Introduction

Approximately 1.3 billion t/a of food produced for human consumption is discarded or wasted worldwide [1], and the EU wastes approximately 8800 t/a of food [2]. Because South Korea has a diverse food culture, food waste accounts for approximately 24.7% of all waste [3]; it is generated in large volumes in homes, restaurants, and food service establishments. Due to such a food culture, food wastes are characterized by high water and salt content [4]. Moreover, food waste leachates (FWLs) generated as wastewater are separated from food waste through gravity and mechanical dehydration during transportation, treatment, and recycling processes. FWLs emit a bad odor and have high organic matter content, which requires a separate treatment [5].

Traditional treatment methods for food waste include landfilling, feeding, and composting. However, feed production from food waste has a negative effect on pigs. In the UK, the illegal supply of food waste to pigs in farms caused foot-and-mouth disease [6]. African swine fever (ASF) is a viral bleeding disease associated with a high fatality rate in pigs [7], and the ASF virus can survive for up to 1000 days in pork products and frozen meat [8]. Furthermore, China prohibited feeding food waste to pigs after discovering that food waste feeds were associated with ASF [9]. The EU and several other countries prohibit feeding food waste to pigs because of infectious diseases [10]. Additionally, in South Korea, wet feed produced from food waste was stopped from being distributed to pig farms in 2019 when ASF viral infections of unknown routes were confirmed in pig farms [11].

Regarding the management of food waste in South Korea, direct landfilling at landfills has been prohibited since 2005 [12], and dum** of organic wastes (food waste, livestock manure, and sewage sludge) in the ocean was prohibited by the London Convention in 2013 [13]. Consequently, as a policy to produce resources from food waste, South Korea has heavily invested in resource-recycling facilities and develo** processes such as feed production and composting. As a result, South Korea has successfully established a resource circulation infrastructure and the recycling rate of food waste has reached > 90% [14].

Policies regarding resource production from food waste are classified mainly into feed production, composting, and energy production. Feed production has a lower effect on the environment than anaerobic digestion or composting processes because raw food wastes are recycled with minimal energy input [15]. Composting results in compost production through controlled aerobic and biologic degradation of biodegradable materials [16] and garners attention in several countries as an eco-friendly and affordable alternative treatment for food waste [17]. Energy production through anaerobic digestion in biogas facilities is useful as a treatment method for various organic wastes including food wastes [18]. European countries, such as Germany and Switzerland, are pioneers of large-scale biogas plant industry [19]. Food waste incurs economic, environmental, and social costs in addition to 3.5 Gt CO2-e of greenhouse gas emitted each; however, biogas facilities have advantages, such as reducing greenhouse gas emissions and highly efficient fuel production [20].

Recently, in South Korea, food waste recycling through feed production has shown a decreasing trend because livestock farms do not favor food waste feeds, and the demand has diminished because of epidemics such as ASF [21]. Furthermore, compost recycling is decreasing as the government supports using organic fertilizers based on livestock manure and oil meal [22] and because of the negative perception of food waste composts. Waste neglect and unauthorized dum** have increased because of poor waste disposal.

The purpose of this study was to establish a treatment methodology for Food Waste Leachates (FWLs), which have seen a significant increase due to changes in the food waste treatment landscape in South Korea following the outbreak of African Swine Fever (ASF). Our approach involved compiling comprehensive FWL statistics, understanding the various pathways of FWL generation, and formulating short- and long-term treatment strategies that take into consideration the characteristics of FWLs and the current status of treatment facilities.

Materials and methods

The statistical collection method of food waste and FWLs treatment status after the ASF outbreak

Since the total amount of food waste leachate, for which there are no national statistics, was collected with the three treatment methods (dry-feed production, composting, and intermediate processing) of food waste, all treatment facilities were surveyed. In particular, the solid phase separated and discharged from intermediate processing of food waste was separately counted as the amount re-inputted into dry-feed production and composting. The amount of food waste leachate treated was calculated as the amount input to the biogasification facility and the amount input to other treatments of incineration facilities and sewage treatment facilities. The number of unconfirmed treatment methods in the national food waste leachate generation was counted as a ‘shortage of food waste leachate treatment’.

The method of aggregating the treatment amount of food waste and food waste leachate was cross-checked through the following two methods. For the primary survey method, information on treatment facilities was collected from the national statistical data “National Waste Generation and Treatment Status” and the waste treatment facility registration system “Allbaro System,” and a survey was conducted on statistics such as the amount of food waste and food wastewater generated and treated, and the amount of solid and liquid discharged in the case of intermediate processing at 316 treatment facilities nationwide. The survey on treatment facilities was conducted between December 10, 2019, and January 10, 2020, and the amount of food waste and food wastewater generated and treated was investigated by classifying them into before and after the outbreak of African swine fever (ASF).

The amount of food waste collected by each treatment method was separately counted through national statistics and verification of “National Waste Generation and Treatment Status” and differences that occurred. In addition, the number of treatment facilities collected in the survey was compared and verified with the statistics of the Allbaro system, and the statistics data of treatment facilities not collected in the survey and the treatment amount of incineration treatment facilities and sewage treatment facilities were also used by the Allbaro system. The ‘Allbaro System’ systematizes the transparent waste treatment process by cross-registering all types of waste from generation to transportation and treatment through the Internet.

As for the 2nd survey method, in the 1st survey, a telephone survey was conducted for insufficient statistics on treatment facilities, and a field survey was conducted for facilities that were still insufficient or uncertain. Telephone surveys on treatment facilities were conducted between January 1, 2020, and February 28, 2020, and on-site inspections were conducted on 30 facilities between February 1, 2020, and March 20, 2020.

Target facilities and sampling method of food waste leachate

Since the sampling method of food waste leachate occurs during food waste treatment, liquid samples generated in treatment facilities such as dry feed production, composting, and intermediate processing and liquid samples input to biogasification treatment facilities, which are most treatment methods, were collected. Target facilities were 15 out of the total facilities (33) for dry feed production, 17 out of the total facilities (85) for composting, 21 out of the total facilities (79) for intermediate processing, and 31 out of the total facilities (68) for biogasification. Samples were refrigerated immediately after collection from facilities.

Analysis methods of physicochemical properties of food waste leachate

All 84 samples were tested twice with nine different analysis items, including total solid (TS), volatile solid (VS), fixed solid (FS), CODcr, total nitron (TN), total phosphorus (TP), volatile fatty acids (VFAs), and so on. TS, VS, and FS were tested using the Korean Waste Standard Examination’s ‘Loss on ignition/Volatile solids and total organics-gravimetry (ES 06301.1b)’ and ‘Humidity and total solid-gravimetry (ES 06303.1)’ [23]. CODcr and SCODcr were determined using the closed reflux titration method (5220 C) [24]. TN and ammonium nitrogen (NH3-N) were analyzed using the oxidation method and UV/visible spectrometry of the Official Testing Method for water pollution [25, 26], and TP was tested using the UV/visible spectrometry and ascorbic acid method of the Official Testing Method for water pollution [27, 28]. Titration methods were used to analyze VFAs, one of the hindrance factors and observation index in the anaerobic digestion system. The Standard method 5560 D gas chromatographic procedure 4.a. was used to determine the amount of VFAs in the sample that was extracted with diethyl ether [24]. A 25 m long, 0.32 mm inner diameter, and 0.5 m thick film FID and DB-FFAP column was installed in the gas chromatograph (Aglient 6890, USA).

The statistical collection method of Food Waste and FWLs treatment status after ASF outbreak

Since the total amount of food waste leachate, for which there are no national statistics, was collected with the three treatment methods (dry-feed production, composting, and intermediate processing) of food waste, all treatment facilities were surveyed. In particular, the solid phase separated and discharged from intermediate processing of food waste was separately counted as the amount re-inputted into dry-feed production and composting. The amount of food waste leachate treated was counted as the amount input to the biogasification facility and the amount input to other treatments of incineration facilities and sewage treatment facilities. The amount of unconfirmed treatment method in the national food waste leachate generation was counted as ‘shortage of food waste leachate treatment’.

The method of aggregating the treatment amount of food waste and food waste leachate was cross-checked through the following two methods. For the primary survey method, information on treatment facilities was collected from the national statistical data “National Waste Generation and Treatment Status” and the waste treatment facility registration system “Allbaro System,” and a survey was conducted on statistics such as the amount of food waste and food wastewater generated and treated, and the amount of solid and liquid discharged in the case of intermediate processing at 316 treatment facilities nationwide. The survey on treatment facilities was conducted between December 10, 2019 and January 10, 2020, and the amount of food waste and food wastewater generated and treated was investigated by classifying them into before and after the outbreak of African swine fever (ASF).

The amount of food waste collected by each treatment method was separately counted through national statistics and verification of “National Waste Generation and Treatment Status” and differences that occurred. In addition, the amount of treatment facilities collected in the survey were compared and verified with the statistics of the Allbaro system, and the statistics data of treatment facilities not collected in the survey and the treatment amount of incineration treatment facilities and sewage treatment facilities were also used by the Allbaro system. The ‘Allbaro System’ systematizes the transparent waste treatment process by cross-registering all types of waste from generation to transportation and treatment through the Internet.

As for the 2nd survey method, in the 1st survey, a telephone survey was conducted for insufficient statistics on treatment facilities, and a field survey was conducted for facilities that were still insufficient or uncertain. Telephone surveys on treatment facilities were conducted between January 1, 2020, and February 28, 2020, and on-site inspections were conducted on 30 facilities between February 1, 2020, and March 20, 2020.

Results and discussion

Characteristics and current status of food waste before the ASF outbreak

The water content in food waste in South Korea varies between 78% and 83% according to the season, as shown in Table 1. As the water content of food waste tends to increase slightly due to crop (fruit) characteristics and weather, NaCl concentration decreases from an average of 1.10–0.68% in this season compared with other seasons [29]. This corresponds to a range of approximately 0.5–2.0% (based on wet matter), which is more than 100 times higher than other organic wastes (e.g., sludge and livestock manure). Because of such characteristics, odor emitted from leachates in the discharge, collection, and transportation processes is a representative pollution source in the surrounding environment.

Table 1 Physicochemical properties of Korean food waste by emission source and season
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The physical properties of food waste show slight differences by season and discharge sites but are mostly consistent. The physical properties are as follows, on average: 72.7% water (~ 68.6–77.5%), 23.4% combustible powder (~ 20.2–28.5%), and 3.9% ash (~ 1.2–7.1%). Regarding the ratios of components, vegetables, and grains comprised 32.9% and 25.0%, respectively, accounting for more than half of all food waste, and the ratio of FWL generated in the collection process was 5.6% [30].

FWLs are generated continuously during the discharge, collection, transportation, and treatment of food waste. Food waste with high water content results in the production of FWLs via gravity. In general, the amount of FWL generated is approximately 60% of the mass of food waste through mechanical dehydration, such as using a screw method. Total solids (TS) and volatile solids (VS) comprise approximately 10% and 8.5% of FWL, respectively. Thus, approximately 85% of FWLs are combustible components. Furthermore, the extremely high concentration of 145,474 mg/L CODcr and 3452 mg/L T-N burdens the sewage treatment plant, which makes it difficult to comply with the water quality standards for Korea’s final effluents (COD < 40 mg/L, T-N < 20 mg/L) [31].

According to the statistics in Tables 2 and 96.2% of food waste was recycled in 2019, and the remaining food from abroad and foods imported without permission were incinerated. In South Korea, direct landfilling of food waste has been prohibited since 2005, marine dum** has been prohibited by the London Convention since 2013 and land treatment is required. Consequently, material recycling methods, such as feed production and composting, are being used; thus, South Korea maintains high annual recycling rates.

Table 2 Treatment status of food waste (2019) (Unit : t/d)
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As reported statistics do not show the amount of waste treated by recycling methods, the facilities that reported the treatment of food waste were compared and classified through field and phone surveys and using the electronic handover system (Albaro). Recycling was classified into seven methods: wet feed production, dry feed production, intermediate processing, composting, exclusive biogasification (with food only), biogas (total organic wastes), and others. Intermediate processing involves screening and discharging foreign matter and separating solids and liquids. Although it is not a recycling method, it was included because it was considered a pretreatment method for recycling. As shown in Fig. 1A, the total number of recycling facilities in 2019 was 346, with 152 wet feed production facilities, 37 dry feed production facilities, and 78 composting facilities comprising material recycling facilities. As shown in Fig. 2A, of the 13,162 t/a of generated food waste, approximately 75% was treated by material recycling in 2019, and biogas production accounted for 14.6%.

Fig. 1
figure 1

Number of food wastes treatment facilities before (A) and after (B) the ASF outbreak

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Fig. 2
figure 2

Food waste treatment amount by treatment facility before (A) and after (B) ASF outbreak

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Status of food waste after ASF outbreak

After North Korea, South Korea officially reported the outbreak of ASF to the World Organization for Animal Health (OIE) (May 2019); the Ministry of Environment of South Korea intensified the response to and monitoring of ASF in border areas, food service centers, and farms at risk of an ASF outbreak in the neighboring countries (interest stage). However, despite such efforts, ASF broke out with an unknown infection route in pig farms in South Korea in September 2019, and as a result, wet feed production for pigs was completely banned.

When the operation of the same target facilities “before ASF” was investigated in 2020, 97 facilities were closed. It was found that most wet feed production facilities were closed, and some wet and dry feed production and composting facilities were converted to intermediate processing facilities. As shown in Fig. 1B in 2020, among the material recycling facilities, there were 34 wet feed production facilities, 32 dry feed production facilities, and 68 composting facilities; the number of material recycling facilities has gradually decreased. It seems that wet feed production facilities changed their business category as the demand for wet feed diminished rapidly due to the outbreak of ASF in South Korea. However, the change in the business category was a temporary measure to sell the existing contracted products.

The wet feed production process involves crushing and heat-treating food waste at 80 ℃ for > 30 min and then feeding it to livestock in a liquid state [32]. The decrease in the number of such facilities has increased the amount of FWLs. Furthermore, there were five less dry feed production facilities, and the treatment amount increased by approximately 18 t/a (Figs. 1 and 2). Dry feed production generates more FWLs than other treatment methods because FWLs are separately discharged before food waste is supplied to the drying equipment, and condensates are generated during the drying process. Moreover, as the number of intermediate processing facilities increased sharply, the treatment of liquid FWLs and solid dewatered cakes increased the processing load. Therefore, short-term measures to treat FWLs are urgently required.

Current status of food waste leachate

Considering the absence of statistics on the generation and treatment of FWLs, we investigated the generation and treatment status of FWLs in 316 food waste treatment facilities in 2019 through field and phone surveys. Representative treatment facilities were selected in the field survey, and FWL generation at the national level was investigated based on the FWL treatment status and empirical data from field operators. The imports and exports of food waste, intermediate processing products, FWLs, and impurities in 2019 were collected through phone surveys.

Generation and treatment volume prediction of food waste leachates

FWL generation after the outbreak of ASF (2019) was determined through field and phone surveys of 197 facilities discharging FWLs by dry feed production, composting, and intermediate processing. As shown in Table 3, dry feed production facilities generated the largest amount of FWLs (2754 t/d: 39.5%), followed by intermediate processing (2360 t/d: 33.9%) and composting facilities (1856 t/d: 26.6%). FWLs accounted for half (50.2%) of the total food waste, and the average amount of FWL generated was 6970 t/d.

Table 3 Food waste and FWL treatment status after ASF outbreak
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A schematic of the FWL treatment process derived from the field and phone surveys is shown in Fig. 3. The biogas production facilities treated raw materials, that is, food wastes that were not pretreated (solid-liquid separation) and simultaneously treated FWLs discharged from other recycling facilities. FWL treatment facilities are classified into biogas production (food only), biogas production (all organic wastes), incineration plants, and sewage treatment plants, and the treatment capacity and operation during the weekend in each facility were investigated. The field and phone survey periods were the same for food waste investigation.

Fig. 3
figure 3

Schematic for the generation and treatment process of food waste, FWLs, and dewatered cakes (2019)

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The intermediate processing facilities discharge FWLs and discard dewatered cakes by separating food waste into solids and liquids, and the dewatered cakes are sent to and treated in dry feed production or composting facilities. The schematic shows the material movement of food waste that is supplied to recycling facilities through intermediate processing.

Figure 3 shows that 5236 t/d (approximately 75%) of FWLs are treated in biogas production facilities, and some FWLs are used as a carbon source in sewage treatment plants and as urea in solution for selective non-catalytic reduction (SNCR), an emission gas treatment facility in incinerators. Biogas production was found to be a critical process for treating FWLs.

The FWLs that were additionally generated after the ASF outbreak (1083 t/d, approximately 15%) were predicted to be stored without being treated or treated by unauthorized or inappropriate processes. This occurred when wet-feed production was reduced in the early stages of the ASF outbreak and switched to other treatments (dry-feed production, composting, and intermediate processing). Consequently, it was expected that FWL treatment problems would occur.

Furthermore, FWL generation changes seasonally according to variations in the generation of food waste [29]. Thus, the shortage of FWL treatment methods is expected to change. When the basic unit of food waste generation by season was estimated, the FWL generation in summer was predicted to be 7959 t/d, 987 t/d higher than the average amount generated and up to 2072 t/d higher than the FWL treatment capacity of 5887 t/d.

Food waste leachate treatment facilities

FWL treatment facilities are classified into biogas production (food only), biogas production (all organic wastes), incineration, and sewage treatment plants. The main FWL treatment process involves the decomposition of organic matter through anaerobic digestion. In some incineration facilities, FWL is used as a substitute for urea in SNCR facilities or as a temperature controller in incinerators. FWL treatment in sewage treatment plants is performed minimally and in small areas, and there was no load on the treatment process.

There are 101 anaerobic digestion facilities operating in South Korea [27], and the type of supplied organic waste classifies the management departments. Twenty-five biogas production (food only) facilities treat food waste as the main raw material, totaling 4014 t/d. Furthermore, 43 biogas production facilities (all organic wastes) treat organic wastes including sewage sludge, livestock manure, and food waste, totaling 3636 t/d.

A total of 6970 t/d FWLs is generated from dry feed production, composting, and intermediate processing. Biogas production (food only) is used to treat 2696 t/d FWLs and biogas production (all organic wastes) to treat 2540 t/d FWLs.

Domestic food waste collection systems mostly operate only on weekdays and intermittently on Saturdays in some areas. We investigated the operation of biogas production facilities treating food waste and FWLs on Saturdays and Sundays and found that 67.6% of the weekday treatment amount was treated on Saturdays and 17.2% of the average treatment amount on Sundays, as shown in Table 4.

Table 4 Prediction of weekend status of food waste treatment
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Policy change

Short-term treatment method: utilization of existing biogas production facilities

South Korean central and local governments have attempted to reduce municipal solid waste by introducing various policies such as resource circulation, the polluter pays principle and publicity; however, municipal solid waste emissions are increasing annually with the increase in population and single-person households. Furthermore, food waste generation is increasing with municipal solid waste generation, and the burden of FWL treatment in the recycling process is growing annually.

The government attempted to gradually expand the anaerobic digestion process that can stably treat organic wastes in accordance with the basic policy of resource circulation and gradually convert the aging and low-efficiency feed production and composting facilities involving the anaerobic digestion process (biogas production facilities). However, as wet-feed production for pig feeding was prohibited suddenly because of the 2019 ASF outbreak, treating approximately 1000 t/d FWL is expected to be difficult. It is difficult to select sites for waste treatment facilities, including biogas production facilities, because of the not-in-my-backyard (NIMBY) movement among residents, and construction completion delays depending on the scale. Hence, short-term solutions are required.

Weekend supply of food waste leachate to biogas production facilities

Because the time dedicated to establishing waste treatment facilities in the short term is insufficient, we examined the weekend supply capacity of the current 68 biogas production facilities for treating food waste. However, the FWL weekend supply scheme only considered the treatment capacity, and economic aspects, such as transportation distance and treatment cost, were not reflected. It is expected that rapid application of such facilities is possible because they treat food waste, and additional equipment or modification of facilities is unnecessary. When the weekend supply was based on the food waste and FWL treatment amount of the target facilities, the additional FWL supply capacity was 8815 t, excluding the existing amount supplied and treated during the weekends.

The shortage of FWL treatment facilities in summer predicted via maximum generation by season was 2072 t/d. It was estimated that up to 10,360 t would accumulate during weekdays (5 days). Consequently, the operation of the weekend supply scheme is expected to resolve approximately 85% of the cumulative amount. Table 5 shows the effects of resolving the seasonal shortage of FWL treatment facilities according to weekend supply, and it is expected that the weekend operation scheme will resolve the shortage in every season except summer.

Table 5 FWL generation and shortage status by season
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Additional food waste leachate supply of sewage-combined biogas production facilities

Twenty combined sewage treatment facilities are in operation to improve the efficiency of anaerobic digesters and to appropriately treat locally generated FWLs. It is expected that all FWLs generated in each season would be treatable if the ratio of FWLs in the operation of digesters is increased (approximately 5% higher) in summer in combined sewage treatment facilities. If the organic matter load is suddenly increased in the current facilities, anaerobic microorganisms in digesters will require some time to adapt to FWLs with high organic matter content and low pH [33]. Therefore, a detailed process for the combination or increase of FWLs and sewage should be prepared, and the amount should be increased stepwise based on the additional supply calculation formula, the number of steps to increase, and a checklist [34]. The South Korean guidelines for the optimal design and operation of sewage sludge anaerobic digesters suggest the following. When 60% FWL is processed in a sewage co-digester, volatile fatty acid levels should be maintained below 3000 mg/L to ensure stable digester operation that meets the guidelines, and the maximum methane yield should be maintained above 0.4 Nm3CH4/kgvs, increasing the production of methane gas [35]. This was adhered to in this study with up to 60% FWL processed in sewage sludge anaerobic digesters, and the maximum methane yield was also maintained at ≥ 0.4 Nm3CH4/kgvs [33]. The FWL combination ratio of anaerobic digesters that treat sewage sludge in South Korea was 10–30%. Hence, it is expected that the optimal operation conditions would be satisfied and biogas production would increase with increased supply of FWLs. Furthermore, according to research on economic efficiency (benefit to cost ratio, B/C) for the combined treatment of sewage sludge and FWL in a an existing sewage treatment facility, this was 3.6 when the FWL combination ratio was 21.7%, and it increased to 4.3 when the FWL supply rate was increased to 29.3%. Hence, financial balance and profitability have stably increased [34].

Long-term treatment method

South Korean policy has pursued the polluter pays principle and the reduction of food waste through various campaigns; however, food waste has steadily increased since 2014. The main cause of the increase in food waste is the change in the dietary lifestyle centered on eating out due to population and single-person household growth [36].

Policies and technologies of Asian countries emphasize separation and treatment rather than preventing the generation of food waste [37]. Furthermore, feed production, incineration, and landfilling were analyzed as unsustainable recycling methods through a strengths, weaknesses, opportunities, and threats (SWOT) analysis because they are associated with various environmental risks. Considering the characteristics, fundamental environment, and economic benefits of food waste, anaerobic digestion (biogas production) was the preferred option [37].

Thus, South Korea also aims to expand biogas production facilities as a long-term goal and is attempting to gradually switch from the resource recycling of food wastes (feed and compost) to biogas production (energy production) by converting facilities through private investment [36].

According to the mid- to a long-term plan of the Korean Government, 1898 t/d of food waste, which accounts for 13% of the total 14,400 t/d food waste in 2017, was treated using biogasification methods. The biogas processing capacity will be expanded to 3270 t/d by 2022. The food waste of biogasification treatment rate is predicted to be 22%. The 2024 long-term plan is expected to process 4460 t/d food waste via biogasification, 31% of food waste discharged in 2017, and if the capacity is expanded to 456 t/d between 2025 and 2027, 4916 t/d food waste can be processed by 2027; the contents are shown in Fig. 4. Accordingly, in 2021, we plan to establish and promote a plan to expand new facilities after 2025.

Fig. 4
figure 4

Biogas production treatment plan for food waste according to the mid- to long-term plan

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Analysis of the expected effects

Greenhouse gas reduction effect

The lifecycle analysis (LCA) results for food waste suggest that treatment by livestock feed and anaerobic digestion has a greater greenhouse gas emission-reduction effect [38]. Furthermore, it was found that biogas production treatment could reduce greenhouse gas emissions by approximately 1.3–2.1 times more than composting treatment [39].

The greenhouse gas reduction impact of converting food waste into biogas was compared to composting. The calculation considered only the greenhouse gases CH4 and N2O in the emission calculation formula, while excluding CO2, which is naturally generated, in both biogasification and composting processes. The calculation method followed the guidelines outlined in the IPCC for national greenhouse gas inventories [40].

Thus, greenhouse gas reduction efficiency is expected to increase as the traditional treatment methods gradually shift to biogas production. Currently, approximately 4200 t/d of food waste and dewatered cakes are treated by composting. If this amount is gradually switched for biogas production, approximately 438 million CO2 of greenhouse gases would be reduced [39].

Production of renewable energy

The recycling of dry-feed products requires fossil fuels, such as LNG and kerosene, to remove the high water content of food waste. A feasibility study showed that when 150 t/d food waste was treated, the variable operating cost was 53,086 won/t for dry feed production and 28,438 won/t for anaerobic digestion. Thus, the variable cost of dry feed production was approximately twice as high [41]. Energy consumption is reduced when the waste heat of incineration plants [42] or microwave drying systems [43] are used to dry waste with high water content, such as sewage sludge. However, the results of the field survey of dry feed facilities revealed that processes for reducing energy use with fossil fuels are required to reduce the water content in food waste.

The anaerobic digestion process increases biogas production if the food waste, which is a biodegradable material, is increased [44, 45]. sewage sludge and food wastes are treated together, the methane yield increases with the food waste treatment ratio [46]. Furthermore, when the FWL supply ratio was increased in a sewage-combined treatment facility, biogas production increased. When the food waste leachate was increased by approximately 8% (110 t/d), the additionally produced methane gas was 3853 Nm3CH4/kg, 19.2% higher on average [34].

Based on the abovementioned results, per Joshi and Visvanathan [37], if approximately 1000 tons of FWL treatment shortage is divided and treated at the sewage co-digestion facilities dispersed in each region, the additional biogas production is estimated to be approximately 63,600 Nm3/d [34]. As shown in Table 6, the additionally generated heat during biogas production is equal to the amount of heat that can be used by approximately 28,389 households at the same time in Korea [47].

Table 6 Biogas production and expected usage according to the increased supply of FWLs
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Advancing sustainable food waste management in South Korea: transitioning towards anaerobic digestion and green new deal initiatives

In South Korea, food waste has been recycled based on the resource circulation policy; however, the demand for food waste shows a decreasing trend because of the negative perception of feed and compost. The government is attempting to expand and gradually switch to the anaerobic digestion process, which can stably treat organic wastes [36]. However, the switch was sudden because of the outbreak of ASF, and we analyzed the effects of prohibiting wet feed production. Additional FWL treatments were generated because of the discontinuation of wet feed production, resulting in an estimated additional FWL generation of approximately 1080 t/d. Furthermore, approximately 2070 t/d FWL is predicted to be additionally generated in summer when the FWL generation peaks.

Therefore, short-term treatment is required. It was estimated that if weekend supply and additional FWL supply schemes are introduced using biogas production facilities, which are currently treating food waste [33], complete treatment would be possible in the summer. However, this was possible for physical treatment without reflecting economic values such as labor, treatment, and transportation costs.

For the long-term treatment of food waste, biogas production by anaerobic digestion (energy recycling) will be expanded while gradually reducing the percentage of material recycling through traditional treatment methods, including wet feed production, dry feed production, and composting. Consequently, greenhouse gas emissions will be reduced, economic efficiency will be improved, and the production of biogas, a renewable energy source, will be increased. This will promote the switch to an eco-friendly, low-carbon economic foundation for the Korean version of the Green New Deal policy [48].