INDUSTRIES / PULP AND PAPER
Paper mill effluent treatment
that saves fibre, water and chemicals.
A paper mill loses money in its effluent: fibre, filler, heat and, in pulp mills, the cooking chemicals in black liquor. We test your streams in our NABL-accredited laboratory and design the ETP, recycling and process chemistry so that more of it goes back to the machine.
In short
How pulp and paper wastewater is treated.
A paper mill effluent treatment plant treats wastewater from pulping, washing, bleaching, stock preparation and the paper machine. It works in four steps: recover fibre with save-alls and screens; settle solids in a primary clarifier; treat biologically, often anaerobic first for recycled-fibre mills and then activated sludge, to cut BOD and COD; and polish colour and remaining COD chemically. In pulp mills, strong black liquor is kept out of the ETP and burned in a chemical recovery boiler. Treated water is increasingly recycled to the mill.
Pulp and paper is one of CPCB's 17 categories of highly polluting industries and must meet the pulp and paper standards under the Environment (Protection) Rules, 1986.
- 3mill types
- Wood-based, agro-based and recycled-fibre mills, each with a different effluent
- 17categories
- CPCB's list of highly polluting industries, which includes pulp and paper
- 30mg/L
- BOD limit for discharge to inland surface water
- ~80%lower
- Evaporation cost with our non-thermal evaporator than with a steam MEE
Where it comes from
Where does pulp and paper wastewater come from?
Indian mills fall into three groups. Wood-based mills make chemical pulp from eucalyptus, bamboo or other wood, usually by the kraft (sulphate) process, and run chemical recovery. Agro-based mills pulp bagasse, wheat straw or rice straw; many are smaller and some have no chemical recovery. Recycled-fibre (RCF) mills repulp waste paper and make kraft paper, board, newsprint and tissue; they are the most numerous. The table shows where effluent arises in each.
| Section | What happens | What it adds to the effluent |
|---|---|---|
| Raw material handling | Wood debarking and chipping, or depithing and washing of bagasse and straw | Bark, sand, silica, soluble organics |
| Chemical pulping | Chips or straw are cooked in caustic soda and sodium sulphide to dissolve lignin | Black liquor: lignin, cooking chemicals, very high COD and colour; must be recovered, not discharged |
| Brown stock washing | Separates pulp from black liquor | Carry-over of black liquor; the dark colour of many mill effluents |
| Bleaching | Whitens pulp with chlorine dioxide, oxygen, peroxide or, in older mills, elemental chlorine and hypochlorite | Chlorinated organics measured as AOX, colour, COD, acid and alkaline filtrates |
| Waste paper repulping and deinking | Pulps waste paper and removes ink, plastics, pins and sand | COD from starch and adhesives, ink, fillers, stickies, plastic rejects |
| Stock preparation | Refining, cleaning and screening of pulp before the machine | Fibre fines, fillers, sand from cleaners |
| Paper machine | Forms and dewaters the sheet; drained water is called white water | Fibre, filler (clay, calcium carbonate), starch, sizing and retention chemicals |
| Utilities | Boilers, cooling towers, water treatment plant | Blowdown, backwash, regeneration waste |
Typical effluent characteristics
| Parameter | Typical range | Why it matters |
|---|---|---|
| COD | 1,000 to 5,000 mg/L; higher in closed RCF mills and agro mills without recovery | Includes lignin and its products, which bacteria break down only slowly |
| BOD | 300 to 2,000 mg/L | Starch, sugars and wood extractives; drives aeration size |
| BOD : COD ratio | About 0.3 to 0.5 | Lower values mean colour and residual COD remain after biology |
| Suspended solids | 300 to 2,000 mg/L | Fibre and filler; much of it can be recovered and reused |
| Colour | Several hundred to several thousand Pt-Co units in chemical pulp mills | Lignin-derived colour passes the biology and darkens rivers |
| AOX | Present where chlorine-based bleaching is used | Adsorbable organic halides are persistent and are limited per tonne of product |
| pH | 6 to 9 combined; alkaline from pulping, acidic from some bleach stages | Needs neutralising before biology |
| Temperature | 35 to 50 °C in closed mills | Too hot for aerobic bacteria without cooling; useful for anaerobic reactors |
| Calcium and TDS | Rise as water circuits are closed, especially in RCF mills | Scaling in anaerobic reactors, pipes and RO membranes |
Black liquor is not effluent in a well-run kraft mill. It holds most of the organic load and nearly all the cooking chemicals, and it is meant to go to chemical recovery. When an ETP receives dark, high-COD water from the pulp mill, the first fix is usually in the washing and recovery area, not in the ETP.
Standards and rules
Which standards apply to paper mill effluent?
The Environment (Protection) Rules, 1986 contain industry-specific standards for pulp and paper mills. Depending on mill size and type, they limit BOD, COD, suspended solids and pH, set a maximum volume of wastewater per tonne of paper, and set an AOX limit per tonne for mills that bleach with chlorine compounds. Where a parameter is not covered, the general standards in Schedule VI apply; the main ones are below.
| Parameter | General standard |
|---|---|
| pH | 5.5 to 9.0 |
| Total suspended solids | 100 |
| BOD (3 days at 27 °C) | 30 |
| COD | 250 |
| Oil and grease | 10 |
| Wastewater per tonne of product | Capped in the pulp and paper standard by mill category |
| AOX | Limited in kg per tonne of paper for mills using chlorine bleaching |
Treatment scheme
A paper mill ETP, stage by stage.
This is the scheme we start from. A recycled-fibre mill usually includes the anaerobic stage; a wood or agro pulp mill relies more on chemical recovery upstream and on tertiary colour removal.
| Stage | What happens | Typical units |
|---|---|---|
| 1. Fibre recovery in the mill | White water passes a save-all that captures fibre and filler and returns them to the stock; clarified water goes back to showers and dilution. | Disc filter, dissolved air flotation (DAF) save-all, or sedimentation save-all |
| 2. Screen and remove grit | Plastic rejects, bark, sand and coarse fibre are taken out. | Bar screen, inclined or rotary screen, grit chamber |
| 3. Primary clarifier | Settles fibre and filler; the sludge is thickened and pressed. | Circular clarifier with scraper; screw press or belt press |
| 4. Cool, neutralise and equalise | Hot effluent is cooled, pH is corrected and flow is evened out. | Cooling tower, pH control, equalisation tank |
| 5. Anaerobic treatment (mostly RCF mills) | Bacteria without oxygen convert much of the soluble COD into biogas, cutting aeration power and sludge. | UASB or internal circulation (IC) reactor, biogas holder, scrubber, flare or boiler use |
| 6. Aerobic treatment | Activated sludge removes the remaining BOD and part of the COD. | Activated sludge process (ASP), extended aeration or MBBR, secondary clarifier; nitrogen and phosphorus dosing |
| 7. Tertiary colour and COD removal | Lignin colour and residual COD are removed chemically. | Coagulation with polyaluminium chloride or ferric salts, flotation or clarifier; ozone or Fenton where needed |
| 8. Recycling and ZLD (where required) | Filtered water returns to the mill; for higher-grade reuse or ZLD, UF and RO follow, with an evaporator on the reject. | Sand filter, UF, RO, MEE or non-thermal evaporator |
Options compared
| Option | Best for | Watch-outs |
|---|---|---|
| UASB or IC anaerobic reactor | RCF mills with high soluble COD; produces biogas | Calcium scaling and granule loss; needs steady pH and temperature |
| Activated sludge (ASP) | All mill types as the main aerobic step | Filamentous bulking with low nutrients; high power |
| Chemical coagulation for colour | Pulp mills and agro mills with lignin colour | Chemical sludge volume; dose rises with colour |
| Ozone | Polishing colour and COD before discharge or reuse | Power and generator cost |
| UF and RO | High-grade reuse, ZLD | Fouling by fines, stickies and organics; calcium scaling |
Our NABL-accredited laboratory tests COD, BOD, TSS, colour, calcium, TDS and nutrients on mill streams, so the ETP and the recycling loop are sized on real loads.
Recovery and reuse
Black liquor, fibre and water: what a mill can recover.
Black liquor chemical recovery
In a kraft mill, the weak black liquor from washing is concentrated in a multiple-effect evaporator and burned in a recovery boiler. The organic matter (dissolved lignin) produces steam and power, and the molten cooking chemicals are recovered. In the causticising plant, lime converts them back to white liquor for the next cook, and a lime kiln regenerates the lime. Chemical recovery is what makes a large pulp mill both economic and treatable.
Agro-based mills find recovery harder because straw and bagasse carry silica, which thickens the liquor and scales evaporators. Black liquor should still be managed separately, not sent to the ETP.
Fibre and filler recovery
Save-alls on the paper machine recover fibre and filler from white water and return them to the stock. Every tonne of fibre caught there is a tonne less to buy, and a tonne less sludge at the ETP.
Water recycling
Clarified white water can replace fresh water for showers and dilution, and treated ETP water can go to pulping, washing and cleaning. As circuits close, calcium, COD and temperature rise, so recycling has to be matched with retention aids, deposit control and biocides. Our paper process chemicals cover these.
Biogas
An anaerobic reactor in an RCF mill produces biogas that can replace part of the boiler fuel once its hydrogen sulphide is scrubbed out. See waste-to-energy and biogas.
Cost
How much does a paper mill ETP cost?
For paper mills, the cost depends on tonnes of paper per day, how much water each tonne uses, the raw material and whether an anaerobic stage, colour removal or ZLD is needed. Use these ranges to budget.
| Plant | Typical scope | Indicative cost* |
|---|---|---|
| Save-all for one paper machine | Disc filter or DAF save-all with pumps and controls | ₹30 lakh to ₹1.5 crore |
| 1 MLD ETP, aerobic | Screening, primary clarifier, ASP, secondary clarifier, sludge press | ₹2.5 to 5 crore |
| 1 MLD ETP with anaerobic stage | As above plus UASB or IC reactor and biogas handling | ₹4 to 8 crore |
| Tertiary and UF/RO, per 500 KLD | Colour removal, filtration, UF and RO for reuse | ₹2 to 4.5 crore |
*Indicative 2026 ranges for equipment, electricals and installation, excluding land, civil works and taxes. Chemical recovery plants for pulp mills are a separate, much larger investment and are not included. Raw material, closure of water circuits and the outlet target change the figure a lot, so we price only after testing. ETP running costs are typically ₹15 to 40 per m³, more with tertiary colour removal.
What drives capital cost
- Water use per tonne of paper
- Wood, agro or recycled fibre
- Anaerobic stage or aerobic only
- Colour and AOX targets
- Recycling, RO or ZLD
What drives running cost
- Aeration power
- Nutrients, coagulants and polymer
- Sludge pressing and disposal
- Cooling of hot effluent
- Membrane cleaning where RO is used
Where mills save money
- Fibre caught at the save-all
- Biogas replacing boiler fuel
- Less fresh water and pumping
- Smaller aeration with an anaerobic stage
- Heat kept in recycled water
We supply retention and drainage aids, pitch and stickies control, felt and wire cleaners and deposit control for white water circuits, alongside the ETP.
Troubleshooting
Common paper mill ETP problems, and how we fix them.
- 01The problem
Sludge bulks in the secondary clarifier and solids spill over the weir.
What we doCheck nitrogen and phosphorus, which paper effluent lacks, correct nutrient dosing and dissolved oxygen, and add a selector zone against filamentous bacteria.
- 02The problem
The outlet meets BOD but stays dark brown and fails on colour or COD.
What we doTrace black liquor losses at washing and recovery, then add jar-tested tertiary coagulation or ozone for the lignin colour.
- 03The problem
The anaerobic reactor scales with calcium and loses its granules.
What we doControl pH and calcium in the feed, add pre-acidification and fine solids removal, and plan regular scale removal.
- 04The problem
Fibre and filler are lost to the ETP, filling the clarifier.
What we doAudit white water flows, repair or add a save-all and improve retention on the machine.
- 05The problem
Recycled water causes slime, deposits and breaks on the machine.
What we doImprove the quality of recycled water and pair it with biocide, deposit control and stickies control matched to your furnish.
What we offer
What Rispri does for paper mills.
Effluent and water testing
COD, BOD, TSS, colour, nutrients, calcium and TDS in our NABL-accredited lab.
Testing services →Paper mill ETP
Primary clarifiers, anaerobic and aerobic biology and tertiary colour removal, designed and built.
Effluent treatment plants →Paper process chemicals
Retention aids, deposit and stickies control, felt and wire cleaners, plus boiler and cooling chemicals.
Water treatment chemicals →Recycling and ZLD
UF, RO and evaporators for mills that must recycle or may not discharge.
ZLD plants →OCEMS
Online monitoring of flow, pH, BOD, COD and TSS connected to CPCB and SPCB servers.
Smart water management →Biogas
Biogas capture, scrubbing and use from anaerobic treatment of mill effluent.
Waste to energy →Buying guide
What to ask before you buy a paper mill ETP.
Use these questions with any supplier, including us.
- Was the design based on my mill's water balance? Flows per tonne, white water loops and seasonal raw material changes.
- How much fibre will be recovered before the ETP? Save-all performance affects every stage after it.
- Is an anaerobic stage worth it for my COD? Ask for the biogas and power figures.
- Which outlet values are guaranteed? BOD, COD, TSS and colour in writing, against my consent and the pulp and paper standard.
- How is calcium scaling managed? In reactors, pipes and membranes.
- How much treated water comes back to the mill, and at what quality? And what chemicals that recycling will need.
- What happens to the sludge? Pressing, reuse options and disposal cost.
- Who runs it after commissioning? Training, O&M, OCEMS upkeep and monthly NABL test reports.
Why Rispri
ETP, chemicals and laboratory under one roof.
Our own NABL lab
We test mill streams before design and certify the outlet after start-up.
Engineering since 1999
1,498+ plants and turnkey projects for 499+ clients across industry, hotels, hospitals and cities.
Process chemicals and water
Paper process, boiler, cooling and RO chemicals from the same team that designs the ETP.
Research partnerships
RO reject recovery and non-thermal evaporation, delivered in collaboration with IIT Bombay, IIT Kharagpur and ICT Mumbai.
Questions
Pulp and paper wastewater FAQs.
What is a paper mill effluent treatment plant?
A paper mill ETP treats wastewater from pulping, bleaching, stock preparation and the paper machine so that it meets the pulp and paper standards and the mill's consent, or can be recycled. It usually combines screening, a primary clarifier, biological treatment and, where needed, tertiary colour removal.
What is black liquor?
Black liquor is the dark liquid left after wood, bamboo or straw is cooked with caustic soda and sodium sulphide to make pulp. It contains dissolved lignin and the cooking chemicals. Kraft mills concentrate it and burn it in a recovery boiler to make steam and regenerate the chemicals.
Why is paper mill effluent brown?
The colour comes mainly from lignin and its breakdown products, which escape from pulping and bleaching. Bacteria remove little of it, so mills use better washing and recovery upstream and chemical coagulation or ozone as a tertiary step.
What is AOX in paper mill effluent?
AOX means adsorbable organic halides: chlorinated organic compounds formed when pulp is bleached with chlorine or chlorine compounds. They persist in the environment, so the pulp and paper standard limits them per tonne of product. Elemental-chlorine-free bleaching reduces AOX sharply.
Why do recycled-fibre mills use anaerobic treatment?
Effluent from recycled-fibre mills is warm and rich in soluble, easily degraded COD from starch. An anaerobic reactor removes much of it without aeration and produces biogas, which lowers power and sludge in the aerobic stage that follows.
What is the CPCB charter for pulp and paper?
It is a CPCB programme for pulp and paper mills, mainly in the Ganga basin, that sets targets for lower water use per tonne, black liquor management, fibre recovery, recycling and ETP performance on a fixed timeline. Mills covered by it should treat the targets as consent conditions.
Is ZLD required for paper mills?
Not for every mill. ZLD is set as a consent condition for some mills, mostly smaller recycled-fibre mills in water-stressed or river-sensitive areas, while others must recycle a large share of treated water. Check your consent to operate.
How much does a paper mill ETP cost?
As an indication, a 1 MLD aerobic ETP costs about ₹2.5 to 5 crore and about ₹4 to 8 crore with an anaerobic stage, excluding land and civil works. Raw material, water use per tonne and the outlet target change this, so we price after testing.
Can treated paper mill effluent be reused?
Yes. Clarified white water and treated effluent are widely reused for pulping, showers and washing. As circuits close, deposits and slime increase, so recycling should be paired with the right process chemicals and, for high-grade uses, UF and RO.
What is done with paper mill ETP sludge?
Primary sludge is mostly fibre and filler and is pressed and often reused in board, co-processed or burned as fuel where permitted. Biological and deinking sludge are handled as your board directs.
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