Quenching Thirst in the Valley

About the author: Steve London is president of Steve London Associates. London can be reached at [email protected] or 215.361.3630.

In what has become a growing trend, the new Point of the Mountain Water Treatment Plant (POMWTP) in Draper, Utah, incorporated a multi-barrier approach to address specific disinfection issues shared by many surface water treatment facilities. The Metropolitan Water District (MWD) of Salt Lake & Sandy integrated a mix of complementary treatment barriers that resolve current issues while instilling the needed capabilities to adapt to changing regulatory requirements. The wholesale district and its consultants carefully evaluated and piloted 22 different design and process alternatives before the adopted concepts translated into formal design and construction at the new plant.

Located approximately 15 miles south of Salt Lake City along the Wasatch Range, the POMWTP facility is the cornerstone of a four-part series of water supply infrastructure projects designed to meet projected demand through and beyond 2025. In addition to the POMWTP plant, the MWD’s $250-million capital program produced a 44-million-gal, open raw water reservoir at the POMWTP site; a 2.6-mile-long, 84-in. raw water pipeline; two 10-million-gal enclosed facilities for storing finished water; and a nearly 12-mile-long, 60-in. finished water aqueduct with a five-unit, 8,000-Hp pump station that interconnects the new plant with the MWD’s Little Cottonwood Water Treatment Plant (LCWTP). The LCWTP also was upgraded significantly with a new ozone disinfection facility, upgraded intake, new grit basin, screen house and chemical feed systems, upgraded flocculation basins, seismic improvements and a 270-ft-diameter, 9-million-gal finished water storage tank. These and related upgrades increased this earlier plant’s hydraulic capacity from 113 million gal per day (mgd) to 143 mgd.

With the MWD’s treatment facilities now interconnected, the district has improved its emergency backup capabilities and gained flexibility for maintaining the 1960-vintage Little Cottonwood facility. The combined capacity of the two plants provides the MWD with assurance that it can meet the daily water requirements for the half-million residents in the growing Salt Lake City Valley that encompasses Salt Lake City, Sandy and other communities.

Raw Water Considerations

The POMWTP draws water from two water supplies in the Provo River watershed. The MWD shares water from the Jordan Aqueduct and the more heavily used Provo Reservoir Canal (PRC) with another water district. It is quality raw water but presents the typical shortcomings of most surface water.

“Turbidity is not a consistent problem, but our raw water can range from a low of less than 1 NTU up to more than 10 NTU during high flows and other incidents,” said Gary Durrant, environmental services specialist for the MWD of Salt Lake & Sandy. “Even in the raw water storage reservoir, where we normally get 3 to 4 NTU, a storm event upstream can increase that to more than 10 NTU.”

“We also experience taste and odor (T&O) events. The PRC is an earthen canal, and weed and algae growth are rapid—to the point of reducing the flow of water. In addition, Deer Creek Reservoir, above the PRC, experiences vigorous algae growth and seasonal turnovers. One can occur as early as January followed by a second in September/October. These turnovers bring more algae downstream, and some types of algae cause the T&O events. In the past, we typically would experience no complaints about T&O until the seasonal turnovers, when our complaint calls would start. Now, by using ozone at both treatment plants, we have eliminated T&O complaints.”

Design considerations ranged from suppressing this T&O problem to using ultraviolet (UV) disinfection to comply with the Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR). The shift toward UV also will help comply with the Stage 2 Disinfection Byproducts (DBP) Rule.

Designed for Now & When

The Rocky Mountain District of Carollo Engineers was the lead consultant for the MWD. A pilot-scale plant was among the measures used during an extensive study of alternatives in the predesign stage. Life-cycle cost analysis became an important tool in selecting preferred suppliers and high-performance technologies.

The POMWTP plant entered service with a rating up to 70 mgd of production, with provisions to increase it to the full 151-mgd maximum design rating. This can be readily achieved because of farsighted oversizing of key elements that reduced both initial and future capital cost requirements.

For example, the UV disinfection system was sized for seven UV reactors; however, only four 40-mgd reactors, with a total of 384 low-pressure high-output (LPHO) lamps, were installed at the outset. The UV units serve the POMWTP as determined by flow volume and UV transmittance, with one unit always reserved as a backup.

Furthermore, in April 2010, UV was approved for use as a primary disinfectant, allowing state credit for Cryptosporidium and Giardia control. It should be noted that this approval is the first in the state of Utah. UV, in tandem with the use of the ozone system, will reduce chlorine DBPs as well.

Goals for the finished water were set at:

• 2.5-log removal of Giardia and Cryptosporidium inactivation;
• Less than 40 ug/L of trihalomethanes and
• Less than 30 ug/L of haloacetic acids.

The tested processes included:

1. Conventional pretreatment (chemical addition, rapid mix, flocculation and sedimentation) followed by microfiltration (MF)/ultrafiltration (UF) membranes;
2. Presedimentation (raw water storage) followed by MF/UF, followed by granular activated carbon; and
3. Conventional pretreatment followed by ozonation, biologically active filters and UV disinfection.

Modeled Efficiency

Extensive modeling, including both bench studies and the operation of a temporary 80-gal-per-minute (gpm) pilot plant, favored the multi-barrier process, according to Durrant. Carollo prepared UV specifications as the basis for a competitive bid between commercial LPHO and medium-pressure technologies. A WEDECO K143 UV system, supplied by ITT Water & Wastewater, was selected for the plant following evaluations of several potential treatment trains.

POMWTP entered service with a hybrid mix of hydraulic flash mixers to rapidly disperse coagulating chemicals into the incoming flow; flocculation; sedimentation to remove suspended particulates; intermediate ozonation; the potential for biological filtration; and the WEDECO UV disinfection system. ITT Water & Wastewater also supplied two WEDECO PDO6000 ozone generators sized for a design production of 950 lb per day.

Carved From the Mountain Range

Situated along the Wasatch Front Range, the site for the zero-discharge plant required 1.4 million cu yd of excavation. The winds that sweep the Traverse Ridge overlooking the plant create a world-ranked launch site for hang-gliding and paragliding which have become a cottage industry for schools that teach the sport in nearby Draper. Like the fabled Icarus, these skyriders sometimes encounter problems aloft and need a place to land quickly. As such, the MWD provided a 5-acre emergency landing pad as a unique part of the plant’s site improvements.

The raw water treatment begins with a low dose of chlorine, ranging from 0.5 to 1.5 ppm, followed by coagulation enhanced by polymers to create the desired flocculation. The water next reaches three sedimentation basins, with a total capacity of 70 mgd, after which it gets a 1-ppm dose of ozone for T&O control.

According to Durrant, in high-level T&O events the plant can turn up the ozone to as high as 3 ppm. After the ozone cycle, the water enters the filter system, distinguished by a Leopold underdrain of plastic laterals with an integral media support porous cap overlaid with 1 ft of sand and 4 ft of anthracite as the media. The filters supplied by ITT Water & Wastewater are rated to treat 4 to 7.8 mgd each, which corresponds to the maximum flow of 8 gpm per square foot of filter area.

“Our goal is to produce finished water below 0.1 NTU, and we generally achieve a value of 0.05 NTU or less,” Durrant said. As for the shift toward UV, Durrant has become an advocate of the nonchemical type of disinfection. The operating costs have been acceptable because the WEDECO system incorporates highly efficient LPHO lamps and has an energy-saving economy mode that allows the system to start at full power and then sequentially extinguish rows of the reactor lamps until the output is at an optimum balance of effectiveness and electricity use.

“We feel good about the UV energy use so far,” he said. “The largest component of operating expense has been the pumps. We can pump up to 78 mgd against 200 ft of head from the south to the north, but the expense can be quite significant.”

“Our philosophy is to stay ahead of the curve in terms of DBP regulations... The U.S. Environmental Protection Agency has made UV disinfection more meaningful as an operational consideration than just the elimination of DBPs.”

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