Two years after raw sewage flooded from a South County wastewater treatment plant into an Oceano neighborhood and backed up into some homes, seven residents have sued the administrator of the facility.

In a lawsuit filed Dec. 18, the residents allege that John Wallace, administrator of the South San Luis Obispo County Sanitation District, and his San Luis Obispo-based engineering firm, Wallace Group, failed to take steps that would have prevented massive flooding in the area in 2010.

The civil lawsuit was filed in San Luis Obispo Superior Court initially by six residents and property owners, including John Carter, Darlene Prebyl, Greg Cobb, Alana Reynolds, and David and Delilah Villalba. A seventh plaintiff, Mary Ehens Stanavage, was added to the suit Dec. 28.

A case management conference has been set for April 23.

The complaint alleges that Wallace and his firm failed to protect the plant’s equipment from flooding, didn’t provide an adequate emergency pump that could operate without shutting down, and didn’t conduct adequate sampling after the spill.

The spill happened Dec. 19, 2010, after rainwater from heavy storms flowed into the treatment plant and caused an electrical short that shut down four influent pumps about 10:30 a.m. Thousands of gallons of sewage spilled into the neighborhood and creeks adjacent to the treatment plant.

Estimates of the spill volume later ranged from about 400,000 gallons to 3 million gallons. The Central Coast Regional Water Quality Control Board — which issued a $1.1 million fine against the sanitation district in October — estimated that 674,400 gallons spilled.

Sanitation district officials later certified six sewer backups where untreated sewage was discharged inside private homes.

“Defendants’ conduct constituted conscious disregard for plaintiffs’ safety in that they had advance knowledge, in some cases years, of the risks posed by the plant yet chose not to make needed modifications and repairs knowing this would place the plaintiffs at risk for exactly the type of harm they ended up suffering,” the suit states.

Wallace referred questions to the district’s attorney, Michael Seitz, who declined to comment until he had a chance to talk to the district’s three-member board at its meeting Wednesday night except to say “that the factual allegations will be strongly disputed.”

The sanitation district serves about 38,000 residents in Arroyo Grande, Grover Beach and Oceano.

Sanitation district officials have maintained that the spill happened because of a series of unforeseen events beyond the district’s control, and disputed that preventive work — namely, a rewiring project — would have kept the spill from occurring.

District officials have also said that the heavy rains diluted the sewage to nearly harmless levels. In a previous interview with The Tribune, Seitz said that fish populations in Arroyo Grande Creek were not harmed and no one filed a claim against the district as a result of the spill.