Electrolytic Manganese Wastewater Treatment: Chromium, Selenium, Ammonia and Sulphate

Electrolytic manganese metal production combines three awkward constituents in one stream: dissolved manganese at concentrations that make precipitation difficult, hexavalent chromium from cathode passivation, and ammonia used for pH control. Add a very high sulphate load from sulfuric acid leaching, and you have an effluent where each treatment step interferes with the others if the sequence is wrong.

Where Each Constituent Enters

Manganese ore — carbonate or oxide depending on the deposit — is leached with sulfuric acid, and the leach liquor is purified before electrolysis. Purification removes iron and heavy metals as a residue, and that residue handling is a solid waste matter in its own right.

Electrolysis produces cathode manganese, and the cathodes are passivated with a dichromate solution to prevent re-dissolution. That passivation step is the source of hexavalent chromium in the wastewater, and it enters through cathode washing. Where several streams need joint treatment, Bottled Water Plant Wastewater Treatment is the more detailed reference.

Ammonia is used to adjust pH in the electrolyte and in leaching. Selenium compounds are added as an electrolyte additive in many operations, contributing a constituent that is toxic at low concentrations and awkward to remove.

Then there is washdown: cathode washing water, floor wash, filter cloth washing and equipment cleaning. Together with electrolyte purge these make up the bulk of the flow.

Chromium First, Deliberately

Hexavalent chromium is the most toxic constituent present and the easiest to remove if you do it in the right place. Reduce to trivalent at low pH — around 2 to 3 with a suitable reducing agent — then raise pH to precipitate chromium hydroxide.

The important point is to do this before manganese precipitation. Manganese precipitation happens at much higher pH, and if you precipitate manganese first you will carry chromium down with it in a mixed sludge that is harder to handle and may leach.

Keep the chromium-bearing sludge separate. It is smaller in volume and its classification is governed by chromium, whereas mixed sludge tends to inherit the worst classification of everything in it.

Manganese Precipitation

Manganese is removed as hydroxide at around pH 10 to 11. It works, but with two complications worth knowing.

First, manganese hydroxide oxidises readily to higher oxides, which changes both the sludge characteristics and the residual solubility. Where oxidation is uncontrolled you can see dissolved manganese rise again after apparently successful precipitation. Aeration control and reasonably rapid solids separation help. If your site also runs this kind of duty, Detergent, Oil and Reuse covers the cost side of the decision in more depth.

Measured Results from Plants We Built

We commissioned a 20 m³/day plant for this duty in Nantong, Jiangsu back in 2022. Feed COD came in around 43,040 mg/L and left at 500 mg/L — 98.8% removal. The train was Fenton + primary sedimentation + DAF + hydrolysis acidification + contact oxidation.

A second unit of the same class, delivered in Shijiazhuang, Hebei during 2022, ran 10 m³/day at 1,750 mg/L feed COD down to 90 mg/L.

Figures are site-measured values from commissioned plants, not modelled estimates.

Second, the high sulphate background means lime addition produces gypsum alongside the manganese hydroxide, and the sludge volume is dominated by gypsum rather than by the metal you are trying to remove. That is unavoidable with lime; using caustic reduces sludge volume and raises cost. The trade-off is usually worth calculating properly for your specific concentrations.

Ammonia and Nitrogen

Ammonia concentrations from electrolyte operations are typically several hundred to a few thousand milligrams per litre. Stripping at elevated pH is the efficient route at those levels, and recovery as ammonium sulphate is often practical given the sulphate already present.

Where ammonia is lower, biological nitrification-denitrification can handle it, but remember that manganese and residual chromium are both inhibitory to nitrifiers. Remove metals first.

Watch for the interaction between ammonia removal and pH: stripping requires high pH, manganese precipitation requires high pH, and chromium reduction requires low pH. Sequence them deliberately rather than letting pH drift.

Sulphate from sulfuric acid leaching is high, often several thousand milligrams per litre, and it accumulates wherever water is reused. This is the constraint that eventually forces a purge even in plants that would otherwise run a closed circuit.

Where sulphate in the discharge is limited, options are limited too: membrane concentration with the reject going to evaporation, or a larger purge with dilution. Neither is cheap, which is an argument for minimising water use in the first place.

Segregate clean cooling water and keep it out of the circuit. On these sites that single measure usually does more for the water balance than any treatment decision.

Residue Storage

Leach residue and treatment sludge both need secure storage with proper lining and leachate collection. Manganese and chromium in particular will migrate if containment is poor, and legacy residue deposits are a common source of long-term groundwater problems at these sites.

Confirm the classification of each solid stream against its intended destination before commissioning, and revisit it when ore source changes — which it often does, and which changes the impurity profile.

Combining Treatment Stages

The same selection criteria reappear in Waste-to-Energy Incineration Plant Wastewater system, which is worth opening alongside this one. 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

Choosing a manufacturer here comes down to one question: who is still accountable once the commissioning team flies home. Our answer is that we build and programme our own equipment, then stand behind it for the life of the plant.

Manufacturing Base in Guangdong

Owned production lines mean we control tolerances, materials and delivery. Standard modular units and bespoke skids are both built by our own people, so schedule commitments are ours to keep.

Certification and Export Practice

CE marking is standard and we work to ISO 9001 procedures. Where a project calls for specific material grades, pressure vessel certification or ATEX-rated equipment, we build to that standard and ship the full documentation pack.

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.

Experience Built the Hard Way

Since the early 2000s we have commissioned plants for food and beverage processors, chemical works, electroplaters, textile mills, mines and municipalities. The lessons that stick are scaling, fouling and sludge handling — those inform every design we issue.

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.

Spares, Service and Commissioning

Membranes, dosing pumps, diffusers, instrumentation and blowers are stocked for the systems we sell. Our control systems support remote diagnostics, and we can put an engineer on site for commissioning, training or an unplanned shutdown.

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.

Talk to Our Engineers

Send through your raw water data and we will advise on the viable process route and realistic operating cost before you commit to anything. We can be reached on WhatsApp: +86 13631765076 or through hkbhp.com.

WhatsApp: +86 13631765076

Frequently Asked Questions

Which discharge standard will the treated water meet?

We design against whatever consent applies at your boundary — GB 8978, local sewer limits, or a ZLD target with no liquid discharge at all. The design basis is agreed in writing before fabrication, so there is no argument later about what "compliant" meant.

Can you work with our existing equipment?

Often yes. We regularly integrate into existing civil structures where the concrete is sound, replacing internals and controls rather than starting over. We will tell you honestly when legacy kit is beyond economic repair.

What does it cost to run, per cubic metre?

Energy dominates, then chemicals, then sludge disposal. We give you a per-cubic-metre breakdown during proposal so you can compare it against your sewer charge — treating too aggressively is a common and expensive mistake.

Can the plant be modified later if our production changes?

Yes, if you tell us about likely expansion up front. Leaving room in the civil works and oversizing the equalisation tank costs far less than retrofitting later. Electrolytic Manganese Wastewater Treatment rarely fails because of bad chemistry — it usually fails because nobody planned for the second production line.