Distribution System Control Strategies

Distribution System Control Strategies

Tank Management/Operations

Flushing

Rerouting Water

Others

optimizing existing booster chlorination (operation, control)

system modifications

tank modifications to enhance mixing, remove DBPs, adding booster chlorination

capital improvements after O&M options are exhausted/evaluated

Tank Management/Operations

Basis for Considering: Tank spreadsheet estimates tank to have:

High water age

Poor mixing

Poor water quality based on sampling: In tank

Immediate vicinity of tank

Other options exist to provide system pressure (i.e., multiple tanks serve same

area, booster pump station, etc.).

Concerns/Implementation Considerations: Maintaining system pressure.

Ability to satisfy peak demands (e.g., fire protection, line breaks, and seasonal

variations).

Drawing tank down (at least initially) may release poor water quality into the

system.

Tank

Man

agem

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ratio

ns Level Modification

Change Fill Rate/Duration

Lower Max Level

Lower Min Level

Lower Both Levels

Faster Rate/ Shorter Duration

Slower Rate/Longer Duration

Remove Tank From Service

Tank Level Modification

Lower Max Level Lower Min Level Lower Both Levels

Pros• Lowers water age (due

to reduced volume in tank)

• Maintains minimum system pressure

Cons• Less water for

fires/emergencies

• Possibly reduces mixing

Pros• Lowers water age

• Improves mixing

Cons• Less water for

fires/emergencies

• Lowers minimum system pressure

Pros• Lowers water age

• May improve mixing (depends on level fluctuation)

Cons• Lowers minimum

system pressure

• Less water for fires/emergencies

Change Tank Fill Rate/Duration

Longer Fill Duration / Slower Fill Rate

Shorter Fill Duration /Faster Fill Rate

Pros• Increased water velocity to improve

mixing

• Tank filled fasterCons• Energy use considerations

• Uncertain impact on water quality

• Less water for fires/ emergencies

• Potential wear on control valve

• May not fill tank before next demand cycle

Pros• Longer mixing time

• Less stress on water mains

Cons• Energy use considerations

• Uncertain impact on water quality

• Possible over-pressurization of water mains

• Possible increased power bill

• May not fill tank before next demand cycle

Remove Tank from Service

Cons• Possible reduced system pressure.

• Less water for fires/emergencies.

• Possible change in ISO rating when re-evaluated.

• Transferring demand to another tank could be a problem.

• Increased energy costs (i.e., pumping costs).

• Empty storage tanks are more vulnerable to weather (especially wind).

Pros• Lowers overall water age in system.

• Transfers demand to another tank (may improve operations of that tank).

Operational Data

Water Quality Data

Flushing

Basis for Considering: Lower disinfectant residuals High disinfection byproducts Area is not influenced by a tank or tank operations cannot be

altered Dead-ends:

Physical Hydraulic

Concerns/Implementation Considerations: Lost revenue Discharge issues (e.g., dechlorination, water conservation)

Hydraulic limitations (e.g., pressure)

Flu

shin

g Unidirectional

Automatic

Blow-Off

Conventional

Flushing – Conventional

Cons• Labor intensive.

• Usually involves large volumes of water, relative to automated flushing.

• Can cause discolored water due to typical high water velocity.

• Usually not metered.

Pros• Immediate water quality improvement.

• Can obtain water quality data over time of flush.

• Can clean/scour the water main.

• Visual verification of water quality.

Flushing – Unidirectional

Cons• Very time intensive/consuming.

• Usually involves large volumes of water, although tends to be less than conventional flushing.

Pros• Immediate water quality improvement.

• Scours/cleans biofilm and debris from mains.

• Controlled flushing, should not disturb adjacent lines.

• Can target specific flushing concerns.

• May reduce chlorine demand and DBP precursors in area.

• May use less water than conventional flushing.

Flushing – Automatic

Cons• Typically, no water quality data is collected except for when

visited (monthly or less).

• Can be tampered with.

• Discharge issues (e.g., dechlorination and erosion).

• “Good” citizens (turning them off).

• If they fail, you don’t know until you visit the station.

Pros• Localized water quality improvement.

• Not labor intensive once installed.

• Water is typically metered.

• Flow is typically slow enough to avoid discolored water issues.

• Some installations can be hidden.

• Easy to control amount of water flushed.

• Reliable designs exist.

Flushing – Blow-off

Cons• Typically no water quality data is collected except for when

visited (monthly or less).

• Can be tampered with.

• Discharge issues.

• “Good” citizens (turning them off).

• If they fail, you don’t know until you visit the station.

• Usually forgotten about.

Pros• Immediate water quality improvement.

• Constantly moving water.

• Flow is typically slow enough to avoid discolored water issues.

Rerouting Water

Basis for Considering: Lower disinfectant residuals

High disinfection byproducts

Parallel lines and/or functional valves exist: to allow for changing the flow of water into/out of an area

to allow for a line to be taken out of service

single line capacity can handle required flow

Concerns/Implementation Considerations: May move problem from one area to another

Flushing may still be needed

Rer

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Parallel Lines

Valve Changes

Rerouting Water – Parallel Lines

Cons• One or more of the pipes is not in good condition.

• Required flow may be too much for one line to handle by itself.

• Must maintain adequate fire protection.

• Connecting the two lines could be difficult and/or expensive:• Required permits• Distance• Rock

Pros• Valve & reroute (assumes two different size parallel lines – i.e., 6” and 3”):

• majority of flow can be handled by larger line, reducing stress on smaller line.

• Rerouting flow through the larger line and into the smaller line helps keep water moving.

• Remove a line from service: reduce underground storage (water age).

Rerouting Water – Valve Changes

Cons• Sufficient valves do not exist or locations of valves are unknown.

• Valves are old and may break when operated.

• No connection to other sections of the system.

• Line sizes are small and cannot handle the additional flow.

• If insufficient records are kept, problems could arise (i.e., hydraulic “bottleneck,” inadvertently opening the valve, etc.).

Pros• Sufficient control valves exist.

• Connections exist or are close enough to connect other areas of the system together.

• Lines are of sufficient size to handle the additional flow.

• Can reroute water through low demand areas, towards high demand area, to keep water moving.

“Others”optimize booster chlorination, capital improvements

Not “primary strategies” because they are system- specific

(i.e., optimize booster chlorination) or not true optimization

strategies (i.e., cost $$).

Optimizing booster chlorination. Basis to consider:

System has booster chlorination and process monitoring/control capability.

Lower (or higher) disinfectant residuals in area of system influenced by

booster chlorine station.

Concerns/Implementation Considerations: May elevate DBP formation.

May still have high water age (i.e., require additional strategies discussed

previously).

“Others”optimize booster chlorination, capital improvements

Capital improvements may include: Adding a mixer in a tank

DBP control in the distribution system

Adding booster chlorination

Others(?)

Should be considered advanced strategies once optimization (O&M) options are exhausted.

Questions?