Zero Liquid Discharge: Comparing Fractional Crystallization Routes by Salinity Band, Salt Composition Decision Tree and Salt Fate

1. Validity first: ZLD has no dedicated national engineering code

Zero liquid discharge (ZLD) has no corresponding national environmental protection standard or national engineering construction code in China. The only primary documents available are of two kinds, both of low hierarchical status, and their status must be stated honestly when cited. This article rests on two of them:

• the Shanxi provincial local standard Technical guideline for centralized treatment and reuse of industrial park wastewater (draft for comment, released for public comment 2025-05), whose clauses 7.5 to 7.9 give the most complete selection logic available today for concentration, crystallization and salt splitting;
• the group standard T/ACEF 127-2024, Technical guidelines for zero discharge engineering of high salt wastewater in thermal power plants (issued 2024-01-11, effective 2024-02-01), whose scope is limited to thermal power plants but which gives concrete concentration stages and crystallization parameters.

Neither is mandatory. When applying T/ACEF 127-2024 outside the power sector, the comparability of water quality and flow boundaries must be demonstrated first.

2. The three-stage skeleton: pretreatment, concentration, crystallization

Clause 7.1 of the Shanxi guideline divides a reuse project into pretreatment, membrane concentration and crystallization-solidification. Clause 7.5 defines the pretreatment tasks before membrane concentration: media filtration, microfiltration or ultrafiltration to reduce turbidity; coagulation-sedimentation, chemical precipitation or ion exchange to reduce hardness; chemical precipitation or ion exchange to remove fluoride and silica. The goal here is not purification but pushing scaling and fouling risk on the membrane system into a controllable range.

T/ACEF 127-2024 defines high-salinity wastewater as wastewater from a power plant drainage system that cannot be economically reused elsewhere after repeated use and whose salt content approaches or exceeds 1%, and defines zero wastewater discharge as a state with no liquid effluent, where salts and pollutants leave as solids for disposal or reuse.

3. Concentration: select the process by total dissolved solids

Clause 7.6 of the Shanxi guideline is the most useful table in the document, splitting the concentration path into three bands by total salt content:

• where total salt is not above 50 000 mg/L, membrane separation should be used for further concentration with permeate reused;
• once concentrated to 50 000 to 100 000 mg/L, either membrane concentration plus crystallization-solidification, or direct crystallization-solidification, should be selected according to water quality and reuse requirements;
• once concentrated to not below 100 000 mg/L, evaporative crystallization-solidification or freeze crystallization with salt splitting should be selected.

Clause 5.2.4 of T/ACEF 127-2024 gives an empirical ladder for power plant conditions that serves as a cross-check: at 1% to 4% salt, ultrafiltration/microfiltration plus seawater reverse osmosis can concentrate to 5% to 8%; at 5% to 8% salt, electrodialysis, high-pressure reverse osmosis, multi-effect evaporation, mechanical vapor recompression (MVR) and low-temperature flue gas concentration can concentrate further to 12% to 20%.

The two are consistent: membranes carry the load up to roughly 5% to 8% (about 50 000 to 80 000 mg/L), beyond which thermal processes or electrodialysis must take over. This is the first inflection point on the ZLD energy curve.

4. Choosing the salt splitting route: the three-branch decision in clause 7.7

Salt splitting is what moves a ZLD project from "it runs" to "it can be accounted for", and it is where investment and operating cost diverge most. Clause 7.7 gives three branches based on the salt composition of the concentrate:

1. where sodium sulfate or sodium chloride alone exceeds 80% of total salt, freeze crystallization should be used to purify and recover the salt;
2. where one of the two is clearly dominant and shows a high fractional crystallization yield during evaporation, thermal sodium sulfate or sodium chloride splitting should be used;
3. where sodium sulfate and sodium chloride are comparable and mixed salt tends to crystallize during evaporation, nanofiltration membrane salt splitting should be used.

Note that the criterion is salt composition, not total salt. Before the salt splitting unit is selected, a full ionic composition analysis (at minimum Na+, Cl- and SO4 2- with a mass balance check) must be completed, otherwise no branch can be assigned. This is the step most often skipped and most often paid for.

Clause 5.3.3 of T/ACEF 127-2024 sets the closure requirement from the power plant perspective: nanofiltration salt splitting is preferred; crystallized salt shall reach at least the second-grade industrial salt standard; no new solid waste shall be generated. The last clause actually moves the mixed-salt characterization question forward into process selection: choose the wrong splitting route and the mixed salt may be identified as hazardous waste under GB 5085.7, collapsing the economics of the whole line. Clause 8.2 of the Shanxi guideline makes the same point, requiring characterization of sludge and mixed salt under GB 5085.7 and, where hazardous, comprehensive use or safe disposal under the relevant national standards.

5. Crystallization and solidification: options and materials

Clause 7.8 of the Shanxi guideline: crystallized salt may be solidified using fluidized bed drying, spray drying or rake drying.

Clause 5.3.1 of T/ACEF 127-2024 lists a wider set: bypass flue evaporation, two-fluid atomizing evaporation drying, rotary atomizing evaporation drying and evaporative crystallization. The quantitative boundaries for bypass flue evaporation are in clause 5.3.2:

• design evaporation per 100 MW capacity should not exceed 1 t/h;
• total extracted high-temperature flue gas should not exceed 3% of the boiler BMCR flue gas flow;
• extracted flue gas temperature should not be below 280 degrees C;
• bypass outlet flue gas temperature should be more than 10 degrees C above the acid dew point.

Clause 5.4.1 adds that when flue gas evaporation drying is used, the impact on unit coal consumption shall be assessed. This is a hidden cost item that is routinely underestimated.

On materials, clause 6.3 requires that areas of metallic surfaces possibly exposed to corrosive media be protected by corrosion- and wear-resistant metallic or non-metallic materials, with metallic options of duplex stainless steel 2205, 2507 or above. Clause 6.4 adds that where metal serves as the pressure-bearing component and a non-metallic lining provides corrosion protection, the bond strength between them shall be analysed.

Product salt quality is judged against GB/T 5462-2015, Industrial salt (issued 2015-10-09, effective 2016-05-01, replacing GB/T 5462-2003, under the National Technical Committee on Salt Industry Standardization). The standard specifies terms and definitions, product classification, technical requirements, test methods, batching and sampling, inspection rules, and packaging, marking, transport and storage.

6. Secondary pollution: do not look only at the water

Clause 8.3 of the Shanxi guideline requires cyclone or wet dust collection for dust-laden off-gas from evaporative crystallization systems, and control of volatile organic compounds at open liquid surfaces in accordance with GB 37822 and sector emission requirements. Clause 8.1 requires enclosure or covering of inlet screening chambers, equalization tanks, primary sedimentation tanks, anaerobic or anoxic tanks and sludge thickening tanks, with collected odour treated to meet GB 14554.

Clause 7.9 states the precondition for ZLD: areas with insufficient water environmental capacity or scarce water resources should adopt zero liquid discharge to achieve wastewater recycling and pollutant reduction. Conversely, projects in areas without either precondition must justify necessity specifically at the environmental impact assessment stage.

7. Checklist and the parameters left to testing

1. Analyse total salt and ionic composition first; fix the concentration stage under clause 7.6 and the splitting stage under clause 7.7.
2. Complete the clause 7.5 pretreatment for turbidity, hardness, fluoride and silica before membrane concentration.
3. Settle the destination of the crystallized salt in advance: grade it under GB/T 5462-2015 and characterize mixed salt under GB 5085.7.
4. Select duplex stainless steel 2205/2507 or above, and verify bond strength for lined systems.
5. Design dust-laden off-gas and odour control together with the main process, not as a retrofit at acceptance.

Parameters with no codified value that must be determined by testing include: optimum membrane recovery and the upper bound of concentrate total dissolved solids (clause 7.6 gives bands, not single-process values); nanofiltration rejection and operating pressure; freeze crystallization temperature and mother liquor recycle ratio; terminal concentration for thermal splitting; and energy consumption for salt drying. These must be established by bench or pilot testing on the actual water; this article provides no recommended values.

The selection logic here is taken from clauses 7.5 to 7.9 and Chapter 8 of the Shanxi provincial local standard Technical guideline for centralized treatment and reuse of industrial park wastewater (draft for comment, 2025-05); the concentration ladder and bypass flue parameters are from T/ACEF 127-2024 (effective 2024-02-01, applicable to thermal power plants only); product salt grading follows GB/T 5462-2015. Neither document is a mandatory national code; engineering design shall follow the environmental impact assessment approval and the current mandatory standards.