Requests for Re-Rating Capacity of
Municipal Wastewater Treatment Facilities

In certain instances, the owner of a wastewater treatment facility may revise the design capacity that has been previously identified and approved by DNR for an existing facility. The design capacity figures are used in WPDES permits, for effluent limit determinations (in some cases), and for facility evaluations conducted as part of the Compliance Maintenance Annual Report (CMAR). Design capacity is normally expressed in terms of a hydraulic loading (flow rate as gallons per day) and organic loading expressed in terms of Biochemical Oxygen Demand (pounds of BOD5 per day). A request to revise either one or both of these may be submitted.

The Department approves the original design flows and loadings for wastewater treatment plants as part of plan reviews conducted under ch. NR 110, Wisconsin Administrative Code. A proposed capacity revision request should be submitted to:

Thomas A. Gilbert, P.E.
Wisconsin DNR
Bureau of Watershed Management
P.O. Box 7921
Madison, WI 53707-7921

The request should include a transmittal letter and two copies of a detailed engineering report to explain and justify the proposed revision. The plan reviewer, the regional wastewater engineer, and other DNR staff as necessary will review the request. A letter documenting the acceptance or denial of a revision will be sent to the facility owner.

Wastewater treatment plants are often designed by employing conservative empirical design methods and it may be justified to re-rate a plant's capacity based on actual performance data. Owners and designers should keep in mind, however, that there are good reasons for conservative design criteria that relate to accounting for the variability of influent flows and loadings that typically occur in any municipal sewerage system.

The "design flow" for municipal wastewater plants in Wisconsin is defined as the average annual flow expected in the design year (the design year is typically the 20th year after facility construction). But any design must also account for the normal variations in flows and loads, and consider the complete hydraulic regime expected in the design year (average flow, plus minimum and maximum flows expected over various time durations). This influent variability translates into a variable effluent quality that must be accounted for in assessing compliance with discharge limits.

For example, based on the typical distribution of effluent quality data associated with activated sludge plants, the long term arithmetic mean effluent BOD concentration must be about 20 mg/l in order to comply with a 30 mg/l BOD monthly average limit at a 95% compliance rate (95 months out of 100 months). This type of design conservatism is necessary to ensure substantial compliance with limits. This factor of conservatism may be inherently built-in as part of certain empirical design criteria, but designers must take care to account for this factor when relying on performance data or other design assessment methods.

DNR plan reviewers will examine re-rating requests with the goal of ensuring that overall effluent limit compliance rates are maintained at acceptable levels. Each unit process in a treatment system will be examined and the overall plant design flow or loading will normally be determined by the one unit process that is most restricting in terms of design capacity. This analysis should include sludge processing and storage units. The proposed new design flow criteria should normally include identification of the annual average design flow, plus the maximum month, maximum week and maximum day design flows. In some cases, the effluent limits will need to be recalculated when the design flow changes.

Design flow is defined in terms of the expected flows and loadings from a service area. This is normally, but not necessarily, the same as the actual capacity of the treatment facility. For example, a plant may have been originally designed with a large capacity allowance for an industrial facility, and then that industry subsequently ceased operations and discharge. In this situation, the design capacity could be re-rated based on the capacity necessary to serve the remaining community for a 20-year time period. The resultant re-rated capacity could then be less than the original rated capacity and less than the actual facility capacity.