CIVE 1200: EXPERIENCING URBAN INFRASTRUCTURE
ASSIGNMENT 12: WASTEWATER AND STORMWATER
1. In class, you briefly learned about the hydraulic residence time (HRT) of a wastewater
treatment tank, which is the average time that water spends in a tank. It is defined as:
HRT =
V
Q
where V is the volume of the tank and Q is the flow rate.
We can use equation to figure out how big the treatment plant has to be. For BostonàDeer
Island plant, the average treatment flow is about 400 million gallons per day (MGD), or 1.8
million m3/day. From class, recall that the primary treatment clarifiers (sedimentation
basins) require up to 3 hours in order to clean the wastewater. (For more background
reading on clarifiers, please refer to the section in our Penn and Parker textbook, pp.
404-406. They denote HRT with the Greek symbol ?.)
a. What is the total volume of clarifier tanks required to treat the average flow, in m3?
Please be sure to keep track of all units.
b. A typical size of a cylindrical clarifier is 30 m diameter and 4 m deep. How many
clarifiers are required?
c. Your previous answers are for average flow, but in practice for all kinds of
infrastructure, we need to design for the peak demands on our systems. In the case of a
large rainstorm, stormwater will run into drains and also be sent to Deer Island for
treatment, as the rain becomes contaminated with all of the things on the road $ust,
engine oil, animal feces, and so on. The addition of stormwater can greatly increase
flows to a treatment plant. Deer Island was actually designed to handle up to 1.2 billion
gallons per day, or 5.5 million m3/day. Recalculate your answer to parts a and b, now
for peak flows.
2. Proper sizing of the urban stormwater infrastructure to carry water away is critical to
prevent flooding and avoid angry residents, or worse, loss of life and property. In your Penn
and Parker textbook, read pp. 165-166 on the development of the ¡tional Method&or
determining peak flows of stormwater. This is a simple, elegant equation, still in wide use.
Letàuse it to calculate the total catchment area for Boston.
a. From problem 1, you already know that the peak flowrate Qp for stormwater is 800
MGD, or the difference between average and peak capacity for Deer Island WWTP.
Express this in cubic feet per second. (Feel free to use an online conversion tool.)
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CIVE 1200: EXPERIENCING URBAN INFRASTRUCTURE
b. Now to calculate the total runoff coefficient CT, Boston of course has lots of different
types of land. Letàsimplify by assuming only three types: (1) downtown business, (2)
single-family residential, and (3) flat, sandy lawns. These have roughly 20, 50, and 30
percent of land area, respectively. Use the midpoint of the ranges for values of C given
in the textbook to calculate an area-weighted average for CT.
c. According to the National Weather Service, the peak intensity for rainfall in our area is 1
inch per hour. Using this value and your answers to parts a and b, calculate the
catchment area for the Deer Island plant.
3. You heard in class that the environmental engineering community is moving from a
paradigm of treating and disposing of waste to one of recovering valuable resources. We are
even renaming our wastewater treatment plants as water and resource recovery plants
(WRRP)! List three resources that can be recovered as valuable products from these plants.
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