Water District Using Large Crane to Replace, Repair Failed Clarifier Components

The DSRSD treatment plant uses four secondary clarifiers near the end of the treatment process  to separate solids from the treated wastewater. Each clarifier has two pairs of arms that move in a circle, like hands on a clock—one on the surface of the water skimming floating material, and another on the bottom of the tank collecting the settled solids. After treatment in the clarifier, the cleaned wastewater is disinfected and either discharged into the San Francisco Bay or diverted to the water recycling plant for additional purification.

When the clarifier failed, the skimming mechanism jammed, twisting the steel arms and cracking the drive gear box. Many of the old components are no longer manufactured.

“A failure of this magnitude is always unwelcome but not unexpected.  Based on the age of the unit and our regular inspections and preventive maintenance, we had been planning to rebuild this clarifier in 2014. When it broke, we had to do it sooner,” said Assistant General Manager/District Engineer Dave Requa.

The budget for rebuilding the clarifier is $924,000. The District rehabilitated another secondary clarifier of the same size in 2006 and chose to install an identical mechanism for this replacement. This approach decreased design costs and will reduce future maintenance costs by having interchangeable parts. The clarifier is scheduled to be back in service by mid-summer. The plant is designed with enough redundant capacity at each treatment step to continue operating when equipment is offline for maintenance or repairs. 

As part of its asset management program, the District recently did comprehensive condition assessments of its other two clarifiers and plans to rehabilitate one of them within two years. The District has over $300 million in water and wastewater infrastructure, some more than 40 years old.