Many NPDES permits include mass loading limits in addition to concentration limits, and every TMDL-driven wasteload allocation is expressed as a mass per time. Converting a concentration result in mg/L to a loading rate in lbs/day is one of the most common calculations in water quality compliance — and one of the most common sources of spreadsheet errors. Water quality software automates these calculations and eliminates the unit conversion mistakes that lead to incorrect DMR values.
The Core Formula
Load (lbs/day) = Concentration (mg/L) × Flow (MGD) × 8.34
Where 8.34 is the weight of one gallon of water in pounds
Example: BOD Daily Loading
A wastewater treatment plant discharges 2.5 MGD with a BOD concentration of 18 mg/L:
Load = 18 mg/L × 2.5 MGD × 8.34 = 375.3 lbs/day
If the permit limit is 500 lbs/day BOD, the facility is in compliance for this day. But if flow increases to 4.0 MGD with the same concentration: 18 × 4.0 × 8.34 = 600.5 lbs/day — an exceedance of the loading limit even though the concentration hasn’t changed.
Unit Conversions
The biggest source of loading calculation errors is incorrect unit conversion. Here are the common conversions:
| From | To | Conversion |
|---|---|---|
| mg/L × MGD | lbs/day | × 8.34 |
| µg/L × MGD | lbs/day | × 8.34 × 10⁻³ (or ÷ 1,000 first) |
| mg/L × GPD | lbs/day | × 8.34 × 10⁻⁶ (convert GPD to MGD first) |
| mg/L × CFS | lbs/day | × 5.394 (1 CFS = 0.6463 MGD × 8.34) |
| mg/L × m³/day | kg/day | ÷ 1,000 |
| lbs/day | kg/day | ÷ 2.205 |
Daily vs. Monthly Loading for DMR Reporting
Daily Maximum Loading
Calculate the load for each sampling day individually (concentration × flow × 8.34). The highest single-day value during the monitoring period is the daily maximum loading reported on the DMR. This represents the worst-case discharge during the period.
Monthly Average Loading
There are two accepted methods for calculating monthly average loading, and they can produce different results:
- Method 1: Calculate daily loads for each sampling day, then average the daily loads. This gives equal weight to each sampling event.
- Method 2: Calculate the monthly average concentration, multiply by the monthly average flow, and apply the conversion factor. This weights each day’s contribution by flow volume.
Your permit or state guidance should specify which method to use. When flow varies significantly from day to day, the two methods can produce noticeably different results.
Cumulative Annual Loading
For TMDL compliance, software calculates cumulative loading by summing daily loads across the compliance period (monthly, seasonal, or annual). This running total is compared against the wasteload allocation to track how much of the annual cap has been consumed and project whether the facility will stay within its allocation.
Special Considerations
Flow Measurement Pairing
For accurate loading, the concentration result must be paired with the corresponding flow measurement from the same period. For 24-hour composite samples, use the average daily flow for that day. For grab samples, use either the instantaneous flow at the time of the grab or the average daily flow, depending on permit specifications. Software links each concentration result to the correct flow value automatically.
Non-Detect Handling in Loading Calculations
When a concentration result is below the detection limit, the loading calculation must use the appropriate substitution value (typically zero or half the detection limit) as specified by the permit or state guidance. Software applies the correct non-detect handling method consistently across all loading calculations.
Percent Removal Calculations
Some permits require percent removal for parameters like BOD and TSS. The formula is: % Removal = ((Influent Load − Effluent Load) ÷ Influent Load) × 100. This requires loading calculations for both influent and effluent, paired by date. Software performs both calculations and computes the removal percentage automatically.
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