Background

Nogales is in the County of Santa Cruz in the south-central region of the state of Arizona. The Nogales International Wastewater Treatment Plant (NIWTP) treats wastewater for the communities of Nogales, Arizona and Nogales, Sonora. The International Outfall Interceptor (IOI) transports wastewater originating in both communities to the NIWTP located in Rio Rico, Arizona. The plant is owned by the City of Nogales, Arizona, and operated by the U.S. Section of the International Boundary and Water Commission (IBWC). In the late 1990s, structural and hydraulic capacity problems with the wastewater collection system was allowing excess amounts of raw sewage from Nogales, Sonora, to enter the system, resulting in wastewater overflows from the collection system to the environment. In addition, more stringent effluent quality standards were expected to be applied to the NIWTP discharge permit, requiring an upgrade to its treatment process.

Scope

The project consisted of expanding the NIWTP to treat a total of 22.2 mgd and upgrading its Modified Ludzack-Ettinger (MLE) process to achieve the necessary nitrogen concentration levels, as well as constructing a new 8.9-mile interceptor with diameters ranging from 48 to 60 inches and replacing approximately 5,900 feet of the wastewater collection system in Nogales, AZ.

The original project scope was later reduced to include only the treatment plant improvements, while the replacement of the IOI was postponed, and rehabilitation of the wastewater lines were out by the City under a separate project.

Benefits

The expanded and upgraded plant is producing a higher quality effluent that complies with current U.S. standards. The treated effluent from the NIWTP is discharged into the Santa Cruz River, where it forms a perennial surface water stream extending 16 miles to the area of Tubac, Arizona, which is helping preserve an important riparian habitat and provide groundwater for downstream users.