The group bid 326mn soles (US$118mn) for the project, Graña y Montero said in a filing to Lima's bourse. The two companies each have a 50% stake in the consortium.
Lima water utility Sedapal had set an estimated budget of 299mn soles for the project.
Work involves designing and building a wastewater treatment plant and installing over 600km of potable water and sewerage pipelines, as well as domestic connections.
The project is the largest to be carried out under the government's Agua Para Todos [water for everyone] program, benefiting some 150,000 residents in Callao's Ventanilla district.