Figure 1 shows the overall research framework and methodology for the four sludge treatment scenarios and the four treatment plant construction options.
System boundary and assumptions
The boundary of the system is that of the cradle-to-grave life cycle, encompassing construction, wastewater collection and transport by pipeline to treatment plants, treatment operation, system maintenance and sludge management. The system limit excludes the demolition of the plant due to the unavailability of Thailand-specific data. The functional unit (UF) is a cubic meter (m3) treated effluents. Effluent meets country regulatory body effluent standards requirements23. Data on the characteristics of tributaries and effluents belong to the years 2016-2017.
Bangkok, the Thai capital, currently has eight centralized sewage treatment plants (i.e. Bangsue, Chatuchak, Chongnonsi, Dindaeng, Nongkaem, Rattanakosin, Sipraya, and Thungkru wastewater treatment plants) and 12 decentralized wastewater treatment plants (i.e. Bangbua, Bangna, Bonkai, Huai-khwang, Hua-mark, KhlongChan, KhlongToei, RamIntra, RomKlao, ThaSai, and Tungsonghong I and II).
Figure 2 shows two scenarios of centralized treatment (C) of sludge: scenarios of dewatering C and treatment with fertilizer C (decomposition) and two scenarios of decentralized treatment (D) of sludge: scenarios of treatment with dewatering D and fertilizer D ( decomposition).
Life cycle inventory (LCI) and sludge treatment
The 2016-2017 average inventory data of the centralized (i.e. C dewatering and C fertilizer) and decentralized (D dewatering and D fertilizer) sludge treatment scenarios are also provided respectively in Fig. Information (IS)24. In the analysis, this study focuses on the eight existing centralized wastewater treatment plants and seven (out of 12) decentralized wastewater treatment plants due to either the temporary closure of the remaining decentralized wastewater treatment plants for renovation or a lack of data. The average capacity of centralized and decentralized wastewater treatment plants is 139,000 and 2,357 m3 per day, respectively. The useful life of the treatment plant and sewer systems is assumed to be 30 years.
Air emissions are calculated in accordance with guidelines from the Intergovernmental Panel on Climate Change and the United States Environmental Protection Agency.25.26. For centralized treatment plants, sludge as a co-product of wastewater treatment is digested anaerobically for biogas and broken down for fertilizer. At the same time, for decentralized wastewater treatment plants, sludge is treated by dehydration for dry organic waste. Due to the unavailability of data, the anaerobic digestion operation is excluded from this study.
Dewatering is a mechanical process to separate solid parts from liquid parts to reduce the moisture content of sludge27. In this study, all centralized and decentralized wastewater treatment plants are equipped with the thickening system to remove moisture from sludge up to 3%.27. After thickening, for the centralized treatment of wastewater, the sludge is transported by truck to Nongkeam Sewage plant to be converted to biogas and fertilizer (ie decomposition). Meanwhile, for decentralized wastewater treatment, the sludge is dried in the sun and used to fill the land (i.e. dewatering).
During decomposition, 70% of the sludge and 30% of the organic matter is composted by the windrow method to improve the quality of the compost24. According to Seleiman et al.28the sludge contains 25.77, 12.98 and 3.40 g of nitrogen, phosphorus and potassium per kg of dry matter.
This current research is based on LCI consequential modeling. For dewatering, sludge is used to fill the land, while sludge is used as a substitute for chemical fertilizers for decomposition.
Life Cycle Impact Assessment (LCIA)
LCI of four sludge treatment scenarios (dewatering C, fertilizer C, dewatering D, fertilizer D) are assessed the environmental impacts using SimaPro’s Stepwise2006 based ecoinvent database. SI Table SI-2 provides details of the median impact categories in the Stepwise2006 method. This research addresses fourteen environmental impacts, including Human Toxicity (Carcinogens), Human Toxicity (Non-Carcinogens), Aquatic Ecotoxicity, Terrestrial Ecotoxicity, Global Warming (Fossil), Respiratory Organics, Respiratory Inorganics , photochemical ozone, acidification, aquatic eutrophication, eutrophication, occupation of nature, non-renewable energy and mining. All environmental impacts are grouped into three categories of damage, namely impacts on the ecosystem, human well-being and resource depletion. Further, the impact on the ecosystem is categorized into atmospheric, lithospheric, and hydrospheric impacts.
Environmental costs are determined by Stepwise monetary weighting factors detailed in SI Table SI-329.30 and converted into the Thai currency of 2020 (THB2020)31 using purchasing power parity (PPP) (i.e., PPP$2,002 and PPP2,002 baht) and Thailand’s Gross Domestic Product (GDP) Deflator Index for 2002 and 2020. Details of currency conversion are provided in SI Table SI-4.
According to ISO14044:20065, the sensitivity analysis aims to assess the reliability of the final results. First, electricity consumption is the main contributor to environmental impacts32, all the sludge treatment scenarios (dewatering C, fertilizer C, dewatering D, fertilizer D) are thus assumed to succeed in reducing electricity consumption by 10%, 20% and 30%. Furthermore, evidence shows that the choice of LCIA method influences environmental impact results.33. Accordingly, this research also performs the sensitivity analysis of the four sludge treatment scenarios (Dewatering C, Fertilizer C, Dewatering D, Fertilizer D), given reduced electricity consumption by 10%, 20% and 30% , using CML-IA baseline and ReCiPe methods at the intermediate level, in addition to Stepwise2006.
Life cycle cost analysis (LCCA)
In the LCCA, the source of income (or cash inflow) is the sale of decomposed sludge fertilizer at a price of 2 THB/kg. For expenditures (or cash outflows), construction costs, including collection system, sewage plant and dewatering system costs, are gleaned from publicly available data and previous publications.34,35,36. Operation and maintenance (O&M) costs include costs for electricity, water supply, chemical reagents, sludge treatment and administrative overhead, e.g. salaries, management fees (Department of Drainage and Sewerage24.
Construction and O&M costs are converted to 2020 Thai Baht (THB2020) based on purchasing power parity (PPP) and the gross domestic product (GDP) deflator index31. The PPP and the GDP deflator index are used to reconcile the differences between the three currencies (USD, EUR and Thai Baht) and several time periods.
The LCA of four sludge treatment scenarios (Dewatering C, Fertilizer C, Dewatering D, Fertilizer D) involves their respective cash inflows and outflows and environmental costs. In this research, the most profitable sludge treatment scenario has the largest LCCA surplus or the smallest LCCA deficit.
Planned wastewater treatment plants
The current total capacity of centralized and decentralized sewage treatment plants in the capital Bangkok is 1,112,000 and 25,000 m3 per day, respectively. The new centralized wastewater treatment plant in min buri is currently under construction and expected to be completed in 2022, with a maximum wastewater treatment capacity of 10,000 m3 per day. In 2021, all existing wastewater treatment plants combined are only capable of treating 68.33% of Bangkok’s municipal wastewater, considering the daily per capita wastewater generation of 0.2 m337 and the population of 8.39 million38.
By 2027, the population of the Thai capital Bangkok is expected to be 8.48 million, with a sewage production of around 1.70 million m3 per day. According to the Department of Drainage and Sanitation24,Japan International Cooperation Agency34it takes two years to build a centralized sewage treatment plant at a cost of THB 3358.27 million2020; and one year for a decentralized wastewater treatment plant at a cost of THB 118.95 million2020. An annual budget of approximately 4,500 million THB2020 is reserved for the construction of new treatment plants (Direction de l’Assainissement et de l’Assainissement24.
Considering budgetary constraints and capacity limits of sewage treatment plants, the Bangkok Metropolitan Administration (BMA) should opt for an environmentally and economically optimal number of future centralized and decentralized sewage treatment plants that match the demand and municipal wastewater treatment supply by 2031. Furthermore, in this research, the environmental and financial costs of the four sludge treatment scenarios (dewatering C, fertilizer C, dewatering D, fertilizer D) are used to determine the optimal combined number of centralized and decentralized treatment plants to be built (i.e. options I, II, III and IV).
In finance, net present value (NPV) is used in capital budgeting and investment planning to determine the profitability of an investment project. Mathematically, the NPV is the present value of future cash flows, discounted at the required rate of return, minus the initial investment. In this research, the required discount rate or rate of return is 10%, given that the discount rate for public infrastructure projects in developing countries is around 10%39. For the sewage treatment plants planned to be built in the capital Bangkok, the sources of revenue are royalties from sewage treatment and the sale of decomposed sludge-based fertilizers, while expenses include operating costs. and maintenance and environmental costs, excluding construction cost since treatment plants are funded public infrastructure projects. state coffers. The sewage treatment fee is 2 THB2020 by m3 Waste40. This study also assumes that the BMA could collect 80% of the fee on treated wastewater.
Figure 1 shows the four options for the construction of wastewater treatment plants for the period 2022-2031: construction of four centralized wastewater treatment plants (option I), construction of three centralized wastewater treatment plants and 60 decentralized wastewater treatment plants ( option II), construction of two centralized treatment plants and 127 decentralized treatment plants (option III), and construction of a centralized treatment plant and 194 decentralized treatment plants (option IV).