Economic sustainability evaluation of cascaded combinations of raw water quality and water treatment processes for drinking water

Abstract

[Objective] Against the background of China's comprehensive victory in poverty alleviation and the overall improvement of drinking water safety, the economic sustainability of safe drinking water supply in some water-scarce and environmentally fragile areas, especially brackish-water areas, remains challenging. Existing studies using brackish water as a drinking water source report large differences in water production costs and high uncertainty, mainly because the relationship among raw-water quality, water treatment processes, and drinking water standards has not been studied jointly, and the reported result reflect only one part of the whole process. [Methods] Starting from the complete chain of drinking water production, the whole-process cascaded combination relationship of " raw water quality-water treatment process-drinking water standard" and its economic evaluation model were constructed. Using raw-water total dissolved solids( TDS) as the key indicator, the model systematically analyzed the water production costs of raw water with different TDS concentrations under different treatment processes for meeting different drinking water standards, and then compared them with the current first-tier water prices in major provincial capitals and municipality with independent planning status in China to evaluate the economic sustainability of different cascaded combinations. [Results] Under China's current drinking water standards, when raw-water TDS was ≤1 000 mg/L, conventional treatment or ultrafiltration could stably meet the standard, with low water production cost and strong economic sustainability. When TDS was 1 000 ~ 1 500 mg/L, nanofiltration was required, and the cost increased but remained within a sustainable range. When TDS further rose to 1 500~2 000 mg/L, reverse osmosis was required to meet the standard, at which point the water production cost increased significantly and economic sustainability approached a critical threshold. When TDS exceeded 2 210 mg/L, the water production cost exceeded the current highest water price of 4. 16 CNY/m³, and economic sustainability became a concern. Application of the method to a prefecture-level city in northwestern China showed that, under existing source-water conditions, conventional processes alone could provide compliant water to only 21. 46% of the population. After introducing nanofiltration, this proportion increased to 58. 92%, while the remaining 41. 08% must rely on the higher-cost reverse osmosis process to ensure drinking water safety. [Conclusion] The complete cascade from raw water to finished water is an inseparable chain in drinking water production. The proposed economic evaluation method for cascaded combinations can provide a scientific method for configuring different treatment processes and selecting water supply sources for drinking water based on raw-water quality, and can serve as a reference for guiding urban and rural water supply planning and ensuring supply safety against the background of continuously rising drinking water standards.

Keywords

raw water quality / drinking water standards / total dissolved solids / water treatment processes / economic sustainability / influencing factors / water supply safety / environmentally fragile areas