The official report on the King’s Beach ultraviolet (UV) pilot program concludes that the technology “effectively treats day-to-day dry weather stormwater flows from the Lynn and Swampscott culverts.”

Water quality issues at King’s Beach date back nearly a century, when Stacey’s Brook, which flows into the beach, was converted into a combined sewer and stormwater system serving both Lynn and Swampscott. Issues have persisted since 1891, and until this year, King’s Beach ranked as the second-most polluted beach in Massachusetts.

In 1985, Lynn invested $65 million into a wastewater treatment facility and worked to ensure no sewer-to-storm drain connections continued to empty into King’s Beach. Similarly, Swampscott has moved with its source elimination work, with the projected investment of $10 million over several years to repair and line sewer mains and service laterals to prevent sewage from entering the storm drains and emptying into Stacey’s Brook culvert.

Installed on June 2, the UV project uses ultraviolet light to kill bacteria in stormwater before it reaches the ocean, aiming to provide a cleaner beach for Swampscott and Lynn residents. The project wrapped up a bit early, on Monday, August 18. 

“UV effectively treats day-to-day dry weather flows from the Lynn and Swampscott culverts,” concluded engineering consultant Kleinfelder. “The pilot’s objectives were met.” 

The UV pilot project was in operation for 59 days, approximately two months. It was designed to operate continuously during both dry and wet weather conditions throughout the summer, with the exception of large rainfall events. For rainfall events greater than a quarter of an inch, the system was designed to remain on but it would not be able to treat all flows. 

The report notes that the “effectiveness of the UV system is reduced during wet weather events, particularly in the first 24 hours, due to increased turbidity in the flow.”

The filtration system treated flows up to approximately 5 million gallons per day. 

“This report is reason for optimism when it comes to a permanent solution for King’s Beach,” said then-interim Town Administrator Gino Cresta, who was in charge of the town while the UV project was in operation. “Our town and Lynn have some work to do in arriving at a long-term strategy, but we now have solid data to inform those discussions.”

Since June 20, water treated through the UV system has consistently tested at or below 10 colony-forming units per 100 milliliters (cfu/100 ml), except on June 28, July 24, and July 26. The Massachusetts Department of Public Health specifies that safe waters have at or under 104 bacterial organisms cfu/100 ml.

Meanwhile, untreated water flowing from the Stacey’s Brook culverts in Lynn and Swampscott have often been above 104 cfu. The majority of the Massachusetts Department of Health’s weekly testing has shown the water quality as acceptable throughout the summer.

In-house testing for the pilot program was conducted daily at five locations, gathering data on untreated stormwater from Lynn and Swampscott individually, the combined flow before and after UV treatment, and ocean water approximately three feet deep near Swampscott’s access ramp.

The average UV Transmittance (UVT), a measure of how well light can penetrate water to inactivate bacteria, was approximately 87.5% in the combined Lynn and Swampscott flow. High UVT are ideal conditions for UV treatment, and typically UV treatment systems at wastewater treatment plants are designed for 55-65% UVT. Swampscott’s high numbers could result in less UV intensity needed to meet the same treatment levels. 

Lynn Mayor Jared Nicholson called this data the “most telling,” in a press release, as the number of exceedances for the bacteria threshold was only eight days, compared to 33 days in 2024, 43 days in 2023, 31 days in 2022, and 23 days in 2021. 

This is also a big difference when compared to the inches of rain, which typically raises bacteria count. According to data from the Beverly Regional Airport Station analyzed by the Swampscott Tides, 2025 saw 7.14 inches of rain in June, July and August. During the same time period there was 6.15 inches in 2024, 14.62 inches in 2023, 4.45 inches in 2022, and 13.23 inches in 2021. 

This means in 2025, there were 1.12 days of exceedances per inch of rain. In 2024, the number was higher at 5.36 days per inch, and 2022 was also recorded higher at 6.97 days per inch. 2023, at 2.94 exceedances per inch, and 2021, at 1.74 days per inch of rain, were more on par with this year’s results, even without a UV pilot project in place.

“UV can reduce the number of days King’s Beach is closed during a summer season even though there are other factors affecting beach closures beyond the discharge from the subject outfalls,” the study states.

“This gives us solid data to examine collaboratively with Swampscott as we try to identify the most effective and practical method to keep King’s Beach open to the public as often as possible,” Nicholson said. “The results show us that this technology is worth considering as a long-term option for King’s Beach.”

That said the report found that “maintenance and engineering issues must be addressed before implementing a permanent UV facility.”

A group of Swampscott and Lynn residents filed a legal notice citing noise concerns from the joint UV treatment project. The group alleges the pilot program violated noise limits, citing a commissioned study that recorded noise levels between 61 and 65 dB, “far in excess of the allowance of 10 dB above” ambient noise, as permitted by the Department of Environmental Protection.

The study did not note noise outside of the fact “blankets were affixed to all fencing that surrounded noise-making equipment.” Instead the results focused on tidal, wet weather and ocean impacts as operational performance-related issues.

Overall, the report concludes that UV treatment can increase the number of days King’s Beach remains open and safe for public use.

“King’s Beach is a treasure for Swampscott and Lynn, and it’s encouraging to see data showing a potential solution for keeping it open and safe,” State Rep. Jennifer Armini (D–Essex) said. “The UV pilot is a model of collaboration between communities when it comes to tackling problems that defy borders.”

Kleinfelder wrote that Swampscott and Lynn need to determine the next steps toward implementation but that studying continuous flow and level monitoring in both culverts for up to a year can increase the overall understanding. The engineering firm also suggested performing sediment analysis of material deposits as well as targeted wet weather testing for UVT. 

“For the first time, our family – and countless others – could safely swim at King’s Beach. This success is a testament to the effective UV treatment, which was secured through the collaboration and hard work of local, state and federal partners, advocacy organizations and community groups,” State Sen. Brendan Crighton (3rd Essex District) said. “As we finalize the report and program results, our continued partnership will allow us to make informed decisions about King’s Beach treatment options, ensuring the beach can be enjoyed for summers to come.”