Effluent Treatment Plant

An effluent treatment plant (ETP) cleans wastewater through a sequence of physical, chemical, and biological steps to remove solids, oils, nutrients, and dissolved contaminants so the treated water is safe to discharge or reuse. Below is an end-to-end walkthrough of the process, what each stage removes, and how plants are operated and monitored.

What ETPs Treat

• Suspended solids such as grit, silt, and fibers.

• Floatables and oils including free oil and emulsified oil.

• Dissolved organics measured as BOD and COD.

• Nutrients including nitrogen and phosphorus.

• Toxic or recalcitrant compounds like dyes, phenols, heavy metals, and surfactants.

• Microorganisms (pathogens).

Process Flow Overview

• Inlet works: screening, grit removal, oil-water separation.

• Flow equalization and pH/alkalinity control.

• Chemical pretreatment: coagulation, flocculation, dissolved air flotation or primary clarification.

• Biological treatment: activated sludge/MBBR/SBR/MBR for BOD/COD and nutrient removal.

• Tertiary polishing: filtration, activated carbon, advanced oxidation, nutrient polishing.

• Disinfection: chlorination, sodium hypochlorite, or UV.

• Sludge line: thickening, stabilization, dewatering, disposal or reuse.

Preliminary and Pretreatment

• Screening: removes rags, plastics, and large debris to protect downstream equipment.

• Grit removal: removes sand and heavy inorganics that cause abrasion and sedimentation.

• Oil-water separation: plate separators or API separators remove free oil and grease.

• Equalization: balances fluctuating flows and loads; provides buffering against shock loads.

• pH and alkalinity control: acid/alkali dosing to bring pH typically to 6.5–8.5 for optimal treatment.

• Coagulation–flocculation: metal salts or polymers agglomerate colloids and emulsions.

• Primary clarification or DAF: separates formed flocs; DAF is preferred for oily or emulsified streams.

Primary Treatment

• Settling of heavier solids to produce primary sludge.

• Skimming of floatables and residual oils.

• Reduces suspended solids and some BOD, improving downstream biological performance.

Secondary (Biological) Treatment Options

• Conventional activated sludge: aeration tanks with return activated sludge; targets high BOD/COD removal.

• MBBR: biofilm carriers increase biomass and resilience in variable industrial effluents.

• SBR: fill–react–settle cycles in a single basin; compact for small plants.

• MBR: membranes replace clarifiers for very low turbidity and bacteria; compact footprint and high-quality effluent.

• Key controls: dissolved oxygen, sludge age, F/M ratio, nutrient balance BOD:N:P≈100:5:1BOD:N:P≈100:5:1.

Typical removals in well-run systems:

• BOD: 85–98%

• COD: 60–90% (higher with pre-oxidation or MBR)

• TSS: >95%

Nutrient Removal

• Nitrogen: nitrification–denitrification via aerobic and anoxic zones; supplemental carbon may be needed.

• Phosphorus: chemical precipitation with alum/Fe salts or enhanced biological phosphorus removal.

• Targets depend on discharge limits; TN and TP polishing may be added when receiving waters are sensitive.

Tertiary and Advanced Treatment

• Filtration: sand, dual-media, or membrane filtration for fine solids.

• Activated carbon: removes color, odors, and residual organics.

• Advanced oxidation: ozone, UV/H2O2, or Fenton to degrade refractory compounds.

• Desalination/reuse: softening or RO where low TDS is required; manage RO reject responsibly.

• Disinfection: chlorine, hypochlorite, or UV to meet microbial standards.

Sludge Treatment and Disposal

• Thickening: gravity or DAF thickening to reduce volume.

• Stabilization: aerobic or anaerobic digestion to reduce odors and pathogens.

• Dewatering: centrifuge, belt press, plate-and-frame press to produce cake.

• End use: composting or land application (if compliant), co-processing, incineration, or secure landfill.

• Centrate/filtrate returns to the headworks and must be accounted for in loading.

Monitoring and Control

• Online instruments: pH, DO, ORP, turbidity/solids, flow, temperature, and sometimes ammonia or nitrate.

• Lab testing: BOD, COD, TSS, oils and grease, nutrients, metals, and microbiology.

• Automation: PLC/SCADA adjusts aeration, recycle rates, chemical dosing based on influent variation.

• Compliance: routine sampling and reporting to demonstrate permit adherence.

Typical Performance Targets

• TSS < 30 mg/L

• BOD < 20 mg/L

• COD < 100 mg/L

• Oils and grease < 10 mg/L

• Ammonia-N < 5 mg/L (where required)

• TN 10–20 mg/L and TP 1–2 mg/L for sensitive waters
  Actual limits depend on local regulations and receiving water sensitivity.

Common Small-Plant Trains (up to ~100 KLD)

Domestic/sewage (STP-like):

• Screen and grit removal

• Equalization with fine screens

• MBBR or SBR biological process

• Secondary clarification (if MBBR) or integrated settling (SBR)

• Pressure sand filter + activated carbon filter

• UV or chlorination for disinfection

• Sludge thickening and dewatering

Industrial with oils/emulsions:

• Coarse screen, grit, oil-water separator

• Equalization with pH control

• Coagulation–flocculation + DAF

• MBBR or MBR for secondary treatment

• Tertiary filtration and activated carbon

• AOP if refractory COD/color persists

• Disinfection and reuse/discharge

• Sludge handling as above

Color/recalcitrant organics (e.g., dyes, pharma):

• Equalization and pH control

• Advanced oxidation pre- or post-biological

• High-rate MBR for deep polishing

• Activated carbon and potential RO for reuse

• Robust sludge management and concentrate handling

STP vs ETP (at a glance)

Practical Operating Tips

• Maintain equalization to dampen shock loads and keep pH within treatment windows.

• Control biomass health: track DO, sludge age, SVI, and F/M; avoid over- or under-aeration.

• Dose chemicals based on jar tests; adjust coagulant/polymer to influent changes.

• Ensure nutrient balance for biological treatment; supplement N/P when treating weak streams.

• Keep a preventive maintenance and calibration schedule for instruments and blowers.

• Plan for safe chemical storage and sludge disposal; verify regulatory compliance before land application.

Example: 100 KLD ETP Equipment List (typical)

• Bar screen and vortex grit chamber

• Equalization tank with mixers and pH control

• Coagulation–flocculation tank with dosing skids

• DAF unit or primary clarifier

• MBBR or SBR basin with blowers and diffusers

• Secondary clarifier (if MBBR) or integral settling (SBR)

• Pressure sand filter and activated carbon filter

• UV or chlorination system

• Sludge thickener and centrifuge/belt press

• PLC/SCADA with online pH, DO, flow, and turbidity meters

If you share your influent characteristics (flow, pH, BOD, COD, TSS, oils, color, nutrients, metals) and reuse/discharge targets, I can propose an optimized process train and sizing assumptions for your capacity