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Why Your Biological Treatment Keeps Failing on Refractory COD

Refractory COD and toxic compounds can stall or damage your biological stage. Here is how to spot the problem and when AOP pre-treatment makes sense.

WASTEWATER TREATMENTCOD REMOVALAOP

Alireza Reyhan

1 min read

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Your biological unit ran well for months. Then removal started dropping, the sludge got thinner, and every upset took longer to recover from. Or maybe effluent COD has been stuck at the same plateau from day one.

These look like two different problems, but they usually share one root cause: your wastewater contains compounds your biology was never designed to handle. Some pass straight through. Others damage the biomass itself.

Failure mode 1: refractory COD passes through

Aromatics, phenolic compounds, naphthenic acids, surfactants, and other complex organics can pass through the aeration tank almost unchanged. The biomass is healthy, but it simply cannot degrade them, so effluent COD sits on a plateau no matter how well you operate.

Failure mode 2: toxic compounds hit the biomass

In petrochemical wastewater, the bigger risk is often inhibition. Phenols, sulfides, solvents, heavy metals, high salinity, and free oil, especially as shock loads, can suppress or kill the microorganisms. The result is biomass loss, bulking or foaming, poor settling, and slow recovery after every upset.

A quick first screen

Look at the BOD₅/COD ratio of your wastewater:

  • Above 0.5: readily biodegradable, biology should work well.

  • Between 0.3 and 0.5: partly biodegradable, expect a COD plateau.

  • Below 0.3: poorly biodegradable, biology alone will not get you there.

These are typical rules of thumb, not design values. And remember that the ratio does not show toxicity: a stream can look acceptable and still inhibit your biomass. A respirometry or inhibition test tells you whether the biology can actually tolerate it.

Why more aeration is not the answer

More air, more biomass, and more chemicals add cost and complexity, but they do not change what the wastewater does to your biology. The problem needs to be addressed before the biological stage.

That is where Advanced Oxidation Processes (AOPs) come in. Used as pre-treatment, they can break refractory compounds down into smaller, more biodegradable molecules and, when properly designed and controlled, reduce toxicity before the stream reaches your biology. Poorly controlled oxidation can leave harmful by-products, which is why proper testing and design matter.

Want to see how it works in practice?

In our free live webinar, we will cover how to recognize refractory and inhibitory wastewater in your plant, when AOP makes technical and economic sense, and how to avoid the most common mistakes.

👉 Register for the related live webinar here: https://waterpro-trainings.com/aops-in-water-and-air-treatment