Town of Washington, Washington Island, Wisconsin


This project was a Facility Plan funded by the state of Wisconsin Department of Natural Resources (DNR) which was conducted by David Venhuizen, P.E., with assistance by Baudhuin Incorporated of Sturgeon Bay, Wisconsin. The facility planning process for the Town of Washington commenced prior to Venhuizen's involvement. The original draft of the plan concluded that the Town should install a central treatment station with discharge into Green Bay, and should use holding tanks and a pump-and-haul operation as its "sewer" system. There was broad dissatisfaction with the recommended course of action, due to its cost, to the distribution of those costs among the population, to the great increase in pumper truck traffic which it would entail, and because it would result in a discharge into prime fishing waters.

This prompted the Town to investigate whether a strategy of using "improved" on-site and/or small collective systems would be a viable alternative to wholesale installation of holding tanks. Any such systems would have to cope with site restrictions-including shallow depth to groundwater and/or bedrock and otherwise "unsuitable" soils-which abound on the island, and would have to accommodate the island's large seasonal population and operate properly through the island's harsh winters. In October of 1990, Venhuizen was asked to make a presentation on such an approach to the Town's Wastewater Advisory Committee. That committee then recommended to the Town Board that Venhuizen be hired to explore this "decentralized" management approach.

By agreement with state regulatory agencies, Venhuizen was required to propose and justify treatment and dispersal systems, then to design several "demonstration" systems, which the Town would install and monitor in order to "prove up" these systems for more general use. Of particular concern to the state at that time was nitrate pollution from on-site systems. Venhuizen submitted the following reports as justification for proceeding with the demonstration program:

  1. A detailed review of the background and capabilities of candidate treatment trains. These included various combinations of septic tanks, anaerobic upflow filters, gravel marshes, and intermittent sand filters.
  2. A discussion of appropriate design flow rate criteria for these systems.
  3. A review of system management practices. (Click here to see this paper)
  4. A proposed protocol for water quality monitoring under the demonstration program.
  5. A preliminary cost effectiveness analysis of the decentralized system versus the previously proposed central station and pump-and-haul option

All these reports were submitted in the spring of 1991. After reviewing them, the state authorized the Town to proceed with design and installation of the demonstration systems. These systems were permitted by the Wisconsin Department of Labor, Industry and Human Relations (DILHR) under an "experimental" system approval process. (Note: Regulatory jurisdiction over private sewage systems has since been transferred to the Wisconsin Dept. of Commerce.) Venhuizen completed design of 9 single user systems in the summer of 1991, and design of 2 collective systems in the summer of 1992. Due to delays in funding, only 2 of the single user systems were installed in the fall of 1991. Another 5 were installed in the summer of 1992. The owners of the other two single user systems withdrew from the program, and it was decided to drop the collective systems from the demonstration program due to funding constraints.

The treatment train chosen for this project consisted of a two-chamber septic tank, an anaerobic upflow filter, and an intermittent sand filter. (See figure below) Five of the systems, all serving year-round users, employed a shallow, narrow-trench pressure-dosed dispersal field. Two seasonal use systems, one serving a marina office and one serving a home, had subsurface drip irrigation dispersal fields. In all the systems, sand filter effluent was recirculated back through the second chamber of the septic tank and/or the upflow filter to accomplish denitrification and to provide hydraulic stability through the diurnal cycle. A pumped recirculation scheme was used.

Performance of each system was monitored from system startup in August 1992 through July of 1994. Samples of effluent out of the first chamber of the septic tank were used to represent system influent. Within the treatment system, upflow filter effluent and sand filter effluent samples were collected, allowing observation of treatment efficiency by the sand filter. Toward the end of the monitoring period, sampling of upflow filter influent (effluent from the second chamber of the septic tank) was instituted to determine the quality of influent the sand filter would receive if the upflow filter were deleted from the system, and if significant nitrogen removal capability would be lost by eliminating the upflow filter. Efforts were made to collect leachate in a monitoring system approximately one foot beneath the dispersal field trench bottoms. As expected, great difficulty was encountered in these efforts to obtain vadose zone samples representative of effluent which has infiltrated through one foot of natural soil. Samples collected by the Town were delivered to the University of Wisconsin at Green Bay for analysis. The University and Venhuizen cooperated on the evaluation and interpretation of results. Venhuizen issued three interim data reports and a final report on the demonstration program.

Results indicated that, when these systems are properly operated and maintained, well in excess of 90% removal of BOD5 and TSS can be routinely expected, and that the majority of the nitrogen can be removed from the wastewater. In excess of 60% total nitrogen removal appears routinely achievable, and more than 80% removal was observed over extended periods in three of the systems. Observation of system function by Venhuizen and Town personnel highlighted operational difficulties and design flaws. In all cases, when the problems were corrected, satisfactory operation was re-established in short order. This resiliency and the relative simplicity of operation were among the factors which recommended the technological approach formulated by Venhuizen for use on Washington Island.

Click here to see a report discussing the results of the Washington Island project, and how these findings were used to generate the high performance biofiltration system concept.

At the conclusion of the demonstration program in 1994 Venhuizen prepared and submitted to DNR a revised facility plan. This plan included a cost effectiveness analysis showing the decentralized option to be highly favorable, a sludge management plan, a proposed plan for Town oversight of individual system management (click here to see this plan), and a proposed plan for accommodating "cluster" systems within the management structure. As an adjunct to the facility plan, Venhuizen produced a paper offering a detailed analysis of soil treatment mechanisms, providing justification on the basis of environmental protection for employing the concepts being proposed on Washington Island. DILHR published this paper and distributed it widely. (Click here to view an updated edition of that paper.) The facility plan was accepted by DNR in 1995. Since that time, the Town has modified the management plan and implemented the decentralized management strategy.