potential for flow rate to impact growth of legionella in ......(some of the) engineering control...
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Potential for flow rate to impact growth of Legionella
in building plumbingWilliam Rhoads
Amy Pruden
Marc Edwards
Water
INTERFace
IGEP at VT
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Opportunistic pathogens (OPs)-infect (mainly) immunosuppressed individuals
• 8k-18k
cases/yr
• $430M/yr
• Caused all 31
reported
respiratory
waterborne
disease
outbreaks
2007-10
• 100
cases/105
people
>60yrs
• $425M/yr
• Only
recently
linked to
drinking
water
2
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Legionella epidemiology
• ~5000 cases/yr reported
• ~9% fatal (5-32%)
• 2011-2012 – 66% potable-water-associated disease outbreaks Legionella related 286%
increase
https://www.cdc.gov/mmwr/volumes/65/wr/mm6522e1.htm#F1_down3
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Domestic plumbing characteristics and stakeholders
Utility
• General water quality
• Distribution system
Building Owner/Operator
• Water use pattern
• Material selection
• In-building treatment
Everyone Else
• Plumbers, consultants, manufacturers, code/standard orgs
Randi Brazeau,Sheldon Masters
4
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(Some of the) Engineering Control Strategies
1) Limiting Nutrient Strategies (e.g., AOC)
2) Secondary Residual Type and Dose
3) Upgrade Water Mains/Corrosion Control
4) In-Building Disinfection
5) Water Heater Set Point
6) Thermal Shock Treatments
7) Pipe Material Selection
8) Flow Control
9) Heater Selection
10)Water Age
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Other important aspects of flow
• What conditions it delivers
• How frequently it occurs
• How the pipes are designed
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Growth dependent on conditions delivered by flow
Conventional wisdom: Flow is better
• When flow deliverers high temperature or chemical disinfectant to taps
Ciesielski et al., 1984; Harper, 1988; OSHA, ASHRAE; Muraca et al., 1987; Stout et al., 1987
Contrary to wisdom: Flow is worse
• When flow delivers ideal growth temperatures and nutrients
Liu et al., 2006
7
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Two identical systems: 40° C vs Variable T (40-60 ° C)
Three use frequencies (water age in distal pipes): High, Med, Low
No disinfectant residual.
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0
1
2
3
4
5
6
35 40 45 50 55 60
Log L
. pneum
ophila
(g
ene c
op
ies/c
m2)
Temperature of Experimental Heater (°C)
Growth dependent on conditions delivered by flow
5.7 logs
9
Biofilm
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Growth dependent on frequency of flow
10
Temp. profiles after use
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Growth dependent on frequency of flow
Regrowth factor in distal taps
No change from recirculating line
10X more L. pneumophila in
distal taps than recirc. line
L. pneum
ophila
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Growth dependent on frequency of flow
Regrowth factor in distal tapsL. pneum
ophila
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Growth dependent on how pipes are designed
50 sec flush through ¾” pipes
• Tcold = 10 °C; Thot = 40 °C; Tmix = 37 °C• @0.5 gpm – 0.38 gallons hot water used – 16 ft of pipe
• @2.2 gpm – 1.65 gallons hot water used – 71 ft of pipe
• Tcold = 30 °C; Thot = 60 °C; Tmix = 37 °C• @0.5 gpm – 0.1 gallons hot water used – 4.2 ft of pipe
• @2.2 gpm – 1.65 gallons hot water used – 18.6 ft of pipe
Conflict between using best management
practice temperature setting, and water age
in individual hot distal pipes
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Opportunistic Pathogen Growth
Sydnor et al. 2012
95% colonized by
Legionella spp.
45% colonized by
Legionella spp.
Cause?
• Materials
• Mixing volume
• Distance to tap
• Flow rates
Devices were removed
and replaced with
conventional devices….
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Hypothesized impact of low flow
• Less volume delivered (higher water age)
• Time for reactions/growth to occur
• Less delivery of disinfectant (thermal or chemical) to distal pipes
• Less delivery of corrosion control
• More biofilm/sediment build up
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Proving the Obvious
0.2 gpm
82% retained
0.6 gpm
60% retained
2.6 gpm
8% retained
Clear PVC seeded with
organics at very low flow
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On-going Experiment at VT
Q = 0.5, 1, 1.5, or 2 gpm
In duplicate
Thot = 49 °C
Tcold = 10-18 °C
Tmix = 37 °C
No disinfectant
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Experimental Design
Completed: Constant draw duration
• 25 second water draw
• 50 second water draw
Approximate hot water pipe volumes replaced during flow
Flow Rate 25 sec 50 sec
0.5 gpm 0.68 1.73
1.0 gpm 1.36 3.46
1.5 gpm 2.04 5.20
2.2 gpm 3.00 7.63
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Total cell counts (flow cytometry)gpm gpm gpm gpm
4.5
4.7
4.9
5.1
5.3
5.5
5.7
5.9
Cold Hot Stag Flush Stag Flush
Log t
ota
l cell
counts
(e
vents
/mL)
25 sec50 sec
0.5 gpm 2.0 gpm
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Legionella growth (by culture)
Flow
Rate
Log
Growth
0.5 gpm 0.29
1.0 gpm 0.59
1.5 gpm 0.27
2.0 gpm -0.14
Max. Hot
Temp.
28.0 °C
42.9 °C
45.4 °C
49.0 °C
25 second flush duration
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Legionella growth (by culture)
Flow
Rate
Log
Growth
0.5 gpm 0.91
1.0 gpm 0.24
1.5 gpm -0.33
2.0 gpm 0.07
Max. Hot
Temp.
48.7 °C
49.2 °C
49.0 °C
49.4 °C
50 second flush duration
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Future conditions to study
• Account for tank turn over/dilution
• Effect of tank temperature setting
• Constant volume draw from each faucet
• Determine effect of insulation
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Quickly reiterate
Lower flow rates may…
• Increase water age at individual taps
• Decrease delivery of disinfectant/corrosion control
Flow rate doesn’t act alone
• Conditions delivered by the flow to the distal tap
• How frequently those conditions are delivered
• How the system is designed
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William Rhoads – [email protected] - @wjrhoads
Marc Edwards – [email protected]
Amy Pruden – [email protected]
Questions? Comments?