Problem: Difficulty meeting stringent discharge limits for ammonia, total dissolved solids, and chloride.
Solution: Biological aerated filters keep levels of all three in check as part of a multibarrier approach.
Nestled in the scenic Lake Tahoe region of eastern California’s Sierra Nevada Mountains, the Tahoe–Truckee Sanitation Agency (TTSA) faces some of the most demanding discharge limits in the country. TTSA operations are complicated, as regional water temperatures range from 7ºC to 20ºC (44ºF to 68ºF), and the population jumps from 35,000 permanent residents to more than 100,000 vacationers during weekends and holidays, causing huge load variations. The agency is located on Lake Tahoe’s north shore, near numerous ski resorts and golf courses.
"Our peak weeks include the Fourth of July and Christmas–New Year’s holidays, but the area also serves as a weekend getaway," said Jay Parker, chief engineer and assistant general manager of TTSA. "As a result, we have large fluctuations in local population which, of course, means large fluctuations in wastewater flows and loadings."
To adhere to strict requirements, TTSA used a pure-oxygen activated sludge process including a clinoptilolite ion-exchange system to remove ammonia. Clinoptilolite is hydrated sodium potassium calcium aluminum silicate, a natural zeolite. This was chosen as the best method to combat ammonia under the temperature and load variations. Though this process was effective at removing ammonia, the ion exchange produced high concentrations of total dissolved solids (TDS) and chloride in the effluent.
With tightening discharge limits, the plant began to look at options that would continue to keep ammonia levels low while also limiting TDS and chloride.
"Our existing plant would have likely been able to meet any tightening of total nitrogen limits if that had been the only change to our permit," Parker said. "But TDS and chloride limits were also tightening, and we realized we had to move away from our existing process to something that would remove the ammonia without adding chloride, which meant we had to go with some type of biological nutrient removal process."
Search and Destroy Ammonia
With assistance from CH2M Hill (Englewood, Colo.) in the search for the right solution, TTSA selected a proprietary submerged-media-bed biological aerated filtration (BAF) process called BIOSTYR®, manufactured by Kruger Inc. (Cary, N.C.). Because this technology had undergone little nitrification and denitrification testing in the United States, TTSA performed a 2-year pilot-scale test of the technology to make sure it would conform to the agency’s nutrient removal requirements.
When it was clear that the BIOSTYR® system met all of their requirements under TTSA’s varying load and temperature conditions, the facility implemented the BAF technology at full scale.
The BAF system is a multicell structure that contains an aerated upflow filter through a submerged media bed of BIOSTYRENE™, buoyant polystyrene beads. As water enters the system from the bottom, it spreads throughout the media. The media both filter the water and treat it biologically — bacteria attach to the beads. The treated water rises to the top of the structure and passes on to downstream processes.
The structure uses a small amount of the treated water for backwashing, which consists of rinsing and air-scouring the media to remove biomass and solids. The BIOSTYR® process is designed for backwash intervals of 24 hours or longer.
TTSA now complements its pure-oxygen activated sludge secondary treatment with BIOSTYR® for tertiary nitrification and denitrification. The system contains eight nitrification cells and four denitrification cells to handle the drastic variations in load and accommodate future increases in demand.
The BIOSTYR® process at TTSA is fully automated by a supervisory control and data acquisition–programmable logic controller control system, which controls and records influent flow, as well as aeration and methanol dosing rates. Operational control strategies are necessary to ensure that the system is prepared to handle the swings in population, said Steve Ahlert, chief operator at TTSA.
"It’s important for us to anticipate those times when our population soars," Ahlert said. "In order to acclimate the cells and prepare the biomass for the higher loadings, we condition the cells for a period of step increases in load. And then, when we’re coming down off of those high loads, more frequent backwashing is often required for a couple of days."
A cost analysis was not performed, because the new system was necessary to meet the stringent discharge requirements, Parker explained. TTSA set goals of 3.0 mg/L total inorganic nitrogen in winter and 2.0 mg/L in summer, which it currently meets with help from the BAF treatment.