Case Study - Arsenic Treatment Technologies
Tucson, AZ
Background: Water Quality Characteristics
Tucson Water serves a population of approximately 600,000 (providing 85% of the Tucson
metropolitan area's potable water supply). The
system utilizes approximately 190 wells, some
of which are operated seasonally to meet the
City's peak water demands. The average annual
water demand is estimated at 102 million
gallons per day (MOD).
Typical Raw Water Quality - Tucson Test
Site1
PH
Arsenic (As(V))
Chloride
Fluoride
Iron
Nitrate (as N)
Silica (as SiOj)
Sulfate
Total Organic Carbon
7.5
0.013 mg/L
24.3 mg/L
1.3 mg/L
<0.04 mg/L
6.63 mg/L
34.5 mg/L
130.6 mg/L
0.37 mg/L
'Norton, M; Chang^Y;& Kommineni, S. "Evaluation of
Micro-Sand-Based Technologies for Arsenic Removal -
Ballasted Sedimentation and Metclean™ Process.
Although it is a large system, Tucson Water can
be compared to smaller community water
systems, since most of its wells serve fewer
than 4,000 customers.
Tucson Water has primarily depended on
ground water, which is affected by naturally
occurring arsenic. The utility has been working
with the Central Arizona Project to incorporate
a surface water source, so that the higher
arsenic ground water can be blended with the
low arsenic surface water. By the end of 2003,
Tucson Water anticipates utilizing the surface
water source for approximately half of its total supply.
Generally, the only existing treatment provided by Tucson Water is disinfection. The utility does
provide additional treatment for water utilized from a groundwater clean-up project contaminated
with TCE (about 5% of the total annual water usage).
Tucson has not had difficulty complying with the current 0.050 mg/L maximum contaminant level
(MCL) for arsenic. The revised 0.010 mg/L arsenic MCL will require the utility to install additional
treatment at one of its wells (the sole source of supply for a small isolated service area). In addition,
the utility anticipates needing to blend water at a number of wells with higher arsenic concentrations
with other low arsenic concentration water supplies.
Pilot Testing
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Tucson Water has tested multiple arsenic treatment technologies at two well sites. Most recently,
the system simultaneously tested four technologies including:
Activated alumina (conventional, iron-modified, and high porosity); and,
Granular ferric hydroxide.
Sponsored by the American Water Works Association Research Foundation (AWWARF), the goal
of this pilot project was to test emerging technologies for arsenic removal. This project was one of
the first to simultaneously test multiple technologies on a large-scale, year-round basis and was run
in conjunction with the pilot testing at the City of Scottsdale's water system.
A skid-mounted apparatus holding four separate, identical fixed-bed columns, was installed at the
Tucson treatment site. Each column, operated in
parallel, contained 25 gallons of adsorbent
media. The flow rate to the skid was 20 gallons Figure 2: Tucson Pilot Testing Unit
per minute (gpm).
Four different proprietary adsorption media
were tested:
• Conventional activated alumina;
• Iron-modified activated alumina;
• High porosity activated alumina; and,
• Granular ferric hydroxide.
The goals of the tests were to discover whether
the adsorbents would allow Tucson Water to
meet the revised 0.010 mg/L arsenic MCL
(taking operation and maintenance, labor, and
personnel costs into account); test whether there
was any seasonal impact on the effectiveness of
the technologies; and establish optimum
operation protocols for full-scale systems.
Ultimately, granular ferric hydroxide proved to
be the most effective for long-term arsenic removal. Conventional activated alumina required
frequent regeneration, and high porosity activated alumina could not be used as a disposable media
due to its short ran time (one month). Iron modified activated alumina lasted 70 days before it had
to be replaced and the replacement process was labor-intensive. The poorer results from the
activated alumina media may be attributable to the high silica concentration and higher pH (72-7.5)
of Tucson's raw water.
Granular ferric hydroxide kept arsenic levels below 0.010 mg/L even after 236 days of operation.
However, its costs are approximately three times higher than costs for activated alumina treatment.
Currently, granular ferric hydroxide supplies must be shipped wet, significantly increasing costs.
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Ion Exchange Technology
Tucson Water pilot-tested indefinite brine recycling (IBR), an ion exchange treatment process. A
single column was installed, and the unit was manually operated. The high sulfate levels in the raw
water limited the effectiveness of IBR.
Micro-Sand-Based Tech
Through funding from the Arsenic Research Partnership (comprised of AWWARF, EPA, and the
Association of California Water Authorities), Tucson Water pilot-tested micro-sand-assisted
oxidation adsorption (MAOA). MAOA consistently removed arsenic from the raw water, though
the treatment process caused high turbidity measurements in the treated water (as high as 8 NTU).
(Higher turbidity levels are often associated with higher levels of disease-causing microorganisms.)
MAOA is an option for Tucson if the technology can be refined in order to prevent these turbidity
problems.
Conclusions
Both activated alumina and granular ferric hydroxide appear to be the best treatment options for
Tucson water. The final report has not been completed and the City has not made any decisions as
to their future approach to arsenic treatment
Preliminary cost estimates completed for the City indicate that each 1 MGD, will cost $1 million in
capital costs and $200,000 to $250,000 in annual operation and maintenance costs. These costs
assume that the media will be disposed of in a landfill as non-hazardous wastes.
Office of Water (4606M) EPA 816-F-03-015 May 2003 www.epa.gov/safewater
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