Calculate HRT using the correct useful volume
For a nominal estimate, HRT = V/Q with useful volume and flow expressed on a consistent time basis. A 200 m³ tank receiving 400 m³/day gives 0.5 days, or 12 hours. This demonstrates the calculation and is not a recommended design residence time.
State which tanks are included, their operating levels and any poorly mixed regions. Nominal residence time does not independently describe short-circuiting or batch feeding. For peak-flow assessment, provide the hourly demand profile as well as daily flow and the available equalisation capacity.
Calculate SRT from inventory and solids discharge
A steady-state approximation is SRT = system solids inventory / daily solids discharge. If 200 m³ contains 8 kg/m³, the inventory is 1,600 kg. Total solids discharge of 80 kg/day corresponds to 20 days. Use the same solids basis in the numerator and denominator.
Include wasting, carryover and other relevant departures. Waste sludge concentration can differ from the bioreactor concentration, so waste volume alone cannot give sludge age. When inventory changes rapidly, this simple expression provides an operational reference and a dynamic balance is needed to interpret the transition.
Solids retention does not replace biological conversion
An MBR separates solids from water, but ammonia and soluble COD performance depend on biology. Temperature, oxygen, available carbon, toxicity and sludge age influence conversion. Permeate with no visible suspended solids may still fail the target for dissolved substances that govern the intended reuse.
Longer SRT can accumulate inert material and change MLSS, viscosity and oxygen transfer. Higher water flow reduces HRT if useful volume is unchanged. Either decision needs review of contaminant loading, oxygen supply and membrane hydraulic capacity rather than an assumption that better separation resolves every process constraint.
Use both times when assessing an external retrofit
Before adding an external MBR, collect tank volumes, levels, flows, MLSS/MLVSS, wasting data and campaign-specific analyses. Draw return flows, membrane tanks and cleaning streams. Define which compartments belong to the biological inventory and which flows remain internal to the selected assessment boundary.
Request separate statements for hydraulic capacity, biological treatment and sludge handling. Agree acceptance quality at representative loads and temperature, net production and effective sludge wasting. A brief flow test demonstrates hydraulic separation under those conditions; it does not establish long-term stability at the proposed new SRT.
Common questions
Are HRT and SRT equal in an MBR?
Usually not. Water passes through the membrane while biomass is retained and removed through controlled wasting.
Can an MBR operate without sludge wasting?
This should not be assumed. Solids are produced and accumulate, so wasting and inventory control belong in the operating balance.