HomeMy WebLinkAbout2001-09-11; Municipal Water District; 509; Capacitive Deionization Technology Field TestingCARLSBAD MUNICIPAL WATER DISTRICT - AGENDA BILL
UPDATE REPORT ON
CAPACITIVE DEIONIZATION TECHNOLOGY
FIELD TESTING BY FAR WEST GROUP AT THE
DEPT. I ENCINA WATER POLLUTION CONTROL FACILITY ENG
RECOMMENDED ACTION:
Update on capacitive deionization technology field-testing by Far West Group at the Encina Water
Pollution Control Facility.
ITEM EXPLANATION:
Capacitive deionization technology (CDT) is a water desalination process using positively and
negatively charged electrodes made of carbon aerogel to adsorb anions and cations in water. Effectively CDT removes sodium and chloride ions (salt) and other ions from the water.
The principle is shown on the attached Figure 1. Lawrence Livermore National Laboratory (LLNL) developed the process in the mid 1990’s. The Far West Group bought the license rights from LLNL
and has further developed CDT by constructing a pilot plant capable of treating up to 1,000 gallons per day.
The Far West Group approached the District with the proposal of field-testing their CDT pilot plant in Carlsbad. To assist in the field-testing effort, the District obtained “financial assistance program”
funding in the amount of $33,000 from the San Diego County Water Authority. Upon completion of
the testing program a report is to be prepared by the District on the effectiveness of CDT in
desalination of groundwater.
In August 2000, after several years of development, the Far West Group delivered their
trailer-mounted CDT process to the Encina Water Pollution Control Facility for field-testing. This site
was selected for testing because there exist groundwater wells that are easily accessible for the
CDT pilot plant. The wells are used to maintain the groundwater level beneath the secondary clarifier tanks. The groundwater in this area is brackish, on the order of 5,000 mg/L in total
dissolved solids (TDS).
Between August 2000 and January 2001, the pilot plant was set up adjacent to the secondary
clarifiers and the District furnished electrical power to the site, connected the wells to the process equipment, and prepared a test program to be used in evaluating the effectiveness of the Far West
Group’s CDT process.
The test program began February 21, 2001 on a part-time basis. The test used a “brick”, which is a
term used by the Far West Group referring to a single process unit containing 1,000 square feet of
carbon aerogel material capable of producing 1,000 gallons per day with a l,OOO-ppm reduction
in TDS. The pilot plant contains different size bricks including l/4-scale and l/2-scale single pass through bricks.
Full-time testing began April 2, 2001. The desalination process was being observed and
satisfactory results were being achieved in salt reduction and efficiencies of the process. However, problems began to occur during this initial part of the testing program. Developing
solutions to these problems were pursued and this effort has been instrumental in advancing the
CDT process by Far West Group. Following is a description of observations made to date and
solutions that are being implemented to solve the problem areas.
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Desalination -Tests were conducted under varying flow rates of 2, 1, and 1.5 liters per minute.
The results showed conductivity was lowered from 5,900 us/cm to 4,500 us/cm. At the flow rate of
2 liters per minute, the adsorption occurred for 40 minutes before the rinse cycle needed to occur.
For the other two slower flow rates, the adsorption both occurred over a period of 100 minutes
before the rinse cycled occurred.
Stainless Steel Corrosion - The carbon aerogel electrical contacts “separator strips” were made of
304 stainless steel to reduce brick cost during the experimental phase. Mass produced carbon
separators are quite inexpensive, however, order quantities of a few hundred are very costly. It was
assumed that corrosion would be very slow using the stainless steel material and allow completion of the experimental program. However, it was learned that severe corrosion still occurred very
quickly on the separator strips. This corrosion caused excessive resistance between the power supply attachments and electrode surface effectively doubling power requirements. As a result of
these problems, Far West Group is making new experimental bricks using graphite in place of
stainless steel. This change will also increase performance of the carbon aerogel.
Algae Growth Inhibition - Treated secondary effluent was processed through the CDT.
The product water showed signs of inhibited algae growth when compared to untreated water.
This is a secondary benefit of the process. The anodes may cause this where a layer of anion rich solution may contain hypo-chlorides necessary for disinfection. Another possibility is the increased
pH caused by CDT was sufficient to inhibit algae growth. This could be beneficial in treating
secondary treated wastewater to produce recycled water for irrigation.
Aerogel Cracking - During manufacture, it was learned that carbon aerogel shrinks during the cure
process and the carbon veil skeleton used to prevent the aerogel material from wrinkling resists this shrinkage. This resistance, however, causes cracks to form in the aerogel. Water retention tests
indicate 50% of electrode volume was in the cracks. However, because crack volumes contain no aerogel, they are inactive for ion storage. In addition, the cracks also create stagnant water
volumes. During ion adsorption, ion transfer to and from retained water in the cracks diminishes the effective storage capacity. During the rinse cycle, the TDS limit is diminished by exchange of input
product water held in the cracks with rinse water. These cracks did not allow the bricks to drain
effectively during backwash operation. After further analysis, Far West Group has changed
their manufacturing process to eliminate these cracks by using a newly patented process referred to
as Sl.
Aerogel Fabrication - The manufacturing process required the use of resorcinol formaldehyde
resin. These chemicals create a hazardous disposal problem during the manufacturing process and
are also high in cost. The new-patented Sl process in manufacturing carbon aerogel eliminates the need for these chemicals and significantly reduces the brick production cost.
Shape of Aerogel Containment Housing - The carbon aerogel electrodes were housed in “brick”
shaped enclosures in the experimental system for ease of manufacture. Even though gravity is sufficient to flow water through the brick, Far West incorporated a gear pump to move water through the brick at higher rates to characterize ion removal. Under a high flow test, the increased
backpressure from downstream valves caused the sidewalls of one brick to crack at the edges causing water leaks. Far West Group is now incorporating the cylindrical design of the production bricks into the experimental system. These can retain their structural integrity under the system
backpressures.
a
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Conclusion -The problems identified above resulted in the Far West Group developing a new
manufacturing process for the electrode material with the following benefits:
b Greater ion storage capacity.
b Significantly lower production costs < 50 % previous cost.
) Minimal EPAIOSHA impact by eliminating resorcinol formaldehyde chemicals used during the
manufacturing process.
Because of the problems that occurred the Far West Group suspended the field-testing on
June 1, 2001. They are now manufacturing new bricks and plan to have these reinstalled in
September 2001. They will renew the testing program at that time and, in working with the District, they have revised the testing program to show completion in January 2002. The District will prepare
a report based on the results of the testing program at that time.
ENVIRONMENTAL REVIEW:
This is an information item only with no environmental impacts.
FISCAL IMPACT:
This is an information item only with no fiscal impacts.
EXHIBITS:
1. Figure 1.
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