Innovation at the Metro District

Phosphorus Initiative

In 2012, Colorado’s Regulation 85 required wastewater treatment facilities above a certain size to begin reducing the amount of phosphorus discharged to receiving streams. The regulation is in effect for the Metro District’s Northern Treatment Plant, and the Robert W. Hite Treatment Facility (RWHTF) will eventually have the same requirement.

If not properly managed, phosphorus removed during the treatment process can have significant cost and maintenance reliability consequences on solids stabilization, processing, and biosolids land application programs. In 2016, the Metro District focused its Phosphorus Initiative research and resources on approaches for managing phosphorus at the RWHTF. The effort ensures the District is prepared to address this permit requirement and also retain reliable and cost-effective operations.

Several technologies and concepts for phosphorus management were evaluated and range from 20th century best practice to leading-edge emerging technology concepts. In the end, the work conducted by Metro District staff found that AirPrex ( is a proven innovation technology which offers the most attractive performance and lifecycle cost attributes for the RWHTF. It simultaneously improved dewatering costs, reduced the phosphorus recycle load back through the treatment process, and will reduce equipment wear and maintenance costs otherwise caused by uncontrolled struvite precipitation. As a secondary benefit, the technology can be operated to harvest phosphorus from the biosolids stream so it can be reused as a fertilizer.

While there are currently no installations in North America using Airprex technology, the concept is being deployed at many larger-scale facilities for phosphorus management. The technology and other similar approaches are commonly applied at wastewater facilities in Germany and the Netherlands.

Return on Investment Graph

Delivery of AirPrex technology for pilot testing at the RWHTF

Phosphorus Initiative infographic