## 2 Methods
### Estimation of the Costs and Benefits of Water Reclamation Projects  
For water reclamation projects for industrial water supply, the direct costs and benefits of these projects included construction costs, operation and maintenance costs, and revenues from the sale of reclaimed water. The information used for the estimation of direct costs and benefits is shown in Table 1.  
For cases that included externalities, the environmental benefits of not discharging WWTP effluent into the river were included as positive externalities. The costs of environmental damage from pollutants in wastewater, such as nitrogen, phosphorus, SS, and COD, were estimated based on their average concentrations in the WWTP effluents of industrial parks in Chonburi in 2018 (Table S3). In the absence of recent information on the costs of environmental damage from water pollutants discharged into water bodies in Thailand, these values were derived from the shadow prices of pollutants discharged into a river in Spain (\(-\)16.353 EURO/kg N; \(-\)30.944 EURO/kg P; \(-\)0.005 EURO/kg SS; and \(-\)0.098 EURO/kg COD) in [35]. The shadow prices of pollutants were estimated from the associated costs of pollutant removal in the treatment process. When a water reclamation project exists, no pollutants are released. The shadow prices of pollutants, therefore, reflect environmental benefits that are positive externalities in the CBA. Equation (1) was then used to account for the exchange rates of the currencies and the difference in gross domestic product based on purchasing power parity (GDP (PPP)) per capita in Spain and Thailand, reflecting the difference in the ability to purchase goods and services per capita in the two countries [36]. The values of the money in 2020 were estimated using Equation (2). \[\text{CFR}_{\text{TH2010}}=\frac{\text{GDP}\ (\text{PPP})\ \text{per capita}_{\text{ TH2010}}}{\text{GDP}\ (\text{PPP})\ \text{per capita}_{\text{SP2010}}}\times\text{E}_{2010} \tag{1}\]  
where  
\[\text{CFR}_{\text{TH2010}}=\text{Thailand's PPP conversion in 2010 (THB/EURO)}\] \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \multicolumn{5}{c}{**Wastewater Flow Rates (m\({}^{3}\)/d)**} \\ \cline{2-5}
**Direct Costs and Benefits** & **5200** & **10,000** & **15,000** & **25,000** \\ \hline Volume of reclaimed water produced (m\({}^{3}\)/d) \({}^{(1)}\) & 3900 & 7500 & 11,250 & 18,750 \\ \hline
**Direct costs** & & & & \\ Construction costs (THB million) \({}^{(2)}\) & 108.2 & 145 & 187 & 332 \\ Operating and maintenance costs (million THB/year) \({}^{(3)}\) & 37.96 & 73 & 109.5 & 182.5 \\ \hline
**Direct benefits** & & & & \\ Water prices (THB/m\({}^{3}\)) \({}^{(4)}\) & 26 & 26 & 26 & 26 \\ \hline \end{tabular} \({}^{(1)}\) We assumed that the filtrate flow rates were 75% of the wastewater flow rates. \({}^{(2)}\) These are the actual construction costs of water reclamation plants in Thailand disclosed by industrial states that used ultrafiltration and reverse osmosis systems, which included the costs of pipeline systems. \({}^{(3)}\) [PERSON] et al. (2020) [34]. \({}^{(4)}\) This is the water price in an industrial estate (Industrial Estate Authority of Thailand).  
\end{table} 
Table 1: Information for estimating the direct costs and benefits of water reclamation projects for industrial water supply, consisting of ultrafiltration and reverse osmosis.