Water Treatment

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The Sebago Lake Water Treatment Facility is located on a 10 acre site near the intersection of Rt. 35 and Rt. 237 in Standish. This $23.4 million facility began treating water in February 1994 using ozone. Ozone is a powerful disinfectant that kills potentially harmful microorganisms and is 99.99% effective against viruses and Giardia. Treatment includes screening, ozonation, chlorination, and corrosion control. Also as a result of a citizen referendum, fluoride is added to the water at the plant to promote dental health.

		Why did the District build this facility?
		What is ozone, and how is it produced?
		How is the water treated with ozone?
		Is this kind of treatment safe for the environment?

Why did the District build this facility?
The federal Safe Drinking Water Act was amended in 1986 in response to concerns about drinking water quality throughout the country. The Act required that drinking water from all surface water supplies be filtered. The District applied for and obtained a waiver from this filtration requirement because of the exceptional water quality of Sebago Lake and the District's comprehensive watershed protection program. This saved customers about $20 - $50 million in capital costs required to construct a filtration plant.

Many other aspects of drinking water quality were addressed by the Safe Drinking Water Act amendments including new standards for disinfection. Disinfection is the destruction of disease causing organisms such as bacteria, viruses, and Giardia. The District decided to switch from chlorine to ozone for primary disinfection to meet these requirements. Ozone is a very powerful disinfectant and it works in much less time than required with chlorine. Utilizing chlorine alone to meet the new disinfection requirements would have increased the formulation of chlorinated by-products which at high amounts are potential health threats to water consumers.

This facility was the first free-standing ozonation plant to be built in New England and the first in the country to meet the disinfection requirement of the Surface Water Treatment Rule with ozone alone. A state-of-the-art laboratory is on site and conducts water quality testing for the lake, treatment process, and distribution systems.

What is ozone and how is it produced?
Ozone consists of three oxygen atoms joined together into a single molecule of ozone. It is produced by the discharge of electricity in air. Lightning produces ozone naturally by splitting oxygen molecules so they can recombine into ozone. This is similar to the process used at the plant to produce ozone.

Ordinary air is brought into the plant with air compressors and dried in two stages using refrigerant and desiccant air dryers. The dry air is then passed through one or more of the ozone generators. Inside the generator are hundreds of aluminum lined glass tubes called "dielectric tubes." The high voltage power is applied to the aluminum lining of each tube. Current passes from the tube, across a small air gap to a stainless steel sleeve. This current splits some of the oxygen molecules into separate oxygen atoms. These atoms then combine with other oxygen molecules to form ozone.

How is the water treated with ozone?
Up to 52 million gallons per day can be pumped from Sebago Lake through the ozone contact tanks. Each contact tank has five chambers where ozonated air can be bubbled up through the water. It is the contact of ozone with the water which accomplishes disinfection. The ozone concentrations are maintained to kill bacteria, viruses, and Giardia. By the time the water reaches the end of the contact tanks, the primary disinfection is complete and the ozone has converted back to oxygen.

The water then flows to a 1.5 million gallon clearwell tank. Chloramines are added in the pipeline to the clearwell to provide long lasting disinfection in the water mains. Other additives are also used to help reduce corrosion. Treated water flows from the clearwell into the transmission mains.

Is this kind of treatment safe for the environment?
To avoid adding ozone pollution to the lower atmosphere, blowers draw air out of the enclosed contact tanks through special equipment which converts any leftover ozone back to oxygen. The "ozone-free" air is then discharged to the atmosphere.

Large electrical generators are located at the facility so that pumping and treatment can continue even when the power fails.