[Mainland China] Amendment to GB18918 Implemented: Wastewater Treatment Plants Enter the Era of Dual Daily-Average and Instantaneous Control

Introduction: Starting from 2026年3月1日, the amendment to the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918—2002) officially came into effect. The amendment adds instantaneous values for Chemical Oxygen Demand, ammonia nitrogen, Total Nitrogen, and Total Phosphorus (4 indicators) (approximately 1.2—2 times the daily average), and changes pH, color, and fecal coliform count (3 indicators) from daily average values to instantaneous values for assessment; exceeding either the instantaneous value or the daily average value shall be judged as exceeding the standard.

A Single Amendment Sheet Ends Two Decades of Enforcement Controversy

For more than twenty years since the standard was issued and implemented, an awkward controversy has persisted in the industry: should enforcement judgments use daily average values or instantaneous values.

Before the amendment was introduced, although relevant documents from the Ministry of Ecology and Environment clearly stated that "on-site instantaneous sampling" could serve as a basis for enforcement penalties, determining whether a wastewater treatment plant exceeded the standard still ultimately required comparison against the 24小时 composite sample required by the standard. This led to two consequences: first, enforcement officers had to work continuously at the enterprise for 24小时, collecting a full 12 samples at a frequency of once every 2小时, resulting in extremely high enforcement costs; second, it objectively tolerated the operating state of "short-term exceedance, average compliance"—as long as the daily average was leveled out and qualified, high-value discharges occurring for several hours in between were difficult to hold accountable.

The amendment resolves this problem in the most direct way: either instantaneous values or daily average values can serve as the basis for exceedance determination and enforcement supervision, and exceeding either limit constitutes exceedance. This means that every real-time reading on automatic monitoring equipment now has independent legal effect.

Four Main Modifications Broken Down Item by Item

The content of the amendment focuses on four directions, with the logic of "giving regulators a more accurate ruler while removing unreasonable costs."

• Adding instantaneous values for 4 major water pollutants—focusing on Chemical Oxygen Demand, ammonia nitrogen, Total Nitrogen, and Total Phosphorus, and based on scientific statistical analysis, setting instantaneous limits slightly higher than daily average values, with a ratio of approximately 1.2—2 times, taking into account the objective patterns of process fluctuations;
• Changing 3 indicators from daily average values to instantaneous values—using daily average values for enforcement of pH, color, and fecal coliform count cannot effectively prevent the impact of instantaneous high values on receiving water bodies; this time, based on the analytical testing characteristics of the indicators and in combination with international common practices, they are changed to instantaneous values;
• Differentiating fecal coliform count by destination—the current Grade I-A limit is mainly set for reuse water and is too stringent for effluent discharged to environmental water bodies; the amendment adjusts the effluent limit for non-reuse destinations to 10000 MPN/L, the same as the Class III surface water quality limit;
• Sampling and monitoring requirements adjusted accordingly—daily average sampling shall be implemented in accordance with the Technical Specification for Wastewater Monitoring (HJ 91.1—2019), changing from the original standard's "at least once every 2小时, 12 samples per day" to 3—6 samples per day, and the applicable requirements for smaller designated town wastewater treatment plants have also been adjusted.

In an official response to reporters' questions, a very apt analogy was used: the daily average value is equivalent to the average heart rate over 24小时, and the instantaneous value is equivalent to the heart rate per minute; both indicate health risks, and controlling both simultaneously is conducive to healthy operation. For wastewater treatment plants, this analogy is not exaggerated at all—it means that every heartbeat of effluent quality will be recorded and judged.

What Is Really Under Pressure Is Not the Process, but Operational Management

An easily misunderstood point is that the amendment does not tighten pollutant discharge limits, nor does it require wastewater plants to start over. The official statement is very clear—after the amendment is implemented, municipal wastewater treatment plants do not need to change treatment processes and will not face increased economic pressure, but they need to improve operational management levels and strengthen the ability to control and respond to abnormal influent and shock loads.

The pressure falls specifically on four aspects:

• Shock load response — Instantaneous shocks such as rainy-season infiltration, illegal discharge of industrial wastewater, and high-concentration influent at night, which used to be "diluted" by daily composite samples, must now be identified and buffered at minute-level;
• Change in dosing method — To avoid instantaneous fluctuations in water quality, chemical dosing is shifting from intermittent, experience-based dosing to automated continuous dosing, and the consumption of phosphorus removal chemicals and carbon sources will increase accordingly;
• Upgrade of online monitoring O&M — Automatic monitoring data is directly used as the basis for enforcement, and regular calibration, spare parts, and data validity review of equipment have changed from "compliance actions" to a "risk defense line";
• Increased manpower input — O&M personnel need to monitor water quality in real time and improve influent and effluent ledgers, and refined management brings a structural increase in human resources.

Together, these four items point to the same category of procurement demand: influent early warning, intelligent dosing, online monitoring quality assurance, and O&M data platforms. For small and medium-sized wastewater treatment plants whose processes have already stably met standards but whose management capability is still at the "manual experience stage," this standard adjustment is a forced digital upgrade.

An Underestimated Item: The Fecal Coliform Limit Has Actually Been Relaxed

Most industry discussion focuses on the pressure brought by instantaneous values, but the amendment actually contains a burden-reduction clause: for effluent not intended for reuse, the limit for fecal coliforms has been relaxed from the strict requirement of Grade A to 10000 MPN/L.

The technical logic behind this modification is clear. The current Grade A fecal coliform limit was set for reuse water, because reuse water comes into contact with people and pathogenic microorganisms must be strictly controlled. However, if the effluent is discharged directly into environmental water bodies and is still controlled according to reuse water standards, it will lead to excessive chlorination — adding large amounts of chlorine-containing disinfectants, with more residual chlorine remaining in the effluent, which instead harms the aquatic ecological environment of the receiving water body.

After the adjustment, there are three positive effects: first, reducing the secondary impact of residual chlorine on aquatic ecology; second, lowering the disinfection cost of wastewater treatment plants; and third, reducing the safety risks in the production, storage, and transportation of disinfectants. Officials summarize it as "achieving an effective balance between ecological and environmental benefits and economic benefits," and this assessment is not exaggerated.

It should be noted that this relaxation applies only to non-reuse destinations. If the effluent is used for reclaimed water reuse, it must still be implemented according to the strict requirements for reuse water. This also reconfirms at the standard level a principle: the microbial indicators of reclaimed water are another set of constraints independent of discharge indicators, and discharge standards cannot be used as reuse standards when undertaking reuse projects.

Indirect Promotion of Reclaimed Water and Resource Recovery

The amendment appears to address only discharge, but in fact it indirectly promotes wastewater resource recovery in two ways.

The first is turning "stability" into an assessable asset. In the past, when wastewater treatment plants supplied reclaimed water externally, water quality fluctuations were often constrained by agreements and lacked a standard basis; after the establishment of the instantaneous value system, "effluent can meet a certain use at any moment" has, for the first time, a verifiable determination method, which provides a technical basis for quality-based pricing of reclaimed water and premium pricing for premium quality.

The second is forcing quality improvement of the upstream pipe network. Instantaneous values have zero tolerance for shock loads, and a large part of shock loads comes from the pipe network — mixed rain and sewage connections lead to a surge in influent volume and concentration dilution during the rainy season, and illegal discharge of industrial wastewater leads to toxic shock to the influent. When a wastewater treatment plant must be responsible for every minute of data, it will proactively require upstream to solve pipe network problems. Objectively, this will accelerate market demand for "plant-network integration" and pipe network inspection and repair.

Conclusion

The value of the GB 18918 amendment does not lie in how strict the limits are — on the contrary, it does not tighten any discharge limit. Its significance lies in changing the granularity of evaluation from "one day" to "one minute", while also removing unreasonable costs such as excessive disinfection. For wastewater treatment plants, this is a shift in assessment from "engineering capability" to "management capability"; for technology and service providers, whoever can make effluent stand up at any moment will gain pricing power under this new rule.

About TIANYI TECH: TIANYI TECH (TIANYI LIMITED) focuses on the fields of wastewater treatment, reclaimed water reuse, and water environment governance, providing municipal and industrial clients with full-chain services such as process design, equipment integration, engineering general contracting, and smart operations. The company continuously tracks global water treatment policies, technologies, and market dynamics, and is committed to helping customers achieve efficient utilization and sustainable management of water resources with reliable solutions. For more information, please visit the company website at www.tianyitech.hk.

Source: Q&A with relevant heads of the Department of Water Ecological Environment of the Ministry of Ecology and Environment and the Department of Urban Construction of the Ministry of Housing and Urban-Rural Development on the Amendment Sheet to the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants (GB 18918—2002) (reprinted by Wuhai Municipal Ecology and Environment Bureau, 2026年10月), and the reply of the Department of Ecology and Environment of Liaoning Province on suggestions regarding the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants (2026年8月).