About 10,500 gallons of partially treated sewage was discharged Friday afternoon into Wahiawa Reservoir, the city said.
The city Department of Environmental Services responded to the discharge of partially treated effluent from the Wahiawa Wastewater Treatment Plant that was not disinfected.
The incident was reported at 2:10 p.m.
The ultraviolet disinfection unit lost electric power for 17 minutes because of a generator failure, the city said in a written news release.
Environmental Services informed the Department of Health, which waived water sampling.
Warning signs will be posted daybreak Saturday at the reservoir.