Denver's Direct Potable Water Reuse Demonstration Project

The Denver Direct Potable Water Reuse Demonstration Project was an initiative aimed at evaluating the feasibility of using treated wastewater as a direct source of drinking water in Denver, Colorado. Conducted between 1979 and 1990, this $30 million project was managed and operated by Denver Water, the city's primary water utility, and was partially funded by the United States Environmental Protection Agency (EPA). The project's primary objectives were to assess the safety, quality, technical feasibility, and public and regulatory acceptance of direct potable water reuse.

A critical component of the project was the establishment of a 1-million-gallon-per-day (mgd) treatment plant, which operated continuously to produce treated water for testing. The plant integrated a comprehensive suite of advanced treatment processes, including lime softening, granular media filtration, ammonia removal and recovery, activated carbon adsorption, ozone and chlorine dioxide disinfection, ultrafiltration, air stripping, and reverse osmosis. These processes were meticulously optimized and evaluated for their efficacy in producing water suitable for direct consumption.

The project addressed several key areas of concern, such as product quality, public health, economic viability, and the technological feasibility of treating wastewater to drinking water standards. One of the most significant aspects of the demonstration was a two-year animal health effects study, which involved the use of treated water to evaluate its safety. This study specifically focused on reproductive health and the potential long-term impacts of consuming treated wastewater on multiple generations of rats and mice.

Another notable feature was the extensive water quality testing program that accompanied the project. The testing monitored a wide range of contaminants, including organic and inorganic chemicals, radiological isotopes, and microorganisms such as bacteria and viruses. The treated water was continuously assessed against stringent EPA and international standards, with Denver's current high-quality drinking water serving as the primary benchmark for comparison. The project was designed to ensure that the treated water met or exceeded these established safety standards.

In addition to the scientific and technical evaluation, the project included a robust public awareness campaign aimed at informing and engaging the public. Tours of the treatment plant were organized to offer citizens firsthand insight into the treatment process, and various media outlets—such as television, radio, and newspapers—were used to disseminate information about the project's progress and findings. Moreover, technical articles and presentations were shared with water professionals and regulatory agencies to promote wider discussion within the industry.

An important element of the outreach effort involved conducting surveys to measure public attitudes towards the concept of potable water reuse. University researchers were engaged to evaluate how the project influenced public perceptions and acceptance of the idea of drinking water sourced from treated wastewater.

Historical Justification

In the 1970s, the water resources of the Denver Water Department (Denver Water) were sufficient to meet the needs of its service area. However, anticipated growth in demand threatened to surpass the department’s existing capabilities. To address this, Denver Water explored several strategies to expand its supply, including the use of wastewater effluent, conservation efforts, and large-scale storage or diversion projects.

New storage and diversion projects located far from the metropolitan area carried an unacceptable cost, and environmental concerns raised doubts about the viability of large-scale diversion. As a result, these options were dismissed.

Conservation, while acceptable to the public, was an unproven alternative. The department had already launched an aggressive campaign, and this would continue. However, the net effect of these efforts was largely speculative, so it could not be relied upon to provide the anticipated water needs.

Colorado water law requires that wastewater be returned to the river from which it originated after one beneficial use. However, courts have ruled that trans-mountain water, which is diverted from one river basin to another, is exempt from this requirement. As a result, Denver had an estimated 100 million gallons per day (mgd) of water available for reuse. After considering various water-use options, such as exchanging wastewater effluent for upstream raw water, industrial reclamation, and non-potable reuse, Denver Water determined that direct potable reuse (DPR) was the only solution that could address the projected supply shortfall and use the volume of water available . Additionally, projections indicated that by the early 2000s, treating wastewater effluent for DPR would be economically competitive with developing new conventional water sources. However, significant research was required before this untested alternative could be implemented.