How this calculator works
Use liquid volume rather than structural tank volume; rectangular mode subtracts freeboard. Applied food loading uses primary effluent entering aeration, while removal percentages compare the untreated influent with downstream samples. F/M uses MLVSS inventory. MCRT divides the specified MLSS inventory by solids lost in waste sludge and final effluent. If your plant includes clarifier inventory, add it to the known-volume or mass basis under its own procedure; this tool models the entered aeration inventory only. Gould sludge age instead uses incoming suspended-solids loading and is not interchangeable with MCRT.
Formula
Load kg/day = Q m3/day × food mg/L / 1,000. F/M per day = load/(V × MLVSS kg/m3). HRT = V/Q. MCRT = V × MLSS/(QwXw + QeXe), with consistent concentration and flow units. Gould age = V × MLSS/(Q × influent SS). Reverse modes rearrange each relation for the named unknown. SVI mL/g = settled mL per litre / MLSS g/L.
Worked example
At Q = 100 m3/hour, V = 1,000 m3 and primary BOD = 200 mg/L, load is 480 kg/day and HRT is 10 hours. MLVSS of 2,400 mg/L gives 2,400 kg inventory and F/M = 0.2 per day. A 300 mL settled volume in a 1 L test with MLSS 3,000 mg/L gives SVI = 100 mL/g.
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Frequently asked questions
Can I compare BOD F/M and COD F/M directly?
No. They use different food analyses. Keep the basis consistent with plant targets and compare like with like.
Are sludge age and MCRT always the same?
Terminology varies. This page labels Gould sludge age separately, using influent suspended solids, while MCRT uses solids leaving the process.
Does a favourable value prove treatment compliance?
No. These are process indicators. Sampling quality, process configuration, permits and other operating measurements still govern interpretation.