December 2009, Vol. 21, No.12

Problem Solvers

Sanitation Agency Installs Biological Aerated Filters To Meet Strict Discharge Limits

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.

Methanol Dosing Technology Helps Denitrification System Pass Rigorous Performance Test

Problem: Removing nitrate–nitrogen and suspended solids without using excess amounts of methanol.
Solution: Installation of a deep-bed denitrification filter.

The Western Carolina Regional Sewer Authority (WCRSA; Greenville, S.C.) decided to install a new tertiary treatment technology into its Lower Reedy Wastewater Treatment Plant in Simpsonville, S.C., to remove nitrate and suspended solids and meet federal and state requirements for improved effluent quality. The plant treats 37,850 m3/d (10 mgd), with peak flow rates of 94,625 m3/d (25 mgd).

Before accepting a new tertiary system, WCRSA conducted a performance test on potential technologies to assess their ability to remove nitrate without using excess amounts of methanol.

Precise feeding of a carbon source, such as methanol for tertiary denitrification, poses a challenge for many tertiary systems, particularly during flow variations. Methanol dosage rates that are too small can lead to excess nitrate concentrations. Methanol dosage rates that overfeed the system can result in elevated effluent biochemical oxygen demand concentrations. For every part per million of methanol fed to the system over what’s needed for denitrification, biochemical oxygen demand can increase as much as 1.5 mg/L.

In an effort to find a system to feed exactly the right amount of methanol, WCRSA decided to conduct a performance test on the TETRA® DeepBed™ Denite® System from Severn Trent Services (Fort Washington, Pa.). The system is a fixed-film biological denitrification process that also serves as a deep-bed filtration system. It integrates with other plant treatment processes for total nitrogen and phosphorus removal.

"The performance test requirements for methanol usage were some of the most stringent requirements ever to be specified for this type of system," said David Slack, business developer for Severn Trent Services’ TETRA business unit.

The plant had to install denitrification filters before its ultraviolet (UV) disinfection system to reduce suspended solids levels entering the UV channels. The design criteria for the plant’s denitrification filters required removal of nitrate and nitrite concentrations combined to less than 1.0 mg/L while optimizing the methanol feed. To meet the methanol usage criteria, the plant used a control system that included an automatic dosing control system. At full scale, six of these filters were installed at the Lower Reedy plant.

The performance test, conducted on three of the filters in March, tested the filters’ ability to provide the required treatment under cold-water conditions, when biological reaction rates are slower and tertiary treatment is challenging. Since the Lower Reedy plant was not equipped with a methanol feed system, a temporary system, including the feed pumps and analyzer, was supplied by Severn Trent Services to execute the test. Filter influent wastewater was supplemented with sodium nitrate to simulate design loading conditions. All performance requirements were met by the system.

The automatic dosing control system introduces methanol to the wastewater to remove nitrate. By also using the patented SpeedBump¨ — a feature that applies backwash water to the bottom of the filter, releasing the trapped gas into the atmosphere and reducing headloss — operators remove the accumulated nitrogen gas without removing the reactor from service.

The methanol dosing ratio of 3.5 lb methanol per pound of nitrate and nitrite removed was slightly higher than the typical dosage ratio of 3 to 3.2, but this resulted from the very high dissolved-oxygen concentrations present in the top of the filters.

The methanol control system worked well and was the key to achieving the required performance results. However, the methanol control system algorithm is based on nitrate readings. For a few days at the beginning of the testing, unusual elevated nitrite concentrations were observed in the secondary effluent feed to the filters. Since nitrite was not part of the control algorithm, the system appeared to slightly underdose methanol. The methanol dosing was increased through an adjustment to the control system setpoint, which ultimately improved overall performance. Therefore, nitrite concentrations must be monitored periodically to ensure that the nitrate-based control system is responding as desired.

For plants with stringent build–operate–transfer limits with low total nitrogen limits, tighter methanol control and reduced risk can become critical to ensuring that the plant meets limits reliably. The accuracy of the proprietary algorithm used to feed methanol during the denitrification process enables the dosing system to yield savings up to 30% in methanol consumption costs, with a company guarantee of no net total organic carbon pickup across the filter system, according to Slack.

 

©2009 Water Environment Federation. All rights reserved.