7 Del. Admin. Code § 7103-97.0
Current through Register Vol. 28, No. 3, September 1, 2024
Section 7103-97.0 - Table 703-2 Nitrogen Balance, Pine Forest and Alfalfa | Pine Forest | Alfalfa |
| Average Daily Flow, ADF (mgd) | 1.0 | 1.0 |
| Average Design Wastewater Loading (in/week) | 1.50 | 2.50 |
| ADF Wetter Area (acres) | 172 | 103 |
| Nitrogen Input from Wastewater (lbscre-year) | 354 | 591 |
| Nitrogen Input from Rainfall and Fixation Rainfall and Fixation | 5 | 5 |
| Total Nitrogen Input (lbscre-year) | 359 | 596 |
| Ammonia Volatilization,@ 5% of ammonia applied (lbscre-year) | 13 | 22 |
| Denitrification, @ 20% of total nitrogen applied (lbscre-year) | 71 | 119 |
| Net Plant Uptake and Storage (lbscre-year) | 75 | 300 |
| Nitrogen Leached by Percolate (l) | 202 | 159 |
| Precipitation (in/year) | 44 | 44 |
| Wastewater Applied (in/year) | 78 | 130 |
| Potential Evapotranspiration (in/year) | 29 | 29 |
| Percolate (in/year) | 93 | 145 |
| Estimated Percolate | 9.6 | 4.8 |
| Total Nitrogen (mg/L | | |
Note that ammonia volatilization is based only on applied ammonia, not total nitrogen as the other components of the nitrogen balance.
97.1 Nitrogen Balance 97.1.1 The nitrogen balance is used to evaluate the range of wastewater loadings possible under different cover crop and management schemes.Table 703-2 presents nitrogen balances for cover crop alternatives of pine forest and alfalfa grass.97.1.2 To meet a percolate total nitrogen limit of 10 mg/L, Table 703-2 indicates a pine forest cover crop will require a design wastewater loading of approximately 1.5 inches/week or less and alfalfa grass will allow a design wastewater loading greater than the suggested maximum of 2.5 inches/week. The final cover crop selected is an economic decision balancing wetted area and storage requirements against operating cost and maximum allowable wastewater loading.97.2 Operating Scheme 97.2.1 The operating scheme for the hypothetical one MGD facility is as follows: 97.2.1.1 The average initial design wastewater loading will be 2.5 inches/week. The actual loading rate will be somewhat less than 2.5 inches/week during normal operation because the additional acreage needed for treating the operational storage, water balance storage and wet weather/emergency storage will be used to treat the normal daily flows. This will be done in order to maintain the cover crop regardless of whether there is any water in storage.97.2.1.2 The cover crop will be alfalfa. The alfalfa will be harvested and sold.97.2.1.3 Normal operation will be five (5) days per week. The flow from the other two days will be stored. sSince the system will normally be operated five days per week, the wastewater volume applied each day is:97.2.1.3.1 (7 days/week) / (5 days/week) ] x 1 MGD = 1.4 MGD97.3 Storage Volume Requirements 97.3.1 As discussed in Subsection 309, the required storage volume consists of three (3) separate storage components.97.3.1.1 Operational Storage97.3.1.2 The operating scheme selected for design calls for irrigation five days per week with storage of two days' flow. The required operational storage is: 97.3.1.2.1 (7 days - 5 days) x 1 MGD = 2 Mgal97.3.1.3 For this example it is assumed that harvesting of the grass will not occur during the wet weather months. Therefore, no additional storage will be needed for fields out of service due to harvesting since the wet weather storage volume will be available. Wet weather harvest and long harvest and vegetation establishment periods will require additional irrigation area or storage volume.97.3.2 Inclement Weather and Emergency Storage 97.3.2.1 Minimum requirements for inclement weather and emergency storage are discussed in Subsection 71.297.3.2.2 For the hypothetical facility, Delta P from Table 309-1 is assumed to be 2.1 inches. The maximum allowable hydraulic wastewater loading in the most critical water balance month (February) from Table 703-1 is 9.3 inches/month. By eq. 309.2: 97.3.2.2.1 [(2.1 in) x (30.4 days/month)] / 9.3 in/month = 6.9 days97.3.2.3 The designer must consider this value as a minimum storage requirement and balance other site and management factors for determination of inclement weather and emergency storage needs. For the purpose of this example, 12 days is selected as the minimum requirement. Therefore, the storage is:97.3.2.3.1 12 days x 1 MGD = 12 Mgal97.3.3 Water Balance Storage 97.3.3.1 As discussed in Subsection 71.3, the water balance storage is a function of hydraulic loading which is a function of the total wetted field area. Therefore, before the water balance storage can be determined the wetted field area must be defined.97.4 Wetted Field Area Determination 97.4.1 The area required for the spray site is the total of four separate components.97.4.2 A(wetted) = A(ADF) + A(WBS) + A(OP) + A(WW/E)97.4.3 Substituting the appropriate loading rates and the appropriate volumes into equations 310.1 and 310.3 results in the following wetted area requirements: 97.4.3.1 A(ADF) = 103.0 acres97.4.3.2 A(WW/E) = 15.6 acres97.4.4 Since the only operational storage is associated with spraying less than 7 days per week: 97.4.4.1 A(OP) = 0.0 acres97.4.4 With these areas determined the next step is to define the necessary water balance storage and the wetted area associated with that storage.97.4.5 The wastewater loading rate (WLR) is: 97.4.5.1 WLR = 7 x 106 gal/wk x (1 cf/7.48 gal) x 12 in/ft x (1 acre/43,560 sf) x 1/118.6 acres = 2.17 in/wk97.4.6 Table 703-3 combines eq. 307.2 and 309.3 to determine the required water balance storage (WBS) for the loading rate of 2.17 in/wk. Storage for the most critical month (February) is 0 as well as for all other months and no additional land is required for reduction of water balance storage.97.4.7 The total area necessary for this land treatment system is:97.4.7.1 A(ADF) 103 acres97.4.7.5 - A(TOTAL) 119 acres97.4.8 Spraying 1.4 Mgal each day for five days per week, the wetted field area will be divided into 23.8 acre sections. For normal flows each field will be loaded at a rate of: 97.4.8.1 (1.4 x 106 gal/day) x (1 cf/7.48 gal) x (12 in/ft) x97.4.8.2 (1 acre/43,560 sf) x (1/23.8 acres) = 2.17 in/wk97.4.9 The average wastewater irrigation period will be:97.4.9.1 (2.17 in/week) / [ (1 day/week) x (0.25 in/hr) ] = 8.7 hr/day7 Del. Admin. Code § 7103-97.0