Statewide (PA) Chlorine Contact Time Tracer Study - Gwin, Dobson ...

Prepared by:
Travis J. Long, Senior Environmental Scientist
James L. Balliet, Secretary, Facilities Planning Director
Mark Glenn, P.E., President
GWIN, DOBSON & FOREMAN, INC.
Consulting Engineers
Altoona, PA

• Ensure disinfection contact time complies with the goals established under
the Interim Enhanced, Long Term 1 and Long Term 2 Surface Water
Treatment Rules and Stage 1 and 2 of the Disinfectants/Disinfection
Byproducts Rule.
• To prepare water suppliers for the “balancing act” of maintaining adequate
pathogen (ex.,giardia) and virus inactivation while avoiding “over
disinfection” that can lead to byproduct formation
• Surface water suppliers may be unaware of the actual short‐circuiting that is
occurring in chlorine contact basins. By default, DEP’s field staff or the water
suppliers had assigned a “rule‐of‐thumb baffling factor” (i.e. short circuiting
estimate) that may not realistically reflect conditions
• Many of the suppliers do not have the technical expertise to conduct tracer
studies, hence PADEP’s efforts to perform studies of select systems.
• This presentation will provide a general overview of tracer studies, selected
methodologies, findings and importance of such efforts to thoroughly
evaluate ones water system

• Gwin, Dobson & Foreman, Inc was selected to provide statewide tracer
studies in 2005 by the PAD Department of Environmental Protection.
• 3‐ year study period (2005‐2008)
• 82 treatment plants were field tested and evaluated for disinfection contact
time along with desk top computations and detailed final reports
• Plants with service area populations of 10–50,000 with capacities ranging
from 1 to 10 mgd.
• More than 140 separate process treatment basins were evaluated to
determine baffling and CT factors
• Basins included contact tanks, clearwells, sedimentation basins, reactor
clarifiers, flocculation units and pipe storage in various geometric shapes,
baffling and inlet/outlet conditions
• For disinfection profiling and actual log‐inactivation, units dedicated for
disinfection (chlorine contact tanks and clearwells) and other unit processes
(sed, floc, filters) were tested analyzed.

• Tracer studies are commonly used for reactor mixing characteristics and
actual detention times for treatment processes and unit operations
• A water system must determine the Contact Time (CT) of each disinfectant
sequence before the first customer to determine the total “inactivation” of
pathogens and viruses.
• Tracer studies allow characterization of system dynamics by:
• Demonstrate thoroughness of mixing chambers
• Determine path and time of travel in conduits
• Various methods and materials can be used to achieve evaluation
• Step Input Method
• Pulse Input Method
• Quantifiable Chemical Material
• Dye Material

• Tracer studies provide a means of disinfection profiling
• Balancing disinfection and proper inactivation
• Maintaining proper inactivation but limiting disinfection byproducts
• Examine the effects of disinfection practice modifications.
• Tracer Studies provide a means of disinfection benchmarking
• Evaluation of significant changes to disinfection practices
▪ Change of points of disinfection
▪ Change to disinfectant used
▪ Change to disinfection process

For the purpose of the study
• identify the type of tracer methodology to be used
• select the tracer chemical that will be introduced at a calculated dosage to
determine the time at which a ten (10) percent rise in concentration of the
tracer from the baseline concentration has occurred.
Once implemented the study will use normalized concentration versus time
profiled to determine the T 10
/ T, which is the detention time required for the
calculation of CT
• Study Considerations and Planning
• Study Methods

• Determine the desired flow rate or rates
• Tracer studies should be performed for at least four (4) different flow rates:
less than average flow; average flow, and two above average flow, with at
least one at 91% of maximum flow ever expected.
• Critical to maintain constant flow rates during study
• Determine process unit volumes or levels to be used
• Critical to maintain constant levels of unit volumes during study
• Identification of accurate injection and sample locations is essential
• Document background concentrations for accurate and precise tracer
evaluation

Effluent Sample
Location
Influent Sample
Location
NaCl Injection

Contact Tank
Effluent Sample
Location
NaCl Injection Pt 2
Clarifier/Filter
Influent Sample
Location
Clarifier
&Filter
Effluent
Sample
Location
Contact
Tank
Influent
Sample
Location
NaCl
Injection
Point 1

Two of the most common methods of tracer addition employed in detention
evaluations are:
• Step Input Method
• Introduction of a tracer chemical at a constant rate until the concentration
at the desired end point reaches a steady state level
• Typically, this method requires feed equipment
• Pulse Input Method
• Introduction of a large instantaneous dose (or pulse) of tracer with samples
taken at the discharge of the unit over time
• This method may not involve feed equipment

• An important step in any tracer study is the selection of a chemical solution to
be used as the tracer.
• The selected tracer should be readily available , that is not adsorbed in the
treatment process, easily monitored, affordable and approved for use in
potable water applications.
• Examples of Tracers:
• Conductivity
• Fluorides
• Chlorides
• Sodium
• Selection and use of a tracer should be discussed and approved by the
regulatory agency prior to use. In addition, public notification may be
required depending upon the tracer selected

• Sodium Chloride (NSF Food Grade Salt) was selected to as the chemical tracer
• Conductivity was the desired tracer indicator
• Rational for selection of Sodium Chloride during statewide study includes
• Readily available and is a conservative tracer since it is not consumed or
removed during the treatment process
• Sodium Chloride is acceptable for use in potable water supplies
• Easily and accurately measured (using portable conductivity meter)
• Was permitted by PADEP
• Sodium Chloride has been successfully demonstrated in over 90 studies state
wide.
• Sufficient background data collection is necessary to assure accurate
evaluation

• Tracer Injection :
• Injection rates during the statewide tracer study were set to deliver 50 to
100 mg/L.
• Secondary drinking water standards for total dissolved solids and chloride
are 500 and 250 mg/L respectively
• Injection duration lasted until 10% of the injection concentration break
through was documented (at a minimum), but preferably until 20% break
through was achieved.
• Monitoring
• Grab samples were generally collected every five minutes
• Again, sufficient background sampling is essential to assuring accurate
evaluations

Conductivity meter used in the tracer study analysis. HACH sensION5
conductivity meter, with ±0.5% of range accuracy.

Solution feed tank and variable speed peristaltic feed pump, allowing precise
delivery/dosage of sodium chloride feed solution

• Important Study Aspects
• Steady state flow rate
• Constant unit process water level/volume
• Determine unit operations to be tested based on preliminary CT values
• Injection and Monitoring Locations
• Injection locations should simulate the disinfection profile
• Injection locations must allow for adequate dilution and mixing
• Effluent monitoring/sampling at common discharge point of unit
• Tracer studies conducted on each section to determine the T 10
for each section.
• To minimize test time and interference with residual tracer material, tracer studies
proceed “upstream” through the units

Example –CT Calculation
• Process Unit
• 194,000 gallon Clearwell (at minimum level)
• Facility Flow Rate
• 4.0 MGD (2,778 gpm)
• Water Quality Conditions
• pH = 7.0
• Temperature = 10 0 C
• Chlorine Residual = 0.8 mg/L

• Theoretical Detention Time (T) Calculation
• 194,000 gal. ÷ 2,778 gpm = 69.83 minutes
• Assumed Baffling Factor (T 10
/T) = 0.8 or 80 %
• Theoretical Detention Time with T 10
/T
• (194,000 gal x 0.8) ÷2,778 gpm = 55.87 minutes

Sample
Time
Actual
Time
CONTACT TIME TRACER STUDY
( CLEARWELL)
Sodium Chloride Concentrations (µS/cm)
Tracer Solution
Conductivity
C 0
Background
Clearwell
Influent
Clearwell
Effluent
Clearwell
Level:
(Water Level
@ Full)
Background1 10:20 38.2 36.1 194000.0
Background2 10:25 38.1 34.4
Background3 10:30 39.0 33.5
Treatment
Flow Rate
(gpm)
0 HR 00 MIN 10:35 38.43 34.67 194,000.00 2,778
0 HR 10 MIN 10:45 114.5 34.8 194,000.00
0 HR 20 MIN 10:55 157.8 35.3 2,778
0 HR 30 MIN 11:05 159.0 38.4 194,000.00
0 HR 40 MIN 11:15 134.9 54.0 2,778
0 HR 50 MIN 11:25 177.0 72.1 194,000.00
1 HR 00 MIN 11:35 171.0 94.9 2,778
1 HR 10 MIN 11:45 154.3 112.1 194,000.00
1 HR 20 MIN 11:55 137.5 121.8 2,778
1 HR 30 MIN 12:05 148.3 130.1 1940,00.00
Average Clearwell Influent (including tracer): 150.48 uS/cm
Average Clearwell Influent Background: 38.43 uS/cm
Average Tracer Conductivity: (150.48-38.43) 112.04 uS/cm
Average Clearwell Background effluent: 34.67 uS/cm
Therefore 10% of C o
= (112.04x0.1) 11.20 uS/cm
Target Concentration for Clearwell Effluent (T 10)
)
= (34.67+11.20) 45.87 uS/cm

• Data from the studies was summarized numerically and graphically
• Graphical analysis was conducted to demonstrate residual concentration versus
time to determine the detention time T 10

• T 10
Calculation for the Clearwell
• Clearwell effluent conductivity + 10% Value of Tracer = Target T 10
Value
• Target T 10
value = 34.67 us/cm + 11.20 us/cm = 45.87 us/cm
• Theoretical DT values
• 69.83 minutes
• 55.87 minutes with assumed T 10
• T 10
/T Calculation
• T 10
for the clearwell as derived from the concentration versus time profile =
35 minutes
• T 10
/T = 35 ÷ 69.83 = 0.50
• QA/QC
• 194,000 gal x 0.50 = 97,000 gal ÷2,778 gpm = 34.92
Thus the assumed or historical T 10
/T factor was to liberal and did not give an
accurate representation of the true detention time and contact effective
volume, thereby providing inaccurate log inactivation values.

• During the study the free chlorine residual was 0.8 mg/L, peak hourly flow
rate was maintained at 2,778 gpm and process unit volume remained steady
at 194,000 gallons.
• T 10
/T was calculated to be 0.5, thus proving the effective volume of this
process unit to only be 97,000 gallons.
• With all the variables determined, the CT provided by the facility can be
calculated as follows:
• CT provided by clearwell = (0.8 mg/L)(97,000 gal. ÷2,778 gpm) = 27.93
mg/L‐min of contact time
• CT provided by piping = (0.8 mg/L)(7,050 gal ÷2,778 gpm) = 2.03 mg/L‐min
of contact time
• CT provided by contact tank = (0.8 mg/L)(100,000 ÷2,778 gpm) = 28.80
mg/L‐min of contact time
• Total CT provide by the chlorine disinfection process units = 58.76 mg/Lmin

EPA Table E-3 CT Values for Inactivation of Giardia Cysts
by Free Chlorine
pH = 7.0
Chlorine Concentration
(mg/L)
Log Inactivation
0.5 1 1.5 2 2.5 3

• In comparing the calculated CT for the facility and the EPA CT values for
inactivation, it is was determined that sufficient CT is being provided under the
tested operating conditions (flow, number of units in operations, basin level)
and water quality conditions (pH, temperature, alkalinity, etc)
• The required CT for 1‐log removal under the stated water quality conditions is
37 mg/L‐minute
• As determined from the study, the total facility CT provided is 58.76 mg/Lminute
• Therefore, the computed Log inactivation value is:
58.76 mg/L‐minute ÷ 37 mg/L‐minute = 1.59
• Please note fluctuation of conductivity measurement during initial stage of
step tracer feed. However, breakthrough (“knee of curve”) is evident and
considered accurate within 1‐2 minutes of contact time and 5‐10% for T10/T

Chlorine
Residual
(mg/l)
Tabulated
1 log CT
(mg/l-min)
Calculated
1 log CT
(mg/l-min)
2,778 gpm
CT Provided
(@2,778gpm)
Yes / No
Calculated
1 log CT
(mg/l-min) @
2,083gpm
CT Provided
(@2,083 gpm)
Yes / No
Calculated
1 log CT
(mg/l-min)
@1,389gpm
0.1 35 7.35 No 9.80 No 14.69 No
0.2 35 14.69 No 19.59 No 29.38 No
0.3 35 22.04 No 29.39 No 44.07 Yes
0.4 35 29.38 No 39.18 Yes 58.76 Yes
0.6 36 44.07 Yes 58.78 Yes 88.14 Yes
0.8 37 58.76 Yes 78.37 Yes 117.52 Yes
1.0 37 73.45 Yes 97.96 Yes 146.90 Yes
1.2 38 88.14 Yes 117.55 Yes 176.28 Yes
1.4 39 102.83 Yes 137.14 Yes 205.66 Yes
1.6 40 117.52 Yes 156.74 Yes 235.04 Yes
1.8 41 132.21 Yes 176.33 Yes 264.42 Yes
2.0 41 146.90 Yes 195.92 Yes 293.80 Yes
2.2 42 161.59 Yes 215.51 Yes 323.18 Yes
2.4 43 176.28 Yes 235.10 Yes 352.56 Yes
2.6 44 190.97 Yes 254.70 Yes 381.94 Yes
2.8 45 205.66 Yes 274.29 Yes 411.32 Yes
3.0 46 220.35 Yes 293.88 Yes 440.70 Yes
CT Provided
(@1,389gpm)
Yes / No

Chlorine
Residual
(mg/l)
Tabulated
3 log CT
(mg/l-min)
Calculated
3 log CT
(mg/l-min)
2,778 gpm
CT Provided
(@2,778gpm)
Yes / No
Calculated
3 log CT
(mg/l-min) @
2,083gpm
CT Provided
(@2,083 gpm)
Yes / No
Calculated
3 log CT
(mg/l-min)
@1,389gpm
0.1 104 7.35 No 9.80 No 14.69 No
0.2 104 14.69 No 19.59 No 29.38 No
0.3 104 22.04 No 29.39 No 44.07 No
0.4 104 29.38 No 39.18 No 58.76 No
0.6 107 44.07 No 58.78 No 88.14 No
0.8 110 58.76 No 78.37 No 117.52 Yes
1.0 112 73.45 No 97.96 No 146.90 Yes
1.2 114 88.14 No 117.55 Yes 176.28 Yes
1.4 116 102.83 No 137.14 Yes 205.66 Yes
1.6 119 117.52 No 156.74 Yes 235.04 Yes
1.8 122 132.21 Yes 176.33 Yes 264.42 Yes
2.0 124 146.90 Yes 195.92 Yes 293.80 Yes
2.2 127 161.59 Yes 215.51 Yes 323.18 Yes
2.4 129 176.28 Yes 235.10 Yes 352.56 Yes
2.6 131 190.97 Yes 254.70 Yes 381.94 Yes
2.8 134 205.66 Yes 274.29 Yes 411.32 Yes
3.0 137 220.35 Yes 293.88 Yes 440.70 Yes
CT Provided
(@1,389gpm)
Yes / No

Water Treatment Facility Daily Log
Sample Daily CT Calculation Spreadsheet
Month
CT Point
Free Chlorine Residual
Date
Minimum
Clearwell
Volume T 10
/T
Effective
Clearwell
Volume
Maximum
Flow Rate
Detention
Time
Minimum
Cl 2
Residual
Minimum
CT Value
Tem
p pH min max avg
CT
Required
Log
Inactivation
Provided
Required
Log
Inactivation
Achieved
gal %/100 gal gpm min mg/L
mg/Lmin
ºC S.U. mg/L mg/L mg/L
mg/Lmin
Log
True/Fals
e
1 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
2 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
3 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
4 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
5 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
6 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
7 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
8 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
9 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
10 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
11 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
12 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
13 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
14 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!
15 0.50 0 #DIV/0! #DIV/0! #DIV/0! #DIV/0!

• Significant design characteristics include: length‐to‐width ratio, the degree of
baffling within the basin and the effect of inlet baffling and outlet weir
configuration.
• These physical characteristics of the contact basins affect their hydraulic
efficiencies in terms of dead space, plug flow and mixed flow proportions.
• The effectiveness of baffling in achieving a high T 10
/T fraction is more related
to the geometry and baffling of the basin than its function.
• For this reason, T 10
/T may be defined for the levels of baffling conditions
rather than for geometry of the basin.

• The purpose of baffling is to maximize utilization of basin volume, enhance
plug flow conditions within the basin and minimize short circuiting.
• Ideal baffling design reduces inlet and outlet velocities, distributes the water
as uniformly (as practicable) over the entire cross section of the basin,
minimizes “dead spaces” in the basin and prevents short circuiting.
• Categories of baffling conditions were developed to describe basin contact
profiles including:
• Unbaffled
• Poor
• Average
• Superior
• Perfect

Baffling Condition T 10
/T Baffling Description Typical Unit Process
Unbaffled 0.1 None, agitated basins, very low length Clearwell, storage tank, no baffles or perforated inlet or
to width ratio, high inlet and outlet
outlet, submerged inlet and outlets.
flow velocities
Poor 0.3 Single or multiple unbaffled inlets and Conventional sedimentation
outlets, few to none intra basin baffles
basins or clearwell with
2 or 3 baffles
Average 0.5 Baffled inlet or outlet with some intra‐ Some sedimentation basins
basin baffles
with several baffles
Superior 0.7 Perforated inlet baffle, serpentine or Filters or clearwell with perforated intra‐basin baffles,
outlet serpentine baffling
weir or perforated launders
“Perfect” (plug flow) 1.0 Very high length‐to‐width ratio Piping (pipeline flow), perforated inlet, outlet
and intra‐basin baffles

Unbaffled Mixing Conditions (rectangular contact basin with
submerged inlet/outlet)

Poor Baffling Condition (circular contact basin with center
column feed)

Average Baffling Conditions (rectangular contact basin with “overunder”
baffles)

Average Baffling Condition (circular contact basin with center top feed)

Superior Baffling Conditions (rectangular contact basin with serpentine
baffles)

Superior Baffling Condition (circular contact basin with center column
feed and slotted baffle ring)

The following table summarizes the results of the statewide tracer study:
Circular Flocculation Sedimentation Rectangular Pipe
Tanks Basins Basins Clearwells Storage
Over 20 % 15% 35% 40% 0%
Under 50% 55% 47% 41% 0%
Confirms 30% 30% 18% 20% 100%
______________________________________________________ _____
Total No.
of Units 10 38 20 60 10
Note: The “Over” category represents those process units that achieved a higher T 10 /T
that was either assumed or designed for, “Under”represents those process units
that achieved a Lower T 10 /T than was either assumed or designed for, and the
“Confirms” category represents those process units that achieved the same T 10 /T
that was either assumed or designed for.

• Confirmed 29% of the units were designed with the correct baffling factors
• Discovered 27 % of the units had higher baffling factors than originally designed
• Therefore, over half of the units (56%) had baffling factors in excess of design
• Almost half of the units (44%) were tested with baffling factors less than design
• Pipe storage achieves the best degree of baffling, but is not suited for larger plants
• Generally, flocculation/sedimentation tanks have lower baffling efficiency and are
not as effective as dedicated contact tanks and clearwells which have some baffling
• Virtually all plants achieve 1‐log removal while the majority of plants achieve 3‐log
• Many chemical injection locations were not as effective as originally believed

• The PADEP state‐wide tracer study project has proven to be a valuable asset
to system operators by providing data on:
• how the facility disinfection process is actually achieved
• the strengths and weaknesses of system baffling and mixing components
• verification of actual basin volumes and baffling factors
• evaluation of operating levels and monitoring devices
• Facility owners are better equipped for the disinfecting “balancing act”
necessary to comply with LT2ESWTR and DBR.
• Facilities have seen the process and methods necessary to conduct a tracer
study and independently replicate the test process in the future

• Tracer studies are a proven and reliable means of evaluating the detention
and contact time through process treatment units
• Can help treatment operators evaluate and monitor disinfection profiling and
achieve the necessary log‐inactivation through varying water quality
conditions
• A means of helping an operator better apply disinfection chemical and reduce
the potential for byproduct formation.
• Can serve as an effective check and balance on process design assumptions
• Tracer studies can be easily repeated to evaluate changes in process
treatment systems or unit operation

Questions