Direct greenhouse gases from domestic wastewater treatment plant in Bandung Regency, West Java, Indonesia

Wastewater treatment contributes direct greenhouse gas (GHG) emissions in the atmosphere. The study aims to estimate the amount of methane (CH4) and carbon dioxide (CO2) emitted from domestic wastewater treatment plants (WWTP) in Bandung Regency throughout the years 2019-2022. Wastewater treatment processes in the WWTP consist of anaerobic, facultative, and maturation ponds. Data series of water quality from each process were collected on a monthly basis from years 2019 to 2022 to estimate direct emissions. Emission factors used in this study were derived from IPCC Guidelines and using the Tier 1 method. The highest GHG emission from WWTP Bojongsoang was about 513.44 tonCO2e in December 2021. The monthly average CO2e emissions from year to year increased about 40.12%. The average GHG emission from WWTP Bojongsoang from the year 2019 to 2022 respectively was about 81.33 tonCO2e/month, 135.21 tonCO2e/month, 229.26 tonCO2e/month, and 193.89 tonCO2e/month. Overall, the percentage of CO2e emission from anaerobic pond was about 93.57%, from facultative pond was about 4.86%, and from maturation pond was about 1.57%. The highest CO2e emissions occurred in the anaerobic pond along with the highest COD reduction also took place in the anaerobic pond.


Introduction
The waste sector contributes greenhouse gas (GHG) emissions from two main activities, namely solid waste treatment and wastewater treatment.One of the GHG sources from the waste sector is domestic wastewater treatment, accounting for 19.28% of the total GHG in the year 2021 from the waste sector in Indonesia [1].Wastewater collection systems.wastewater treatment plants and discharging wastewater to the environment lead to direct GHG emissions from its biological processes and indirect emissions due to energy consumption [2].Direct and indirect GHG emissions were influenced by various factors, such as used treatment processes and treatment scales.Direct GHG emission has a positive correlation with the amount of pollutants removed [3].Based on IPCC methodology, only CH4 and/or N2O are calculated, excluding CO2 emissions of biological treatment processes as these are generally derived from biogenic organic matter in human excreta or food waste and should not be included in the national total.However, some research has shown that a significant amount of fossil CO2 is directly emitted from WWTPs, and assuming that all direct CO2 emissions are biogenic may underestimate GHG emissions [4].
The wastewater treatment plant (WWTP) Bojongsoang is located in Bandung Regency, West Java Province, Indonesia.It has operated since 1992 and its service population is about 500,000 persons.The unit processes in WWTP Bojongsoang consist of anaerobic, facultative, and maturation ponds.Anaerobic ponds were the main source of on-site emissions with 66% of the total contribution and 33% for facultative ponds, followed by the energy consumption of the pumping station as off-site GHG emissions [5].Previous study showed that GHG emission from WWTP Bojongsoang from Set A was about 38.26 tonCO2e [6] based on water quality data taken in April 2018 [7].There are still limited papers that calculate GHG emissions from WWTP in Indonesia.Developing countries, such as Indonesia, still face several problems in quantifying of GHG emissions.The data collection of GHG emissions are essential to define the mitigation strategy or to utilize the GHG emission as the source of energy.In this study, a more detailed analysis was conducted to estimate GHG emissions from WWTP Bojongsoang based on water quality data on a monthly basis throughout the year 2019 to 2022.Due to data limitation, the system boundary in this study is limited only to estimate GHG emission from Set A. This study excludes GHG emissions from Set B, from sludge, and also from electricity consumption.The results of this study are expected can be useful as consideration to define GHG emissions mitigation strategies in WWTP Bojongsoang.

Research Method
WWTP Bojongsoang has two compartments of treatment, namely Set A and Set B. Each stage consist of three anaerobic ponds, two facultative ponds, and two maturation ponds.The average inflow of WWTP Bojongsoang was about 79,953 m 3 /day, with the highest inflow occurring in December.The treatment stages of WWTP Bojongsoang are presented in Figure 1.

Figure 1. Treatment stages in WWTP Bojongsoang
Methane (CH4) and carbon dioxide (CO2) were calculated based on the load of COD removed from each stage of treatment.In this study, GHG emission was calculated by using these equations.
Where: MCOD treated,i = load of COD treated from each stage of treatment (kg) EF CH4,i = emission factor for CH4 from each stage of treatment (g CH4/kg COD) EF CO2,i = emission factor for CO2 from each stage of treatment (g CO2/kg COD) ∆COD i = concentration of COD treated (mg/L) Q = inflow (m 3 /day) Tier 1 was applied as the method in this study.Emission factors were derived from IPCC Refinement in 2019 and presented in  27.9 Carbon dioxide (CO2) 1

Results and Discussion
Water quality data from each stage of treatment in Set A were collected throughout the year 2019 to 2022 on a monthly basis and the data was sourced from Tirtawening of Municipal Waterwork.The concentration of COD in the inlet was increased from the year 2019 to 2022 with the average increasing by about 13% per year.The highest average COD concentration in the inlet was about 375.13 mg/L in 2021.The efficiency of WWTP Bojongsoang showed good value that increased by about 15% per year.The highest average efficiency of COD concentration was about 84.41% in 2022.In this study, the load of treated COD from each stage of treatment was calculated by computing COD concentration reduction from each stage.WWTP Bojongsoang has two compartments of treatment, namely Set A and Set B. Each stage consists of three anaerobic ponds, two facultative ponds, and two maturation ponds.The highest load of treated COD occurred in the anaerobic pond (71.25%), followed by maturation ponds (14.83%) and facultative ponds (13.92%).
The reduction of COD concentration in the year 2019 occurred about 63.26% in anaerobic ponds, especially from anaerobic pond 3. Anaerobic pond 3 emitted the highest GHG with an average of 75.60 tonCO2e/month in 2019.%.The highest GHG emissions occurred in December meanwhile the lowest occurred in April.The average percentage of GHG emission in 2019 from anaerobic ponds 1317 (2024) 012006 IOP Publishing doi:10.1088/1755-1315/1317/1/0120064 was about 92.25%, from facultative ponds was about 3.96%, and from maturation ponds was about 3.09GHG emissions from each stage of treatment in the year 2019 are presented in Figure 2.

Figure 2. GHG emission of WWTP Bojongsoang in 2019
The reduction of COD concentration in the year 2020 occurred about 69.51% in anaerobic ponds and the highest COD reduction took place in anaerobic pond 3. The average of GHG emissions from anaerobic pond increased to 125.55 tonCO2e/month in 2020.Similar to year 2019, the highest CO2e still occurred in December.The amount of GHG emissions were doubled in December 2020 compared to December 2019.Based on the discharge data, the amount of discharge also increased significantly in December 2020.Higher discharge leads to higher amount of pollutant loading.The average percentage of GHG emission in 2020 from anaerobic ponds was about 92.85%, from facultative ponds was about 5.59%, and from maturation ponds was about 1.56%.GHG emissions from each stage of treatment in the year 2020 are presented in Figure 3.The reduction of COD concentration increased to 78.0% in anaerobic ponds in year 2021.Anaerobic pond 3 emitted twice as much in 2020 with an average of 217.05 tonCO2e/month.The highest GHG emissions also still occurred in December while the amount of GHG emissions rose significantly to 513.4 ton tonCO2e.The average percentage of GHG emission in 2021 from anaerobic ponds was about 94.68%, from facultative ponds was about 4.57%, and from maturation ponds was about 0.75%.GHG emissions from each stage of treatment in the year 2021 are presented in Figure 4.

Figure 4. GHG emission of WWTP Bojongsoang in 2021
The reduction of COD concentration in the year 2022 occurred about 74.90% from anaerobic pond.Anaerobic pond 3 emitted the highest GHG with an average of 181.83 tonCO2e/month.The average percentage of GHG emission in 2022 from anaerobic ponds was about 93.78%, from facultative ponds was about 5.32%, and from maturation ponds was about 0.90%.GHG emissions from each stage of treatment in the year 2022 are presented in Figure 5.The highest GHG emission from WWTP Bojongsoang was about 513.44 tonCO2e in December 2021.The monthly average CO2e emissions from year to year increased about 40.12%.The average GHG emission from WWTP Bojongsoang from the year 2019 to 2022 respectively was about 81.33 tonCO2e/month, 135.21 tonCO2e/month, 229.26 tonCO2e/month, and 193.89 tonCO2e/month.Overall, the percentage of CO2e emission from anaerobic pond was about 93.57%, from facultative pond was about 4.86%, and from maturation pond was about 1.57%.Some alternatives can be applied to reduce GHG emissions from wastewater treatment.The biogas generated from wastewater treatment can be used as a natural gas substitute or converted into heat and electricity.Anaerobic treatment for waste-to-biogas conversion systems is the most popular technology for energy recovery from wastewater treatment [8].The combined anaerobic and aerobic system followed by biogas recovery for the source of energy also can reduce GHG emissions up to 61.72% [9].Many factors that influenced GHG emissions such as socio-economic factors such as GDP, household consumption expenditure, household food consumption expenditure, and population were found to highly correlate with urban CH4 emissions from municipal WWTPs.Such findings suggest that controlling the residential discharge of municipal wastewater is a promising avenue for control of wastewater CH4 emissions [10].

Conclusion
The concentration of COD in the inlet of WWTP Bojongsoang increased by about 13% per year from 2019 to 2022.The efficiency of treatment in WWTP Bojongsoang also increased about 15% per year with the highest efficiency occurred in the year 2022.The reduction of COD concentration mostly occurred in anaerobic ponds with the percentage of COD reduction ranging from 63.26% -78.00%.The monthly average GHG emission from 2019 to 2022 was about 81.33 tonCO2e/month, 135.21 tonCO2e/month, 229.26 tonCO2e/month, and 193.89 tonCO2e/month respectively.Overall, the percentage of CO2e emission from anaerobic pond was about 93.57%, from facultative pond was about 4.86%, and from maturation pond was about 1.57%.It is suggested to estimate the GHG emission from WWTP Bojongsoang by using the higher method such as Tier 2 or Tier 3. Different types of unit process used in WWTP affect the amount of GHG emission.It is essential to conduct more studies to determine a suitable emission factor value for each WWTP considering WWTP configuration and regional weather conditions.Estimating the GHG emission from indirect sources

Table 1 .
Emission factor for each stage of treatment GHG emissions were accounted as CO2 equivalent (CO2e) in ton by multiplying the GHG emission of CH4 and CO2 with the value of Global Warming Potential (GWP).The values of GWP were derived from the Sixth Assessment Report of IPCC and presented in Table2.

Table 2 .
Emission factor for each stage of treatment

Table 3 .
COD concentration at the inlet and outlet of WWTP Bojongsoang