Answering the Challenge

The city of San Diego shares a common problem with many other Western cities—not enough readily available water supplies. With continued growth, supplying adequate water in the future with the quality needed is an impending challenge.

San Diego took a major step in the late 1990s by equipping a new high technology wastewater reclaim plant with an electrodialysis reversal (EDR) system from Ionics, Inc., to desalinate tertiary treated wastewater. By providing this reclaimed water with less than 1,000 ppm and low sodium levels, San Diego was able to use an existing 47-mile-long pipeline system to deliver irrigation water to golf courses, plant nurseries, parks, highway green belts and homeowner associations.

Until the late 1990s, the city relied on only one wastewater treatment plant to treat all of the municipalities’ wastewater flow. The Pt. Loma facility used conventional treatment and then discharged its treated effluent to the Pacific Ocean. Rapid growth in what had been undeveloped areas in the northern regions of the city was producing significant increases in wastewater flow.

Under guidance from EPA and other legal jurisdictions, San Diego developed plans to build a second treatment plant to take some of the load off Pt. Loma. At the same time, forward thinking and prudent planning in the city resulted in designing the new North City Water Reclamation Plant (NCWRP) so that it could be tied in with an existing pipeline system to send state of California DHS Title-22 reclaim water out to numerous irrigation and industrial water users.

The new plant would help lessen the load on the use of fresh municipal drinking water.

While the irrigation water would be treated for biological and health considerations, the water could have sodium levels that would be too high. High sodium water would damage close cut grasses on golf course putting greens and it would stunt normal development of plants in nursery operations. Therefore, the city looked for ways to cost effectively reduce the salinity and the sodium levels in the new irrigation water supply.

Challenges abound

San Diego initially looked at using desalination with microfiltration and reverse osmosis (MF-RO). Before committing itself to this approach, the city looked around to see if a less expensive alternative was possible. They found that EDR had been successfully used at a number of wastewater locations to desalinate reclaimed irrigation water.

EDR offered the opportunity to use a less expensive single membrane treatment process, with a higher water recovery to make better use of the pretreated wastewater supply. While MF-RO requires chemical pretreatment to control salt scale buildup on the RO membranes, EDR would operate with no chemical addition to the feedwater—one advantage of EDR. Overall, MF-RO water recovery would be about 80% while EDR would operate at 85% recovery, a second advantage.

Several contractors bid on the membrane treatment process. When all the bids were opened, the least expensive MF-RO option was capital cost priced at $4,775,975. EDR cost $3,569,000 or 25% less than MF-RO.

Considering these factors, the city elected to use an EDR 2020 system from Ionics for the NCWRP irrigation water desalination process. The initial EDR system went online in May 1998 and it produced 2 mgd of water with low enough total dissolved solids (TDS) that non-desalted water could be blended in, and up to 3.3 mgd of less than 1,000 ppm water was delivered to the city’s irrigation water supply system.

Results and developments

When San Diego’s the initial system started, it had a single stage membrane stack design. Approximately 55% of the salt was removed in this single stage process, then EDR product water was blended up to 3.3 mgd at less than 1,000 ppm TDS.

As the city continued to sell more reclaimed irrigation water, the need for EDR product flow increased.

In 1999, the EDR system was expanded with the addition of a third unit (a two stage, 1.1 mgd EDR unit) and with the retrofitting of second stage membrane stacks to the original EDR equipment. This brought total EDR product flow up to 3.3 mgd of still better quality product. With a more aggressive blending of desalted and nondesalted product made possible by the two stage EDR system’s better desalted water quality, the facility was now rated at up to 7 mgd of less than 1,000 ppm irrigation water.

Continued development of this alternate irrigation water supply led San Diego to purchase still more EDR capacity in the fall of 2003. Two options were available. The first used the conventional approach of adding more “land based” EDR systems, which required a major rework of the site, with more conventional building of sunshades, concrete pads and extensive installation work. The second approach called for the installation of fully preassembled EDR systems within 53 ft-enclosed trailers. On arrival at site, a simpler retrofit using preassembled trailers allowed for a quick installation of two 1.1 mgd EDR systems. Overall, the site area required would be smaller, and the expansion could be done far more easily. Therefore, the city chose the EDR “trailerized” approach from Ionics.

When the latest EDR units go online shortly, total EDR production will be 5.5 mgd. With blending, up to 12 mgd of irrigation water will be supplied to a further expanded irrigation water pipeline infrastructure system, sending water to additional golf courses and new home developments.

This latest NCWRP expansion also included adding enough concrete pad area and water treatment site pipeline capacity to bring in a third EDR trailer in the future. This version of the plant will have 6.6 mgd of EDR capacity and up to 15 mgd total blended flow.

Since 1998, the city of San Diego increased water rates for its drinking water supply. Through the resulting market driven conservation, overall water usage has dropped somewhat and the average TDS of tertiary treated wastewater has increased. While EDR was originally intended to operate during the dry season, gradually increasing TDS levels are having the EDR units run for a greater percentage of the year.

Results at the NCWRP have proven positive for both the city and for the EDR process. Based on the success of San Diego, other municipal agencies in southern California are looking at EDR to treat their municipal wastewater. In some cases, where final high TDS brines have to be trucked from remote treatment sites to the closest available sewer connection, EDR’s inherent ability to operate at higher water recovery than MF-RO is an advantage.