Effluent Treatment Plant (ETP)
Treating industrial effluent to meet discharge norms.
An ETP treats wastewater from industrial processes via physical, chemical and biological stages before discharge or reuse, meeting local pollution board norms (BOD, COD, TSS, pH, oil & grease).
How it works
- 1
Screening & oil-grease trap remove solids and floating oil.
Coarse and fine bar screens (25 mm then 6 mm) catch rags, plastics and fibre that would choke pumps. The oil-and-grease trap gives a quiet, low-velocity zone where free oil floats and is skimmed — critical because oil coats biomass and kills the aeration stage downstream.
- 2
Equalization tank balances flow and pH.
The equalization tank holds 8–12 hours of flow with coarse-bubble mixing. It flattens the shock loads of batch discharges, blends acidic and alkaline streams, and lets the plant be fed at a steady, pumped rate — the single biggest factor in stable ETP performance.
- 3
Coagulation/flocculation with alum/PAC + polymer.
In the flash mixer, alum or PAC (or ferric) at 100–400 ppm is dispersed within seconds at high G-value; it neutralises the negative charge on colloids. In the slow flocculator, anionic polymer (0.5–2 ppm) and gentle mixing let the destabilised particles grow into settleable flocs. Lime or caustic holds pH in the 6.5–7.5 window where the coagulant works.
- 4
Primary clarifier separates settled sludge.
In the primary clarifier the flocs settle under 1–1.5 m/h surface loading; a slow scraper drags sludge to a central hopper while clarified water leaves over a V-notch weir. This removes 60–70 % of TSS and 25–35 % of BOD before biology, cutting the aeration power bill.
- 5
Biological treatment (aeration/MBBR/MBR) reduces BOD/COD.
Biological treatment does the real COD/BOD destruction. Blowers feed fine-bubble diffusers to hold 1.5–2.5 mg/L dissolved oxygen while a mixed bacterial culture (MLSS 3000–4500 mg/L) oxidises organics. MBBR adds free-floating plastic carriers with attached biofilm; MBR replaces the clarifier with an ultrafiltration membrane and runs at 8000–12000 mg/L MLSS. Nutrients are dosed to keep BOD:N:P at about 100:5:1.
- 6
Secondary clarifier and tertiary polishing (sand/carbon filter, UV).
The secondary clarifier separates biomass from treated water; most of it returns as RAS to keep MLSS constant and the surplus is wasted as SAS. Tertiary polishing — pressure sand filter, activated carbon, and UV or chlorination — trims residual solids, colour and pathogens so the outlet meets CPCB/state norms or can feed a recycle/RO stage.
Key components
Manual or mechanically raked screen, 25 mm coarse followed by 6 mm fine. Protects pumps and diffusers. Screenings are drained and disposed of daily; a blinded screen causes upstream flooding.
8–12 h retention with coarse-bubble air grid or submersible mixer to prevent settlement and septicity. Feeds the plant at constant rate through a pump with VFD or a weir box.
Flash mixer: 30–60 s at high-speed impeller for coagulant dispersion. Flocculator: 15–30 min at low speed with paddle or picket-fence stirrer for floc growth. Jar tests set the actual dose — never dose by habit.
Circular or tube-settler units with rotating bridge scraper, central feed well and peripheral V-notch weir. Primary removes chemical sludge, secondary separates biomass. Keep the sludge blanket below 1 m and never let it rise into the weir.
Activated sludge basin with fine-bubble membrane diffusers fed by twin-lobe or screw blowers, DO probe and VFD control. Key control numbers: DO 1.5–2.5 mg/L, MLSS 3000–4500 mg/L, SVI 80–150 mL/g, F/M 0.2–0.4, sludge age 10–20 days.
Gravity thickener, sludge holding tank with polymer dosing, then a filter press, centrifuge or belt press to reach 25–35 % dry solids. Filtrate returns to equalization; the cake goes to a TSDF or authorised co-processor with a manifest.
Typical operating parameters
Problem solving videos
Safety notes
- •H₂S in sumps — gas monitors required.
- •Confined space entry permit for tanks.
- •PPE for chemical handling (alum, lime, polymer).
Maintenance schedule
- •Daily: DO, pH, MLSS, sludge blanket check.
- •Weekly: diffuser inspection, dosing pump calibration.
- •Annual: tank de-sludging & structural inspection.
Problem solving matrix
| Issue | Likely cause | Fix |
|---|---|---|
| High outlet COD | Shock load / low MLSS | Increase RAS, dose nutrients |
| Bulking sludge | Filamentous growth | Adjust F/M, chlorinate RAS |
Learn everything with Engineer Muhammad Alam
Discuss every utility — compressors, chillers, boilers, WHRB, ETP and more — one-to-one on WhatsApp. This is not just one call: you can call freely and chat as many times as you need. We are always with you to help.