Down With DBPs

New England town takes proactive approach to reducing disinfection byproducts in its drinking water

By Ben Rollins, Tim Barber, and Steve Sullivan

For its ability to produce safe drinking water and reduce the spread of waterborne disease, the U.S. Centers for Disease Control has hailed disinfection as one of the greatest public health achievements of the 20th century. However, when reacting with organic matter, traditional disinfectants such as chlorine can produce harmful disinfection byproducts (DBPs) that water treatment facilities need to mitigate in order to comply with maximum contaminant levels (MCLs).

Dartmouth, Massachusetts, already had taken several actions to reduce DBPs in its drinking water. But when the town faced increased DBP levels despite system upgrades — as well as an emergency facility shutdown and an expensive water purchase — it decided to take a more holistic look at its entire water system. After performing water quality sampling for a year and reviewing distribution system storage, Dartmouth now has a roadmap for implementing needed improvements, reducing its dependency on purchased water, and working proactively toward ensuring future water supply rather than reactively responding to emergencies.

Meeting Demand and MCLs

Settled in 1652, the Town of Dartmouth, Massachusetts, is a coastal community with a 2020 population of nearly 34,000. The Dartmouth Water Division supplies drinking water to more than 24,000 residents from 14 groundwater wells that feed three water treatment facilities. Nearly 320 km (200 mi) of water main connect the water sources to Dartmouth’s water consumers. There are five water storage tanks (WSTs) in the distribution system, four of which are located in the town’s high-service area, with the fifth tank in the southern part of town (see Figure 1).

The town’s average daily demand is approximately 9.5 ML/d (2.5 mgd). Dartmouth purchases water from a neighboring community during periods of high demand.

The treatment process is similar at all three facilities. Each maintains and operates pressure vessels for iron and manganese removal, along with chemical treatment. They use sodium hypochlorite as the primary disinfectant, then mix sodium hypochlorite and ammonium sulfate to form chloramines for the secondary disinfectant as the water leaves the facilities. The utility changed the secondary disinfectant from free chlorine to chloramines in April 2021.

The U.S. Environmental Protection Agency (EPA) Stage 2 Disinfection Byproduct Rule (DBPR), which became effective in 2006, focuses on monitoring for and reducing concentrations of two classes of DBPs: total trihalomethanes (TTHM) and haloacetic acids (HAA5). For community water systems, Stage 2 DBPR changes the compliance requirements from a systemwide running annual average to a locational running annual average (LRAA). This regulation requires that Dartmouth test for DBPs at four sampling sites each quarter.

The MCLs for TTHM and HAA5 are 80 ppb and 60 ppb, respectively. Since the update from Stage 1 to Stage 2 DBPR, the town has experienced several MCL exceedances and entered into two Administrative Consent Orders (ACOs) with the Massachusetts Department of Environmental Protection (DEP).

Mitigating DBPs

DBP concentration is dependent on the level of organics already in the water, the chlorine concentration applied to the water for disinfection, and the reaction time. Increasing all three of these factors typically increases the production of DBPs. Theoretically, a water treatment facility can reduce DBP concentrations by reducing the organics in the water, the applied chlorine dosage, or the reaction time. Facilities can treat DBPs after production as well; for example, TTHMs are volatile organic compounds that can be removed via aeration.

Even before it made its first LRAA calculations for DBPs, Dartmouth took several actions to reduce DBPs in the distribution system, including

• installing mixing systems in three of the town’s five WSTs;
• increasing pointed flushing in high-water-age/low-water-use areas, including the Reed Road area;
• reducing untreated water flow from wells with relatively higher total organic carbon (TOC); and
• reducing chlorine application at the Old Westport Road facility, which has high levels of organics, during the lower-demand winter months.

Dartmouth calculated the first DBP LRAAs in August 2013. There was one TTHM MCL exceedance at the Reed Road sampling site, where the LRAA was about 86 ppb.

Massachusetts DEP issued the first ACO in March 2014, and within 3 months, Dartmouth and its consultant, Weston & Sampson (Reading, Massachusetts), completed an evaluation to reduce TTHM concentrations in the distribution system on immediate, short-, and long-term bases. The team identified several long-term options to help achieve this goal.

Dartmouth experienced two more MCL exceedances in early to mid-2015 and entered into a second ACO in 2016. The town then took a two-pronged approach to tackle its TTHM issue: installing WST mixing and aeration systems at the Allen Street WSTs and converting from free chlorine to chloramines at all three treatment facilities. The capital costs of this approach were relatively low compared to the other options for reducing TTHM.

From late 2017 to mid-2018, the team worked on designing these projects and secured funding through the state’s Drinking Water State Revolving Fund. Dartmouth installed the Allen Street tank mixing and aeration systems in 2019 and did construction work for chloramine conversion from mid-2019 to early 2021. Before the town brought these systems online, the typical TTHM LRAA results at the DBP sampling sites ranged from 60 to 80 ppb. After system startup, the TTHM LRAA dropped to about 20 to 40 ppb (see Figure 2).

However, in May 2021, the town recorded a HAA5 concentration at the Reed Road sampling site of 110 ppb — nearly double the MCL and exceeding the limit for the second time. This one result pushed Dartmouth over the MCL for four consecutive quarters (see Figure 3), prompting public notifications to consumers. Dartmouth experienced another headache in October 2021, when there was a positive bacteria hit at the Old Westport Road facility. The town had to take this source offline and make an emergency water purchase to meet demand.

Generally speaking, it is expensive to react to emergency scenarios because they need to be fixed immediately. When the town shut down the Old Westport Road facility in 2021, it had to spend more money on purchased water to meet demand. In 2022, the town estimated that to produce 3.8 million L (1 million gal) of its own water cost approximately USD $800, compared to more than $8,000 to purchase the same amount of water. Due to the 2021 emergency scenario, the town spent upward of $1.4 million — $800,000 more than its annual water-purchase budget — to buy water. This meant that it had $800,000 less to spend on critical capital improvement projects.

These issues — along with relatively high organics, iron, and manganese in some of the untreated water sources; hydraulic and permitting constraints; and some color observations from consumers — led Dartmouth to take a step back and examine its water system holistically.

Evaluating Everything

The team embarked on a comprehensive water system review, with a focus on water quality and quantity. Dartmouth’s goals were to reduce its water supply reliance on others, bolster its town-owned sources and distribution system infrastructure, and be proactive instead of reactive. A proactive approach would allow Dartmouth to prepare for the future, prioritize needed capital improvement projects, and put plans in place to fund them.

Weston & Sampson drew on several resources to evaluate Dartmouth’s water quality and treatment system. To get a clearer picture of the water quality in the town’s distribution system, Water Division employees completed sampling over the course of a year. The consultant reviewed the data collected along with historical data provided by Dartmouth. Its observations included the following.

• The town should reduce the use of relatively high-TOC wells and/or consider organics removal.
• Water purchased had relatively higher levels of DBPs and organics compared to the town’s own sources. The town should bring the Old Westport Road facility back online and reduce its purchased water.
• All three facilities need chlorine contact (CT) and backwash supply storage tanks.
• Buildup of iron and manganese in the untreated water mains that come into the facilities hinders production.

Scrutinizing Storage

In general, DBP concentration increases with water age. Therefore, an effective strategy to reduce DBPs in an area of the distribution system is reducing the water age locally and bringing in fresher, relatively new water from sources. Viable ways of doing this are reducing storage volume and increasing WST turnover. The consultant did a preliminary review of the water storage volume in Dartmouth’s distribution system, with an emphasis on the Reed Road area and northern portion of town.
There currently is 23.8 million L (6.3 million gal) of storage in the town’s five WSTs and 22.7 million L (6.0 million gal) in the town’s high-service area, but only 14.8 million L (3.9 million gal) of this supply is considered usable. For water to be usable, it must be stored at an elevation that produces a minimum street level pressure of 140 kPa (20 lb/in.2) at the highest elevation in the service area.

A preliminary review of these primary storage components found that the town is short in equalization storage but sufficient in fire and emergency storage. During the system evaluation, Dartmouth noted that to maintain sufficient water pressure in certain areas of town, WST water levels need to remain at or near the higher end of operating ranges.

The location and size of the Old Fall River Road WST in the northern section of town pose a water quality challenge. Along with the Cross Road WST, these tanks do not get the recommended 3-day turnover. As the Old Westport Road facility turns on and feeds water into the distribution system, these tanks get filled first and cannot draw down as the Allen Street WSTs do. High volume, low tank turnover, and high water-age result in relatively higher levels of DBPs.

In September 2022, the team decided to take Old Fall River Road offline temporarily to determine its effect on water quality in the area. Once they did, the HAA5 concentration was lower, as seen in the Reed Road sample results, and it continued to be relatively low. In February 2023, the town brought the Old Westport Road facility back online, which reduced the HAA5 concentration even more. The consultant recommended further investigation into the town’s storage and turnover rates to help Dartmouth keep its water age to a minimum.

Investing in the Future

The full water-system evaluation informed the consultant’s recommendations for operational and capital improvements (see Table 1, and Table 2).

Operational improvements. Recommendations included taking the Old Fall River Road WST offline, which was a effective, as DBP concentration was reduced. The town also brought the Old Westport Road facility back online to reduce reliance on purchased water and bring relatively lower-DBP water into the distribution system.
Another key to Dartmouth’s success is focusing on individual wells and pumps. The consultant recommended that the town conduct annual inspections and make source well and pump improvements to maintain the necessary water flow to the treatment facilities. The town could meet the average daily demand if it were able to withdraw the amount authorized, but hydraulic and permitting constraints currently lower individual well production (see Figure 4). Annual inspections would provide documentation of well and pump conditions and help prioritize well maintenance to ensure the town is using its available sources fully.

Capital improvements. At one facility, a CT pipe already was installed during the chloramine conversion project. However, the other two facilities still need CT to ensure 4-log virus treatment. Achieving CT would allow the town to maintain facility output and keep the facilities online when there are positive bacteria hits in the untreated water.

The mains that connect wells to facilities also are critical. At both Chase Road facilities, these mains are undersized and restrict flow; at one of these facilities, the mains enter the water treatment process at different locations. The consultant advised the town to upsize the mains and combine pretreatment processes for these two facilities.

The consultant also recommended dedicated backwash supply storage at all three facilities, which currently use distribution system water for backwashing. During high-flow backwash steps, facilities experience complete flow reversals that might be interfering with finished-water chemical dosing. Installing a separate, dedicated backwash supply would allow facilities to feed the distribution system at a more regular rate, making it easier to maintain proper chemical feed for chloramination.

Ensuring Continued Supply

Dartmouth now has a blueprint to implement needed improvements and reduce its dependency on purchasing water from others. Since the water system evaluation was completed in 2022, the town has prioritized adding CT and backwash supply storage tanks. At press time, Dartmouth has accepted public bids, entered into a contract with a General Contractor, and is in the early stages of construction for a CT pipe and a backwash supply storage tank at one of its facilities.

Recently, the town also completed a water and sewer rate study, in which it included all its high-priority capital improvement projects along with their estimated construction costs. Dartmouth currently is planning ahead to fund these projects.

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Ben Rollins, P.E., is a Project Manager in the Portsmouth, New Hampshire, office of Weston & Sampson (Reading, Massachusetts).

Published in Water Environment Technology, February 2025.