MLSS Inventory Calculator
Wasting and SRT control start with knowing how many pounds of biomass you carry. Enter MLSS and volume for up to four aeration basins to get your total system inventory.
System MLSS Inventory
| Item | Value |
|---|
How the calculation works
Formulas
Inventory per basin (lb) = 8.34 × volume (MG) × MLSS (mg/L)
Total inventory (lb) = sum of all basin inventories
Weighted-average MLSS (mg/L) = total lb ÷ (8.34 × total MG)
8.34 lb per MG per mg/L is the weight of water: one million gallons of water at 1 mg/L carries 8.34 lb of that constituent.
Inventory per basin (lb) = 8.34 × volume (MG) × MLSS (mg/L)
Total inventory (lb) = sum of all basin inventories
Weighted-average MLSS (mg/L) = total lb ÷ (8.34 × total MG)
8.34 lb per MG per mg/L is the weight of water: one million gallons of water at 1 mg/L carries 8.34 lb of that constituent.
Notes & limitations
The 8.34 factor is the standard water-weight conversion used throughout wastewater process control (Metcalf & Eddy; WEF operator training materials).
- Inventory is the “mass in the system” side of solids retention time: SRT = total inventory ÷ (solids wasted per day + solids lost in effluent per day). Use this result with an SRT / wasting-rate calculator to set your WAS pump rate.
- Leave a basin blank (or zero) when it is out of service; it contributes nothing to the total.
- This covers aeration-basin MLSS only. Clarifier sludge blankets hold additional inventory — include a blanket estimate for a full mass balance on plants with deep blankets.
- Use lab MLSS values from representative composite samples; a single grab can mislead wasting decisions.
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