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9.35 A 2.5-MGD wastewater treatment plant is currently running at 80% capacity during the
annual maximum day servicing a city of 38,500 people with 26.7 miles of sewers. During the
next ten years, it is expected that new residential developments for 15,000 people along with 6.5
more miles of sewer will be built. The sewer is projected to have an I/I equal to 8,500 gpd/mile.
a) Project the maximum daily demand for the wastewater treatment plant after the new
development is built. b) Should the wastewater treatment plant capacity be increased?
Solution:
a)
Total current water use 2,500,000 gpd 0.8 2,000,000 gpd
Current I / I 8500 gpd / mi 26.7mi 227, 000 gpd
Current domestic use 2, 000, 000 gpd 227,000 gpd 1, 773,000 gpd
1, 773, 000 gpd
Per capita use 46 gpcd
38,500 people
Fo
= ×=
= ×≈
= −=
= =
( )
( )
rcast water use :
Domestic use 46 gpcd 38,500 15, 000 2, 461, 000 gpd
I / I 8500gpd / mi 26.7 6.5 282, 200 gpd
Total forcasted use 2, 461, 000 282, 200 2,743, 200 gpd
=×+=
= × +=
= +=
b) Yes, the treatment plant should be increased.
9.36 A residential community with a population of 15,000 is planning to expand its wastewater
treatment plant. In 20 years, the population is estimated to increase to 23,000 residents, and
1,000 students per day are expected to commute to proposed junior college from outside the area.
A new industry will also move in and contribute an average flow of 350,000 gpd and maximum
day flow of 420,000 gpd. The present average daily flow into the plant is 1.45 million-gpd. The
average inflow and infiltration (I/I) is 6 gal/capita day and maximum day I/I is 42 gal/capita day
(rainy day). Residential per capita water use is expected to be 15 percent less in 20 years due to
in-house water saving strategies. The demand factor for domestic (residential use only)
wastewater is determined to be 2.4 for the maximum day. Compute the future average and
maximum day flow rates. Hint: Compute the present per capita flow rates first; [total flow rate –
I/I] divided by current population.
Solution:
9.37 List five advantages to precipitating struvite from the nitrogen and phosphorus found
primarily in urine discharged to municipal wastewater.
Solution:
9.38 For the following influent water quality of a wastewater treatment plant that employs
struvite recovery that is 70% effective in nutrients, determine which nutrient (N or P) in the
influent is limiting for struvite precipitation and why? The influent contains [NH4+-N] (80.5 mg
N/L) and PO4--P] (20.7 mg P/L).
Solution:
9.39 Assuming a homeowner installs a 60-gallon rain barrel at their home which has roof with
215 ft2 of surface area, how much rainfall (in feet) could be stored? Assume that only 90% of
the rain that falls on the roof enters the rain barrel due to leaky downspouts.
Solution:
9.40 Assume that the roof area for a residential home is 12 ft. × 30 ft. If a green roof is placed
on the home, what percentage of a 0.5 in. rain event will be stored on the roof if the growing
medium has a water-holding capacity of 0.25? What is the volume of water (in gallons) that is
stored during this rain event?
Solution:
9.41 What area (in sq. ft.) is needed for two bioretention cells used to collect rainwater coming
from a household roof? The roof has dimensions of 30 ft. × 40 ft. It drains to two downspouts,
each of which will be routed to a bioretention cell. Assume the soil surrounding the home is silty
and the cell will be dug to a depth of 6 in.
Solution:
Area of residential nonpermeable size
bioretention cell area factor
= ×
Using size factor provided in Table 8-9,
2
30 40 0.25 300 × ×=ft ft ft
Since there are two cells, each will have an area of 150 ft2 with a depth of 6 in.
9.42 A 1-acre paved parking lot measures 50 ft. × 20 ft. What volume of bioretention cell is
required (in cu. Ft.) that can handle a first flush from the nonpermeable pavement of 0.5 in.?
Assume the soil porosity is 0.30.
Solution:
2
The volume first nonpermeable
of rainwater to flush area
be stored
= ×
9.43 Select a specific location on your campus that has a building and associated parking lot.
Redesign this area, incorporating at least three low-impact development techniques. Besides
thinking about management of stormwater, also consider the movement of people and vehicles,
and the use of native plant species.
Solution:
Students’ responses will vary.
9.44 Size a rain garden for your current home, apartment, or dormitory to treat stormwater that
originates from the roof.
Solution:
Students’ responses will vary based on the area of their home, apartment, or dormitory. They
9.45 The average cost of delivering an acre-foot of treated water in a water scarce region is
$5,900 and the cost of delivering an acre-foot of reclaimed water in the same region is $6,400
(due to treatment and transport). Given that the following factors can be credited to reclaimed
water, what is the range of cost of reclaimed water as compared to treated water as a percentage?
Increased potable water supply: $300-$1000/acre-foot
Water supply reliability: $100-$140/acre-foot
Effluent disposal savings: $200-$2000/acre-foot
Downstream effects: $400-$800/acre-foot
Energy conservation: $0-$240/acre-foot
Solution: 𝑀𝑖𝑛𝑠𝑎𝑣𝑖𝑛𝑔 =$300 +$100 +$200 +$400 +$0 =$1,000/𝑎𝑐𝑟𝑒−𝑓𝑜𝑜𝑡
9.46 Assume the energy requirements to treat wastewater using a mechanical process is 1
million kWhr per million gallons of water treated. According to eGRID, the carbon dioxide
equivalent emission rate is 1,324.79 lb CO2e/MWh in Florida and 727.26 lb CO2e/MWh in
California. Estimate the carbon footprint of treating 50 million gallons of wastewater Florida
and California. Ignore line losses in your estimate (you may have to go back to Chapter 2 to
review carbon footprints and eGRID).
Solution:
