental Kitchen Residue Recycle Limited, situated at Ho Sheung Heung, Sheung Shui, treats about 10 tpd of food waste using drying process. Meanwhile, the HKOWRC, which is located at Kam Tsin Tsuen, Sheung Shui, collects food waste from both schools and hotels. The treatment capacity of the HKOWRC is about 12 to 15 tpd of food waste. Food waste
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collected from the schools has lower nutritional value and is sold to pig farms for direct feeding. Meanwhile, food waste collected from the hotels has higher nutritional value and is used to produce dried swine feed. For the production of swine feed from food waste with higher nutritional value, the operational processes consist of grinding, 48-h fermentation using effective microorganisms (EM), followed by drying. For these two companies, it is estimated that 10 tonnes of food waste can produce 1 tonne of dried swine feed. The swine feed is sold at 2500 HKD/tonne to two local pig farms. Nevertheless, there is a conundrum in turning food waste to swine feed in Hong Kong, primarily due to hygiene issue. The Hong Kong Agriculture, Fisheries and Conservation Department does not encourage the use of food waste as swine feed in Hong Kong. This is mainly attributed to the fact that the UK Government has banned its use in 2001 (DEFRA 2012), followed by the European Union in 2003 (EC 2002), in order to avoid diseases transmission such as bovine spongiform encephalopathy (i.e., mad cow disease) and hand, mouth, and foot disease. Besides, the market demand of swine feed in Hong Kong is limited. Currently, there are 43 pig farms in Hong Kong, in which it can only consume about 170 tpd of food waste. In Hong Kong, a plant named Kowloon Biotechnology Limited currently collects 20 tpd of food waste to produce fish feed using drying process. The fish feed is sold to four to five fish pond owners in Hong Kong and made cheaper than the imported fishmeal from mainland China. Meanwhile, another company named South China Reborn Resources (Zhongshan) Company Limited is constructing a food waste processing plant with a treatment capacity of 200 tpd of food waste in Ecopark in order to produce high protein content feed for aquaculture using fermentation and drying processes. The plant is expected to be commissioned in 2015, and the feed product will be exported to the USA, European Union, Japan, Taiwan, and mainland China.1 The valorization of food waste to fish feed in Hong Kong is a promising alternative. There are, however, some challenges for valorizing food waste to fish feed in Hong Kong. On the one hand, the fish feed demand in Hong Kong is not high. It is estimated that the fish feed demand from food waste in Hong Kong is about 10 tpd of food waste. On the other hand, export of fish feed products to the other regions such as mainland China can be difficult, mainly due to the license and regulatory issues. A feed factory license from the HKSAR Government is required to apply for feed import license from the Ministry of Agriculture of the Chinese Central Government (AQSIQ 2014). In addition, the regulation has become stricter as the Quality and Technology Supervision Bureau of the Chinese Central Government 1 Based on a local newspaper, the plant owner of the Kowloon Biotechnology Limited will work with the South China Reborn Resources (Zhongshan) Company Limited once the food waste processing plant in Ecopark is operated (Mingpao 2012).
requires the HKSAR Government to register with the Chinese Central Government before importing any feed products to mainland China. This requires communication between both governments, and thus hinders feed trade between cities. Table 1 provides the summary of food waste recycling facilities, food waste conversion rate, expected consumption of food waste, and challenges for compost, swine feed, and fish feed. In Asian countries such as South Korea, Taiwan, and Japan, more than 40 % of the food waste is recycled into compost, swine feed, and fish feed to partly substitute for chemical fertilizers and conventional feed ingredients. Nevertheless, the recycling of food waste into compost, swine feed, and fish feed in Hong Kong is low mainly due to the diminishing agricultural and fisheries industries. Hence, the current food waste recycling rate in Hong Kong is very low compared with these Asian countries. Renewable biogas generation from proposed organic waste treatment facilities As aforementioned, the local market of compost, pig farms, and fish ponds can only consume 340, 170, and 10 tpd of food waste, respectively. Hence, the total recycling rate of food waste for these three products is about 520 tpd. Based on the assumed food waste collection rate of 1800 tpd and the market demand of compost, fish feed, and swine feed in Hong Kong, there is still about 1280 tpd of collected food waste remained to be recycled. In this light, the feasibility of turning food waste into a source of renewable energy should be explored. Figure 3 summarizes the expected amount of food waste to be converted to compost, swine feed, fish feed, and biogas for renewable energy production in Hong Kong. According to the recently published government report entitled A Food Waste and Yard Waste Plan for Hong Kong 2014–2022, the HKSAR Government envisages that Hong Kong needs to build a network of Organic Waste Treatment Facilities (OWTF) in different locations in Hong Kong between 2014 and 2024. The OWTF aims to considerably reduce the amount of food waste and lessen the burden of landfills. The OWTF will adopt anaerobic digestion technology which stabilizes the food waste via biological decomposition of food waste in the absence of oxygen. Food waste is decomposed into less harmful inorganic products, producing biogas that can be recovered for energy generation. The proposed sites for the first phase and second phase of the OWTFs are located at Siu Ho Wan of North Lantau and Shaling at North District, respectively (HKEPD 2013). For phase 1 OWTF, it is expected that 200 tpd of food waste will be treated for the production of biogas. The biogas produced by the OWTF will be used as renewable energy for electricity generation. Meanwhile, phase 2 OWTF will have a treatment capacity of 300 tpd of food waste. However, it should be emphasized that the proposed phase 1 and phase 2 OWTFs
Environ Sci Pollut Res Table 1 Summary of value-added products (i.e., compost, swine feed, and fish feed) regarding existing or proposed plant, food waste conversion rate, expected consumption of food waste, and challenge faced in Hong Kong Type of Recycling plant value-added product
Plant Food waste to product operational conversion rate capacity (tpd)
Compost
Kowloon Bay Pilot Composting Plant
1.4
Swine feed
Green Environmental Kitchen Residue Recycle Limited Hong Kong Organic Waste Recycling Centre
10
Fish feed
12 to 15
Kowloon Biotechnology Limited 20 South China Reborn Resources 200 (Zhongshan) Company Limited
Expected Challenge faced consumption of food waste in Hong Kong (tpd)
10 tonnes of food waste 340 to produce 4 tonnes of compost 10 tonnes of food waste 170 to produce 1 tonne of dried swine feed
10 tonnes of food waste 10 to produce 2 tonnes of dried fish feed
are only able to recycle 500 tpd food waste. It is, therefore, estimated that three or four more OWTFs, each with a proposed capacity of 200 to 300 tpd, are required to be built in pursuance of accommodating the remaining 780 tpd food waste (i.e., remaining amount of collected food waste after fulfilling the demand of compost, swine feed, fish feed, phase 1 and 2 OWTFs in Hong Kong). The energy recovery system in the OWTF is an added credit to the food waste recycling as it enhances the use of renewable energy in the fuel mix for electricity generation in Hong Kong, thus reducing the environmental pollution and greenhouse gas emissions. Nevertheless, the HKSAR Government is obligated to collaborate with the power companies in Hong Kong (i.e., China Light and Power and Hong Kong
Fig. 3 Assumed food waste collection rate from domestic and C&I sectors and expected amount of food waste to be converted to compost, swine feed, fish feed, and biogas generated from Organic Waste Treatment Facilities (OWTF) in Hong Kong
Low demand of compost due to diminishing agricultural sector in Hong Kong. (1) Limited and declining of swine feed in Hong Kong. (2) Due to hygiene issue, the Hong Kong Agriculture, Fisheries and Conservation Department does not encourage the use of food waste as swine feed in Hong Kong. (1) Low demand of fish feed in Hong Kong. (2) Export of feed products to the other regions such as mainland China can be difficult, mainly due to the license and regulatory issues.
Electric) for employing the electricity generated from the OWTF. Negotiations with the power companies in transmitting the excess electricity to the grid are necessary, and there is a possibility that the power companies might not purchase the electricity generated from the OWTF. In addition, the electricity is generated via a combined heat and power (CHP) in the OWTF. The efficiency of electricity generation via CHP, however, is low while the heat generated is not used in Hong Kong. Besides turning the biogas into electricity, another alternative is to purify and upgrade the biogas to city gas and used as a cooking gas for gas cooker in domestic households. This utilization of biogas demonstrates the beneficial reuse of energy from waste in an environmentally friendly manner. Yet, the conversion of biogas to city gas faces the same caveat as like turning the biogas into electricity. The HKSAR Government also needs to deal with the Hong Kong and China Gas Company Limited (i.e., Towngas Company) for taking on the biogas. Similar to the power companies, there is a likelihood that the Towngas Company might not involve in the acquisition of biogas generated from the OWTF. While food waste can be sent to the proposed OWTF for further treatment, it is also proposed that co-digestion of food waste and sewage sludge in a sewage treatment work can be another approach to recycle food waste. Food waste codigestion with sewage sludge is a well-established and promising valorization technology that has been applied in many countries such as Sweden, Germany, Switzerland, and South Korea (Gallert et al. 2003; Kim et al. 2004; Murto et al. 2004; Appels et al. 2011). Figure 4 shows the process flow diagram of the co-digestion of food waste and sewage sludge for biogas generation. After separating food waste from other MSW
Environ Sci Pollut Res Fig. 4 Process flow diagram of the co-digestion of food waste and sewage sludge in sewage treatment works
in the refuse transfer station, a portion of the collected food waste can be delivered to the sewage treatment works in Hong Kong (e.g., Shatin Sewage Treatment Works and Yuen Long Sewage Treatment Works) for anaerobic co-digestion with sewage sludge. By co-digesting the food waste with the sewage sludge, it accelerates the growth of anaerobic microorganisms and hence improves the digestion performance (Iacovidou et al. 2012). The addition of food waste into sewage sludge enhances the biogas output of the co-digestion process, providing more energy for electricity generation (Sosnowski et al. 2008). This is attributed to the fact that food waste has three times the methane production potential than sewage sludge (USEPA 2013). The high electricity demand in sewage treatment works has always been an issue needed to be resolved as sewage treatment is an energy-intensive process. The current biogas production from sewage sludge treatment in Hong Kong is still not able to meet the overall on-site electricity demand in the sewage treatment work (DSD 2011; 2013). In view of the higher calorific value of the co-digestion process, it is, therefore, expected that more energy can be recovered to fulfill the on-site electricity demand. Taking the example from a co-digestion plant in YongYeon, South Korea, it is reported that 180 tpd food waste is codigested with sewage sludge and around 11 Mm3 biogas (61 % methane by volume) is produced annually (Ejlertsson and Magnusson 2013). With the sewage sludge to food waste mixing ratio of 10:3 (by weight) and hydraulic retention time of 20 days, it is projected that about 350 tpd food waste can be codigested with sewage sludge in Hong Kong (the current weight of sewage sludge in Shatin Sewage Treatment Works and Yuen Long Sewage Treatment Works is about 1160 tpd). Assuming that the lower heating value of methane is 10 kWh/m3 and the CHP efficiency is 60 %, it is estimated that the energy generation from the co-digestion process is around 78 GWh/year, which contributes to about 0.2 % of total electricity consumption in Hong Kong. However, it is worth noting that negotiations with power companies are still needed if there is excess electricity to be transmitted to the electricity grid. This is the hurdle of converting the biogas generated from the OWTF to electricity in Hong Kong.
Food waste as renewable biogas fuel for vehicle use More alternatives should be explored in order to utilize the biogas generated from the OWTF and sewage treatment works. In this light, it is recommended to further purify and upgrade the biogas to a quality suitable for use as a vehicle biogas fuel. With the advent of advanced biogas upgrading technologies (e.g., pressure swing adsorption, water scrubbing, and amine scrubbing), the methane (present at around 50–70 % by volume in raw biogas) and carbon dioxide (25– 45 % in raw biogas) can be separated effectively and efficiently, turning the biogas into vehicle biogas fuel. The upgraded biogas fuel typically has a methane content of around 98 % by volume (Patterson et al. 2011). Table 2 shows the lower heating value (LHV) of biogas fuel compared with other fuels. The upgraded biogas fuel is expected to have higher LHV than the synthetic natural gas (i.e., treated landfill gas) for Towngas and biogas from sewage sludge due to higher methane content in the biogas fuel, as well as conventional fuels such as natural gas, liquefied petroleum gas, and liquefied natural gas. From the environmental perspective, the use of biogas fuel for vehicle use reduces the reliance on foreign fossil fuel supplies, as well as significantly provides lower emissions of greenhouse gases, nitrogen oxides, and particles than petrol and diesel. Papacz (2011) reported that biogas-fuelled vehicles can reduce greenhouse gas emissions by between 75 and 200 % compared with fossil fuels. Table 3 summarizes the average exhaust emissions for passenger car fleet based on different types of fuels. The biogas fuel offers environmental benefits compared with petrol and diesel, as well as releases less CO2 emissions compared with compressed natural gas. In view of economic perspective, Murphya et al. (2004) stated that the use of biogas as a vehicle fuel has an economic advantage over its use in generating heat and electricity. The biogas has the potential to save 0.39 EUR/Nm3 (3.77 HKD/ Nm3) as a petrol substitute, 0.28 EUR/Nm3 (2.71 HKD/Nm3) as a diesel substitute, 0.19 EUR/Nm3 (1.84 HKD/Nm3) in a combined heat and power plant, and 0.143 EUR/Nm3 (1.38 HKD/Nm3) for electricity-only production.
Environ Sci Pollut Res Table 2
Energy content (lower heating value) of biogas fuel compared with other fuels
Type of fuel
Lower heating value (MJ/kg)
Lower heating value (kWh/kg)a
Biogas fuel from food waste for vehicle use (98 % methane by volume) Synthetic natural gas for Towngas (treated landfill gas, 80 % methane by volume) Biogas from sewage sludge (60 % methane by volume) Liquefied petroleum gas (propane) Liquefied petroleum gas (butane) Natural gas Liquefied natural gas
50.1 31.4 18.7 45.6c 45.3c 48.0d 44.2d
13.92b 8.71b 5.19b 12.67 12.58 13.33 12.28
a
1 MJ=0.2778 kWh
b
It is assumed that the lower heating value of methane is 10 kWh/m3 , while carbon dioxide has zero (Swedish Gas Centre 2012). The energy content of biogas is therefore directly related to the methane concentration. In other words, assuming a biogas composition with 80 % methane, the lower heating value is around 8.0 kWh/m3 . The densities of methane and carbon dioxide are 0.680 and 1.871 kg/m3 , respectively. Assuming that the main compositions of biogas comprise CH4 and CO2 only, the densities of biogas fuel from food waste, synthetic natural gas, and biogas from sewage sludge are 0.704, 0.918, and 1.156 kg/m3 , respectively
c
Adapted from Boisen and Lage (2009)
d
Adapted from Carbon Disclosure Project (2012)
The HKSAR Government can import biogas-fuelled cars which are readily manufactured by some major car manufacturers such as Ford, Mercedes-Benz, Volvo, Volkswagen, and etc. The biogas fuel can be firstly used by the government service cars in the government departments, as the HKSAR Government is not required to deal with any private companies for its implementation. For example, the HKSAR Government can introduce biogas refuse collection vehicles for the Food and Environmental Hygiene Department, biogas fire trucks for the Fire Services Department, biogas ambulance for the Department of Health, and biogas police car for the Hong Kong Police Force. The use of biogas fuel for vehicle use can be followed by the private cars used by the Hong Kong people. To encourage the use of biogas-fuelled cars among the Hong Kong people, the HKSAR Government can provide economic incentive and tax reductions to the Hong Kong people if biogas-fuelled cars are bought. Hafeez (2013) stated that the biogas generated from 1 kg of food waste equals to about 0.13 L petrol. This biogas allows a private car, with an engine size of 2501–3500 cm3 and an average consumption of 14 L/100 km (EMSD 2013), to drive Table 3
0.93 km/kg treated food waste. Considering the 1280 tpd food waste treated in the OWTF, it is estimated that an equivalent of 166,000 L petrol will be produced daily. Assuming the private car is travelled 100 km/day, the biogas fuel generated from food waste can be used to fuel approximately 12,000 private cars/day, which is about 2.6 % of total private cars (there are 464,595 licensed private cars as at the end of June 2013) in Hong Kong. Using the CO2 (wellto-wheel) emission factors of petrol and biogas fuel from Table 3, the application of biogas fuel in the 12, 000 private cars will reduce the CO2 emissions by about 96,000 tonne/year, which is equivalent to 1.3 % of the total emitted amount by the transport sector in Hong Kong in 2011. In addition, the switch from petrol to biogas fuel in the 12,000 private cars will reduce the CO, NOx, and SO2 emissions by 4,092 kg/year, 492 kg/year, and 1,200 kg/year, respectively. To further enhance the use of biogas fuel, the HKSAR Government can deal with the Hong Kong city buses (e.g., Kowloon Motor Bus) and taxis companies in favor of promoting the use of biogas fuel in public transports.
Average exhaust emissions for passenger car fleet
Type of fuel
CO (g/km)
CO2 (kg/km)
CO2, well-to-wheela (kg/km)
NOx (g/km)
Particular matter (g/km)
SO2 (g/km)
Petrol Compressed natural gas Biogas fuel
4.4 0.99 0.99b
0.23 0.09 0
0.27 0.13 0.05
0.45 0.04 0.04b
0.0047 0.0047 N.D.c
0.0014 0.0004 0.0004b
The table is adapted from Rydberg et al. (2010). The passenger car fleet is applied in urban area of Sweden. a
Well-to-wheel incorporates the feedstock or fuel production and processing, fuel delivery or energy transmission, and vehicle operation itself.
b
Since there is no data specifically for biogas fuel, the emissions of biogas fuel are assumed to be the data for compressed natural gas due to similar composition. c
N.D. stands for no data.
Environ Sci Pollut Res Fig. 5 Photos of (a) biogasfuelled car; (b) biogas-fuelled public bus; (c) biogas-fuelled car filling station; and (d) biogasfuelled bus filling station in the vicinity of Linköping city, Sweden
Regarding the use of biogas fuel generated from food waste, it has been widely practiced by some countries such as Sweden and France. Sweden represents the most advanced nation in terms of deployment of biogas upgrading technology with 32 biogas upgrading plants (Patterson et al. 2011). Figure 5 shows the photos of the biogas-fuelled cars and biogas-fuelled filling stations located within the vicinity of Linköping city. A biogas plant in Linköping has an annual treatment capacity of 100,000 tonnes food waste, produces 4.7 million m3 of upgraded biogas fuel that is used in 64 buses and a number of heavy and light duty vehicles (IEA Bioenergy 2014). The biogas in Linköping is distributed via well-developed distribution grids. The grids connect the refueling stations with the production plants and minimize road-bound transportation. The food waste produces biogas fuel to about 6 % of the vehicles use, including the fleet of city buses and taxis in Linköping. It is reported that each bus running on biogas fuel contributes to reducing CO2 emissions by 90 tonnes/year and NOx emissions by 1.2 tonnes/year (Francis and Bell 2008). Similarly, a biogas plant named as Organic Recovery Centre (ORC) has been established in Lille, France, to treat 100,000 tonnes/year of food waste using anaerobic digestion since 2007. This biogas plant produces upgraded biogas to refuel 100 urban buses and 10 waste collection trucks (BCEP 2014). To offer high quality of biogas for transport fuel use, national standards have been developed for grid injection of biogas or utilization as a vehicle biogas fuel in some European countries such as Sweden, Switzerland, Germany, and France (Patterson et al. 2011).
Conclusions This paper demonstrates a proposed food waste management framework for separating, collecting, and valorizing food waste in Hong Kong. The proposed framework considers the application of an optic bag system and valorization of food waste to various valuable materials and renewable energy, offering a more holistic approach in addressing the food waste issue in Hong Kong. In this paper, the benefits and ways of implementation of the optic bag system in Hong Kong have been explicitly discussed. It is hoped that the simplicity of the optic bag system can motivate the Hong Kong public to practice food waste separation at source. This paper also highlights the valorization of food waste to different value-added products, such as compost, swine feed, fish feed, and renewable biogas (i.e., electricity and city gas) in Hong Kong. In this paper, it is recommended to upgrade the biogas generated from the OWTF to biogas fuel for vehicle use. Based on this study, it is found that the valorization of food waste to biogas fuel for vehicle use can substantially reduce the emissions of greenhouse gas and air pollutants in Hong Kong. Through the implementation of the proposed framework of food waste collection and recycling for renewable biogas fuel production, the epitome of the waste-towealth concept for sustainable food waste management in Hong Kong can be hopefully accomplished.
Compliance with Ethical Standards The authors declare that they have no conflict of interest.
Environ Sci Pollut Res
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