7 Del. Admin. Code § 7103-69.0

Current through Register Vol. 28, No. 4, October 1, 2024

Section 7103-69.0 - Determination of Design Wastewater Loading(s)

69.1 General. The design wastewater loading is a function of:

69.1.1 Precipitation

69.1.2 Evapotranspiration

69.1.3 Design percolation rate

69.1.4 Nitrogen loading and other constituent loading limitations

69.1.5 Groundwater and drainage conditions

69.1.6 Average and peak design wastewater flows and seasonal fluctuations

69.1.7 Therefore, developing the design wastewater loading is an iterative process.An initial value is selected from water balance calculations and used to determine wetted field area.

69.1.8 This loading is then compared to nitrogen and other constituent loading limitations (reference Subsection 308). If the initial value exceeds these limitations, the design wastewater loading is reduced and the process is repeated. This iterative process is illustrated in Subsection 700.

69.1.9 The Department limits design wastewater loadings to a maximum of 2.5 inches/week and instantaneous wastewater application rates to 0.25 inches/hour. Requests for higher loadings will be evaluated on a case- by-case basis. The design wastewater loading may be fixed at a constant rate or may vary monthly but it must account for site specific wastewater flow, climatic, drainage and vegetation management limitations.

69.2 Water Balance

69.2.1 Maximum allowable monthly wastewater loadings are determined from the following water balance equation:

D(allowed) = (Evap + Perc) - Precip eq. 307.2.

Where,

D(allowed) = Maximum allowable hydraulic wastewater loading (in/month).

Evap = Potential evapotranspiration (in/month)

Perc = Design percolation rate (in/month):

Precip = Design precipitation (in/month).

69.2.2 Example water balance calculations are presented in Section 700. From these, critical water balance months, i.e(r) months with the smallest allowable hydraulic wastewater loading, are identified.

69.3 Potential Evapotranspiration

69.3.1 Reliable field data for evapotranspiration are difficult to obtain. Therefore, values for average monthly potential evapotranspiration generated from vegetative, soil and climatological data are used in water balance calculations. A list of evapotranspiration references is presented in Subsection 603. For row crops, the Department recommends use of the modified Penman Method calibrated for local conditions. For forage and forested systems, or when data for other methods are not available, the Thornthwaite equation is preferred. The Thornthwaite equation and values for all US Weather Service long-term stations are presented in the Appendices.

69.3.2 The method used to estimate average monthly potential evapotranspiration for water balance calculations must be referenced in the Design Development Report.

69.4 Five-Year Return Monthly Precipitation

69.4.1 The Department requires the use of five-year return, monthly precipitation values in water balance calculations. Five-year return values are defined as the 80th percentile value in a 30-year ranked listing of historical monthly precipitation data. This corresponds to:

Precip(avg) + (0.85 x std. dev. eq. 307.4

Where,

Precip(avg) = 1/2 Average monthly precipitation from 30 or more years historic record std. dev. = Standard deviation for same.

69.4.1 Factoring design precipitation into the water balance equation for winter months must consider conditions such as form of precipitation, soil freezing, etc. Wastewater not irrigated must be stored.

69.4.2 Records of both monthly precipitation and temperature are available for all of Delaware from the National Climatological Center of the National Oceanic and Atmospheric Administration in Asheville, North Carolina. The source of precipitation data used for design must be referenced in the Design Development Report. Mean and design precipitation values for each of the US Weather Service long-term stations in Delaware are presented in Table 702-3.

7 Del. Admin. Code § 7103-69.0