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 Disinfectants
Types of Chlorine
Chemistry
Application / Dosages
Contact Time v. Residual Cl
How to disinfect well – e.g. calculating CTs, &
How to disinfect a well.
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DHS Drinking Water Program
Purpose of Disinfection
Inactivation of Pathogens
Bacteria – Coliforms, Campylobacter
Viruses – Norwalk, Hepatitis A/B
Giardia
(parasitic protozoa)
Cryptosporidium (parasitic protozoa)
Regulatory Requirements
Residual Maintenance vs. Disinfection
Why does your system use disinfectant??
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DHS Drinking Water Program
Common Disinfectants
(In order of prevalence)
CHLORINE
Ultraviolet Light (UV)
Ozone (O3)
Mixed Oxidants (MIOX)
Chloramines (Cl2 plus ammonia)
DHS Drinking Water Program
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Chlorine
Types (All must be NSF approved):
Gas:
Solid:
Liquid:
Larger systems, safety concerns,
99.5 % available chlorine
Calcium Hypochlorite (Ca(OCl)2)
15% to 70% available chlorine
Sodium Hypochlorite (NaOCl)
5% to 15 % available chlorine
Common bleach = 5.25%
(Shorter half-life at higher concentrations)
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DHS Drinking Water Program
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 50/50
Colder water temps and high pH make
disinfection less efficient.
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DHS Drinking Water Program
Relative Amount of Hypochlorous
Acid & Hypochlorite Ion
1
OCl-
HOCl
Fraction of Free Chlorine
0.8
0.6
0.4
0.2
0
3
4
5
6
7
pH
8
9
10
11
12
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DHS Drinking Water Program
Effects of pH on Residual Type
Free Chlorine
50%
OCl-
90%
OCl-
50%
HOCl
10%HOCl
pH
7.5
8.5
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DHS Drinking Water Program
Strengths of Residual Chlorine
Compounds
OCl(80x weaker)
Chloramines (combined)
(60x – 200x weaker)
Increasing Strength
HOCl
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Interfering Agents
Iron (Fe)
Manganese
(Mn)
Turbidity
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DHS Drinking Water Program
When to Increase Chlorine
Dosage
Increase
Chlorine
Dose When
You Have:
Increase in:
• pH
• Turbidity
• Fe & Mn
Decrease in:
• Temperature
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DHS Drinking Water Program
Calculating Chlorine Residual
Chlorine Dose
- Chlorine Demand
Chlorine Residual
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DHS Drinking Water Program
Calculating Chlorine Residual
(continued)
Free Chlorine Residual
+ Combined Chlorine Residual
Total Chlorine Residual
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DHS Drinking Water Program
Breakpoint Chlorination Curve
100
100
88 minutes
90
83 minutes
80
70
Zone 1
Zone 2
Combined
Chlorine Only
e.g., NH2Cl
Destruction of
Combined
Residual
Chlorine
60
50
40
Zone 3
60
Free Chlorine
40
30
20
Time for 99% inactivation of
B. metiens spores [minutes]
mg
Free Chlorine Residual [ /L]
80
20
Breakpoint!
10
2.7 minutes
0
20
40
60
80
100
Chlorine Dose [ mg/L]
120
140
0
160
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DHS Drinking Water Program
Same Breakpoint Curve
(with Nitrogen Compounds)
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DHS Drinking Water Program
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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DHS Drinking Water Program
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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DHS Drinking Water Program
Why Is Residual Maintenance
NOT “Disinfection”?
Cl
Cl
0.2 ppm Cl
residual
throughout
distribution
system
Cl
Cl
Cl
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DHS Drinking Water Program
Why Isn’t Residual Maintenance “Disinfection”?
Disinfection requires
Contact Time
Contamination
Point
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DHS Drinking Water Program
Chlorine Applications
Continuous Disinfection
Must be proportional to flow
Shock Disinfection
One-time dose
Well or reservoir disinfection
Repairs
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DHS Drinking Water Program
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 or
50 ppm for 6 hours
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DHS Drinking Water Program
Well Disinfection - Shock
50 ppm for 24 hours
Mix chlorine with water before
introducing into well
Re-circulate if possible
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DHS Drinking Water Program
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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DHS Drinking Water Program
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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DHS Drinking Water Program
Disinfection of Storage Tanks
May be necessary if sampling indicates
Access to tank (sight only, arm’s length,
or physical entry)
Tank drainage
Where to?
Discharge may be regulated
Pump out if no drain
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DHS Drinking Water Program
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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DHS Drinking Water Program
Chlorine Equipment
Gas cylinders (and safety equipment)
Chemical feed pumps
Tablet dispensers / Erosion chlorinators
Test kit
DPD-type, color wheel or digital
Orthotolodine (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
NaOCl
100%
SALT
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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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DHS Drinking Water Program
CT Calculation
CT
=
CT
= [mg/L] X [minutes]
= a number (no units expressed)
Cl2 residual multiplied by
contact time
Example: 0.2 mg/L times 30 minutes
CT = 6
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DHS Drinking Water Program
The Magic Number 6
CT required = 6
30 minutes * 0.2 ppm Cl
Assumptions: Water Temperature ≥ 10oC (50oF)
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DHS Drinking Water Program
Required CT Values
GW vs. SW
Groundwater –
Minimum CT = 6
Adequate for bacteria / virus kill
Surface Water –
Required CT varies with:
pH, temp, and log-inactivation requirement
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DHS Drinking Water Program
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 ideal
I.e., 100% credit for pipe volume
Storage tanks likely have short-circuiting
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DHS Drinking Water Program
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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DHS Drinking Water Program
Chlorination Concerns
Disinfection by-products (DBPs)
THMs and HAA5s
DBP production encouraged by:
High chlorine levels
Long contact time
Warm weather
High source water TOC
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DHS Drinking Water Program
UV
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DHS Drinking Water Program
Ultraviolet Light
Effective non-chemical disinfectant
Suitable for small systems with few
distribution lines
Must meet plan review requirements:
Minimum dosage 38 mWsec/cm2
Intensity monitor
Automatic shut-off
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DHS Drinking Water Program
UV Pros & Cons
Advantages:
No chemicals, therefore no DBP’s
No contact time required
Simple
Low operation & maintenance cost
Disadvantages:
No residual that protects distribution system
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DHS Drinking Water Program
Ozone (O3)
Disinfection and taste/odor control
Produced by electricity and air or pure O2
Inactivates Cryptosporidium
Must be generated on-site
Leaves minimal disinfectant residual
Costs 4 times more than conventional
disinfection, yet cheapest LT2 option
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DHS Drinking Water Program
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-site
Requires electricity, rock salt, and water
Special equipment needed, difficult for
small systems
Produces less DBPs
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DHS Drinking Water Program
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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DHS Drinking Water Program
Questions???
Start Talking.
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