Sand Trap

Filtering CAP water through sand may dramatically improve its quality, but at what cost?

A high-quality product at the lowest possible price may be one goal of every water company in Tucson. But how they try to achieve that objective using Central Arizona Project water could be vastly different.

In order to comply with the requirements of the voter-approved 1995 Water Consumer Protection Act, the Tucson City Council chose to implement the recently inaugurated $77.5 million Avra Valley basin recharge project. Over time, the blending of CAP and groundwater there will result in slowly diminishing quality, shrinking that area's groundwater supply, and requiring rate hikes of about 5 percent annually for the next several years.

A consortium of northwest-side water companies--Marana, Metropolitan, Oro Valley and Flowing Wells--is now looking at another way of dealing with CAP water. Based on the recommendations of a U.S. Bureau of Reclamation report, later this summer near Twin Peaks the group will begin a nine-month pilot study of slowly running CAP water through sand to remove organic particles that can clog very fine filters. These filters can then be used to reduce the amount of inorganic material in the water, meaning that at the end of the dual process water companies can virtually dial up any quality of water they want, based on what their customers demand and how much they are willing to pay.

According to its supporters, slow-sand filtration offers several advantages as a CAP pretreatment method to prevent filter clogging. Unlike other options, it requires no chemical additions in order to work. Thus it is less expensive, and possibly more healthful, than other alternatives. It does, however, need enough land area to allow for sufficient water to be processed.

Brad DeSpain, utilities director for the Town of Marana, says of the upcoming trial project, "Slow-sand as a pretreatment seems to have worked well in other cases and the cost is reasonable." Once the pilot study is completed, DeSpain indicates the four water companies will review the results and decide what steps to take next.

Although it now serves groundwater to only about 1,000 customers, Marana's water company may soon grow dramatically. DeSpain says negotiations are underway for the town to buy out Tucson Water holdings within Marana's boundaries. If those talks succeed, DeSpain expects they may be providing water to almost 10,000 customers by the end of next year.

The financial and legal incentives for eventually using CAP for drinking purposes is the reason Marana and its partners are exploring the use of a slow-sand pretreatment on the canal water. They are doing this even though a report recently prepared by Tucson Water officials and two of their consulting firms for a "membrane technology" conference in San Antonio concluded that the slow-sand approach had the least reduction in CAP water particulate levels of the three options tested.

DeSpain was not impressed by this conclusion. In his opinion, Tucson Water is biased against slow-sand filtration because, he says, "They are not set up to do that at their treatment plant," on Ajo Way.

Longtime local water maverick and vocal critic of Tucson Water Brent Cluff wasn't convinced by the report, either. "The test results were good," he says, "but the conclusions were lousy." He points out several glaring discrepancies in the study and believes the results clearly indicate that the slow-sand process was getting better at the end of the test period while the preferred option was getting worse.

Tucson Water deputy director Marie Pearthree, one of the authors of the study, admits there were errors in some of the data-collection graphs. But she stresses that the utility was impartial in its approach to the slow-sand technique and that an independent third party conducted the study. "It came out the way it came out," she insists.

While he believes the money spent by Tucson Water to drill new wells in Avra Valley was valuable for drought-proofing the CAP system, Cluff maintains that operating the project on a daily basis for water delivery is literally pouring money down a hole. He is especially critical of the high energy costs required to pump the recharged water out of the ground.

Instead, Cluff has long favored use of a slow-sand, reverse osmosis treatment for CAP water, which he says would be cheaper, safer and healthier. But Cluff, who is now president and chief engineer of a company that sells reverse osmosis systems, believes his business involvement prevented Tucson Water from supporting the idea.

There may be other reasons, though, why Tucson Water officials didn't want to pursue the slow-sand/nanofiltration option. As Brad DeSpain pointed out, it didn't mesh well with the existing $100 million treatment plant. Plus, the vacant land needed to implement a slow-sand process ranges from Pearthree's 100 acre estimate to Cluff's 15 acre figure. Finally, the process produces waste water that may be very expensive to dispose of.

In response to the passage of the Water Consumer Protection Act, a study prepared by the Bureau of Reclamation looked at the potential of Tucson Water using a nanofiltration system for its CAP allocation. The draft report was completed in 1998 but never finalized. Skeptics contend this was done for local political reasons. Bureau of Reclamation officials say, however, they simply ran out of money and now promise the report should be finished by the end of July.

Parts of the earlier study looked at what could be done with the waste water from the filtration process. This water would have a high level of inorganic particulates in it, almost 15 times higher than that now found in some local groundwater.

The study reviewed 12 disposal options and focused on four alternatives: use it for mining operations, inject it into deep wells, blend it with existing sewage effluent, or pipe it to the Colorado River at Yuma. The preferred option was the very expensive pipeline.

Brent Cluff believes that wouldn't be necessary. "The pipeline would never get past Pinal County," he states, "because the farmers there would use the water." While the Bureau of Reclamation doesn't think this is likely, given the high mineral content of the water, Cluff says, "Let's give the farmers a chance to see if they'll use it."

Marana's DeSpain also sees possible agricultural uses for the waste water. "I feel that with low amounts of waste, we could work with the local Irrigation District to use it," he says. "Or it could be blended with groundwater and used on farms after more research is done."

Comments (0)

Add a comment

Add a Comment