Definition of Large
Private Onsite Wastewater Treatment System (POWTS)

The definition for a "Private Onsite Wastewater Treatment System" (POWTS), in s. Comm 81.01(194), Wis. Adm. Code, states that a POWTS has the same meaning as that given for a "private sewage system" defined under s. 145.01 (12), Wis. Stats. The statutory definition of a private sewage system includes broad language that the Department of Natural Resources and Department of Commerce have interpreted and clarified in interagency Memorandum of Understanding (MOU). The definition of a POWTS, and a "Large POWTS", and applicable regulatory procedures are based on the following general factors:

1. Type of wastewater
2. Type of discharge into the environment
3. Size of system ("large" vs. "small")

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Type of Wastewater

A POWTS is intended to treat domestic wastewater. If a system treats wastewater that is 100% non-domestic wastewater, jurisdiction for regulation rests solely with the DNR. For a mixture of domestic and non-domestic wastewater, the agencies will use a joint plan review process to determine whether the mixed wastewater can be treated by a Commerce approved system, or if system modifications, or a DNR permit will be required for proper disposal.

"Non-domestic wastewater" means the type of wastewater that does not originate solely from humans and domestic activities such as sanitary, bath, laundry, dishwashing, garbage disposal and the cleaning of domestic areas or utensils. Non-domestic wastewater includes, but is not limited to wastes collected from non-residential garages used for storage, maintenance or washing of motor vehicles; commercial food processing, commercial laundromats, animal shelters or kennels, animal rendering, metal fabricating, electric component manufacturing, chemical manufacturing, and other industrial or commercial process water. Wastewater from restaurants is considered to be domestic wastewater.

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Type of Discharge into the Environment

A POWTS is a system that discharges effluent below the surface of the ground (typically subsurface soil absorption fields). Any system with a final discharge into surface waters (rivers, streams, etc.) or with a final discharge upon the ground surface, such as spray irrigation or infiltration lagoons, are regulated by the DNR.

A system employing drip irrigation (with buried distribution lines) or any non-conventional method of infiltration can still be considered to be a POWTS as long as the system is designed to not result in any exposure of final treated water upon the ground surface.

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Size of System (Definition of a "Large POWTS")

A small non-municipal POWTS is subject solely to Commerce regulations. A "Large" POWTS is subject to a joint DNR/ Commerce plan review process and to a DNR Wisconsin Pollutant Discharge Elimination Permit (WPDES) operating permit. Under s. NR 200.03, Wis. Adm. Code, a system serving residential development will be classified as large if it serves more than 85 bedrooms.

A system serving non-residential development will be classified as large if its "design wastewater flow" exceeds 12,000 gallons per day (gpd). Note that "design wastewater flow" is defined by Comm 83 criteria as 150% of the "daily effluent application" which is essentially an average flow. Therefore, a system designed for an average flow of 8,000 gpd, would normally have a "design wastewater flow" rate of 12,000 gpd. For existing systems, design flows may be premised on actual flow metering data.

A system serving a combination of residential and non-residential development will be classified based on a design flow calculated by prorating the number of residential bedrooms on the basis of 85 bedrooms equaling 12,000 gpd and adding the estimated non-residential wastewater flow.

In accordance with the current DNR/Commerce Memorandum of Understanding (MOU) and s. NR 200.03(5), any two systems will be classified as one "large system" for purposes of DNR permitting if:

1. Each of the two systems are individually designed for more than the equivalent of two families, and
2. Both systems are under the same ownership, and
3. The distribution cell of one system is less than 1,500 feet from the distribution cell of the other system, and
4. The combined number of residential bedrooms served exceeds 85, or for non-residential, if the combined design wastewater flow exceeds 12,000 gpd.

The two system criteria applies to both existing and new systems, thus a new system will be evaluated with consideration of another existing system if both systems satisfy the criteria listed above.

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