The Twentynine Palms Water District serves a population of approximately 17,000 through a system of nine potable wells. The system comprises 7,600 water meters, with about 95 percent of them residential. Like many water systems in Southern California, the water district utilizes equipment to remove fluoride from its water in order to comply with the state mandate of fluoride levels of less than 2 milligrams per liter. The district utilizes a 3 million gallon per day activated alumina fluoride removal system – the largest of its kind in the United States. Chlorine also is added to the water supply at each of the nine well sites.
Arsenic, which is commonly found in many western states’ groundwater, was also present in the Twentynine Palms wells, with levels ranging from undetected to 12 parts per billion (ppb) in eight of them. In the ninth, however – a 30-year-old well -- arsenic levels averaged 19 ppb. In 2007, the Twentynine Palms Water District became one of the United States’ only water utilities to address the problem of arsenic in its drinking water by installing a closed-loop adsorptive arsenic removal system featuring zero backwash water discharge. The system, a SORB 33® arsenic removal system utilizing Bayoxide® E33P media, was provided by Severn Trent Services and installed by the water district under the direction of its general manager, Mike Wright.
When Wright began to investigate an arsenic removal solution to address the elevated arsenic levels, he consulted with the State Department of Health Services, the state agency responsible for enforcing the California Environmental Quality Act (CEQA). Wright’s goal was to comply with CEQA standards as cost effectively as possible. By creating a closed-loop system in which media backwash water was recovered and recycled, Wright reasoned he could create significant opportunities for savings for the city by utilizing a solution that was not bound to CEQA standards. Of additional interest to Wright was utilizing a unique arsenic removal system with tangential environmental benefits.
After investigating several arsenic removal systems, the Twentynine Palms Water District selected the SORB system with Bayoxide media. “We wanted a reliable system,” said Wright, “and our additional considerations were capital costs, waste handling and operations and maintenance. With several wells scattered throughout our service area, we needed a system that would be practical for small well applications. The closed-loop system allowed for very simple construction, compliance with the regulatory agencies and ensured low O&M costs.”
The SORB 33 arsenic removal system is simple to operate and requires virtually no labor. It consists of one or more pressurized vessels constructed of steel. Contaminated water enters the vessels and passes through a robust ferric oxide media, Bayoxide E33 (granular) or Bayoxide E33P (pellets). As water passes through the media, arsenic is adsorbed and removed to a level below the EPA standard. The system requires no cleaning, no regeneration and no complex, labor-intensive steps. Unlike other arsenic removal technologies such as ion-exchange resins, which require frequent backwashing with acid solutions and create a great deal of highly acidic and hazardous waste in the backwash process, no chemicals are used in the SORB 33 backwash procedure. Since the feed water is used for backwashing, the backwash water quality will bear similar characteristics to the feed water.
For the Twentynine Palms arsenic removal system, Severn Trent Services provided two five-foot SORB 33 adsorption vessels. The system’s footprint measured 14’ (L) x 7’ (W) x 10’ (H). Each adsorption vessel has a design flow rate of 130 gpm. The water district is using each vessel to its full capacity to treat 260 gpm of the well’s 325 gpm capacity and is bypassing 65 gpm. Severn Trent Services designed the system with a backwash recovery zero discharge system so that water produced during the backwash process is returned to the head of the treatment system and run through the Bayoxide E33P media. The backwash/rinse wastewater holding tank has a capacity of 3,200 gallons.
Based on a review of water quality provided by the water district, Severn Trent Services guaranteed 118,000 bed volumes or the equivalent of 121.9 million gallons of usage from the media before change-out was required. The company estimated that the media installed in September 2007 would be exhausted in September 2009.
According to Wright, Severn Trent Services representatives said the SORB system operates most efficiently when the feed water’s ambient pH is in the range of 7.0 to 7.5. While water from the Twentynine Palms Water District’s contaminated well had a pH of 7.9, the SORB system has been working extremely well without any pH treatment.
Wright had been told that backwashing would be needed every three months. “But the system has been in operation since September 2007, and as of July 2008 the pressure increase over the media bed has not required that we backwash yet. So far, the system is exceeding our expectations and is operating very well. Arsenic in the water treated by the SORB system measures 0 ppb, and when effluent from the two vessels is combined with the raw bypass water, arsenic levels average about 3.4 ppb.
“An additional positive feature of our closed-loop SORB system is that it can be easily retrofitted to accept more flow just by adding additional vessels and some piping. As a matter of fact, we’re planning just such a retrofit of our system in the near future.”
For more information, e-mail info@severntrentservices.com.
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