Repair costs were the last thing that the city of Henderson needed. It was 2008, and Nevada’s second-largest city had just completed an upgrade and expansion project at the award-winning Kurt R. Segler WRF. The plant was ready to serve the city’s 270,000 plus residents — and then the new submersible mixers began to fail.
At a repair cost of $3,000 to $4,000 per mixer, the failures soon became a major issue. But what started out as a nightmare turned out to be a dream come true. Four years later, not only has the plant managed to plug the leak in its maintenance budget, but it is also saving around $180,000 per year in energy and chemical costs.
A great beginning
In 2008, Kurt R. Segler WRF finished an expansion project to better serve the area’s growing population. Reclaimed water is used to irrigate city golf courses, a cemetery and highway medians. The water is also used to control dust and compact soil at construction sites. The water is supplied to ponds in the Henderson Bird Viewing Preserve, which is located at the Kurt R. Segler WRF site. Treated effluent that is not used as reclaimed water is discharged to the Las Vegas Wash, which carriers the effluent to Lake Mead. The incoming flow to Kurt R. Segler WRF is domestic wastewater, along with small amounts of flow from industry. (A pretreatment program regularly samples effluent from industrial dischargers.)
The expansion created a West Complex that processes two Plug Flow Activated Sludge (AS) trains and an East Complex that processes one Complete Mix AS train. The plant uses the modified Johannesburg biological nutrient removal (BNR) process. Preliminary treatment involves fine screens for grit removal, followed by Biochemical Oxygen Demand (BOD) removal and nitrification of ammonia in AS aeration basins. Nine BNR cells in the West Complex and eight in the East Complex process phosphorus removal and nitrogen reduction. Chemical coagulation and precipitation, either in solids contact clarifiers or lamella plate settlers, provide secondary phosphorus removal; sand filtration occurs through either monomedia or continuous backwash filters. Aside from improving and adding to the AS capacity with BNR, the project also included the addition of ultraviolet (UV) disinfection capabilities. (Hypochlorination is still an option in case of emergencies.)
With the completion of the project, the plant increased its flow output capacity from about 21 mgd to 32 mgd. The facility was ready for the increased demand — until trouble cropped up in the new submersible mixers that had been installed during the upgrade. Twenty-three of these 12-horse-power (hp), 480-Volt mixers were installed throughout the BNR cells in both complexes.
Howard Analla, treatment process manager for Kurt R. Segler WRF, explains that the trouble started within the first year. “We were still in the warranty period when we started experiencing problems,” he says. Unfortunately, the problems only increased as time went on. The mixers continued to short out, for no apparent reason.
Adrian Edwards, wastewater operations manager for the plant, recalls: “It was probably a good two-and-a-half or three years of sending the mixers out to a local repair company, dealing with the manufacturer to try and find out what the cause of the problem was. They made a few trips out here to look at the installation and look at our maintenance practices — but nothing much materialized.”
At a cost of $3,000 to $4,000 for each repair, Analla and Edwards were feeling the pinch.
As far as the team at Kurt R. Segler WRF could tell, the problem seemed to be that on some units water was somehow breaching the mixer shaft seals; while on others it was wicking its way down the electrical connection and into the motor windings. At a loss for a solution and facing mounting repair costs, Analla and Edwards decided to contact the team at Clark County Water Reclamation, the sister facility to Kurt R. Segler WRF. Analla and his team wondered whether the county plant might have experienced similar issues, and if so, whether it had found a reasonable solution.
The answer? Yes, the county’s mixers had suffered from the same moisture problem as the one at Kurt R. Segler WRF. The solution had been to switch to a different mixer: the Amamix submersible mixer manufactured by KSB. The new mixers seemed immune to the mechanical seal and cable wicking issues, and better yet, they operated on half the horsepower of the previous devices.
Analla and Edwards found the latter difficult to believe. But after more discussion with the county team and input from the Henderson engineering staff, they decided to investigate the Amamix mixers. They contacted Quadna, the local KSB representative and service center, which stocks the mixers.
In conjunction with the plant and Quadna, KSB ran the numbers — the plant’s basic dimensions and the total suspended solids (TSS) estimates — and suggested the 6-HP Amamix C4138/48 UDC mixer. This horizontal submersible mixer has a propeller design that is optimized to deal with the energy input in wastewater treatment processes. Furthermore, the machine is self-cleaning and only requires an oil change only every two years.
Still somewhat dubious, the Kurt R. Segler WRF team bought one Amamix as a replacement and trial.
The results were gratifying. The mixer worked fine and seemed to be pulling its weight with half the horsepower. Best of all: no moisture problems, and no mixer failure.
“We saw good progress and results out of that, so we proceeded with purchasing,” says Edwards. “The fact that the county had an open contract made the process much easier, because we were able to take advantage of that contract to purchase the equipment at a good price. Plus, we saved on employee time, since we didn’t have to go through purchasing or get engineering involved.”
Analla says, “Our initial decision to go with the KSB mixer was based on just the maintenance costs [of the existing mixers]. We were still hesitant about the difference in mixing power, even though it had worked for Clark county.
“Once we figured out that the reliability was there, we were able to buy 16 new mixers. The plan was to replace the mixers in the initial three BNR chambers [nine mixers total in the anaerobic zones]. Then, we would replace the others as they dropped. Within about two weeks, we started seeing positive results as far as lower phosphorous numbers coming out of the process.”
The cost savings in energy alone were impressive. With just seven new mixers installed, energy costs dropped from $166,450 per year to $136,880 per year. When all 23 mixers were finally replaced, the costs dropped even further — to $84,630 per year. Analla’s team was pleasantly surprised, to say the least. Not only were they saving tens of thousands in repair costs; they were also saving close to $100,000 in energy expenses!
More savings were in store. Once the anaerobic zone mixers were all replaced, the team noticed an improvement in secondary effluent phosphorous. Ortho-P had previously ranged from 0.2 to 0.3 mg/L — well within National Pollutant Discharge Elimination System (NPDES) permitting requirements. Soon after the mixers were replaced, the Ortho-P numbers dropped to 0.08 to 0.12 mg/L. This improved operation allowed the plant to reduce the amount of alum used in biological processing, without compromising its excellent results.
“That was when it started falling into place,” says Analla. The alum savings averaged around $270 each day, or a whopping $98,000 per year. Between the energy savings and the reduction in chemical costs, Kurt R. Segler WRF is saving around $180,000 per year, not to mention the elimination of thousands in mixer-repair costs. Analla and Edwards estimate that the mixers will have achieved an equal return on investment (ROI) within the first year. In the current economic environment, that is like mixing wastewater into cash.
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