Thermal Power Plant Wastewater: Standards and Compliance Points

Thermal power plants do not have a dedicated, standalone heavy metals discharge standard. Their obligations are assembled from a general wastewater standard, an industry-specific guideline for flue gas desulfurization (FGD) wastewater, and increasingly, local requirements and permit conditions. Understanding this layered structure is essential, because the compliance logic for heavy metals differs from that of conventional pollutants in one decisive respect: the point of measurement.

The foundational rule: GB 8978 and the “first-class pollutant” principle. China’s Integrated Wastewater Discharge Standard (GB 8978–1996) classifies mercury, cadmium, total chromium, hexavalent chromium, arsenic, lead, and nickel as first-class pollutants. These substances are subject to a rule that applies regardless of industry, discharge method, or the receiving water body’s functional category. Samples must be taken at the workshop or workshop treatment facility outlet, not at the plant’s final discharge point. Dilution with other waste streams or with cooling water does not create compliance; a exceedance at the workshop outlet is a violation on its face. 

The concentration ceilings for the relevant metals are fixed in Table 1 of the standard, and they serve as the baseline against which all thermal power plant discharges are judged.

The industry-specific layer: DL/T 997 for FGD wastewater. The principal heavy-metal-bearing stream in a coal-fired plant is FGD wastewater. Trace elements in flue gas—mercury, cadmium, lead, arsenic—concentrate in the limestone-gypsum slurry as it recirculates, making the blowdown from this system the most concentrated heavy-metal stream in the plant. The electric power industry guideline DL/T 997 (currently the 2020 revision) sets control values for this stream that align with GB 8978 Table 1, covering total mercury, cadmium, chromium, arsenic, lead, nickel, and zinc. In practice, FGD wastewater treatment facility outlets are required to satisfy both GB 8978 Table 1 and DL/T 997, with the stricter of the two governing where they differ.

The widening perimeter: local standards and permit conditions. The compliance picture is not closed at the national level. Provinces with serious heavy-metal concerns have issued local standards that can reach thermal power plants. Thallium, not listed in GB 8978 Table 1, has become a regulatory focus in several provinces, with local industrial wastewater thallium standards imposing limits at workshop or production facility outlets. Separately, discharge permits frequently write in concentration limits that are stricter than the technology-based standard, and where an applicant proposes a tighter limit, the permit is issued on that basis. A plant that checks only the national numbers may therefore be non-compliant against its own permit.

Monitoring and the practical meaning of compliance. Self-monitoring requirements reinforce the workshop-outlet principle. Under HJ 820, FGD wastewater that is not reused must be monitored at the workshop or workshop treatment facility outlet; where the stream is discharged externally, monthly monitoring is typically required, while internal reuse may allow quarterly frequency. The FGD treatment facility outlet is the control point, not the plant’s general wastewater outlet.