Gresham Smith-Designed Agat-Santa Rita Wastewater Treatment Plant Complete

July 9, 2019

Gresham Smith is pleased to announce the commissioning of Guam Waterworks Authority’s (GWA) Agat-Santa Rita Wastewater Treatment Plant. The new plant replaces an old, obsolete 0.9 million gallon per day (mgd) plant to provide GWA the critical capacity needed to serve southern Guam, as well as improve effluent quality. Agat-Santa Rita expands dry- and wet-weather capacity to 4.6 and 13.3 mgd, respectively.

The plant houses the first municipal ultraviolet (UV) disinfection system in Guam, which is an environmentally-friendly disinfection solution that eliminates formation of unwanted chemical byproducts. The high quality effluent produced by this new facility is discharged through a deep ocean outfall pipe.

“Ease of operations and maintenance are crucial, and since Guam is an island, we also needed a solution that improves discharge quality to help minimize impact on coral reefs and marine life,” commented Miguel C. Bordallo, P.E., general manager, Guam Waterworks Authority. “Gresham Smith and our design team brought us a solution that addresses the unique conditions we face, and spent the time to get into the weeds with our operators on what our challenges are and how they could be addressed. They partnered with local engineers to develop a great solution for GWA.”

“This wastewater facility is a critical piece of infrastructure needed to support the needs of the community in southern Guam. As we approached this design, we looked at how we could best address today’s capacity needs and regulatory requirements while helping GWA be more adaptable and flexible in the future,” said Ken Baker, senior vice president, Gresham Smith. “We made provisions in the plant layout for future chemical treatment including coagulation and tertiary filtration. If this provision is needed in the future, GWA will have capabilities to further reduce orthophosphate following the biological nutrient removal process.”

GWA celebrated the first on-island facility with secondary treatment capabilities and UV disinfection with a ribbon cutting on June 28. Villages of Agat and Santa Rita mayors, representatives of the Governor, and EPA Region 9 Administrator Mike Stoker, among other dignitaries, were in attendance to celebrate this momentous achievement for GWA.

Gresham Smith came up with a flexible, efficient process design. The facility has the capability to remove nitrogen and phosphorus using an oxidation ditch activated sludge process with anoxic/anaerobic selectors and secondary clarification. With consideration for constructability and value, the firm’s design team repurposed an existing influent pump station for improved performance while also designing all-new equalization pumping and a 1.0 million gallon storage basin to handle peak wet weather flows to the plant. Other process components include fine screening and grit removal to extract particulates at the start of processing. The Agat-Santa Rita facility was also designed to comply with EPA’s 503 biosolids rules for Class B biosolids by employing gravity belt thickening, aerobic digestion and centrifuges for dewatering.

This project is designed to address the Department of Justice’s consent order for improvements across the island. Gresham Smith was the lead process designer, working as a subconsultant to Dueñas, Camacho & Associates (DCA). The firm is working with DCA on several other projects in Guam including a progressive design build for Umatac-Merizo overland wastewater treatment plant, the Northern District Wastewater Treatment Plant, a sewer improvement project, which eliminates the Baza Gardens wastewater treatment plant by re-routing wastewater to the new Agat-Santa Rita facility, and the Route 3 design build project to rehabilitate 45,000 linear feet of sewer. Gresham Smith recently completed a study on the use of peracetic acid (PAA) as an interim disinfection process at the Agaña Wastewater Treatment Plant, which is chemically-enhanced primary treatment plant.


Gresham Smith-designed wastewater treatment plant
Gresham Smith-designed wastewater treatment plant

Gresham Smith-designed wastewater treatment plant