The growing community of Castle Rock, CO has experienced exponential population growth during the last ten years and faced corresponding pressure on existing infrastructure. With the price of real estate rapidly rising and encroaching sub- divisions gobbling up available land, the Plum Creek Wastewater Authority (PCWA) addressed a problem that is becoming common for public utilities in growing suburbs--the need to expand the treatment plant within extremely tight space constraints while upgrading to meet current regulations. Plum Creek had to find a way to double the plant capacity and meet the federally mandated TMDL limits on nitrogen and phosphorus levels.

“Plum Creek brought us on board to help them plan and create a state-of-the-practice and operator-friendly facility that fit in the limited space,” says Dave Oerke, vice president and senior project manager with the Denver-based engineering firm of Rothberg, Tamburini & Winsor, Inc. (RTW).

Working with RTW, the Plum Creek staff began exploring technology. The preferred solution included biological nutrient removal (BNR) with deep oxidation ditches that would take up less land, meet tight effluent regulations and allow for future growth. Oxidation ditches are easier to operate and more stable than conventionally-activated sludge systems. They provide significant load equalization, have lower operating costs with denitrification and predictable effluent quality; but deep ditches also require efficient mixing. Although oxidation ditches with horizontal brush rotor mixers were evaluated, they were not as efficient, especially during the winter months and maintenance and operating costs were cause for concern. An alternate solution, the Landox Process, was explored. The technology has been used in the U.S. on a limited basis and more commonly in Europe. The PCWA staff and RTW met with representatives from equipment manufacturer, WesTech Engineering of Salt Lake City, UT who had experience with the process. “During the design phase,” Oerke says, “we performed significant research and toured facilities in Germany and Holland.”

The Landox Process combines the oxygen transfer efficiency of fine bubble aeration and the efficient mixing of the unique Landox vertical drum. While oxidation ditches with horizontal brush rotors become mixing-limited at 12 to 14 ft, this vertical mixing drum allows for a 21-ft side water depth needed for this application. This depth had only previously been successfully achieved in the systems installed in Germany and Holland. “The Landox system provides a reliable, low-energy mixing solution for deep ditch installations,” says Mark G. Biesinger, chief process engineer for WesTech Engineering.

Construction was carefully sequenced by PCWA, RTW and Garney Construction Company so that existing treatment processes and activities were successfully kept on line while the new ones were constructed.

The plant with its new facilities met the effluent permit limitations on the first day of operation. “We’re very proud of that,” Oerke says. “Most plants want these kinds of results, PCWA got them.”