Reclaimed Water for Industrial Recirculating Cooling: How Two National Standards Divide the Job, and Five Differences That Matter

1. Two standards, two different jobs

Using reclaimed water as make-up for an industrial recirculating cooling system means satisfying two national standards at once, and they do not control the same thing. GB/T 50050-2017, Code for design of industrial recirculating cooling water treatment (approved by Ministry of Housing and Urban-Rural Development Announcement No. 1576 of 2017, effective 2018-01-01, replacing GB 50050-2007) governs the result indicators at the heat exchanger and the quality threshold when reclaimed water is fed directly into an indirect open system. GB/T 19923-2024, Reuse of urban recycling water - Water quality standard for industrial uses (issued 2024-03-15, effective 2024-10-01, replacing GB/T 19923-2005) governs the quality of the reclaimed water as delivered. Neither substitutes for the other: meeting GB/T 19923-2024 does not guarantee a clean heat exchanger, and vice versa.

2. Result side: four hard indicators and two preconditions

Clause 3.1.5 sets the control indicators for heat exchange equipment in indirect open systems, together with two preconditions that are very often overlooked: tube-side velocity should exceed 1.0 m/s, shell-side velocity should exceed 0.3 m/s, and the cooling-water-side wall temperature of the heat transfer surface should not exceed 70 degrees C. These are not minor. Low velocity causes deposition; high wall temperature accelerates both scaling and corrosion. If either is out of control the numbers below cannot be held. Subject to them:

• corrosion rate on the water side of the heat transfer surface of carbon steel equipment shall be less than 0.075 mm/a;
• corrosion rate for copper alloy and stainless steel equipment shall be less than 0.005 mm/a;
• adhesion rate on the water side shall not exceed 15 mg/(cm2.month), or 20 mg/(cm2.month) in oil refining;
• fouling thermal resistance on the water side shall not exceed 3.44x10-4 m2.K/W.

Clause 3.1.6 tightens fouling thermal resistance for closed systems to less than 0.86x10-4 m2.K/W, with the corrosion rate taken from item 6 of clause 3.1.5. Microbiology is covered by clause 3.1.12: total heterotrophic bacteria should not exceed 1x10^5 CFU/mL and biological slime should not exceed 3 mL/m3.

The scaling criterion is clause 3.1.10: for indirect open and direct cooling systems, when the sum of calcium hardness and total alkalinity exceeds 1100 mg/L (as CaCO3), or when the stability index RSI is below 3.3, sulfuric acid dosing or softening should be applied. Cycle of concentration is set by clause 3.1.11: the design value for indirect open systems should not be below 5.0 and the operating value should not be below 3.0; for direct cooling systems the design value should not be below 3.0.

3. Source side: five differences that matter

Clause 6.1.3 and Table 6.1.3 of GB/T 50050-2017 state that when reclaimed water is used directly as make-up for an indirect open system, the water quality should comply with that table, or be determined from tests and operating data from similar projects. Table 6.1.3 has 16 items: pH (25 degrees C) 6.0 to 9.0; suspended solids not above 10.0 mg/L; turbidity not above 5.0 NTU; BOD5 not above 10.0 mg/L; COD not above 60.0 mg/L; iron not above 0.5 mg/L; manganese not above 0.2 mg/L; chloride not above 250 mg/L; calcium hardness (as CaCO3) not above 250 mg/L; total alkalinity (as CaCO3) not above 200 mg/L; ammonia nitrogen not above 5.0 mg/L (not above 1.0 mg/L where the heat exchanger uses copper alloy); total phosphorus (as P) not above 1.0 mg/L; total dissolved solids not above 1000 mg/L; free chlorine 0.1 to 0.2 mg/L at the end of the make-up line; oil not above 5.0 mg/L; total bacterial count below 1000 CFU/mL.

Table 1 of GB/T 19923-2024, column for make-up water of indirect open recirculating cooling water systems, gives: pH 6.0 to 9.0; turbidity not above 5 NTU; BOD5 not above 10 mg/L; COD not above 50 mg/L; ammonia nitrogen not above 5 mg/L (below 1 mg/L for copper alloy heat exchangers); total nitrogen not above 15 mg/L; total phosphorus not above 0.5 mg/L; total alkalinity (as CaCO3) not above 350 mg/L; total hardness (as CaCO3) not above 450 mg/L; total dissolved solids not above 1000 mg/L; chloride not above 250 mg/L; sulfate not above 250 mg/L; iron not above 0.3 mg/L; manganese not above 0.1 mg/L; silica not above 30 mg/L; fecal coliform not above 1000 MPN/L; total residual chlorine 0.1 to 0.2 mg/L.

The differences, item by item

1. COD: 60.0 mg/L versus 50 mg/L. GB/T 19923-2024 is stricter.
2. Total phosphorus: 1.0 mg/L versus 0.5 mg/L, a factor of two. Phosphorus is both a nutrient for biological slime and a precursor of calcium phosphate scale, making it the most sensitive item for reclaimed make-up.
3. Hardness and alkalinity are defined differently: GB/T 50050-2017 limits calcium hardness to 250 mg/L and total alkalinity to 200 mg/L, while GB/T 19923-2024 limits total hardness to 450 mg/L and total alkalinity to 350 mg/L. They cannot be compared directly; each must be checked against its own definition.
4. Iron and manganese: 0.5/0.2 mg/L versus 0.3/0.1 mg/L. Elevated iron and manganese in reclaimed water raise the adhesion rate directly and encourage iron bacteria.
5. Suspended solids: the foreword of GB/T 19923-2024 states that the suspended solids limit was deleted, while Table 6.1.3 of GB/T 50050-2017 still requires not above 10.0 mg/L. Suspended solids must therefore be controlled to GB/T 50050-2017 and must not be relaxed just because the reuse standard dropped the item.

4. Corrosion, scaling and microbiology are coupled

Controlling corrosion while ignoring microbiology leaves slime blocking corrosion inhibitor from the metal surface, so no dosing rate will work. Controlling scaling while ignoring corrosion produces rust deposits that are harder to remove than calcium carbonate. GB/T 50050-2017 puts adhesion rate and corrosion rate in the same clause (3.1.5) precisely for this reason. A practical troubleshooting order runs backwards: check cycle of concentration and blowdown first (is the master switch out of control), then make-up water quality (is hardness and solids entering at source), and only then verify whether the three chemical dosing lines are balanced.

5. Cycle of concentration: the code gives a floor, not a ceiling

Clause 6.1.5 is explicit: with reclaimed water as make-up, the cycle of concentration shall be determined by test or by reference to operating experience from similar projects, taking into account reclaimed water quality, circulating water quality targets, the chemical treatment formulation and heat exchanger materials. The code gives only a floor - design not below 5.0 and operation not below 3.0 for indirect open systems - and no ceiling. The ceiling is in effect set inversely by clause 3.1.10, through the 1100 mg/L sum of calcium hardness and total alkalinity and the RSI limit of 3.3.

This means cycle of concentration is not a single-objective "higher saves more water" problem. Reclaimed make-up already carries more alkalinity, hardness and chloride than fresh water, so pushing the cycle up usually hits the scaling criterion before it delivers water savings. Design values for reclaimed make-up are generally more conservative than for fresh water.

6. Parameters the code explicitly leaves to testing

• cycle of concentration (clause 6.1.5);
• the reclaimed water quality indices themselves - clause 6.1.3 permits determination "from tests and operating data from similar projects";
• selection of the reclaimed water treatment process (clause 6.2.1), which requires technical and economic comparison plus reference to operating experience or tests.

Two further clauses are non-negotiable engineering boundaries. Clause 6.1.6 requires the reclaimed water distribution network to be an independent system, strictly prohibits any connection to potable water piping, and requires water quality and quantity monitoring facilities. Clause 6.2.2 requires the influent to the reclaimed water treatment system to meet the Grade 2 criteria of GB 18918 or the Grade 1 criteria of GB 8978.

7. Checklist for the designer

1. Check the reclaimed water as delivered against Table 1 of GB/T 19923-2024, focusing on total phosphorus, iron, manganese and COD.
2. Check the water at the make-up point against Table 6.1.3 of GB/T 50050-2017, adding suspended solids, calcium hardness and total alkalinity.
3. Apply the clause 3.1.10 scaling judgment to decide whether acid dosing or softening is needed.
4. Set the lower bound of cycle of concentration from clause 3.1.11 and the upper bound by back-calculating clause 3.1.10, then take the intersection.
5. Arrange corrosion coupons and a monitoring heat exchanger under clause 3.1.5 and assess both indicators together.
6. Establish routine monitoring of heterotrophic bacteria and biological slime under clause 3.1.12.
7. For any parameter outside the clauses, follow clause 6.1.5: run tests or draw an analogy from comparable projects, and state the basis in the design documents.

All values in this article are taken from the official texts of GB/T 50050-2017 (MOHURD Announcement No. 1576, effective 2018-01-01) and GB/T 19923-2024 (effective 2024-10-01). The statement that the suspended solids limit was deleted is based on the technical changes listed in the foreword of GB/T 19923-2024 relative to the 2005 edition.