All Free Civil Engineering MCQs with Answers

• A. Oxygen used by microbes to decompose biodegradable organic matter
• B. Oxygen dissolved in water after complete physical treatment
• C. Chlorine needed to destroy disease-causing organisms
• D. Suspended solids removed during sedimentation

Explanation: BOD5 measures the dissolved oxygen consumed by microorganisms while decomposing biodegradable organic matter over five days at a specified…

• A. Only settleable mineral particles
• B. Biodegradable and many non-biodegradable oxidizable substances
• C. Only pathogenic microorganisms in the sample
• D. Only dissolved oxygen already present in the sample

Explanation: COD uses a strong chemical oxidant and accounts for many substances that can be chemically oxidized, including some compounds not readily…

• A. To destroy all viruses by chemical oxidation
• B. To remove dissolved salts through membrane separation
• C. To destabilize colloidal particles so they can form larger flocs
• D. To increase the dissolved oxygen concentration of water

Explanation: Coagulation neutralizes or reduces the electrical forces that keep colloidal particles dispersed.

• A. Most dissolved chloride ions
• B. Dissolved hardness ions
• C. Volatile organic gases
• D. Settleable suspended solids

Explanation: Sedimentation removes particles that can settle under gravity during the basin detention period.

• A. Remaining fine suspended particles and floc
• B. Most dissolved sodium and chloride
• C. All taste-producing organic compounds
• D. The entire oxygen demand of raw sewage

Explanation: Rapid sand filters remove floc particles and other fine suspended matter that escape sedimentation.

• A. Increase water hardness for pipe protection
• B. Inactivate pathogenic microorganisms
• C. Remove all suspended solids before filtration
• D. Convert biodegradable matter into settleable sludge

Explanation: Chlorination is a disinfection process intended to inactivate disease-causing bacteria and other microorganisms.

• A. Prevent oxygen transfer in the aeration tank
• B. Increase the amount of grit entering the primary clarifier
• C. Maintain a sufficient population of microorganisms in the aeration tank
• D. Convert treated effluent into drinking water

Explanation: Return sludge brings active microorganisms from the secondary clarifier back to the aeration tank.

• A. Chemical precipitation of hardness
• B. Tertiary disinfection
• C. Aerobic digestion
• D. Primary sedimentation

Explanation: Primary sedimentation allows heavier suspended and settleable solids to separate from wastewater by gravity.

• A. An impermeable liner with leachate collection
• B. Open dumping without compaction
• C. Direct discharge of rainfall into nearby drains
• D. Burning all waste inside the landfill cell

Explanation: A liner limits downward movement of contaminated liquid, while leachate collection allows that liquid to be removed and treated.

• A. Dissolved organic matter from wastewater
• B. Particulate matter from industrial exhaust gases
• C. Pathogens from treated drinking water
• D. Carbon dioxide from ordinary ventilation air

Explanation: An electrostatic precipitator electrically charges particles and collects them on oppositely charged plates.

• A. Aeration with compressed air
• B. Screening through coarse bars
• C. Lime-soda softening
• D. Disinfection with ultraviolet light

Explanation: Lime-soda softening precipitates calcium and magnesium compounds, which cause hardness.

• A. Mixing water violently to break particles
• B. Heating water to destroy microorganisms
• C. Adding oxygen to oxidize organic matter
• D. Gently agitating water to form larger flocs

Explanation: Flocculation uses gentle mixing so destabilized particles collide and form larger, settleable flocs.

• A. It requires water with adequate clarity
• B. It cannot inactivate any bacteria
• C. It always increases water hardness
• D. It produces large quantities of sludge

Explanation: Ultraviolet light works effectively only when suspended particles do not shield microorganisms from the radiation.

• A. Increase the alkalinity of water
• B. Prevent microbial regrowth during conveyance
• C. Remove dissolved calcium from water
• D. Improve the settling of suspended solids

Explanation: A chlorine residual helps control microorganisms that may enter or regrow within storage tanks and pipelines.

• A. Separate sewer system
• B. Intercepting sewer system
• C. Combined sewer system
• D. Pressure sewer system

Explanation: A combined sewer system conveys sanitary sewage and stormwater together.

• A. Pressure membranes rejecting salts
• B. Chlorine reacting with dissolved solids
• C. Screens removing floating debris
• D. Microorganisms growing on a fixed medium

Explanation: A trickling filter is an attached-growth biological process in which microorganisms form a biofilm on stones or synthetic media.

• A. To stabilize organic matter and produce biogas
• B. To increase the volume of raw sludge
• C. To remove all dissolved salts from sludge
• D. To convert sludge directly into drinking water

Explanation: Anaerobic digestion biologically stabilizes sludge in the absence of oxygen and produces biogas containing methane.

• A. Open dumping
• B. Composting
• C. Magnetic separation
• D. Pyrolysis of metals

Explanation: Composting is an aerobic biological process that converts biodegradable organic waste into a relatively stable soil-like material.

• A. Discharging untreated leachate to a river
• B. Burning mixed waste without energy capture
• C. Recovering metals from discarded electronic equipment
• D. Storing all waste permanently in open heaps

Explanation: Recovering metals from electronic waste returns useful materials to production and reduces the need for virgin resources.

• A. Using a dry filter to remove dissolved salts
• B. Using gravity alone to settle all fine particles
• C. Using ultraviolet light to remove carbon monoxide
• D. Using liquid contact to remove particles or gases

Explanation: Wet scrubbers bring polluted gas into contact with a liquid that captures particles or absorbs soluble gaseous pollutants.