This presentation borrows heavily from an AWWA learning module for small systems entitled "Distribution System Operations and the Impact of a Flushing Program on Water Quality"
The AWWA module was funded by the EPA Much of the text and a limited number of illustrations are
taken from that presentation The layout/organization of the content and most of the
illustrations are original and are intended to improve upon the original
What's the purpose of flushing?
Flushing is usually considered a corrective measure
Flushing is better seen as a tool to maintain water quality
Why flush?
Flush to address customer complaints
Flush to remove sediment and loose deposits
Flush to expel contaminants after a backflow incident
Flush to scour distribution lines
Flush to decrease water age in dead-end mains
Flush to restore chlorine residuals
Flush to prevent nitrification
Flush to respond to nitrification
A look at flushing programs
Who here has an active flushing program?
What triggers a round of flushing in your system?
A four-step flushing program
1: Determine if flushing is appropriate
2: Plan and manage flushing
3: Collect data
4: Evaluate and revise the program
Flushing isn't always appropriate
Ask these questions first
Do you use unfiltered surface water?
Do you use undisinfected ground water?
Do you have high iron and/or manganese?
Do you have positive results for coliform?
Do you have elevated heterotrophic plate counts?
Is your water chloraminated?
Has treatment changed in a way that could affect water quality?
Do your customers complain a lot?
Do parts of your system have low disinfectant residuals?
Are you aggressive about exercising valves, hydrants, and tanks?
Is your source water corrosive?
Does sediment accumulate in your storage facilities?
If you answered "Yes", flushing can help
To recap: Flushing helps in these cases
Unfiltered surface water Undisinfected ground water High iron High manganese Coliform HPCs Chloramines Treatment changes Valve exercise Corrosive source water Sediment
If none of those apply, try something else
Five alternative options
Alternative 1: Source treatment
Alternative 2: Booster disinfection
Alternative 3: Valve exercise
Alternative 4: Pipe lining
Alternative 5: Storage tank elevation adjustment
But let's suppose flushing is appropriate
If you're going to flush, you need a plan
Plan your flushing with clear objectives
Objectives for water quality
What are you trying to achieve? What has to improve? How much improvement is enough?
Objectives for cost
It may not have reached the customers, but the water still has value
So does your time and labor
Plan for water quality
Flush with clean water behind you
Plan for hydraulic consequences
Plan for mechanical consequences
Determine flushing approach
Approach 1: Conventional
Approach 2: Unidirectional
Approach 3: Continuous blow-off
Three approaches in detail
Approach 1: Conventional flushing
Most common method
Requires minimal pre-design
Just keep opening hydrants
Open hydrants in the distribution systemuntil criteria are met
Criteria
Disinfectant residual Discoloration Turbidity
Think ahead
Don't draw poor-quality water into high-quality areas Most important for nitrification remediation
No isolation means low velocity
What can you expect from conventional flushing?
Restoration of disinfectant residual
Expulsion of some poor-quality water in some areas
But conventional flushing has shortfalls
Shortfall 1: Customer complaints
Shortfall 2: Wasted water
Shortfall 3: Improvements are minimal
Shortfall 4: Improvements are short-term
Shortfall 5: Risk of increased coliform incidents
Shortfall 6: Chlorinated water discharge
Is there a better way than conventional flushing?
Approach 2: Unidirectional flushing
Unidirectional flushing can be isolated in the distribution system
System-wide or as-needed
Velocity matters
Velocities 3 fps or greater
Remove silt and sediment Reduce disinfectant demand