United States of America
I. Kruger Inc.

4001 Weston Pkwy
Cary, NC 27513

Phone: (919) 677-8310
Fax: (919) 677-0082

Oconomowoc, WI

Replacing and retrofiting aging media filters


The Oconomowoc, WI Wastewater Treatment Facility (WWTF) needed 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 million gallons per day to 12 million gallons per day in order 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.

Plant Overview

The WWTF is located in Waukesha County, WI. The original plant dated back to 1935. In 1975 the facility relocated and upgraded from a trickling filter biological treatment system to activated sludge. Fine bubble aeration was installed in the late 1980's for further improvement. Additional projects for sludge processing, de-chlorination, filtrate storage/handling, and fine screening were completed between 1990 and 2005. The current wastewater process train consists of screening, pumping, grit removal, primary settling, activated sludge, secondary settling, tertiary filtration, disinfection, and oxygen uptake prior to discharge to the Oconomowoc River. The facility also includes air floatation thickening, anaerobic digestion, gravity thickening, and agricultural land application for sludge treatment.

The WWTF provides treatment for residential, commercial, and industrial wastewater generated in Oconomowoc as well as several other neighboring sanitary districts. The plant is designed for an average day flow of 4.0 million gallons per day and a peak flow of 9.0 million gallons per day. The facility averages approximately 2.5 million gallons per day and has experienced peak flow rates up to 11.9 million gallons per day. The facility must meet effluent limits of 10 mg/L of total suspended solids (TSS) prior to discharge.

Process Solution

Together with Ruekert/Mielke, the City determined that Kruger's Hydrotech Discfilter system would be the ideal replacement for the existing filters. Four (4) of the model HSF2214-1F Discfilter units with an integrated PLC control system would be provided by Kruger. The system would be designed to meet 10 State standards by maintaining loading rates ≤ 5 gpm/sf during peak flow conditions with one of the units out of service. The Discfilter system would provide a significant project cost savings compared to conventional treatment alternatives. 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 million gallons per day. 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 Kruger system.

The City installed the Discfilter 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 Discfilters. The first two (2) Discfilters 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 Hydrotech Discfilters. The project was completed in March 2009 when the final two (2) Discfilters were commissioned.


The facility has consistently met permit limits since completing the filter project. The influent to the filter system typically averages approximately 10 to 15 mg/L of TSS with peaks up to 35 to 40 mg/L, while the Hydrotech Discfilter system produces effluent with average TSS < 3 mg/L. While the sand filters typically averaged approximately 60% removal of TSS, data collected by the facility after Discfilter startup has demonstrated the system's ability to routinely achieve 80% to 90% removal of TSS. Additionally, the backwash volumes have been reduced significantly and filtration system maintenance has dropped to a minimum.

The Oconomowoc WWTF staff enjoys the straight-forward and user friendly design that allows routine inspection and maintenance to be performed with ease. Another benefit of the system is the added flexibility that having 4 units provides compared to only 2 units with the previous sand filters. This allows the facility to maintain, inspect, and repair the filter equipment without such a large reduction in capacity when one unit is removed from service.

Finally, the Operations Manager for the WWTF, Tom Steinbach, has seen another benefit that was not anticipated. "The old filters had a large water surface area that was exposed inside the filter building, resulting in high humidity. The enclosed Discfilter design has removed the exposed water surface area and dramatically reduced the humidity in the building, resulting in an unexpected reduction in our energy cost for heating. This is just another example of how the Discfilters are helping us save money at the plant."

The project to install the Hydrotech Discfilter system at the Oconomowoc WWTF is an excellent example of using Discfilter technology to successfully replace and retrofit a facility's aging media filters. This solution provided by Kruger, a Veolia Water Solutions & Technologies company, is enabling the facility to easily meet their stringent effluent limits, increase filtration capacity, and reduce cost while maintaining the existing facility footprint and hydraulic profile.