Quick answer: First establish whether your COD is particulate or soluble, because the answer decides everything. Particulate COD comes out cheaply with coagulation and settling or flotation. Soluble COD needs biological treatment, and only the stubborn remainder needs advanced oxidation — which is where budgets get destroyed if applied too early.
Step 1: Split your COD before choosing equipment
COD is a measurement, not a substance. Two effluents with identical COD readings can need completely different plants. Before anything else, have the laboratory report total COD and filtered (soluble) COD on the same sample.
- Particulate COD = total minus soluble. It is organic matter carried on suspended solids, and it is the cheap fraction — coagulation, flocculation, settling or dissolved air flotation remove it without any biology.
- Soluble COD is dissolved. It passes straight through filters and settling tanks, and only biological treatment or chemical oxidation will touch it.
If most of your COD is particulate and someone is quoting you an advanced oxidation system, ask why. The reverse is equally common: a plant with high soluble COD fitted only with a clarifier will never meet its limit no matter how much coagulant is dosed.
Step 2: Take the cheap removal first
Every kilogram of COD removed physically is a kilogram the biological stage does not have to aerate. Aeration is typically the largest energy consumer in an aerobic plant, so the order of operations has a direct effect on your electricity bill.
| Stage | Removes | Relative cost per kg COD |
|---|---|---|
| Screening and settling | Coarse and settleable particulate COD | Lowest |
| Coagulation + flocculation + DAF or clarifier | Fine particulate, colloidal, oil-associated COD | Low |
| Anaerobic digestion | Bulk soluble COD at high strength | Low — and produces biogas rather than consuming air |
| Aerobic treatment | Remaining soluble COD, plus ammonia | Moderate — aeration energy dominates |
| Advanced oxidation (ozone, Fenton) | Refractory COD that biology will not touch | Highest by a wide margin |
Work down this list, not up it. The most expensive mistake in COD reduction is buying an oxidation system to do work that a coagulant and a settling tank would have done for a fraction of the running cost.
Step 3: Decide anaerobic, aerobic, or both
Strength decides this. High-strength effluent — food processing, brewing, starch, palm oil, some chemical streams — is normally treated anaerobically first. Aerating that much load is expensive and generates a large volume of surplus sludge; anaerobic digestion removes the bulk of it at much lower energy cost, produces far less sludge, and yields biogas.
Aerobic treatment then polishes what remains and handles ammonia, which anaerobic treatment does not remove. For dilute effluent, aerobic alone is usually simpler and cheaper overall. Sizing both stages is covered in how to size an industrial wastewater treatment plant.
Step 4: Check what is actually limiting the biology
When biological COD removal underperforms, the cause is rarely “not enough tank”. Work through these first:
- Nutrients. Biology needs nitrogen and phosphorus in rough proportion to carbon. Sugar, starch and some chemical effluents are carbon-rich and nutrient-poor, and performance stays flat until nutrients are dosed.
- pH. Outside the workable band the biomass slows sharply. Check it at the reactor, not at the inlet.
- Temperature. Biological rates fall as temperature falls. A plant that passes in summer can fail in winter with nothing else changed.
- Toxicity. Cleaning chemicals, biocides, solvents and disinfectants inhibit the biomass. A rising dissolved oxygen reading with unchanged aeration often means the biomass has stopped working — a warning that is frequently misread as good news.
- Sludge age. Too short and you wash out the slower organisms; too long and settleability degrades.
These five explain the large majority of underperforming biological stages, and four of them cost nothing to check.
Step 5: Only then consider advanced oxidation
Some COD is refractory — biology genuinely will not remove it. Dye intermediates, certain pharmaceutical and chemical residues, and some surfactants fall in this category. For these, ozone, Fenton or similar processes work, at high energy and chemical cost.
Two things worth knowing before buying one:
- Advanced oxidation is usually most economical as a partial treatment — breaking refractory molecules into biodegradable fragments that a following biological stage then removes cheaply. Oxidising all the way to carbon dioxide is rarely the economical route.
- Run a bench test on your actual effluent first. Refractory COD varies enormously between sites, and vendor performance figures are generated on someone else’s water.
Don’t forget the sludge
Every COD removal step above produces solids. Coagulation produces chemical sludge, biology produces surplus biomass, and both have to leave the site. A plant that solves its COD problem and creates an unmanaged sludge problem has moved the cost rather than removed it — see how much sludge a wastewater plant produces and how the five treatment stages size against each other.
Frequently asked questions
Why is my COD still high after coagulation?
Almost certainly because your COD is mainly soluble rather than particulate. Coagulation removes suspended and colloidal matter; dissolved organics pass straight through. Ask the laboratory for total and filtered COD on the same sample — if the two numbers are close, coagulation was never going to fix it and you need biological treatment.
Does anaerobic treatment reduce COD more than aerobic?
It removes the bulk of the load more cheaply on strong effluent, but it does not finish the job. Anaerobic digestion is the economical first stage at high strength because it uses far less energy and produces far less sludge, but residual COD and ammonia remain and need an aerobic stage afterwards.
What is refractory COD and how do I know if I have it?
It is organic matter that biological treatment will not remove. You suspect it when biological performance plateaus well above the limit while all the usual factors — nutrients, pH, temperature, toxicity, sludge age — check out. Confirm with a bench biodegradability test before spending money on oxidation equipment.
Can I reduce COD without biological treatment?
Only if your COD is predominantly particulate. In that case coagulation, flocculation and flotation or settling can be enough. If a significant fraction is soluble, the choice is biological treatment or chemical oxidation, and oxidation is far more expensive to run.
Why does my COD removal get worse in winter?
Biological reaction rates fall with temperature, so the same reactor removes less load in cold weather. The effect is strongest on the slower-growing organisms. Options include increasing sludge age, insulating or covering tanks, or designing the plant for the winter case from the start rather than the annual average.
We added more coagulant but the water got worse. Why?
Overdosing coagulant can re-stabilise particles instead of destabilising them, so floc quality falls and settling worsens. It also increases sludge volume without removing more COD. Run a jar test against your current influent rather than adjusting the dose by trial and error on the full-scale plant.
Send your COD profile, flow rate and discharge limit and we will propose the stage sequence rather than a single unit: request a treatment proposal.