Upload
gavin-paul
View
227
Download
5
Tags:
Embed Size (px)
Citation preview
Assessing Distribution System Performance:
Sampling and Monitoring
Outline
Sampling Approach
Field Methods
Representative Sampling:
What, Why, Where
Critical Sites – identifying, sampling considerations, and the process
Optimization Monitoring:
What / Where, Frequency
Summary
Assessing Distribution System Performance
Objective: To develop an accurate “picture” of water quality in the distribution system.
Optimization Monitoring
Representa-tive Sample Sites
Sampling
Approach
Your Sampling Practices
How long do you flush a sample tap before you collect a sample?
Sampling Approach: Our Observations
Inconsistent practices for flushing a sample tap before sample collection:
Temperature: Note change by “feel.”
Time: All sites are considered “equal” (i.e., 5 to 15 minutes, 2 songs on the radio).
Flush until you get a “good” sample.
Practices vary by operator.
Who Cares?Regulatory vs. Optimization Sampling
For regulatory purposes: Frequently, the focus is on getting a “good” number to meet state regulations
For optimization purposes: The focus is on getting a “representative” number in order to: Accurately characterize distribution system water quality –
both spatially and temporally
Make important process control decisions in the distribution system
The sampling procedure used for optimization can be applied to regulatory sample locations (taps)
Calculated Flush Time Approach
A consistent & technically sound approach for sample collection.
Sample should represent water in the distribution system main immediately adjacent to the sample location.
Flush/waste the water in the service line before collecting a sample.
Avoid over-flushing, which may pull in water from another area of the distribution system.
Based on article: Collect Representative Distribution System Samples, Opflow, January 2009
IntendedSample Site
6” main12
” m
ain
Se
rvic
e L
ine
Key Elements of the Optimization Sampling Approach
Provides guidance for sample taps and hydrants.
Does not use temperature or chlorine residual as indicator of “adequate” flushing.
Time-based flushing approach using the volume of the service line length, diameter, and pre-determined flow rate.
General Sampling ApproachCan be applied to Taps or Hydrants
1. Estimate the length and diameter of the pipe that is to be flushed.
2. Determine flush time based on an estimated service pipe length, diameter, and flow-rate.
3. Slowly open the tap/hydrant At a tap: immediately start the timer and
verify that the flow is at the desired rate of 2 gpm if tap sampler is not used (e.g., by quickly timing the fill of a measuring cup)
At a hydrant: once the hydrant is fully open, open the flush valve and start the timer. The flow control valve will regulate the flow to 20 gpm
4. After twice the calculated flush time (CFT), or time specified by the rule of thumb, stop flushing and collect the water sample(s).
Tap Sampler
Step 1: Estimate Pipe Length and Diameter (Tap)
Service LineDiameter:Estimate based water system staff knowledge, a system/site map, and/or best judgment.
Length:Measure the distance from the meter/curb stop to building (at approximately where the service enters the building).
Premise PlumbingDiameter:(Ideally) go into the building to estimate the pipe diameter.
Length:(Ideally) go into the building to estimate the pipe length.
Note: If the tap is close to where the service line enters the building, the volume of premise plumbing may be negligible.
MainService Line
Estimating pipe lengthand diameter at tap sampleSites can be difficult. Hydrants are preferred for investigative sampling.
Premise Plumbing
Recommended approach for calculating flush time ~ especially for taps sampled on a routine basis (e.g., TCR).
• Table shows CFT in minutes
• Can be determined for any length and pipe diameter
• Assumes 2 gpm flushing rate
• Determine CFT from matrix
• Flush time = 2 times the CFT
(adjust if the flush rate ≠ 2 gpm)
Tap CFT Matrix
Step 2: Determine Flushing Time- Calculated Flush Time (CFT)
Tap Approach
Example
20’ of 3/4” Line
30’ of 1/2” Line
Premise Plumbing:Pipe Length = 30’, Diameter of Pipe = 1/2”CFT = 0.2 minutes
Distribution System Main
CFT = 0.2 + 0.2 = 0.4 minutes
Total flush time = 2* CFT
= 0.8 minutes or 48 seconds
Service Line:Pipe Length = 20’, Diameter of Pipe = 3/4”CFT = 0.2 minutes
Assumes the flush rate is 2 gpm
Step 3 and 4: Flush, Then Collect Water Quality Sample(s) and Record Data
Flush at ~ 2 gpm (verify flow-rate if needed) for total flush time (previous slide).
Measure chlorine residual, temperature and pH and record the results on a data collection sheet.
Also, record the characteristics of the site: Description of sample site (e.g., Chevron Station
600 Main St.).
CFT (*2) (it might be used in the future).
Type of customer demands (residential, commercial, rural area, old industrial area).
Pressure zone (if applicable).
Others (e.g., characteristics of water sample; site was recently flushed during seasonal flushing program, main pipe material and age).
Notes and Recommendations
The procedure provides a reasonable quick and easy estimate of flush time along with a controlled safety factor.
CFTs are only estimates because “precise” measurements of pipe length and diameter would be difficult to obtain.
Try to avoid sample locations where piping diameter/length are unknown.
Follow a Similar Approach When Sampling from Hydrants
Easier to estimate hydrant service pipe diameter and length than internal building plumbing.
Hydrant Sampler
https://www.youtube.com/watch?feature=player_detailpage&v=2u0rR9TTENw
Representative Sample Sites
What: Optimization sample sites that represent water quality throughout your distribution system.
Optimization Monitoring
Representa-tive Sample Sites
Sampling
Approach
Representative Sample Sites: Why
Sampling throughout the distribution system helps ensure that your disinfection barrier is in-place!
Consider your system… Do you think your current sample sites represent
water quality throughout the system?
Do you have data to confirm that you have a disinfection barrier throughout your entire system?
Do you think some investigative sampling could help you better understand your system?
This System Thought Their Disinfection Barrier was Robust, Based on Their Historical Data
Chlorine Performance (TCR Sites Only)
22
4
1
0
3
Monitoring at Critical Sites Showed Something Different…
Your system’s representative sample sites should include some combination of: Plant effluent and/or master meter
Your regulatory sites (i.e., for TCR and DBP sampling)
Critical sites Areas in your system that have poor water
quality
These are determined (confirmed) by investigative sampling in your system
Representative Sample Sites: Where
Areas influenced by (draining) tanks: With high turnover time (confirmed or suspected)
Operated in series
Standpipes (generally are poorly mixed)
Hydraulic “problem areas”:
On dead-ends ~ actual and looped (subdivisions)
Towards the “extremities” of the distribution system (hydraulically far from the treatment plant)
Near pressure zone boundaries (i.e., closed valve creates a “hydraulic” dead-end)
Areas where water from different sources is blended
Critical Sites Might Include…
Areas with aging pipes (old cast iron lines) Low demand, low flow, stagnant areas Water quality problem areas:
At TCR / DBP sites with historically low chlorine residual, high DBPs, coliform occurrences
Areas with persistent customer complaints In general,
These criteria should be considered when selecting potentially critical sites (i.e., have low chlorine residual); however, water quality data is needed to confirm.
Critical Sites (cont.)
Low chlorine residual = Critical site(high DBPs, high water age, microbiological risk)
Critical SitesOther Considerations
When conducting investigative sampling in your system for optimization, you may measure a free chlorine residual < 0.2 mg/L (especially at a critical sample location!)
Measure total chlorine to assess relative to the regulatory requirement
Write-down the data to determine whether this is a critical site, or a one-time incident
If the residual is extremely low, respond to increase the residual.
Document (write down) what is done in this area!
Identification of Critical Sites: Final Thoughts
Flexibility to sample at as many investigative sampling sites as you want to identify water quality problem areas in your system
Based on your investigative sampling results, select a minimum of ten potential sites to monitor on a routine basis (consider monthly during warmer months)
Review your sampling results to identify which sites will become part of your routine optimization monitoring program
What could you do with your data?
Assess whether you are meeting the:
disinfection optimization goals
DBP optimization goals (and Stage 2 compliance
requirements)
Longer-term, ongoing monitoring data helps tell us
whether process adjustments (operational changes) are
needed to improve water quality
Assess the impacts of your distribution system
optimization efforts.
However - it is difficult to do the above if the data is not
collected or lost!!
Data Collection SheetSample
Site Name(Lat & Long)
Date TimeFree
Chlorine(mg/L)
Temp.(C)
CFT*2 (min)
CFT Notes
Comments(street location, sample location [faucet, hose, hydrant], description of sample site [i.e., remote, high demand, dead-end, residential, etc.], pipe material, pipe age, pipe diameter, pressure zone, and influence of which tank)
EXAMPLE:121 Main St.36.5042 Lat-80.674754
Long
7/31/84 14:35 2.10 22.4 2.2
Hydrant Site – 6’ of 6” hydrant lead +9’ of 6’’ line = 15’ of 6’’ line = 1.1*2 = 2.2
flush time (@ 20gpm)
Site located in a remote area off of main transmission line, old 6” cast-iron pipe, in low-pressure zone, likely influenced by Tank A
Optimization Monitoring:Where are we heading?
What: Long-term, routine sampling at representative sites throughout your distribution system, using the optimization sampling approach, to generate water quality data!
Optimization Monitoring
Representa-tive Sample Sites
Sampling
Approach
Field Methods Used for Optimization Monitoring
Objectives of Optimization Monitoring
To characterize water quality throughout all areas of the distribution system
Hopefully… ensures that the water quality goals (chlorine residual, DBPs) are consistently met
May… identify areas of poor water quality
Definitely… raises system’s awareness of issues in the distribution system!
Ensures integrity of the distribution system (disinfection) barrier is maintained.
Field Methods Used For optimization work, parameter(s) should be:
Useful for process monitoring (i.e., can be used to assess impact of process/operational changes).
Easy to measure and provide relatively quick results.
A surrogate for the parameter of interest (e.g., chlorine).
Primary Parameters: Free chlorine as surrogate for water age and DBP formation.
Total chlorine, pH, and temperature also measured (to support/verify the primary parameters).
Secondary Parameters: TTHM and HAA5 (at regulatory sites and entry point to the
distribution system).
Optimization Monitoring Routine sampling at representative sites
helps us to: Develop water quality data and trends over time
Have a basis for assessing performance relative to water quality goals
Have data needed before process control is implemented
Confirm that your distribution system barrier is in-place
Steps to Assure Data Integrity
Collect representative samples (i.e., using the CFT approach throughout your distribution system)
Be familiar with the field test equipment and test methods
Routinely calibrate (or check) field instruments
Use clean glassware Use reagents within the expiration date
Summary
Establishing a consistent and technically sound sampling approach is the foundation for distribution system optimization
Free chlorine residual is a surrogate for bacteriological water quality, DBPs, and water age. pH, temperature, total chlorine (and DBPs, less frequently) are also useful parameters to assess distribution system water quality
Sample at representative sites (system influent + compliance + critical sites)
Consider determining the CFT and utilizing for your routine sample locations (for TCR, DBPs, etc.)