South Davis Sewer District officials broke ground Thursday for the construction of a $7 million expansion of the district's north sewer treatment plant.

The project will include the expansion of a clarifier system, a basin where waste water is treated with chlorine, construction of more sludge drying beds, and the replacement of equipment in a airless digester system where solid waste is decomposed. A new electrical system, including a standby generator, is also planned, said Dal D. Wayment, district manager.Joseph F. Patrick, sewer district board vice chairman, said that even with the expansion project the district hopes to maintain some of the lowest sewer rates in the state. Monthly rates in the district will remain about $3 per connection.

"Because the bid came in so low we are not having to raise rates as much as we thought we would," Patrick said.

The north plant, located at 1800 W. 12th North, dumps treated waste water into the Great Salt Lake. The north plant expansion is part of an overall $11.8 million project that also includes renovation at the district's south treatment plant in North Salt Lake.

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

Current construction estimates of $11.8 million are well below the original estimate of $13 million. Officials attribute it to lower construction costs caused by a competitive bidding atmosphere and a recent decision by the U.S. Labor Department to reduce union wages for sewer and water facility construction in Wasatch Front counties, Wayment said.

"When the project came in $1 million cheaper it was better than our wildest hopes," District Board Chairman Vernon B. Carr said.

Work at the north plant is being done by Nelson Brothers Construction Co. and is expected to increase plant capacity from 5.8 million gallons per day to 12 million gallons per day, Wayment said.

"By the end of 1990 we hope to have everything complete and on line ready for the next 20 years of growth," Wayment said.

Carr said good maintainance at the plant has saved the district from replacing about $15 million worth of equipment. Instead of replacing the entire plant they will only need to add to the existing equipment.

The plant, originally built in the 1950s, was expected to last only 20 years but has lasted 30 years, Patrick said. Even though it has been exceeding flow levels it was designed for, staff has kept if from exceeding federal waste standard levels, Patrick said.

The project is being funded through a state revolving loan fund. Some $4.5 million of the project will be loaned during the 1988 fiscal year.