More than 100 workers and a dozen private contractors are scrambling to repair the critical Sand Island Wastewater Treatment Plant that was flooded by Hurricane Ana, which left behind 20 million gallons of untreated sludge that overwhelmed the plant Sunday morning and left it without power.

After 5,000 gallons of raw sewage poured out of manholes into Hono­lulu Harbor on Sunday morning, sewage treatment plant workers discovered a series of below-ground rooms, tunnels and critical panels under 6 to 8 feet of effluent and water that reached as high as door sills.

The crippled plant represented the worst of Ana’s damage on Oahu, Mayor Kirk Caldwell said.

"Before Ana left she left us a good kick in the butt," Caldwell said before leading reporters on a tour of the plant. "The good news is there were no injuries to anyone."

Caldwell hopes repairs will be made before the end of the week. He had no estimate for the costs for overtime, outside contractors and repairs to the plant’s electrical system. But Caldwell said he did not expect the price tag to reach into the millions.

Even as workers continued to clean out the sludge left behind by Ana on Monday, about 70 million gallons of new raw sewage was being processed by gravitational flow without electrical assistance.

The plant serves Hono­­lulu’s dense urban core from Hawaii Kai to Red Hill and normally processes 60 million to 70 million gallons of urban sewage on a typical day.

But around 9 a.m. Sunday morning, stormwater from Ana overwhelmed the plant for about 15 minutes with 240 million gallons of combined water and sewage.

Normally, sewage flows into the plant through two "channels." But one is down for repairs, and the other could not handle the sudden surge of water and sewage.

"One channel can only hold half of that, maybe 110 to 120 million gallons," said Markus Owens, spokes­man for the city’s Department of Environmental Services. "You just had too much coming."

The overflow from the working channel spilled into the adjacent one under repair, which allowed the sewage to pour into concrete rooms and tunnels below that house electrical panels that were all damaged.

Then around 11 a.m. another stream of 130 million to 140 million gallons of sewage poured through the plant.

Ana also knocked out power to the Synagro Bioconversion Facility next door that normally handles the sludge left behind after water is sent to be cleaned via ultraviolet rays.

Workers on Monday continued to pump out the sludge and transport it manually to Synagro, which had its power back up Monday afternoon.

No new storms are forecast for the next seven days, Caldwell said, giving officials time to make both repairs and contingencies for the next storm.

"This is the worst damage we’re going to see from Ana," Caldwell said. "We definitely want to avoid this from happening in the future. The good news: It was all contained."

The city remains under a federal consent decree to make sweeping sewer upgrades by 2035.

City officials, meanwhile, reopened Hanauma Bay, the Wai­pio Soccer Complex, Central Oahu Regional Park and the Hono­lulu Zoo, which had all been closed because of Hurricane Ana. The West Loch Municipal Golf Course remained closed.

From Friday afternoon through Sunday night, the National Weather Service said, Ana dumped 11.05 inches of rain on Manoa’s Lyon Arboretum and 11.67 inches in Keaumo on Hawaii island. A high-surf advisory remained in effect for all islands until 6 a.m. Tuesday.