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Guidance for requesting effluent limits for wastewater facility planning studies

This guidance is provided for use by wastewater facility designers who need to request effluent discharge limits as part of a facility planning study for any new or modified sewerage system project subject to ch. NR 110, Wis. Adm. Code. Chapter NR 110 is applicable to all sewerage systems except systems treating industrial wastewater or "private sewage systems" regulated by the Department of Commerce. This guidance is not applicable to limit questions arising from routine Wisconsin Pollutant Discharge Elimination System (WPDES) permit reissuance, or to requests for alternative phosphorus limits.

The department normally provides planning limits only for conventional pollutants for facility planning. The calculation of limits for toxic substances may occur separately through the WPDES permit issuance process. Conventional limits are normally sufficient for facility planning and design purposes; however, designers should be aware of and consider the impacts of other potential limits to the extent possible.

Although the methods for determining land application limits differ from the methods used for surface waters, the procedures in this guidance may also be used for planning limit requests for proposed discharges to groundwater via a land application system. Groundwater standards are in ch. NR 140, Wis. Adm. Code, and effluent limits for municipal and domestic wastewater land application systems are in ch. NR 206. In general, these standards and limits do not vary for different discharge locations, but in some instances, alternative limits may be established due to specific site circumstances. For example, if the land application site is located in an area where groundwater migrates a short distance and then emerges into surface waters, the department may impose alternative effluent limits.

When should planning effluent limits be requested?

Identifying the effluent limits that will apply to various potential discharge points is essential to enable an analysis to determine the most cost-effective means of disposing of treated effluent as is required under s. NR 110.09(2)(g). Requests for planning limits should be made early in the planning process, as soon as a reasonable design flow estimate and discharge locations can be identified. The department recognizes that initial design flow estimates may be subject to further refinement as a planning analysis proceeds, and that limits may also need corresponding revision. When numerous discharge locations are potentially viable, DNR staff should be consulted to screen the options to enable a more efficient focus on the most promising locations. The department routinely encourages "planning initiation meetings" with community officials and their engineering consultant when a planning effort is just beginning. The scope of an effluent limit request may be one of the primary topics at these meetings.

Submittal and department review procedures

Requests for planning limits should include a cover transmittal letter and a report addressing the information listed below. Requests should be sent to:

Jason Knutson
Wastewater Section Chief
Wisconsin DNR
Bureau of Water Quality
PO Box 7921
Madison, WI 53707-7921

The assigned plan review engineer will conduct a review of the proposed service area, population projection and design flows. Once these are determined to be acceptable, the request will be transferred to appropriate department staff for determination of applicable water quality standards and calculation of planning effluent limits. The planning limits determination will be documented in a memo and returned to the department plan reviewer, who will transmit the planning limit memo to the applicant, along with any other pertinent comments.

Information necessary for planning effluent limit requests

General information

Includes the following:

  • name of existing or proposed facility (include owner contact information);
  • county;
  • WPDES permit number (if existing facility);
  • summary of current limits (if existing facility); and
  • identify if proposal is for a new discharge or increase to an existing discharge. If the latter, identify the existing design flow and location.

Proposed discharge location

A map showing the proposed discharge location should be provided (7.5 minute quadrangle maps are recommended). If multiple receiving streams or discharge locations are being considered, each location should be identified on the map.

Proposed facility design flows

Wastewater facilities are normally designed to treat the maximum loading condition expected to occur within a 20-year planning period. Because flow rates into wastewater plants are variable, a design condition is defined by expressing average flow rates that will occur over different time periods, within the year of the maximum loading condition. These design flows are then used to design sewerage system components to provide compliance with WPDES permit limits and other performance objectives. Additional detailed information on design flow determinations is available from the Guidance for Wastewater Treatment Facility Design Flow Determinations.

At a minimum, the "Annual Average Design Flow" must be provided to enable a limit determination under ch. NR 210 for conventional limits for planning purposes. The other listed design flows do not necessarily need to be included for the initial limit request, but should be identified in the final facility plan report as necessary to evaluate alternatives in accordance with NR 110 requirements. Facility plan reports should always identify the Maximum Month Design Flow because this will be used by the department for the Compliance Maintenance Annual Report (CMAR) assessment.

  1. Annual Average Design Flow: The average of the daily flow volumes anticipated to occur for a continuous 12-month period, expressed as a daily average. (Note: This is not a dry weather flow. It is the total yearly flow volume discharged, expressed as a daily average.)
  2. Maximum Month Design Flow: The largest volume of flow anticipated to occur during a continuous 30-day period, expressed as a daily average.
  3. Maximum Week Design Flow: The largest volume of flow anticipated to occur during a continuous seven-day period, expressed as a daily average.
  4. Maximum Day Design Flow: The largest volume of flow anticipated to occur during a one-day period, expressed as a daily average.
  5. Maximum Hour Design Flow: The largest volume of flow anticipated to occur during a one-hour period, expressed as a daily or hourly average.
  6. Peak Instantaneous Flow: The maximum anticipated instantaneous flow.

The maximum month, week and day design flows may be necessary to establish mass limits or limits for toxic or other "non-conventional" substances. The maximum hour flow or peak instantaneous flow are not normally necessary for limit calculations, but are typically necessary for design purposes and should be included in the facility plan report if applicable for the specific project design.

Although the above list does not include a dry weather flow, determining a dry weather flow ("base flow") is typically necessary as part of analyzing flow components (base flow, infiltration, and inflow), and it should normally be identified in a facility plan report. Any quantified flow used for facility design purposes must be identified and explained in a facility plan report.

The planning effluent limit request should contain the design flow calculations and an accompanying explanation of the assumptions and methods used to derive the design flows. Information on sewer service areas and population projections should also be included.

Potential effluent discharge options

In some cases it may be beneficial to consider effluent storage to enable improved control of discharge rates or the time periods of discharge. For example, storing flows such that discharge periods could be limited to one or two months of the year. Another discharge option could involve coupling a land application discharge with a surface water discharge such that the discharge rate to the stream could be reduced during the land application season.

Any proposal for a unique discharge scheme should be clearly explained and design flows adjusted accordingly. For any facility that does not discharge continuously, the average design flow during the discharge period should be provided.