The International Wastewater Treatment Plant (IWTP) is a sewage treatment plant developed by the International Boundary and Water Commission (IBWC) in the South Bay area of San Diego, California. Construction began on a 75-acre site (30 ha), west of San Ysidro in the Tijuana River Valley. The project, authorized by the U.S. Congress in 1989 and formally agreed between the two countries in July 1990, was part of a regional approach to solve long-standing problems, particularly the flow of sewage-contaminated water into the ocean via the Tijuana River.