Nanofiltration Salt Fractionation and Brine Minimization: What HJ 579-2010 Does and Does Not Specify

1. Where This Standard Applies and Where It Does Not

HJ 579-2010, Technical Specifications for Membrane Separation Process in Wastewater Treatment, was approved and issued by the former Ministry of Environmental Protection on 12 October 2010 (Announcement No. 73 of 2010) and took effect on 1 January 2011. Clause 1 states that it covers microfiltration, ultrafiltration, nanofiltration and reverse osmosis applied to wastewater treatment and advanced treatment for reuse, and explicitly excludes membrane bioreactor processes and charged membranes. An NF plus RO salt-fractionation and brine-minimization train may therefore cite it; anything involving MBR may not.

The standard also separates membrane performance loss into three defined mechanisms (3.2 to 3.4): membrane degradation, meaning loss of performance through oxidation or hydrolysis; membrane fouling, meaning deposition of organic pollutants, micro-organisms and their metabolites; and membrane scaling, meaning precipitation of salts on the membrane surface once their solubility product is exceeded. In high-salinity streams all three occur together, so pretreatment and cleaning strategies must address each separately.

2. The Physical Basis of Salt Fractionation Is in Table 4

Salt fractionation works because nanofiltration and reverse osmosis sit at different points on the rejection scale. Table 4 of HJ 579-2010 gives the following values:

• Nanofiltration (NF): filtration precision 0.001 to 0.003 micrometre, molecular weight cut-off 200 to 1 000 u; function is removal of multivalent ions, part of the monovalent ions, and organic matter of 200 to 1 000 Daltons; applications include removal of hardness, colour and radium from well water, partial removal of dissolved salts, and process-stream concentration.
• Reverse osmosis (RO): filtration precision 0.000 4 to 0.000 6 micrometre; removal of dissolved salts and organic matter above 100 Daltons.

In other words, the NF rejection window falls exactly where multivalent ions are retained while part of the monovalent ions pass, which is the window that allows sodium chloride to be separated from sodium sulfate. RO retains and concentrates dissolved salts as a whole. Quoting these two rows in a mass balance is far safer than quoting vendor brochures.

3. Feed Water Limits: Table 3 Is Binding

Clause 4.1.5 requires feed water to nanofiltration and reverse osmosis systems to meet Table 3, and where the limits are exceeded, additional pretreatment is mandatory. Table 3 gives: polyamide composite membrane (PA), turbidity 1 NTU or less, SDI 5 or less, residual chlorine 0.1 mg/L or less; cellulose acetate membrane (CA/CTA), turbidity 1 NTU or less, SDI 5 or less, residual chlorine 0.5 mg/L or less.

On temperature, clause 5.1.3 sets an operating range of 5 to 45 degrees C at pH 2 to 10, and below 35 degrees C where pH exceeds 10. If an alkaline softening route is used upstream, the tighter temperature cap applies.

4. Pretreatment Clauses Mapped to the Risks of High-Salinity Water

Clause 5.3 gives five pretreatment rules that translate directly into design decisions:

1. 5.3.1 to prevent chemical oxidation damage: remove residual chlorine or other oxidants by activated carbon adsorption or by dosing a reducing agent such as sodium bisulfite, keeping residual chlorine at or below 0.1 mg/L.
2. 5.3.2 to prevent colloidal, slime and particulate fouling from iron and aluminium corrosion products: use a dual-media filter with anthracite and quartz sand.
3. 5.3.3 to prevent biological fouling: apply physical or chemical disinfection to the feed.
4. 5.3.4 to control scaling: acid dosing controls carbonate scale effectively, while antiscalant dosing or strong-acid cation resin softening controls sulfate scale effectively.
5. 5.3.5 microfiltration or ultrafiltration producing silt density index of 3 or less and turbidity of 1 NTU or less effectively prevents colloidal and particulate fouling and blockage of modules.

Note that 5.3.4 separates carbonate scaling from sulfate scaling. In salt-fractionation projects the real ceiling in the concentration stage is usually supersaturation of calcium sulfate or strontium sulfate, so softening and antiscalant selection must be built around sulfate scale, not carbonate scale alone.

5. Design Calculations That Can Be Cited Directly

• 6.1.5 on the low-pressure side use PVC pipe and fittings, on the high-pressure side use stainless steel pipe and fittings, to resist corrosion from acid and chlorine dosing.
• 6.1.7 pressure vessel array ratios for a single-stage multi-pass NF or RO system are preferably 2:1, 3:2, 4:2:1 or scaled up proportionally.
• 6.3.2.1 take 25 degrees C as the design temperature; permeate flow rises or falls by 2.5 percent for every 1 degree C rise or fall.
• 6.3.2.2 the number of elements equals the design permeate flow divided by the maximum element permeate flow and by a design safety factor of 0.8.
• 6.3.3.1 for systems of 50 m3/h or more, the design flow of the feed header equals the sum of the design flows into every pressure vessel; 6.3.3.2 permeate branch and header velocity is preferably 1.0 m/s or less.
• 6.3.5.1 the control system must monitor feed pressure, feed conductivity, permeate flow, permeate conductivity, concentrate flow and concentrate pressure; 6.3.5.2 a residual chlorine monitor is required on the feed line and must interlock with the reducing-agent dosing unit; 6.3.5.3 the high-pressure pump needs a low-pressure switch on the inlet and a high-pressure switch on the outlet; 6.3.5.4 where acid is dosed for pH, upper and lower pH cut-out switches are required.

6. The Concentrate Stream Cannot Be Skipped

Clause 6.1.6 requires concentrate from the membrane system to be treated before compliant discharge. Clause 6.4 adds that concentrate may be routed to the biological treatment system or collected together with chemical cleaning wastewater and backwash water from media and activated carbon filters, and recommends a train of equalization, reaction, sedimentation, hydrolysis-acidification and contact oxidation, with 6.4.3 requiring the treated concentrate to meet national or local discharge standards. Minimizing volume is only half the answer; where the concentrate goes must be answered in the same document.

7. Commissioning and Chemical Cleaning Triggers

Clause 7.2.2 can be lifted straight into a commissioning programme: flush pressure 0.2 to 0.4 MPa; flush flow 0.6 to 3.0 m3/h for a 100 mm pressure vessel and 2.4 to 12.0 m3/h for a 200 mm vessel; low-pressure flushing of at least 30 min for elements containing preservation liquid and at least 6 h for dry elements; pressure ramp below 0.07 MPa per second on first start of the high-pressure pump; and 24 to 48 h of continuous running to record baseline performance data.

Appendix C.2.1 sets three triggers for chemical cleaning of NF and RO systems: permeate flow down by 10 percent, differential pressure up by 15 percent, or salt passage up by 5 percent. Meeting any one of them calls for cleaning.

8. What This Standard Does Not Give You

Two gaps must be stated honestly. First, HJ 579-2010 gives no monovalent-to-divalent rejection figures for nanofiltration membranes and no process parameters for salt crystallization routes. Those belong to element selection and mass balance work and must come from bench or pilot testing on the actual water and from membrane manufacturer data; they cannot be read off a table in this standard. Second, charged membranes are outside its scope, so a fractionation scheme that relies on charged nanofiltration membranes needs a different basis.

As a cross-reference, GB/T 30888-2014, Technical Specification for the Treatment and Reuse of Textile Wastewater by Membrane Separation (issued 30 September 2014, effective 1 February 2015, current), is another current voluntary national standard covering microfiltration, ultrafiltration, nanofiltration and reverse osmosis, and can be read alongside HJ 579-2010 on process selection principles. It likewise provides no salt-split rejection values.

Bottom line: a nanofiltration salt-fractionation project can safely cite HJ 579-2010 for feed limits, pretreatment, design calculations, instrumentation and cleaning triggers. Rejection ratios, crystallization routes and energy figures must come back to test data and membrane element documentation; the standard offers no substitute.