.webp)
Treated Wastewater Smell: The Investigation That Ruled Out the Biological System
Updated: 19 hours ago
Overview
A bad smell in treated wastewater does not always mean the biological system has failed. In this ETP, COD removal had improved and the biomass had recovered, yet the treated water still carried an unpleasant odour. The investigation showed that the problem was occurring further down the treatment line. Residual sulphides, other odour-causing compounds or conditions inside treated-water storage can cause a treated wastewater smell even after biological treatment. This blog explains how we traced the odour source, what operators should check after the biological stage, and when oxidation treatment may help with a remaining treated water foul smell.
Table of Contents
The biological tank was working. COD removal had improved. The biomass had recovered.
But there was still one problem.
The treated water smelled bad.
At first, it was easy to suspect the biological process. After all, a treated wastewater smell often makes operators wonder whether the ETP has failed somewhere upstream.
But our investigation pointed somewhere else. The biological stage was doing its job. The remaining odour was present further down the treatment line.
That changed where we looked for the cause and how we treated it.
Why Does Treated Wastewater Still Smell?
When operators ask why does treated wastewater still smell, poor biological treatment is only one possible cause.
Wastewater can carry some odour-causing compounds beyond the biological stage.
Sulphides, amines and other organic compounds may remain depending on the type of industrial wastewater and the treatment process.
A post treatment odour can also develop when treated water sits in a storage tank for too long.
This is why smell should be investigated separately from the normal ETP treated water quality checks.
Water can look clear and still carry an unpleasant odour.
Our First Check: Was the Biological System Failing?
We started with the obvious place: the biological tank.
Poor aeration, weak biomass and incomplete organic breakdown can all contribute to a wastewater odour problem.
So we checked the biological stage rather than assuming it was working correctly.
But the findings did not match a biological failure. Treatment had improved and the active biomass had recovered. The remaining smell was being noticed in treated water further along the process.
This pointed towards wastewater odour after biological treatment, rather than an untreated biological load being the only cause.
The Investigation Moved Downstream
Once the biological process was ruled out as the main source, we followed the water through the later treatment stages.
This is an important step when investigating causes of odour in treated wastewater.
Operators should check where the smell first becomes noticeable. Is it present after clarification? Does it appear after filtration? Is it only noticeable in RO permeate or inside a treated-water tank?
The location can tell you a great deal.
In our case, the remaining foul odour was associated with the treated water after later treatment stages. That meant repeatedly changing the biological process would not have addressed the actual problem.
Residual Compounds Can Survive Biological Treatment
Biological treatment is very useful for biodegradable organic matter, but it does not mean every odour-causing compound will disappear.
Residual sulphide wastewater can remain where sulphide has not been fully treated. Other industrial compounds may also pass into later treatment stages.
This is where industrial wastewater polishing may be required.
Polishing means treating the water further after the main treatment stage to deal with something that remains, such as colour, dissolved contaminants or odour.
For a treated water foul smell, the polishing method should match the compound causing it.
Could the Storage Tank Be Creating the Smell?
A treated water storage tank odour should also be investigated before blaming the biological tank.
Treated water can remain inside a tank before reuse or discharge. If residual biodegradable matter is still present and the water becomes stagnant, local low-oxygen conditions can develop.
Sludge or deposits inside the tank may add to the problem.
If the water smells normal when it enters the tank but smells bad later, the storage tank itself deserves attention.
This distinction can prevent unnecessary changes to an ETP that is otherwise operating correctly.
How to Remove Smell from Treated Water
When deciding how to remove smell from treated water, start by finding the odour source and likely compound.
If oxidisable compounds are still present, oxidation treatment wastewater methods may be considered.
For this application, NutriServe Rapid Oxidizer ETP can be used for malodour in untreated and treated industrial effluent. The oxidising additive reacts with odour-causing compounds and organic matter and can be used for continuous or intermittent dosing depending on the wastewater.
The dose should be based on the wastewater characteristics rather than copied from another ETP.
Can Oxidizers Eliminate Wastewater Odour?
The answer to can oxidizers eliminate wastewater odour depends on what is causing the smell.
An oxidizer for wastewater treatment can react with certain odour-causing compounds and change them chemically. This is different from adding fragrance to cover the smell.
In a documented 600 KLD textile ETP case, NutriServe Rapid Oxidizer was used to treat foul odour remaining in RO permeate after the biological treatment stage.
But odour neutralization wastewater treatment still needs a source check first. Oxidation will not correct a dirty storage tank, poor sludge handling or another mechanical problem.
When Should You Look Beyond the Biological Tank?
A recurring treated wastewater smell does not automatically mean the microorganisms have failed.
If COD, biomass and other biological indicators look normal, follow the water downstream.
Check clarification, filtration, membrane treatment and storage. Test for likely odour compounds where required.
For remaining oxidisable odours, NutriServe Rapid Oxidizer ETP may be considered as part of the final odour treatment stage.
Conclusion
The most useful part of this investigation was finding out where the problem was not.
The biological tank had recovered, yet the treated water foul smell remained further downstream.
That meant the next job was not to keep changing the biological process. It was to investigate residual compounds and the later treatment stages.
When dealing with wastewater odour after biological treatment, follow the water through the plant. Check where the smell starts, what compounds may remain and whether industrial wastewater polishing is required.
Finding the right source can save a lot of unnecessary changes elsewhere in the ETP.
Download product brochure:
Eliminate Treated Water Odour at the Source
If treated wastewater still smells despite meeting BOD and COD targets, the problem may not be your biological system. Residual sulphides and stagnant storage conditions can create odours after treatment. A targeted oxidation solution quickly neutralizes these compounds, keeping treated water fresh without modifying your existing ETP.
