Fish and other aquatic life need an adequate level of dissolved oxygen (DO) in the water and a suitable habitat to survive. So when the U.S. Environmental Protection Agency (EPA) mandated $112 million in treatment plant improvements to increase DO levels in a portion of the South Platte River, the Metro Wastewater Reclamation District (Metro District) started work to develop creative solutions. Working with CDM, the Colorado Department of Public Health and Environment (CDPHE), and EPA, the Metro District developed a non-traditional, natural approach to increasing DO and improving water quality and aquatic habitat in a 26-mile reach of the river designated as Segment 15. The plan is saving the Metro District $70 million in construction costs, and over $4 million per year in operation and maintenance costs.
CDM's multidisciplinary team of engineers and scientists, working closely with Metro District staff, addressed the DO problem by first evaluating alternatives to in-plant removal of ammonia, nitrate, and nitrite. This study concluded that in-stream reaeration structures could be designed to increase DO levels at a significantly lower cost than treatment plant improvements. In addition, CDM conducted field and laboratory studies to develop a new site-specific DO standard for Segment 15. Although the new standard is lower than the warm water Class II standard originally specified for Segment 15, CDM and the Metro District were able to prove to the regulators that the new DO criterion would still be protective of Segment 15's native aquatic life.
To increase DO concentrations, CDM designed channel improvements consisting of three low-head drop structures configured to maximize the tailwater bubble path. These structures, which incorporate boat chutes for safe boater passage and fish migration, also increase the stability of the river channel. Other improvements include the development of backwater and off-channel wetlands and other aquatic habitat, the design of a pedestrian bridge, the restoration of 2 miles of stream bank, and the protection and relocation of several utilities.
The benefits of this natural, in-stream approach include a significant savings to Metro District ratepayers, an improved/expanded natural habitat for fish species and other aquatic life, enhanced boater and recreational experience and safety, and a reduction in channel and river bank erosion.