Well Water Treatment: Ozone Helps Solve Water Problems at Bassi Ranch
The setting is idyllic: Twenty-four large hillside sites for upscale homes in a rural, gated community with views of the Pacific Ocean. The weather is perfect with warm, sunny days and cool evenings.
But the water is a different story. An analysis of the raw well water (Table 1) shows a pH of 7.9 and 30 color units with >20 mg/L hydrogen sulfide, 5.6 mg/L iron and 0.6 mg/L manganese. Odor was detected at 10 threshold odor number (TON), indicative of the high hydrogen sulfide levels. To meet these water problems head-on, the homeowners formed the Bassi Ranch Mutual Water Co. in 1992.
The original treatment system consisted of chlorine gas injection directly out of the well head, capable of 60 gpm at 250 psi. There was no precise, mechanical means of controlling the chlorine dosing. The water then was piped a distance of about 1.5 miles and up 300 vertical feet to a 36-inch diameter by 72-inch filter tank filled with manganese greensand. The final stop was the 68,000-gallon galvanized steel storage tank, which fed the 12 completed homes by gravity.
Poor overall water quality, described by homeowners as "inconsistent at best," prompted them to seek alternatives to the simple chlorine/filtration system. According to Rich Walker, the Bassi Ranch Mutual Water Co.’s homeowner representative, it was difficult to maintain proper chlorine levels. "Half of the time, the smell of chlorine was so bad, you just couldn’t drink the water. But the rest of the time, it was the smell of sulfur that got to you."
In addition to inconsistent water quality, other reported drawbacks to the system included problems associated with delivery and on-site use of chlorine gas and the difficulty of handling potassium permanganate.
By January of 1999, the 12 Bassi Ranch homeowners knew something had to be done to improve the quality of their water. The homeowners association was very close to approving a new system, which would have offered little improvement; it only called for the replacement of the undersized filter tank with a larger one. With proposals finalized and work about to begin, a conversation between a homeowner and his children’s nanny gave the water company a new option–ozone. As the homeowner spoke casually about the water problems, his nanny said that she too had noticed the poor water quality while doing the family’s laundry. She also mentioned that she had previously worked as a receptionist for ClearWater Tech, a local manufacturer of ozone water treatment equipment. The nanny’s boss, who is on the board of directors for the Bassi Ranch Homeowners Association, called ClearWater Tech the following day and preliminary water system designs began immediately.
Because of the type of contaminants involved, the new system (Figure 1) was designed primarily to enhance two critical system functions: oxidation capability and filter capacity. Also, the water is a designated source for fire suppression, so rapid recovery was an important consideration. Pumping on demand from the well head at 60 gpm and 250 psi, raw water now flows into stage one of the treatment train–aeration. Used for initial hydrogen sulfide removal, the aeration system employs twin Mazzei air injection venturis and two 120-gallon tanks. Following aeration, the water flows past the ozone injection point. Fed by oxygen generators, the ozone system produces 100 grams per hour at 6 percent concentration. The ozone injector is a Mazzei #1583, plumbed into a 2-inch bypass line. Past the injector, the water flows through two parallel-plumbed 120-gallon contact tanks to achieve the desired contact time. From there, the water enters two 42-inch diameter by 72-inch multimedia (depth) filters, used to remove the contaminants rendered insoluble following oxidation by ozone. The backwash mode is microprocessor-controlled using the Filter Wizard, a system that allows for automatic backwashing based on water usage. The final step is applying the sodium hypochlorite residual sanitizer via chemical feed pump to achieve a 0.5 to 0.1 mg/L residual. The water then is stored in the original 68,000-gallon galvanized steel storage tank.
Compared to the original chlorine gas/ filtration system, the new aeration/ ozonation/filtration package is providing much more consistent water quality and is effectively handling the high iron, hydrogen sulfide and manganese levels found in the raw water. The treated water, tested by an accredited independent lab as recently as July 14, now has "not-detected" amounts of hydrogen sulfide, iron, odor and color. (See Table 2.) The manganese level now is at .05 mg/L. The new water treatment train includes on-site production of ozone, the system’s primary oxidizer. This is especially important at a time when according to the Bassi Ranch Mutual Water Co., gas chlorine has become increasingly difficult to obtain. Also, the new system allows for treated water to be used for backwashing, eliminating the possibility of contamination with untreated water. Finally, potassium permanganate is no longer required because manganese greensand is not used in the filter media.
The use of ozone in small water systems such as Bassi Ranch has grown significantly in recent years. While the exact number of such systems is unknown, one report suggests that there may be 55,000 regulated systems nationwide. Another report, produced by the U.S. Environmental Protection Agency (EPA), indicates that as many as 200,000 small systems may be operating.1 The actual number may be even higher because the definition of "public water system" varies from state to state. For example, the EPA defines a public water system as one having a minimum of 15 connections or serving 25 people for at least 60 days per year. The State of Washington’s definition uses a minimum of just two connections.2
Despite the fact that approval requirements for ozone-based small community drinking water systems vary from state to state, the technology remains a very viable one. In fact, the EPA has identified ozone as "Small System Compliance Technology" under the Safe Drinking Water Act.3
The homeowners at Bassi Ranch now have a good reason to smile. The 12 homes, which now use an average of 400,000 gallons of water per month during the winter and as many as 900,000 to one million gallons per month during the summer, are enjoying consistently good water for all of their household needs. Once all 24 Bassi Ranch homesites are built, an estimated 1.6 to 1.8 million gallons will be used per month during the peak season. At that time, the water treatment system will require an expansion retrofit, and considering the results that are being achieved, it simply will be a larger version of what is in place today.
1 USEPA website, www.epa.gov.
2 Larson, Kenneth. "Using Your Ozone Expertise to Open Doors Into Small Municipal Systems," Water Conditioning & Purification Magazine. February 2000, pg. 97.
3 Rice, R.G., P.K. Overbeck and K. Larson. "Ozone, an Effective and Affordable Choice for Small for Small Water Systems," a presentation at the American Water Works Association Convention. 1999.