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Pulp and Paper Wastewater Treatment Process: How We Fixed High COD Loads
Updated: 2 days ago
Overview
A high COD reading does not always mean a biological treatment plant needs more bacteria. In a paper mill, COD can come from easily biodegradable material as well as lignin, fibres, sizing agents, colour compounds and other substances that microorganisms may break down slowly. This changed how we looked at our pulp and paper wastewater treatment process. Instead of treating COD as one problem, we began asking what was creating it, where it entered the ETP and which part of the load the biological stage could actually remove.
Table of Contents
Why Is Paper Mill Wastewater Difficult to Treat?
Paper mill wastewater changes according to the raw material, manufacturing process and finished product. It can contain suspended fibres, cellulose, lignin-related compounds, starches, surfactants and other organic matter.
Lignin can be particularly difficult for microorganisms to break down because of its complex chemical structure. This means a biological plant may remove much of the biodegradable organic matter while leaving some harder-to-degrade compounds in the treated water.
That distinction matters when dealing with high COD wastewater treatment. For a broader look at the problems operators can face, see our guide to pulp and paper industry wastewater treatment. A high COD value tells you how much oxygen would be needed to oxidise the measured organic and oxidisable material. It does not tell you how much of that material can be consumed by the microorganisms in your aeration tank.
What Are the Stages of Pulp and Paper Wastewater Treatment?
The paper industry effluent treatment process normally begins with screening and flow control. Equalisation may then be used to reduce sudden changes in flow and pollutant concentration.
Primary clarification can remove suspended solids and fibres before the water reaches the biological stage. Depending on the mill, biological treatment may use activated sludge, SBR, MBBR, MBR or another reactor arrangement.
In the biological stage, microorganisms consume biodegradable organic matter. Secondary clarification then separates the biological solids from the treated water. Some mills also require tertiary treatment for colour, fine solids or other substances that remain after secondary treatment.
There is no single pulp and paper ETP process that suits every mill. The production process, wastewater composition and discharge requirement must be considered before selecting or changing treatment stages.
Where Did Our Biological Treatment Start Struggling?
Our first mistake was looking mainly at the final COD figure.
A COD increase can have several causes. Production may have increased, more fibres may have reached the biological stage, chemical use may have changed, or the primary clarifier may not have been removing enough solids.
The biological stage itself may also be under pressure because of low active biomass, insufficient dissolved oxygen, unsuitable temperature, sudden loading changes or poor sludge control.
We began checking several measurements together:
Inlet COD and BOD
COD after primary treatment
TSS and fibre carryover
pH and temperature
MLSS and MLVSS
Dissolved oxygen
Sludge settling
Changes in production and chemical use
This gave us a better basis for finding the cause of poor COD removal.
Understanding how aerobic bioculture works in wastewater treatment can also help operators assess whether weak microbial activity is contributing to poor COD removal.
Why Does Biological Treatment Fail in Paper Mill Wastewater?
Biological treatment can struggle when a large part of the organic load is difficult for the microbial population to degrade.
Lignin is one example. Cellulose is generally more biodegradable under suitable conditions, but its behaviour depends on the wastewater and the treatment conditions. Other substances, including some sizing agents, surfactants and colour-related compounds, may also affect biological treatment.
A second issue is the amount of active biomass available. If the organic load rises quickly but the microbial population does not increase at a similar rate, the aeration tank may not remove COD at the required rate.
Low oxygen, poor mixing, temperature changes and excessive solids carryover can make the situation worse.
If oxygen availability is a concern, our guide to low dissolved oxygen in aeration tanks explains the common causes and checks operators can make.
How Can COD Removal Be Improved in Paper Industry Wastewater?
Start by finding out what type of COD is entering the biological stage.
BOD/COD testing can provide a useful indication of the biodegradable fraction. More detailed tests, such as respirometry, can provide further information about how microorganisms respond to the wastewater.
The next step is to check the treatment stages before the aeration tank. Better screening and primary solids removal can reduce fibre entering biological treatment. Equalisation can reduce sudden changes in organic loading.
The aeration tank should then be checked for dissolved oxygen, MLSS, MLVSS, sludge age, loading and settling behaviour.
Where settling problems are present, our guide to sludge bulking and MLSS control explains how sludge condition and microbial balance can affect treatment performance.
Where the microbial population is weak or poorly suited to the wastewater, bioaugmentation may be considered. Microbial performance can also be affected by micronutrient deficiency in wastewater treatment, so nutrient availability should be considered when biological activity remains weak. Amalgam Biotech states that BactaServe Pulp and Paper contains selected microorganisms intended to assist with the breakdown of lignin, cellulose, sizing agents, surfactants and fibrous organic matter in paper mill ETPs.
What Changed in Our Approach?
We stopped asking, “How do we remove more COD?”
Instead, we asked, “Which COD are we failing to remove, and what is causing it?”
That question changed our testing process.
For example, if COD drops sharply after primary clarification, suspended solids or fibres may be a major contributor. If primary treatment performs well but COD remains high after aeration, the biological stage needs closer examination.
If the biological stage removes a large part of the biodegradable load but final COD remains high, the remaining material may contain compounds that are less suitable for biological oxidation.
This is why COD should be tracked at several points through the pulp and paper wastewater treatment process rather than only at the final outlet.
Conclusion
Biological treatment is an important part of pulp and paper wastewater treatment, but it cannot be expected to remove every substance.
The right process depends on the wastewater entering the plant. Activated sludge may work well for one mill, while another may require MBBR, MBR, SBR, anaerobic treatment or a combination of stages.
The same principle applies when selecting microbial treatment. BactaServe Pulp and Paper is one product intended for paper mill ETP applications, but its suitability should be assessed against the actual wastewater characteristics and operating conditions.
The main lesson from a high COD reading is simple: test the wastewater before changing the treatment programme. A number on a laboratory report can tell you that something is wrong. It cannot, by itself, tell you what needs to change.
Frequently Asked Questions
How does the pulp and paper wastewater treatment process work?
It commonly includes screening, equalisation, primary solids removal, biological treatment and clarification. Some mills also require tertiary treatment.
How to improve COD removal in paper industry wastewater?
First identify whether the COD is biodegradable. Then check primary solids removal, biomass, dissolved oxygen, loading, temperature and sludge conditions.
What are the stages of pulp and paper wastewater treatment?
Common stages include screening, equalisation, primary clarification, biological treatment and secondary clarification. Further treatment may be required depending on the final water-quality requirement.
Can biological treatment remove lignin?
Microorganisms can break down some lignin-related compounds, but lignin is difficult to degrade. Other treatment stages may be needed depending on the wastewater.
What is the best biological process for pulp and paper wastewater treatment?
There is no single answer. Activated sludge, SBR, MBBR, MBR and other biological processes can be suitable depending on wastewater characteristics, organic loading and the required treated-water quality.
