Homework #7

CVEG 3243 Homework #7 Due Friday, March 14
Read the “Toilet to Tap” article on the class web site and write a one paragraph summary.
Read the “Lift Station” article on the class web site and explain why the lift station was built.
You flush the toilet in a) La Canada and b) Pomona, California. Where does the liquid go? Where does the sludge go?
Which LA County Sanitation District plants do some sort of reuse of their wastewater? See
Text problem 6-7
For problems 5 - 8
220 mg/L BOD
4 mg/L
# = ??
Design a 8 MGD (design flow) wastewater treatment plant which includes a grit chamber, two parallel primary clarifiers, a
number of aerated lagoons, and a disinfection system. The influent BOD is 220 mg/L. The effluent requirement is 4 mg/L
BOD. 20% of the influent BOD is removed in the primary treatment.
Size the grit chamber. It should have a velocity of approximately 0.8 ft/s, be about 3 feet deep, and have a hydraulic
residence time of approximately 1 minute.
 What are the actual velocity and t0 with the design flow (i.e., do design calculations, choose size, then
calculate v and t0 to see if they fall within design specifications)?
Size the primary clarifiers. They each take half of the flow. The particles have a settling velocity of 7 ft/hr. The
clarifiers should be circular and be about 12 feet deep.
 What is the flow velocity in the basins?
 What is the actual detention time at the design flow?
 What is the overflow rate?
About 20% of the BOD is removed in the primary clarifiers. The remainder of the BOD reduction is in the aerated
lagoons. The lagoons work as CSTR ( C  1  kt ) with a first-order decay rate constant of 12.0/day. Each lagoon should
be 12 feet deep and cover an area of 0.5 acre.
 What should the residence time in the lagoons be?
 How many lagoons are needed?
99.99% (“four nines”) disinfection is required. To eliminate short-circuiting, a plug flow contact basin is desired. In the
disinfection process, the number of viruses (per volume) present in the water at the applied chlorine dose follows firstorder decay kinetics (Chick’s law): N/No = e-kt with k = 1.0 min-1.
 Design and size the chlorine contact basin.
Note: problem 9 has nothing to do with the previous problems
9. A high-rate trickling filter is to be designed for a 6.0 MGD wastewater treatment plant with an influent BOD of 220
mg/L. The filter is to be 10 feet deep and can handle a hydraulic loading (flow divided by plan area) of 1000 gpd/ft2 and
a BOD loading (BOD applied divided by filter volume) of 150 lb/day per 1000ft3.
a). Which loading, hydraulic or BOD, controls the design?
b). If the flow is split between four circular filters, what is the diameter of the filters?
c). If an effluent BOD limit of 4 mg/l is required, what is the required BOD removal efficiency of the plant?
CVEG 3243 Homework #7 Due Friday, March 14
10. Nitrification / Denitrification
Nitrogen can exist in seven states of oxidation. The five nitrogen-containing compounds that are of most interest to us for this
exercise are organic nitrogen, ammonia (NH4+), nitrite (NO2-), nitrate (NO3-), and nitrogen gas (N2). Nitrification is the
biological oxidation of ammonium to nitrite and then to nitrate under aerobic (with oxygen) conditions. Denitrification is
the anoxic conversion of nitrate to nitrogen gas and must follow nitrification. Wastewater treatment systems can remove
nitrogen by cycling back and forth between nitrification and denitrification.
Complete the following sentences:
Nitrification is the ______________ (oxygen condition) conversion of ______ to ______ then ________.
Denitrificaton is the ____________ conversion of _____________ to ______________.
Write the reactions for nitrification and denitrification.
The nitrification step (eqn 5-49) is actually two steps:
_________ + 3O2 -----Nitrosomonas---- > ________ + 2H2O +4H+
_________ + O2 ------Nitrobacter------- > ________
The denitrification step can be accomplished by a broad range of facultative and anaerobic bacteria such as
_______ + organic matter -----Pseudomonos---- > ______ + CO2 + H2O
For schematic below showing dissolved oxygen concentrations in two stages of a wastewater treatment unit, list
the nitrogen process (nit or denit), conversions, and products for each section.
=3-8 mg/L
DO =~ 0
In biological nitrogen removal, where does the nitrogen go when it leaves the wastewater?
In biological phosphorus removal, where does the phosphorus go when it leaves the wastewater?
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