HomeMy WebLinkAboutTTHM Removal Evaluation_PresentationTrihalomethanes Removal
Evaluation
TETRA TECH
Introduction & Purpose
Background With the onset of EPA’s Stage 2 Disinfectants and
Disinfection Byproducts Rule, the Town was proactive
in investigating and implementing process
modifications to enhance treatment efficiency and
effectively reduce regulated DBPs .
Motivation Although the system is in compliance with the Stage 2
rule, the potential for exceedance still exists
particularly of total trihalomethanes (TTHMs) during
the summer.
Purpose The purpose of this work was to assist the Town to
develop and evaluate alternative DBP control strategies
and provide recommendations for an effective DBP
control strategy.
2
How Do DBPs Form?
Free Chlorine
(Cl2)
Dissolved Organics
(TOC)
Temp Br
Time
Factors Contributing to Higher
DBP Formation
An unintended consequence of chemical disinfection is
the formation of DBPs.
Disinfection Byproducts
(DBP’s)
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Trihalomethanes (TTHMs)
Types of TTHMs:
Chloroform
Dichlorobromomethane
Dibromochloromethane
Bromoform
Maximum Contaminant Level:
80 μg/L
Haloacetic Acids (HAA5)
Types of HAAs:
Dichloroacetic Acid
Trichloroacetic Acid
Chloroacetic Acid
Bromoacetic Acid
Dibromoacetic Acid
Maximum Contaminant Level:
60 μg/L
Regulatory compliance is based on a
locational running annual average (LRAA).
Types of DBPs
TETRA TECH 4
DBP Control Strategies
DBPs
Flushing
Modified Disinfection
‒Move Cl2 location
‒Reduce Cl2 dose and/or time
‒Use an alternative disinfectant
Remove DBP precursors (aka TOC)
‒Coagulation Optimization
‒GAC Adsorption
‒Biological Active Carbon (BAC)
‒Ozone-BAC
‒MIEX / Ion Exchange
‒Membranes
Remove DBP after formation
‒Aeration
‒Biofiltration
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Water Quality and Treatment Goals
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Constituent Goal Basis Concentration
Goal
Goal
Achieved?
Treatment
Requirement
TTHM 80% of MCL 64 µg/L No 40% to 50%
HAA5 80% of MCL 48 µg/L Yes Goal Met
TOC Meet DBP
Goals ≤ 2.0 mg/L No 50% to 65%
Technologies Identified & Evaluated
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Spray Aeration
GAC/BAC
Ozone BAC
Ion Exchange
Spray Aeration Treatment Integration
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Spray Aeration Treatment Overview
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Two Key Considerations:
TTHM Speciation
1
TTHM Re-Formation
2
TTHM Speciation
TETRA TECH 10
60%27%
12%
2%
Chloroform Bromodichloromethane
Dibromochloromethane Bromoform
WTP SITE
BOUNDARY
Spray Aeration Treatment Example
CUSTOMERSTOTAL REGULATED DBPS (µg/L)CONTACT TIME (DAYS)
DISINFECTION
80 µg/L THM Limit
TTHM FormationTTHM Formation
TETRA TECH 11
WTP SITE
BOUNDARY
Spray Aeration Treatment Example
AERATIONTOTAL REGULATED DBPS (µg/L)CONTACT TIME (DAYS)
80 µg/L THM Limit
DISINFECTION
TTHM Removal
CUSTOMERS
TETRA TECH 12
WTP SITE
BOUNDARY
Spray Aeration Treatment Example
AERATIONTOTAL REGULATED DBPS (µg/L)CONTACT TIME (DAYS)
80 µg/L THM Limit
DISINFECTION
CUSTOMERS
AERATION
TETRA TECH 13
GAC/BAC Treatment Integration
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GAC/BAC Treatment Overview
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Two Key Considerations:
Empty Bed Contact Time
1
Carbon Change Out
Frequency
2
Biologically Active Carbon (BAC)
Absorption Biological
Removal
A
B
C D
E
F
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Ozone-BAC Treatment Integration
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Ozone-BAC Treatment Overview
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Two Key Considerations:
Empty Bed Contact Time
1
Biodegradable TOC
Fraction
2
Ion Exchange Treatment Integration
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Fixed Bed Ion Exchange Overview
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Two Key Considerations:
Effectiveness
1
How Frequently Does it
Have to be Regenerated
2
Treatment Performance
TETRA TECH 21
Water Quality/
Treatment
DBP Precursor
(i.e. TOC)
Removal
TTHM Reduction Goal Achieved?
Spray Aeration Does not Remove
TOC 20% to 40%Mostly
GAC
Adsorption 30% to 70%30% to 70%Yes
BAC 25% Typical
(5% to 60% Range)8% to 50%Possibly
Ozone-BAC 25% Typical
(5% to 60% Range)15% to 50%Possibly
Ion Exchange 30% to 60%30% to 60%Yes
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Conceptual Cost Opinions
Alternative at 2 MGD
Capacity
Construction
Cost Opinion
($)
Unit
Construction
Cost Opinion
($/gal)
Relative
Operating cost
Spray Aeration System
within Clearwell $770,000 $0.40/gal Minimal
$$$$
Spray Aeration System
within Bear Hill and
Scituate Hill Tanks
$1,600,000 $0.80/gal Minimal
$$$$
GAC/BAC Filtration $2,000,000 $1.00/gal Moderate
$$$$
Ion Exchange $3,900,000 $1.95/gal Moderate
$$$$
Ozone-BAC $5,800,000 $2.90/gal Moderate/High
$$$$
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Recommendations
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Phase II: Organic Removal
‒Installation of organic removal technology capable of removing 50%to
65% of the combined filtered water TOC levels.
‒Install a 1.5 to 2.0 MGD expandable to 3.0 MGD GAC or ion exchange
technology.
‒GAC could be allowed to convert into biological mode during the higher
quality winter/spring months.
‒If GAC/BAC is selected, the design could be phased to allow the
subsequent installation of ozonation, if required at a later phase.
Phase I: Spray Aeration
‒Perform bench-top evaluation of spray aeration.
‒Installation of spray aeration within the clearwell.
‒Installation of additional spray aeration at the Bear Hill and Scituate
Hill tanks or proceed with Phase II.
Q&A