In the Mojave Desert 50 miles northeast of Las Vegas lies the fertile Moapa
Valley. Nourished by a network of artesian springs, the land has for years
produced a variety of crops, including carrots, spinach, cantaloupe, potatoes,
alfalfa and onions. The Valley’s lush marshes have attracted many
new residents to the nearby towns of Moapa, Overton, Logandale and Warm
Springs – a spillover from Las Vegas’ dynamic growth.
The local water authority – Moapa Valley Water District (MVWD) – and
its predecessor water authorities have wisely planned for the region’s
present and future water needs. In fact, MVWD’s general manager was
recently honored as the Nevada Rural Water Association’s 2008 Water
Manager of the Year, in part for his leadership in helping the District
bring a new arsenic treatment facility on line – the only rural water
provider in the state to do so before a mandated January 23, 2009 deadline.
MVWD serves a population of 3,000 household customers – about 10,000
people – and 300 commercial accounts. The 177-mile water pipeline
system distributes more than one billion gallons of water to Moapa Valley
customers each year in a service area that covers 79 square miles.
Selecting an Arsenic Treatment Solution: Total Cost the Key
When the USEPA mandated that drinking water utilities achieve a new, lower
arsenic level by January 2006, many water authorities around the country
were given extensions enabling them to complete the necessary work. The
Nevada Department of Environmental Quality set the January 2009 deadline
extension for rural water districts in the state, including MVWD.
The District evaluated both coagulation/filtration and adsorptive filtration
media as potential arsenic treatment solutions because of their proven
performance at other sites across the country. In 2005, MVWD conducted
a pilot test of an adsorptive system and was pleased with the results.
The key finding for the District was the total cost of the competing systems.
In a 10-year cost analysis, they estimated that use of a ferric oxide adsorptive
media would cost five to six percent less than the coagulation/filtration
process.
When project engineer, Bowen, Collins & Associates of Draper, Utah,
completed the engineering study in 2008, MVWD general manager Brad Huza
reaffirmed the District’s decision to construct two SORB
33® arsenic removal systems using Bayoxide® E33
media, both provided by Severn Trent Services. Construction of the
facilities began in April 2008. Five 12-ft-diameter SORB adsorption vessels
were installed at the Arrow Canyon well water treatment plant. Construction
of this 6.48-mgd facility was completed in January 2009. Two 12-ft-diameter
vessels were installed at the 2.52-mgd Baldwin Springs spring water treatment
plant, where construction was completed in April 2009.
Adsorption Treatment a Simple Process
The SORB 33 arsenic removal system is a fixed-bed adsorption system using
Bayoxide media, a granular ferric oxide media for the adsorption of dissolved
arsenic. The system employs a simple “pump-and-treat” process
that flows pressurized well or spring water through a fixed-bed pressure
vessel containing the media where the arsenic removal occurs. Both arsenite
(arsenic III) and arsenate (arsenic V) oxyanions are removed from water
via a combination of oxidation, adsorption, occlusion (adhesion) or solid-solution
formation by reaction with ferric oxide ions. Water at both the Arrow
Canyon and Baldwin Springs treatment sites is pre-chlorinated before
it flows into the SORB 33 system.
The arsenic levels at both Arrow Canyon and Baldwin Springs is 17 µg/L,
low enough to allow for bypass/blending. But for the first several months
of operation, the District treated the entire flow through the systems’ vessels
in order to achieve a non-detectable level of arsenic. This helped give
operators time to become comfortable using the new system and learn the
bypass/blending procedure. At Arrow Canyon, the design flow rate per adsorber
is 900 gpm; at Baldwin Springs, 875 gpm. Both systems utilize a media bed
depth of 3.2 ft.
In September, MVWD began bypassing a portion of the pre-treatment flow
to achieve arsenic levels in the blended water of 7 µg/L. To achieve
this level, the Arrow Canyon system bypasses 1,125 gpm, or 25 percent of
the combined well flow rates; the Baldwin Springs system bypasses 470 gpm,
or 27 percent of the combined springs flow rates.
The Moapa Valley adsorbers are operated in parallel flow configuration.
If there is variation in the flow rate to either of the adsorbers and balancing
is desired, the effluent butterfly valve on the higher flow adsorber(s)
can be positioned to distribute more flow to the lower flow adsorber(s).
The pressure differential through the adsorbers is monitored, and when
the differential on either adsorber exceeds the high setpoint (normally
10 psi), an alarm sounds indicating high pressure differential. After a
15-minute backwash is conducted to reclassify the compacted media, the
adsorber is returned to service.
The differential pressure gauge measures the differential across each
adsorber. A discrete signal is sent to the well’s or spring’s
SCADA system to notify the MVWD operators that an adsorber has a high differential
value requiring attention.
Every one to four months, depending upon the water's quality, each adsorber
is taken out of service for backwashing to expand the compacted media bed
and to remove solids that may have built up within the bed. Otherwise,
there is no other non-service action required until the media is exhausted.
Media life ranges from six months to six years depending upon the system’s
utilization factor and on the influent water’s arsenic level and
the presence and concentration of other ions in the water that could shorten
the media’s arsenic adsorption capacity.
“Flawless” Performance
During the first seven months of operation, with the SORB systems operating
to achieve non-detect arsenic levels, Huza called the arsenic removal
systems’ performance “flawless.” According to Huza, “The
SORB technology has allowed the District to achieve and maintain our
system in compliance with the new arsenic standard in a very cost effective
manner. Because of the ease and reliability of operation, the District
has been able to minimize rate increases and bolster consumer confidence
in the water being delivered – both of which are positives for
our customer base.”
For more information, e-mail info@severntrentservices.com.