Using a DO meter in an ETP — and cutting the power bill
Short answer: DO stands for Dissolved Oxygen. In the aeration tank of an ETP or STP it should be 1.5 to 3 mg/L. Below that the bacteria weaken and treatment fails; above it you are simply wasting electricity.
That is the real benefit here — in an ETP the blower consumes 60 to 70 percent of the electricity, and DO monitoring can produce substantial savings.
What happens in an aeration tank
Bacteria consume the organic load, but they need oxygen to survive. The blower supplies air, oxygen dissolves into the water, and the bacteria keep working.
- DO too low (below 1 mg/L): bacteria weaken or die, BOD is not reduced, discharge fails
- DO correct (1.5–3 mg/L): bacteria work efficiently
- DO too high (4+ mg/L): no further benefit, just wasted electricity
The electricity saving — actual numbers
Suppose your blower is 5 HP running 24 hours:
- 5 HP ≈ 3.7 kW, over 24 hours ≈ 89 units per day
- At ₹8 per unit that is ₹712 per day, roughly ₹21,000 a month
If DO is running at 5 mg/L and you bring it down to 2.5, blower running time falls considerably. Even a 20 percent saving is around ₹4,000 a month.
Target DO levels
| Location | DO (mg/L) |
|---|---|
| Aeration tank (general) | 1.5 – 3.0 |
| MBBR aeration | 2.0 – 4.0 |
| SBR — aeration phase | 2.0 – 3.0 |
| SBR — anoxic phase | Below 0.5 (deliberately) |
| Discharge to a watercourse | Above 4.0 (commonly) |
DO meters available
| Model | Feature | Price |
|---|---|---|
| D.O. Meter with sensor | Basic online meter | ₹68,250 |
| D.O. Meter with RS485 | Data to panel or SCADA | ₹72,450 |
| D.O. sensor (separate) | Sensor replacement only | ₹61,950 |
Where to place the sensor
- In the middle of the aeration tank — not in a corner, where mixing is good
- Not directly above a diffuser — bubbles striking the sensor give a falsely high reading
- Depth: around half the tank depth
- Easy to withdraw — for cleaning and calibration
Sensor care — this really matters
- Clean weekly — biofilm builds up on the sensor and makes it read low. You then run the blower harder, which is the opposite of what is needed
- Membrane cap — some sensors have a cap that needs annual replacement
- Calibrate in air (at 100 percent saturation), monthly
- Never let it dry out
Cheaper alternatives
If ₹68,000 is not in the budget right now:
- Use a portable DO meter two or three times a week and set the blower timer accordingly
- Put the blower on a timer — for example 45 minutes on and 15 off. Even this saves a useful amount
- Use an energy meter panel — the WASTEWATER NXT 2.0 (₹77,700) shows electricity consumption continuously
What to monitor alongside DO
DO alone does not give the whole picture. In an ETP you also want:
- pH controller — between 6.5 and 8.5, or the bacteria will not work
- Turbidity meter — outlet quality
- Electromagnetic flow meter — discharge flow for pollution board records
- MLSS testing — how much biomass is present (a laboratory test)
A common situation
On many plants DO reads perfectly well yet treatment still fails. The usual reasons are:
- The sensor is dirty and giving a false reading
- Mixing is poor — DO is adequate in one part of the tank but not another
- MLSS is low — there are too few bacteria, so oxygen is of no use
- pH is wrong — the bacteria are not working despite the oxygen
So DO should always be read alongside pH and MLSS.
Set up ETP monitoring
Send your plant KLD, blower HP and current electricity bill, and we will estimate how quickly a DO meter would pay for itself.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
What DO level should an aeration tank have?
Generally one and a half to three mg per litre. Below that the bacteria weaken and treatment fails, and above it there is no additional benefit, only wasted blower electricity.
How does a DO meter save electricity?
In an ETP the blower accounts for sixty to seventy percent of electricity consumption. If DO is running higher than necessary, reducing blower operation produces substantial savings. On a large plant the meter can pay for itself within about fifteen to eighteen months.
How often should a DO sensor be cleaned?
Weekly. Biofilm builds up on the sensor and makes it read low, which leads you to run the blower harder and waste more electricity. Regular cleaning is therefore essential.
DO is correct but treatment is still failing — why?
Common reasons are a dirty sensor giving a false reading, poor mixing so that DO is adequate in only part of the tank, or low MLSS meaning too few bacteria. Incorrect pH also stops bacteria working despite adequate oxygen.
Is there a cheaper alternative to an online DO meter?
You can check with a portable DO meter two or three times a week and set the blower timer accordingly. Simply putting the blower on a timer also produces worthwhile savings, and an energy meter panel shows consumption continuously.