7 Del. Admin. Code § 7103-95.0

Current through Register Vol. 28, No. 3, September 1, 2024

Section 7103-95.0 - Example Calculations

95.1 Municipal Wastewater

95.1.1 Introduction and Assumptions

95.1.2 Design of slow rate land treatment systems is a process of balancing site limitations against construction and operating costs.

The following example calculations are for a hypothetical one (1) MGD facility in the Central area of Delaware. The example illustrates the basic computations required and the relationship between variables.

95.1.3 The following assumptions were made. They must not be used for real world systems without verification.

95.1.4 A small municipal system with little industrial input and an average design flow of one (1) MGD without significant seasonal fluctuation.

95.1.5 The land treatment site is moderately well drained with seasonal high groundwater after consideration of mounding due to irrigation more than 5 feet below the surface. The most limiting layer in the soil profile occurs at a depth of 2 to 4 feet. Testing for saturated vertical hydraulic conductivity indicates an average permeability for this layer 0.2 inches/hour.

95.1.6 The annual average precipitation is 44.3 inches. Evapotranspiration occurs at the potential evapotranspiration as computed by the Thornthwaite equation.

95.1.7 Nitrogen concentrations in effluent from the preapplication treatment system are as follows:

Total Nitrogen as N 20 mg/L

Ammonia Nitrogen as N 15 mg/L

95.1.8 Nitrogen is applied to the site through rainfall and fixation at a rate of 5 lbscre-year.

95.1.9 Maximum loss to ammonia volatilization is 5 percent of the total ammonia applied. Maximum loss to denitrification for pine forest is 20 percent of the total nitrogen applied. Maximum loss to denitrification for alfalfa is 15 percent of the total nitrogen applied.

95.1.10 Net uptake and removal of nitrogen in pine forest with understory growth is 75 lbscre-year. Nitrogen uptake and removal for Alfalfa is 300 lbscre-year.

95.1.11 Delta P from Table 309-1 is assumed to be 2.1 inches.

95.2 Design Percolation

95.2.1 As stated above, the average permeability of the most limiting soil layer is 0.2 inches/hour. As this limiting layer occurs at a depth less than 5 feet, 10 percent of this value will be used for design (reference Subsection 306.2). The design percolation rate becomes:

95.2.1.1 0.10 x (0.2 in/hr) x (24 hr/day) = 0.48 in/day.

95.3 Water Balance

95.3.1 Water balance calculations for the hypothetical one MGD wastewater irrigation system are presented in Table 703-1. This table makes use of eq. 307.2 to determine maximum allowable monthly hydraulic wastewater loadings.

95.3.2 Thornthwaite potential evapotranspiration and 5-year return monthly precipitation values for Dover are used in Table 703-1. The table indicates that for the assumed site conditions, the month with the lowest wastewater application [D(allowed)] is February with a maximum allowable wastewater loading of 9.3 inches, corresponding to 2.2 inches/week. Therefore, an annual design wastewater loading greater than 2.2 inches/week will require water balance storage in February. Conversely, no water balance storage will be required for a design wastewater loading less than 2.2 inches/week (reference Subsection 71.3. Wastewater loadings equal to D(allowed) for each specific month could be applied for each respective month without requiring water balance storage.

7 Del. Admin. Code § 7103-95.0