Across California and Nevada,
implementation of the Stage 2
Disinfectants and Disinfection
By-product Rule is spurring utilities to
explore a range of strategies for improving
water quality in their water systems. Many
utilities are investigating ways of lowering
total organic carbon (TOC) in their water
or altering treatment plant processes to
reduce the production of disinfection
byproducts (DBPs). Unfortunately, many
of these treatment plant changes are large
capital projects that will take several years
The most common DBP, trihalomethanes
or THMs, is a function of both raw water
quality AND disinfectant levels and water
age. Thus, some utilities are also examining
ways in which operational and technological
changes in their distribution system
can lower DBP levels. In many cases, high
THM levels are present in only one part of a
water distribution system, most commonly
where water age is the highest or source
water quality is a challenge. By deploying
new technologies in distribution system
water storage tanks, a few utilities have
discovered that THM levels can be brought
20 SOURCE winter 2013
Lowering THM Levels and
Achieving Stage 2 DBP Rule
Compliance with In-Tank Aeration
Monterey, California, is a seaside town that
enjoys cool weather, picturesque beaches
and, for the most part, excellent water quality.
However, one part of the system, the Ryan
Ranch Business Park, faced water quality
challenges. Ryan Ranch was fed by three
wells separate from the rest of the Monterey
system, and treated water was stored in a
single 500,000 gallon above-ground, steel
storage tank. While raw water TOC levels
were low, the well water was known to
contain elevated levels of bromide.
Beginning in 2010, TTHM levels spiked, and
the running annual average for the Ryan
Ranch system rose dramatically (Figure 1).
The dominant THM species was bromoform.
After accumulating three quarters of elevated
levels, water quality managers calculated that
they needed to achieve a TTHM level of less
than 50 ppb for the June 2011 measurement
for the locational running annual average
(LRAA) to remain in compliance. Historical
estimates suggested that without a major
intervention, actual TTHM levels at that time
would be around 140 ppb.
The precise causes of the dramatic increase
in TTHMs in the Ryan Ranch tank were
uncertain, but several factors likely contributed
to the problem:
The combination of high temperatures and
low turnover likely led to thermal stratification
during some of the year. Thermal stratification
leads to high water age and high
rates of residual consumption, both of which
can elevate THM levels.
The use of source water high in bromine
likely stimulated the formation of brominated
THM species such as bromoform.
The tank had been periodically washed out,
but it had not been chemically cleaned to
By Peter S. Fiske, Ph.D., and Leslie Jordan
Figure 1. Historic TTHM levels (ppb) per quarter
at Ryan Ranch. Locational running annual
averages were expected to exceed the MCL
in Q2 2011.
Figure 2a. Interior conditions of the Ryan Ranch
tank prior to TRS.
Figure 2b. Locations where the interior coatings
had failed in the Ryan Ranch tank (these were
Figure 2c. Application of the chemical cleaning
agent to remove organic and inorganic
deposits on interior surfaces.