A Fix for Ammonia

An integrated fixed-film activated sludge facility helps a Colorado sanitary district achieve consistent compliance despite a harsh climate

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The Fairplay (Colo.) Sanitation District faced a challenge with inconsistent ammonia removal. The treatment plant consisted of aerated lagoons totaling more than 7.4 million gallons at about 10,000-feet elevation.


Influent water temperatures were regularly recorded between 43 and 50 degrees F. In addition, groundwater table fluctuations affected the performance of the aeration lagoons.

Because of the harsh climate, with freezing temperatures most of the year, the plant was not achieving nitrification consistently. The lagoons brought the additional challenges of excess algae growth in summer and freezing in winter.


The district solved the problem by constructing an AnoxKaldnes Hybas biological treatment process that consistently treats the wastewater even during harsh weather. The plant (300,000-gpd maximum month design, 100,000-gpd average daily flow) now easily meets ammonia permit limits that range seasonally from 20 to 45 mg/l.


The plant became operational in winter 2008. Ammonia levels in effluent discharged to the Middle Fork of the South Platte River are around 0.1 mg/l, from influent typically containing 20 to 30 mg/l, according to David Stanford, president of H2O Consultants, who operates the plant for the district.


Small and efficient

The district leaders were concerned with finding a solution to work in the harsh climate and provide consistent effluent quality. The process would also need to be able to meet effluent quality regulations well into the future.


The district’s engineer worked with Kruger Inc., a Veolia Water Solutions & Technologies company, to design an integrated fixed-film activated sludge (IFAS) process with internal recycle for partial denitrification. The small footprint maximizes heat retention by not allowing the wastewater time to cool off before treatment. The completely covered process allows effective treatment while providing winter protection for both the operators and equipment.


The facility includes an anoxic reactor, 125,000 gallons of aerobic volume, and two rectangular clarifiers connected with the headworks, pumping systems, laboratory, disinfection and ancillary equipment in a single structure. The new flow scheme consists of:

• Headworks, including comminutor, 3-mm spiral screen and grit removal.

• Influent pump chamber.

• Two trains each of one anoxic reactor and two aerobic reactors containing AnoxKaldnes K1 media.

• Secondary clarification.

• UV disinfection.


One of the existing lagoons was converted to an aerated sludge holding pond for waste activated sludge (WAS). The IFAS reactors are 15 feet square by 16 feet side water depth (SWD) with media fills of 65 percent in the first reactor and 38 percent in the second. The design mixed liquor suspended solids is 3,000 mg/l.


The biological system is designed to treat screened influent down to 10-mg/l soluble BOD5 and 1.0-mg/l effluent NH3-N. The return activated sludge (RAS) rate has a maximum of 150 percent of influent flow, and the internal recycle design rate is 70 percent.


The plant is designed to have manpower requirements comparable to the previous lagoon system. An operator makes a walk-through each day and spends a half day on site twice per week. Because of the remote location and harsh climate, the plant design focuses on simplicity and consistent performance with little operator attention.


Process description

Hybas is the trademarked name for the Kruger AnoxKaldnes IFAS technology. The core principle of the process is the use of nonclogging biofilm carriers placed in an aerobic reactor with activated sludge mixed liquor. An engineered aeration system mixes the media and mixed liquor in the reactor.


The carrier media does not require backwashing, has a low head-loss, and provides a high specific surface area for biofilm development. The media is retained within the reactor using stainless steel retention screens, while the mixed liquor passes through and is settled in the secondary clarifiers. An added benefit of the IFAS principle is that it increases nitrification capacity without increasing the solids loading rate to the clarifiers.


The biofilm carrier elements are made of high-density polyethylene and have a specific gravity of about 0.95. The addition of media to the aeration basins makes it possible to maintain nitrifying biomass in a much smaller footprint than with conventional activated sludge systems.


A volume of 6,427 cubic feet of AnoxKaldnes K1 media, aeration grids and media retention sieves have been installed for the 0.3-mgd plant upgrade. The pre-denitrification zone is designed to combine nitrified internal recirculation, raw influent, and RAS to achieve total nitrogen removal and partial BOD reduction upstream of the aerobic IFAS zones.


Simple and effective

Stanford is pleased with the ammonia removal and overall performance of the plant, designed by Burns and McDonnell Engineering. “I have been a licensed wastewater operator since 1978, and this is the easiest, most forgiving mechanical wastewater plant I have ever operated,” he says.


“Andrew Waddoups, our engineer from Burns and McDonnell, told me at the beginning that this plant would depend on only two things: aeration and sludge wasting. As long as you waste from the plant at the proper rate on a continuing basis, and make sure your blowers are operating correctly, the process will perform.


“You have a circular plastic media with a whole lot of surface area, and your nitrifying bacteria adhere to the plastic. When you examine the media, you can see with the naked eye the biomass growing on it. When you keep such a large biomass in play — when you’re running at 3,000-mg/l MLSS — your uptake of ammonia is awesome.”


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