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Monroe L. Weber-Shirk
School of Civil and
Environmental Engineering
Multiple Barriers, Multiple ExposuresMultiple Barriers, Multiple Exposures
Perspectives from the North and the South
Multi-Barrier ApproachMulti-Barrier Approach
If we implement multiple barriers, then failure in any one barrier won’t result in catastrophic failure of the overall process
Multiple barriers increase the reliability of the overall process
The concept of multiple barriers is often used to refer to water purification steps, but it can also be extended to any interventions used to break pathogen exposure routes
Barriers: The Water Purification Perspective
Barriers: The Water Purification Perspective
Watershed protectionParticle removal
Coagulation, flocculation and sedimentationFiltration
DisinfectionDistribution system
Cross contamination protectionResidual disinfectant
Treatment Train ComparisonTreatment Train Comparison
Alternative 1- one unit process, which, when it operates properly, achieves 6 logs of removal ____________________
Alternative 2 - three independent unit processes in series, each of which, when it operates properly, achieves only 2 logs of removal__________________________________________________________________
Membrane filtration
Conventional filtration
ChlorinationUV disinfection
Barriers with FailureBarriers with Failure
Let’s also assume that each unit process completely fails 1% of the time
“Normally”, both trains produce 6 logs of removal but, let’s look closer
6 log removal really means that pf = 6
log out
in
Cpf
C
Negative log of the fraction remaining
1.E+00 1.E+02 1.E+04 1.E+06 1.E+08
Seconds of Operation
6
4
2
0
pfTrain 2Train 1
Process A
99%, 6 logs
1% , 0 logs
Process A
99%, 6 logs
1% , 0 logs
Process B
99%, 2 logs
1%, 0 logs
Process B
99%, 2 logs
1%, 0 logs
Process C
99%, 2 logs
1%, 0 logs
Process C
99%, 2 logs
1%, 0 logs
Process D
99%, 2 logs
1% , 0 logs
Process D
99%, 2 logs
1% , 0 logs
Train No. 1 Train No. 2
263 hours263 hours
158 min.158 min.
32 sec.32 sec.
361 days361 days
350 days350 days
87.6 hours87.6 hours
Comparison of one and three barrier trains with same nominal removal
Comparison of one and three barrier trains with same nominal removal
Relative performanceRelative performance
Increasing the number of process barriers substantially reduces the risk of __________ in return for a small compromise in the time at which the nominal design performance is achieved
This analysis assumes failures are independent events!
Imagine if every electrical power failure resulted in everyone in your city becoming ill!
total failure
Add It Up to Meet the Target Protection Level
Add It Up to Meet the Target Protection Level
Multiple Barrier approach as codified in EPA regulations
Treatment technique summationBarrier trading
Choose which barrier to implementCan trade different approaches
Crypto Requirements for Filtration Plants
Crypto Requirements for Filtration Plants
1 (40 CFR 141.709 and 40 CFR 141.720)2 Systems may use any technology or combination of technologies from the microbial toolbox.3 Systems must achieve at least 1 log of the required treatment using ozone, chlorine dioxide, UV, membranes,bag/cartridge filters, or bank filtration.4 Total Cryptosporidium treatment must be at least 4.0 log.5 Total Cryptosporidium treatment must be at least 5.0 log.6 Total Cryptosporidium treatment must be at least 5.5 log.
If your Cryptosporidium concentration (oocysts/L) is... bin pf
And if you use the following filtration treatment in full compliance with existing regulations, then your additional treatment requirements are...
Conventional Filtration Treatment (includes softening)
Direct Filtration Slow Sand or Diatomaceous Earth
Filtration Alternative Filtration Technologies
< 0.075 1 3 No additional treatment No additional treatment
No additional treatment
No additional treatment
> 0.075 and < 1.0 2 4.0 1 log treatment2 1.5 log treatment2 1 log treatment2 As determined by the State 2,4
> 1.0 and < 3.0 3 5.0 2 log treatment3 2.5 log treatment3 2 log treatment3 As determined by the State 3,5
> 3.0 4 5.5 2.5 log treatment3 3 log treatment3 2.5 log treatment3 As determined by the State 3,6
DRAFT - LONG TERM 2 ENHANCED SURFACE WATER TREATMENT RULE TOOLBOX GUIDANCE MANUAL
EPA Crypto EPA Crypto
Maximum oocyst concentration is 0.0001/L
So for every 10,000 liters of water consumed, one person could contract Cryptosporidiosis…
But not all oocysts are viable…
0
1
2
3
4
5
6
0.01 0.1 1 10
Influent concentration (oocysts/L)
pf r
equi
red
0.000001
0.00001
0.0001
0.001
Eff
luen
t con
cent
rati
on
(ooc
ysts
/L)
pf
Cout
Filtration Credit for CryptoFiltration Credit for Crypto
The total Cryptosporidium treatment required for Bins 2, 3, and 4 is 4.0 log, 5.0 log, and 5.5 log, respectively
The additional treatment requirements are based on a determination that conventional, slow sand, and diatomaceous earth filtration plants in compliance with the IESWTR or LT1ESWTR achieve an average of 3 log removal of Cryptosporidium
Therefore, conventional, slow sand, and diatomaceous earth filtration plants will require an additional 1.0 to 2.5 log additional treatment to meet the total removal requirement, depending the source water contamination
DRAFT - LONG TERM 2 ENHANCED SURFACE WATER TREATMENT RULE TOOLBOX GUIDANCE MANUAL
Why isn’t Crypto pf measured for each plant?
Barrier EquivalencyBarrier Equivalency
Mathematically correct (adding pf is like multiplying fraction remaining)
Are removal and inactivation equivalent?
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
multiple barriers
p(fr
acti
on r
emai
ning
)
disinfection (ozone,UV, chlorine dioxide)Conventional treatment
Bank filtration (25 footsetback)Watershed controlprogram
Achieving pf for Crypto
We need to understand how each of these technologies work!
Barrier Order of OperationBarrier Order of Operation
The underlying assumption is that the barrier fraction remaining can be multiplied
Does order of operation matter?Mathematically: ____Water treatment: ________________________
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
multiple barriers
p(fr
actio
n re
mai
ning
)
Conventional treatment
disinfection (ozone,UV, chlorine dioxide)Bank filtration (25 footsetback)Watershed controlprogram
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
multiple barriers
p(fr
acti
on r
emai
ning
)
disinfection (ozone,UV, chlorine dioxide)Conventional treatment
Bank filtration (25 footsetback)Watershed controlprogram =
YES! Ask for proof after treatment technologies
NO!
Cryptosporidium Treatment CreditCryptosporidium Treatment Credit
Source Toolbox Components
Watershed control program 0.5 log credit
Pre-Filtration Toolbox Components
Bank filtration 0.5 log credit for 25 foot setback; 1.0 log credit for 50 foot setback.
Presedimentation basin with coagulation
0.5 log credit for new basins with continuous operation and coagulant addition.
Two-stage lime softening 0.5 log credit for two-stage softening with coagulant addition.
Treatment Performance Toolbox Components
Combined filter performance 0.5 log credit for combined filter effluent turbidity ≤ 0.15 NTU in 95% of samples each month.
Individual filter performance 1.0 log credit for individual filter effluent turbidity ≤ 0.1 NTU in 95% of daily maximum samples each month (excluding 15 minutes following backwash) and no filter >0.3 NTU in two consecutive measurements taken 15 minutes apart.
Additional Filtration Toolbox Components
Bag filters 1 log credit with demonstration of at least 2 log removal efficiency in challenge test;
Cartridge filters 2 log credit with demonstration of at least 3 log removal efficiency in challenge test;
Membrane filtration Log removal credit up to the lower value of the removal efficiency demonstrated during the challenge test
Second stage filtration 0.5 log credit for a second separate filtration stage; treatment train must include coagulation prior to first filter.
Slow sand filters 2.5 log credit for second separate filtration process. No disinfectant residual present in influent.
Additional BarriersAdditional Barriers
Wastewater treatment New water distribution systems disinfected prior to use Distribution system always pressurized to prevent
infiltration Prevention of siphoning Vigilance against cross connections Residual chlorine
Protection against recontamination (in the home or in the distribution system)
Providing residual protection against microbial regrowth Are these microbes pathogens? Is there a health risk to microbial regrowth?
Research!
No Absolute Barriers?No Absolute Barriers?
Did you notice that each barrier is given a pf rating?
No barriers have absolute protection Any claims that a treatment technology provides
an absolute barrier (100% removal) should be evaluated skeptically
Usually “100% removal” means the detection limit was too high
There is a world of difference between 99% removal and 100% removal
pf = 2 vs. pf = infinity!
Barrier SummaryBarrier Summary
Don’t depend on a single barrier The protection from each barrier can be
characterized by its pf EPA’s pf values may be overly simplistic, but the
concept of additive pf values is useful A fundamental understanding of how each process
achieves its pf is required to understand how processes can be combined into treatment trains
Coming soon!
Multiple Exposures: Pathogen loadMultiple Exposures: Pathogen load
Watersheds:InhabitedAgriculture including Livestock and Poultry
HouseholdsLatrines (or lack thereof) Hand washing (not as convenient)Livestock and Poultry
Multiple ExposuresMultiple Exposures
Water treatment Urban areas
Conventional centralized water treatment (large scale similar practices in the North)
Incomplete coverage Rationing
Small communities and underserved urban areas Inadequate central treatment Unreliable point of use treatment
Distribution system Water rationing Cross contamination Household storage
Global South ContextGlobal South Context
HondurasUrban: Tegucigalpa (1 million)
Central water treatmentSanitation and water supply in the poor
neighborhoods
Rural poor: Vara de Cohete (300)
MexicoUrban: Mérida (1 million)
CoagulationCoagulation
Sedimentation (Lamella)Sedimentation (Lamella)
Filter bedsFilter beds
Filter ControlsFilter Controls
ChlorinationChlorination
Tegucigalpa LatrinesTegucigalpa Latrines
PilaPila
RainwaterRainwater
Vara de CoheteVara de Cohete
Masa StoneMasa Stone
Corn GrinderCorn Grinder
Fuego y CalFuego y Cal
Laundry dayLaundry day
Rural LatrineRural Latrine
MéridaMérida
Urban Household Water TanksUrban Household Water Tanks