Hierarchy of Loading Calculations

The Loading Tool calculates DMR pollutant loadings using a quantity or concentration value and a wastewater flow, reported in the DMR data. There are a total of two quantity and three concentration data fields that can be populated in the DMR data (i.e., five measurement value fields):

  • Quantity 1 (kg/day)
  • Quantity 2 (kg/day)
  • Concentration 1 (mg/L)
  • Concentration 2 (mg/L)
  • Concentration 3 (mg/L)

The quantity measurement fields may represent average, maximum, or total values, while the concentration measurement fields may represent minimum, average, or maximum values. Facilities may use a variety of measurements to populate the above five measurement value fields. For example, a facility can use a monthly average, daily average, 30 day geometric average, etc. to represent the average quantity. The DMR data contain statistical base codes to indicate the types of measurements reported.

EPA's goal for calculating pollutant loads is to characterize the average pollutant mass discharges to receiving streams; therefore, the Loading Tool selects the appropriate DMR measurement data field using a hierarchy that gives priority to the average values. Calculating annual pollutant loadings using concentrations requires the use of wastewater flow data, which increases the uncertainty of the calculation. Therefore, the Loading tool measurement selection hierarchy generally prioritizes average quantities over average concentrations.

The only exception to this rule is if the permit limit is on a concentration basis and there is no corresponding mass-based quantity limit. In this case, the loading tool will prioritize concentrations over quantities. In order to calculate the load over limit estimates, the Loading Tool selects the same DMR measurement as the permit limit, which could be either a concentration or quantity. Although the pollutant loadings are calculated by prioritizing measurements that are the same measurement as the permit limit, average measurements are still prioritized over minimum, total, and/or maximum measurements.

Pollutants in facility wastewater discharges are typically measured at regular intervals called monitoring periods (e.g., monthly, quarterly). After the Loading Tool selects the appropriate measurement values, the following values are calculated for each pollutant at individual outfalls (discharge points) at the facility over a monitoring period:

  • Average Daily Load (kg/day);
  • Average Concentration (mg/L); and
  • Average Wastewater Flow (Million Gallons/day or MGD).

The Loading Tool then calculates the following values for each monitoring period, assuming that the discharge occurs continuously for the duration of the monitoring period. Note that for intermittent discharges reported in DMRs, the Loading Tool adjusts the number of days in the monitoring period using the duration of discharge parameter. The following general equations are used to calculate the load discharged during a monitoring period:

Monitoring Period Load (kg/monitoring period)


or

 

Monitoring Period Wastewater Flow (MG/monitoring period)

 


Load Over Limit Calculations

Where permit limits are available, the Loading Tool calculates the Load Over Limit (LOL) for the monitoring period for each pollutant. Load over Limits are provided in the results of Custom Search.

Key considerations:

  • Load over Limit is calculated for each monitoring period (e.g., month, quarter). The difference between the average load and permitted load for each monitoring period are then summed to generate an Annual Load over Limit (kg/year).
  • Load over Limit is blank for parameters that have requirements to monitor (i.e., measure) discharges, but do not have numeric permit limits.
  • Load over Limit represents reported data. The Load over Limit is not estimated for monitoring periods where no data is reported.

This calculation is done by summing the monthly differences between the average load and the permit limit in two ways, as illustrated by Figures 1 and 2 below. Both options subtract the DMR measurement value from the corresponding permit limit value. Option 1 (Figure 1) sets any negative differences equal to zero, while Option 2 (Figure 2) retains the calculated negative value.

Figure 1: Load Over Limit (Option 1)

 

Figure 2: Load Over Limit (Option 2)

The following examples demonstrate how the Load over Limit is calculated, using loadings data from the Facility Loading Calculations monitoring period table. See Section 3.2 of the Technical Users Background Document for the Water Pollutant Loading Tool for more specific information on the calculations.

Load Over Limit Flag - Indicates there was one or more exceedances of permit effluent limits for this pollutant during the reporting year.

Example 1. Load over Limit Calculation for Pollutant Discharges Without Effluent Exceedances during a Year
Monitoring PeriodDischarge InformationBelow Detection Limit?Measurement TypeAvg Daily ValueLimitWastewater Flow (MGD)Number of DaysMonitoring Period Load (lb/period)Load EquationDaily Load Over Limit (lb/day)1LOL Option 1 (lb)2LOL Option 2 (lb)2
1/31/2014Occurred? YesNoQuantity (kg/day)2.5421.20.1631174Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-18.660-578.46
Reported? Yes
Estimated? No
2/28/2014Occurred? YesNoQuantity (kg/day)2.40821.20.1528149Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-18.7920-526.176
Reported? Yes
Estimated? No
3/31/2014Occurred? YesNoQuantity (kg/day)3.04721.20.1431208Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-18.1530-562.743
Reported? Yes
Estimated? No
4/30/2014Occurred? YesNoQuantity (kg/day)3.687.0740.1430244Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-3.3940-101.82
Reported? Yes
Estimated? No
5/31/2014Occurred? YesNoQuantity (kg/day)3.0117.0740.1331206Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-4.0630-125.953
Reported? Yes
Estimated? No
6/30/2014Occurred? YesNoQuantity (kg/day)2.337.0740.1130155Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-4.7440-142.32
Reported? Yes
Estimated? No
7/31/2014Occurred? NoNoQuantity (kg/day) 7.074031 No Load Calculated   
Reported? Yes
Estimated? No
8/31/2014Occurred? NoNoQuantity (kg/day) 7.074031 No Load Calculated   
Reported? Yes
Estimated? No
9/30/2014Occurred? NoNoQuantity (kg/day) 7.074030 No Load Calculated   
Reported? Yes
Estimated? No
10/31/2014Occurred? YesNoQuantity (kg/day)0.7121.20.173148.67Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-20.490-635.19
Reported? Yes
Estimated? No
Annual Load from Monitoring Data (lb/yr)1,184.67Annual Load over Limit (lb/yr)0-2,672.66

Example 2. Load over Limit Calculation for Pollutant Discharges with One Effluent Exceedance during a Year
Monitoring PeriodDischarge InformationBelow Detection Limit?Measurement TypeAvg Daily ValueLimitWastewater Flow (MGD)Number of DaysMonitoring Period Load (lb/period)Load EquationDaily Load Over Limit (lb/day)1LOL Option 1 (lb)2LOL Option 2 (lb)2
1/31/2014Occurred? YesNoQuantity (kg/day)0.722.220.0763149.6Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-1.50-46.5
Reported? Yes
Estimated? No
2/28/2014Occurred? YesNoQuantity (kg/day)1.672.220.1128104Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-0.550-15.4
Reported? Yes
Estimated? No
3/31/2014Occurred? YesNoQuantity (kg/day)0.722.220.113149.6Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-1.50-46.5
Reported? Yes
Estimated? No
4/30/2014Occurred? YesNoQuantity (kg/day)0.862.220.09063057Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-1.360-40.8
Reported? Yes
Estimated? No
5/31/2014Occurred? YesNoQuantity (kg/day)0.682.220.1033146.5Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-1.540-47.74
Reported? Yes
Estimated? No
6/30/2014Occurred? YesNoQuantity (kg/day)0.722.220.1013048Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-1.50-45
Reported? Yes
Estimated? No
7/31/2014Occurred? YesNoQuantity (kg/day)0.812.220.0913155.8Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-1.410-43.71
Reported? Yes
Estimated? No
8/31/2014Occurred? YesNoQuantity (kg/day)1.0882.220.1023174.4Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-1.1320-35.092
Reported? Yes
Estimated? No
9/30/2014Occurred? YesNoQuantity (kg/day)3.0382.220.100430Load Over Limit Flag 201Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)0.81824.5424.54
Reported? Yes
Estimated? No
10/31/2014Occurred? YesNoQuantity (kg/day)0.772.220.0783152.7Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-1.450-44.95
Reported? Yes
Estimated? No
11/30/2014Occurred? YesNoQuantity (kg/day)0.772.220.0933051Daily Load (kg/day) * Number of Days * 2.205 (lb/kg)-1.450-43.5
Reported? Yes
Estimated? No
Annual Load from Monitoring Data (lb/yr)789Annual Load over Limit (lb/yr)24.54-384.652

Example 3. Load over Limit Calculation for Pollutant Discharges with Effluent Exceedances during all Monitoring Periods where Discharges Occurred during a Year
Monitoring PeriodDischarge InformationBelow Detection Limit?Measurement TypeAvg Daily ValueLimitWastewater Flow (MGD)Number of DaysMonitoring Period Load (lb/period)Load EquationDaily Load Over Limit (lb/day)1LOL Option 1 (lb)2LOL Option 2 (lb)2
1/31/2014Occurred? NoNoConcentration (mg/L) 6.2031 No Load Calculated   
Reported? Yes
Estimated? No
2/28/2014Occurred? NoNoConcentration (mg/L) 6.2028 No Load Calculated   
Reported? Yes
Estimated? No
3/31/2014Occurred? YesNoConcentration (mg/L)6.36.23031Load Over Limit Flag 48,899Concentration (mg/L) * Flow (MGD) * 3.785 (L/gal) * Number of Days * 2.205 (lb/kg)25776776
Reported? Yes
Estimated? No
4/30/2014Occurred? NoNoConcentration (mg/L) 6.2030 No Load Calculated   
Reported? Yes
Estimated? No
5/31/2014Occurred? YesNoConcentration (mg/L)6.86.23231Load Over Limit Flag 56,298Concentration (mg/L) * Flow (MGD) * 3.785 (L/gal) * Number of Days * 2.205 (lb/kg)1604,9674,967
Reported? Yes
Estimated? No
6/30/2014Occurred? YesNoConcentration (mg/L)7.26.22930Load Over Limit Flag 52,279 Concentration (mg/L) * Flow (MGD) * 3.785 (L/gal) * Number of Days * 2.205 (lb/kg)2427,2617,261
Reported? Yes
Estimated? No
7/31/2014Occurred? NoNoConcentration (mg/L) 6.2031 No Load Calculated   
Reported? Yes
Estimated? No
8/31/2014Occurred? NoNoConcentration (mg/L) 6.2031 No Load Calculated   
Reported? Yes
Estimated? No
9/30/2014Occurred? YesNoConcentration (mg/L)8.36.22530Load Over Limit Flag 51,953Concentration (mg/L) * Flow (MGD) * 3.785 (L/gal) * Number of Days * 2.205 (lb/kg)43813,14513,145
Reported? Yes
Estimated? No
Annual Load from Monitoring Data (lb/yr)209,429Annual Load over Limit (lb/yr) 26,14926,149

1 Daily Load Over Limit (lb/day) = Average Daily Value - Limit Value
2 Load Over Limit = Daily Load Over Limit * Number of Days

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