Reclaimed Water Quality Standards Compared: Current Versions, Classification Logic and Limit Interfaces of GB/T 18920, 18921 and 19923

1. Verify the current versions of the three standards first

Three recommended national standards in the "urban wastewater reclamation and reuse" series are most commonly used in engineering. All are administered by the Ministry of Housing and Urban-Rural Development and managed by the National Technical Committee on Urban Water Supply and Drainage (SAC/TC 434). Confirming the version is the first step in selection:

• GB/T 18920-2020, Urban wastewater reclamation and reuse - Water quality standard for urban miscellaneous use: issued 2020-03-31, effective 2021-02-01, fully replacing GB/T 18920-2002;
• GB/T 18921-2019, Urban wastewater reclamation and reuse - Water quality standard for scenic environment use: issued 2019-06-04, effective 2020-05-01, fully replacing GB/T 18921-2002. Its review conclusion dated 2026-03-31 is revision, and revision project 20261919-T-333 on reclaimed water quality for scenic environment use is currently being drafted;
• GB/T 19923-2024, Urban wastewater reclamation and reuse - Water quality standard for industrial uses: issued 2024-03-15, effective 2024-10-01, fully replacing GB/T 19923-2005, which was abolished on 2024-10-01.

All three share the same six-chapter structure: scope, normative references, terms and definitions, water quality indices, sampling and monitoring, and safe use. The foreword of GB/T 18920-2020 further notes that this revision removed the chapter on standard implementation and supervision and made safe use a separate chapter.

2. The classification logic differs completely

The three standards are not three tiers of the same scale; they classify by end use, which is the core of selection:

• GB/T 18920-2020 covers reclaimed water for toilet flushing, vehicle washing, urban greening, road sweeping, fire fighting and construction. Table 1 gives limits for two classes, toilet flushing and vehicle washing and urban greening, road sweeping, fire fighting and construction, the former being stricter;
• GB/T 18921-2019 covers reclaimed water for scenic environment use, classified by ornamental or recreational use crossed with river, lake or water feature type. Lower water body mobility and higher likelihood of human contact mean stricter limits;
• GB/T 19923-2024 covers reclaimed water used as an industrial water source, classified by make-up water for indirect open recirculating cooling systems, boiler feed water, process water and product water, with control items organized around scaling, corrosion and microbial slime.

3. Control items unique to each standard

3.1 Microbiological indicators are not expressed the same way

This is where cross-standard comparison most often goes wrong:

• GB/T 18920-2020: the basic control item is Escherichia coli, with a limit of absent (MPN/100 mL or CFU/100 mL), paired with total chlorine of at least 1.0 mg/L at the plant outlet and at least 0.2 mg/L at the end of the distribution network;
• GB/T 18921-2019: uses faecal coliforms, with a limit of 1000 per litre for ornamental water features, plus a residual chlorine requirement of 0.05 to 0.1 mg/L for recreational water bodies;
• GB/T 19923-2024: uses faecal coliforms of at most 1000 MPN/L, paired with total residual chlorine of 0.1 to 0.2 mg/L measured at the connection to the user's piping.

The foreword of GB/T 18920-2020 states that this revision changed the item "total coliforms" in Table 1 to "Escherichia coli" and revised the values for turbidity, five-day biochemical oxygen demand, ammonia nitrogen, total dissolved solids and total chlorine. Design documents must use the indicator names from the current version and must not carry forward "total coliforms" from the 2002 edition.

3.2 Scaling and corrosion items found only in GB/T 19923-2024

The industrial water standard uniquely includes, for make-up water for indirect open recirculating cooling systems, boiler feed water, process water and product water: total alkalinity (as CaCO3) of at most 350 mg/L, total hardness (as CaCO3) of at most 450 mg/L, chloride of at most 250 mg/L, sulfate (as SO4) of at most 250 mg/L, silica of at most 30 mg/L, iron of at most 0.3 mg/L, manganese of at most 0.1 mg/L and petroleum oils of at most 1.0 mg/L, with new limits for total nitrogen of at most 15 mg/L and total phosphorus of at most 0.5 mg/L plus fluoride and sulfide. The foreword also notes that the suspended solids limit was deleted and that water quality indices were reorganized into basic control items and selective control items with simplified water classification.

One footnote is easily missed: for make-up water to an indirect open recirculating cooling system whose heat exchangers are made of copper alloy, ammonia nitrogen must be below 1 mg/L. For industrial users with copper alloy heat exchangers this is a hard constraint that directly determines whether an advanced ammonia removal unit is needed.

3.3 Sensory and distribution network items found only in GB/T 18920-2020

Odour (no unpleasant sensation), dissolved oxygen of at least 2.0 mg/L, and total chlorine of at least 1.0 mg/L at the outlet and at least 0.2 mg/L at the network end are characteristic of the urban miscellaneous use standard; neither of the other two sets a dissolved oxygen requirement. GB/T 18920-2020 also added limits for chloride and sulfate as selective control items.

3.4 Eutrophication control items found only in GB/T 18921-2019

The scenic environment standard controls both total phosphorus and total nitrogen, tightening with water body type. For ornamental water features and lakes: total phosphorus at most 0.3 mg/L, total nitrogen at most 10 mg/L, ammonia nitrogen at most 3 mg/L, five-day biochemical oxygen demand at most 6 mg/L, turbidity at most 5 NTU. For river-type bodies, both ornamental and recreational, the values relax to total phosphorus at most 0.5 mg/L, total nitrogen at most 15 mg/L, ammonia nitrogen at most 5 mg/L, five-day biochemical oxygen demand at most 10 mg/L, turbidity at most 10 NTU. All three standards share pH 6 to 9 and colour (GB/T 18920-2020: at most 15 degrees for toilet flushing and vehicle washing, at most 30 degrees for greening, sweeping, fire fighting and construction; GB/T 18921-2019: at most 20 degrees; GB/T 19923-2024: at most 20 degrees). GB/T 18921-2019 additionally sets a basic requirement: no floating matter and no unpleasant odour or taste.

4. Quick comparison of limits (transcribed values)

The lists below compare the basic control items of the three standards for their typical uses. The full official texts of these three national standards are not available for online reading in the national standards full-text system, so the values are transcribed from item-by-item quotations of Table 1 of each standard in environmental impact assessment documents and approval documents published by local governments. Formal design should rely on the official printed standard texts.

• GB/T 18920-2020, toilet flushing and vehicle washing: pH 6 to 9, colour at most 15 degrees, no unpleasant odour, turbidity at most 5 NTU, BOD5 at most 10 mg/L, ammonia nitrogen at most 5 mg/L, anionic surfactants at most 0.5 mg/L, dissolved oxygen at least 2.0 mg/L, total dissolved solids at most 1000 mg/L, total chlorine at least 1.0 mg/L at the outlet and at least 0.2 mg/L at the network end, Escherichia coli absent;
• GB/T 18920-2020, urban greening, road sweeping, fire fighting and construction: pH 6 to 9, colour at most 30 degrees, turbidity at most 10 NTU, BOD5 at most 10 mg/L, ammonia nitrogen at most 8 mg/L, anionic surfactants at most 0.5 mg/L, dissolved oxygen at least 2.0 mg/L, total dissolved solids at most 1000 mg/L, Escherichia coli absent;
• GB/T 18921-2019, ornamental water features: pH 6 to 9, BOD5 at most 6 mg/L, turbidity at most 5 NTU, total phosphorus (as P) at most 0.3 mg/L, total nitrogen (as N) at most 10 mg/L, ammonia nitrogen (as N) at most 3 mg/L, faecal coliforms at most 1000 per litre, colour at most 20 degrees. For river-type bodies the corresponding values relax to BOD5 at most 10, turbidity at most 10, total phosphorus at most 0.5, total nitrogen at most 15 and ammonia nitrogen at most 5; recreational water bodies carry a residual chlorine requirement of 0.05 to 0.1 mg/L;
• GB/T 19923-2024, make-up water for indirect open recirculating cooling systems, boiler feed water, process water and product water: pH 6.0 to 9.0, colour at most 20 degrees, turbidity at most 5 NTU, BOD5 at most 10 mg/L, COD at most 50 mg/L, ammonia nitrogen at most 5 mg/L, total nitrogen at most 15 mg/L, total phosphorus at most 0.5 mg/L, anionic surfactants at most 0.5 mg/L, petroleum oils at most 1.0 mg/L, total alkalinity at most 350 mg/L, total hardness at most 450 mg/L, total dissolved solids at most 1000 mg/L, chloride at most 250 mg/L, sulfate at most 250 mg/L, iron at most 0.3 mg/L, manganese at most 0.1 mg/L, silica at most 30 mg/L, faecal coliforms at most 1000 MPN/L, total residual chlorine 0.1 to 0.2 mg/L.

5. Four practical conclusions for design selection

1. Define the use first, then the standard. The three standards are parallel rather than successive. Different supply branches of the same reclamation plant may have to comply with GB/T 18920 and GB/T 19923 simultaneously, so the boundary must be clearly defined at the network branch with separate monitoring points.
2. Microbiological indicators cannot be substituted for one another. Escherichia coli "absent" and faecal coliforms "at most 1000" are two entirely different logics: one is a detection-limit logic, the other a count-limit logic, and the monitoring methods and sampling frequencies differ accordingly.
3. Industrial users must complete a scaling and corrosion check. The five items total alkalinity, total hardness, chloride, sulfate and silica appear only in GB/T 19923-2024; compliance assessment must also check the circulating water quality at the applicable concentration factor.
4. Respect version dates. GB/T 18920-2002 and GB/T 18921-2002 were replaced by the 2020 and 2019 editions; GB/T 19923-2005 was abolished on 2024-10-01; GB/T 18921-2019 is already on a revision programme, so later limits may change. Contracts should stipulate that the current valid version governs.

6. Interface with discharge standards

Reclaimed water quality standards and the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants (GB 18918-2002) belong to different stages: the former constrain water quality at the reclaimed water user, the latter constrain effluent discharged to environmental water bodies. The most common design error is treating GB 18918 Class 1A as the reclaimed water quality target. Their control items do not correspond: GB 18918 Class 1A sets no requirements for total alkalinity, total hardness, chloride, sulfate, silica, dissolved oxygen or total chlorine, and does not distinguish river-type from lake-type scenic water bodies. At least one local reclaimed water master plan has compared the two item by item and concluded outright that the Class 1A values of GB 18918-2002 do not satisfy GB/T 18921-2019. In addition, under the pollutant permit technical specifications, no pollutant discharge quantity is permitted where the effluent goes to reuse, but that does not exempt the reclaimed water itself from meeting its own quality requirements.

Note: standard status information is taken from the national standards information public service platform; limit values are transcribed from item-by-item quotations of Table 1 in environmental impact assessment and approval documents published by local governments. Formal design should rely on the official standard texts.