Olathe Wastewater Facility Expansion Features Biological Nutrient Removal

Several years ago, the Environmental Protection Agency (EPA) began an initiative to control excess amounts of nitrogen and phosphorus pollution entering the Gulf of Mexico through watersheds. Although phosphorus and nitrogen are nutrients that are needed for aquatic plant and algae growth, excess amounts can have the opposite effect by depleting aquatic growth. Surface water affected by excess nutrients can become green with algal blooms (a process called eutrophication), have an unpleasant odor, and create anaerobic conditions, which depletes the aquatic life.

A booming agriculture industry using pesticides was one of the primary contributors of excess nutrients being introduced into lakes, rivers and even groundwater resources. States located in the heart of farmland were under pressure to restore their surface water quality.

Kansas, for example, a leader in wheat, grain and beef production, has 244 streams and 187 reservoirs with nutrient-related impairments. Based on Kansas Department of Health and Environment data, point sources such as municipal wastewater treatment plants (WWTPs) collectively account for 18 percent of total nitrogen and 25 percent of total phosphorus loading in the state.

The city of Olathe, Kansas, along with many other cities in the state, decided to take a proactive approach to decrease nutrient levels by upgrading their treatment facilities.

Cedar Creek Wastewater Treatment Plant, located in Olathe, welcomed the opportunity to revamp its existing facility. Built in 1984, the plant had already undergone several expansions to meet increasing water demand and was due for another. Designed for a 2.5-million-gallons-per-day (MGD) capacity, the original plant contained a headworks building with bar screening and grit removal, two oxidation basins equipped with mechanical aerators, and two 50-foot final clarifiers.

In 1997, the capacity was increased to 3.0 MGD with the addition of a final clarifier and ultraviolet disinfection. In 2005, the plant also added a centrifuge dewatering system and aerated sludge holding basins. The last addition to the WWTP occurred in 2008 when an effluent pumping station was added.

Despite the upgrades and consistently meeting effluent limits, the plant was hydraulically overloaded with average flows around 3.75 MGD. In addition to lowering effluent nutrient levels and therefore preventing eutrophication in nearby water channels, the facility had the opportunity to increase the hydraulic capacity to 5.25 MGD.

Expansion Becomes Necessity

In June 2010, the city broke ground on the Cedar Creek WWTP expansion. The project would increase the plant's capacity by 1.5 MGD, for a total capacity of 5.25 MGD, which would reduce the frequency of wet-weather discharge overflow and provide biological nutrient removal (BNR) to lower effluent nitrogen and phosphorus concentrations. The structural upgrades included two parallel trains, including the new BNR treatment process.

Xylem equipment was implemented strategically throughout the existing treatment facility. The BNR plant design incorporated a Sanitaire Fine Bubble Aeration system and Flygt Submersible Mixers to maximize the introduction of dissolved oxygen (DO) into the wastewater.

Fine bubble diffused aeration technology was selected because of its ability to produce smaller bubbles that have a larger surface area and longer residence time in the water being treated. The result is higher oxygen transfer efficiency, which equates to energy savings. The mixer aided the diffusion process by dispersing the created DO throughout the wastewater. By combining aeration and mixing technologies, the oxygen uptake/demand was tightly controlled and able to drive the biological reaction towards nutrient removal.

To remove nitrogen and phosphorus, the plant added a new five-stage biological process within the BNR plant. The multi-stage process uses nitrification/denitrification to reduce nitrogen compounds and the biological uptake process to reduce phosphorus. Sanitaire's coarse bubble diffused aeration system was selected to satisfy the mixing requirements in the last zone of the BNR trains while maintaining a DO tension. This aeration produces larger air releases, which help bacteria growth needed to consume dissolved phosphorus and nitrogen. At the end of the train, Flygt PP Pumps are used to re-circulate a percentage of the forward flow back to the in-line fermenter for biological phosphorus uptake.

Throughout the entire BNR treatment process, WTW instrumentation, which includes DO meters and probes, accurately measures the oxygen levels. Based on the oxygen readings, the air volume is controlled in each train of the BNR process. Monitoring and controlling the air volume is critical for achieving desired nutrient removal and also for minimizing operating costs.

Current Operations

Expansions at Cedar Creek WWTP have been a huge success and worked to optimize the process and operational performance at the facility. The BNR multi-step treatment process achieved lower-than-expected nitrogen and phosphorus levels.

With the first robust treatment system for nitrogen and phosphorus removal in the country, Cedar Creek WWTP is now meeting the most stringent EPA effluent requirements for nitrogen and phosphorus.

Additionally, the plant is experiencing up to 50-percent energy savings from implementing the fine bubble diffused aeration system compared to its previous aeration system technology. Continuous monitoring and controlling of the oxygen levels has also contributed to operation and maintenance savings. Storage in the expanded extraneous flow basins has dramatically reduced the frequency of wet-weather discharges. If discharging is necessary, extraneous flow basin effluent quality is improved, as both primary treatment and disinfection are now provided.

Plant operators are satisfied with the performance of one of the most sustainable, energy-efficient systems on the market. Joe Foster, Olathe wastewater superintendent, said, "The design of the expansion incorporates innovative technologies and serves the needs of Olathe residents now and into the future."

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