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