Question

In a rapid sand filter, the time for reaching particle break through ($T_B$) is defined as the time elapsed from start of filter run to the time at which the turbidity of the effluent from the filter is greater than 2.5 NTU. The time for reaching terminal head loss ($T_H)$ is defined as the time elapsed from the start of the filter run to the time when head loss across the filter is greater than 3 m.
The effect of increasing the filter loading rate (while keeping all other conditions same) on $T_B$ and $T_H$ is

• A. $both{T}_{B}and{T}_{H}decreases$
• B. $both{T}_{B}and{T}_{H}increases$
• C. $T_{B}increasesand{T}_{H}decreases$
• D. $T_{B}decreasesand{T}_{H}increases$

Answer 1

The correct option is A $both{T}_{B}and{T}_{H}decreases$

Since velocity will be more due to larger loading rate, shear forces experienced by collected particle will be more. Hence particle detachment will be more, leading to a decrease in $T_B$

Since particle loading rate on the filter and rate of particle collection in the filter will increase, the porosity of the filter bed decrease more rapidly, leading to more rapid head-loss build up and hence a decrease in $T_H$.