Suspended Solids in Drinking Water Source Standards

In China’s drinking water source management, the suspended solids (SS) standard is not a straightforward value written into the primary national water quality standard. The current Environmental Quality Standards for Surface Water (GB 3838–2002) does not include suspended solids among its basic controlled parameters, so no direct SS limit can be read from that standard itself.

In practice, however, SS is evaluated by reference to the Water Quality Standards for Surface Water Resources (SL 63–94). Under SL 63–94, suspended solids are divided into five grades. The second grade, which applies to centralized drinking water sources, sets a limit of 25 mg/L; the third grade, applicable to general industrial water and general fish habitats, sets a limit of 30 mg/L. Accordingly, for surface-water sources serving residential water supply, the commonly used benchmark for suspended solids is 25 mg/L.

This benchmark comes with an important qualification. SL 63–94 was formally abolished in 2020, but no replacement standard has explicitly taken over its function for suspended solids. Consequently, the 25 mg/L value continues to be cited in numerous source-water assessments and environmental impact evaluations, functioning as a de facto reference rather than a directly enforceable limit under GB 3838. In source areas where the first-grade protection zone is managed toward a Class II water quality objective, this 25 mg/L reference value operates as a supplementary constraint consistent with the protection goal.

The practical significance of this reference value lies less in strict compliance than in early warning. When SS approaches or exceeds 25 mg/L, it signals possible soil disturbance, sediment resuspension, or erosion in the source catchment—conditions that can increase treatment burden and provide attachment sites for microorganisms and pollutants. Thus, while the 25 mg/L figure is a reference rather than a statutory standard value, it remains a useful operational threshold for protecting drinking water sources.