Test: Environmental Engineering- 2 - Civil Engineering (CE) MCQ

As per Government of India’s Effective size (D₁₀) of filter for slow sand filter is in the range of (0.2 to 0.3) mm and Coefficient of uniformity for this filter media is in the range of (3 – 5)

Water of zero hardness can be obtained from ion exchange method only. It is also called zeolite process or base exchange treatment.

The cleaning of slow sand filter is done by scrapping off the top sand layer and cleaning of rapid sand filter and pressure filter is done by passing air and by back - washing.

For particle removal, setting velocity > Surface loading rate.

Settling velocity is depended upon the particle size & SLR is on the surface area which depends on length and width.

So efficiency of particle removal does not depend upon the depth of the tank.

Large G values with short times tend to produce small, dense flocs, while low G values and long times produce large, lighter flocs. Since large, dense flocs are more easily removed in the settling basin, it may be advantageous to vary the G values over the length of the flocculation basin. The small dense flocs produced at high G values subsequently combine into larger flocs at the lower G values. Reduction in G values by a factor of 2 from the influent end to the effluent end of the flocculator has been shown to be effective.

Area of clarifier

⇒ D = 13.5 m

Break point chlorination = Chlorine demand = 1.5 mg/L 

Combined Cl₂ residual at that time = 0.3 mg/L

Chlorine dosage = 2 mg/L

∴ Free chlorine residual = Dose – demand

= 2 – 1.5

= 0.5 mg/L

∴ Total chlorine residual = Free chlorine residual + combined chlorine residual

= 0.5 + 0.3

= 0.8 mg/L

Power input (P) = μVG²

Where, μ = dynamic viscosity of water

V = volume of the tank

G = velocity gradient

P = 39.0625 kW

The settling velocity can be determined through the equation,

where H = depth of the tank, L = length of the tank, V_s  = Settling velocity, and V_H = Flow through velocity

Now, using Stokes law,

 

where G = specific gravity of the particle, ν =kinematic viscosity of the particle, g = acceleration due to gravity, d = diameter of the particle

For 95% removal efficiency,

d = 9.92 × 10⁻⁵m
d = 0.0992.mm