Question

4. Design an aerated grit chamber installation (2 tanks in parallel) for an average flowrate of 0.3 m3/s and a peak flowrate of 1.0 m3/s. The average depth is 3 m, the width-to-depth ratio is 1.5:1, and the detention time at peak flow is 3.5 min. The aeration rate is 0.4 m3/min per m of the tank length. Determine the dimensions of the grit chambers and the total air required

Answer 1

Answer

given,

average flow rate = 0.3 m³/s

peak flow rate = 1 m³/s

width to depth ratio = 1.5 : 1

detention time = 3.5 min

average depth = 3 m

aeration rate = 0.4 m³/min/m

dimensions of grit chamber = ?

volume of grit chamber = $\dfrac{1 \times 3.5 \times 60}{2}$

                                       = 105 m³

average depth = 3 m

width-to-depth ratio = 1.5:1,

width = 1.5 × 3 = 4.5 m

length of the grit chamber = $\dfrac{105}{3\times 4.5}$

                                            = 7.78 m

increase length by 20% to account for inlet and outlet of the tank

length = 1.2 × 7.78 = 9.34 m

dimension of grit chamber = 9.34 m × 4.5 m × 3 m

air required = aeration rate × length

                    = 0.4 m³/min /m × 9.34

                    = 3.736 m³/min