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.
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.
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]
Solids balance and SRT basis
Documentation revision: 2026-10-02. This page documents the calculation basis so an engineer can trace the inputs, units, assumptions, and review needs. Results are preliminary screening estimates; they are not a permit determination, OEM guarantee, or construction/procurement approval.
Method and source basis
SRT is calculated as solids inventory within the selected boundary divided by daily solids leaving that boundary. The page uses V × MLSS for inventory, subtracts entered effluent solids loss from target solids loss, and converts required WAS solids to WAS flow using the wasting-point concentration.
The sources below provide technical context or analytical/design methodology. They do not endorse this calculator's defaults, and any jurisdiction-specific requirement applies only where that authority has jurisdiction.
- U.S. EPA, Activated Sludge Units technical guidance
- U.S. EPA, Membrane Bioreactors Wastewater Management Fact Sheet (2007)
- WEF Manual of Practice No. 8, Design of Water Resource Recovery Facilities
- Metcalf & Eddy/AECOM, Wastewater Engineering: Treatment and Resource Recovery, 5th ed.
Assumptions, limitations, and review actions
- Define the boundary explicitly and include all relevant compartments, recycle lines, permeate/effluent solids, inert and chemical solids, growth/decay residues, and intermittent wasting on a consistent solids basis.
- MLVSS/MLSS is used here for reporting; WAS flow is based on total MLSS. Do not treat the result as an automatic biological SRT, nitrification SRT, or permit/design limit.
- A negative residual solids balance is a measurement or boundary warning, not proof that WAS demand is zero. Review flow-weighted samples, MLSS/MLVSS, WAS concentration, permeate TSS, and wasting records.
Professional review checkpoint
Before using the result for specification or operation, reconcile it with representative site data, the applicable permit and design standard, the selected equipment or membrane supplier's current data, and a qualified wastewater/process engineer. Record the input basis, date, units, and any pilot, bench-test, or comparable full-scale evidence used to select the assumptions.