Water Treatment Plant Reflections

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?