Construction could begin as soon as Aug. 1 on the first part of a $10.8 million expansion and rehabilitation of South Davis County Sewer Improvement District's two treatment plants.

According to district manager Dal Wayment, bids will be opened Thursday for the expansion of the north treatment plant at 1800 W. 12th North in West Bountiful. The north plant expansion is expected to cost $7.8 million.The $3 million rehabilitation of the south plant at 2500 W. Center St. in North Salt Lake City is still in the planning stages. Bids for that part of the project are expected to be let early in 1989, Wayment said.

The district serves residents in Bountiful, West Bountiful, North Salt Lake, Centerville and other unincorporated areas of south Davis County.

The expansion should handle expected South Davis County population growth during the next 20 years. The plants will be designed to serve 100,000 people - 30,000 more than now live in the area.

"The No. 1 problem is that we are operating 20 percent over designed capacity," Wayment said.

The 25 year-old systems have required frequent repairs and replacements. The waste water that the plants dump into the Great Salt Lake after processing the sewage has also failed federal Environment Protection Agency guidelines. The EPA has put the district on a schedule that requires full compliance by the end of 1990. Construction is expected to be completed before then.

The Utah Bureau of Water Pollution Control will fund the project with a 20-year 3 percent loan through a revolving loan fund. The district recently raised its rates by $3 a month and increased the cost of sewer hookups to help repay the loan.

Wayment said he hopes after the expansion project is completed that the district board will approve cuts in rates. He said the project may cost less than its estimated price tag because of the current competitive construction market and a recent federal Labor Department decision that sets a new union wage for sewer and water projects in Salt Lake and Davis counties.