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Color Improves but COD Remains High: What to Check
2026,10,27
A frequent situation in industrial wastewater treatment is that after coagulation, flocculation and settling, the water looks clear, the apparent color is gone, and the operator expects the COD result to drop accordingly. When the laboratory result comes back high, the usual first reaction is to add more coagulant, switch to a stronger polymer, or double the dose.
Before adjusting chemicals, separate two questions: has the color been removed, and has the COD actually been reduced? These two observations are not the same step, and conflating them leads to wasted dose and a target that the current unit cannot reach.
1. Color removal is not COD removal
Coagulation and flocculation mainly act on suspended particles, colloids, and some emulsified oils. What you see as color often comes from these materials, so the water looks clear quickly.
COD, however, measures oxygen demand from both suspended and dissolved organic matter. A large part of COD in many wastewaters is dissolved organic compounds that do not form settleable flocs. Removing visible color does not break down those dissolved molecules.
· Visible color from colloids / suspended particles -> usually removable by coagulation
· True color from dissolved compounds (dyes, humic substances, solvents, organics) -> often unaffected by coagulation alone
· Suspended COD -> partly removable; dissolved COD -> needs a different step
2. The fastest check: split the sample
Take one fresh sample from the affected point and analyze it in two ways before changing any dose:
| Sample | Treatment before COD test | What it tells you |
| A: raw as received | No filtration, analyze as sampled | Total COD at the point |
| B: filtered sample | Filter through 0.45 µm paper before the COD test | Dissolved COD passing through the filter |
| Difference A - B | Calculated from the two results | Suspended / filterable COD portion |
If result B is close to result A, most of your COD is dissolved, and raising coagulant dose will only add sludge without moving the number.
3. Troubleshooting checklist
| Check | What to record and compare |
| Sampling point | Confirm where the COD sample is taken: after flocculation but before settling, after settling, or at the final outlet. Color and COD should be read at the same point. |
| Apparent vs true color | Compare filtered and unfiltered color. If filtered water is still colored, the color source is dissolved, not colloidal. |
| pH at dosing point | Record pH before and after coagulant addition. Coagulants behave very differently outside their working pH window, but the correct window must be found by trial, not assumed. |
| Dose vs settled sludge | Note settled sludge volume after 30 minutes. A rising sludge volume without COD improvement suggests overdosing rather than under-dosing. |
| Short-circuit / mixing | Check whether flocs are actually formed before the settling zone, or whether raw water passes through without retention time. |
| Influent load changes | Any new product, CIP cycle, cleaning batch, or return stream (backwash, filtrate) that brings dissolved organics into the line. |
4. What coagulation can and cannot solve here
· It can: lower turbidity, remove colloidal color, reduce suspended COD, separate emulsified oil droplets that flocculate.
· It usually cannot: mineralize dissolved organic molecules, remove soluble salts, or break down small-molecule solvents.
· It can make the water look clear while dissolved COD remains high. This is expected behavior, not a unit failure.
When the target is dissolved COD removal, look at the right downstream step (biological treatment, oxidation, adsorption, or membrane separation) rather than increasing the coagulant dose.
5. What to record before contacting us
| Field | Example to provide |
| Wastewater source | Which process or industry, e.g. textile, food, chemical, landfill leachate. |
| Sample split results | Total COD and filtered COD from the same point, with sampling date. |
| Color observation | Apparent color at outlet, and filtered color after 0.45 µm. |
| Current chemicals | Coagulant type/grade and polymer, dosing points. |
| Operating conditions | Flow, pH, and settling observation at the same time. |
| Target | Which COD limit or discharge standard you are trying to meet. |
6. How we can help
We do not recommend a dose based on a single COD number. With the split sample data above, we can tell whether your problem is primarily a coagulation issue, a dissolved-COD issue, or a sampling / pH issue, and suggest the right grade or trial sequence rather than a generic number.
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