How this calculator works
Laboratory mode subtracts tare masses, divides each residue by the sample volume it represents and subtracts fixed from total suspended solids. If separate aliquots were used, enter both represented volumes. Reverse laboratory modes assume dry and ash residues represent the same aliquot. Industrial mode divides applied COD load by a chosen COD-based daily F/M ratio; it is a process-planning reconstruction, not a substitute for laboratory measurement. Use working tank volume, excluding freeboard, or reconstruct it from rectangular dimensions or flow times HRT. Additional inverse modes solve the industrial mass balance for flow, COD, F/M or working volume.
Formula
MLSS = net dry mass/sample volume. Fixed solids = net ash mass/represented sample volume. MLVSS = MLSS - fixed solids. In kg and m3, multiply kg/m3 by 1,000 for mg/L. Industrial load (kg/day) = Q (m3/day) × COD (mg/L)/1,000. Biomass kg = load/F:M; MLVSS = biomass/working volume.
Worked example
A 100 mL sample leaving 300 mg dry solids and 100 mg ash contains 3,000 mg/L MLSS, 1,000 mg/L fixed solids and 2,000 mg/L MLVSS. At 2,400 m3/day and 200 mg/L COD, load is 480 kg/day; F/M of 0.2 implies 2,400 kg biomass, or 2,400 mg/L in 1,000 m3.
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Frequently asked questions
Is all volatile residue living biomass?
No. Organic debris and non-living material may contribute. MLVSS is an operational measurement used as a biomass proxy.
Can fixed solids exceed dry suspended solids?
Not in a consistent mass balance. Check tare subtraction, represented sample volumes, units and the laboratory procedure.
Can I use a BOD-based F/M ratio with COD?
No. Food loading and the selected F/M ratio must use the same analytical basis. This industrial mode labels COD explicitly.