Fall 2002
CE 326 Principles of Environmental Engineering
Wastewater Engineering Problems
Due date: Nov. 11
1.
A wastewater-treatment plant must process an average flow of 90,000 m3/d with a peak
flow of up to 150,000 m3/d. Three clarifiers will be required by the treatment plant.
Design criteria for surface overflow rates have been set by the state regulatory agency at a
maximum of 40 m/d for average conditions and 100 m/d for maximum conditions.
Determine the surface area of the primary clarifier if
it is a (a) circular basin
(b) long-rectangular basin (Length = 3 x Width)
If the depth of the circular clarifier is 3 m, what is the detention time and weir overflow
rate for both the average and maximum flow conditions?
2.
Solve Problem 5-27
3.
Using equations 5 - 28 and 5 -30 in your book
(i) Plot the change in effluent soluble BOD5 concentration as a function of the solids
residence time (SRT), c for a completely mixed activated sludge system.
(ii) Also on your plot, show how the MLVSS concentration would change with c and
the BOD5 removal efficiency.
Explain what happens to the effluent BOD5 and biomass at very short c values?
Assume a wastewater with an influent soluble BOD5 of 200 mg/L and a HRT c of 5
hour. The wastewater has the following characteristics: m = 0.12 h-1, Ks = 130 mg/L as
BOD5, Y = 0.45 mg VSS/mg BOD5, kd = 0.005 h-1.
(Suggestion: use a spreadsheet program and use c values between 4 hours and 40 days.
4.
The following kinetic parameters have been determined by pilot-plant studies.
Y = 0.6, kd = 0.06 h-1, m = 2.5 d-1, Ks = 100 mgBOD5/L
Using these parameters, a conventional activated sludge plant is to be designed to treat
13,000 m3/d of primary settled sewage. The effluent from the primary settling tank has a
BOD5 of 200 mg/L and suspended solids of 100 mg/L. The effluent from the activated
sludge plant should be 5.0 mg/L of soluble BOD5 and 30 mg/L of suspended solids. The
plant is to operate at a MLVSS of 2,500 mg/L with a solids retention time of 10 days. If
the aeration tank is 5.5 m wide and 5.5 m deep, determine its length.
5.
Go to Iowa DNR web site on wastewater design standards
http://www.state.ia.us/dnr/organiza/epd/wastewtr/design.htm
Provide a listing of the design criteria needed for a completely mixed digester and a
moderately mixed digester for sludge handling and disposal.