An upgrade and expansion at the Kurt R. Segler Water Reclamation Facility in Henderson, Nev., included 23 submersible mixers installed in a modified Johannesburg biological nutrient removal system. Sixteen cells in two trains of the West Complex and eight in the East Complex provided phosphorus removal and nitrogen reduction.
The mixers began shorting out within the first year. “We spent almost three years working with the manufacturer to find the cause,” says Adrian Edwards, wastewater operations manager. Repairing a mixer cost $3,000 to $4,000.
“The problem seemed to be water breaching the mixer seals, wicking down the electrical connection, and into the motor windings,” says Howard Analla, treatment process manager. Seeking a solution, the two contacted the Clark County Water Reclamation Facility in Las Vegas, since it used a similar process and equipment.
The plant had experienced the same problem and had solved it by switching to Amamix submersible mixers (KSB USA). Clark County operators also reported that they used lower mixing energy for the process. The new mixers installed at the Kurt R. Segler plant stopped the short outs and used half the horsepower of the original units, saving the plant some $180,000 per year in energy and chemical costs.
The expansion increased the facility’s design capacity from 24 to 32 mgd and created a West Complex with two plug-flow activated sludge trains and an East Complex with one complete-mix activated sludge train.
Chemical coagulation and precipitation, either in solids contact clarifiers or lamella plate settlers, provide supplemental secondary phosphorus removal. Sand filtration occurs through monomedia or continuous backwash filters. A UV process provides additional disinfection. When the 12 hp, 480-volt submersible mixers went online, they increased the plant’s electric bill by $11,000 per month. Analla and Edwards found it difficult to believe that mixers operating on half the horsepower could be as effective as larger units, so they contacted Quadna, the KSB representative in Las Vegas. A company technician evaluated the plant’s dimensions and TSS estimates, then suggested the 6 hp C4138/48 UDC mixer.
The 16-inch, self-cleaning propeller on the horizontal submersible mixer moves 6,100 gpm at 850 rpm. The unit has triple seals — two mechanical seals and a lip seal — to keep water from breaching the mechanical seals or wicking through the whip-like cable where the blade nicks it. In addition, a watertight, resin-embedded cable entry system works with a plug-like connection. The mixer requires an oil change every two years.
The team bought one mixer as a trial. It mixed the cell with half the horsepower and never failed. Confident in its reliability, the team purchased 13 more mixers, then replaced seven of the nine in the anaerobic zones.
“With just seven mixers installed, energy dropped from $166,450 per year to $136,880,” says Analla. “After we replace all 23, costs will decrease to $84,630 per year and we’ll save tens of thousands in repair bills.”
Within two weeks of the seven mixers going online, operators saw secondary effluent phosphorous levels decrease from 0.25 mg/l to 0.10 mg/l, enabling them to use less alum. Alum savings averaged $270 per day, or $98,000 per year.
“The reductions in energy and chemicals are saving us around $180,000 per year,” says Analla. “We estimate an equal return on investment within the first year.”