Q. What is an Effluent Treatment Plant (ETP)?
An Effluent Treatment Plant (ETP) is a system designed to treat industrial wastewater (effluent) before it is discharged into the environment or reused.
Its main purpose is to remove harmful contaminants, such as chemicals, oils, suspended solids, and toxic materials, from wastewater generated by manufacturing and industrial processes.
Key Points:
Objective:
To make the wastewater environmentally safe and compliant with pollution control norms.
Process Stages:
Preliminary Treatment: Screening, grit removal.
Primary Treatment: Sedimentation, oil & grease removal.
Secondary Treatment: Biological treatment using microorganisms.
Tertiary Treatment: Advanced filtration, disinfection, and polishing.
Outcome:
Reduces water pollution, promotes water reuse, and ensures compliance with statutory regulations (e.g., CPCB / SPCB norms).
Application:
Used in industries such as textiles, pharmaceuticals, chemicals, food processing, and refineries.
Q. What is the purpose of ETP?
Answer:
“The purpose of an ETP is to purify industrial wastewater, ensuring safe discharge, regulatory compliance, and environmental protection.”
- To treat industrial wastewater and remove pollutants before discharge or reuse.
- To protect the environment from harmful chemical and biological contaminants.
- To ensure compliance with pollution control regulations set by CPCB/SPCB.
- To prevent water pollution and safeguard nearby water bodies and soil.
- To reduce the load on municipal treatment systems by pre-treating industrial effluent.
- To enable recycling and reuse of treated water, supporting water conservation.
- To enhance corporate sustainability and promote an organization’s green image.
Q. What is the difference between ETP, STP, and CETP?
Answer:
“ETP treats industrial waste, STP handles domestic sewage, and CETP is a common facility for collective industrial effluent treatment.”
ETP (Effluent Treatment Plant):
Used to treat industrial wastewater containing chemicals, oils, and heavy metals.
Designed as per the specific process waste of each industry.
Ensures treated effluent meets pollution control norms before discharge or reuse.
STP (Sewage Treatment Plant):
Used to treat domestic wastewater (from toilets, kitchens, washrooms, etc.).
Focuses on removing organic matter, solids, and pathogens.
Commonly used in residential complexes, offices, and institutions.
CETP (Common Effluent Treatment Plant):
A shared facility that treats effluent from multiple small or medium industries.
Economical solution for industries that cannot afford individual ETPs.
Managed collectively under industrial associations and monitored by pollution control boards.
Q. What are the main stages of effluent treatment?
Answer:
“Effluent treatment involves preliminary, primary, secondary, and tertiary stages to ensure complete purification before safe discharge or reuse.”
The effluent treatment process generally involves four main stages designed to remove physical, chemical, and biological impurities effectively.
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1. Preliminary Treatment:
- Removes large solids, grit, and debris using screens and grit chambers.
- Protects pumps and equipment from damage.
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2. Primary Treatment:
- Involves sedimentation and oil–grease separation.
- Reduces suspended solids and floating materials.
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3. Secondary (Biological) Treatment:
- Uses microorganisms to break down dissolved organic matter.
- Processes include Activated Sludge Process, Trickling Filters, or MBBR.
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4. Tertiary (Advanced) Treatment:
- Final polishing stage for removal of residual nutrients, color, and pathogens.
- Techniques include filtration, disinfection (UV/chlorination), and RO.
Q. What parameters are measured in ETP?
“ETP parameters such as pH, BOD, COD, TSS, TDS, and heavy metals are measured to ensure effective treatment and environmental compliance.”
In an Effluent Treatment Plant, key parameters are measured to monitor the efficiency of treatment and ensure compliance with environmental standards.
pH: Indicates acidity or alkalinity of the effluent.
BOD (Biochemical Oxygen Demand): Measures the amount of biodegradable organic matter.
COD (Chemical Oxygen Demand): Represents total organic and inorganic pollutants.
TSS (Total Suspended Solids): Determines the level of suspended particles.
TDS (Total Dissolved Solids): Indicates dissolved salts and minerals in water.
Oil & Grease: Checks for hydrocarbon contamination.
DO (Dissolved Oxygen): Shows oxygen availability for biological treatment.
Heavy Metals (like Pb, Cr, Cd): Monitored in industries handling chemicals or metals.
Ammonia, Nitrates & Phosphates: Tested to prevent nutrient pollution.
Turbidity & Color: Evaluated to assess clarity and visual quality of treated water.
Q. What is BOD?
“BOD is the measure of oxygen needed by microorganisms to break down organic matter in wastewater — a vital indicator of organic pollution.”
BOD (Biochemical Oxygen Demand) is the amount of oxygen required by microorganisms to decompose organic matter present in wastewater under aerobic conditions.
It is a key indicator of the organic pollution level in effluent or sewage.
Higher BOD means more organic pollution and lower water quality.
Measured in mg/L (milligrams per liter), typically over a 5-day incubation period at 20°C.
Helps assess the efficiency of biological treatment in ETP/STP.
Used to design, monitor, and control wastewater treatment processes.
Q. What is COD?
“COD is the measure of oxygen needed to chemically oxidize pollutants in wastewater — a key indicator of total contamination and treatment performance.”
COD (Chemical Oxygen Demand) measures the total amount of oxygen required to chemically oxidize both organic and inorganic pollutants in wastewater.
It indicates the overall pollution load of the effluent.
Determined by using a strong oxidizing agent (like potassium dichromate) in acidic conditions.
Expressed in mg/L (milligrams per liter).
Higher COD values represent greater contamination and lower water quality.
Used to evaluate treatment efficiency and to design or optimize ETP and CETP processes.
COD is usually higher than BOD, as it measures all oxidizable substances, not just biodegradable ones.
Q. Explain the working principle of an Effluent Treatment Plant (ETP).
The ETP works on the principle of removing physical, chemical, and biological contaminants from industrial wastewater.
The process includes screening, sedimentation, aeration, biological degradation, and filtration.
Each stage treats specific pollutants, resulting in clean and compliant discharge.
Final treated water can be reused or safely discharged to the environment.
Q. What is the function of the equalization tank?
To collect and homogenize wastewater from different sources.
Balances flow rate and pollutant concentration to ensure steady treatment performance.
Prevents shock loading on downstream units.
Provides consistent influent quality to primary and biological treatment stages.
Q. What is the purpose of an aeration tank?
Provides oxygen to microorganisms for the biological breakdown of organic pollutants.
Promotes the growth of aerobic bacteria that decompose organic matter.
Ensures effective BOD and COD reduction.
Maintains mixing and suspension of biomass (MLSS) in the wastewater.
Q. What is MLSS and why is it important?
MLSS (Mixed Liquor Suspended Solids) is the concentration of suspended solids (mainly microorganisms) in the aeration tank.
Represents the active biomass responsible for treating wastewater.
Optimum MLSS ensures efficient biological degradation.
Low MLSS reduces performance; excessive MLSS can cause sludge bulking and poor settling.
Q. What is the function of the clarifier?
Separates treated water from biological sludge after aeration.
Allows solids to settle at the bottom, producing clear overflow water.
The settled sludge is either recycled back to the aeration tank or sent for sludge handling.
Ensures high-quality effluent clarity.
Q. What is the role of a sludge recirculation system?
Returns a portion of settled sludge from the clarifier to the aeration tank.
Maintains optimum biomass concentration (MLSS) for continuous biological activity.
Prevents microbial washout and supports stable treatment efficiency.
Excess sludge is removed periodically for dewatering and disposal.
Q. What is BOD and COD? How are they different?
BOD (Biochemical Oxygen Demand): Measures oxygen needed by microorganisms to break down organic matter biologically.
COD (Chemical Oxygen Demand): Measures oxygen required to chemically oxidize both organic and inorganic substances.
Difference:
BOD indicates biodegradable load, COD indicates total pollution load.
COD values are always higher than BOD.
Q. What is the acceptable BOD and COD limit for discharge?
As per CPCB norms:
BOD: ≤ 30 mg/L
COD: ≤ 250 mg/L
Actual limits may vary depending on industry type and local regulations.
Q. What is the purpose of the flash mixer and flocculator?
Flash Mixer: Rapidly mixes coagulants (like alum or lime) with wastewater for particle destabilization.
Flocculator: Slowly stirs the water to form larger flocs by aggregation of fine particles.
Together, they enhance solid-liquid separation in the sedimentation process.
Q. Explain the function of the sand and carbon filter.
Sand Filter: Removes suspended solids, turbidity, and fine particles from treated water.
Activated Carbon Filter: Adsorbs color, odor, organic compounds, and residual chlorine.
These filters act as the final polishing units before reuse or discharge.
Q. What is DO and its importance?
Dissolved Oxygen — required for microorganisms to survive in biological treatment.
Ideal range: 2–4 mg/L.
Q. What is the function of an equalization tank?
To collect and homogenize wastewater, balancing flow and concentration before further treatment.
Q. What is the function of a bar screen?
To remove large floating solids (plastics, rags, wood) that can damage pumps or block pipelines.
Q. What is the function of an aeration tank?
To supply oxygen for microbial growth and biological oxidation of organic pollutants.
Q. What are common aeration systems?
Surface aerators and diffused aeration systems (fine bubble diffusers).
Q. What is the function of a clarifier?
To settle suspended solids and biomass after biological treatment, separating treated water and sludge.
Q. What is sludge recirculation (RAS)?
Returning a portion of settled sludge from the clarifier to the aeration tank to maintain MLSS concentration.
Q. What is waste activated sludge (WAS)?
The portion of excess biomass removed from the system to control solids buildup and maintain biological balance.
Q. What is the role of lime in ETP?
Used for pH correction (neutralization) and precipitation of heavy metals.
Q. What is the role of alum or PAC?
Used as a coagulant to remove suspended solids and colloidal particles during primary or tertiary treatment.
Q. What is the purpose of a flocculator?
To gently mix coagulated water to promote the formation of larger flocs that can settle easily.
Q. What is the purpose of an oil and grease trap?
To separate free oil and grease from wastewater before biological treatment.
Q. What is a neutralization tank?
Used to adjust pH by adding acid or alkali to bring it within the desired range (usually 6.5–8.5).
Q. What is a DAF (Dissolved Air Flotation) unit?
Removes suspended solids and oil by attaching air bubbles to particles, allowing them to float and be skimmed off.
Q. What is a sand filter and carbon filter used for?
Sand filter: Removes fine suspended solids.
Activated carbon filter: Removes odor, color, and residual organics.
Q. What is sludge and how is it treated?
Sludge is the solid residue from wastewater treatment.
Treated by thickening, dewatering, drying, or disposal.
Q. What is SVI (Sludge Volume Index)?
Measures sludge settling characteristics in the clarifier.
Formula:
SVI = (Settled sludge volume in mL/L ÷ MLSS in mg/L) × 1000
Q. What is F/M ratio?
Food-to-Microorganism Ratio — balance between organic load and microbial mass.
Formula:
F/M = (Influent BOD × Flow) / (MLSS × Volume of aeration tank)
Q. What is the typical pH range maintained in ETP?
Between 6.5 and 8.5 for optimum biological activity.
Q. What is the purpose of tertiary treatment?
To polish treated water — removing residual solids, color, nutrients, or pathogens before discharge or reuse.
Q. What is ZLD (Zero Liquid Discharge)?
A treatment approach where no liquid waste is released — water is recovered and recycled, solids are managed separately.
Q. What causes foaming in the aeration tank?
Low F/M ratio, excess filamentous bacteria, or presence of surfactants in influent.
Q. What causes high COD in treated water?
Insufficient aeration, poor microbial activity, shock load, or chemical carryover.
Q. What causes low DO in aeration tank?
Blower malfunction, excess MLSS, or high organic load.
Q. What causes odor issues in ETP?
Anaerobic conditions, sludge accumulation, or poor aeration.
Q. What are typical ETP outlet standards (CPCB)?
pH: 6.5–8.5
BOD: <30 mg/L
COD: <250 mg/L
TSS: <100 mg/L
Oil & Grease: <10 mg/L
Q. What is hydraulic retention time (HRT)?
Time wastewater remains in a tank.
Formula:
HRT = Volume of tank (m³) / Flow rate (m³/hr)
Q. What is the role of microorganisms in ETP?
They biodegrade organic pollutants, converting them into CO₂, water, and biomass.
Q. What happens if MLSS is too high?
Poor aeration, sludge bulking, and poor settling.
Q. What happens if MLSS is too low?
Incomplete oxidation, high BOD/COD in outlet.
Q. What is the purpose of an equalization blower?
To aerate wastewater in the equalization tank to prevent settling and odor formation.
Q. What is the role of return activated sludge (RAS)?
To maintain biomass concentration and ensure consistent biological treatment efficiency.
Q. What is the purpose of preventive maintenance in ETP?
To avoid equipment breakdown, maintain treatment efficiency, and ensure compliance.
Q. What safety precautions are needed in ETP?
Use PPE (gloves, mask, goggles).
Follow MSDS for chemicals.
Avoid confined space entry without permit.
Q. What is the difference between aerobic and anaerobic treatment?
Aerobic: Uses oxygen; faster and odor-free.
Anaerobic: Without oxygen; slower but energy-efficient (biogas generation).
Q. What are common ETP chemicals?
Lime, alum, PAC, NaOH, HCl, chlorine, polymers.
Q. What is the purpose of backwashing in filters?
To remove trapped solids and restore filter efficiency.
Q. What action will you take if outlet pH < 6.0?
Stop discharge, check pH control system, dose alkali (lime/NaOH) to neutralize.
Q. What is the first step if COD suddenly increases?
Check inlet load, aeration, MLSS, pH, and adjust chemical dosing or recycle rate.
Q. What daily records are maintained in ETP?
Inlet/outlet parameters, chemical consumption, flow readings, equipment logbooks, and maintenance activities.
Q. What is the role of a dosing tank in ETP?
Used for preparing and dosing treatment chemicals such as coagulants, flocculants, and pH adjusters.
Ensures accurate chemical dosing for optimal treatment efficiency.
Prevents overdosing or underdosing, which can affect process performance.
Controlled dosing helps maintain consistent effluent quality.
Q. What is a bar screen and what type of waste does it remove?
A bar screen is a mechanical filter placed at the inlet of ETP.
Removes large floating solids, such as plastics, rags, paper, and debris.
Prevents clogging and damage to downstream pumps and pipelines.
It’s part of the preliminary treatment stage.
Q. What chemicals are commonly used in ETP?
Coagulants: Alum, Ferric Chloride, Poly Aluminium Chloride (PAC).
Flocculants: Poly Electrolytes.
pH Adjusters: Lime, Caustic Soda, Sulphuric Acid, Hydrochloric Acid.
Disinfectants: Chlorine, Sodium Hypochlorite.
Defoamers & Nutrients: For process stabilization.
These chemicals help in coagulation, neutralization, and disinfection.
Q. What is the role of lime and alum in ETP?
Lime:
Used to adjust pH and neutralize acidic effluent.
Aids in precipitation of heavy metals and hardness.
Alum:
Acts as a coagulant to destabilize suspended particles.
Promotes floc formation for better solid-liquid separation.
Q. What is the function of a neutralization tank?
Used to adjust the pH of incoming effluent to a neutral range (6.5–8.5).
Prevents corrosion or scaling in downstream units.
Ensures optimal conditions for biological treatment.
Acids or alkalis are dosed automatically based on pH sensors.
Q. What is sludge and how is it treated?
Sludge is the semi-solid residue generated during wastewater treatment.
Contains biological solids, chemicals, and inert matter.
Treated through thickening, dewatering, drying, and safe disposal.
Methods include sludge drying beds, filter presses, or centrifuges.
Q. What is a sludge drying bed and how does it work?
A sludge drying bed is a filtration-based system used to dewater sludge naturally.
Sludge is spread on sand beds; water drains through sand and evaporates.
The dried sludge is then collected and disposed of or reused as manure.
It is a low-cost and simple sludge management method.
Q. Explain the working of a dissolved air flotation (DAF) unit.
DAF removes oil, grease, and suspended solids by air flotation.
Air is dissolved under pressure and released to form microbubbles.
Bubbles attach to solid particles, making them float to the surface.
The floated layer is skimmed off, and clear water is discharged.
Q. What is the purpose of an oil and grease trap?
Separates and removes free oil and grease from wastewater.
Works on the principle of density difference — oil floats on water.
Prevents oil clogging in pipelines and improves treatment efficiency.
Usually installed at the preliminary stage of the ETP.
Q. What is a lamella clarifier and where is it used?
A lamella clarifier is a compact sedimentation device with inclined plates.
Increases surface area for faster settling of suspended solids.
Used in primary or tertiary treatment to remove fine particles.
Offers high efficiency in less space, ideal for compact ETP designs.
Q. What parameters are measured in ETP on a daily basis?
pH – to check acidity or alkalinity.
BOD (Biochemical Oxygen Demand) – organic load indicator.
COD (Chemical Oxygen Demand) – total pollution load.
TSS (Total Suspended Solids) – solids concentration.
TDS (Total Dissolved Solids) – dissolved salts level.
DO (Dissolved Oxygen) – aeration efficiency check.
Oil & Grease, Temperature, Flow Rate – operational monitoring.
Ensures consistent process performance and regulatory compliance.
Q. How is pH monitored and controlled in ETP?
Online pH sensors continuously monitor pH in real time.
Automatic dosing systems add acid or alkali based on pH feedback.
Manual sampling done for cross-verification.
Maintained typically between 6.5 to 8.5 for effective treatment.
Q. What instruments are used for flow measurement?
Electromagnetic flow meter – most common, accurate for wastewater.
Ultrasonic flow meter – used for non-contact measurement.
Orifice meter or Rotameter – for smaller flow lines.
Flow measurement helps control retention time and process balance.
Q. How do you calculate retention time of a tank?
Retention time represents the average time wastewater stays in a tank.
Formula:
Retention Time (hours) = Volume of Tank (m³) / Flow Rate (m³/hr)
Helps in process design and efficiency evaluation.
Q. What is the formula for hydraulic retention time (HRT)?
Formula:
HRT = Volume of Tank (m³) / Flow Rate (m³/hr)
Indicates contact time between wastewater and microorganisms or treatment chemicals.
Longer HRT allows better treatment efficiency.
Q. What is sludge volume index (SVI) and how is it calculated?
SVI measures the settling characteristics of activated sludge.
Formula:
SVI (mL/g) = (Settled Sludge Volume in 30 min, mL/L) / (MLSS, g/L)
Ideal range: 80–150 mL/g.
Helps assess sludge settling quality and clarifier performance.
Q. How do you calculate MLSS and MLVSS?
MLSS (Mixed Liquor Suspended Solids):
Total suspended solids in the aeration tank.
Determined by filtering, drying, and weighing a known sample volume.
MLVSS (Mixed Liquor Volatile Suspended Solids):
Portion of MLSS that is organic (biomass), measured by igniting dried MLSS at 550°C.
MLVSS ≈ 70–80% of MLSS, indicating biological activity.
Q. What is the function of a DO (Dissolved Oxygen) meter in the aeration tank?
Measures oxygen concentration available for microorganisms.
Ensures efficient biological degradation of organic matter.
Helps maintain optimal DO range (2–4 mg/L).
Prevents energy wastage from over-aeration or process failure from low oxygen.
Q. What happens if DO levels fall below required limits?
Microbial activity reduces, causing poor BOD/COD removal.
Leads to sludge bulking and foul odor generation.
May result in process instability and incomplete treatment.
Continuous DO monitoring avoids these operational issues.
Q. How is chlorine dosing controlled in ETP?
Chlorine dosing is used for disinfection in the final stage.
Controlled through automatic dosing pumps based on flow rate and residual chlorine levels.
Residual chlorine is maintained between 0.2–0.5 mg/L.
Ensures pathogen-free and compliant discharge water.
Q. What are the CPCB or SPCB standards for effluent discharge?
The Central Pollution Control Board (CPCB) and State Pollution Control Boards (SPCB) set discharge standards for treated effluent.
Major limits for industrial discharge into inland surface water:
pH: 5.5 – 9.0
BOD: ≤ 30 mg/L
COD: ≤ 250 mg/L
TSS: ≤ 100 mg/L
Oil & Grease: ≤ 10 mg/L
TDS: ≤ 2100 mg/L
Ensures environmental protection and legal compliance.
Q. What is the permissible pH, TDS, and COD in treated water?
pH: 6.5 – 8.5 (neutral range for safe discharge)
TDS (Total Dissolved Solids): ≤ 2100 mg/L
COD (Chemical Oxygen Demand): ≤ 250 mg/L
These limits ensure treated water is safe for reuse or discharge into the environment.
Q. How is effluent sample collected and analyzed?
Samples are collected using grab or composite sampling methods.
Containers must be clean, labeled, and preserved (acidified or cooled) as per guidelines.
Analyzed in a certified laboratory using APHA or BIS standard methods.
Parameters like pH, BOD, COD, TSS, TDS, oil & grease are tested.
Q. What are the common water quality parameters tested in a lab?
Physical: pH, Temperature, Turbidity, Color.
Chemical: BOD, COD, TSS, TDS, Oil & Grease, Hardness, Chlorides, Sulphates, Nitrates.
Biological: Coliform Count, DO, Pathogen Presence.
These tests assess overall water quality and compliance.
Q. How do you ensure treated water meets environmental norms?
Conduct daily and periodic monitoring of key parameters.
Maintain calibrated instruments and accurate records.
Perform lab analysis as per CPCB guidelines.
Implement preventive maintenance and process control in ETP.
Ensure treated water reports are submitted to SPCB regularly.
Q. What is Zero Liquid Discharge (ZLD)?
ZLD is a treatment approach where no wastewater is discharged from the plant.
All effluent is treated, recycled, and reused, leaving zero liquid waste.
Achieved through evaporation, RO systems, and condensate recovery.
Promotes sustainability and regulatory compliance.
Q. What are the advantages of achieving ZLD in industries?
Complete elimination of water pollution.
Water recovery and reuse, reducing freshwater demand.
Compliance with environmental laws and avoidance of penalties.
Improved corporate image and sustainability credentials.
Long-term cost savings through resource optimization.
Q. What happens if untreated effluent is discharged?
Severe environmental damage to water bodies and soil.
Health hazards to humans and aquatic life.
Legal actions, heavy fines, or closure of operations by SPCB/CPCB.
Loss of reputation and public trust for the organization.
Q. How often should ETP performance be monitored and reported?
Daily: In-house monitoring of pH, flow, DO, and appearance.
Weekly/Monthly: Laboratory testing of BOD, COD, TSS, and TDS.
Quarterly: Comprehensive analysis for submission to SPCB/CPCB.
Regular reporting ensures transparency and compliance.
Q. What are the penalties for non-compliance with effluent norms?
Under the Water (Prevention & Control of Pollution) Act, 1974:
Imprisonment up to 6 years and/or fines up to ₹1 lakh per day for continued violation.
Disconnection of utilities (water, electricity) by SPCB.
Revocation of consent to operate or plant closure.
Also leads to reputational and financial losses.
Q. What causes foaming in the aeration tank?
Caused by excessive growth of filamentous bacteria.
Can occur due to high organic load or low DO levels.
Use of detergents or surfactants in influent may increase foaming.
Controlled by maintaining balanced MLSS, proper aeration, and nutrient ratio.
Q. What happens if sludge is not removed regularly?
Leads to sludge bulking and poor settling in clarifier.
Causes reduced treatment efficiency and foul odors.
May overload biological systems, increasing BOD/COD in outlet.
Regular sludge removal ensures stable process performance.
Q. What causes high BOD or COD in outlet water?
Insufficient aeration or low DO levels.
Overloaded system or high influent concentration.
Malfunctioning clarifier or poor sludge settling.
Chemical dosing errors or process imbalance.
Regular monitoring and proper control restore efficiency.
Q. What can cause a drop in MLSS levels?
Excessive sludge wasting or washout from clarifier.
Low nutrient levels affecting microbial growth.
Toxic influent shock load killing microorganisms.
Maintain optimum sludge recirculation and nutrient balance to stabilize MLSS.
Q. How do you handle shock loads in ETP?
Use equalization tank to buffer sudden flow or concentration changes.
Reduce flow rate temporarily to increase retention time.
Add nutrients or oxygen to support microbial activity.
Monitor pH, DO, and MLSS closely to prevent process upset.
Q. What are common reasons for odor problems in ETP?
Anaerobic conditions in tanks due to low DO.
Accumulated sludge or poor housekeeping.
Hydrogen sulfide (H₂S) formation from sulfur compounds.
Prevented by aeration, regular sludge removal, and deodorizing agents.
Q. How can you improve aeration efficiency?
Maintain DO levels between 2–4 mg/L.
Use fine bubble diffusers for better oxygen transfer.
Perform regular cleaning of diffusers to prevent clogging.
Optimize blower operation and airflow rate to save energy.
Q. What preventive maintenance is done on blowers and pumps?
Lubricate bearings and check oil levels regularly.
Inspect belts, filters, and couplings for wear.
Monitor vibration and noise levels for early fault detection.
Ensure clean air filters and unclogged suction lines.
Periodic maintenance improves efficiency and lifespan.
Q. What is backwashing and why is it important in filters?
Backwashing reverses water flow through filters to remove trapped particles.
Prevents clogging and pressure drop in sand and carbon filters.
Ensures consistent filtration performance and extended filter life.
Should be done regularly based on pressure differential.
Q. What are the symptoms of poor floc formation in a clarifier?
Cloudy or turbid effluent with floating solids.
Slow settling or carryover of sludge in overflow.
Light, dispersed flocs instead of dense, compact ones.
Caused by improper coagulant dosing, low pH, or shock loads.
Corrected by optimizing chemical dosing and mixing conditions.
Q. Name major equipment used in an ETP.
Bar Screen & Grit Chamber – for solid removal.
Equalization & Neutralization Tanks – for flow and pH balance.
Flash Mixer & Flocculator – for coagulation and floc formation.
Primary & Secondary Clarifiers – for solid-liquid separation.
Aeration System (Blowers & Diffusers) – for biological treatment.
Sand & Carbon Filters – for polishing.
Sludge Handling Units (Filter Press, Centrifuge, Drying Beds) – for dewatering.
Q. What is the function of a diffuser in the aeration tank?
Distributes fine air bubbles uniformly into wastewater.
Enhances oxygen transfer efficiency for microbial activity.
Helps mix biomass and wastewater evenly.
Prevents dead zones and supports stable biological treatment.
Q. What are the types of aerators used?
Mechanical Aerators: Surface aerators, brush aerators, turbine aerators.
Diffused Aeration Systems: Fine bubble, coarse bubble diffusers.
Jet Aerators: Combine mixing and aeration using pressurized air and water.
Selection depends on plant size, oxygen demand, and energy efficiency.
Q. What is the difference between mechanical and diffused aeration?
Mechanical Aeration:
Uses rotating impellers or paddles to agitate water and introduce air.
Provides strong mixing but lower oxygen transfer efficiency.
Diffused Aeration:
Air is passed through porous diffusers, forming fine bubbles.
Higher oxygen efficiency, energy-saving, and uniform distribution.
Q. What are the types of pumps used in ETP?
Centrifugal Pumps: For raw and treated water transfer.
Submersible Pumps: For sump or tank drainage.
Dosing Pumps: For chemical feeding.
Sludge Pumps: For thick and viscous sludge handling.
Pump type depends on viscosity, head, and flow requirements.
Q. What is the function of a belt filter press?
Used for mechanical dewatering of sludge.
Sludge is sandwiched between belts and pressed to remove water.
Produces semi-dry sludge cake suitable for disposal.
Reduces sludge volume and handling cost.
Q. What is the role of an ultrasonic flow meter?
Measures flow rate of liquid without direct contact.
Works using ultrasonic sound waves to detect velocity difference.
Suitable for dirty or corrosive effluents.
Provides accurate, maintenance-free, and non-intrusive flow monitoring.
Q. How does a pH analyzer work?
Uses a glass electrode and a reference electrode to measure hydrogen ion concentration.
Converts voltage difference into pH value.
Provides continuous pH monitoring for automatic chemical dosing control.
Essential for neutralization and process stability.
Q. What are typical instruments used for process control in ETP?
pH Analyzer – for acidity/alkalinity control.
DO Meter – for aeration efficiency.
Flow Meter – for monitoring influent and effluent flow.
TSS/TDS Analyzer – for solids measurement.
Level Sensors & Pressure Gauges – for tank and pump control.
Ensure automation, safety, and compliance.
Q. Explain the role of SCADA or PLC in ETP automation.
PLC (Programmable Logic Controller): Controls pumps, valves, and dosing automatically.
SCADA (Supervisory Control and Data Acquisition):
Monitors process parameters (pH, flow, DO, etc.) in real-time.
Provides data logging, alarms, and remote control.
Enhances operational efficiency, accuracy, and safety through automation.
Q. What are the main hazards in ETP operation?
Chemical exposure from acids, alkalis, and coagulants.
Toxic gas emissions (H₂S, chlorine).
Slips and falls on wet floors.
Electrical hazards near water systems.
Biological exposure from untreated sludge.
Proper PPE, ventilation, and safety training are essential.
Q. What PPE is required while handling chemicals?
Safety goggles or face shield – eye protection.
Chemical-resistant gloves and apron – prevent skin contact.
Respirator or mask – against harmful fumes.
Safety shoes and helmet – for personal protection.
Emergency eyewash and shower should be nearby.
Q. How do you handle chemical dosing safely?
Always read MSDS (Material Safety Data Sheet) before handling.
Use calibrated dosing pumps and proper containers.
Add chemicals to water, not vice versa, to prevent splashing.
Ensure adequate ventilation and PPE use.
Label and store chemicals securely and separately.
Q. What precautions are needed during chlorine handling?
Use chlorine-resistant gloves, mask, and goggles.
Store cylinders in cool, ventilated areas away from heat.
Check for leaks with ammonia solution, never with open flames.
Install emergency chlorine scrubbers where necessary.
Ensure trained personnel handle and connect cylinders.
Q. What is confined space entry, and where does it apply in ETP?
A confined space is an enclosed area with limited entry and poor ventilation.
Applies to tanks, sumps, and manholes in ETP.
Requires gas testing, ventilation, standby attendant, and permit system.
Prevents asphyxiation and toxic gas exposure.
Q. How do you ensure safe operation of blowers and pumps?
Perform regular maintenance and lubrication.
Check for vibration, overheating, and leaks.
Operate within rated pressure and flow limits.
Ensure electrical earthing and emergency shut-off switches.
Keep guards and covers intact on moving parts.
Q. How is sludge disposal handled safely?
Dewater sludge using filter press or drying beds.
Transport in sealed containers or covered trucks.
Dispose of at authorized landfill or co-processing unit.
Operators should wear gloves, boots, and masks.
Maintain records of sludge quantity and disposal location.
Q. What are environmental risks of untreated effluent?
Water and soil contamination with toxic chemicals.
Oxygen depletion in rivers causing aquatic death.
Groundwater pollution from seepage.
Human health risks through contaminated drinking water.
Legal penalties and damage to company reputation.
Q. How do you ensure ETP operator safety?
Provide regular training on chemical handling and emergency response.
Enforce use of PPE and safety procedures.
Maintain good housekeeping and signage.
Conduct periodic medical check-ups and safety audits.
Establish emergency exits, alarms, and first-aid facilities.
Q. What emergency measures are taken during an effluent overflow?
Stop influent flow to prevent further overflow.
Divert excess to standby or storage tanks.
Contain and recover spilled effluent immediately.
Notify authorities and management as per emergency plan.
Investigate the cause and take corrective preventive actions (CAPA).
Q. What is the difference between aerobic and anaerobic treatment?
Aerobic Treatment:
Uses oxygen-dependent microorganisms to break down organic matter.
Produces CO₂, water, and biomass.
Faster and produces low odor effluent.
Anaerobic Treatment:
Occurs without oxygen, microorganisms produce biogas (CH₄ + CO₂).
Suitable for high-strength wastewater.
Slower process but energy-efficient through methane recovery.
Q. What is the role of microbes in biological treatment?
Microorganisms degrade organic pollutants into simpler, harmless compounds.
Convert BOD and COD into biomass, CO₂, and water.
Maintain process balance and nutrient recycling.
Essential for stable operation of aeration tanks and bio-reactors.
Q. How do you maintain MLSS concentration?
Control sludge wasting rate to maintain desired MLSS (2,500–4,000 mg/L).
Adjust sludge recirculation from clarifier.
Ensure proper aeration and nutrient availability.
Avoid toxic influents that kill microorganisms.
Q. What is the purpose of F/M ratio and how is it calculated?
Purpose: Indicates the balance between food (organic load) and microorganisms (biomass) in biological treatment.
Formula:
F/M = (Q × S₀) / (V × X)
Where,
Q = Flow rate (m³/day)
S₀ = Influent BOD (mg/L)
V = Aeration tank volume (m³)
X = MLSS (mg/L)
Optimum F/M ratio ensures efficient BOD removal and healthy biomass growth.
Q. What are the advantages of using MBR (Membrane Bioreactor)?
Combines biological treatment and membrane filtration.
Produces high-quality effluent suitable for reuse.
Compact design with smaller footprint.
No need for secondary clarifier.
Provides better control over biomass and solids retention.
Q. What is SBR (Sequential Batch Reactor) technology?
Time-based biological treatment process operated in cycles.
Steps include Fill → React → Settle → Decant → Idle.
Allows aeration and clarification in one tank.
Flexible for variable loads and space-saving design.
Q. What is MBBR (Moving Bed Biofilm Reactor)?
Uses plastic media carriers in aeration tanks for biofilm growth.
Increases surface area for microbial activity.
Resistant to shock loads and easy to operate.
Suitable for retrofit of existing plants to increase capacity.
Q. Compare MBR and MBBR processes.
MBR:
Uses membrane filtration for separation.
Produces superior quality effluent.
Higher cost and maintenance.
MBBR:
Uses biofilm carriers for biological degradation.
Simple, robust, and cost-effective.
Requires secondary clarification.
Q. What is RO (Reverse Osmosis) and how is it used in ETP?
RO is a membrane separation process that removes dissolved salts, ions, and impurities.
Used in tertiary treatment or ZLD systems for final polishing.
Produces permeate (clean water) and reject (concentrated brine).
Ensures high-purity water recovery for reuse.
Q. What are the challenges of ZLD (Zero Liquid Discharge) systems?
High capital and operating costs due to energy-intensive evaporation.
Scaling and fouling in RO and evaporators.
Complex sludge and salt management.
Requires continuous monitoring and skilled operation.
Despite challenges, it ensures complete wastewater reuse and zero discharge.
Q. What records are maintained in ETP operation?
Daily logbook for flow, pH, DO, and operating parameters.
Chemical consumption register.
Maintenance and calibration records.
Sludge generation and disposal records.
Laboratory test reports for influent and effluent quality.
Monthly compliance reports to SPCB/CPCB.
Q. How do you log chemical consumption?
Record daily quantity of chemicals used (lime, alum, PAC, chlorine, etc.).
Maintain date, batch number, and dosing rate.
Compare with flow and quality variations to optimize usage.
Helps in inventory control and cost monitoring.
Q. What is a daily effluent monitoring report?
A report showing daily analysis results of treated effluent.
Includes pH, BOD, COD, TSS, TDS, DO, oil & grease, and flow rate.
Ensures continuous compliance with discharge norms.
Reviewed by plant head and environmental officer daily.
Q. What is a calibration record and why is it important?
A document verifying accuracy of instruments like pH meter, DO meter, and flow meter.
Maintained after scheduled calibration by internal or external agencies.
Ensures data reliability, audit readiness, and process accuracy.
Calibration frequency as per manufacturer or regulatory norms.
Q. What documentation is required for statutory audits?
ETP operation logbooks and daily reports.
Effluent analysis results (in-house and third-party).
Chemical consumption records.
Sludge disposal manifests and transporter details.
Calibration, maintenance, and compliance certificates.
Consent to Operate (CTO) and renewal documents.
Q. What will you do if outlet pH is below 6.0?
Immediately stop discharge to prevent non-compliance.
Check neutralization tank pH and dosing system.
Increase alkali dosing (lime/caustic) to restore balance.
Record incident and report corrective action in logbook.
Q. How do you respond to a blower failure in the aeration tank?
Switch to standby blower immediately to maintain DO levels.
Check for power, motor, or mechanical issues.
Ensure aeration resumes within minutes to prevent anaerobic conditions.
Record downtime and schedule preventive maintenance.
Q. What action will you take if COD in outlet water increases suddenly?
Verify sampling and testing accuracy.
Check inlet quality for shock load or toxic contamination.
Inspect aeration and MLSS levels for biological performance.
Optimize chemical dosing and increase retention time.
Monitor continuously until parameters stabilize.
Q. How will you handle sludge overflow in the clarifier?
Stop sludge return temporarily to control overflow.
Check sludge blanket level and underflow pump operation.
Increase sludge wasting or dewatering to restore capacity.
Inspect for mechanical faults or hydraulic overload.
Resume operation once normal level is achieved.
Q. How do you optimize ETP operation for cost and efficiency?
Regularly monitor key parameters to avoid reprocessing.
Automate dosing and aeration systems for energy saving.
Recycle treated water to reduce freshwater use.
Maintain proper MLSS and F/M ratio for biological stability.
Conduct preventive maintenance to avoid downtime.
Review chemical usage and process trends monthly for continuous improvement.
Q. Explain the process flow of your ETP plant.
“Our ETP follows a systematic physical, chemical, and biological process to ensure treated water meets all regulatory discharge standards.”
Collection & Screening: Effluent is collected from various sources and passed through bar screens to remove large solids and debris.
Equalization Tank: Balances flow and pollutant concentration to ensure steady treatment.
Neutralization Tank: pH is adjusted using acid or alkali dosing for optimal conditions.
Coagulation & Flocculation: Chemicals (lime, alum, PAC) are added to remove suspended solids and colloidal impurities.
Primary Clarifier: Allows heavy solids and sludge to settle, producing clarified water.
Aeration Tank: Biological treatment occurs using microorganisms and diffused aeration to reduce BOD and COD.
Secondary Clarifier: Separates biological sludge from treated water; part of the sludge is recycled.
Tertiary Treatment: Sand and activated carbon filters polish the effluent, removing color, odor, and fine solids.
Disinfection & Discharge: Chlorine or UV is used for final disinfection before reuse or safe discharge.
Q. Which parameters do you monitor daily and why?
“Daily parameter monitoring helps maintain treatment efficiency, detect abnormalities early, and ensure consistent compliance with CPCB norms.”
pH: To maintain neutral conditions for biological and chemical treatment.
BOD & COD: To monitor organic and total pollutant load.
TSS & TDS: To control suspended and dissolved solids in discharge.
DO (Dissolved Oxygen): To ensure proper aeration efficiency.
Flow Rate: To maintain correct retention time in each tank.
Oil & Grease: To prevent interference with aeration and biological processes.
Daily monitoring ensures process stability, efficiency, and environmental compliance.
Q. What actions do you take to maintain compliance?
Continuous monitoring of critical parameters using online sensors.
Regular calibration of instruments like pH, DO, and flow meters.
Optimized chemical dosing to maintain desired effluent quality.
Proper sludge management to avoid overflow or contamination.
Periodic laboratory testing and record submission to SPCB.
Preventive maintenance of pumps, blowers, and filters for uninterrupted operation.
Training of operators on safety and environmental procedures.
Q. What is the design basis of an ETP?
“ETP design is based on wastewater characteristics, treatment objectives, and regulatory discharge standards.”
Based on quantity and quality of wastewater generated.
Considers flow rate (m³/day), BOD, COD, TSS, oil & grease, and pH levels.
Design ensures optimum treatment efficiency and compliance with CPCB/SPCB norms.
Includes retention time, loading rates, hydraulic balance, and sludge management needs.
Q. How do you calculate the hydraulic retention time (HRT)?
Formula:
HRT (hours) = Volume of Tank (m³) / Flow Rate (m³/hr)
Represents the average time wastewater remains in a treatment unit.
Ensures sufficient contact time for pollutant removal.
Q. What is the difference between F/M ratio and SVI?
F/M Ratio (Food to Microorganism):
Indicates the balance between organic load and microbial mass.
Helps control biological treatment efficiency.
SVI (Sludge Volume Index):
Measures the settling characteristics of sludge.
Helps assess clarifier performance.
In short: F/M controls biological activity; SVI evaluates sludge settling quality.
Q. What is the typical F/M ratio maintained in an aeration tank?
Generally maintained between 0.2 to 0.5 kg BOD/kg MLSS/day.
Low F/M = stable process, less sludge; High F/M = overloading and poor treatment.
Proper F/M ensures optimum microbial activity and BOD reduction.
Q. What is the role of return activated sludge (RAS)?
Recycles settled biomass from secondary clarifier back to the aeration tank.
Maintains required MLSS concentration for biological treatment.
Prevents washout of microorganisms and ensures process stability.
Q. What is the purpose of waste activated sludge (WAS) removal?
Removes excess sludge generated during biological treatment.
Prevents overgrowth of microorganisms and maintains MLSS balance.
Reduces oxygen demand and operational issues.
Disposed of safely after dewatering or drying.
Q. What are the key design parameters for an aeration tank?
Flow rate and organic load (BOD/COD).
F/M ratio and MLSS concentration.
Hydraulic Retention Time (HRT).
Air requirement and oxygen transfer rate.
Type of aeration system (diffused or mechanical).
Designed for maximum efficiency and minimal energy use.
Q. How do you determine the air requirement in the aeration tank?
Based on oxygen demand for microbial activity and oxygen transfer efficiency (OTE).
Formula:
Air Required (m³/hr) = (O₂ Demand / (OTE × 0.232))
where 0.232 = oxygen fraction in air.
Also depends on BOD load, MLSS, and DO levels required.
Q. What is the ideal DO (Dissolved Oxygen) range in an aeration tank?
Maintained between 2.0 – 4.0 mg/L.
Ensures sufficient oxygen for microbial respiration and avoids anaerobic conditions.
Low DO → poor treatment & odor; High DO → energy waste.
Q. How do you calculate the oxygen transfer efficiency (OTE)?
Formula:
OTE (%) = (O₂ Transferred to Water / O₂ Supplied by Air) × 100
Represents the effectiveness of aeration system.
Influenced by bubble size, diffuser depth, temperature, and tank mixing.
Higher OTE = better aeration and lower energy cost.
Q. What is the significance of MLSS and MLVSS in biological treatment?
MLSS (Mixed Liquor Suspended Solids): Represents the total suspended solids (microorganisms + inert matter) in the aeration tank.
MLVSS (Mixed Liquor Volatile Suspended Solids): Represents the organic (biological) portion of MLSS—mainly active microorganisms.
Indicates biomass concentration and biological activity in the system.
Optimum MLSS (2500–4000 mg/L) ensures efficient organic load removal.
Q. How do you calculate sludge age (SRT)?
Formula:
SRT (days) = (Mass of MLSS in Aeration Tank) / (Mass of Sludge Wasted per Day)
Indicates average time microorganisms remain in the system.
Longer SRT = stable operation and lower sludge production; too long = aged, less active sludge.
Q. What is a shock load and how does it affect ETP performance?
A sudden increase in flow or pollutant concentration beyond design limits.
Causes upset in biological activity, low DO, and poor treatment efficiency.
Can result in high COD/BOD in outlet or foaming.
Managed by using equalization tanks, gradual flow control, and nutrient balance.
Q. What is the importance of equalization tank retention time?
Provides uniform flow and concentration to downstream processes.
Reduces shock load impact and maintains process stability.
Typical retention time: 8–12 hours.
Ensures consistent effluent quality and smoother operation.
Q. How do you design a clarifier (settling tank)?
Based on surface overflow rate (SOR) and detention time.
Formula:
Area (m²) = Flow (m³/day) / SOR (m³/m²/day)
SOR typically 20–30 m³/m²/day for secondary clarifiers.
Design ensures effective solid-liquid separation and clear effluent.
Q. What factors affect the settling efficiency of sludge?
Sludge concentration (MLSS).
Floc size and density.
pH and temperature of wastewater.
Hydraulic loading rate and inlet flow distribution.
Presence of filamentous bacteria (can cause bulking).
Q. What are common design considerations for sludge drying beds?
Bed area based on sludge volume and drying rate.
Layering: Gravel, sand, and underdrains for drainage.
Slope: 1–2% for easy water runoff.
Drying time: 7–15 days depending on climate.
Provision for leachate collection and odor control.
Q. Explain the principle of a dissolved air flotation (DAF) system.
Works on air flotation principle — air is dissolved under pressure and released to form microbubbles.
Bubbles attach to suspended solids or oil, causing them to float to the surface.
The floated sludge layer is skimmed, and clarified water is discharged.
Effective for oil, grease, and fine solids removal.
Q. What is the difference between primary clarifier and secondary clarifier?
Primary Clarifier:
Removes settleable solids and floating matter before biological treatment.
Reduces BOD by 25–35%.
Secondary Clarifier:
Separates biological sludge after aeration.
Returns active sludge (RAS) and removes excess (WAS).
Both ensure efficient solid-liquid separation at different treatment stages.
Q. What is a tertiary treatment and why is it needed?
Tertiary treatment is the final polishing stage after secondary treatment.
Removes residual solids, nutrients, color, and pathogens.
Involves sand filtration, carbon filtration, RO, and disinfection (UV/Chlorine).
Ensures effluent is safe for reuse or compliant discharge.
Q. What is the purpose of coagulants and flocculants in ETP?
Coagulants destabilize fine particles and colloids in wastewater.
Flocculants bind these destabilized particles into larger flocs for easy settling.
Together, they enhance solid–liquid separation, improve clarity, and reduce turbidity and suspended solids.
Q. Name commonly used coagulants in ETP.
Alum (Aluminium Sulphate)
Ferric Chloride (FeCl₃)
Ferrous Sulphate (FeSO₄)
Poly Aluminium Chloride (PAC)
Lime (Ca(OH)₂) — used for pH correction and partial coagulation.
Q. What is the function of alum in wastewater treatment?
Acts as a coagulant to destabilize suspended particles and colloids.
Helps form flocs that settle easily in clarifiers.
Reduces turbidity, color, and phosphate levels in wastewater.
Works best in a slightly alkaline pH range (6.5–8.0).
Q. What is the difference between alum and ferric chloride?
Ferric chloride is more powerful; alum is more economical and suitable for general use.
Alum:
Mild coagulant, effective in clear or lightly polluted water.
Produces less sludge and works at neutral pH.
Ferric Chloride:
Strong coagulant, effective for highly turbid or industrial wastewater.
Works over wider pH range and produces denser flocs.
Q. What is polymer dosing and when is it required?
Polymers (flocculants) are used to enhance settling and dewatering efficiency.
Added after coagulants to form larger, stronger flocs.
Common in clarifiers, DAF units, and sludge dewatering systems.
Used when fine particles or poor settling sludge are present.
Q. How do you determine the correct dosage of chemicals?
Conduct jar testing to simulate treatment performance.
Adjust dosage based on turbidity removal, pH, and settling rate.
Optimize to achieve maximum clarity with minimum chemical use.
Review dosage regularly with changes in influent quality.
Q. What is jar testing and why is it performed?
A laboratory simulation test to determine the optimum chemical type and dose.
Multiple beakers are used to test various dosages and mixing conditions.
Helps identify ideal coagulant and flocculant doses before full-scale operation.
Prevents overdosing and chemical wastage.
Q. What is the role of lime in ETP?
Used for pH correction of acidic effluent.
Aids in coagulation, precipitation of heavy metals, and hardness removal.
Provides alkalinity to support biological treatment.
Also used as a cost-effective neutralizing agent.
Q. How do you control pH during neutralization?
Install online pH sensors linked with automatic dosing pumps.
Use acids (H₂SO₄/HCl) for high-pH effluent and alkalis (lime/NaOH) for low-pH effluent.
Maintain pH between 6.5 – 8.5 for optimum treatment.
Ensure mixing and response time for uniform adjustment.
Q. What is the difference between acidic and alkaline effluent treatment?
Acidic Effluent Treatment:
Neutralized using alkaline chemicals (lime, caustic soda).
Aim: Raise pH to neutral range.
Alkaline Effluent Treatment:
Treated using acidic chemicals (sulphuric or hydrochloric acid).
Aim: Lower pH to neutral range.
Both ensure balanced conditions for biological and chemical processes.
Q. Why is chlorine used in tertiary treatment?
Chlorine is used for disinfection to kill harmful bacteria, viruses, and pathogens.
Ensures treated water is safe for reuse or discharge.
Provides residual protection in the distribution system.
It is a cost-effective and proven method for final treatment.
Q. What are the disadvantages of excessive chlorine dosing?
Causes formation of toxic by-products (like trihalomethanes).
Leads to corrosion of pipelines and equipment.
Produces strong odor and taste in water.
Can be harmful to aquatic life upon discharge.
Must always maintain residual chlorine between 0.2–0.5 mg/L.
Q. What is dechlorination and when is it required?
Dechlorination is the process of removing residual chlorine after disinfection.
Prevents toxic effects on aquatic ecosystems.
Commonly done using sodium bisulfite, sulfur dioxide, or activated carbon.
Required when treated water is discharged into natural bodies or reused for biological treatment.
Q. What are the effects of overdosing coagulants?
Increases sludge volume and handling cost.
Causes low pH and may damage equipment.
Produces poor floc formation or re-stabilization of particles.
Results in turbid or colored effluent and higher operating cost.
Proper dosing must be verified through jar testing.
Q. What is the role of PAC (Poly Aluminium Chloride)?
Acts as an efficient coagulant with faster floc formation than alum.
Effective over a wide pH range (5.0–9.0).
Produces denser flocs and less sludge.
Commonly used for color, turbidity, and organic matter removal.
Provides superior performance in compact ETP designs.
Q. What precautions must be taken during chemical handling?
Always wear PPE (gloves, goggles, apron, mask).
Refer to Material Safety Data Sheet (MSDS) before use.
Use dedicated tools and containers for each chemical.
Ensure adequate ventilation and avoid inhaling fumes.
Keep emergency eyewash and shower stations nearby.
Q. How do you store and prepare chemical solutions for dosing?
Store chemicals in dry, well-ventilated areas away from heat and moisture.
Use non-reactive containers and clear labeling.
Prepare solutions by adding chemical to water, not vice versa.
Stir gently to ensure complete dissolution.
Maintain daily preparation logs and use fresh solutions to avoid degradation.
Q. What is the ideal mixing time in a flash mixer?
Ideal mixing time is typically 30 to 60 seconds.
Ensures uniform dispersion of coagulant before flocculation.
Excess mixing breaks flocs; insufficient mixing leads to poor coagulation.
Speed and duration depend on flow rate and coagulant type.
Q. What is zeta potential and why is it important in coagulation?
Zeta potential measures the electrical charge on particle surfaces in water.
Indicates the stability of suspended particles.
Coagulants neutralize this charge to promote particle aggregation.
Optimum zeta potential (near zero) ensures effective coagulation and settling.
Q. What is the difference between mechanical and static mixers?
Mechanical Mixer:
Uses motor-driven impellers to create turbulence.
Allows control over mixing intensity and time.
Suitable for variable flow conditions.
Static Mixer:
Has stationary blades or baffles in a pipe; mixing occurs as fluid passes through.
No moving parts, low maintenance.
Ideal for continuous flow and consistent dosing.
Q. Explain the difference between attached and suspended growth systems.
“Attached growth uses fixed biofilms; suspended growth relies on free microbial flocs for pollutant removal.”
Attached Growth System:
Microorganisms grow on solid media surfaces (like MBBR carriers, trickling filters).
Biofilm remains fixed, and wastewater flows over it.
Handles shock loads better and requires less sludge handling.
Suspended Growth System:
Microbes are freely suspended in wastewater (like activated sludge process).
Easier to control but more sensitive to load variations.
Q. What are aerobic, anaerobic, and anoxic zones?
Aerobic Zone: Presence of oxygen, microbes oxidize organic matter and convert ammonia to nitrate.
Anaerobic Zone: No oxygen present; microbes degrade organics to produce biogas (CH₄ + CO₂).
Anoxic Zone: No dissolved oxygen but nitrates present; used for denitrification (conversion of nitrate to nitrogen gas).
“Aerobic = oxygen present, Anaerobic = no oxygen, Anoxic = no DO but nitrate available.”
Q. What microorganisms are involved in biological treatment?
“Bacteria perform the main treatment; protozoa and fungi support by stabilizing and clarifying the process.”
Bacteria: Primary decomposers of organic matter (aerobic & anaerobic).
Protozoa: Consume dispersed bacteria and clarify effluent.
Fungi: Break down complex organics, especially under low pH.
Nitrifying & Denitrifying Bacteria: Convert ammonia → nitrate → nitrogen gas.
Q. What happens when MLSS becomes too high or too low?
Too High MLSS:
Causes poor oxygen transfer and sludge bulking.
Reduces clarifier efficiency and increases energy use.
Too Low MLSS:
Leads to insufficient biomass for pollutant breakdown.
Causes poor BOD/COD removal.
Optimum MLSS: 2500–4000 mg/L for stable operation.
Q. What causes sludge bulking in the aeration tank?
Excess filamentous bacterial growth due to low DO, nutrient deficiency, or high organic load.
Imbalanced F/M ratio or poor sludge age control.
Leads to poor settling and turbid effluent.
Controlled by adjusting DO, nutrient dosing, and sludge wasting.
Q. How can you control filamentous bacterial growth?
Maintain DO > 2 mg/L in aeration tanks.
Optimize F/M ratio and nutrient balance (BOD:N:P).
Apply chlorination of return sludge if excessive filaments persist.
Avoid low pH, shock loads, or high grease content.
Q. What is the purpose of nutrient dosing (N & P)?
Provides essential nutrients (Nitrogen & Phosphorus) for microbial growth.
Supports enzyme production and cell synthesis in biological treatment.
Prevents process imbalance and poor BOD removal.
Especially important in industrial wastewater low in natural nutrients.
Q. What is the typical BOD:N:P ratio maintained?
Ideal ratio = 100 : 5 : 1
100 parts BOD (carbon source)
5 parts Nitrogen
1 part Phosphorus
Maintains balanced microbial metabolism and stable biological performance.
Q. What is nitrification and denitrification?
Nitrification:
Aerobic process converting ammonia (NH₃) → nitrite (NO₂⁻) → nitrate (NO₃⁻) by Nitrosomonas and Nitrobacter.
Denitrification:
Anoxic process converting nitrate (NO₃⁻) → nitrogen gas (N₂) by denitrifying bacteria.
Together, they remove nitrogen compounds and prevent eutrophication.
Q. What causes foaming in the aeration tank?
Excess filamentous bacteria (e.g., Nocardia) or high surfactant levels in influent.
Low DO or nutrient imbalance.
Over-aeration or excessive sludge age.
Controlled by adjusting MLSS, DO, and applying antifoam agents if necessary.