7 Del. Admin. Code § 7103-99.0

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

Section 7103-99.0 - Table 703-4 Irrigation Schedule

Month	DesignaPrecip(in/week)	D(allowed)b(in/week)	Maximumc Irrigation Period(hr/week)
January	1.1	2.4	9.6
February	1.0	2.2	8.8
March	1.2	2.4	9.6
April	1.1	2.5	10.0
May	1.2	2.5	10.0
June	1.2	2.5	10.0
July	1.6	2.5	10.0
August	1.7	2.5	10.0
September	1.6	2.5	10.0
Ocotober	1.6	2.5	10.0
Novmember	1.1	2.4	9.6
December	1.2	2.3	9.2

99.1 Weekly fraction of five-year return, monthly precipitation used in design. This rainfall depth is accounted for in D(allowed).

99.2 Maximum allowable hydraulic wastewater loading, each spray field. If rainfall during the week (7 days) prior to irrigation is less than the design precipitation, spray fields may be loaded up to D(allowed) in order to dispose of water diverted to storage. If rainfall during the week preceding irrigation exceeds the design precipitation, D(allowed) must be proportionally reduced and water diverted to storage.

99.3 Based on D(allowed) and the application rate of 0.25 in/hr. This is the maximum number of hours each spray field may be operated per week when rainfall during the week (7 days) prior to irrigation is less than the design precipitation. When rainfall exceed the design precipitation, both D(allowed) and the irrigation period must be modified.

99.4 The maximum wastewater irrigation period will be:

99.4.1 (2.50 in/week) / [ (1 day/week) x (0.25 in/hr) ] = 10.0 hr/day

99.5 Irrigation Schedule

99.5.1 The simplest method for operating a wastewater irrigation system is to control the irrigation period by time clock. Maximum irrigation periods correspond to the maximum allowable hydraulic wastewater loading for each month. This insures the correct volumes of wastewater are applied and stored.

99.6 Table 703-4 is an example irrigation schedule for the hypothetical one (1) MGD facility. The table is based on Tables 703-1 and 703-3 and is intended as a general guide for operation. Daily application periods are adjusted according to antecedent rainfall and soil moisture conditions. Wastewater is not applied during periods of heavy rain or saturated soil. Conversely, dry weather allows long irrigation periods and disposal of wastewater diverted to storage.

99.7 Industrial Wastewater

99.7.1 Introduction and Assumptions

99.7.1.1 Industrial wastewaters can be quite variable and may require a different design approach than municipal wastewaters to determine design and operational parameters. Although many industrial wastewaters will have nitrogen and/or hydraulic loading as their limiting constituents, other constituents may have to be considered in the design.

99.7.1.2 The following example illustrates an industrial wastewater in which metals are the limiting constituents. The same site assumptions as used for the municipal wastewater example will be used for this example as well. The following additional assumptions are made:

99.7.1.2.1 Flow averages 20,000 gallons per week.

99.7.1.3 The wastewater consists of industrial process waste, no domestic wastes from the plant are included.

99.7.1.4 Solids content of the waste is low (less than municipal) and does not require screening prior to land application.

99.7.1.5 The hydraulic loading calculations completed for the municipal wastewater example apply to this example as well.

99.7.1.6 The waste characterization is as follows:

	mg/L
Calcium	3.1
Magnesium	.64
Sodium	12.0
Cadmium	.008
Copper	3.5
Nickel	.014
Lead	.048
Zinc	.069
Total Nitrogen	32.7
Total Phosphorus	8.4
pH	7.0 (standard units)

99.8 Site Assimilative Capacities

99.8.1 Calculation of assimilative capacities for nitrogen, phosphorus, hydraulic loading, and metals were conducted and it was determined that metals were the potential design or land limiting constituents. In accordance with the cumulative metal loadings listed in Table 2 of the Guidance section, the cumulative loadings for the site with a cation exchange capacity of 5 meq/100 g soil or less are as follows:

99.8.1.1lb/ac

Lead	500
Zinc	250
Copper	125
Nickel	125
Cadmium	4.4

99.8.1.2 Furthermore, cadmium loadings cannot exceed 0.44 lbc/yr (0.5 kg/ha/yr) for food chain crops.

99.8.2 The site available for wastewater application is 9 acres and it is decided to use the entire area to provide flexibility in operation. Based on calculations using the above cumulative metal loadings, the site life for each of the metals is:

99.8.2.1

Site Life	Years
Lead	1070
Zinc	2640
Copper	36
Nickel	125
Cadmium	105

99.8.2.2 The cadmium loading would be 0.04 lbc/yr, less than the permissible loading of 0.44 lbc/yr.

99.8.2.3 Nitrogen loading is 42.4 lbsc/yr and the hydraulic loading is approximately 0.5 in/wk.

99.8.3 Because the sodium concentration appears high in comparison with calcium and magnesium the sodium adsorption ratio (SAR) of the waste should be calculated. Refer to the Guidance section for a discussion of SAR and the procedure for calculation. Based on the waste characteristics, the SAR is 1.6. This value is less than the critical value of 12 for the type soils described for this example and therefore does not limit the wastewater application.

99.8.4 Since wastewater loadings are low for this example waste and site, calculations of storage requirements as outlined in the municipal wastewater example are not necessary. However, determinations of storage requirements for winter and wet weather and emergency conditions should be determined to provide flexibility in operations and not lead to severe curtailment of plant operations. Considering these factors and examination of the weather records, a decision to provide one-weeks storage was made with consideration that plant operations will be reduced if there is a longer period during which the waste cannot be land applied. Additional storage would be necessary if storage or curtailment of plant operations cannot be accepted by the applicant.

Amended October 15, 1999

B. Regulations

7 Del. Admin. Code § 7103-99.0