Trihalomethanes Removal Evaluation June 26, 2019

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. 1. The 2004-2006 modifications worked since the data demonstrates that you are in compliance. 2. But at some locations the concentration are approaching the MCL particularly of total trihalomethanes (TTHMs) during the summer season when higher TOC levels and temperatures can drive their formation. TETRA TECH

How Do DBPs Form? An unintended consequence of chemical disinfection is the formation of DBPs. Free Chlorine (Cl2) Dissolved Organics (TOC) Disinfection Byproducts (DBP’s) Temp Br Time Factors Contributing to Higher DBP Formation TETRA TECH

Types of DBPs 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). TETRA TECH

DBP Control Strategies 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 DBPs TETRA TECH

Treatment Requirement Water Quality and Treatment Goals Constituent Goal Basis Concentration Goal Goal Achieved? Treatment Requirement TTHM 80% of MCL 64 µg/L No 40% to 50% HAA5 48 µg/L Yes Goal Met TOC Meet DBP Goals ≤ 2.0 mg/L 50% to 65% TETRA TECH

Technologies Identified & Evaluated Spray Aeration GAC/BAC Ozone BAC Ion Exchange TETRA TECH

Spray Aeration Treatment Integration TETRA TECH

Spray Aeration Treatment Overview Two Key Considerations: TTHM Speciation 1 TTHM Re-Formation 2 TETRA TECH

TTHM Speciation TETRA TECH

Spray Aeration Treatment Example 80 µg/L THM Limit TOTAL REGULATED DBPS (µg/L)  TTHM Formation WTP SITE BOUNDARY  TTHM Formation CUSTOMERS DISINFECTION CONTACT TIME (DAYS) TETRA TECH

Spray Aeration Treatment Example 80 µg/L THM Limit TTHM Removal  TOTAL REGULATED DBPS (µg/L) WTP SITE BOUNDARY CUSTOMERS DISINFECTION AERATION CONTACT TIME (DAYS) TETRA TECH

Spray Aeration Treatment Example 80 µg/L THM Limit TOTAL REGULATED DBPS (µg/L) WTP SITE BOUNDARY CUSTOMERS DISINFECTION AERATION AERATION CONTACT TIME (DAYS) TETRA TECH

GAC/BAC Treatment Integration TETRA TECH

GAC/BAC Treatment Overview Two Key Considerations: Empty Bed Contact Time 1 Carbon Change Out Frequency 2 TETRA TECH

Biologically Active Carbon (BAC) Absorption Biological Removal F E C D B A TETRA TECH

Ozone-BAC Treatment Integration TETRA TECH

Ozone-BAC Treatment Overview Two Key Considerations: Empty Bed Contact Time 1 Biodegradable TOC Fraction 2 TETRA TECH

Ion Exchange Treatment Integration TETRA TECH

Fixed Bed Ion Exchange Overview Two Key Considerations: Effectiveness 1 How Frequently Does it Have to be Regenerated 2 (1) How frequently do we have to regenerate it (2) TETRA TECH

Treatment Performance 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% Yes BAC 25% Typical (5% to 60% Range) 8% to 50% Possibly Ozone-BAC 15% to 50% Ion Exchange 30% to 60% ≈ Make this into a decision flow chart ≈ TETRA TECH

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 GAC/BAC Filtration $2,000,000 $1.00/gal Moderate Ion Exchange $3,900,000 $1.95/gal Ozone-BAC $5,800,000 $2.90/gal Moderate/High Based on cost curves, vendor based estimates, and experience with analogous systems. 15% Contractor Overhead & Profit, 7% General Requirements, mobilization and bonding 30% Construction Cost Contingency TETRA TECH

Recommendations Phase I: Spray Aeration Phase II: Organic Removal 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. 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. Decreased your organic removal capacity with integration of aeration with GAC To reduce the operating cost of any of the phase II alternatives, you could operate the additional treatment seasonally. TETRA TECH

Q&A June 26, 2019