The City of Tijuana, is located at the northwestern-most part of the United Mexican States in the state of Baja California. Its population has grown from 21,977 in 1940 to 1,092,468 in 1990. This rapid growth has made it difficult to construct, operate and maintain a wastewater collection system that keeps pace with the increasing population. The terrain is generally higher in elevation in Mexico than in the United States resulting in natural drainage from South to North. As a result of this natural drainage pattern, intermittent transboundary sewage flows from Mexico to canyons and drains in the United States are common. Such transboundary discharges present a serious threat to the health and well-being of inhabitants in both countries and to their beneficial use of the waters of the Tijuana River and of the ocean beaches near the boundary in the San Diego-Tijuana area. To alleviate this problem, Mexico has been taking steps to improve the Tijuana disposal system. under the procedures in Commission Minute No. 261, concluded under the 1944 Water Treaty in which the two Governments agree to give preferencial attention to the solution of the border sanitation problems.
In the 1930's through the 1960s, an international collector and septic tank system with a shoreline discharge provided a solution. In the 1960s Mexico constructed two pump stations and two pressure line systems along with an open channel along the western slopes near the coast, to provide an ocean discharge of Tijuana's sewage without treatment at a point some 5.6 miles directly south of the international boundary.
In 1965 the United States and Mexico concluded IBWC Minute No. 222 which authorized the construction of an emergency connecting pipeline between the main collector line in the City of Tijuana, Baja California and a branch collector line of the San Diego Metropolitan Sewage System. This connection provided, at times of breakdown in the Tijuana pumping plants or facilities, the safe disposal of Tijuana sewage in the San Diego system to avoid a serious unsanitary condition as might be caused by an overflow of waste water onto lands in the City of San Diego and in the streets in the City of Tijuana. The connection, constructed under authorization in the Act of August 19, 1935, was completed in 1966.
The connection consists of a valved turnout pipe in Mexico, extending northward 300 feet (91.4 m) to the international boundary, thence a pipeline continuing northward in the United States a distance of 4,277 feet (1,304 m) partially under the Tijuana River floodplain to the San Ysidro branch collector line in the United States. The installation includes a metering station in the United States. Each Government paid the costs of the works it its territory. Mexico made use of the connection intermittently through 1975 and extensive use of the connection until 1998, with the construction of the South Bay International Wastewater Treatment Plant (SBIWTP).
The Government of Mexico under the terms of Commission Minute No. 270 constructed between 1985 and 1990 a new sewage conveyance system generally following the alignment of the 1965 system and a new treatment plant at a point directly 4.0 miles south of the international boundary. The system consisted of a new Pumping Plant No. 1 near the border in Tijuana with a design pumping capacity of 64 mgd, and an average daily operational pumping capacity of 48 mgd. The pumping plant was followed by a 42 inch diameter concrete asbestos lined steel pressure line near the boundary extending westwards approximately two miles. The pressure line discharges to a 60 inch gravity line and then to a concrete lined conveyance channel. The collection system also receives sewage flows from developments in the canyon areas known in Mexico as Canon de los Mataderos and the Canon de los Laureles (Smuggler's Gulch and Goat Canyon in the United States), from a pumping station in the Playas de Tijuana part of the Mexican city, and from Maquiladoras in the western part of Tijuana. Estimated sewage contributions to the collection system, other than Pumping Plant No. 1, are from 5 to 7 mgd. The gravity pipe and open canal system contain five siphons before the Tijuana treatment plant, and there are four siphons after the treatment facility. The treatment plant, know as the San Antonio de los Buenos Wastewater Treatment Plant, provides treatment of 25 mgd of Tijuana sewage before discharging to the surf zone. An additional 17 mgd second module was canceled, in favor of joining the United States in the construction, operation and maintenance of the South Bay International Wastewater Treatment Plant (SBIWTP) under the terms of IBWC Minute No. 283.
The South Bay International Wastewater Treatment Plant (SBIWTP) is located on a 75-acre site near the international border and provides for advanced primary treatment of 25 mgd of Tijuana sewage. The SBIWTP construction was completed in April, 1997; the South Bay Ocean Outfall was completed in January, 1999, at which time the SBIWTP became fully operational. . The Government of Mexico contributed $16.8 million toward construction of the SBIWTP and currently contributes $1.1 million toward the annual operation and maintenance costs. Funding for the U.S. share of construction costs was appropriated through the Environmental Protection Agency in the amount of $239.4 million. Of that amount, $225.5 million had been obligated as of 2002, of which $89.2 million was given to the City of San Diego and the Corps of Engineers to construct the South Bay Ocean Outfall; $8 million was given to the Corps of Engineers for environmental work; and $127.4 million was given to the USIBWC for the costs associated with the construction of the SBIWTP and related infrastructure. Mexico's share of the construction cost was that amount that Mexico would have had to pay to construct and maintain a plant at the Rio Alamar, and the construction of a 144 inch diameter land and ocean discharge outfall with canyon collectors. Both countries share in the operation and maintenance of the SBIWTP. Mexico is expanding its sewage collection system, and constructing additional works necessary to collect and convey Tijuana's sewage. These future facilities will be operated and maintained at Mexico's expense.
The State of Baja California has constructed a parallel conveyance and treatment system in Tijuana similar to that constructed under IBWC Minute No. 270 which includes pumping facilities, pressure lines and rehabilitation of the San Antonio de Los Buenos Wastewater Treatment Plant. The rehabilitation of the treatment plant provides secondary treatment to an average daily flow of 25 mgd discharged at the shore at a point 5.6 air miles south of the international boundary. The system will be interconnected with the (SBIWTP) and will follow safeguards against transboundary pollution established by the United States and Mexico in Commission Minute No. 298 signed on December 2, 1997. The system was developed in a partnership coordinated by the Border Environment Cooperation Commission (BECC)and the North American Development Bank (NADBANK), the IBWC, the Environmental Protection Agency (EPA)and Mexico's Comision Nacional del Agua (CNA) , with support from the State of California and the City of San Diego.