Environmental Engineering FOURTH EDITION

204 ENVIRONMENTAL ENGINEERING

b. What is the settling velocity in feet per day of the critical particle, assuming

9.16 A rectangular primary clarifier (settling tank) is 10 m wide, 25 m long, and

a. What is the design overflow rate?
b. Would all particles having a settling velocity of 0.01 cdmm be removed?
9.17 A treatment plant receives wastewater at a flow rate of 0.5 m3/s. The total
surface area of the primary clarifiers is 2700 m2 and the retention time is 3 h. Find the
overfiow rate and the depth of the clarifiers.
9.18 A transoceanic flight on a Boeing 747 with 430 persons aboard takes 7 h.
Estimate the weight of water necessary to flush the toilets if each flush uses 2 gal.
Make any assumptions necessary, and state them. What fraction of the total payload
(people) would the flush water represent? How could this weight be reduced, since
discharging, for obvious reasons, is illegal?
9.19 A family of 4 wants to build a house on a lot for which the percolation rate
is 1 .00 mm/mm. The county requires a septic tank hydraulic retention time of 24 h.
Find the volume of the tank required and the area of the drain field. Sketch the system,
including all dimensions.
9.20 Are the following design and expected performance for a primary clarifier
reasonable? 0 0.150 m3/s, influent SS 3 10 ma, raw primary sludge concentration 4%
solids, SS removal efficiency = 60%, length = 10m, width = 15 m, depth = 2.2 m.
If not, suggest modifications.
9.21 A wastewater contains soluble phosphate at a concentration of 4 mg/L. How
much ferric chloride would theoretically be necessary to precipitate this nutrient?
9.22 A community with a wastewater flow of 10 mgd is required to meet effluent
standards of 30mg/L for both BOD5 and SS. Pilot plant results with the influent
BOD5 = 250mg/L estimate the kinetic constants at Ks = lOOmg/L, 1 = 0.25 day,
and Y = 0.5. MLSS are maintained at 2000 ma. What is the hydraulic retention time,
the sludge age, and the required tank volume?

that the entire tank is used for clarification.

5 m deep. The flow is 0.5 m3/s.