HILO, Hawai'i — Hilo shoreline areas reopened yesterday after tests showed bacterial levels had declined to "acceptable levels" after the largest sewage spill in at least a decade from the Hilo Wastewater Treatment Plant.

A total of 2 million gallons of treated — but unchlorinated — sewage was released from an outfall outside the Hilo breakwater between 4:27 p.m. Friday and 7:15 a.m. Saturday after a pump and alarm control system failed, prompting a bypass of part of the treatment process.

The outfall where the partially treated sewage was released is a half-mile to three-quarters of a mile offshore, said Lyle Hirota, deputy director of the wastewater division of the Big Island's county Department of Environmental Management.

Hirota said repairs to the malfunctioning electronic equipment were made Saturday, and wastewater officials are looking into possible improvements that could create a redundant backup system to prevent any similar spills.

"We are looking at ways to make sure that hopefully it doesn't happen again," Hirota said.

Hirota said in his 10 years with the wastewater system, "we've never had a bypass of this volume ... that I'm aware of."

Signs warning ocean users of contaminated water were posted from the base of the Hilo breakwater to Puhi Bay in Keaukaha. They advised the public not to do any swimming, fishing or boating. Those signs were removed yesterday morning.

Reach Kevin Dayton at kdayton@honoluluadvertiser.com.