For nearly 70 years, the public water system in the Town of
Sterling, Massachusetts operated without incident, delivering clean water to
more than 2,000 homes. Because the water from the town’s well field was
clean and free of contamination, there was no need for a permanent disinfection
system. In fact, for many years the only treatment necessary was the addition
of potassium hydroxide to make the water slightly alkaline to minimize
corrosion in the distribution system.
That all changed in September 1999 when a storm caused by
Hurricane Floyd and the state’s increasing beaver population combined to
cause bacterial contamination in the town’s water supply. Though the
water has passed all tests for contamination since December 1999, the
Department of Public Works in Sterling began to study available disinfection
technologies and recently installed a new ultraviolet (UV) system that kills
coliform bacteria without the use of chlorine or other chemicals.
The Town of Sterling
Sterling, Massachusetts is a New England town of 7,000
people located about an hour’s drive west of Boston. The Department of
Public Works provides water to about two-thirds of the town’s homes, for
a total of more that 2,000 service connections. The remaining homes in the town
draw water from private wells.
The town’s water system was installed in the mid 1930s
and has been expanded several times since then. Today, it consists of four
wells, a 750,000- and a 250,000-gallon storage tank with a third under
construction, and 65 to 70 miles of pipe in the system. Three of the
town’s four wells are in a well field in West Sterling. The fourth well,
used only for backup purposes, is located several miles away.
Sterling’s water system is capable of delivering up to
1,500 gallons per minute (GPM) of water, though on average only about a third
of that capacity is used. The average daily water usage is about 500,000
gallons, but peak daily demand can be as high as 1.5 million gallons of water.
The remnants of Hurricane Floyd hit the Sterling area in
September 1999, bringing with it large amounts of rain. In the past, such
rainfall would have flowed away in the nearby Stillwater River without any
effect on Sterling’s well fields and water supply. However, the law of unintended
consequences took hold and Sterling was faced with its first contamination
problem resulting from new state regulations restricting the use of beaver
Beaver Trapping Restrictions Cause Water Problems
In 1996, citizens of Massachusetts voted to pass the
Wildlife Protection Act. As part of that act, animal trappers were prohibited
from using specific types of traps to catch beaver and other animals. The
result was an exponential growth in the state’s beaver population. Along
the Stillwater River, the animals built 17 dams within the first four miles
downstream from Sterling’s well field. The largest dam (located just 400
yards downstream from the well fields) was four to six feet high and in some
areas nearly 100 yards wide.
In the past, water from the Stillwater River flowed slowly,
but steadily, away from the well field, rarely getting closer than 200 feet
from the town’s wells. However, when Hurricane Floyd dumped several
inches of rain on the area in a matter of hours, the river got closer to the
well fields than ever before. Because of the network of beaver dams, the river
came within 20 feet of the wells.
Within days after the rainfall, water from Sterling’s
wells showed low levels of E. Coli and other coliform bacteria. The water
department’s first action was to immediately begin emergency chlorination
of the water system and issue a boil alert to town residents. Since the
Sterling Water Department had never chlorinated its water before, the emergency
procedures required employees to manually add the chemical to the water from
After five days of treating the water and flushing the water
distribution system to eliminate the bacterial contamination, the water was
once again safe for residents to use without boiling. However, continuing tests
showed occasional low levels of coliform bacteria in the wells, forcing the
town to continue the chlorination procedures.
Breaching Dam Ends Contamination
Three months after the initial contamination, the Sterling
Water Department received permission from the state to breach the two nearest
beaver dams and return the Stillwater River closer to its pre-hurricane levels.
As soon as the dams were breached and the water receded from the well fields,
the coliform contamination stopped. This process confirmed the suspicion that
because the water had been closer to the town wells, it was able to enter the
wells without undergoing the natural filtration that occurs when groundwater
passes through sand and gravel.
Despite the absence of contamination, the Massachusetts
Department of Environment Protection ordered Sterling to install a permanent
water disinfection system. Although chlorination was the most obvious choice,
the Superintendent of Public Works had recently attended a presentation focusing
on a small ultraviolet light (UV) disinfection facility recently installed at a
nearby municipal well supply and decided to look at that treatment technology
There were several factors that led the Sterling Department
of Public Works to look for other alternatives to chlorination. Among those
can be a dangerous chemical and its fumes could pose a serious risk to
employees if it were spilled or if the chlorine leaked.
chlorine is injected into water that contains organic materials, it can create
byproducts that may be carcinogenic.
systems are labor intensive and must be monitored closely.
residents have long enjoyed the taste of their water and disliked the taste
imparted to it by the temporary chlorination system.
raw water supply has low levels of iron and magnesium that are oxidized with
the addition of chlorine causing colored water and staining laundry and
UV Disinfection Is the Answer
The disinfection system that showed the most promise for
Sterling was ultraviolet (UV) light. Though UV has been used throughout Europe
for many years and is used in the United States in recreational vehicles and
other specialized applications, its use in municipal water systems has been
UV disinfection works by exposing microorganisms to
ultraviolet light at a specified intensity over a period of time. Once they
have been exposed, the bacteria are unable to reproduce and are therefore
unable to cause illness in humans.
When used in municipal water systems, UV disinfection offers
advantages such as
are no chemicals introduced into the town’s water supply, so the water
would continue to smell and taste as it has for the past 70 years.
disinfection systems are easy to operate and maintain. They are not labor
intensive—maintenance is as simple as changing a light bulb.
UV system would be safe for both residents and water department employees.
Though the use of UV disinfection is only now emerging in
the United States for the treatment of potable water, one of the few systems in
the country, designed by Fay, Spofford and Thorndike, Inc. of Burlington,
Massachusetts, had recently been installed in nearby Westford, Massachusetts.
After reviewing the results of that installation, the Sterling Department of
Public Works decided to engage the engineering firm of Fay, Spofford and
Thorndike to design and provide services during construction of a UV facility
to handle all three of Sterling’s main wells.
Town residents voted overwhelmingly to approve funding for
the new UV system. The Department of Public Works then prepared an application
for a subsidized construction loan through the state Department of
Environmental Protection (DEP) and the Massachusetts Drinking Water State
Revolving Loan Fund Program. That application was approved and the Town
received a zero percent interest loan for the full construction cost of the
project. The town and the engineers then set up and conducted a pilot study to
help determine if the iron and manganese would foul the UV light sleeves in the
With funding in place and the piloting successfully
completed, the engineers worked closely with DEP to design the system and
construction of the UV Treatment Plant was begun in February 2001. The UV
system provided by Severn Trent Services included a programmable logic
controller (PLC) design that allows water department employees to control and
monitor the wells, pumps, storage tanks and UV disinfection system remotely
from a personal computer in their office several miles away from the well
fields, or from a laptop computer through a dial-up telephone connection.
The project was lengthy and complex, but in the end the town
has ended up with a state-of-the-art disinfection facility and the residents
are once again enjoying the clean, safe, good tasting Sterling water that they
have enjoyed for the last seventy years.
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