Civil war—Day 19-Oct 15-PWD-Quality of water& treatment of water

MPN index is measure of one of the following:

• Coliform bacteria
• BOD₅
• Dissolved oxygen content
• Hardness

Answer explanation

MPN (Most Probable Number) test estimates the concentration of coliform bacteria, indicating fecal contamination.

The following data pertain to a sewage sample: Initial DO = 10 mg/L, Final DO = 2 mg/L, dilution = 1%. The BOD of the given sewage sample is —

• 8 mg/L
• 100 mg/L
• 800 mg/L
• 10 mg/L

Answer explanation

BOD = (DO₁ − DO₂) × Dilution Factor = (10 − 2) × 100 = 800 mg/L.

Which one of the following processes of softening water may require recarbonation?

• Lime–soda process
• Sodium cation exchange process
• Demineralization process
• Zeolite process

Answer explanation

In the lime–soda process, excess lime increases pH; recarbonation neutralizes it using CO₂.

Which of the following tests of water employs Erichrome Black T as an indicator?

• Hardness
• COD
• Residual chlorine
• DO

Answer explanation

Erichrome Black T is used in EDTA titration to determine total hardness (Ca²⁺ and Mg²⁺).

The permissible limit in the absence of an alternate source for hardness of drinking water (in mg/L) as per IS 10500:2012 is —

• 200
• 400
• 300
• 600

Answer explanation

Desirable limit = 200 mg/L; permissible limit in absence of alternate source = 600 mg/L (IS 10500:2012).

After suspended matter causing colour is removed by centrifugation, the colour obtained is called —

• True colour
• Apparent colour
• Brackish colour
• Pure colour

Answer explanation

True colour is that remaining after removing suspended matter by filtration or centrifugation.

Which one of the following is not a specific criterion for calculating surface overflow rate in sedimentation tank design?

• Total quantity of water to be treated
• Total surface area available in the tank
• Total length of the tank
• Total depth of the tank

Answer explanation

Surface overflow rate depends on flow rate and surface area; depth affects detention time, not overflow rate.

A water treatment plant has a flow rate of 432 m³/hr. What is the required plan area of a Type-I settling tank to remove 90% of particles with settling velocity 0.12 cm/s?

• 90 m²
• 111 m²
• 120 m²
• 100 m²

Answer explanation

Vs = 0.0012 m/s = 4.32 m/hr; for 90% removal Vo = 4.32/0.9 = 4.8 m/hr; A = 432/4.8 = 90 m².

Which one of the following represents the correct sequence of water treatment?

• Screening, sedimentation, disinfection, filtration
• Screening, filtration, sedimentation, disinfection
• Screening, sedimentation, disinfection, aeration
• Screening, sedimentation, filtration, disinfection

Answer explanation

The conventional order: Screening → Sedimentation → Filtration → Disinfection.

Chlorine usage in the treatment of 20,000 m³/day is 8 kg/day. The residual after 10 min contact is 0.20 mg/L. The chlorine demand of the water (in mg/L) is —

• 0
• 0.2
• 0.4
• 0.6

Answer explanation

Applied chlorine = 0.4 mg/L; residual = 0.2 mg/L; demand = 0.4 − 0.2 = 0.2 mg/L.