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EGCE 572 - Water Treatment and Design
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Complete all questions.
Turn in your responses to the questions in order and stapled.
There is no need to look up regulations for Ohio. Where the choice is left to you, use
values from our textbook.
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The town of Yellow Springs, Ohio (pop: 3,487), home of YSI Incorporated and Dave Chappelle, is
planning to upgrade their water treatment plant (WTP). The source water will come from the
Little Miami River, a river that flows through agricultural lands. It is characterized by high
turbidity, fecal coliforms, and protozoa. Hardness, alkalinity, and salinity are low. Preliminary
testing has shown no issues with excessive nutrients, pesticides, or other anthropogenic
contaminants.
Design Flow
Max. Flow
pH
Alkalinity
229 LPCD
458 LPCD
6.7
1.55 meq/L
Winter Temp.
Summer Temp.
TDS
Turbidity
3°C
19°C
185 mg/L
75 – 175 NTU
As a newly-hired engineer, you have been tasked with the preliminary design of the new water
treatment plant. Here are your tasks.
1) Sketch a basic drawing of the treatment train including the relevant unit operations and
chemical feeds. This is intended as an “outline” to guide you in what you need to design.
2) Design rapid mix, for the coagulation step, that is separated into two (2) basins.
a) What volume (m3) must each basin be if the Gt value is 60,000 and the velocity gradient
is 916 s-1?
b) Should this be calculated with design flow or maximum flow and why?
c) If the plan view of each basin is square and the wetted depth is 2.5m, what are the
length and width (m)?
d) What power input is required to meet the design velocity gradient?
3) An alum (Al2(SO4)3 x 14 H2O) dose of 105 mg/L is added during rapid mix and coagulation.
a) The alum is delivered as a slurry with 13% solids by weight and a specific gravity of 1.13.
What flow rate of the slurry will be added to the rapid mixers (in L/hr)? Should this be
calculated with design flow or maximum flow, and why?
b) How much sodium bicarbonate (NaHCO3) would need to be added in kg per day to
negate the removal of alkalinity?
c) If dewatering of sedimentation residuals brings the solids content of the sludge up to
65%, what is the wet mass of aluminum hydroxide sludge that will be disposed in metric
tonnes per week?
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4) Design conventional flocculation with horizontal paddle wheels and a velocity gradient of
60 s-1 in each cell. Flocculation will be split into two (2) trains with four (4) stages in each
train. Each stage is to have one (1) paddle wheel. Paddle boards will be 89 mm wide and
there will be two (2) boards per paddle arm with four (4) arms per wheel (8 boards total per
wheel). Outer radii of the paddle boards shall be 0.60 m, and 1.10 m.
a) Total residence time for flocculation will be 90 minutes. Choose which flow to use and
justify your choice. Using that flow, calculate the volume of each stage. Leaving 0.5m of
clearance on all sides of the paddle wheel, what are the dimensions of each flocculation
stage? Design the length:depth ratio to be 1:1. What length are the paddles (m)?
b) Sketch the layout of the flocculation system. Include relevant dimensions.
c) What power input (in kW) will be required for the entire flocculation system? Should
the equipment be designed for winter or summer temperatures? If electricity is $0.10
per kilowatt-hour, what will be the annual electrical cost of flocculation? For electrical
cost, assume downtime and variation of water temperature are negligible factors.
d) What rotational velocity (in rpm) will be required to obtain the desired velocity
gradient? Assume that due to the shape of the basins the paddles are moving at 70%
velocity relative to the water.
5) The sedimentation basins will remove 95% of particles at design flow. A pilot plant has
given the following particle counts after flocculation of Little Miami River water.
Settling Velocity (m/hr) # of Particles
0.2
251
0.6
389
1.0
821
1.4
1127
1.8
1350
2.2
980
2.6
268
3.0
50
3.4
0
3.8
0
a) What design overflow rate is required to meet the desired particle removal rate (m3/m2hr)? (hint: use data analysis tools in Excel to save time with this calculation)
b) What percentage of particles will be removed at maximum flow?
c) Design the dimensions of the sedimentation basins (m). Use two (2) basins. Use the
same width as the flocculation tanks and a reasonable depth. What is the detention
time in sedimentation (hrs)?
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6) A dual-media filter is to be installed using a design filtration rate of 15 m/hr. Samples of the
filter media were sifted through a stack of sieves and the results are shown below.
Sieve
8
10
12
14
16
18
20
25
30
35
40
45
50
Pan
Sieve Opening (mm)
2.36
2.00
1.70
1.40
1.18
1.00
0.85
0.71
0.60
0.50
0.425
0.355
0.300
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Anthracite Weight (g)
11
57
162
281
320
241
67
52
1
0
Sand Weight (g)
0
4
57
125
279
153
75
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a) The density of the anthracite and sand are 1.7 g/cm3 and 2.65 g/cm3, respectively. The
anthracite layer is 1.5 m thick and the sand layer is 1.0 m thick. What is the clean-bed
head loss of this filter (m)?
b) What are the uniformity coefficients of the anthracite and sand media? What does this
tell you about the suitability of these two particular media for filtration?
c) Calculate the UFRV, UBWV, UFWV, and recovery for this filter if run length is 48 hours,
backwash rate is 40 m/hr, backwash duration is 20 minutes, and filter-to-waste duration
is 20 minutes.
d) Determine a reasonable design for the number of filter cells and the dimensions of each
cell (m). Justify your response.
7) Assume that Ohio regulations require 6-log removal of Crytosporidium during disinfection
and that the disinfection is occurring with first-order rate kinetics.
a) What would be the Ct required for free chlorine? How about for ozone?
b) Based on your understanding of this problem, which of those two disinfectants would
be preferable? Why is it preferred? Are there other considerations to ensure safe
drinking water if that disinfectant is used?
c) How long (m) would a 0.65 m diameter pipe contactor need to be to ensure a proper
contact time to reach the required disinfection goal? Choose a disinfectant dose based
on maximum values permitted while minimizing DBP formation.
d) Will dispersion be a significant factor to achieving disinfection goals in that pipe?
Choose a reasonable value for the friction factor.
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8) Now that you have designed the entire water treatment plant, draw out a plan-view plant
layout with an emphasis on maximizing space efficiency. You may either sketch it out by
hand or draw it using computer software. Be sure to keep all processes to scale and include
all processes that you just designed. I am not requesting a schematic, but the actual plan
for the plant you propose to build. This should take a bit of time, particularly when it comes
to maximizing space efficiency.
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