ETP stages explained — from raw effluent to discharge
Short answer: An ETP works in three broad stages — primary removes solids physically and chemically, secondary uses bacteria to consume the organic load, and tertiary polishes the water to meet discharge standards.
Most ETP failures are in the secondary stage, because that is the one that depends on living bacteria, and bacteria are sensitive to pH, oxygen and shock loads.
Primary treatment
Screening
Coarse solids, rags and debris are removed by bar screens. Simple, but essential — anything that gets past here damages pumps downstream.
Oil and grease trap
Floating oil is skimmed off. Oil passing into the aeration tank coats the bacteria and stops them working.
Equalisation tank
This is more important than it looks. Effluent arrives in batches with varying pH and strength; the equalisation tank evens it out so the biological stage receives a steady load.
Neutralisation
pH is corrected to 6.5 to 8.5, because bacteria will not work outside that range. Acid or alkali is dosed automatically using a pH controller with ETP sensor (₹28,350) and a dosing pump.
See our pH correction guide.
Coagulation and flocculation
Chemicals are dosed to make fine suspended particles clump together into larger flocs that will settle.
Primary clarifier
The flocs settle out as sludge and are removed.
Secondary treatment — the biological stage
This is where the dissolved organic load (BOD and COD) is actually removed, by bacteria consuming it.
Aeration tank
A blower supplies air so the bacteria have oxygen. Target dissolved oxygen is 1.5 to 3 mg/L.
- Below 1 mg/L — bacteria weaken, treatment fails
- Above 4 mg/L — no benefit, just wasted electricity
The blower typically consumes 60 to 70 percent of the plant's electricity, so DO monitoring pays for itself. See our DO meter guide.
Secondary clarifier
Biomass settles out. Part is returned to the aeration tank to maintain the bacterial population; the surplus is removed as waste sludge.
Tertiary treatment
- Sand or multigrade filter — removes remaining suspended solids
- Activated carbon filter — removes colour and residual organics
- Disinfection — chlorination or UV
- RO — where the water is to be reused
Instruments needed
| Instrument | Where | Why |
|---|---|---|
| pH controller (ETP) | Neutralisation, aeration | Bacteria need 6.5–8.5 |
| DO meter | Aeration tank | Blower control, energy saving |
| Turbidity meter | Outlet | Discharge quality |
| Electromagnetic flow meter | Inlet and discharge | Load and compliance records |
| Telemetry modem | Discharge point | Data to pollution board |
ETP sensors are different from standard ones — see our ETP sensor guide.
Where ETPs most commonly fail
- pH outside range — bacteria stop working; everything downstream then fails
- Shock load — a strong or toxic batch kills the biomass
- Inadequate DO — blower undersized or DO sensor dirty and reading falsely
- Sludge return stopped — biomass washes out
- Low MLSS — too few bacteria for the load
- Oil reaching the aeration tank — coats the biomass
- Undersized equalisation — no buffering against variation
What to monitor daily
| Test | Target |
|---|---|
| pH — all stages | 6.5 – 8.5 |
| DO in aeration | 1.5 – 3.0 mg/L |
| MLSS | As per design |
| Sludge settling (30 min) | Indicates biomass health |
| Outlet turbidity | Per discharge norms |
| Outlet pH | Per discharge norms |
Recovering after a biomass failure
If the bacteria have been killed, recovery takes time:
- Identify and fix the cause first — otherwise it happens again
- Correct pH and DO
- Seed with biomass from a working plant if available
- Feed the plant gradually, at reduced load, while the population rebuilds
- Monitor MLSS as it recovers
This typically takes two to four weeks, which is why prevention matters so much more than cure here.
Automation
An ETP has many pumps, blowers and dosing systems running in sequence. A control panel handles the interlocks and sequencing — see our ETP panel guide.
ETP instrumentation and chemicals
Send your plant KLD and effluent type, and we will recommend the instruments and dosing systems you need.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
What are the main stages of an ETP?
Primary treatment removes solids physically and chemically, secondary treatment uses bacteria to consume the organic load, and tertiary treatment polishes the water to meet discharge standards.
Why is the equalisation tank important?
Effluent arrives in batches with varying pH and strength, and the equalisation tank evens it out. Without adequate buffering a shock load can reach the aeration tank directly and kill the biomass, after which recovery takes weeks.
Why do most ETP failures happen in the secondary stage?
Because it depends on living bacteria, which are sensitive to pH, oxygen levels and shock loads. If pH goes outside six point five to eight point five, or dissolved oxygen falls too low, the bacteria stop working.
Why does sludge return matter?
Part of the settled biomass is returned to the aeration tank to maintain the bacterial population. If the return stops, the population washes out and treatment fails within days.
How long does it take to recover after the biomass dies?
Typically two to four weeks. The cause must be fixed first, then pH and DO corrected, the plant seeded if possible, and fed gradually at reduced load while the population rebuilds.