remove biofilms. The presence of biofilms
and sediment provide additional organic
matter that can react with disinfectant to
In-tank aeration has been demonstrated
to be effective in substantially lowering
THM levels in finished water storage tanks.
In-tank aeration works by exposing water to
air. Volatile DBPs, such as THMs, evaporate
out of the water and into the air and escape
out of the tank. But while there are several
different aeration technologies, they differ in
size, cost and effectiveness.
For example, many engineers initially
considered bubble aeration systems for
THM removal. Bubble aeration systems are
attractive because they are a familiar technology
and the effects of bubble aeration
are easy to calculate: the amount of THMs
removed from the water is directly related to
the amount of air injected. However, bubble
aeration systems have two distinct disadvantages.
First, bubble aeration systems tend
to have high power consumption relative
to other systems. Second, bubble aeration
systems preferentially remove the most
volatile THM species (chloroform) and leave
behind the least volatile (bromoform), which
is the species of greatest health concern.
In contrast, spray aeration and some types
of surface aeration have roughly the same
level of effectiveness across all THM species:
bromoform is removed at roughly the same
rate as chloroform. But these systems have
one principal disadvantage: predicting
their level of effectiveness requires detailed
computer modeling and experimentation.
Operators at Monterey had considered
an in-tank aeration system to lower THM
levels, but the initial cost estimate they
received was not attractive. Complicating
the issue was the fact that there was only a
limited amount of power available at the
Ryan Ranch tank, and a conventional aeration
system would have required a substantial
electrical service upgrade.
Monterey decided to go with a staged
approach, using a combination of energyefficient
aeration and mixing combined
with a thorough tank cleanout and chemical
clean (Figure 2). The total system was
designed to use less than 30 amps at 120V,
and leave power to spare for other systems.
The combination of power constraints and
the high goal for THM reduction (~60%
reduction) left no room for error.
During the washout, crews discovered locations
where the interior coatings had failed.
These sites were cleaned and repaired.
The chemical cleaning step involved the
application of a proprietary chemical solution
designed to remove the organic and
inorganic deposits on the walls and floor
of the tank. The combination of washout,
repair and chemical cleaning was designed
to bring the tank interior as close as
possible to its original, as-designed condition.
Installation and start-up of the in-tank
aeration system was completed on June 23,
2011, just five days before the Q2 compliance
sample was to be taken.
On July 11, 2011, the staff at Monterey
received their results: TTHM levels were
measured at 49.2 ppb (~65% reduction),
which brought their LRAA to just below the
MCL. While this was a great initial result,
operators at Monterey were concerned
that THM levels might rise again once the
system stabilized and water age increased.
Subsequent results (Figure 3) allowed
everyone to breathe a big sigh of relief.
THM levels remained low for the rest of
the summer. Measurements taken at the
compliance site in the system (Upper Ragsdale
Court) also showed the direct benefits
of the in-tank aeration system (Figure 4):
THM levels remained below 40 ppb for
subsequent quarters of measurement.
It is impossible to know with certainty which
parts of the project made the greatest contribution
to the reduction of THMs at the Ryan
Ranch tank. Mixing, aeration and chemical
Figure 2d. Installation of the submersible mixer.
Continued on page 23