Water Quality Parameters for Municipal Wastewater Treatment Plants

Municipal wastewater treatment plants (WWTPs) must conduct systematic water quality testing to ensure treatment efficiency meets regulatory requirements and prevents secondary pollution of receiving water bodies. Under China’s Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918‑2002) and its latest amendment, the testing system covers physical, chemical, and biological parameters, totalling more than 30 core indicators.

1. Physical Parameters

Suspended Solids (SS): Measured by the gravimetric method, SS indicates the content of insoluble particulate matter, directly affecting the transparency and ecological health of the receiving water.

Color and Turbidity: Color is tested by the dilution multiple method or platinum‑cobalt colorimetry, with a limit of 30 times; turbidity is measured by the scattered light method.

Temperature: Water temperature critically influences microbial activity in biological treatment processes, typically ranging between 10°C and 25°C in municipal WWTPs.

2. Chemical Parameters

Oxygen Demand Indicators: Chemical Oxygen Demand (COD) reflects the total amount of organic matter and reducing substances in water. Biochemical Oxygen Demand (BOD₅) represents the pollution load of microbially degradable organic matter. Together they form the basis for assessing organic pollution levels. According to the first‑level A standard of GB 18918‑2002, COD is limited to ≤50 mg/L and BOD₅ to ≤10 mg/L (daily average).

Nutrients: Ammonia nitrogen (NH₃‑N), total nitrogen (TN), and total phosphorus (TP) are key nutrient indicators. Their control serves to prevent eutrophication in rivers and lakes, avoiding excessive algae growth that degrades water quality. The first‑level A standard limits ammonia nitrogen to ≤5 mg/L, TN to ≤15 mg/L, and TP to ≤0.5 mg/L.

3. Biological Parameters

Fecal Coliforms (FC): This indicator assesses the degree of fecal contamination and its potential threat to human health. The first‑level A standard limits FC to ≤1,000 CFU/L. Under the 2025 amendment, FC is also assessed only as an instantaneous value.

Total Bacterial Count: This can serve as a reference indicator for evaluating water cleanness and purification effectiveness; an elevated count suggests poor disinfection performance.

For WWTPs receiving pharmaceutical, chemical or mixed industrial wastewater, testing also extends to specific heavy metals (lead, cadmium, mercury, hexavalent chromium, arsenic) and toxic organic compounds.