Household food waste in Surakarta City, Indonesia: Quantity, global warming potential (GWP), and management

Households represent one of the main contributors to food waste in urban areas. Surakarta is one of the cities in Indonesia where a significant portion of household food waste is disposed of in landfill sites. The piles of food waste can become an environmental issue if left unmanaged, as they generate greenhouse gas emissions that contribute to global warming. The aim of this research is to determine the quantity, emission estimations, and management of household food waste in Surakarta City. A total of 151 household samples had their food waste collected over a period of 7 days. The method for measuring the quantity of food waste involved direct measurements using a scale, calculating food waste emissions using the GWP formula, and collecting management data through a questionnaire. The research results indicate that, on average, households dispose of 0.499 kg/week of food waste and estimate food waste emissions to be 0.789 kg CO2e/week. In addition to being discarded, some food waste is managed as animal feed and fertilizer. Implementing proper and sustainable management can lead to a reduction in the quantity of food waste sent to landfills.


Introduction
Food waste is one of the types of waste generated in large quantities by humans.According to the report entitled Food Waste Index Report 2021 issued by the United Nations Environment Programme (UNEP), approximately 931 million tons of food waste were generated globally in the year 2019.A total of 61% of food waste is contributed by the households, followed by the food service and retail sectors at 26% and 13%, respectively [1].
Food waste is a massive problem on a worldwide scale.It plays a significant role in causing global warming.During its decomposition in the landfill, food waste releases greenhouse gas emissions.Additionally, the process of producing and transporting food also contribute to emissions.FAO found that around 4.4 gigatons of greenhouse gas (GHG) emissions are produced annually by global food loss and waste (FLW), accounting for about 8% of total anthropogenic GHG emissions [2].
Indonesia ranks among the top countries that produce the most food waste.According to data from The Economist Intelligence Unit, Indonesia was reportedly the second-largest producer of food loss and waste (FLW) in the world in 2017 [3].An estimated 300 kilograms of FLW are generated annually per capita in Indonesia.The average emissions generated from FLW in Indonesia from 2000 to 2019 are 2,324.24kg CO2-eq/ton FLW.This statistic accounts for 7.29% of total greenhouse gas emissions in Indonesia [4].
Bappenas [4] suggested that Global Warming Potential (GWP) is one of the indicators of greenhouse gas (GHG) emissions, including carbon dioxide (CO2) and methane (CH4).The GWP is measured in carbon dioxide equivalent (eq).Each food category has a different Global Warming Potential (GWP) value [5].The total GWP value will vary depending on how each food category's GWP value differs.A study carried out in the United Kingdom by Jeswani et al. [6] demonstrated that although the amounts of meat and fish waste produced are fewer than those of vegetable and root waste, the total GWP value of meat and fish waste is higher than the total GWP value of vegetable and root waste.Research conducted by Chen et al [7] also indicates that even though meat products make up just 6% of the total weight of food waste on average, they play a significant role in contributing to 57% of greenhouse gas emissions.As a result, strategies and goals for preventing food waste should not be formulated solely based on the volume of waste but should also take into account their environmental impact [6].
Food loss and waste occur throughout the entire food system, but households playing a substantial role in the overall quantity of food waste [8].Indonesia leads in producing household food waste in Southeast Asia, producing an annual total of 20.9 million tons.The Indonesian's household food waste reach 77 kg/capita/year [1].Data from the examination of Bappenas [4] have identified carbohydrates are the most frequently leftover food category in Indonesian households, making up 41.55% of uneaten items.
Efficient management of household food waste is essential to prevent adverse environmental effects.Nevertheless, not all households are inclined to manage their food waste responsibly.Based on the questionnaire survey conducted by Bappenas [4], it is found that the predominant behavior of households towards food leftovers is discarding them or providing them to livestock.
Surakarta City is one of the cities in Indonesia located in Java Island.The city has experienced rapid development in various fields.The population of Surakarta City in 2022 is recorded at 523,008 people [9].A relatively high population can impact the quantity of food waste.It leads to a rise in landfill waste volume, particularly when not properly managed.
This research attempts to determine the amount of household food waste and the associated emission values in Surakarta City based on food categories.Moreover, the current specific data on the quantity and emission potential of household food waste in Surakarta are still lacking.Additionally, this study aims to gain insights into the existing practices of food waste management undertaken by households in Surakarta City.

Location, Time and Sample of the Study
This research was conducted in 151 households in Surakarta City.The samples were taken from five districts: Pasar Kliwon, Jebres, Banjarsari, Laweyan, and Serengan.This research was conducted over two months, specifically from May to June 2023.The data were collected using simple random sampling.The determination of the number of sampled households referred to SNI 19-3964-1994 on the Method of Sampling and Measurement of Urban Waste Generation and Composition [10].Here are the equations used:

Food Waste Sampling
The sampling was assessed using direct measurement methods, employing scales.Food waste samples were collected from each household over a period of 7 days [11][12][13].Plastic bags were provided to collect the food waste at each household chosen for sampling one day before the scheduled waste collection.The following day, the researcher gathered the plastic bags containing food waste from each household.The food waste was sorted into categories, such as vegetables, rice, fruits, meat, and others, to determine the composition of the food waste.Using a digital scale, the weight of each food category was measured and recorded.Certain unavoidable food waste, such as fish bones, chicken bones, and fruit peels, were not included in the study and therefore not weighed.

Food Waste Emissions Calculation and Analysis
The calculation of emission values for food waste was calculated using the Global Warming Potential (GWP) based on the assumption of CO2 emissions for specific food categories.These GWP values (CO2 emission assumptions) for distinct food categories, were derived from a research paper authored by Clune et al. [5].Here are the equations used: Description: GWPFW : the Global Warming Potential values for specific food categories discarded over the course of a week FWi : the amount of food waste from specific food categories discarded every week [kg] GWP : the CO2 emission assumptions for specific food categories By employing these formulas in calculations, the food categories that have the highest and lowest contributions to CO2 emissions could be identified.The data is subjected to descriptive analysis and the findings are elucidated through interpretations presented in tables and diagrams to clarify the discussion.

Data Collection for Food Waste Management
The data collection for food waste management was carried out by distributing questionnaires to households from which food waste samples were obtained.A total of 151 respondents participated, with a focus on housewives or individuals responsible for managing food waste in their respective households.The analysis of the food waste management data was analyzed descriptively.

Demographic Characteristics of Household Food Waste Respondents in Surakarta City
In this study, the total number of respondents is 151 individuals.The majority of the respondents are female, with very few male respondents.The respondents' ages are diverse, with the highest percentage falling within the 51-60 age group.The most frequently reported highest level of education is high school (SMA).The predominant occupation is housewife, accounting for 43% of respondents.Additionally, the majority of respondents have an income of less than 1,000,000 rupiahs.Detailed data is available in Table 1.

Quantity, Emission Value/GWP, and Household Food Waste Management in Surakarta City
The findings from the sampling of food waste in 151 households reveal that household food waste in Surakarta City encompasses 11 categories, including rice, vegetables, potatoes, cereals, legumes, fish, eggs, chicken, beef, fruits, and cream (Table 2).Rice stands out as the predominant food category left uneaten in the households.It constitutes 64.2 percent of the total amount of food waste generated by 151 households.Given that rice is a fundamental dietary item often prepared in larger quantities than other food types, it presents a greater likelihood of transforming into food waste compared to other categories.In addition to rice, vegetables are a notable category left uneaten, accounting for 20.3%.Conversely, cream and beef represent the least prevalent household food waste categories in this research, each contributing 0.1% to the total food waste.On average, households produce 0.901 kg of food waste per week.  2 illustrates that not all of the food waste produced by households in Surakarta City is thrown away; instead, a portion of it is put to use.The total amount of food waste discarded by 151 households is 75.328kg, with the remaining 60.726 kg being repurposed as animal feed and fertilizer.The discarded quantity of food waste exceeds the utilized amount, indicating that many people are still reluctant to manage or make use of food waste.Based on the survey findings, 58.1% of respondents opt to dispose of their food waste in the garbage, 41.4% choose to use it as animal feed, and 0.5% prefer to convert it into fertilizer.Rice constitutes the primary portion of disposed food waste (Figure 3).In Serbia, however, a European country, the most common household food waste category that is discarded the most is bread.[14].In Italy, fruits are the most frequently discarded category of food waste, whereas rice is among the least discarded categories.[15].Cream and beef stand out as the least discarded food waste categories among households in Surakarta City.The limited disposal of beef can be linked to its high cost, causing individuals to hesitate in throwing it away.Moreover, since beef is not a staple in every household, it contributes to a lower overall quantity of food waste.On average, the amount of food waste discarded per week by households in Surakarta City is 0.499 kg, as shown in Table 3.This figure is less than the corresponding averages in Serbia and Canada, where households in Serbia, the average disposal of food waste per week is 1.54 kg [14].At the same time, the average amount of food waste discarded per week by Canadian households is 2.98 kg [12].Variations in the average quantity of discarded food waste can be influenced by disparities in consumption patterns and habits related to food waste management.
Referring to Table 3, employing household food waste as animal feed and fertilizer has the potential to decrease the volume of food waste disposed of.Rice emerges as a food waste category commonly addressed or repurposed by households, with a utilization rate of 58.4% of the total rice waste generated.In contrast, other categories of food waste are not as extensively managed or repurposed.Household food waste in Surakarta City is discarded by the community into garbage bins, and then collected by waste collection officers from the sub-district or the Environmental Agency of Surakarta City.Subsequently, this household waste is transported to the final disposal site, Putri Cempo landfill, by these personnel.Food waste that remains unprocessed or unused by the community contributes to the overall landfill volume.Additionally, accumulations of food waste in landfills can generate methane gas, adding to the production of greenhouse gases.The impact on greenhouse gas emissions is not limited to the time the food spends in the landfill; it also extends to various stages, including production, post-harvest handling and storage, processing and packaging, distribution and marketing, as well as consumption [4].Greenhouse gas emissions are the cause of global warming.The emission values are indicated by the Global Warming Potential (GWP) values.
According to Table 3, households in Surakarta City, on average, could produce emissions of 0.789 kg CO2e/week from the disposal of their food waste.This emission level is less than the findings of Djekic et al. [14] in Serbia, where the corresponding figure is 3.46 kg CO2e/week.The difference can be attributed to the larger quantity of household food waste in Serbia.Figure 4 illustrates that rice stands out as the food category with the highest emission value in this study, reaching a percentage of 77.8%.The considerable quantity of discarded rice, nearly half of the total household food waste, significantly contributes to the elevated emission value.
Having larger quantities does not necessarily mean higher emission values for all food categories.In this study, despite chicken meat food waste having a smaller quantity compared to cereals and legumes, its emission value or weekly GWP is higher than that of cereals and legumes.Similarly, although the quantity of beef food waste is smaller than that of potatoes, its emission value exceeds that of potatoes.This discrepancy arises from the different assumed CO2 emission values (GWP) assigned to each food category, affecting the total emission or weekly GWP values.According to the meta-analysis conducted by Clune et al. [5], grains, fruits, and vegetables exhibit the lowest GWP values, while meat from ruminants has the highest GWP value.
The volume of discarded vegetable food waste is notably high, with the second-highest emission value.However, this emission value is relatively modest, not surpassing twice the amount/weight of the waste.This is attributed to the low GWP value for vegetables, specifically 0.37 [5].In contrast, beef, despite having a minimal quantity of food waste, exhibits an emission value that is 26 times the amount of waste.This raise concerns if the quantity/weight of beef waste increases.Effectively reducing waste or disposing of food with high GWP stands as a viable strategy to alleviate adverse environmental impacts [6].
According to the research findings, it is crucial to adopt effective management or utilization practices for household food waste.Implementing proper and sustainable management can lead to a reduction in the quantity of food waste sent to landfills.In addition, sustainable food waste management aims to minimize environmental impact and maximize economic and social benefits.The best options for managing food waste include anaerobic digestion, animal feeding, composting, and thermal treatment with energy recovery [16,17].Food waste management must also be accompanied by efforts to prevent and reduce food waste production to achieve significant environmental and economic savings [18].

Conclusion
Each week, households in Surakarta City, on average, dispose of 0.499 kg of food waste, contributing to an estimated emission of 0.789 kg CO2e/week.Rice, being the most commonly discarded food, has the highest emission value.The predominant disposal method for food waste is the landfill, while a portion is repurposed as animal feed and fertilizer.Implementing proper and sustainable food waste management practices can effectively decrease the volume of waste directed to the landfill.

5 Figure 1 .
Figure 1.The sampling locations for household food waste.

Figure 2 .
Figure 2. Proportion of the quantity of household food waste in Surakarta City that is used and discarded.

Figure 3 .
Figure 3. Percentage of each category of household food waste in Surakarta City that is discarded.

Figure 4 .
Figure 4. Percentage of Weekly GWP for Each Category of Household Food Waste Discarded in Surakarta City.

Table 1 .
Demographic characteristics of household food waste respondents in Surakarta City.

Table 2 .
The quantity of household food waste in Surakarta City for a week based on sampling results (151 households).

Table 3 .
Weekly CO2 emission/GWP value of discarded household food waste in Surakarta City based on sampling results (151 households).