GPSC Civil Engineering (Combine Exam) Paper Solution (03-05-26) – Page 17

161) Consider the following statements about Activated Sludge Process (ASP):

1. In extended aeration variant, Sludge Retention Time (SRT) is typically 20-30 days.
2. Higher F/M ratio results in better effluent quality but more sludge production.
3. Nitrification in ASP requires SRT greater than 10 days at 20°C.
4. Return sludge ratio (R) is always maintained below 0.25 in conventional ASP.

Which of the statements given above are correct?

1. 1, 2 and 3 only
2. 2 and 4 only
3. 1 and 3 only
4. 1, 3 and 4 only

162) Which one of the following sequences correctly represents the order of unit operations in a conventional surface water treatment plant for a moderately turbid river water?

1. Screening → Aeration → Plain sedimentation → Coagulation → Filtration → Disinfection
2. Screening → Coagulation → Flocculation → Sedimentation → Filtration → Disinfection
3. Screening → Flocculation → Plain sedimentation → Filtration → Chlorination → Aeration
4. Aeration → Screening → Coagulation → Filtration → Sedimentation → Disinfection

163) Consider the following statements regarding BOD and COD of wastewater:

1. COD of a sample is always greater than or equal to its BOD5.
2. The ratio BOD5/COD < 0.3 suggests the wastewater is not suitable for biological treatment.
3. Nitrogenous oxygen demand (NOD) is included in the measurement of BOD5 when inhibitor is not used.
4. Theoretical oxygen demand (ThOD) is always greater than COD.

Which of the statements given above are correct?

1. 1 and 2 only
2. 1, 2, 3 and 4
3. 2, 3 and 4 only
4. 1, 2 and 3 only

164) Consider the following statements about sludge digestion:

1. Mesophilic anaerobic digestion operates in the temperature range of 35-37°C.
2. The C:N ratio of sludge, fed to an anaerobic digester, should ideally be between 20:1 and 30:1.
3. In a two-stage digestion system, biogas is primarily collected in the second stage.
4. Volatile solids reduction of 60-65% is typical for well-operated anaerobic digesters.

Which of the statements given above are correct?

1. 1, 2 and 4 only
2. 1 and 3 only
3. 2, 3 and 4 only
4. 1, 2, 3 and 4

165) A star is observed at its upper culmination when it is North of zenith. The latitude of a place of observation is 30°N and declination of the star is 50°N. The zenith distance of the star is

1. 10°
2. 20°
3. 40°
4. 80°

166) Consider the following statements about the design of a rapid sand filter:

1. The recommended filtration rate for a conventional rapid sand filter is 4-6 m³/m²/hr.
2. Backwashing is done with filtered water at a rate sufficient to expand the bed by 25-50%.
3. The L/d ratio (length to effective diameter) of filter sand should be between 100 and 150 for effective filtration.
4. Negative head development in a filter indicates that the underdrain pressure has become positive.

Which of the statements given above are correct?

1. 1 and 2 only
2. 1, 2 and 3 only
3. 2 and 4 only
4. 3 and 4 only

167) Consider the following statements about Imhoff tanks used in wastewater treatment:

1. Imhoff tank combines sedimentation and sludge digestion in the same unit but in separate compartments.
2. The slot between the settling and digestion chamber is designed to prevent digested gas bubbles from entering the settling zone.
3. Imhoff tanks are best suited for large towns with population above 50,000.
4. Scum is removed from the gas vent/scum port which is separate from both the settling and digestion chambers.

Which of the statements given above are correct?

1. 1 and 2 only
2. 1, 2 and 4 only
3. 2, 3 and 4 only
4. 1, 2, 3 and 4

168) As per IS 10500:2012 (Drinking Water Standard), which of the following statements is correct regarding permissible limits?

1. The desirable limit for fluoride is 1.5 mg/L and the permissible limit is 2.0 mg/L.
2. Turbidity’s desirable limit is 5 NTU and permissible limit is 25 NTU.
3. The maximum permissible limit for nitrate (as NO₃⁻) is 100 mg/L.
4. The desirable limit for Total Dissolved Solids (TDS) is 500 mg/L

169) Which of the following statements about the Sludge Volume Index (SVI) is correct?

1. An SVI value > 150 mL/g indicates excellent sludge settling characteristics.
2. SVI is calculated as (volume of sludge after 30-min settling in mL) divided by (suspended solids in g/L).
3. A low SVI (< 50 mL/g) always indicates bulking sludge in the secondary clarifier.
4. SVI is dimensionless and its value is independent of initial mixed liquor suspended solids (MLSS).

170) Consider the following about Noise Pollution (Regulation and Control) Rules, 2000 and noise measurement:

1. The permissible ambient noise level in a residential area during daytime is 55 dB(A).
2. L₉₀ is the noise level exceeded 90% of the time and represents background noise level.
3. A doubling of distance from a point source reduces the sound pressure level by approximately 6 dB.
4. The A-weighting network in a Sound Level Meter is used to measure infrasound (< 20 Hz).

Which of the statements given above are correct?

1. 1, 2 and 3 only
2. 2 and 3 only
3. 1 and 4 only
4. 1, 2, 3 and 4