Municipal Tertiary Filtration
Retrofit of Eimco Rotoco Filter

Plant History: The Sausalito-Marin City Sanitary District is an existing facility that has undergone several upgrades through the years. Sitting immediately adjacent to San Francisco Bay, the facility footprint is very limited. With the major emphasis on bringing the facility into discharge compliance, the SMCSD installed Parkson DynaSand continuous backwash filtration units. The SMCSD later opted to purchase Eimco Rotoco filters to meet increased flow requirements.

As a result of less than satisfactory service from the existing filters, Applied Process Technology, Inc. presented a proposal to SMCSD to convert an Eimco Rotoco Filter to a Centra-flo design, explaining that the Centra-flo design would eliminate the many operational problems being experienced with the current units.

SMCSD, in conjunction with their consulting engineer, Kennedy Jenks Engineers, elected to accept the Applied Process Technology proposal in July 1997, The conversion of the Eimco Rotoco Filter was completed in September 1997, and the converted "Centra-flo" Filter was put into service on December 1997.

Plant Description: The SMCSD facility is designed for an average daily flow of 2.0 MGD with a peak flow of 3 MGD. The treatment system consists of screening, clarification, bio-towers, secondary clarifiers, filtration, and disinfection. Treated effluent is discharged into San Francisco Bay. Plant discharge permits require effluent contain average total suspended solids (TSS) of not more than 30 mg/l, and BOD less than 30 mg/l.

Tertiary Filters: Clarified discharge from the secondary clarifiers is pumped to the filter feed manifold common to all four 64 ft2 continuous backwash, moving bed sand filters. The Centra-flo units have been operated at up to 6 gpm/ft2 since early in 1998.

The SMCSD does not have the ability to add chemicals to aid in the filtration process at this time.

Filter Startup: The first Centra-flo was originally put on line in October 1997. The retrofit of the existing filters took approximately 2 weeks, start to finish. Before SMCSD agreed to retrofit the second filter, the Centra-flo had to prove that the filter could perform and consistently produce filtrate that met the requirements of the permit. The Centra-flo did indeed meet the performance requirements. The Centra-flo was put on-line as the primary filter and was continually fed at a rate of 400 USgpm (6.25 gpm/ft2). The Centra-flo produced filtrate with up to 25% less solids than the Parkson DynaSand when it was running at a lower flow rate.

After ten (10) months of successful operation, SMCSD decided to retrofit the Eimco Rotoco filter and the unit was converted in September 1998.

Filter Operation: Operating data collected by SMCSD shows that the Centra-flo has met the performance requirements.