A March explosion in a power generator at the Central Contra Costa Sanitary District sewage treatment plant has left it without its primary source of electricity while operators complete repairs.

General Manager Ann Farrell said the plant, east of Martinez at the Highway 4-Interstate 680 interchange, has been running on more expensive PG&E power since the March 29 blast inside a unit that burns natural gas in a turbine and supplies the plant with most of its electricity.

Operators are waiting to receive parts to finish repairing the 18-year-old unit. But until they have a fix, they will have to keep relying on secondary power that is costing $8,000 per day. The district had been paying $5,000 per day for the backup electricity, but that rate ended May 1, when "peak power" rates went into effect.

Farrell estimated repairs will cost $600,000 and said property insurance should cover that and other expenses, including the cost of PG&E electricity. The district has a $250,000 deductible, but Farrell said there should be no measurable impact to customers.

No one was seriously injured in the explosion, which occurred during routine maintenance. Four workers went to the hospital with headaches and to have their hearing checked, Farrell said.

A contract worker had been trying to restart the 3-megawatt Solar Turbine unit when it blew up. Farell said gas that is normally purged had accumulated in the unit's heat recovery and exhaust system, and that a spark from the ignition system caused the blast.

Workers immediately took the unit offline, and it remained on PG&E power, which they use during maintenance performed by contracted experts.

The plant treats an average of 45 million gallons of water per day for customers in 13 cities and towns, including much of the I-680 corridor and Lamorinda. The district hopes to be generating its own power again by mid-July.