Chlor-Alkali Plant Wastewater Treatment: Chlorate, Bromate and Sulphate Purge

A membrane-cell chlor-alkali plant looks close to closed loop, and mostly is. Brine recycles, caustic and chlorine leave as product, and the only real aqueous outputs are a purge, a scrubber stream and various intermittent drains. The reason that modest volume still needs real treatment is that the contaminants are awkward: residual chlorine species, chlorate accumulating in the loop, bromate formed from trace bromide, and sulphate that has nowhere else to go.

The Streams, One at a Time

Depleted brine is the main continuous purge. Raw salt brings in sulphate, and sulphate accumulates because it has no outlet — so the loop has to be bled. Sulphate in the purge runs 5 to 15 g/L, along with chlorate at a few grams per litre and whatever bromide the salt carried.

Tail gas scrubbing produces the second stream. Chlorine-containing vent gas is scrubbed with caustic, giving a sodium hypochlorite solution. If it is sold as bleach, it is a product; if not, it is a wastewater with free chlorine, chlorate and high pH. The third group is intermittent: ion exchange regeneration backwash from brine softening with calcium and magnesium, floor wash, RO or NF reject, and cooling water blowdown. Where several streams need joint treatment, Fertilizer Manufacturing Wastewater Treatment is the more detailed reference.

Treating these together is a mistake. Hypochlorite and depleted brine need opposite things — one is an oxidant to destroy, the other is a salt solution to separate.

Dechlorination First, Always

Free chlorine species wreck downstream biology and damage membranes, so they come out first. Vacuum dechlorination strips most of the dissolved chlorine gas, and the remaining hypochlorite is chemically reduced with sodium sulphite, sodium bisulphite or hydrogen peroxide, or passed through activated carbon. For the reuse angle specifically, Flue Gas Desulfurization Wastewater Treatment system goes further than we can here.

Sulphite dosing needs control rather than excess. Under-dose and residual oxidant attacks whatever follows; over-dose and the surplus sulphite becomes a COD load and consumes oxygen in the receiving water. Oxidation-reduction potential control with a trim dose is the right answer and is not expensive.

Activated carbon is the fallback where chemical dosing is undesirable. It reduces hypochlorite to chloride with no added dissolved solids, but the carbon is consumed, so it becomes a solid waste stream and a media replacement cost.

Chlorate and Bromate: The Ones Regulators Watch

Chlorate forms wherever hypochlorite sits, by disproportionation, and it is present in almost every chlor-alkali aqueous stream at some level. It is also increasingly regulated in drinking water, which matters because chlorate is poorly rejected by reverse osmosis and passes through to the permeate. If a plant discharges to a watercourse used downstream for potable abstraction, chlorate can become the binding constraint.

Reference Data from 42 Commissioned Plants

Across 42 commissioned plants where we hold complete COD records, influent COD ranged from 200 to 172,000 mg/L and treated effluent from 50 to 5,000 mg/L. Average removal across that set is 87.1% — not a marketing figure, but the measured mean.

That average is worth pausing on. It sits well below the 95%-plus numbers most suppliers quote, because the set includes genuinely difficult streams. On some electroplating and municipal duties the installed configuration only reaches about half the influent COD, and saying that up front is more use to you than a number the plant will never hold.

Destruction routes exist. Strong acid with a reducing agent — sulphur dioxide, ferrous iron — reduces chlorate to chloride, and concentrated hydrochloric acid at elevated temperature does the same. Both need corrosion-resistant construction and careful handling. Biological reduction is gentler: perchlorate- and chlorate-reducing bacteria will take chlorate to chloride under anaerobic conditions with a suitable electron donor, in the same kind of reactor used for perchlorate removal from groundwater.

Bromate is the more serious one. Trace bromide in the salt is oxidised through to bromate wherever hypochlorite or ozone is present, and bromate is a probable human carcinogen with a guideline value in the low tens of micrograms per litre. It is not destroyed by biological treatment under normal conditions. The practical control is upstream — limit bromide in the salt specification, avoid ozone anywhere in the water loop, and keep hypochlorite contact times short.

Sulphate Removal with Nanofiltration

The reason the brine loop needs a purge is sulphate, and nanofiltration removes it without removing the chloride the plant actually wants. NF membranes carry a charge that rejects divalent sulphate strongly while passing monovalent chloride and sodium, so a sulphate removal unit takes a slipstream, returns the permeate to the loop and sends a small high-sulphate concentrate to disposal or evaporation.

This changes the purge from a steady bleed into a much smaller concentrate stream, and that is usually enough to turn a discharge problem into a manageable one. Recovery rates are limited by gypsum scaling on the membrane — sulphate and calcium in the same water will precipitate as the concentrate is concentrated — so antiscalant dosing and a conservative recovery target are standard.

Where NF is not justified, the old answers still apply: barium chloride or barium carbonate precipitation, and calcium aluminate treatment. Both produce a solid waste stream containing barium or aluminium, which is why most new capacity goes to membranes.

Legacy Technologies and Their Footprints

Not every plant is membrane cell. Diaphragm cells use asbestos diaphragms and their wastewater can carry fibres, requiring dedicated filtration and handling under asbestos rules. Mercury cells are being phased out globally, and where one operated, mercury in the brine circuit and in the cell room floor drainage remains a contamination issue long after shutdown — it binds to sediment and requires isolation rather than treatment.

Graphite-anode cells, long retired, left another legacy: chlorinated dioxins and furans in the sludge and in the cell room structure. Remediation of those sites is a demolition and containment problem, not a wastewater problem, and it should be scoped separately from the operating plant. We have written about the downstream implications separately in Rendering Plant Wastewater Treatment system.

Combining Treatment Stages

Where several streams need joint treatment, metal pickling line wastewater treatment is the more detailed reference. Most facilities do not run a single clean stream, and a shared equalisation and biological stage is usually the economical answer once the streams are chemically compatible.

Why Choose Baihuipu as Your Manufacturer

Picking a supplier for this kind of plant is mostly about who carries the risk when something goes wrong. We have built this equipment for more than twenty years, and we are the ones who fabricated it, programmed the controls and will still answer the phone three years from now.

Own Factory, Real Capacity

Tanks, skids, control panels and membrane housings are built in our own Guangdong facility. Nothing critical is subcontracted, which is why we can commit to delivery dates and hold quality across a batch instead of hoping the shop floor gets it right.

Two Decades of Field Data

Food and beverage, chemicals, electroplating, textile dyeing, mining, municipal work — we have commissioned across all of them. The useful part is not the project count, it is having seen how plants fail in year seven and designing those failures out.

Twenty-Plus Export Markets

We have shipped to the United States, Canada, Russia, Indonesia, India, Nigeria and Vietnam among others. Crating, export documentation and certificates are handled in-house, which keeps customs surprises to a minimum.

We Engineer the Whole Train

Screening, equalisation, biological or chemical treatment, membranes, evaporation, brine handling — the complete line comes from one organisation. Process design, mechanical, electrical and PLC programming sit under the same roof, so there is no gap for finger-pointing at handover.

Documentation You Can Actually Audit

Each system ships with as-built drawings, material certificates, welding records and a functional specification. If a client's own consultant wants to review the process calculation basis, we hand it over rather than treating it as proprietary.

Spare Parts and After-Sales Support

We carry membranes, pumps, blowers, diffusers and instruments for every system we supply. Remote support through the PLC is standard on most builds, and on-site service covers commissioning, training and urgent callouts.

Let's Look at Your Numbers

Share your water analysis and site constraints and we will give you a straight assessment of the process route and the realistic operating cost before you commit to anything. Contact our engineers at WhatsApp: +86 13631765076, or find us at hkbhp.com.

WhatsApp: +86 13631765076

Frequently Asked Questions

How do you handle sludge from this process?

Sludge handling is where most budgets quietly blow out. We size dewatering against realistic solids production, and we usually recommend a filter press for this duty. The disposal route — landfill, incineration, or a licensed contractor — should be confirmed before commissioning.

Is chlor-alkali treatment suitable for water reuse?

Usually yes, once the dechlorination first, always stage is stable. Reuse normally means adding membrane polishing to bring conductivity and COD down to your process water specification. We would need to see your reuse water limits to size it properly.

What happens if the incoming chlor-alkali load exceeds the design figure?

Load swings are the most common reason plants underperform. We set the design envelope from your worst-case samples, not your average ones, and specify enough buffer in the biological stage to ride out shocks. If your process genuinely shifts up permanently, we can usually add capacity in stages.