Data collection
Data quality should be characterized by quantitative and qualitative aspects. Data collection was based on site inspections. In the present study, time-related coverage was from 2005 to 2009; however, plant data were collected with the age of data within the latest year, specifically 2008. Literature data were collected with the time scope within the last 5 years. Data were collected by plant survey at a facility in Sainggok, Busan City for anaerobic digestion, in Shincheon, Daegu City for co-digestion, and in Nowon, Seoul metropolitan for dryer-incineration to satisfy the expanded system boundaries of each scenario as shown in Figure4. Up- and downstream databases, which were connected with technosphere, were taken by the Ministry of the Environment and the Ministry of the Knowledge Economy, Korea. In the case of non-existing data, data inventories were directly constructed with site inspection data, or overseas inventory supported by SimaPro 7.1 was used. The analysis was conducted in accordance with the methods stipulated in ISO 14040[22, 23].
Life cycle inventory analysis
Discharge stage
Dryer-incineration. Food wastes were dried using a garbage dryer at the discharge stage by a discharger. Two types of the popular garbage dryers which dominated at the market share were installed in ten of the sample houses and were tested by residents. Under the data from the user surveys, 75.9% weight reduction in efficiency resulted from one operation on 1 day and in the next, which is 3.35 min of continuous activity. Electricity consumption was averaged at 648.884 kW h/tonne of food waste using both stirring warm air drying and warm air circulation drying.
Separate collection stage
Food wastes were kerbside collected by a 5-tonne-capacity garbage truck and transported to a transfer station. The distance for collection depended on the collection systems. However, the round-trip distance was accepted, and the average distance of 94.4 km was directly measured in Jungnang District, the study region. Fuel efficiency was 4.8 km/L, and the load was 5 tonnes/truck. In the dryer-incineration scenario, about 0.24 tonne of dried food waste was collected.
Transportation stage
The study area, Jungnang, has had its own transfer station. Food wastes were transported from a transfer station to each facility by an 11-tonne truck. The distances from the transfer station to each facility were as follows: around 5.21 km to the anaerobic digestion facility for the anaerobic digestion scenario and around 3.97 km to the STP for the co-digestion scenario. In the case of dryer-incineration scenario, around 8.42 km was the calculated distance to the Nowon energy recovery facility; however, the weight of the dried food wastes was around 0.24 tonne.
Treatment stage
Anaerobic digestion. The target facility treated 200 tonnes of food waste by anaerobic digestion per day. Approximately 14.29 tonnes of biogas and 22 tonnes of biosolids were produced for a day. About 40 tonnes of processed water was recycled. Around 200 tonnes of wastewater and 21.2 tonnes of screenings were produced. The quality of wastewater was 5,440 mg/L of BOD, 4,520 mg/L of SS, 2,400 mg/L of T-N, and 19 mg/L of T-P. The wastewater was pretreated before moving to the STP. For this process, 13,645 kW h of electricity usage was required. Produced biogas was converted to 48,735 kW h of electricity and 40.79 tonnes of steam per day. Steam was used to warm the digester. Electricity typically covered 13,645 kW h of daily usage for the process operation, and a surplus of 35,089 kW h was sold at Korea Power Exchange. Air emissions from the combustion of generated biogas were calculated by IPCC emission factors. Produced CO2 was not counted on GHG because it had a biogenic origin. It was investigated that 242.64 kW h of electricity, 4,280,000 kcal/h of steam, and 0.11 tonne of biosolids were produced from 1 tonne of food waste. In this study, surplus electricity and the biosolids were analyzed by an avoided impact approach. The gate-to-gate data of the scenario is shown in Table2.
Table 2
Gate-to-gate data for the scenarios at treatment stage
Co-digestion. The selected facility treated about 200 tonnes of food waste per day with mixing sewage sludge in the STP. The co-digestion process was divided into two major sections, pretreatment and co-digestion. The pretreatment covered sorting, shredding, and dewatering. Pretreated food wastes, which were in pulp status, were injected to an anaerobic digester with primary sewage sludge. On the basis of plant data from July 2007 to June,2008, an average of 193 tonnes of food waste per day was pretreated, and then 293 m3 of pulp-phase food waste and 1,206 m3 of sewage sludge per day were co-digested by anaerobic digestion. Finally, 11,150 m3 of biogas per day was produced. An average of 3,700 kW h of electricity per day was required. Around 213 tonnes of processed water per day was needed to move the pulp-phase pretreated food waste easily to the anaerobic digester. Approximately 77 tonnes (around 40% of the daily treated food waste) of screenings and dewatered cakes per day were sorted and finally landfilled. In this system, pretreated food wastes (flow rate at 250 m3/day, volatile solid (VS) 39 kg/L) and the first sludge from a primary sedimentation tank (flow rate 1,300 m3/day, VS 12.9 g/L) were mixed in the anaerobic digester. Sewage sludge and pretreated food wastes were injected to the digester in a ratio of 5:1, and the ratio of volatile solid concentration of sludge and food wastes was 1:3. Biogas production in 2006 was 7,236 m3 from sewage sludge alone; this increased to 9,745 and 9,559 m3 of biogas in 2007 and 2008, respectively, when co-digestion with food wastes was introduced. It was represented that co-digestion with the food waste system produced around 34% of biogas more than sludge digestion alone.
○ Allocation for biogas quantification. Biogas production was calculated by allocation. In 2008, a daily increase, around 2,323 m3, was produced from 196 tonnes of food waste; therefore, it was estimated that 11.85 m3 of biogas was produced from 1 tonne of food waste. It was 13.32 m3 in 2007 (Table3). It was converted to 8.92 kg of weight based from the average value of 12.59 m3. CO2-eq of 186 kg was generated as GHG from co-digestion treatment process. GHG production from the combustion of biogas, which was generated for this process, was calculated using IPCC emission factors. If generated CO2 was a biogenic origin, then this CO2 was excluded on the counted GHG.
Table 3
Biogas production from the co-digestion
○ Allocation for electricity quantification. Electricity usage was calculated by allocation. According to the visiting surveys at selected STPs, the station for pumping to a grit chamber, facility operation (including settler and clarifier), and aerator operation needed 27.75%, 8.86%, and 37.46% of total electricity usage, respectively. In the case of the sludge treatment process, dewatering, sludge thickening, and deodorization needed 19.89%, 4.54%, and 1.50% of electricity, respectively. Therefore, 74% and 26% of electricity were used for sewage treatment process and sludge treatment process, respectively. Another selected STP showed that 50% of electricity was consumed for aeration, and 32% and 9% of electricity was consumed by the facility operation and pump station, respectively. Only 8% of electricity was used for anaerobic digestion. Therefore, it was assumed that an average of 17,511 kW h (17%) was used for sludge treatment by anaerobic digestion. Allocation was followed by flux rate, therefore, it was estimated that 3,502 kW h, which was one fifth of electricity, was consumed for anaerobic digestion per day. The facility treated around 196 tonnes of food waste a day; therefore, it was assumed that daily electricity for anaerobic digestion per 1 tonne of food waste was 17.87 kW h. In conclusion, 18.88 kW h, which was already calculated for pretreatment of 1 tonne of food waste, and 17.87 kW h, which was calculated by allocation here for anaerobic digestion, were counted for this scenario. The sum of two electric values, 36.75 kW h, was estimated as electricity usage for 1 tonne of food waste treatment by co-digestion.
We calculated that 36.75 kW h of electricity was used and 8.92 kg of biogas was produced for the treatment of 1 tonne of food waste by allocation. Steam, which was converted from the captured biogas, was used for digester warming. The environmental impact of the recycled processed water and steam was not considered because it was already counted on the system. It was estimated that 141 kg of sludge was discharged from 1 tonne of food waste. The gate-to-gate data is shown in Table2.
Dryer-incineration. Reduction efficiency was 75.9% of food waste; therefore, the water containing food wastes after drying with the use of a garbage dryer reduced in weight from 1 to 0.24 tonne (Table2). Volume was also reduced; however, ultimate constituents were not changed. Produced fly ash and bottom ash were transported to the Sudolwon landfill site for final disposal. As the ash from complete incineration contained no organic carbon when it arrived at the landfill, the burial of ash would generate no landfill gas (LFG). Carbon was not completely removed by incineration, but it can be assumed that the landfill of incinerator ash resulted in no LFG emissions[24]. Therefore, we estimated that GHG from the ash at the landfill was not produced; however, GHG by transportation was added. Around 364 kg of CO2, 7.99 kg of CH4, and 2.75 kg of N2O were produced. The produced CO2 was non-biogenic and was produced from energy consumption. However, CO2 by incineration of food waste was not counted as GHG because it was biogenic in origin. The caloric value (wet based) of 1 tonne of dried food waste was 2.92 Gcal, and energy recovery would be 9,047 MJ if the average recovery rate of 74% was applied (Table4). Therefore, it was estimated that around 657 kg of GHG was reduced.
Table 4
Energy recovery rate of the dryer-incineration
Disposal stage
The distances of transportation from facilities to final disposal sites were calculated. It was assumed that the screenings were landfilled and the sludge was incinerated. The distances from each facility to the landfill site and resource recycling (incineration) facility were calculated respectively by GIS. One-way distances were 46 and 11 km for anaerobic digestion, 47 and 12 km for co-digestion from the facility to the landfill and resource recycling facility, respectively, and the distance was approximately 50 km between the landfill and incineration plant.
Life cycle impact assessment analysis
Characterized impact on the global warming impact category was assessed using GWP for 100 years[25].
Anaerobic digestion
The indicator, GWP, represented that the treatment and disposal stages mainly contributed to the environment. Directly discharged CO2 for digestion was biogenic in origin; therefore, it was not counted as GHG. In addition, produced methane was captured. However, the treatment stage still contributed to GWP the most because chemicals and electricity usage were sources of GHG. Screenings were finally landfilled, and wastewater was treated chemically or biologically. These final disposals also contributed to global warming. This anaerobic digestion system treated 1 tonne of food waste and produced 0.07 tonne of biogas. The biogas was converted to 244 kW h of electricity and 204 kg of steam. About 68.23 kW h of electricity was primarily used for the facility operation. Surplus electricity of 175 kW h was sold at Korea Power Exchange. Produced steam was used for warming a digester. About 110 kg of biosolids was also produced. It was sold as a raw material of compost. For this scenario, 211 kg of CO2-eq of GHG were discharged from 1 tonne of food waste. Around 8.8, 0.7, 131, and 70.1 kg of CO2-eq of GHG were produced in the collection, transportation, treatment, and disposal stages, respectively (Table5).
Table 5
GHG emissions (kg CO
2
-eq/tonne) at each stage
Co-digestion
The indicator, GWP, represented that disposal stages mainly contributed to the environment. Screenings, which were produced for the pretreatment process, were disposed by landfilling. LFG was the major reason of global warming in the co-digestion scenario. For this scenario, 259 kg of CO2-eq of GHG were discharge from 1 tonne of food waste. Around 8.8, 0.5, 37.7, and 212 kg of CO2-eq of GHG were produced in the collection, transportation, treatment, and disposal stages, respectively (Table5). Landfilling of the screenings was contributed to GWP the most in this scenario.
Dryer-incineration
Electricity was consumed for the garbage dryer operation. Produced CO2 in the incineration process was biogenic in origin; therefore, it was excluded from the GHG counting. In addition, combusted waste heat was recovered and converted to steam. This system treated 1 tonne of food waste and produced 2.16 Gcal of heat energy. It was sold and used in the residential area for room heating with hot water heating. For this scenario, 342 kg of CO2-eq of GHG were discharged from 1 tonne of food waste. Around 321, 8.8, 1.1, 10.3, and 0.7 kg of CO2-eq of GHG were produced in the discharge, collection, transportation, treatment, and disposal stages, respectively (Table5).
Avoided impact analysis
Avoided impact was analyzed in order to calculate efficiencies of GHG reduction relevant to alternatives. The commercial production of steam and electricity was analyzed by LCA. The result showed that 0.04 and 0.50 kg of CO2-eq of GHG were produced when 1 MJ of steam and 1 kW h of electricity were manufactured, respectively. About 178 kg of CO2-eq by anaerobic digestion and 657 kg of CO2-eq by dryer-incineration from 1 tonne of food waste, respectively, were calculated by avoided impact analysis. In the case of dryer-incineration, GHG was reduced by waste heat recovery; however, GWP was still high through the full stages. It was because a garbage dryer operation required electricity usage.
This comparison of three different food waste disposal scenarios found that the GWP of each scenario before the offset of generated electricity is around 211 kg of CO2-eq for anaerobic digestion, 259 kg of CO2-eq for co-digestion, and 342 kg of CO2-eq for dryer-incineration from 1 tonne of food waste, respectively, all based on the wet weight. GWP of dryer-incineration in the treatment stage was low because CO2 production from incineration was biogenic in origin; however, GWP in the discharge stage was the highest because electricity usage for a waste dryer operation in the discharge stage contributed to GWP. Anaerobic digestion, co-digestion, and dryer-incineration scenarios were higher in GWP in that order.
Around 110 kg of biosolids, 244 kW h of electricity, and 0.2 tonne of steam were produced by the anaerobic digestion scenario. Biosolids were used as a raw material in the composting industry. Steam was not included in analyzing the by-product because it was recycled in the system boundary. Around 70 kW h of produced electricity was used for facility operation; therefore, 174 kW h of electricity could be sold in a market. Incineration scenario produced 2.16 Gcal of waste heat. It was converted to steam and supplied to residential areas. Steam of the co-digestion scenario was recycled within the system boundary. Therefore, steam production was excluded in the list of by-products. Environmental credit was 33 kg of CO2-eq for anaerobic digestion and −315 kg of CO2-eq for dryer-incineration from 1 tonne of food waste, respectively (Table6).
Table 6
Alternative effects of scenarios
In the present study, the indicator, GWP, was used for environmental impact analysis. We analyzed six impact categories, ADP, AP, EP, GWP, ODP, and POCP for the scenarios as shown in Table7. The results show that GWPs of all scenarios affect the environment the most although POCP of dryer-incineration is high.
Table 7
Environmental impact for each impact category of scenarios
We analyzed the B/C ratio of the scenarios and found that it was 0.11 for anaerobic digestion and 0.26 for dryer-incineration in the previous research[26]. We could not find it for co-digestion because the by-product of co-digestion was recycled within the system boundary.