Dueñas, Camacho & Associates, Inc.


Agat, Guam


Wastewater Engineering

Serving the Agat and Santa Rita areas on the Island of Guam, the existing Agat-Santa Rita Wastewater Treatment Plant (WWTP) was operating 40 percent over capacity. As a subconsultant to Dueñas, Camacho and Associates, Inc., Gresham Smith was selected to design a new greenfield Biological Nutrient Removal (BNR) facility to improve wastewater service to Guam’s southern villages.

acre site
mgd of wastewater treated per day during dry weather
mgd treated per day during wet-weather conditions
Agat-Santa Rita Wastewater Treatment Plant under construction

No Snail’s Pace

The project was driven by a tight schedule to help the Guam Waterworks Authority (GWA) meet deadlines set by a Consent Decree enforced by the US EPA. As the new wastewater treatment plant was to be situated on a 44.37-acre site surrounded by wetlands and critical habitat for an endangered snail species, we took special care when siting the plant to avoid encroachment and disturbance of the delicate habitat. Designed to treat 1.6 million gallons of wastewater per day during dry weather and up to 9.3 million gallons per day during wet-weather conditions, the new facility can accommodate future wastewater flows from a nearby U.S. Navy installation—a connection that will increase the dry and wet weather design flows to 4.6 and 13.3 million gallons per day, respectively.

Oxidation ditches at Agat-Santa Rita Wastewater Treatment Plant

Complex & Compliant

The advanced replacement facility uses Biological Nutrient Removal via what is known as an oxidation ditch process with an “anoxic selector and secondary clarification” to achieve all-important compliance with the National Pollutant Discharge Elimination System (NPDES) permit. Effluent is discharged through a deep-ocean outfall pipe. A repurposed pump station on the existing Agat site—and a new pump station on the Baza Garden site that eliminated the Baza Gardens WWTP—delivers raw wastewater to the facility; the latter by way of a cross-island force main/booster station system. Other major process components include influent flow equalization, fine screening, grit removal, and Guam’s first municipal ultraviolet disinfection process.

Aerial view of Agat-Santa Rita Wastewater Treatment Plant

Improving, Protecting & Preserving

Better serving residents of southern Guam, the new Agat-Santa Rita Wastewater Treatment Plant not only improves wastewater treatment capacity and local water quality but also helps protect and preserve the island’s precious native coral. An additional environmental objective included in this project is a biosolids management process designed to produce disposable residual that meets the 40 CFR 503 biosolids rule for a Class B product.