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Carbon Source for Wastewater Treatment: Why Commissioning Failed Until We Fixed Aeration
Updated: 1 day ago
Our ETP failed commissioning twice even though the aeration equipment was running and bacteria had been added. MLSS simply would not build up as expected. The investigation eventually showed that the wastewater entering the biological tank did not contain enough biodegradable organic matter to feed the microorganisms. The bacteria were present, but they were short of carbon. We then reviewed the BOD COD ratio in biological systems and introduced a measured carbon source for wastewater treatment. This blog explains how low BOD can delay commissioning, why MLSS may remain low, and how controlled external carbon dosing can support biomass development.
Table of Contents
The first commissioning attempt looked normal at the start. The aeration tank had been seeded, the blowers were running and operating conditions were being checked.
But MLSS was hardly building.
We tried again. The second attempt gave us the same problem.
That forced us to look beyond the bacteria and aeration equipment. The influent itself was short of biodegradable organic matter. We had microorganisms in the tank, but not enough usable carbon to support their growth.
The missing part was a suitable carbon source for wastewater treatment.
Why Does MLSS Not Build Up During ETP Commissioning?
If you are asking why does MLSS not build up during ETP commissioning, first check whether the microorganisms have enough food.
BOD represents biodegradable organic matter that microorganisms can use. When incoming BOD is very low, bacteria have less carbon available for growth and metabolism.
This can lead to BOD-deficient wastewater treatment, where the biological tank has bacteria and oxygen but still struggles to establish enough active biomass.
Low MLSS during commissioning can have other causes too. Sludge washout, unsuitable pH, low dissolved oxygen, toxic compounds and nutrient shortage should also be checked.
Our Aeration Tank Had Air, but the Biology Was Starving
Aeration alone cannot build biomass.
Aerobic bacteria need oxygen, carbon and suitable nutrients. If one of these is missing, microbial growth can slow.
In our case, the problem was microbial carbon starvation in STP and ETP-style biological treatment. The incoming organic load was too low to support the biomass we were trying to establish.
This explained the slow MLSS development in the aeration tank.
Adding more bacteria without correcting the carbon shortage would have left the new microorganisms facing the same problem.
What the BOD:COD Ratio Told Us
The BOD COD ratio in biological systems can help operators understand how much of the organic load is readily biodegradable.
COD may be present while BOD remains low. In that situation, the total organic load can look reasonable on paper, but much of it may not be easily used by the biomass.
That matters during low BOD wastewater biological treatment.
If the BOD:COD ratio is poor, operators should investigate the wastewater characteristics before assuming the biological culture itself is at fault.
How to Fix Low BOD in Biological Wastewater Treatment
When looking at how to fix low BOD in biological wastewater treatment, the first job is to confirm that carbon shortage is actually the problem.
We checked:
Plant parameter | Why we checked it |
Influent BOD | Available biodegradable organic load |
Influent COD | Total oxidisable load |
BOD:COD ratio | Indication of biodegradability |
MLSS and MLVSS | Biomass development |
DO | Oxygen available to aerobic bacteria |
pH | Conditions for microbial activity |
Sludge settling | Biomass retention or loss |
Once low biodegradable carbon was confirmed, external carbon addition for slow biomass growth became a practical option.
Why We Moved to a Liquid Carbon Source
For controlled carbon addition, we used NutriServe BOD Serve 100.
It is a liquid formulation based on complex polysaccharides and is intended for organically deficient wastewater. According to the product specifications, 1 kg contributes approximately 1 kg of COD with proportionate BOD.
Its liquid form allows liquid carbon source dosing in activated sludge through a dosing pump.
That matters during commissioning because the amount added can be changed according to plant load rather than relying on irregular manual feeding.
How External Carbon Supports Biomass Growth
A carbon source for the activated sludge process provides organic material that heterotrophic bacteria can use.
When carbon is genuinely limiting, supplying it can support biomass growth in ETP operation and help MLSS and MLVSS develop.
But more carbon is not automatically better.
Excess carbon can increase oxygen demand and may leave unwanted BOD or COD in the treated water. The dose should be linked to actual influent conditions and treatment requirements.
This is why an external carbon source for ETP operation should be measured rather than added by guesswork.
What About Carbon Dosing for Denitrification?
Carbon has another important use in biological treatment.
Denitrifying bacteria require an available electron donor, commonly an organic carbon source, to convert nitrate to nitrogen gas. Where wastewater lacks enough suitable carbon, a liquid carbon source for denitrification may be added.
NutriServe BOD Serve 100 can also be used for this purpose and is described by its manufacturer as a non-flammable methanol alternative for wastewater treatment.
For plants considering a safe alternative to methanol for denitrification, non-flammability can reduce one handling concern, though normal chemical storage and dosing procedures are still required.
Why Controlled Dosing Matters During Commissioning
A new biological tank can change quickly.
As MLSS development in the aeration tank increases, the amount of carbon required may also change. Influent BOD may rise once the factory reaches normal production.
This means the commissioning dose should not automatically become the permanent dose.
Operators should keep checking BOD, COD, MLSS, MLVSS and DO.
The product page states that NutriServe BOD Serve 100 can be used during commissioning as well as routine operation and requires no modification to the existing biological system.
Conclusion
Our commissioning problem was not fixed by adding more bacteria or simply running the blowers for longer.
The microorganisms were short of usable organic carbon.
Once BOD-deficient wastewater treatment was identified, the focus shifted to supplying a measured carbon source for wastewater treatment and watching how the biomass responded.
For any plant facing slow commissioning, do not assume low MLSS means the bacterial culture has failed. Check BOD, COD, DO, nutrients, sludge retention and pH first.
If carbon shortage is confirmed, NutriServe BOD Serve 100 can be considered as a controlled external carbon source for commissioning, biomass development or denitrification.
Amalgam Biotech is a Pune-based manufacturer of bioculture, process additives, and odour control solutions, helping industries across India and international markets achieve stable and compliant wastewater treatment performance.
Build Stronger Biomass with the Right Carbon Source
Slow MLSS growth, poor BOD removal, and failed commissioning are often signs of carbon-deficient wastewater, not equipment failure. A controlled external carbon source helps restore microbial activity, accelerate biomass development, and stabilize biological treatment for reliable plant performance.
