Appendix II: Detailed Assessment Approaches and Methodologies

B. Wastewater – Capacity Template and Reuse – Spray Irrigation

​Wastewater Capacity Template

1. Identify future growth to be served by WWTPs: # of EDUs

2. Identify Service Areas:

​Planned Service:

Existing or 3-to-5 year planned service - # EDUs 

10-year service - #EDUs

No-Planned Service Area​ -#EDUs – Need W/S Plan amendment

3. Identify WWTPs: 

​Example table- Refer to the tables provided in the Capacity Management Plans for greater detail

4. Compute Nutrient Loads:

Major Plants (0.5 mgd or greater):

Total Nitrogen: FLOW (mgd) x 4.0 (mg/l) x 8.334 x 365 days/yr 

Total Phosphorus: FLOW (mgd) x 0.3 (mg/l) x 8.334 x 365 days/yr 

Minor Plants (less than 0.5 mgd):

Total Nitrogen: FLOW (mgd) x 18.0 (mg/l) x 8.334 x 365 days/yr

Total Phosphorus: FLOW (mgd) x 3.0 – 6.0 (mg/l) x 8.334 x 365 days/yr

5. For new plants, or plants that expand beyond their nutrient cap or other limitation, develop plans for expansion or offsets considering current requirements, laws, and information from state agencies:

Identify potential limitations:


b. Load Cap

c. Compliance issues, including Combined Sewer Overflows (CSOs)/Sanitary Sewer Overflows (SSOs)

d. NPDES requirements

e. Water Supply and Wastewater Capacity Management Plans

f. Inflow/Infiltration

Develop a plan of action:​

a. Upgrade WWTPs

b. Plan expansion (amend Water and Sewer Plan)

c. Identify offsets

d. Limit # of EDUs to be developed

e. Water Supply and Wastewater Capacity Management Plans 

Update Comp Plan to reflect planned actions ​

​​Wastewater Reuse – Spray Irrigation

Option A - Preliminary Spray Irrigation Site Capacity Estimate

Table 2. Estimates of application rates and site capacities based on soil drainage


​​* Soil drainage characteristics can be found in the “General Soil Map” provided in each County Soil Survey

** Soil type and soil permeability can be found in the soil map and soil property table provided in each County Soil Survey

*** Application rates shown in this table are for County planning purposes only and are not to be used for permit applications and system designs.

Option B - Refined Spray Irrigation Site Capacity Estimate

  1. Determine soil type, soil permeability, land slope, depth to bedrock and ground water table depth from County Soil Survey.
  2. Eliminate area with slope in excess of 15% in open field and 25% in wooded area.
  3. Eliminate area with ground water table less than 4 ft or less than 2 ft in Eastern Shore.
  4. Eliminate area with depth to bedrock less than 4 ft.
  5. Eliminate area with the most limiting layer soil permeability of less than 0.2”/hr shown in the County Soil Survey.
  6. Delineate the suitable area (keep 100 ft. away from streams and 25 ft to home structures) and measure the acreage.
  7. Use Table 2 in Option A to determine the application rate (H) based on the soil type and Equation 1 below to determine the initial site capacity (Q).
  8. Reduce site capacity by 25% for reserve purpose and determine the final site capacity, Qf (Qf =75% Q).
  9. Add up final site capacities (Qf) determined from Step 8 for all suitable lands with various soil types in the county.

Equation and Example

Q = [A x 27,154 x (365 –G) x H] / [365 x (E+F)] (1)


A is area in acres (from Step 6 above)

Q is the flow in gallons per day

E+F is the loading cycle (loading plus rest periods) in days per week

E is the loading period in days per week

F is the rest period in days per week

G is the storage requirement in days per year (90 days for Washington, Allegany and Garrett Counties, 60 days for other counties)

H is the application rate (loading rate) in inches per week Conversion factors:

365 = days per year

27,154 = gallons per acre-inch

The following example of these calculations assumes 100 acres suitable spray irrigation area at 1”/week application rate and a 60-day storage time:

Q = [100 x 27,154 x (365 –60) x 1”/wk] / [365 x (7 days)] = 324,147 gal/day ​

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