The hazmat incident was reported at about 9:10 p.m. at Vista Cove Assisted Living near Arroyo Drive and Santa Anita Street. Four apartment buildings with 20 apartments were affected.
Eighty-six non-ambulatory patients were inside the facility. A fleet of ambulances arrived in case of evacuations.
More than 30 patients were initially moved to the second-floor and were told to shelter in place.
All residents were later to be taken to a sister facility nearby. Ten people were to be moved to a facility in Arcadia.
No injuries were reported.
The sewage issue was expected to be restored overnight.