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Biological process calculator

Solids Balance & SRT Calculator

Set a target SRT and calculate the required waste-activated-sludge (WAS) solids and flow rate from your bioreactor solids inventory and defined solids-loss terms.

Target-SRT design mode: The selected SRT, bioreactor inventory, WAS concentration, and any permeate solids loss determine the required WAS flow. For an MBR, permeate suspended-solids loss is often very low, but it is included explicitly. Use representative average values; verify final control settings with plant data and a qualified process engineer.

Input parameters
Enter average values; WAS flow is calculated from the target SRT.
m³ of mixed-liquor volume included in the SRT boundary
mg/L
decimal fraction; default 0.80
days; used to calculate the target total solids-loss rate
Solids-loss terms
mg/L at the wasting point
kg MLSS/day; use 0 when negligible
Calculated results
All solids terms are expressed on an MLSS basis unless stated otherwise.

Enter values and select Calculate to view the solids balance.

Equations used

The calculation treats SRT as the solids inventory within the selected process boundary divided by the rate at which solids leave that boundary.

MLSS inventory = V × MLSS ÷ 1,000
Target total solids loss = MLSS inventory ÷ Target SRT
Required WAS solids = Target total solids loss − Effluent solids loss
Required WAS flow = Required WAS solids × 1,000 ÷ XWAS
Target SRT = MLSS inventory ÷ (Required WAS solids + Effluent solids loss)

Scope and interpretation

The calculator operates in target-SRT design mode: it returns the required WAS flow, rather than treating it as an independent input. The SRT result represents the defined bioreactor boundary. It does not include solids in external tanks, pipes, or other vessels unless their volume and solids inventory are included in the input. The SRT relation and the role of sludge wasting are consistent with practical activated-sludge SRT control guidance. Source: YSI SRT control guidance [1]