Monroe L. Weber-Shirk
School of Civil and
Environmental Engineering
Water Treatment Plant Reflections
Water Treatment Plant Reflections
alumalum
Flocculation
Clear Well
Sedimentation
OverviewOverview
Trouble shooting guideHydraulic challengesSurface tensionStartup requirementsRapid Mix/Flocculation/SedimentationWrap up
When it doesn’t work!When it doesn’t work!
You are creating a complex systemAny component that fails can lead to failure
in the systemHow do you identify the source of a
problem?Attempt to identify the events that could
cause a failure in advance
Scientific Method of Troubleshooting
Scientific Method of Troubleshooting
The Scientific method:Clearly identify the problemCreate hypothesesDesign experiments to test the hypothesesDraw conclusions based on the data
How can you choose which components to test first? Intuition?Ask which component could cause the observed
symptomsRequires an understanding of how the system works!
Modular ApproachModular Approach
How can you build a complex system with the greatest probability of ultimate success?
Break a system down into its components and test individual pieces
Only add components to the system after the components have been tested
Begin with the system as simple as possibleWhat would the simplest operating rules be?Add unit processes one by one
Hydraulic Challenges (getting the water to go where you want it to go)
Hydraulic Challenges (getting the water to go where you want it to go)
LeaksConnections not sufficiently tight Improvised connections lacking a good seal
OverflowsCaused by water not going where you thought it was
goingOpen channel flows (air and water) – coming up
Excessive head lossTubing size too smallFilter clogging
Simplified WTP SchematicSimplified WTP Schematic
Can the flow accumulator be on the bench top? What controls the water level in the sedimentation tank? Why does the water flow through the filter? How would you start up the plant and get water to flow
through the filter the first time? How much head loss can the filter cause before the
system fails?
Flocculation
Clear Well
Sedimentation
Improved WTPImproved WTP
Why is this better?
Max head loss? How could you
measure head loss through the filter?
Clear Well
Open Channel Flow
Flocculation
Sedimentation
0.01
0.1
1
10
100
0.1 1 10 100
length scale (mm)
surf
ace
tens
ion/
wei
ght
Relative Strength of ForcesRelative Strength of Forces
3Fg glr=
F ls s=
0.0500.0550.0600.0650.0700.0750.080
0 20 40 60 80 100
Temperature (C)
Sur
face
tens
ion
(N/m
)
0.0500.0550.0600.0650.0700.0750.080
0 20 40 60 80 100
Temperature (C)
Sur
face
tens
ion
(N/m
)
Surface tension Gravity
Stable* Unstable?
* water column over air won’t break
Open Channel Flows: Water and Air
Open Channel Flows: Water and Air
All overflows tubes are open channel flowMinimum inside diameter for sink drain is
6.35 mm (¼”)Minimum inside diameter for line where
water level moves up and down based on filter head loss is 9.5 mm (3/8”)
What happens if the tubing isn’t large enough for open channel flow?
WTP
Leak PreventionLeak Prevention
Clear well overflow line must be horizontal or sloping down (can’t go up and down)
What happens if head loss through the filter increases too much?
Check each tank or tube with an opening to the atmosphere and ask: What could cause an overflow at this location? How could we design the system to reduce risk of failure
Turn off the manual supply valve when you aren’t using the plant
Make sure that all valves are off when the plant isn’t being used.
WTP
PointersPointers
Pressure sensors must be kept dry (they can fail if one drop of water soaks into the terminals)
Use manual valves to make it easy to drain tanks Make sure you are using the most recent method
file in your folder! Save a new version of the method file every time
you make changes Label all processes Label the alum stock bottle Label all containers containing fluids
Plant LayoutPlant Layout
Design a plant layout that is easy to followTubing lengths can be changed so you can
place your devices anywhere you want themTurbidity sensors are more stable on the lab
benchBeware of large diameter horizontal tubes
containing particlesWhy? WTP
Startup RequirementsStartup Requirements
Stamp module must be on the computer side of the bench divider
Must prove that excessive head loss will cause the filter to backwash before causing a flood!
Must show that backwash won’t empty clear well Must show that the plant switches between states
correctly
Operating CriteriaOperating Criteria
Initial down flow rate of 5 m/h (40.9 ml/min) If you increase the down flow rate make absolutely sure
that the plant doesn’t overflow
Use your water treatment plant design homework to calculate the best stock concentration of alumThe best stock concentration might change if you
significantly change the plant flow rate
Plan to have someone check on the plant at least once per day (alum stock!)
• Hydraulic Jump: Hydraulic Jump creates turbulence and thus help better mixing.
• Mechanical mixing
Inflow
Chemical feeding
Chemical feeding
Inflow
Back mix impeller flat-blade impeller
Rapid MixingRapid Mixing
• In-line flash mixing
Coagulant
Goal?
Poor
Excellent
Flocculation DesignFlocculation Design
Goal: produce large flocs from tiny particles Mechanism:
Small particles collide by Brownian motionLarge particles collide with small particles by
differential sedimentation Need to keep large particles in suspension!
Vertical velocity needs to exceed floc terminal velocity Residence time: 10 to 30 minutes (but this is based
on an old theory that incorrectly emphasized shear as the transport mechanism)
Flocculator designsFlocculator designs
Vertical bafflesHorizontal bafflesMechanical mixing
Based on transport by shear
How can we get more collisions?_________________________Increase particle concentration
How?
Tapered upflowTapered upflow
If the vertical velocity gradually decreased particles would “hang out” at the depth where their terminal velocity matched the vertical velocity.
Velocity at bottom of flocculator must be high enough to suspend largest floc
Velocity at top of flocculator must be low enough so that medium sized flocs are trapped
This technique is used as a sedimentation tank! The process combines flocculation and sedimentation. But it probably isn’t the best design for a sedimentation tank.
Particle removal?
Water inlet
36 - 100 m/dayWater inlet
36 - 100 m/day
Flocculator volume and velocityFlocculator volume and velocity
VolumeGiven plant flow rate of 80 mL/minResidence time of 10 minutesVolume is 800 mL
VelocityUse baffles to distribute flow evenly at inlet to avoid
high velocities that would cause mixingHow do you find the velocity that will capture large
flocs?
Floc Density and Velocity (Approximate)
Floc Density and Velocity (Approximate)
floc w
w
0.001
0.01
0.1
0.1 1 10
floc diameter (mm)
floc
den
sity
10
100
1000
floc
term
inal
vel
ocit
y (m
/day
)
floc density
Vt (m/day)
Vertical velocity in FlocculatorVertical velocity in Flocculator
Vertical velocity of 100 m/dayJet action at bottom to keep particles
suspendedResidence time of 10 minutes (although this
might not be a necessary constraint)How far would water travel?
100 10 min H= 0.7
1 1440 mint
m dayV m
day
100 10 min H= 0.7
1 1440 mint
m dayV m
day
Tapered vertical flow flocculatorTapered vertical flow flocculator
Bottom velocity – 1000 m/dayTop velocity – 100 m/day
2 21 12 1
2 2
3
r h r hh r r
r r
22
3
hr
32 1
2 1 22
3
rh r r r
r
r1
r2
This might be more complicated than necessary
Transfer into Sedimentation tankTransfer into Sedimentation tank
Critical connection!Make sure shear doesn’t break flocsI don’t have information on floc strength How does pipe size affect shear?
Shear in pipe flowShear in pipe flow
f 2
32 lVh g p
d
0 8Vd
dvdy
2
4QV
d
0 3
32Qd
Laminar flow (check Re!)
Velocity
Shear (wall on fluid)
2
0 4
dl d p
Force balance
High shear in small pipes!
Sedimentation TankSedimentation Tank
Inlet and outlet baffles to get more uniform velocity through tank
Lamella to increase surface area of tank?Lamella spacing must be larger than the
flocs you are trying to captureCritical velocity designed to capture small
flocsCombine flocculation and sedimentation?
Competition SubmissionCompetition Submission
Submit 5 copies by 5 pm on Thursday (May 5). Executive Summary
A one page cover letter to the judges where you introduce your design firm, identify the members of your design team, and describe the important features of your water treatment plant.
Final plant schematic all valves, sensors, tanks, and pumps with correct relative elevations.
Clearly show the elevation of the laboratory bench top. Description of your design and your design process
How did you size the unit processes in your plant? How did you test the plant? How did you use data that you acquired to modify the design? What are the special features of your plant that make it the best?