Reclaimed Water for Farmland Irrigation: How GB 5084-2021 and GB 20922-2007 Work Together

1. Reclaimed water irrigation must pass two tables at once

When reclaimed water from a municipal wastewater treatment plant is used for farmland irrigation, one table is not enough. The current GB 5084-2021, Standard for irrigation water quality - approved by the Ministry of Ecology and Environment on 2021-01-09, issued on 2021-01-20 and effective on 2021-07-01, replacing GB 5084-2005, GB 22573-2008 and GB 22574-2008 - states plainly in clause 4.2 that where reclaimed water from a municipal wastewater treatment plant is used for farmland irrigation, the provisions of GB 20922 shall also be met. GB 5084-2021 governs the water at the irrigation abstraction point, while GB 20922-2007 governs the reclaimed water itself; both must be satisfied.

2. Scope of GB 5084-2021: decide first whether the table applies

Clause 1 states that the standard applies to the supervision and management of water quality where surface water or groundwater is used as an irrigation source, and that where municipal sewage (excluding industrial wastewater and medical wastewater), unused livestock and poultry breeding wastewater, agricultural product processing wastewater or rural domestic sewage enters an irrigation channel, the water quality at the nearest downstream irrigation abstraction point is supervised against this standard.

Two points are frequently misread. First, the nearest downstream irrigation abstraction point is the compliance location, not the discharge point - clause 4.3 repeats that discharges of municipal sewage and the other listed wastewaters to irrigation channels must ensure that the water quality at the nearest downstream abstraction point meets the standard. Second, municipal sewage mixed with industrial or medical wastewater is excluded: the official interpretation published by the Ministry of Ecology and Environment states that the revised standard prohibits industrial wastewater and medical wastewater, or municipal sewage mixed with them, from entering farmland irrigation channels, consistent with Article 58 of the Law on Prevention and Control of Water Pollution.

3. The sixteen basic control items of Table 1

Clause 4.1.1 makes the basic control items mandatory and subject to Table 1. The main limits are as follows, in mg/L unless stated otherwise:

• pH 5.5 to 8.5; water temperature not above 35 degrees C;
• Suspended solids: 80 for paddy crops, 100 for dry land crops, and 60 for processed, cooked or peeled vegetables and 15 for raw eaten vegetables, melons and herbaceous fruits;
• BOD5: 60 for paddy, 100 for dry land, and 40 and 15 for the two vegetable categories;
• CODCr: 150 for paddy, 200 for dry land, and 100 and 60 for the two vegetable categories;
• Anionic surfactants: 5 for paddy, 8 for dry land, 5 for vegetables;
• Chloride (as Cl-) 350; sulphide (as S2-) 1;
• Total salt 1000 in non-saline-alkali areas and 2000 in saline-alkali areas;
• Heavy metals and metalloids: total lead 0.2, total cadmium 0.01, chromium (hexavalent) 0.1, total mercury 0.001, and total arsenic 0.05 for paddy and vegetables and 0.1 for dry land;
• Hygienic indicators: faecal coliforms 40000 MPN/L for paddy and dry land, 20000 for processed, cooked or peeled vegetables and 10000 for raw eaten vegetables, melons and herbaceous fruits; roundworm eggs 20 per 10 L, with 20 and 10 per 10 L for the vegetable categories.

The vegetable column is the most frequently overlooked part of the standard: for the same indicator the requirement for raw eaten vegetables is often a fraction of that for dry land crops, and for COD it tightens from 200 to 60.

4. The twenty selective control items of Table 2 and who selects them

Clause 4.1.2 states that the selective control items are chosen and implemented by the local ecology and environment authority together with the agriculture and rural affairs and water resources authorities according to the type of irrigation water and the crop. The same item may therefore be exempt in one province and mandatory in another.

The foreword notes that this revision added nine selective control items: total nickel, chlorobenzene, 1,2-dichlorobenzene, 1,4-dichlorobenzene, nitrobenzene, toluene, xylene, isopropylbenzene and aniline. Typical limits are: cyanide (as CN-) 0.5; fluoride 2 in general areas and 3 in high fluoride areas; petroleum oils 5 for paddy, 10 for dry land and 1 for vegetables; total copper 0.5 for paddy and 1 for dry land; total zinc 2; total nickel 0.2; selenium 0.02; boron in three tiers of 1, 2 and 3 according to crop sensitivity; benzene 2.5, toluene 0.7, xylene 0.5, isopropylbenzene 0.25, aniline 0.5, chlorobenzene 0.3, 1,2-dichlorobenzene 1.0, 1,4-dichlorobenzene 0.4 and nitrobenzene 2.0.

5. GB 20922-2007: a four tier classification on the reclaimed water side

GB 20922-2007, The reuse of urban recycling water - Quality of farmland irrigation water, is a mandatory national standard issued on 2007-04-06 and effective on 2007-10-01, administered by the Ministry of Housing and Urban-Rural Development through the national technical committee on water supply and drainage. The national standards information platform records a review conclusion of revision on 2022-11-02, and the standard remains current.

It classifies irrigation by crop into fibre crops, dry land cereal and oil crops, paddy cereals and open field vegetables. For the basic control items, BOD5 is 100, 80, 60 and 40 mg/L respectively, CODCr is 200, 180, 150 and 100 mg/L, and suspended solids are 100, 90, 80 and 60 mg/L. These values are quoted from a sewage outfall demonstration report published on a Zhejiang provincial government cloud platform, which reproduces Table 1 of GB 20922-2007 item by item; the authoritative values remain those of the official standard text.

Placing the two tables side by side clarifies their division of labour: GB 20922-2007 addresses only farmland irrigation water sourced from municipal wastewater treatment plant effluent and is written in terms of reclaimed water quality at the point of supply, whereas GB 5084-2021 is written in terms of water quality at the irrigation abstraction point and covers all irrigation sources including surface water and groundwater. Reclaimed water must still meet GB 5084-2021 after it enters the channel because other flows may join the channel along the way.

6. Monitoring, implementation and supervision

Clause 5.1: the monitoring site layout and sampling methods for both basic and selective control items shall comply with NY/T 396, the technical specification for environmental quality monitoring of agricultural water sources, until a dedicated monitoring specification for irrigation water quality is issued. Clause 5.2 and Table 3 list the analytical methods item by item, for example HJ 828 dichromate method or HJ/T 399 fast digestion spectrophotometry for COD, HJ 347.2 multiple tube fermentation for faecal coliforms, HJ 775 sedimentation concentration for roundworm eggs, and HJ/T 51 gravimetry for total salt.

Chapter 6 assigns implementation: the standard is supervised and implemented by the ecology and environment authorities of people's governments at all levels together with the agriculture and rural affairs and water resources authorities. The foreword also opens the door to local standards: provincial governments may set local standards for items not covered, and may set stricter local standards for items already covered, which must be filed with the national ecology and environment authority.

7. Three things the designer must calculate independently

Three issues are left to the designer. First, the match between irrigation volume and crop water demand: the standard governs quality, not quantity. Second, the long term accumulation of salts and the sodium adsorption ratio: GB 5084-2021 sets only a total salt limit, so salinisation risk under long term irrigation must be assessed through soil monitoring and field trials. Third, water quality changes during conveyance and distribution, especially residual chlorine decay and microbial regrowth, for which the standard provides no control parameters.

The compliance procedure reduces to three steps: confirm the water source type and the location of the nearest downstream abstraction point under clause 1; check the sixteen basic control items in Table 1 and, under clause 4.1.2, determine which selective control items apply locally; and finally, for reclaimed water from a municipal wastewater treatment plant, apply clause 4.2 and check the corresponding crop category limits in GB 20922-2007. Only when all three steps pass is the water quality judgement complete.