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Guidance for wastewater treatment facility design flow determinations

Planning assumptions

In accordance with s. NR 110.09(1), Wis. Adm. Code, a 20-year planning period shall be used for wastewater facility planning. The 20-year period shall commence with the anticipated startup of the proposed new facilities. A sewer service area shall be identified. If the facility is located in an NR 121 designated planning area, or in a community larger than 10,000 population, then the sewer service area must conform to the recommendations of the applicable area-wide water quality management plan as approved by the Department. The Regional Planning Commission or other designated agency responsible for implementation of the area-wide plan must document that sewer service area and population projections conform to the area-wide plan.

As indicated in s. NR 110.09(2)(j)4, it is possible to stage the construction of treatment facilities over time periods of less than 20 years. This must be justified, however, by a cost-effective analysis performed over a 20-year planning period. If a staged project is to be evaluated, the effluent limits should be obtained for both the 20-year design flow and the design flow for any lesser staged design period. Both sets of limits would be necessary in order to complete the cost-effectiveness comparison of the staged alternative and the normal 20-year design alternative.

Population projections

Requirements for population projections are addressed in s. NR 110.09(2)(j)1. Population projections for service areas within NR 121 designated areas, or in communities larger than 10,000 population, must conform with area-wide water quality management plans and receive approval from the applicable Regional Planning Commission or other designated agency. All other projects must conform to the official Wisconsin Department of Administration population estimates and projections made in accordance with s. 16.96, Wis. Stats. If population projections for specific areas are not available, or need to be broken down into more detail, a linear extrapolation of trends from the recent past should be used. Any deviation from a linear projection method must be clearly justified.

Derivation of design flows

Many factors need to be considered to predict future wastewater facility flows and loadings in a manner that will ensure a cost-effective design and facility service life. Although NR 110 provides certain criteria for flow projection assumptions and procedures, it does appropriately allow for designer discretion, and the exercise of good professional judgment by a qualified professional engineer is important. The ch. NR 110 requirements for determinations of design flows are summarized in this remainder of this guidance.

The "average design flow" is defined in s. NR 110.03(12g)(b). Designers often use various types of dry weather flows and may refer to them as "average daily flows". Dry weather base flows may still be used as appropriate for design purposes, but the "average" flow that the DNR will use for effluent limit determinations and as the permit "design flow" must be the total annual average flow as described.

Facility design flows shall be developed according to the procedures of s. NR 110.09(2)(j). These procedures require design flow calculations to be based on existing wastewater flow data, when such data is available and is valid. The most recent flow data set covering a minimum three-year time period is recommended. Flow data may need to be adjusted to account for system bypassing events or inaccurate flow metering. The data set should be analyzed to identify a per capita average flow and various peaking factors. These can then be applied to projected population increases to obtain design flow estimates, including various maximum and peak design flows. Allowances for future increases of per capita flow over time are not allowed under s. NR 110.09(2)(j)2m.a.

Any maximum or peak design flow should normally be expressed as one of the following (exceptions may be acceptable based on specific design circumstances):

  • Maximum month design flow: The largest volume of flow anticipated to occur during a continuous 30-day period, expressed as a daily average.
  • Maximum week design flow: The largest volume of flow anticipated to occur during a continuous seven-day period, expressed as a daily average.
  • Maximum day design flow: The largest volume of flow anticipated to occur during a one-day period, expressed as a daily average.
  • Maximum hour design flow: The largest volume of flow anticipated to occur during a one-hour period, expressed as a daily or hourly average.
  • Peak instantaneous flow: The maximum anticipated instantaneous flow.

As required by s. NR 110.09(2)(j)2, flow estimates need to account for residential, commercial, institutional, industrial, and infiltration and inflow (I/I) sources . Additional capacity for treatment of hauled septage wastes may also be approved where justified. If wastewater flow records are not available, water supply records may be used to estimate the "average daily base flow" (ADBF), which is the average flow excluding I/I. If adequate water supply records are not available, the ADBF may be estimated by using 60 to 70 gallons per capita per day (gpcd) for areas with populations of 5,000 or less. The ADBF may be estimated using 65 to 80 gpcd for other cities and towns. These figures account for commercial and institutional sources that would typically be associated with the residential population. Exceptions to these figures may be approved where large commercial and institutional flows exceed 25% of the total estimated ADBF.

A flow component for sewer system infiltration and inflow will need to be added to the ADBF to obtain the total "annual average design flow." NR 110 does not prescribe an exact method for estimating future I/I. Some designers have proposed use of the new sewer construction quality test leakage rate of 200 gallons per inch of pipe diameter per mile per day as a method to estimate future I/I. This criterion is a standard for new construction and is not intended to be a method of estimating future I/I. Non-excessive I/I quantities may be substantially greater than that associated with the 200 gpd/in./mi. criteria. The designer will need to consider characteristics of the specific collection system and its hydrogeological setting to estimate potential future infiltration flows.

In accordance with s. NR 110.09(2)(j)3, flow projections for industrial contributions should account for existing industrial and future documented industrial flows, and may include a nominal flow allowance for future unplanned industrial expansions. This allowance for unplanned industrial flow should normally not exceed 5% (or 10% for service areas less than 10,000 population) of the total non-industrial design flow. The unplanned industrial flow will be allowed only if area-wide management plans, land-use plans and local zoning provide for such industrial growth.

When flow records do not exist, certain flow peaking factors are prescribed in Table 2 of s. NR110.15(4)(c). The specific language in this section indicates the peaking factors should be used to estimate the "maximum design flow rate." It is important to recognize that these peaking factors are actually intended to be used to derive the maximum hour design flow, not maximum day. The factors are based on the ratio of peak hour flows to annual average flows, and the factors apply to the entire annual average wastewater flow (i.e. not to a base flow component). It should also be noted that in s. NR 110.13 (1)(b) and (c), a sewer design criterion of 100 gallons per capita per day and certain peaking factors are specified. These criteria are intended only for sewer design and should not be used in place of the s. NR 110.09(2)(j) procedures applicable to treatment facilities.