Journal of Environmental Protection
Vol.05 No.07(2014), Article ID:46340,10 pages
10.4236/jep.2014.57061

Greenhouse Gas Emissions and Cost Analyses for the Treatment Options of Food Waste and Human Excrement

Sora Yi1*, Kee-Young Yoo2

1Department of Urban Planning Research, Daejeon Development Institute, Daejeon, Korea

2Department of Safe and Environment Research, Seoul Institute, Seoul, Korea

Email: *sora@djdi.re.kr

Copyright © 2014 by authors and Scientific Research Publishing Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).

http://creativecommons.org/licenses/by/4.0/

Received 25 March 2014; revised 23 April 2014; accepted 17 May 2014

ABSTRACT

This study suggested environmental and economic evaluations by developing a scenario according to the various treatment options of food waste in Korea. In particular, the study evaluated the possibility about the combined treatment of food waste and human excrement after using food waste disposers (FWDs). The scenario including only composting (133 kg CO2 equiv./ton-household organic waste) or only FWDs (125 kg CO2 equiv./ton-household organic waste) was superior to the other scenarios in the environmental aspect and the scenario including only composting (101 USD/ton-household organic waste) was superior to the other scenarios in the economic aspect. However, the study discovered that 52% of greenhouse gas emission was reduced when sewage pretreatment was conducted in houses after using FWDs and also when biogas was collected on site and utilized in the private power station. Furthermore, the energy saving effect due to recovery of biogas has found to be larger in the environment aspect than in the economic aspect.

Keywords:

Food Waste Disposer, Food Waste Drying Machine, Composting, Biogasification, Automatic Vacuum Waste Collection System

1. Introduction

Food waste and human excrement, i.e., human biological waste is household organic waste which is highly valued as the production of biomass such as biosolids, biogasification, and bioethanol [1] -[4] . In Korea, 97.1% of the total food waste generation has been separately collected food waste from municipal solid waste (MSW) and recycled to animal feed and compost since 2005 [5] . Due to food waste separation from MSW, MSW composition in landfill sites and incineration plants has been changed. As a result, the efficiency of waste to energy in landfill sites and incineration plants was differed by changing of biodegradable composition and lower heating value (LHV), respectively. The existing study has reported that the LHV of municipal solid waste increases by 6.4% when 32.8% of the household food waste used the food waste disposers (FWDs). In that report, the ratio of household combustible waste of the total combustible waste was 58.6%, the ratio of household food waste of the household combustible waste was 44.0% [6] .

Since food waste was collected separately from the municipal solid waste, unsanitary problems, including unpleasant odors, germs, insects, and feeling of aversion from curbside waste, have increased. Due to these problems, many households tend to install food waste disposal units (FWDs or food waste decomposers by using drying system) such that food waste can be treated before it is taken out [7] . Recently, FWDs are supplied to large scale apartment complexes via the built-in system in general. However, the applicability of these units is not high due to the burden of additional electricity and water demands as well as the early purchase cost [8] .

Figure 1 shows sewer systems in Seoul, Korea. The FWDs give effects for wastewater loadings and biogas production in the public wastewater treatment plants (WWTPs) [9] - [11] .

Seoul has already carried out pilot studies in order to treat food waste in the public WWTPs. The studies were conducted by introducing domestic in-sink FWDs to 447 households in apartment houses with the sewage pretreatment facilities and 538 households in apartment houses with the sewage pretreatment facilities combined with human excrement. The result of the pilot studies has found that the combined treatment of food waste with human excrement lowers the effluent biological oxidation demand (BOD) concentration (food waste: 1055.1 mg/L, food waste and human excrement mixture: 804.1 mg/L), and increases SS concentration (respectively, 905.7 mg/L, 1564.4 mg/L) and n-Hexane concentration (respectively, 195.9 mg/L, 240.5 mg/L), more than the food waste treatment [12] [13] . If it is considered that the separate sewer system for the separated treatment of human excrement covers 14% of the sewer service area in Seoul, it needs to introduce this combined treatment of food waste with human excrement in order to reduce the environmental burden as well as to promote the convenience of the citizens.

Meanwhile, food waste has shown various environmental and economic evaluation results according to the treatment methods [14] [15] . In particular, the waste has been evaluated such that the benefit/cost (B/C) was 0.85 when domestic in-sink FWDs were used, B/C was 0.19 when the food waste was separated and put out for collection, B/C was 0.39 when the food waste was separated and put out for collection after using the food waste drying machines (FWDMs), and B/C was 0.18 when the automatic vacuum waste collection (AVWC) system was utilized [16] .

Therefore, this study suggested environmental and economic evaluations about the various treatment methods of food waste and the combined treatment of food waste with human excrement after installing domestic in-sink FWDs. The environmental evaluation indicated as greenhouse gas (GHG) emissions by methane gas production and the energy equivalent and net energy consumption, which is required in the treatment. The economic

Figure 1. Sewer systems in Seoul.

evaluation is indicated as construction and operation costs, which are required in the treatment. Further, the additional social cost considering the value of domestic labor of separating and handling food waste.

2. Materials and Methods

2.1. Functional Unit and Scope

This study analyzed the properties of food waste and human excrement and the characteristics of the handling process and treatment of the household organic waste; scenarios which reflected the characteristics were developed. The functional unit is a measure of the function of the studied system and it provides a reference to which the inputs and outputs can be related. In this study, the functional unit of food waste was set to 1 ton of food waste from 6667 persons, which was based on 0.15 kg/person∙day, in order to calculate the parameter. Further, the functional unit of human excrement was set to 0. 6667 ton (= 0.7 ton in the text below) from 6667 persons, because one human produces 0.1 kg of human excrement per day.

To evaluate GHG emissions and cost for the treatment options of food waste and human excrement mixture, the functional units set the quantity of human excrement per population (6667 persons) that handles 1 ton of food waste. Therefore, the functional unit for calculating parameters is 1.7 tons of household organic waste, which is the total amount of about 1 ton of food waste and 0.7 ton of human excrement. It is necessary in order to compare the scenarios of 1 ton of food waste and 0.7 ton of human excrement, which are transferred and treated in the different processes, with the scenarios of 1.7 tons of food waste and human excrement mixture, which are combined and treated in one system. The parameters based on 1 ton of food waste were calculated in order to reflect the social cost of separating and putting out the food waste for collection.

Net energy consumption (= Energy consumption in process minus energy recovery in process) was reflected in energy use. The GHG evaluation calculated the emissions from each process as well as the saved effect due to the recovery of biogas (the alternative effect based on the Korean power plants using LNG). The economic evaluation calculated the costs per each process and the saved effect due to the recovery of biogas.

As it was assumed that public sewer is the combined system and human excrement is treated in a separate sanitary treatment plant by collecting from the septic tanks, most of the scenarios did not include the use of energy, GHG emissions, and the costs according to the public wastewater treatment on human excrement. For the scenarios including the combined treatment, treated wastewater is flowed into public WWTPs; however, the part of public WWTPs of human excrement was ignored, according to the mass balance which had been applied before.

2.2. Properties and Characteristics of Household Organic Waste

2.2.1. Characteristics of Food Waste

There are big differences in the amount of food waste generation change to according to the building types or sources. Generally, the amount of generation from residential and commercial (restaurants) sectors is 0.33 kg/person/day [17] , but it range from 0.12 to 0.25/kg/person/day according to the types of houses [13] [17] [18] . There are also differences in the properties of food waste according to the areas and the seasons, yet, fruits and vegetables represent the highest share of them, 55% to 75% (Table 1).

Food waste is treated by dividing it into public and private facilities. In the public facilities, the following occurs: animal feed (26.1%), composing (65.4%), and others (biogasification) (8.5%). Animal feed are of great importance in the private facilities because they make up more than half of total amount: animal feed 58.8%, composing 38.2%, and others 3.2%.

The result from the survey of this research reveals that 20.7% of total respondents in Seoul are now using food waste disposal units: FWDMs (9.1%); compost bins (8.0%); FWDs (2.1%); and others (1.5%). Further, 62.2% of total respondents plan to use them: FWDMs (26.0%); FWDs (24.3%); compost bins (6.8%); and others (0.4%) in the future.

2.2.2. Characteristics of Human Excrement

Human excrement is generated in blair and flush toilets in Korea. The excrement in blair toilets is referred to as raw human excrement; the one from the flush toilet is processed through the septic tank sludge. Men expel urine and feces seven times throughout the day, including one time of feces and six times of urine per day. Human excrement, when calculated, is 1 L/person/day; feces account for 0.1 L/person/day. BOD of expelled human excrement is more than 20000 mg/L and SS is 27,500 mg/L [19] .

Table 1. Properties of food waste in Korea.

As shown in Table 2, BOD of septic tank sludge, which is carried into the Seoul human excrement treatment facilities, is higher than that of the US, as 8674 to 11343 mg/L [20] [21] .

2.3. Scenario Development

A total of six scenarios were suggested by applying the applicable conditions that were most practical in Seoul City; the contents of the concrete scenarios are shown in Table 3. Composting was selected, except for animal feed, in the current food waste treatment methods because Korea is similar to a traditional agrarian country and thus, it can secure more stable facilities and supply and demand chain than the ones for animal feed.

This study included the options to install FWDMs or FWDs in individual houses because there were intentions to purchase FWDMs or FWDs in the previous survey. The AVWC systems, which have been recently distributed to housing complexes, were added in the transport option.

In the developed scenarios, household organic waste is divided into food waste, human excrement, and food waste and human excrement mixture (Table 4). Energy (electric power and diesel) and material (public tap water) are consumed in each transfer and treatment processes, GHG is emitted accordingly, and costs are required. Scenario 6 also evaluated the reduction level of GHG by recovering biogas. If food waste is put out after it has been dried in a FWDM, energy, which is necessary for the use of elevators in apartments, for the collection and transport, and for the transport of the waste to aerobic compositing facilities, will be reduced. If food waste is sent to the public WWTPs after sewage pretreatment in houses, energy for public WWTPs operation will be significantly reduced.

2.4. Key Factors Analysis

2.4.1. Possibility of a Combined Treatment of Household Organic Waste

This study evaluated the characteristics of anaerobic digestion and biogas generation on mixed liquids of grinded food waste and human excrement mixture by the biochemical methane production (BMP) test.

The characteristics of the used samples are shown in Table 5 and the reaction formula to generate methane, which was calculated based on the measured data, was Equation (1).

(1)

The ingredient content in gas, which was calculated based on this formula, was 66.7% of CH4 and 33.3% of CO2. The early ingredient content of gas in headspace, which was corrected by considering the solubility of gas ingredients, was 84% of CH4 and 16% of CO2. The amount of accumulated methane generation was largest when grinded food waste was 94 mL, and non-thickened human excrement with flush water was 51 mL. The amount of methane generation based on inflow of CODCr for grinded food waste and for non-thickened human excrement with flush water was 0.342 L CH4/g COD and 0.423 L CH4/g COD, respectively.

The existing study showed the range of 0.333 to 0.347 L/g CODCr [22] on the methane gas generation of mixed samples. Methane gas generation, which was collected after the mixed samples were digested for 20 days, was 0.35 L/g CODCr per person, thus, the amount of methane gas generation was 24.85 L/person/day for 71 g/day (food waste 35 g + human excrement 36 g) [23] .

Table 2. Properties of septic tank sludge and raw human excrement.

*Based on TS.

Table 3. Characteristics of the developed scenarios.

Table 4. Energy and material consumption, GHG emissions, and recovery in various scenarios.

○: Consumption in full operation, ◎: Emissions in full operation, △: Consumption in half or less operation, ×: Insignificant consumption/emissions, and ●: Recovery in full operation. *S1, S2, S3, S4, S5, and S6 stand for scenarios 1 - 6, respectively.

Meanwhile, the amount of CH4 generation per person in a septic tank was calculated as 0.423 L CH4/g CODCr × 1/2.4 (g CODCr/g TS) [24] × 27 (TS, cases of toilets in US) [25] × 70% (storage solid material) [24] × 0.12 (room temperature)/0.27 (temperature range of 37˚C - 41˚C) [26] = 1.481 L CH4/person/day; the weight was 1.481 L × 16 g/22.4 L = 1.058 g/person/day.

2.4.2. Parameters in Process Units

Parameters were calculated by dividing them largely into transfer and treatment processes in Table 6, Table 7, and Table 8. The parameters were calculated as consumption, emission, and recovery by GHC source (electric

Table 5. BMP test of food waste and human excrement.

Table 6. Parameters for transfer processes.

Table 7. Parameters for treatment processes.

Table 8. Parameters for the social costs and conversion factors.

power, diesel, tap water, methane gas) for functional unit: per 1 ton of food waste; per 0.7 ton of human excrement; or per 1.7 tons of mixture.

3. Results and Discussion

3.1. The Environmental Analysis

Figure 2 shows GHG emissions and net energy consumption from 1 ton of household organic waste according to various scenarios. In Scenario 4 (125 kg CO2 equiv./ton-household organic waste), the grinded food waste discharge to the public WWTP (Human excrement was sent to the existing septic tank) was the best practice. There were increasing emission of GHG in the following order: Scenario 1 (Compositing, 133), Scenario 2 (AVWC system + Compositing, 215), Scenario 5 (FWDs + Pretreatment + Public WWTP, 298), Scenario 6 (FWDs + Biogasification + Public WWTP, 351), and Scenario 3 (FWDM + Compositing, 886).

The evaluation of net energy consumption identified that energy consumption of Scenario 4 was the smallest at 36 kWh/ton-waste. The order of energy consumption was as follows: Scenario 1 (125), Scenario 2 (320), Scenario 6 (445), Scenario 5 (470), and Scenario 3 (2105).

In Scenario 6, GHG emissions were 351 kg CO2 equiv./ton-household organic waste in the transfer and treatment processes. However, 167 kg of CO2 equiv.ton-household organic waste was actually generated if the fact that the avoided impact (or saved effect, the LNG alternative effect) of methane gas recovery of 184 kg of CO2 equiv./ton-household organic waste was considered.

3.2. The Economic Analysis

Figure 3 shows the cost analysis. Scenario 1 (101 USD/ton-household organic waste) was the best, followed by Scenario 3 (221), Scenario 4 (245), Scenario 5 (450), Scenario 2 (558), and Scenario 6 (592). If the cost of 17 USD/ton-waste by using biogas in the private power station was applied, the net cost was 575 USD/ton-waste.

However, the result, which analyzed the social cost by reflecting the value of domestic labor about handling and separating food waste for collection of 10 USD/month, has found that the cost of Scenario 4 (343 USD/ton- household organic waste) was most efficient followed by Scenario 3 (416), Scenario 1 (481), Scenario 5 (547), Scenario 6 (689; net cost 672), Scenario 2 (938), in order.

Figure 2. GHG emissions and net energy consumption in various scenarios.

Figure 3. Cost analysis in various scenarios.

By considering domestic labor of 34 USD/month, the cost of Scenario 4 (575 USD/ton-household organic waste) was the most efficient followed by Scenario 5 (779), Scenario 3 (881), Scenario 6 (922; net cost 905), Scenario 1 (1388), and Scenario 2 (1845) in order.

4. Conclusions

In the environmental aspect, the scenarios including only compositing and only FWDs have been proved to be superior. The utilization of biogas in the private power station after the FWDs and domestic sewage pretreatment in the apartments was better in the GHG emissions and energy consumption than the discharge to the public WWTP after the FWDs and domestic sewage pretreatment in the apartments.

The scenario including only composting was superior to the other scenarios in the economic aspect. In considering the only transfer and treatment processes, the study discovered that the method to put out food waste by using elevators was less expensive, yet, it incurred higher social cost on handling and separating food wastes; thus, it was more efficient to use FWDs. Furthermore, the energy saving effect due to recovery of biogas has found to be larger in the environment aspect than in the economic aspect.

Acknowledgements

This project was supported by the Seoul Metropolitan Government, Republic of Korea.

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NOTES

*Corresponding author.

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