Meeting national standards for trihalomethane levels in drinking water created issues for the Main Street Water Treatment Plant in Yuma, Ariz.

Short-circuiting and thermal stratification in the three 3-million-gallon tanks at the booster pump station produced inconsistent chlorine residuals. Operators drew down the 50-foot-deep, 100-foot-diameter tanks to change out the layers, but the process also pulled in stale water.

Daily sampling showed that chlorine was 0.65 to 0.70 mg/L going into the tanks, but only 0.40 mg/L coming out. “The setpoint for distribution is 0.80 to 0.85 mg/L, so we’d add 10 to 15 pounds of gas chlorine every day to trim the discharge,” says chief operator Bill Barbieri.

The operators also considered installing a 45-degree elbow at each inlet to divert water upward, but the solution was expensive and its effectiveness questionable. “THM levels were creeping up toward our 64 µg/L alert level,” says Barbieri. “We needed a better way to mix the tanks while reducing chlorine usage and becoming more efficient.”

At a conference, Barbieri networked with representatives from a Nevada water treatment plant who had installed SolarBee units to mix, destratify, and disperse chlorine in their tanks. “They were really excited about the result and told me how the mixers worked,” says Barbieri. The city bought one SB2500v12 solar-powered circulating unit for each tank. Since their installation, operators add just 1.5 to 3 pounds of chlorine daily to meet the setpoint.

 

What’s in the box?

The 40 mgd (design) Main Street plant distributes 16 mgd. “Before the economy crashed, we put out 28 to 33 mgd in summer and 20 mgd in winter,” says Barbieri. “Over the last two years, we’ve cut year-round production by 50 percent or more, and that reduced chlorine usage to 150 to 200 pounds a day.”

To prepare the tanks for the mixers, the city hired a contractor to enlarge the access hatches in the steel roofs from two feet square to four feet. “It saved money in the long run,” says Barbieri. “If the machines need servicing, SolarBee technicians just lift them out instead of working on them in the tanks.”

The units have a stationary intake assembly that remains on the tank floor. An expandable hose connects the intake to the floating impeller, enabling it to self-adjust to varying water levels. Rotating at 80 rpm, the impeller moves 2,500 gpm in a near-laminar flow pattern that completely mixes the water column. The pattern also ensures consistent dispersal of chlorine at the walls, in the bottom three feet of the tank, and at the floor.

Mounted on each tank roof are solar panels, a digital control panel, and a battery to store energy for continual operation. The panel has daily and seasonal run-time algorithms and includes programming for auto-reverse, anti-jam, and self-cleaning. The mixers in the tanks are driven by a high-torque, direct-drive, permanent-magnet motor with no gearbox to reduce energy consumption.

The city accepted the company’s maintenance and service program. “Technicians come on a regular schedule and take care of everything,” says Barbieri. “It makes life simple, enabling us to focus on other things.”

 

Installation

The technicians installed one mixer per day. “The biggest part of the setup was maneuvering a crane into position to lower the units into the tanks,” says Barbieri. “They are a mile from the Main Street plant and in an isolated area.”

Before the technicians turned on the units, they did basic temperature testing to check for stratification. They returned after three days to retest the tanks and found no stratification.

“It took a few days to stabilize everything, but the numbers have remained consistent since then,” says Barbieri. “We’ve eliminated short-circuiting and stratification. Chlorine usage at the booster pump station is down by more than 50 percent, and although THMs are still forming, it isn’t happening as quickly.”