Reading the GB 18918-2002 Amendment: Instantaneous Limits, Dual-Track Compliance and Online Monitoring Requirements

1. Core Changes in the Amendment

The amendment to GB 18918-2002, Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant, issued by the Ministry of Ecology and Environment, makes five substantive changes:

• It adds Table 4, maximum allowable discharge concentration for basic control items (instantaneous values).
• It removes colour, pH and faecal coliforms from Table 1, adding a transitional note to Table 1 instead.
• The air pollutant tiering is reduced from three tiers to two, with tier three and its limits deleted from the former Table 5.
• Applicability for small towns is clarified: plants below 500 cubic metres per day (excluding 500), other than those in towns that are seats of county-level government, follow the local standard set or designated by the provincial government; where no local standard exists, this standard may be applied by reference.
• New clause 6.3: discharge concentrations must not exceed the daily mean values in Tables 1 to 3 nor the instantaneous values in Table 4.

2. Instantaneous Limits in Table 4

Maximum allowable instantaneous concentrations, in mg/L except for pH and items with stated units:

• COD: Class 1A 75; Class 1B 90; tier 2 130; tier 3 140.
• Total nitrogen as N: Class 1A 20; Class 1B 25; not set for tiers 2 and 3.
• Ammonia nitrogen as N: Class 1A 10 (15); Class 1B 15 (20); tier 2 30 (35); not set for tier 3.
• Total phosphorus as P: for plants built on or before 31 December 2005, Class 1A 1.5, Class 1B 2.5, tier 2 5, tier 3 6; for plants built on or after 1 January 2006, Class 1A 1, Class 1B 1.5, tier 2 5, tier 3 6.
• Colour (dilution factor): Class 1A 30; Class 1B 30; tier 2 40; tier 3 50.
• pH: 6 to 9.
• Faecal coliforms in MPN per litre: Class 1A 10 to the power 3 for reuse and 10 to the power 4 for non-reuse; Class 1B 10 to the power 4; tier 2 10 to the power 4; not set for tier 3.

Notes: where influent COD exceeds 350 mg/L, removal must exceed 60%; where BOD exceeds 160 mg/L, removal must exceed 50%. For ammonia nitrogen, the value outside brackets applies above 12 degrees Celsius and the value in brackets at or below 12 degrees Celsius.

3. Transitional Provision for Older Plants

Plants built on or before 31 December 2005 must meet the limits applicable to plants built on or after 1 January 2006 from 1 January 2028. In practice this tightens the Class 1A total phosphorus limit from 1.5 to 1 mg/L, so upgrade designs must reserve margin well in advance.

4. The Dual-Track Compliance Rule

Clause 6.3 of the amendment states that discharge concentrations must not exceed either the daily mean values in Tables 1 to 3 or the instantaneous values in Table 4. The engineering consequence is direct: a plant whose daily means are consistently compliant is still in violation if any instantaneous sample exceeds Table 4. Compliance focus shifts from average concentration to peak concentration, which affects:

• Shock load resilience: larger biological volume margin or a more capable equalisation tank.
• Automatic control quality: lag in phosphorus precipitation dosing, carbon dosing or nitrate recycle translates directly into instantaneous exceedances.
• Validity of online monitoring: data must represent genuine instantaneous concentrations.

5. Sampling, Monitoring and Automatic Monitoring Requirements

• Outfalls require automatic flow metering, automatic proportional samplers, and automatic analysers for pH, water temperature and COD.
• Dischargers must carry out self-monitoring under HJ 1083, retain original monitoring records and publish results.
• Sampling follows HJ 91.1, HJ 493, HJ 494 and HJ 495. Daily means are analysed on composite samples at the frequency specified in HJ 91.1; where compositing is not permitted, every sample in 24 hours is analysed and averaged arithmetically. Instantaneous values are analysed on a grab sample taken under HJ 91.1.
• Outfall and monitoring point design, construction and maintenance follow environmental monitoring regulations and HJ 1405.
• Determination uses the method standards listed in Table 5; later national monitoring standards also apply where suitability is met.

6. Analyser Range Setting

Clause 5.1.1 of HJ 355-2019, the operating technical specification for online monitoring systems for CODCr and ammonia nitrogen, requires that analyser range be set rationally against actual discharge concentrations, with the upper limit set at two to three times the applicable discharge limit. A public reply from a provincial ecology and environment department confirms that after the amendment takes effect, the upper range limit may be set reasonably according to the daily mean limits in the pollutant discharge permit together with actual on-site discharge.

7. Consequences for Process Selection

• MBR, with its very low permeate suspended solids and turbidity, has a structural advantage against Table 4, but margins must still be verified against instantaneous rather than daily mean values.
• The Class 1A total nitrogen instantaneous limit of 20 mg/L, against a daily mean of 15 mg/L, raises the bar for anoxic zone volume and carbon dosing; low C/N influent calls for an enlarged anoxic zone and denitrifying phosphorus removal.
• A total phosphorus instantaneous limit of 1 mg/L demands accurate automatic control of chemical dosing, ideally with two dosing points linked to online phosphate monitoring.
• The faecal coliform limit for reuse is 10 to the power 3 MPN per litre, one order of magnitude stricter than the 10 to the power 4 MPN per litre for non-reuse, so disinfection must be sized for the reuse pathway.

References

• Amendment to GB 18918-2002 (MEE, in Chinese)
• Yunnan Provincial Department of Ecology and Environment: public reply on analyser range setting after the amendment (in Chinese)
• HJ 2010-2011, Technical Specifications for Membrane Biological Processes
• HJ 576-2010, Technical Specifications for Anaerobic-Anoxic-Oxic Activated Sludge Process