An average operating data for conventional activated sludge treatment plant is as follows:

Waste water flow $3500$ cubic metre per day

Volume of aeration tank $10900$ cubic metre

Influent BOD $250m\frac{g}{1}$

Effluent BOD $20m\frac{g}{1}$

Mixed liquor suspended solids $($MLSS$)2500m\frac{g}{1}$

Effluent suspended solids $30m\frac{g}{1}$

Waste sludge suspended $9700m\frac{g}{1}$

Quantity of waste sludge $220$ cubic metre per day.

What will be the aeration period? Also find the efficiency of BOD removal.