Disc Filter Technology Provides Successful Replacement for Existing Media

Shortly after the most recent upgrades in 2005, the facility realized the need to repair or replace the existing shallow bed traveling bridge sand filters. The sand filters had reached the end of their lifespan and needed significant maintenance improvements to continue to be viable. In addition, the facility needed to upgrade the filtration capacity from 9 mgd to 12 mgd to allow treatment of peak capacities. The City of Oconomowoc worked with engineering firm Ruekert/Mielke to evaluate various technologies to replace the existing filters.

Together with Ruekert/Mielke, the city determined in 2007 that Kruger’s Hydrotech Discfilter system would be the ideal replacement for the existing filters. Four of the model HSF2214-1F filter units with an integrated PLC control system were provided by Kruger. The system was designed to meet 10 States Standards by maintaining loading rates ≤ 5 gpm/sf during peak flow conditions with one of the units out of service.

The system consists of woven cloth filter elements installed on multiple discs, and uses an inside-out flow pattern. Water to be treated flows by gravity into the filter segments from the center drum. Once solids have accumulated on the inside of the media, the discs are cleaned by a counter-current backwash system.

As a result of the compact footprint and low operating headloss of the Hydrotech units, the filters could be located within the existing filter basins while providing the increased capacity to allow treatment up to 12 mgd. In addition, the city anticipated reductions in operation and maintenance costs as a result of the lower backwash volumes and reduced equipment repair costs associated with the system.

The city installed the new filtration system in a phased approach. First, they removed half of the existing sand filters from service in order to clean out those filter cells and install two of the new filters. They were installed in one side of the existing filter building and were commissioned in November 2008. Once these units were online, the city decommissioned the rest of the existing filters and the basins were prepared for the remaining disc filters. The project was completed in March 2009 when the final two filters were commissioned.