Project Overview

Boulder’s Water Resource Recovery Facility (WRRF) was constructed in the late 1960s to treat wastewater generated by the city before discharging treated water to Boulder Creek. The 25 million gallon per day facility has undergone several improvements over the years to adapt to changing regulations, enhance resource recovery, improve energy efficiency, and modernize aging assets.

Colorado is adopting increasingly stringent nutrient limits, and Boulder will soon be required to meet increasingly tight nitrogen limits, as well as new limits for phosphorus. The WRRF is already out-performing the expected nitrogen limit, and is participating in an incentive program further reducing nitrogen in the discharge. This project will extend the treatment capabilities for the facility to also include phosphorus reductions.

Project Background

Detailed planning for nutrient reductions began in 2012 with the Nutrient Compliance Study #1. This study set a framework for the facility to address upcoming limits in phases, based on anticipated regulatory timelines. This study recommended a chemical solution for phosphorus reduction. Since the completion of that study, the city conducted several pilot/demonstration studies to further evaluate chemical phosphorus removal at the WRRF.

As technologies have evolved over the last ten years, the city chose to re-evaluate options with an update in 2021. This update led to a more sustainable removal solution than the original study and assessed the whole-plant impacts with improved sidestream processes. The recommendations included in the update formed the basis for this project. The resulting project scope includes three main elements:

  • Upgrades to the secondary treatment system,
  • Improvements to sidestream treatment processes; and
  • Revitalization of existing facility assets. Work for this project will be confined within the existing facility.

Phosphorus Removal Upgrades

The secondary process at the WRRF is the part of the plant primarily responsible for organics and nutrient removal. 12 alternative technologies were evaluated to implement phosphorus reductions at the WRRF. Generally, each of these options fell into one of three categories:

  • Chemical: processes where a chemical is added to remove phosphorus from the wastewater, and resulting particulates are removed by settling
  • Biological: processes that encourages the growth of organisms that uptake higher amounts of phosphorus, then the organisms are removed by settling
  • Biological with intensification: biological processes with technologies that reduce footprint or operate in a more energy efficient way.

The city screened these alternatives based on modeled treatment performance, as well as financial, operational, environmental and community impacts. The recommended low dissolved oxygen aerobic/oxic alternative is an intensified biological process technology on the leading edge of the industry. This process combines the performance of a conventional biological process with decreased energy usage in a moderate footprint. For Boulder, this will improve effluent quality, reduce energy usage and avoid constructing additional process basins. The infrastructure required for this technology will require a small, unoccupied building to house new process and electrical equipment and two small storage tanks. The remainder of the project will be isolated to existing structures and buried piping and conduits on the site.

Sidestream Nutrient Management

Biological phosphorus removal is expected to increase maintenance requirements in the solids process due to scaling in pipes, valves and tanks. Solids handling improvements will be phased in as needed to mitigate these impacts. The implementation plan makes use of existing assets in the near term with minimal capital expense, while building more advanced systems in future years.

This plan initially includes converting an existing solids holding tank into a redundant post aerobic digester so operators can drain and clean the existing unit without affecting plant performance. A pilot-scale, calcium-based chemical feed system will support applied research for staff to test different approaches for a more advanced, full-scale system. Additionally, this approach allows staff to continue to monitor market conditions and assess whether phosphorus recovery becomes economically viable in the future.

Existing Asset Revitalization

The project includes asset revitalization in the primary and solids treatment processes. The respective capacities of these systems will remain unchanged. The extents of these efforts are dependent on available funding. This rehabilitation allows the city to maximize its beneficial use of existing infrastructure, extending its useful lifespan.


The project will be funded through a combination of a loan from the Clean Water State Revolving Fund (SRF) and bond financing, both paid back by utility bills. The SRF loan provides opportunities for a portion of the loan to be forgiven through the Bipartisan Infrastructure Law, as well as lower interest rates for green infrastructure components of the project.

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