Guidance for a Large POWTS
Preliminary Design Report

It is recommended that a Preliminary Design Report be prepared and submitted to the Department of Natural Resources (DNR) and the Department of Commerce (Commerce) for a project proposal involving a large Private Onsite Wastewater Treatment System (POWTS) that will receive a DNR WPDES permit.

The recommended content of the report is as follows:

1. Facility Ownership and Management - Identify the entity that will be the legal owner of the proposed facility when it starts operating (discharges effluent). Provide owner name and contact information. Describe the expected facility management arrangements and any management entity if different than the owner.

2. Site Information - A site map that indicates existing or proposed buildings, all water supply wells, property boundaries, the proposed large onsite system, and other existing onsite sewage systems on the property or within 1,500 feet of the proposed system. Topographical contours and any available information on groundwater flow direction should be included.

(Note: As part of the review process, the DNR will use the site information to check information available from the Wisconsin Source Water Assessment Program (SWAP). This is a database covering approximately 11,500 public water systems in the state. If the SWAP information indicates a proposed wastewater system is not within a land area potentially tributary to a public water supply well, then the system may qualify for a DNR General Permit (WI 0062901-1, opens in window). But if the proposed system is located within a source water protection area, then a DNR individual (specific) permit may be required.)

3. Location and Design of Infiltration Systems - Options for different locations and designs of dispersal components should be addressed with the intent of maximizing treatment and dispersion of the contaminant plume prior to the point of groundwater standards application. It is also highly desirable to chose a site that avoids potential down gradient impacts on water supply wells to the greatest extent possible. As specified in s. NR 140.22, Wis. Adm. Code, the groundwater standards apply at a horizontal distance of 250 feet from the infiltration area, or at the property boundary, or any water supply well, if the property boundary or well is closer than 250 feet.

   Conservative absorption field designs that will enhance soil treatment mechanisms or plume dispersion are recommended. This may include providing low hydraulic loading rates and applying the discharge with frequent small doses. Locating a distribution line at shallow depths, such as in drip-irrigation systems, may also enhance treatment.

4. Treatment System Design Issues - Design information pertaining to the proposed treatment system should be provided. The report should specifically address expected performance, operational needs, and sampling provisions. Although basic operational needs should be addressed, it is not necessary to include a complete management plan with the preliminary Design Report. But the management plan must be included with the submittal of construction plans. Information on how compliance will be provided with specific limits or standards may also be necessary as follows:

1. Demonstration of Nitrogen Removal - The nitrogen removal requirement will normally apply and will significantly affect the types of onsite systems determined to be acceptable. A conventional septic tank and soil absorption field would not be expected to meet this standard under normal circumstances. There are, however, certain treatment systems that are marketed for small-scale application that can be configured to provide nitrogen removal.

   The Department can accept custom-engineered wastewater treatment systems that are designed to have nitrogen removal capabilities. This is typically accomplished by incorporating nitrification/denitrification processes as provided in Sequencing Batch Reactor (SBR) systems or other activated sludge processes that include both oxic and anoxic reactors. It must also be recognized, however, that custom-designed systems typically have more complex and extensive operation and maintenance needs. As a result, it is often preferable that smaller scale treatment systems employ "package" Aerobic Treatment Units (ATUs) that are specifically designed for small-scale applications and require less operator attention. ATUs can use a suspended growth treatment process or a "fixed film" process and ATUs can be configured to provide nitrogen removal.

   Systems employing "fixed film" treatment processes may be most suitable for small-scale applications. A "fixed-film" process provides for the growth of biomass on the surface of a media (such as rock, sand, or specially designed plastic media). Fixed-film systems are better suited for small applications because maintenance and controls are relatively minimal and can be more fully automated. Plus, the process of septic tank storage and effluent recirculation can compensate for transient loadings often encountered with smaller systems.

   The DNR has already accepted certain ATUs designed for nitrogen removal. In general, many ATU manufacturers include models for nitrogen removal and it is anticipated that many of these would be acceptable to the DNR for permitting of large POWTS. To assess whether specific ATU designs or systems are acceptable the Department will generally expect a minimum removal efficiency of 65% for total nitrogen.

   List of acceptable ATUs for large POWTS (24KB, 2 pg. PDF, opens in new window)

2. Demonstration of Compliance with Chloride groundwater standard of 250 mg/l - It is expected that the chloride standard would be met for any domestic wastewater system if the served dwellings have efficient water softening systems, i.e., a demand based regeneration system that minimizes spent chloride wastes into the sewer system. In some instances, sampling of an existing wastewater flow, or an assessment of a planned water softening system will need to be conducted during the design phase for the wastewater system to establish probable compliance with the chloride standard.



3. Demonstration of Compliance with Biochemical Oxygen Demand (BOD5) and Fecal Coliform Bacteria - If a system discharges into a subsurface soil absorption component with at least 3 feet of unsaturated suitable soil (from infiltrative surface to limiting condition), then the treatment component's efficiency for BOD5 and fecal coliform will typically not be a concern. A system may, however, be designed with a higher hydraulic loading rate, and/or less than 3 feet of unsaturated soil, if the effluent applied into the absorption field meets the criteria of s. Comm 83.44. This criteria includes a maximum monthly effluent BOD5 and Total Suspended Solids of 30 mg/l or less, and fecal coliform equal to or less than 10,000 cfu/100ml (monthly geometric mean). For a proposed design of this sort, information must be provided to demonstrate the system's capability to comply with these criteria.
5. Conformance with Area-Wide Water Quality Management Plans - For projects located within planning areas designated under NR 121 authority, or in communities larger than 10,000 population with regional sewer service area plans, the applicable Regional Planning Commission or other designated planning agency (opens in new window) must be contacted. The planning agency should be asked to comment on whether the proposal conflicts with Sewer Service Area Plans or other elements of an Area-Wide Water Quality Management Plans. The Department may not approve a discharge conflicting with such a plan in accordance with s. 283.31(2)(d), Statutes.

6. Environmental Assessment - Each submittal should include a general description of the environmental features of the proposed site (identify water resources and describe existing land use and characteristics). If an individual (specific) WPDES Permit is necessary, then additional environmental impact information should be provided to enable a Department environmental assessment as required for permit issuance under ch. NR 150, Wis. Adm. Code. See the Resources Impacts Summary Outline (opens in new window) for a comprehensive outline to assist applicants with providing a specific description of a proposal (not everything listed in the outline would necessarily apply to a specific proposal). If the system qualifies to obtain a General WPDES Permit, the additional environmental information will typically not be necessary. When the preliminary design report is first prepared it may be uncertain if a general or specific permit will apply. It is recommended that applicants include the general description for an initial submittal. Additional information can be subsequently provided if it is determined to be necessary.