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Practice SetEnvironmental Engineering

Environmental Engineering MCQ Practice Set — 50 Questions with Answers

50 exam-oriented Environmental Engineering multiple-choice questions with the correct answer and a clear explanation for each. Frequently asked in AE Level Civil Engineering, JE Level Civil Engineering. Solve the full set below for free — no login required.

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  1. 1
    Environmental EngineeringMEDIUM

    The 'chemical oxygen demand' (COD) test measures:

    AOnly the dissolved oxygen available
    BOxygen equivalent of ALL oxidisable organics (by K2Cr2O7) — faster than BOD; includes non-biodegradable fraction
    CBiochemical oxygen demand over 5 days
    DTotal dissolved solids only

    Answer: B. Oxygen equivalent of ALL oxidisable organics (by K2Cr2O7) — faster than BOD; includes non-biodegradable fraction

    Explanation: COD: all organics (biodegradable + non-biodegradable + toxic) oxidised by chemical oxidant (K2Cr2O7 in acidic condition with Ag2SO4 catalyst and heat for 2 hours). Results in g O2/L equivalent. COD > BOD always (COD includes non-biodegradable). Ratio BOD/COD: > 0.5 = biodegradable; 0.3–0.5 = moderate; < 0.3 = non-biodegradable/toxic waste (needs special treatment). COD result in hours vs BOD5 in 5 days.

  2. 2
    Environmental EngineeringHARD

    The 'Moving Bed Biofilm Reactor' (MBBR) is a biological treatment process where:

    AMedia is fixed and water flows through it (like trickling filter)
    BBuoyant plastic carriers with biofilm move freely in aeration tank — no sludge recycle needed
    COnly for primary sedimentation
    DChemical dosing only (no biological process)

    Answer: B. Buoyant plastic carriers with biofilm move freely in aeration tank — no sludge recycle needed

    Explanation: MBBR: plastic carrier media (polyethylene, specific gravity ~0.95) suspended in aeration tank by airflow and mixing. Biofilm grows on media surface (attached growth). No sludge recirculation needed (attached growth). Compact footprint. Advantages over activated sludge: nitrification + BOD removal; no clarifier sludge recycle needed; no sludge bulking; easy to upgrade existing tanks. Used in India for STP upgrades.

  3. 3
    Environmental EngineeringMEDIUM

    The 'electrostatic precipitator' (ESP) works on the principle of:

    AFiltration through cloth bags (like fabric filter)
    BCorona discharge ionises particles; electric field attracts them to grounded collection plates
    CCentrifugal separation (like cyclone)
    DWater scrubbing

    Answer: B. Corona discharge ionises particles; electric field attracts them to grounded collection plates

    Explanation: ESP: particles passing between high-voltage electrodes (discharge electrode at −30 to −60 kV DC, collecting plate grounded). Corona discharge ionizes air → ions attach to particles → charged particles attracted to collecting plates. Plates rapped (vibrated) → dust falls to hopper. Efficiency > 99% for particles > 1 μm. Used for: coal power plants (fly ash), cement kilns, steel plants. High capital cost, low operating cost.

  4. 4
    Previous-year standardMEDIUM

    For population 50000 and water demand 135 LPCD, daily water requirement is

    A370.37 MLD
    B13.50 MLD
    C3.38 MLD
    D6.75 MLD

    Answer: D. 6.75 MLD

    Explanation: Daily water requirement = Population × per capita demand. = 50000 × 135 L/day = 50000 × 135 / 10⁶ MLD = 6.75 MLD MLD (million litres per day). LPCD = litres per capita per day. IS 1172 recommends 135 LPCD for cities without full flushing, 150 LPCD with.

  5. 5
    Original practiceMEDIUM

    A circular sewer of diameter 0.5 m runs full at velocity 0.6 m/s. Discharge is nearly

    A0.236 m³/s
    B0.118 m³/s
    C0.300 m³/s
    D0.059 m³/s

    Answer: B. 0.118 m³/s

    Explanation: Discharge in a circular sewer running full: Q = A × V = (π D² / 4) × V. D = 0.5 m, V = 0.6 m/s. Q = π × 0.5² / 4 × 0.6 = 0.118 m³/s m³/s. For a sewer running full, the cross-sectional area equals π D²/4. Self-cleansing velocity is typically ≥ 0.6 m/s to prevent silt deposition.

  6. 6
    PYQ/PYQ-PatternMEDIUM

    A sedimentation tank receives 10000 m³/day over surface area 250 m². Surface overflow rate is

    A10250 m³/m²/day
    B80 m³/m²/day
    C40 m³/m²/day
    D20 m³/m²/day

    Answer: C. 40 m³/m²/day

    Explanation: Surface Overflow Rate (SOR) = Q / A_s (m^3/m^2/day), where Q = daily flow rate and A_s = plan surface area of sedimentation tank. Particles with settling velocity Vs >= SOR are completely removed in an ideal rectangular tank. Typical SOR: 25–50 m^3/m^2/day for plain sedimentation. Increasing A_s for given Q improves particle removal efficiency. SOR=Q/A=10000/250=40 m³/m²/day.

  7. 7
    Environmental EngineeringMEDIUM

    The method which is most widely used for analysing and designing the pipes of all types of complex water distribution network is

    AEquivalent pipe method
    BHardy-cross method
    CCircle method
    DElectric network analyser method

    Answer: B. Hardy-cross method

    Explanation: The Hardy-Cross method is the most widely used iterative method for the analysis of complex pipe networks.

  8. 8
    Environmental EngineeringMEDIUM

    The permissible limit of arsenic concentration in domestic water supplies is

    A0.1 ppm
    B0.5 ppm
    C0.01 ppm
    D0.05 ppm

    Answer: C. 0.01 ppm

    Explanation: According to current WHO and BIS (IS 10500:2012) standards, the acceptable limit for arsenic in drinking water is 0.01 mg/L (10 ppb).

  9. 9
    Air PollutionMEDIUM

    NAAQS (National Ambient Air Quality Standards) for PM2.5 (annual average):

    A40 μg/m³
    B60 μg/m³
    C25 μg/m³
    D15 μg/m³

    Answer: A. 40 μg/m³

    Explanation: NAAQS 2009: PM2.5 — annual mean 40 μg/m³ (residential/rural/other); 24-hr standard 60 μg/m³. PM10: annual 60 μg/m³, 24-hr 100 μg/m³. WHO guidelines are stricter (PM2.5 annual: 5 μg/m³, 2021).

  10. 10
    Environmental EngineeringMEDIUM

    The 'fluoride' content in drinking water (IS 10500) should be within:

    A0 mg/L (fluoride always harmful)
    B0.6–1.5 mg/L — below 0.6: caries risk; above 1.5: fluorosis risk per IS 10500
    C5 mg/L maximum (no minimum)
    DAlways 100 mg/L

    Answer: B. 0.6–1.5 mg/L — below 0.6: caries risk; above 1.5: fluorosis risk per IS 10500

    Explanation: IS 10500: Fluoride permissible range — minimum 0.6 mg/L (below this: dental caries risk); desirable limit 1.0–1.5 mg/L; maximum permissible 1.5 mg/L (above: dental/skeletal fluorosis). Defluoridation: Nalgonda technique, activated alumina adsorption. Fluorosis is endemic in Rajasthan, AP, Telangana, Gujarat (high fluoride groundwater areas).

  11. 11
    Environmental EngineeringHARD

    The 'sequencing batch reactor' (SBR) for wastewater treatment operates in:

    AContinuous flow mode like conventional ASP
    BTime-based cycles (fill, react, settle, decant) in a single tank — no secondary clarifier needed
    COnly physical settling — no biological treatment
    DGravity filtration only

    Answer: B. Time-based cycles (fill, react, settle, decant) in a single tank — no secondary clarifier needed

    Explanation: SBR: a fill-and-draw activated sludge system operating in time-sequenced cycles: Fill (influent enters) → React (aeration for BOD removal) → Settle (sludge settles) → Decant (clear effluent withdrawn) → Idle. All in one tank. Advantages: flexible operation, no secondary clarifier needed, can achieve nitrification and denitrification. Used for small flows and intermittent industrial discharges.

  12. 12
    Environmental EngineeringEASY

    As per IS 10500 (Drinking Water Standards), the desirable limit for Total Dissolved Solids (TDS) is:

    A200 mg/L
    B500 mg/L
    C1000 mg/L
    D2000 mg/L

    Answer: B. 500 mg/L

    Explanation: IS 10500: TDS desirable limit = 500 mg/L, permissible (in absence of alternate) = 2000 mg/L. High TDS causes scale, corrosion, salty taste, and laxative effect. TDS measured by evaporation of filtered sample at 180°C. Reverse osmosis effectively removes TDS.

  13. 13
    Environmental EngineeringMEDIUM

    Egg-shaped sewer section is advantageous because it:

    AMaintains higher velocity at low flow
    BEliminates manholes
    CCarries no sewage
    DRequires no excavation

    Answer: A. Maintains higher velocity at low flow

    Explanation: Narrow bottom improves self-cleansing during low dry-weather flow.

  14. 14
    RecarbonationMEDIUM

    After lime-soda softening, recarbonation is commonly used in water treatment to:

    AStabilize water by reducing excess alkalinity and pH
    BIncrease suspended solids before filtration
    CConvert all chlorides into hardness
    DRemove the need for sedimentation tanks

    Answer: A. Stabilize water by reducing excess alkalinity and pH

    Explanation: Recarbonation adds carbon dioxide after softening to stabilize water and reduce excess lime/alkalinity.

  15. 15
    Sewage Peaking FactorMEDIUM

    The peak factor (ratio of peak flow to average daily flow) used in sewerage design for a city with population 100,000 is approximately:

    A1.0
    B1.5
    C2.5
    D5.0

    Answer: C. 2.5

    Explanation: Peak factor decreases with population. For population 100,000: peak factor = 2.5-3.0. For small towns (<5000): 4-5. Formula: PF = 1 + (14/(4+sqrt(P))) where P in thousands (Babbit and Baumann). Peak flow governs sewer pipe diameter design, while average flow is used for treatment plant capacity.

  16. 16
    Air Quality Index CPCBMEDIUM

    As per CPCB, the Air Quality Index (AQI) value of 200-300 is categorized as:

    AGood
    BModerate
    CPoor
    DVery Poor

    Answer: C. Poor

    Explanation: CPCB AQI categories: 0-50 = Good (green); 51-100 = Satisfactory; 101-200 = Moderate; 201-300 = Poor; 301-400 = Very Poor; 401-500 = Severe (red). AQI is calculated for PM2.5, PM10, NO2, SO2, CO, O3, NH3 separately; the maximum sub-index is the AQI.

  17. 17
    Solid Waste Calorific ValueMEDIUM

    The calorific value of municipal solid waste (MSW) determines the feasibility of:

    AComposting
    BLandfill disposal
    CWaste-to-Energy (WTE) incineration
    DOpen burning

    Answer: C. Waste-to-Energy (WTE) incineration

    Explanation: WTE feasibility requires minimum calorific value of ~1200-1500 kcal/kg (Indian MSW is ~800-1200 kcal/kg due to high moisture and organics). Indian MSW: moisture 40-60%, organics 40-60%, inerts 20-40%. Higher income cities have higher CV. Composting works for high-organic MSW.

  18. 18
    Imhoff TankMEDIUM

    An Imhoff tank combines sedimentation and sludge digestion in two separate zones. The upper zone:

    ADigests sludge at elevated temperature
    BProvides quiescent sedimentation for 2-4 hours; settled solids slip into the lower digestion zone
    CAerates the wastewater
    DDisinfects with chlorine

    Answer: B. Provides quiescent sedimentation for 2-4 hours; settled solids slip into the lower digestion zone

    Explanation: Imhoff tank: upper compartment = sedimentation (2-4 h detention); lower compartment = anaerobic sludge digestion (separated by slot, prevents gas bubbles rising into settling zone). Advantages: simple, no mechanical parts. Disadvantages: inefficient digestion, limited gas recovery.

  19. 19
    MLVSS SignificanceMEDIUM

    Mixed Liquor Volatile Suspended Solids (MLVSS) in the activated sludge process represents:

    ATotal inorganic solids in the aeration tank
    BThe active biomass (microorganisms) fraction of MLSS, used in kinetic design
    CThe settled sludge volume
    DThe final effluent suspended solids

    Answer: B. The active biomass (microorganisms) fraction of MLSS, used in kinetic design

    Explanation: MLSS = MLVSS + inert ash. MLVSS (volatile = ignites at 550 deg C) represents active microbial biomass. Typical MLVSS/MLSS ratio = 0.7-0.8. In kinetics, process design uses MLVSS. Typical MLSS in conventional AS: 1500-3000 mg/L; extended aeration: 3000-6000 mg/L.

  20. 20
    Trickling Filter LoadingMEDIUM

    A high-rate trickling filter (HRTF) differs from a standard-rate filter in that HRTF has:

    ALower hydraulic loading, finer media
    BHigher hydraulic loading (recirculation) and coarser media (40-100 mm), with recycle ratio 1-8
    CNo recirculation
    DLower BOD removal efficiency always

    Answer: B. Higher hydraulic loading (recirculation) and coarser media (40-100 mm), with recycle ratio 1-8

    Explanation: High-rate TF: hydraulic loading = 10-40 m3/m2/day (with recirculation), media size 40-100 mm, recycle ratio 1-8. Standard TF: hydraulic loading 1-4 m3/m2/day, media 25-75 mm, no recirculation. HRTF gives >85% BOD removal in 2-pass systems.

  21. 21
    Population Forecast GeometricMEDIUM

    In the geometric increase method for population forecasting, the population after n decades is:

    AP_n = P_0 + n x (dP/dt)
    BP_n = P_0 x (1 + r/100)^n
    CP_n = P_0 + n x Ig
    DP_n = (P_0 + P_f)/2

    Answer: B. P_n = P_0 x (1 + r/100)^n

    Explanation: Geometric (exponential) method: P_n = P0 x (1 + Ig/100)^n, where Ig = geometric growth rate per decade (%). Used for cities in growth phase. Arithmetic: P_n = P0 + n x Ia. Geometric gives higher projections; arithmetic is conservative.

  22. 22
    Rapid Sand Filter DesignMEDIUM

    The normal filtration rate for a rapid sand filter (RSF) in water treatment is:

    A0.1 to 0.4 m/h (slow sand filter rate)
    B100 to 150 litres/m2/hour (2.5 to 4 m/h)
    C500 m3/m2/day
    D1 m/h

    Answer: B. 100 to 150 litres/m2/hour (2.5 to 4 m/h)

    Explanation: Rapid sand filter (RSF): filtration rate = 100-150 L/m2/h (about 2.5-4 m/h, 60-100 m3/m2/day). Much faster than slow sand filter (0.1-0.4 m/h). RSF requires coagulation and sedimentation before filtration. Backwashing at 600-900 L/m2/h to clean the filter.

  23. 23
    GeneralMEDIUM

    The permissible pH range for potable water is commonly around

    Aexactly 7 only
    B1 to 2
    C12 to 14
    D6.5 to 8.5

    Answer: D. 6.5 to 8.5

    Explanation: Drinking water standards generally keep pH near neutral, commonly 6.5-8.5.

  24. 24
    GeneralMEDIUM

    In water distribution, dead-end system has the disadvantage of

    Aconstant pressure everywhere
    Bstagnation at pipe ends
    Cminimum pipe length always
    Dno valves required

    Answer: B. stagnation at pipe ends

    Explanation: Low-flow dead ends can cause poor water quality due to stagnation.

  25. 25
    GeneralMEDIUM

    Oxidation pond treatment depends mainly on

    Aonly high pressure aeration
    Bonly reverse osmosis
    Conly chlorination
    Dalgae-bacteria symbiosis and sunlight

    Answer: D. algae-bacteria symbiosis and sunlight

    Explanation: Algae supply oxygen and bacteria oxidize organics in ponds.

  26. 26
    Public Health and Environmental EngineeringHARD

    Plain sedimentation removes mainly:

    Adissolved salts
    Bviruses completely
    Csuspended settleable solids
    Dhardness completely

    Answer: C. suspended settleable solids

    Explanation: Sedimentation removes particles that settle under gravity.

  27. 27
    Environmental EngineeringHARD

    Coagulation in water treatment is aided by rapid mixing because it:

    Asettles sludge instantly
    Bdisperses coagulant uniformly
    Creplaces filtration
    Dremoves dissolved salts completely

    Answer: B. disperses coagulant uniformly

    Explanation: Rapid mixing quickly distributes coagulant and promotes destabilisation of colloids before flocculation.

  28. 28
    Environmental EngineeringHARD

    In activated sludge process, MLSS denotes:

    Amixed liquor suspended solids
    Bminimum lateral sewer slope
    Cmaximum lime saturation state
    Dmunicipal liquid settling speed

    Answer: A. mixed liquor suspended solids

    Explanation: MLSS is the concentration of suspended biological solids in the aeration tank mixed liquor.

  29. 29
    Environmental EngineeringHARD

    Alum is used in water treatment mainly as a:

    Afilter medium
    Bdisinfectant gas
    Csoftening resin
    Dcoagulant

    Answer: D. coagulant

    Explanation: Alum destabilises colloids and forms flocs that settle or are filtered out.

  30. 30
    Environmental Engineering, Estimation, Contracts and EquipmentMEDIUM

    A transit mixer transports ready-mixed concrete while agitating it.

    Ameasuring road speed
    Bdrilling piles only
    Ctransporting/agitating RMC
    Dtesting pH

    Answer: C. transporting/agitating RMC

    Explanation: Transit mixers keep concrete mixed during transport.

  31. 31
    Environmental Engineering, Estimation, Contracts and EquipmentEASY

    In RRB JE context, work-charged establishment is:

    Apermanent cadre only
    Btemporary staff charged directly to a specific work
    Cperformance test
    Dbank guarantee

    Answer: B. temporary staff charged directly to a specific work

    Explanation: temporary staff charged directly to a specific work is the correct association for work-charged establishment.

  32. 32
    Environmental Engineering, Estimation, Contracts and EquipmentMEDIUM

    The best description of measurement book is:

    Aattendance register only
    Bofficial record of measurements of work done
    Ccontractor's personal diary only
    Dcement test register only

    Answer: B. official record of measurements of work done

    Explanation: official record of measurements of work done is the correct association for measurement book.

  33. 33
    Environmental Engineering, Estimation, Contracts and EquipmentEASY

    Scraper is earthmoving equipment that cuts, loads, hauls and spreads soil.

    Awater testing
    Brail welding
    Ccut-load-haul-spread earthwork
    Dconcrete vibration only

    Answer: C. cut-load-haul-spread earthwork

    Explanation: Scrapers combine several earthwork operations.

  34. 34
    Environmental EngineeringMEDIUM

    The most common shape of sewer for separate sanitary system is:

    ARectangular open channel
    BTriangular notch
    CCircular
    DParabolic arch only

    Answer: C. Circular

    Explanation: Circular sewers are structurally efficient and hydraulically convenient.

  35. 35
    Environmental EngineeringEASY

    The unit of population equivalent is based on:

    AChlorine dose only
    BFlow velocity only
    COrganic load expressed as BOD
    DPipe diameter only

    Answer: C. Organic load expressed as BOD

    Explanation: Population equivalent converts waste load to equivalent domestic population load.

  36. 36
    Environmental EngineeringHARD

    Trickling filter recirculation is provided mainly to:

    AIncrease grit load
    BRemove all oxygen
    CStop biological growth
    DImprove wetting and dilute influent

    Answer: D. Improve wetting and dilute influent

    Explanation: Recirculation improves contact, distribution and treatment stability.

  37. 37
    Environmental EngineeringMEDIUM

    Inverted siphon in sewerage is used when sewer must pass:

    ABelow an obstruction under pressure
    BThrough a filtration unit
    CAbove a road in open channel
    DOver a roof

    Answer: A. Below an obstruction under pressure

    Explanation: An inverted siphon carries sewage under pressure below obstacles.

  38. 38
    Environmental EngineeringHARD

    Chloramines compared with free chlorine generally provide:

    AInstant turbidity removal
    BComplete hardness removal
    CZero disinfection
    DLonger-lasting but weaker residual action

    Answer: D. Longer-lasting but weaker residual action

    Explanation: Combined chlorine residual persists longer but disinfects more slowly.

  39. 39
    Environmental EngineeringMEDIUM

    The effective size of filter sand is represented by:

    AD60
    BD90
    CD10
    DD30

    Answer: C. D10

    Explanation: Effective size is the sieve size through which 10 percent by weight passes.

  40. 40
    Environmental EngineeringEASY

    Trickling filter is an example of:

    AAnaerobic digestion only
    BAttached growth biological treatment
    CScreening
    DChemical precipitation only

    Answer: B. Attached growth biological treatment

    Explanation: Microorganisms grow on media and oxidize wastewater as it trickles through.

  41. 41
    Environmental EngineeringHARD

    Sludge age in activated sludge process means average time solids remain in:

    AGrit chamber only
    BAeration system
    CIntake well
    DDistribution pipe

    Answer: B. Aeration system

    Explanation: Sludge age or SRT indicates biomass retention time.

  42. 42
    Environmental EngineeringMEDIUM

    The disinfection efficiency of chlorine in water treatment

    Ais not dependent on pH value
    Bis increased by increased pH values
    Cremains constant at all pH values
    Dis reduced by increased pH value

    Answer: D. is reduced by increased pH value

    Explanation: At higher pH, less hypochlorous acid is available, so chlorine disinfection efficiency reduces

  43. 43
    Environmental EngineeringMEDIUM

    Imhoff cone is employed to determine:

    Asuspended solids
    Bvolatile solids
    Csettleable solids
    Dfilterable solids

    Answer: C. settleable solids

    Explanation: The Imhoff cone is used to measure settleable solids in wastewater

  44. 44
    Environmental EngineeringMEDIUM

    If chlorine demand of a water sample is 0.2 mg/l and the bleaching powder contains 30% chlorine, the amount of bleaching powder needed for 1 litre of water is:

    A0.14 mg
    B0.06 mg
    C0.133 mg
    D0.67 mg

    Answer: D. 0.67 mg

    Explanation: Required bleaching powder = chlorine demand/available chlorine fraction = 0.2/0.30 = 0.67 mg

  45. 45
    Environmental EngineeringMEDIUM

    In general, pH value of fresh sewage remains:

    Aless than 1
    Bmore than 14
    Cbetween 1 and 7
    Dbetween 7 and 14

    Answer: D. between 7 and 14

    Explanation: Fresh sewage is usually alkaline or near neutral; among the options, between 7 and 14 is appropriate

  46. 46
    Environmental EngineeringMEDIUM

    The growth of algae is useful in:

    Aseptic tank
    Bslow sand filter
    Coxidation pond
    Dsludge digestion tank

    Answer: C. oxidation pond

    Explanation: Algae supply oxygen through photosynthesis in oxidation ponds, supporting aerobic bacteria

  47. 47
    Environmental EngineeringMEDIUM

    Storage of water by impounding is required where:

    Aplenty of water is available in the stream in all seasons
    Bexcess suspended and dissolved matter are present in the water
    Cthere is large variation in quantity of the river flow from time to time
    Dwater availability is insufficient

    Answer: C. there is large variation in quantity of the river flow from time to time

    Explanation: Impounding storage balances seasonal/temporal variation in river flow

  48. 48
    Environmental EngineeringMEDIUM

    If the pH value of sewage is 7

    Ait is acidic
    Bit is alkaline
    Cit is neutral
    Dnone of these

    Answer: C. it is neutral

    Explanation: pH = 7 indicates a neutral solution (neither acidic nor basic). In sewage treatment, pH 7 means the effluent is neutral — the hydrogen ion concentration [H⁺] equals the hydroxyl ion concentration [OH⁻]. Ideal sewage pH for biological treatment is 6.5–8.5. Strongly acidic (pH < 5) or alkaline (pH > 10) conditions inhibit biological activity in sewage treatment.

  49. 49
    Environmental EngineeringMEDIUM

    The advantage of COD test over BOD test is

    Ait oxidizes all matter including toxins
    Bit cannot oxidize benzene and pyridine
    Cit is relatively a quick process
    Dbiodegradable matter cannot be distinguished from the rest

    Answer: C. it is relatively a quick process

    Explanation: COD can be measured in a few hours, whereas BOD needs several days

  50. 50
    Environmental EngineeringMEDIUM

    Thermal power plants mostly produce

    ACO
    BCO₂
    CNO₂
    DPAN

    Answer: B. CO₂

    Explanation: Combustion in thermal power plants produces large quantities of carbon dioxide

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