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DISINFECTION TECHNOLOGIES FOR

SMALL WATER SYSTEMS

Silver Falls

24 May 2006

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Some Presentation Topics(not necessarily in order or complete)

Why do we Disinfect?

What, When & Where do we disinfect:Chlorine & Other Common DisinfectantsTypes of ChlorineChemistry

How to disinfect well – e.g. calculating CTs, & How to disinfect a well.

Application / DosagesContact Time v. Residual Cl

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Purpose of Disinfection

Inactivation of PathogensBacteria – Coliforms, CampylobacterViruses – Norwalk, Hepatitis A/BGiardia (parasitic protozoa)Cryptosporidium (parasitic protozoa)

Regulatory RequirementsResidual Maintenance vs. DisinfectionWhy does your system use disinfectant??

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Common Disinfectants(In order of prevalence)

CHLORINE

Ultraviolet Light (UV)

Ozone (O3)

Mixed Oxidants (MIOX)

Chloramines (Cl2 plus ammonia)

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ChlorineTypes (All must be NSF approved):

Gas: Larger systems, safety concerns,

99.5 % available chlorineSolid: Calcium Hypochlorite (Ca(OCl)2)

15% to 70% available chlorineLiquid: Sodium Hypochlorite (NaOCl)

5% to 15 % available chlorine

Common bleach = 5.25%(Shorter half-life at higher

concentrations)

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Chlorine Chemistry Tips

Chlorine + Water = Hypochlorous acid (HOCl), a strong disinfectant.

Depending on water temperature and pH, this acid may disassociate to hypochlorite ion (OCl-), a less effective disinfectant.(HOCl is eighty times more potent than OCl-)

At pH of 7.5, acid to ion ratio is 50/50Colder water temps and high pH make

disinfection less efficient.

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Relative Amount of Hypochlorous Acid & Hypochlorite Ion

0

0.2

0.4

0.6

0.8

1

3 4 5 6 7 8 9 10 11 12pH

Fra

ctio

n of

Fre

e C

hlor

ine

HOCl OCl-

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Effects of pH on Residual Type

pH

7.5 8.5

Free Chlorine

50% OCl-

50% HOCl

90% OCl-

10%HOCl

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Strengths of Residual Chlorine Compounds

HOCl

OCl-

(80x weaker)

Chloramines (combined)(60x – 200x weaker)

Increasing S

trength

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Interfering Agents

Iron (Fe)

Turbidity

Manganese

(Mn)

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When to Increase Chlorine Dosage

Increase Chlorine

Dose When You Have:

Increase in:• pH• Turbidity• Fe & Mn

Decrease in:

• Temperature

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Calculating Chlorine Residual

Chlorine Dose

- Chlorine Demand

Chlorine Residual

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Calculating Chlorine Residual (continued)

Free Chlorine Residual

+ Combined Chlorine Residual

Total Chlorine Residual

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Breakpoint Chlorination Curve

0

10

20

30

40

50

60

70

80

90

100

20 40 60 80 100 120 140 160

Chlorine Dose [mg/L]

Fre

e C

hlor

ine

Res

idua

l [m

g / L]

0

20

40

60

80

100

Tim

e fo

r 99

% in

activ

atio

n of

B.

met

iens

spo

res

[min

utes

]

2.7 minutes

88 minutes

83 minutes

Zone 3Zone 2Zone 1

Breakpoint!

Free ChlorineCombined

Chlorine Only

e.g., NH2Cl

Destruction of Combined Residual Chlorine

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Same Breakpoint Curve(with Nitrogen Compounds)

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Disinfection Contact Time v. Residual Maintenance

What’s the Difference??

Generally speaking, we require:

Disinfection with Contact Time for contaminated Sources.

Residual Maintenance for Distribution.

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Chlorine Residual

Measure free chlorine residual daily.

Must be detectable everywhere at all times.

High levels of iron, manganese, or turbidity will increase chlorine demand.

Use DPD-type test kit (colorimeter/digital)

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Why Is Residual Maintenance NOT “Disinfection”?

0.2 ppm Cl residual

throughout distribution

system

Cl

Cl

Cl

Cl

Cl

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Why Isn’t Residual Maintenance “Disinfection”?

Contamination Point

Disinfection requires Contact Time

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Chlorine Applications

Continuous DisinfectionMust be proportional to flow

Shock DisinfectionOne-time doseWell or reservoir disinfectionRepairs

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Common Chlorine Doses

0.2 to 1.0 mg/L in distribution system

Max allowable = 4.0 mg/L

Using bleach (5.25% chlorine): 1 cup bleach/100 gal water = 25 ppm

1/3 cup bleach/1000 gal water = 1 ppm

Reservoir cleaning:10 ppm for 24 hours or50 ppm for 6 hours

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Well Disinfection - Shock

50 ppm for 24 hours

Mix chlorine with water before introducing into well

Re-circulate if possible

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Well Disinfection Dose (cont’)

6” diameter well = 1.5 gallons/foot of depth

Need well depth & static water level (SWL)

2 cups 5% bleach / 100 gallons water = 50 ppm

Pump to waste.

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Shock Chlorination Example

Well = 200 feet deep; SWL = 120 feet

80’ of water in 6” casing = 120 gallons of water in the well

Since 2 cups of 5% bleach per 100 gallons of water will produce 50 ppm, use slightly over 2 cups.

Flush after 24 hours.

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Disinfection of Storage Tanks

May be necessary if sampling indicates

Access to tank (sight only, arm’s length, or physical entry)

Tank drainageWhere to?Discharge may be regulatedPump out if no drain

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Disinfection Doses for Storage Tanks

(per AWWA Std. C652 and OAR 333)

10 ppm for 24 hours (with tank full)

50 ppm for 6 hours “ “

200 ppm sprayed or brushed on interior surfaces of empty tank

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Chlorine Equipment

Gas cylinders (and safety equipment)Chemical feed pumpsTablet dispensers / Erosion chlorinatorsTest kit

DPD-type, color wheel or digitalOrthotolodine (OTD) method or test

strips not acceptable

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Chemical Feed Pumps

Positive-displacement type (LMI, BlueWhite)

Peristaltic (Stenner)

Meter-driven (no electricity)

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LMI Feed Pump

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Wallace & Tiernen Feed Pump

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SCBA

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Proper Storage

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On-site Hypochlorite Generator

SALT

NaOCl

100%

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“Electrocution” Chamber

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Disinfection “CT Values”

Definition:

CT values are a measurement of the effectiveness of chlorine disinfection against bacteria, viruses, and protozoa.

Concentration * Time = C*T

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CT CalculationCT = Cl2 residual multiplied by

contact time

CT = [mg/L] X [minutes]

= a number (no units expressed)

Example: 0.2 mg/L times 30 minutes

CT = 6

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The Magic Number 6

CT required = 6

30 minutes * 0.2 ppm Cl

Assumptions: Water Temperature ≥ 10oC (50oF)

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Required CT ValuesGW vs. SW

Groundwater –Minimum CT = 6Adequate for bacteria / virus kill

Surface Water –Required CT varies with:

pH, temp, and log-inactivation requirement

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Disinfection Contact Time(not to be confused with CT Calculation)

Determined by Tracer Study or interim estimate

Time = “effective” volume of contact chamber/flow

Time = [minutes] = [gal] / [gpm]

Pipe flow is idealI.e., 100% credit for pipe volume

Storage tanks likely have short-circuiting

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How Do You Really Measure Contact Time? T = V/Q

1. Determine peak demand flowrate Does system have an effluent flowmeter?

2. Look at the configuration of system.3. Make estimate based on tank

configuration & length of pipe Assume 10% of tank volume if unknown.

4. Calculate contact time using effective tank volume and effluent flowrate.

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Chlorination Concerns

Disinfection by-products (DBPs)THMs and HAA5s

DBP production encouraged by:High chlorine levelsLong contact timeWarm weatherHigh source water TOC

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UV

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Ultraviolet Light

Effective non-chemical disinfectantSuitable for small systems with few

distribution linesMust meet plan review requirements:

Minimum dosage 38 mWsec/cm2

Intensity monitorAutomatic shut-off

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UV Pros & Cons

Advantages:No chemicals, therefore no DBP’sNo contact time requiredSimpleLow operation & maintenance cost

Disadvantages:No residual that protects distribution system

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Ozone (O3)

Disinfection and taste/odor controlProduced by electricity and air or pure O2

Inactivates CryptosporidiumMust be generated on-siteLeaves minimal disinfectant residualCosts 4 times more than conventional

disinfection, yet cheapest LT2 option

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Onsite O3 Generation

Liquid O2

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Demo SS Injection w/ Nozzles

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MIOX

“Mixed Oxidants” – mostly hypochlorite, ozone, and peroxides

Produced on-siteRequires electricity, rock salt, and waterSpecial equipment needed, difficult for

small systemsProduces less DBPs

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MIOX Process

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MIOX Salt for Brine Solution

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MIOX Manifold

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PLAN REVIEW

Required for ALL disinfection projects.

NSF certification

Contact regional engineer for specifics

If don’t know whether PR was done: Ask!

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Questions???Start Talking.