Denver’s Metro is on the road to a utility of the future

March 23, 2017 Art Umble

Art Umble explains how Denver’s Metro is better serving its customers and finding sustainable solutions.

The Metropolitan Wastewater Reclamation District of Denver (Metro), in Colorado, USA has embarked on an exciting fifteen-year journey to a sustainable future for its water reclamation facility (210 MGD design rating) serving its 1.8 million customers in the Denver metro region. In doing so, Metro is demonstrating its leadership in integrating innovative technology to meet these objectives, and MWH, now part of Stantec, is providing strategic research, facility planning, process/detailed design, and construction services for Metro in this endeavor.

Recently, several new regulatory initiatives were promulgated focused on enhancing the quality and ecology of the South Platte River, the receiving stream for Metro’s treated effluent. These regulations impose much lower effluent nitrogen, phosphorus and temperature levels for the discharge. It became immediately clear that Metro had two choices. It could invest hundreds and hundreds of millions of dollars in proven conventional technology approaches consisting of numerous new infrastructure process components, or it could accept risks associated with emerging technologies and push innovation. This, in turn, would save hundreds of millions of dollars in infrastructure, allow for the re-purposing of existing assets, create new revenue streams for the utility, and all the while produce even higher quality effluent than conventional system approaches for discharge into the South Platte River.

The innovative approach included first crafting a “roadmap” to the “Utility of the Future” for Metro. This roadmap consists of a hierarchy of objectives that Metro embraces as sustainable best practices. The backbone of this roadmap has three major initiatives, ordered by their propensity to meet short and long term regulatory, environmental and financial requirements, with a focus on recovering as many resources present in the wastewater stream as practical:

1) nutrient removal and recovery

2) energy management

3) water reuse.

The methodology embraced by Metro was simple: identify potentially disruptive, applicable technologies, conduct bench research and pilot those technologies at demonstration and full-scale to prove performance and applicability, and evaluate and select the preferred technologies through a detailed “sustainable return on investment” methodology. In late 2011, Metro took the first steps on the roadmap path with a full-scale pilot for a sidestream Enhanced Biological Phosphorus Removal (EBPR) process. This technology, which provides for maximum release of phosphorus from microbes thereby enhancing the maximum phosphorus uptake in the subsequent mainstream reactor.  By utilizing a sidestream, no existing reactor volume, necessary for nitrogen removal in the mainstream, had to be relinquished for the EBPR. Furthermore, a rich carbon source necessary to drive the sidestream EPBR process was obtained by repurposing existing tanks, used for the gravimetric thickening of primary sludge, as fermenters.  Because of the success of the pilot, full design proceeded in 2016 for the upgrade, with construction in 2017, at an overall savings to Metro in the range of $70 million.

With the EBPR process in place, Metro could proceed to the next step in its roadmap to sustainability, phosphorus recovery. Metro historically experiences significant maintenance problems associated with the deposition of struvite (magnesium-ammonium-phosphate) resulting from phosphorus release from sludges during anaerobic digestion. With the implementation of EBPR, this problem would be expected to escalate, and in fact was indeed found to be the case during the pilot. Conventional means for controlling struvite typically include addition of chemicals, such as iron chloride, something that Metro had done in the past at significant annual cost.  Metro instead saw this challenge as an opportunity. By intentionally controlling the formation of struvite via a sidestream phosphorus recovery technology, the phosphorus could be recovered and sold as a fertilizer product to the agriculture sector.  In so doing, the nuisance deposition could be drastically reduced, resulting in huge savings in annual O&M costs, lengthening the service life of equipment that no longer experienced damage from the struvite deposits, and generating a new revenue stream for the utility via sale of the recovered struvite product to the fertilizer industry.  Furthermore, by “breaking the phosphorus cycle” within the plant through the recovery step, the effluent quality of the treated water discharged to the South Platte is enhanced significantly.

Regulatory requirements for nitrogen discharged to the South Platte River will be ratcheted down substantially over the next decade. Here again Metro has embraced this as an opportunity rather than a constraint. Conventional means for removing nitrogen typically requires expansions to infrastructure and significant inputs of energy and organic carbon to drive the process. A large source of nitrogen at the Metro plant is from recycle streams from the processing of solids with anaerobic digestion. Metro recognized that if this nitrogen-rich stream can be treated, huge savings on energy could be realized since that nitrogen would no longer be treated in the mainstream. Furthermore, by reducing the nitrogen in the recycle streams, effluent nitrogen quality would also be improved.  In 2012, Metro embarked on a pilot study of an emerging technology that utilizes the anammox bacteria, a remarkable organism that removes nitrogen with less than 50% of the required energy input and without need for an organic carbon co-substrate.  The success of this pilot research promoted the full-scale design of an Anita-MOX™ process in 2016, again utilizing existing tankage assets at the site, thereby eliminating the need for costly, new infrastructure. Construction and startup for the process is scheduled for summer of 2017.  Overall, the implementation of this technology will save Metro potentially more than $100 million in new treatment infrastructure if conventional approaches would have been pursued.

Metro continues to push forward with initiatives for sustainability that is making the utility a beacon to the industry. Evaluations for initiatives including:

1) the extraction of heat from the treated effluent to reduce the temperature in the discharge to protect sensitive aquatic life in the South Platte River, and recovering that thermal energy to heat buildings and processes (such as digesters) on site

2) providing large quantities of effluent water to Denver Water for further treatment for reuse within the local community

3) use of alternative disinfection technologies (such as peracetic acid) to reduce operational costs while enhancing disinfection effectiveness

4) application of thermal hydrolysis processes for the reduction of sludge quantities while resulting in increased on-site energy generation energy potential due to the increased production of biogas and producing a “Class A” biosolids

5) the potential for using natural system technology such as algae in lieu of conventional tertiary treatment methods for further reduction of nutrients to very low levels, with the opportunity to generate new revenues from the sale of the algal biomass to animal and aquaculture feed and biofuel markets.

Metro is truly a leader in innovation that is demonstrating the limitations and constraints of conventional approaches. MWH, now part of Stantec, is proud to be working directly alongside Metro as its trusted partner on its roadmap to its Utility of the Future.

Content was originally published by MWH Global, which is now part of Stantec.

About the Author

Art Umble

As the global lead for wastewater practice, Art Umble's position involves developing strategies and providing solutions for complex wastewater treatment challenges. His role? To connect our Company’s global resources in process design to promote collaboration and deliver comprehensive knowledge and expertise to our clients.

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