Paper Mill Wastewater Treatment: Why COD Stays High

Your effluent treatment plant is operating, but the outlet report still shows high COD. The water may look clearer than it did at the inlet, yet the result remains above your treatment target.

For paper mills, the next step is to identify what is reaching the plant, what each treatment stage removes and which part of the remaining pollution needs further treatment.

Paper mill wastewater treatment works best when the investigation connects production records, representative laboratory results and actual plant operation. That evidence helps you decide whether the priority is source control, an operating correction or a treatment upgrade.

What Does High COD Tell a Paper Mill?

COD means chemical oxygen demand. It is a laboratory measure of the oxygen equivalent required to chemically oxidise substances in a water sample.

COD can include contributions from biodegradable substances and compounds that are harder to break down biologically. BOD, or biochemical oxygen demand, measures oxygen demand associated with biological processes under specified test conditions. The two results therefore provide different information.

A high COD result identifies a treatment concern, but it does not identify the cause by itself. Read it alongside flow, BOD, suspended solids, pH and relevant production information.

Why the Mill’s Process Matters

A recycled-paper operation, a coated-paper mill and an integrated pulp-and-paper mill can produce different wastewater streams. The assessment needs to reflect the raw materials, additives and processes actually used at your site.

IFC’s pulp-and-paper guidance distinguishes between mill types and discusses measures such as fibre recovery, management of process spills and separate treatment of coating wastewater where appropriate.

Prepare a simple map of the drains feeding your ETP. Identify which streams run continuously, which arrive in batches and which change during cleaning or product changeovers.

Keep process effluent and domestic sewage clearly identified in that map. Their treatment requirements need to be assessed for the intended system.

Four Reasons COD May Remain High After Treatment

1. The Incoming Load Has Changed

Compare current operating conditions with the assumptions used to design the ETP.

Has production increased? Has the mill changed paper grades or raw materials? Are more frequent cleaning cycles sending concentrated batches to treatment?

Record the wastewater volume as well as its concentration. A plant’s daily hydraulic capacity does not describe every aspect of its ability to handle pollution.

For an illustrative calculation:

COD load in kg/day = flow in m³/day × COD in mg/L ÷ 1,000

At 100 m³/day and 1,000 mg/L COD, the incoming load is 100 kg/day. At 200 m³/day and 500 mg/L, it is still 100 kg/day.

The concentration has halved, but the daily COD load is unchanged. This is a calculation example, not a BioSynk project result.

Ask the process engineer to assess both daily load and short periods of unusually high inflow.

2. Fibre and Other Solids Are Passing Through

Review the screens, fibre-recovery equipment and primary separation stages. Check whether they are performing as intended and whether collected material is being removed consistently.

IFC identifies suspended-solids removal as a primary treatment function, followed by biological treatment for organic pollution and further treatment where required.

Ask the laboratory and treatment engineer whether comparing total and filtered COD would help distinguish solids-associated material from the fraction passing through the selected filter. Interpret the results alongside total suspended solids and the sampling method.

This assessment can help establish which treatment stage needs attention.

3. Biological Treatment Is Not Performing as Intended

A running pump or blower does not establish that biological treatment is achieving its design performance.

For an aerobic process, ask the operating team to review aeration and dissolved oxygen against the process requirements. The wider review should also consider pH, temperature, biomass condition, nutrient balance, retention time and possible inhibitory inputs.

Check whether outlet deterioration coincides with a production event, a chemical change, an equipment interruption or a change in plant operation.

Use the findings to plan controlled corrective work. Changes to dosing or operating settings should follow the plant’s design requirements and the process engineer’s assessment.

4. Some Remaining Compounds Need a Different Treatment Step

Biological treatment may remove part of the organic load while leaving more persistent compounds.

Research on wastewater from a coated-paper mill found that COD and colour responded differently to treatment. It illustrates why success against one parameter should not be assumed to establish the same result for another. The study’s laboratory results are not a performance promise for another mill.

Ask what evidence supports the proposed next step. Depending on the effluent and treatment objective, the investigation may consider chemical treatment, adsorption, oxidation or other polishing processes.

The selection should follow testing of your wastewater and an assessment of how the process would operate at your plant.

Collect Evidence From Each Treatment Stage

A final-outlet report tells you where the water ended up. A planned sampling programme can help explain how it got there.

Work with your laboratory and process engineer to choose sampling points, timing and methods. Account for the time water takes to move through the plant when interpreting inlet and outlet results.

Prepare a review pack containing:

Information	Why it helps
Raw materials, paper grades and production schedule	Provides context for changing wastewater characteristics
Flow records and batch-discharge timings	Helps identify hydraulic and loading variations
Stage-by-stage laboratory results	Shows where performance changes through the ETP
Equipment and operating logs	Helps connect interruptions or settings with outlet results
Chemical consumption and sludge records	Supports assessment of operating costs and residuals
Applicable treatment targets	Defines what the upgrade must achieve

Ask the laboratory to flag issues with sampling, preservation, analytical range or interference before drawing conclusions from an unusual result.

Keep records from routine production and known problem periods. Label each sample with the relevant operating conditions.

Define the Required Water Quality Before Choosing an Upgrade

Write down the intended destination of the treated water.

A proposal for discharge treatment and a proposal for a particular process-reuse application may require different treatment arrangements. IFC’s guidance makes effluent composition, required quality and discharge destination part of technology selection.

If reuse is planned, identify the equipment or process that will receive the water and obtain its quality requirements. Include parameters relevant to that use, rather than relying on COD alone.

For example, a study examining tertiary treatment of pulp-and-paper wastewater identified conductivity and calcium, alongside COD, as important constraints on higher-quality reuse in the mill investigated.

Ask how the proposal addresses residual streams as well. A membrane system, for example, needs a defined plan for its concentrate, cleaning requirements and associated operating costs.

What Should an ETP Upgrade Proposal Explain?

Request a proposal that connects the identified problem to the recommended work.

It should state the design flow, incoming wastewater assumptions, outlet targets and operating conditions under which performance will be evaluated.

Ask the supplier to explain:

• Which existing equipment can remain in service.

• What operating changes or additional stages are proposed.

• What treatability testing or pilot work supports the selection.

• How variable production conditions will be accommodated.

• What chemicals, energy, staffing and maintenance are expected.

• How sludge, spent media or concentrate will be managed.

• How commissioning and performance verification will be carried out.

Relevant project evidence should include the wastewater type, inlet conditions, treatment arrangement and dated test results. A photograph of clear water provides limited information about treatment performance.

Discuss Your Paper Mill’s Wastewater With BioSynk

BioSynk’s paper-and-pulp service page describes effluent treatment, colour removal, COD reduction, water-reuse support and retrofit solutions for existing ETPs.

If your paper mill is experiencing high outlet COD, prepare your latest laboratory reports, typical daily flow, production details and current process diagram. Include the discharge or reuse target you need to achieve.

Discuss paper mill wastewater treatment with BioSynk and request an assessment of the current treatment stages and potential improvements.

Call: +91 95512 61154.

Give the team the operating evidence behind the problem so the discussion can focus on a treatment plan suited to your mill.