Climate change impact assessment of various wastewater treatment facilities: A case study in Bandung City, Indonesia

Untreated wastewater discharge poses notable environmental and public health problems, among others the production of CH4, CO2, and N2O gas emissions contributing to climate change phenomenon. The implementation of wastewater treatment facilities can minimize the GHG emissions resulting from the effluent. However, different technologies and systems perform different results. Using the Life Cycle Assessment method of analysis, this study took Bandung City as a case study and selected the seven most common wastewater treatment facilities found in the city. The results showed that individual hybrid systems with the combination of the conventional septic tank, conveyance by septage truck, and sludge treatment in the Septage Treatment Plant (HS1) and individual on-site system with the conventional septic tank (OFS1) produced substantial global warming impact, resulting in 0.78 and 0.71 kg CO2 eq respectively per 1 m3 treated wastewater. This amount was sixfold that of communal on-site systems with biofilter tanks (OFS2) and 2-3 times larger than other remaining systems. On the other side, the system that produced the lowest amount of GHG emissions performed by OFS2 (0.11 kg CO2 eq).


Introduction
In recent years, climate change has inevitably become a prominent phenomenon, and it is expected to accelerate further in the near future.Wastewater discharge to the environment adds to the existing exacerbated issue of climate change.In 2014, the IPCC recorded that waste and wastewater account for around 3% of global greenhouse gas (GHG) emissions [1].Releasing wastewater to the surroundings generates negative impacts among others Methane (CH4), Carbon dioxide (CO2), and Nitrous oxide (N₂O) gas emissions contributing to climate change phenomenon.Additionally, untreated wastewater discharge poses notable environmental and public health problems.Around 44% of generated wastewater is still discharged back into the environment without being well treated [2].
Wastewater treatment facilities can minimize the GHG emissions resulting from the effluent.However, different technologies and systems perform different results.Choosing the most sustainable wastewater treatment infrastructure is essential because all wastewater treatment and various technologies generate direct and indirect emissions [3].Different processes and options of technologies IOP Publishing doi:10.1088/1755-1315/1353/1/012002 2 used in wastewater treatment result in different impacts and levels of climate change.Accordingly, a comparison study of carbon footprint assessment and evaluation of impact quantification for system and technology options in wastewater treatment are crucial factors to be taken into consideration.
A study conducted by Kim et al., (2019) compared the effects of untreated and treated wastewater effluent on the GHG emissions downstream of Mekong River, Cambodia, and Han River, Korea [4].Their experiment showed a significant distinction in GHG emissions (measured by biodegradable dissolved organic carbon or BDOC and CO2 concentration) from the effluent generated from treated and untreated wastewater.The GHG emissions observed from untreated wastewater were 31 times higher in sewage than in treated wastewater.This indicates that GHG emissions could be reduced substantially by constructing and implementing wastewater treatment facilities.
Nevertheless, GHGs are still emitted during wastewater treatment.It includes CO2 from oxidation with aerobic process, CH4 from anaerobic processes that can take up to 3-19% of global anthropogenic methane emissions, and N₂O that is being associated with nitrification and/or denitrification (NDN) processes [5].These GHGs contributes to climate change when released into the atmosphere.Therefore, in addressing that challenge in the wastewater context, some mitigations strategies should be implemented, among others by choosing the most efficient treatment processes so that it can enhance treatment efficiency to reduce GHGs emissions in its wastewater treatment process.
Globally, major development has been made regarding wastewater treatment and management since ancient times.Nonetheless, providing a reliable, adequate, affordable, and environmentally friendly system is still a challenge for lower-income countries.To achieve their sustainable agenda, many lower-income countries embarked on a series of initiatives to reform their sanitation sector policies [6].This reform is translated into the emergence of a diverse practice in water and sanitation systems, generating the rise of multiple hybrid combinations of service delivery options [7].
Indonesia, with Bandung City as a case study, serves as a good example of a low-middle-income country with a diverse range of wastewater treatment facilities.There are at least seven types of wastewater treatment facilities, ranging from: (1) Off-site Wastewater Treatment Plant (WWTP) system with stabilization ponds or ONS1; (2) Individual on-site system with conventional septic tank or OFS1; (3) Communal on-site system with biofilter tank (OFS2); (4) Communal on-site system with anaerobicaerobic baffled reactor (AABR) or OFS3; (5) Individual hybrid System with OFS1 + septage collection service + sludge treatment plant or HS1; (6) Communal hybrid System with OFS2 + septage collection service + sludge treatment plant or HS2; and (7) Communal hybrid System with OFS3 + septage collection service + sludge treatment plant or HS3.
This study aims to assess the most climate-friendly wastewater treatment facilities that are readily available and tailored made with the Indonesian context by performing GHGs emission calculation and comparison among seven wastewater treatment facilities found in the case study area.Assessing the impact resulted and recognizing the most climate-friendly facility could promote towards broader context and ultimate goal of sustainable infrastructure planning.

2.
Method This study utilized the Life Cycle Assessment (LCA) method to assess the climate change impact translated by global warming potential.LCA is a comprehensive analytical tool to calculate environmental impacts of products, process, and/or services from raw material acquisition, through production and use, to waste management [8] [9].
The functional unit for this research was 1 m 3 of treated wastewater generated from households and the time frame was 30 years.The system boundary comprised construction, operation (treatment), maintenance, and transportation (collection) phase as shown in the following Figure 1.Global warming potential (GWP) was calculated from GHG emissions in kg CO2 eq for each phase of the treatment system.This research implemented a modelling approach that employs the technical principles and standards of seven systems to be assessed: (1) Off-site Wastewater Treatment Plant (WWTP) system with stabilization ponds or ONS1; (2) Individual on-site system with conventional septic tank or OFS1; (3) Communal on-site system with biofilter tank (OFS2); ( 4 No spatial distribution map can be generated from the analysis as there is no spatial pattern in the location distribution of the wastewater treatment facility types, meaning that it is evenly distributed throughout Bandung City.Those seven types of facilities mentioned earlier are the most common facilities that can be found in the study area.Therefore, those facilities were chosen purposively.

3.
Study area context and characteristics of wastewater treatment systems Bandung City was selected to represent Indonesia (a lower-income country) in this research.As the capital of West Java Province, Bandung is one of Indonesia's metropolitan cities with a population of 2,444,160 and a population density of 14,608.57people per km 2 in 2020.As mandated in the Presidential Regulation 45/2018 regarding Spatial Plan of the BMA, BMA consist of five regions (West Bandung Regency, Bandung Regency, some parts of Sumedang Regency, Bandung City, and Cimahi City) [10].In the context of wastewater treatment facilities, typical characteristics of the infrastructure in the BMA are listed in the following Table 1.Up until this paper is written, there is only one off-site or centralized system found in the BMA, located in Bojongsoang District, Bandung Regency (ONS1) that has the coverage area of some parts in Bandung City.Despite of the location that is outside of the Bandung City, this facility has the treatment capacity that can only serves for around 15% of households in Bandung City in 2020.Referring to the Presidential Regulation 45/2018, there should be ideally more centralized systems distributed across the BMA and it is planned to have four systems in West Bandung Regency, ten systems in Bandung Regency, one system in Sumedang Regency and Cimahi City, and eventually two systems in West Bandung City.
Bandung City is the core of the Bandung Metropolitan Area and serves as the center of the metropolitan, leading in the infrastructure system including wastewater treatment facility.Many pilot projects of sanitation system improvement led this city to have various sanitation and domestic wastewater treatment systems.A survey conducted by the Statistics Bureau of Bandung Municipality in 2020 found that only 12.78% of households were connected to an off-site system (Wastewater Treatment Plant) using the sewage network and 41.80% of households already installed or were connected to a septic tank.Almost 50% (45.41%) of households still practiced open defecation.Open defecation refers to directly discharging human excreta into the environment, mostly into the surface water.

4.
Results and discussion

Inventory data
The following are the inventory data per system where the GHG emissions are produced.

4.1.1.
Off-site system with WWTP (ONS1).ONS1 treated the wastewater with a waste stabilization pond system.The main elements known in the construction phase consisted of cast iron, concrete, plastic PVC, reinforced steel, and stainless steel.In the treatment phase, this system required electricity input of 0.04 kWh/m3 and emitted CH4 of 0.206E-02 kg/m3.As the system is an off-site facility, it requires an electricity input of 0.0778 kWh/m3 to transport the wastewater.

4.1.2.
Individual on-site system with conventional septic tank (OFS1).OFS1 is a materialintensive system in terms of the use of construction material per m3 of treated wastewater as it is an individual on-site system covered on a household scale.It is enormously made of brick, sand, cement, and gravel split stone.It also used a small amount of cast iron, plastic PVC, and reinforced steel for its construction.No electricity is needed during all phases of this system.This system emitted 0.366E-02 kg/m3 of CH4 into the air during the treatment phase and this resulted in the highest CH4 emission compared to other systems.

4.1.3.
Communal on-site system with biofilter tank (OFS2).Same as OFS1, OFS2 required no electricity for all phases, and it emitted 0.217E-02 kg/m 3 from its treatment process.In terms of construction material, OFS2 was made of brick, cast iron, cement, concrete, plastic fiberglass, HDPE, PVC, reinforced steel, and sand.

4.1.4.
Communal on-site system with Anaerobic Aerobic Baffled Reactor (AABR) (OFS3).OFS3 shared similar characteristics with OFS2 but it was equipped with an additional treatment process of aerobic.Therefore, the need to generate oxygen for the aerobic process had the consequence of requiring electricity (0.13 kWh/m 3 ) in the treatment process.CH4 was emitted during the treatment phase with the amount of 0.217E-02 kg/m 3 .DS3 was constructed of brick, cast iron, cement, concrete, gravel, split stone, plastics (PET, PP, and PVC), reinforced steel, and sand.

OFS1, OFS2, OFS3 + HS (conveyance by septage truck + sludge treatment in Septage Treatment Plant
).The hybrid system is basically a combination of an off-site and on-site system.The on-site systems consisted of OFS1, OFS2, and OFS3 combined with the off-site systems in the forms of sludge collection and transportation as well as the Septage Treatment Plant (STP).Hybrid system, particularly for the part of the off-site system utilized Moving bed biofilm reactor (MBBR) for its treatment process.Therefore, the sludge collection as well as the STP required electricity for the maintenance phase with the amount of 4E-07 kWh/m 3 .The sludge transportation used diesel fuel as an input of 6.39E-03 l/m 3 .There were two types of emissions which were CH4 in the treatment phase (HS1: 0.137E-02 kg/m 3 , HS2 and HS3: 0.082E-02 kg/m 3 ) and N2O (5.25E-04 kg/m 3 ) in the maintenance phase.

4.2.
Global warming potential HS1 and OFS1 produced substantial global warming impact, resulting in 0.78 and 0.71 kg CO2 eq respectively per 1 m 3 treated wastewater.This amount was sixfold that of OFS2 and 2-3 times larger than other remaining systems.OFS1 and HS1 are the systems that use intensive clay brick construction material.The small service coverage of this system caused these systems to require more construction material per functional unit.This intensive usage of clay brick as a construction material accounted for the largest contribution to global warming.The burning phase of the red bricks production accounted for the largest amount of CO2 emissions [11].
The operational (treatment) phase was also a major contributor in ONS1, OFS2, and OFS3 and the second factor in OFS1.CH4 emission was seen as the main attribution.This CH4 was attributed to the anaerobic process during wastewater treatment.OFS1 was found to be the system that does not practice regular desludging, thus leading to sludge accumulation inside the septic tank.The more sludge inside the system during a long period of time, the more CH4 is emitted.The same process and contributor occurred to HS1 and ONS1 with lower CH4 emission.HS1 worked with the same process as OFS1, yet as a hybrid system, it is coupled with regular septage collection once in 2-5 years, meaning that the more intense sludge is emptied, the less CH4 produced.Frequent sludge removals from the system can make additional CH4 removal up to 62.5% [12] [13].However, the hybrid systems (the on-site systems coupled with MBBR) also resulted in consequences to the global warming potential, as the nitrifier denitrification process caused by nitrite accumulation in the MBBR emitted N2O [14].Influent transportation (collection) from the service areas to the WWTP made use of an off-site sewer network and six centrifugal pumps.The operation of those pumps in ONS1 required a considerable amount of electricity usage.This electricity is generated by coal-fuelled power.Carbon is a major element of coal and in principle is the source of heat.The coal combustion process in electricity generation forms CO2.CO2 emission in the ONS1 transportation phase accounted for almost 40% of global warming potential in ONS1.
An emission of a GHG will raise GHG concentration in the atmosphere, leading to increased radiative forcing, which in turn will eventually increase the global average temperature [13].The increase in global temperature will alter biome distribution and river discharge, which ultimately will cause damage to the ecosystem with the disappearance of terrestrial species and marine species [13].With regard to human health damage, the increase in global temperature will increase the risk of flood and diseases for example malnutrition, malaria, and diarrhea [13].

Conclusion
Climate change impact in this study was translated by the global warming potential calculated from GHG emissions.HS1 and OFS1 produced substantial global warming impact, resulting in 0.78 and 0.71 kg CO2 eq respectively per 1 m 3 treated wastewater.GHG emissions in CO2 eq in all systems were contributed mainly by electricity usage.The electricity generated in Indonesia primarily came from the coal combustion process.The burning phase in producing material constructions was also responsible for the CO2 emissions.CH4 emission to air in the operational or treatment phase and N2O in the maintenance phase for the GHG emissions also accounted for the GHG emissions.Meanwhile, the system producing the lowest GHG emissions produced OFS2 (0.11 kg CO2 eq).It can be concluded that OFS2 is the most efficient or environmentally friendly regarding global warming potential as this system did not require electricity in all phases.Using this treatment will also bring advantages for the neighborhood areas, not only because it will reduce electricity needs but also because it will be an excellent example for other areas to follow.
) Communal on-site system with anaerobicaerobic baffled reactor (AABR) or OFS3; (5) Individual hybrid System with OFS1 + septage collection service + sludge treatment plant or HS1; (6) Communal hybrid System with OFS2 + septage collection service + sludge treatment plant or HS2; and (7) Communal hybrid System with OFS3 + septage collection service + sludge treatment plant or HS3.We collected a set of data acquired from primary and secondary sources.Primary data were gathered through in-depth interview and site observation to the Bojongsoang WWTP for obtaining the data of ONS1.Secondary data in the form of technical guidelines, budget plans, and detailed engineering designs for each type of wastewater treatment facility in the city were mainly used to translate the systems into technical data.The secondary data were obtained in 2021 from Land, Human Settlement, and Public Housing Agency (DPKP3) of Bandung City Government and Technology Center for Sanitation, the Ministry of Public Works and Housing, Republic of Indonesia for six systems of OFS1, OFS2, OFS3, HS1, HS2, and HS3.

Table 1 .
Characteristic of wastewater treatment facilities.

Table 2 .
Global warming potential resulted from each type of facility/system (kg CO2 eq/m 3 ).