Utility Reduces DBPs While Controlling Algae Growth
by Angela Cheung and Brent Howe
Sunlight-blocking basin covers reduce chlorine usage, yielding lower DBPs and cost savings too.
In the fall of 2004, the Operations Division of the
Santa Clara Valley Water District (the District)
started a program called the Operations Council on
Excellence. The program was created to support
the District’s vision of a cleaner, greener, and leaner
operation. At the time, two of the District’s three
water treatment plants, Penitencia and Santa
Teresa, were undergoing construction to replace
chlorine with ozone for primary disinfection. With the
switchover to ozone, the plan was to reduce or
eliminate chlorine feed at the head of the plant, thus
reducing the formation of disinfection byproducts
(DBPs). The challenge was that any attempts in the
past to reduce chlorine feed resulted in algal growth
on the basins walls, launders, tube settlers, etc. At
times, algal growth was significant enough to have
caused in-plant taste and odor production.
This case study describes how the District met the
challenges it faced in controlling algal growth while
reducing DBP formation. The water retailers served
by the District were expecting substantial reductions
in DBPs from ozonation. If pre-chlorination were to
continue at the same rate for algae control, the
benefits of ozonation would be less pronounced.
District staff knew that DBP reduction with algae
control could be accomplished in two ways: through
the use of alternative oxidants to chlorine or by
creating a physical barrier to block the sunlight that
algae requires to grow. Both solutions were found to
be used effectively at water treatment plants
contacted as part of the District’s investigations.
Ultimately, District staff chose to go with basin
covers because the cost of the basin cover
installation can be recovered in about five years
through hypochlorite reduction. The idea also
encompasses the District vision of “getting cleaner,
greener and leaner.”
District staff evaluated three types of basin covers:
1) buoyant fabric covers floating directly on the
water, 2) high-strength fabric covers tensioned
across a basin opening and suspended above the
water, and 3) covers consisting of a structure
spanning the basin opening. Fabric, fiberglass and
aluminum cover materials were considered. All of
the cover styles would block sunlight and thus
control algae. Similarly, all the cover styles were
manufactured with materials that were NSF 61
approved for drinking water applications. The
Penitencia plant team expressed a strong
preference for retractable, structurally supported
covers because the basins are frequently taken in
and out of service, and this cover style is the easiest
for basin viewing by the operations staff and
accessing by the maintenance staff.
“The structurally supported covers are easier to
open and close than the other options we
considered”, Steve Cancilla, Senior Plant
Maintenance Mechanic.
Structurally supported covers block sunlight over tube settlers to control algae
Fabric covers retract to expose tube settlers
Structurally supported covers consist of a retractable
fabric sheet tensioned over supporting aluminum
arches. A decision was made to install these covers
on nine 21' x 31' openings over the plant’s tube
settlers which were especially prone to algae
attachment. This installation would also serve as a
test case for other potential applications elsewhere
in the District.
The cover supplier custom-designed the structurally
supported covers to include special features chosen
by the plant team, and to fit around the various pipes
and light standards existing around the basin
openings. The nine covers were installed in four
days in June 2006 while the basins remained in
service. Results were immediately apparent, and
operators were able to cut back chlorine feed even
before the covers were completely installed.
“The covers were installed quickly, without
disruption, while our basins were in service”,
John Cook, Water Plant Supervisor.
Five months after the covers were installed, plant
staff retracted the covers and found the tube settlers
to be free of algae. Plant operators have also
reduced pre-chlorine feed by more than half. A
small amount of chlorine is still used to provide
“With our covers, DBP levels are down and we
are saving money through reduced chlorine
feed”, Angela Cheung, Senior Engineer.
coagulant aid and to control algal growth in
uncovered portions of the treatment trains. In the
first six months of cover installation, the plant saved
$24,000 from lower hypochlorite usage compared to
the year before, in line with the projection of $50,000
annual chemical cost savings. The cost to cover the
tube settlers at Penitencia was $258,990, which
included material, installation, and field support by
the manufacturer. With a life expectancy of 15 to 20
years, the District is expected to recover the cost of
the cover installation, and save an additional
$500,000 over the life of the covers. In addition,
with the help of the basin covers, once the plant
switched to ozonation, total trihalomethanes (THMs)
and haloacetic acids (HAA5) leaving the plant are
now consistently below 40 ug/L and 10 ug/L,
respectively.
The Santa Clara Valley Water District is now well
positioned to meet customer demand for a healthy,
cost-efficient reliable water supply produced in an
environmentally sensitive matter.
Angela Cheung is a Senior Engineer with the Santa Clara Valley Water District. Brent Howe is a Product Manager
with Geomembrane Technologies, Inc.