INDUSTRIES / STEEL AND METALS
Steel plant wastewater treatment
that recirculates water and recovers scale and acid.
A steel plant moves huge volumes of water through gas cleaners, casters, mills and cooling towers, and a few small streams such as coke oven effluent and spent pickle liquor carry most of the pollution. We test each circuit in our NABL-accredited laboratory, then design the treatment that keeps water recirculating and toxic streams under control.
In short
How steel plant wastewater is treated.
Steel plant wastewater treatment handles three kinds of water. Indirect cooling water for furnaces, moulds and motors stays clean and is recirculated through cooling towers with chemical treatment. Direct contact water from blast furnace and converter gas cleaning, continuous casting and rolling picks up iron oxide, scale and oil, and is settled, de-oiled, filtered, cooled and recirculated. Process effluents are the toxic ones: coke oven by-product plant effluent, with phenols, cyanide, thiocyanate and ammonia, is treated in an ammonia still and a biological plant; spent pickling acid is regenerated or neutralised to precipitate iron and metals. Integrated plants must meet the iron and steel industry standards under the Environment (Protection) Rules, 1986.
- 1.0mg/L
- Phenolic compounds limit under the general effluent standards
- 0.2mg/L
- Cyanide limit under the general effluent standards
- 50mg/L
- Ammoniacal nitrogen limit for discharge to inland surface water
- 17categories
- Of highly polluting industry that must run online monitoring; integrated iron and steel is one
Where it comes from
Where does steel plant wastewater come from?
An integrated plant makes iron from ore and coal in coke ovens, sinter plants and blast furnaces, then steel in converters. A secondary plant melts scrap or sponge iron in an electric arc or induction furnace. Both then cast and roll. The integrated route has far more effluent, mainly because of the coke ovens.
| Section | Water use | What it adds |
|---|---|---|
| Coke oven and by-product plant | Gas cooling, ammonia liquor, tar and benzol recovery; coke quenching | Phenols, cyanide, thiocyanate, ammonia, oil and tar, high COD; the most toxic stream in the plant |
| Blast furnace gas cleaning | Wet scrubbers clean and cool the top gas | Fine iron oxide and carbon dust, zinc and lead, some cyanide and ammonia; warm |
| Converter (BOF) gas cleaning | Wet scrubbers on converter off-gas | Fine iron oxide, lime; often high pH |
| Slag granulation | Water quenches molten slag into granules for cement | Fine slag, sulphides, high pH; heat |
| Continuous casting | Mould cooling (indirect) and spray cooling of the strand (direct) | Scale and oil in spray water |
| Hot rolling and re-rolling mills | Descaling sprays, roll and table cooling | Mill scale, lubricating and hydraulic oil |
| Pickling and cold rolling | Acid removes scale before cold rolling, galvanising or wire drawing; rinsing; rolling emulsions | Acid, dissolved iron, zinc and other metals; spent emulsions; nitrate and fluoride from stainless pickling |
| Furnace and equipment cooling | Closed circuits for EAF panels, induction coils, moulds and motors | Cooling tower blowdown: dissolved salts, inhibitors |
| Raw material yards, utilities and township | Dust suppression, DM and softener regeneration, sewage | Runoff solids, regeneration salts, BOD |
Typical characteristics of the main streams
| Stream | Typical quality | Why it matters |
|---|---|---|
| Coke oven effluent after ammonia still | COD 2,000 to 6,000 mg/L; phenols 200 to 1,000 mg/L; thiocyanate several hundred mg/L; ammonia 50 to 300 mg/L; cyanide 5 to 50 mg/L | Toxic to fish and to unadapted bacteria; needs a dedicated biological plant |
| Blast furnace gas cleaning water | Suspended solids from several hundred to several thousand mg/L; zinc and lead; hardness | Sludge settles fast but scales pipes and spray nozzles |
| Rolling mill scale pit water | Scale from hundreds to thousands of mg/L at the pit; oil tens to hundreds of mg/L | Oil and fine scale block filters and nozzles if not removed |
| Pickling rinse water | pH 1 to 3; dissolved iron hundreds of mg/L; zinc where galvanising | Must be neutralised; metals limit reuse and discharge |
| Spent pickle liquor | A few percent free acid with very high dissolved iron | Hazardous waste; best regenerated or sent to an authorised recycler |
| Cooling tower blowdown | TDS 1,500 to 4,000 mg/L; hardness, silica, inhibitors | Main dissolved salt outflow from the plant |
Secondary steel units, such as induction furnace and re-rolling mills, have no coke ovens or blast furnaces. Their water use is mainly closed cooling for coils and moulds, which needs soft or demineralised water, plus rolling mill direct cooling with scale and oil, cooling tower blowdown and sewage. Many state boards require these units to discharge no process water.
Standards and rules
Which standards apply to steel plant effluent?
Integrated iron and steel plants have industry-specific standards under the Environment (Protection) Rules, 1986, revised by MoEF&CC in 2012, with separate limits for the coke oven by-product plant, sinter plant, blast furnace, steel melting shop and rolling mills. Secondary units are usually held to the general standards and to no-discharge conditions in their consent. The general standards for the parameters that matter most in steel are below.
| Parameter | General standard |
|---|---|
| pH | 5.5 to 9.0 |
| Suspended solids | 100 |
| Oil and grease | 10 |
| BOD (3 days at 27 °C) | 30 |
| COD | 250 |
| Ammoniacal nitrogen | 50 |
| Phenolic compounds | 1.0 |
| Cyanide | 0.2 |
| Hexavalent chromium | 0.1 |
| Total chromium | 2.0 |
| Dissolved iron | 3.0 |
| Zinc | 5.0 |
| Lead | 0.1 |
| Fluoride | 2.0 |
Treatment scheme
Steel plant water, circuit by circuit.
The rule in steel is to treat each circuit just enough to send its water back, and to keep the toxic process streams separate from the large recirculating flows.
| Circuit | Treatment | Typical units |
|---|---|---|
| 1. Coke oven effluent | Tar and oil removed; free and fixed ammonia stripped with steam and lime or caustic; then biological treatment that removes phenols, thiocyanate, cyanide and ammonia, often with nitrification and denitrification; tertiary polish | Tar decanter, ammonia still, equalisation, activated sludge in anoxic and aerobic stages, clarifier, sand and carbon filters |
| 2. Blast furnace and BOF gas cleaning water | Solids settled with coagulant and polymer; pH and scale control; water cooled and returned to the scrubbers; sludge dewatered | Thickener or clarifier, cooling tower, anti-scalant dosing, filter press |
| 3. Casting and rolling mill direct water | Coarse scale settled in the scale pit; oil skimmed; fine scale removed in a clarifier and pressure filters; water cooled and returned to sprays | Scale pit, oil skimmer, settling tank or clarifier, pressure sand filters, cooling tower |
| 4. Pickling | Spent acid regenerated or sent to an authorised recycler; rinse water neutralised with lime or caustic to precipitate iron, zinc and other metals; clarified and filtered | Acid regeneration plant or ferrous sulphate crystalliser; neutralisation tanks, clarifier, filter press |
| 5. Indirect cooling circuits | Soft or DM water in closed loops; open towers with scale and corrosion inhibitors and biocide | Softener or DM plant, plate heat exchangers, cooling towers, dosing |
| 6. Cold mill emulsions | Emulsions split chemically or by ultrafiltration; oil recovered; water to the main ETP | Emulsion breaking tanks, DAF or UF |
| 7. Blowdown and final polish | Combined residual effluent treated for reuse in slag granulation, dust suppression or, after RO, as make-up | Clarifier, filters, UF, RO; evaporation where zero discharge applies |
Options for the toughest problems
| Option | Best for | Watch-outs |
|---|---|---|
| Single-stage activated sludge | Phenol and thiocyanate removal in coke oven effluent | Ammonia is often not removed fully; sensitive to cyanide and phenol shocks |
| Anoxic and aerobic stages (nitrification and denitrification) | Meeting ammonia and total nitrogen limits | Needs long sludge age, alkalinity and careful control |
| HCl regeneration | Large hydrochloric pickling lines; recovers acid and iron oxide | High capital cost; justified by acid volume |
| Lime neutralisation of pickling waste | Small and medium lines, galvanisers, wire plants | Large volume of iron hydroxide sludge to dispose of as hazardous waste |
| Selective metal adsorbents | Polishing zinc, nickel, chromium and other metals after precipitation; recovering them | Trialled on your effluent first |
Our NABL-accredited laboratory tests phenols, cyanide, thiocyanate, ammonia, COD, oil, suspended solids, iron, zinc, chromium and other metals, so each treatment stage is sized on measured values.
Recirculation and recovery
What a steel plant can recover from its water.
Most of the water in a steel plant should never leave it. Direct and indirect circuits run in loops with only a small blowdown, and several things removed from the water have value.
Mill scale
Scale from scale pits, clarifiers and filters is mostly iron oxide. Once de-oiled, it goes back to the sinter plant or is sold, so better oil removal raises its value.
Acid and iron
Regenerating hydrochloric acid gives back acid and an iron oxide product. Sulphuric acid pickling liquor can yield ferrous sulphate crystals. Both remove a hazardous waste and a neutralisation bill.
Coke oven by-products
The by-product plant already recovers tar, benzol and ammonia (as ammonium sulphate) from coke oven gas. Efficient tar removal and ammonia stripping also make the biological plant smaller and more stable.
Metals
Zinc from galvanising rinses and nickel or chromium from stainless pickling can be captured rather than buried in sludge. Our heavy metal recovery technology uses selective, reusable polymer adsorbents that remove over 90% of target metals from acidic effluent.
Water
Treated coke oven effluent and blowdown can go to slag granulation and dust suppression; after RO they become cooling tower make-up. Where zero discharge is required, our non-thermal evaporator handles the RO reject using air and the plant's abundant waste heat.
Cost
How much does steel plant water treatment cost?
Costs vary more by circuit than by plant size. These ranges help with budgets for the packages most steel and metal units buy.
| Package | Typical scope | Indicative cost* |
|---|---|---|
| Induction furnace or re-rolling mill water system | Softener, closed cooling, scale pit with oil skimming, filter, cooling tower | ₹20 to 80 lakh |
| Pickling rinse neutralisation, 20 to 50 KLD | Neutralisation, clarifier, filter press, polishing filter | ₹25 to 70 lakh |
| Rolling mill direct water treatment, 100 to 300 m³/hour | Clarifier, pressure filters, oil removal, cooling tower | ₹1.5 to 5 crore |
| Coke oven effluent biological plant | Equalisation, multi-stage biology, tertiary polish | Priced after load study and treatability tests |
| Township STP, 500 KLD to 1 MLD | MBBR or SBR with filtration and disinfection for reuse | ₹70 lakh to 2.5 crore |
*Indicative 2026 ranges for equipment, electricals and installation, excluding land, civil works, plant-wide piping and taxes. Acid volume, metals, coke oven load and the level of recirculation change the figure a lot, so we price only after testing. Acid regeneration and full ZLD are separate, larger investments sized on your volumes.
What drives capital cost
- Recirculating flow rates
- Coke oven load and nitrogen limits
- Acid volume and type
- Metals to remove or recover
- Discharge, reuse or ZLD
What drives running cost
- Lime, caustic and polymer
- Aeration power in the BOD plant
- Hazardous sludge disposal
- Cooling water chemicals
- Filter and pump maintenance
Where plants lose money
- Oil left in mill scale
- Spent acid neutralised, not recovered
- Phenol shocks killing the biology
- Hard water scaling induction coils
- Blowdown run higher than needed
We test the circuit, find the cause and supply the fix: softeners, cooling chemicals, filters, oil removal or a biological upgrade.
Troubleshooting
Common steel plant water problems, and how we fix them.
- 01The problem
Phenol, cyanide or ammonia breaks through the coke oven biological plant.
What we doCheck tar removal and ammonia still performance first, add equalisation, then restore sludge age, alkalinity and oxygen so nitrifiers survive. Test thiocyanate, which often explains high COD.
- 02The problem
Induction furnace coils and mould jackets scale and fail.
What we doUse softened or DM water in the closed loop, keep it closed, and treat it with corrosion inhibitor; keep hard tower water on the other side of a plate heat exchanger.
- 03The problem
Rolling mill spray nozzles and filters choke with oily scale.
What we doImprove oil skimming at the scale pit, add a clarifier with polymer before the filters, and fix backwash so fine scale does not cycle round the loop.
- 04The problem
Gas cleaning water lines and scrubbers scale.
What we doControl pH and hardness, dose anti-scalant matched to the water, and set blowdown on measured conductivity.
- 05The problem
Pickling sludge volume and disposal cost keep rising.
What we doCut drag-out with counter-current rinsing, separate spent acid for recovery, and dewater the remaining sludge in a filter press.
- 06The problem
Zinc or chromium in the outlet is above limit despite neutralisation.
What we doReduce hexavalent chromium before precipitation, hold the right pH for each metal, and polish with selective adsorbents.
What we offer
What Rispri does for steel and metal plants.
Water and effluent testing
Phenols, cyanide, ammonia, oil, metals and cooling water chemistry in our NABL-accredited lab.
Testing services →Effluent treatment plants
Coke oven biology, neutralisation, clarifiers, filters and oil removal, designed and built.
Effluent treatment plants →Heavy metal recovery
Selective, reusable polymer adsorbents that capture metals from acidic effluent.
Heavy metal recovery →Cooling and treatment chemicals
Scale and corrosion inhibitors, biocides, coagulants, polymers and descalers.
Water treatment chemicals →OCEMS
Online effluent monitoring linked to CPCB and state board servers.
Smart water management →Buying guide
What to ask before you buy steel plant water treatment.
Use these questions with any supplier, including us.
- Was each circuit tested separately? Coke oven, gas cleaning, mill and pickling water are different problems.
- What recirculation rate and blowdown are designed for? And how blowdown will be controlled.
- How are phenol, cyanide, thiocyanate and ammonia removed? Ask for the biological design basis.
- What happens to spent acid? Regeneration, recycling or neutralisation, with the cost of each.
- How is oil kept out of the scale and the filters? Skimming, clarification and filter backwash design.
- Which outlet or reuse values are guaranteed? Against the iron and steel standards and your consent.
- Which sludges are hazardous, and where will they go? Volumes, authorisation and disposal routes.
- Who supplies chemicals and tests the water after start-up? Monthly NABL reports and on-site support.
Why Rispri
Testing, chemistry and plants for every circuit.
Our own NABL lab
We test each circuit before design and certify outlet and reuse quality after start-up.
Engineering since 1999
1,498+ plants and turnkey projects for industries, institutions and cities.
Research partnerships
Heavy metal recovery and low-energy evaporation developed in collaboration with IIT Bombay, IIT Kharagpur and ICT Mumbai.
Chemicals, softeners and O&M
Cooling chemicals, softeners, spares and trained operators from one team.
Questions
Steel plant wastewater FAQs.
What wastewater does a steel plant produce?
An integrated plant produces coke oven by-product effluent, blast furnace and converter gas cleaning water, slag granulation water, casting and rolling mill water with scale and oil, pickling waste, cooling tower blowdown and sewage. Secondary steel units mainly produce cooling blowdown, rolling mill water and sewage.
How is coke oven effluent treated?
Tar and oil are removed first, then ammonia is stripped in an ammonia still. The effluent is equalised and treated biologically, usually in anoxic and aerobic stages that remove phenols, thiocyanate, cyanide and ammonia, followed by clarification and filtration.
Why is coke oven wastewater so hard to treat?
It contains phenols, cyanide and thiocyanate, which are toxic to bacteria and fish, along with high ammonia. The biology must be acclimatised, protected from shock loads and run with long sludge age to remove both carbon and nitrogen reliably.
How is rolling mill scale pit water treated?
Coarse scale settles in the scale pit, where oil is skimmed off. Fine scale is removed in a clarifier and pressure filters, and the water is cooled in a cooling tower and returned to the mill sprays. The recovered scale goes to the sinter plant or is sold.
What happens to spent pickling acid?
Spent pickle liquor is hazardous waste. Large hydrochloric acid lines regenerate it into fresh acid and iron oxide; sulphuric acid liquor can be crystallised to ferrous sulphate; smaller units send it to authorised recyclers or neutralise it with lime and dispose of the sludge.
What standards apply to steel plant effluent?
Integrated iron and steel plants follow the industry-specific standards under the Environment (Protection) Rules, 1986, revised in 2012, with separate limits for each sub-plant. Other units follow the general standards, such as phenols 1.0 mg/L, cyanide 0.2 mg/L and ammoniacal nitrogen 50 mg/L, and the conditions of their consent.
Do steel plants need OCEMS?
Integrated iron and steel plants are among CPCB's 17 categories of highly polluting industry and must run online continuous effluent and emission monitoring connected to CPCB and the state board. Boards may also ask other large units to install it.
How is blast furnace gas cleaning water treated?
The scrubber water is sent to a thickener or clarifier where iron oxide and carbon dust settle with the help of coagulant and polymer. The clear water is cooled, dosed against scale and returned to the scrubbers, and the sludge is dewatered.
What water should be used in induction furnace cooling?
Induction coils and moulds need soft or demineralised water in a closed loop, because hardness scales the narrow copper passages and causes failures. The closed loop is cooled through a heat exchanger by an open cooling tower with its own chemical treatment.
Can heavy metals be recovered from steel plant effluent?
Yes. Zinc from galvanising and nickel or chromium from stainless pickling can be captured with selective adsorbents or recovered by precipitation and regeneration, instead of ending up in hazardous sludge.
Can steel plants achieve zero liquid discharge?
Many already recirculate almost all of their water. Zero discharge adds treatment of the remaining blowdown and treated effluent through RO, with the reject evaporated. Waste heat in steel plants can lower the cost of evaporation.
How much does a pickling effluent treatment plant cost?
As an indication, a 20 to 50 KLD pickling rinse neutralisation plant with clarifier and filter press costs about ₹25 to 70 lakh, excluding civil work. Acid volume, metals and disposal routes change the figure, so we price after testing.
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