The city of Rowlett, Texas, experienced an unauthorized wastewater ...
Utah wastewater facility meets discharge requirements with aeration system
Originally designed for a 0.75-mgd capacity, the Grantsville Wastewater Treatment Plant receives residential wastewater from almost 10,000 residents in Grantsville, Utah, which is located just west of Salt Lake City. In 2009, the plant had significant trouble meeting the city’s discharge requirements for ammonia, TSS and BOD.
According to Ron Griffin, the plant’s operator, treated wastewater would violate discharge requirements so regularly that he had to withhold it for six months each year to limit violations.
“Something within every six months was a problem—either our ammonia or our BOD,” he said. “So I was only discharging six months out of the year. I would discharge for three months, hold for three months, discharge for three and then hold for three.”
Identifying the cause
The cause of the problem could be traced to a surface aerator in the plant’s primary treatment pond. The aerator created an aerobic zone in the top 4 or 5 ft of the pond and maintained an anaerobic zone throughout the rest of the 10-ft column. The anaerobic process was slow and sluggish, and the byproducts—hydrogen sulfide and methane—caused odor. The plant needed a more efficient system that would allow a continuous discharge of treated effluent and have the capacity to meet future requirements for ammonia removal. It had two options: Convert existing nonaerated ponds to aeration or construct new ponds that meet state requirements.
Selecting a solution
“It was much more cost-effective to use our existing footprint and ponds and just add the [aerators],” said Griffin.
Upon further evaluation, Environmental Dynamics International (EDI) proposed using an Intermittently Decanted Extended Aeration Lagoon (IDEAL), a lagoon-based extended aeration system that treats raw wastewater using a batch reactor process. The IDEAL process allows continuous inflow of raw wastewater, which then undergoes a sequenced aeration reaction/mixing phase, followed by a settling phase. Finally, the water is decanted and discharged to polishing ponds.
After removing the existing surface aerator from the primary pond, construction workers installed the IDEAL. Equipment required for the upgrade included EDI’s floating lateral aeration system with FlexAir 88S assemblies, paired with Magnum fine bubble diffusers, a BioReef BioCurtain and four 24-in.-diameter decant pipes with electric actuated valves, a system control panel and three 220-hp turbo blowers.
During construction, workers met a significant challenge: a 4-ft sludge blanket at the bottom of the cells.
“We had to pull about 30 different blocks that held the old surface aerators in place, and they had to be eliminated so we could put in these deep, fine-bubble diffusers,” said Griffin.
Improved treatment process
The system went online in July 2010. With the new treatment process, wastewater comes in through the head works and goes through a Huber screen to remove all of the large particles. It then goes to the primary cell, comprised of the IDEAL.
“The system has big air pumps that [aerate] from the very bottom of the pond, so it turns the entire 10 ft of the elevation into an aerobic zone,” said Griffin.
The aerobic treatment process is much more aggressive and robust. Since air is applied at the bottom of the lagoon instead of from the top down, the process is more efficient. Because the byproducts are carbon dioxide and water vapor, odor is eliminated.
After this process is completed, the water is decanted from the first cell, traveling through two partially mixed cells and one quiescent cell. It is disinfected with a UV light before discharge. The water is then discharged into Blue Lakes—a small wildlife area north of the plant—and is mostly used for irrigation of pasture.
The system upgrade has resulted in increased plant capacity—now 1.5 mgd—and the quality of the effluent has drastically improved.
“With the upgrade, the ammonia went down to less than 1 mg/L, and nondetect on the BOD,” said Griffin. “I’m literally getting a nondetect on my report about 80% of the time on my BOD, TSS and ammonia, and even E. coli. It’s made a real fine quality of water.”