The Portland Water District recently completed a project that added new treatment at its Sebago Lake Water Treatment Facility. The facility treats water from Sebago Lake to make it safe for drinking, cooking, and bathing for more than 200,000 people in the Greater Portland area. Since the facility was built in 1993, water from Sebago Lake has been disinfected using a combination of ozone (a form of oxygen) and chloramines (a chlorine-based compound).

In 2014, UV light treatment was added as an additional form of disinfection. Many of you are probably familiar with the terms “ultra-violet” or “UV” from buying sunglasses, but how and why is UV light used in water treatment?

UV light is a form of energy that is similar to light that humans can see but is invisible and contains more energy. The sun is the major source of UV light in nature. The energy emitted by the sun contains visible light rays, UV rays, and infrared rays (heat). UV light can also be man-made, like the lamps used in tanning beds. As many know, UV rays cause damage to human skin cells, resulting in suntan, sunburn, and sometimes skin cancer, and that is why we wear sunscreen to protect ourselves.

In addition to damaging human cells, UV light also damages other types of cells making it useful as a water disinfectant. It works to make water safe for drinking by penetrating potentially harmful cells (bacteria, viruses, and other germs) and destroying their DNA. With its DNA destroyed, it is impossible for a cell to reproduce which makes it unable to infect people and make them sick.

So now we know the “how” of UV disinfection, but what about the “why?” If the water is already being treated and is safe for people to use, why add another type of treatment? The answer is as ongoing research across the country identifies new contaminants, treatment must evolve to address the risks they may pose. The Environmental Protection Agency made a new rule requiring utilities that use surface water, such as a lake, for drinking water to install additional treatment to protect against cryptosporidium (crypto) infection.

Crypto is a microscopic parasite that can cause serious illness and is found in human and animal fecal matter. It can be found in recreational lakes as well as pools and hot tubs that have not been properly disinfected. Because crypto includes a durable microscopic shell – or cyst – form, it is more resistant to common disinfection methods such as chlorine. UV treatment has proved to be very effective at destroying it. Although crypto has not been detected in the water we pull from Sebago Lake, there is still a risk that it could be present due to people swimming and wildlife using the lake. The addition of UV treatment will increase drinking water disinfection to a level where a crypto infection is nearly impossible!

The construction project is complete and a new treatment chamber has been installed that houses two units equipped with 84 UV bulbs set to the optimum wavelength for disinfection. As water flows past the bulbs, the UV rays will destroy bacteria, crypto, and other potentially harmful cells. Drinking water from Sebago Lake will continue to be treated with ozone and chloramines as well, so it will be disinfected in three different ways. In fact, along with the addition of UV treatment, the ozone system is being upgraded to a high-efficiency system that will save electricity and provide reliable ozone treatment into the future. With the completion of the project, the Sebago Lake Treatment Facility will be the second largest UV facility in New England and another barrier of protection will be added to safeguard public health.

Laurel Jackson is a water resources specialist at the Portland Water District. She can be reached at ljackson@pwd.org.