HRT & SRT Calculator
Calculate Hydraulic Retention Time (HRT) and Sludge Retention Time (SRT) — the two most fundamental parameters in biological wastewater treatment design and process control.
Free · HRT · SRT · Biomass AssessmentBioreactor Parameters
Enter your biological reactor parameters to calculate HRT and SRT for activated sludge systems (CAS, MBBR, MBR, SBR).
Mixed Liquor Suspended Solids
Suspended solids in treated effluent
Daily volume of mixed liquor wasted from the system (from RAS or aeration tank)
How to Use This Calculator
- 1Enter the aeration tank volume (m³) and influent flow rate (KLD) to calculate HRT.
- 2Enter the MLSS concentration in the tank and the sludge wastage rate (m³/day) wasted from the system daily.
- 3Enter the effluent TSS — the suspended solids in the clarified effluent leaving the system (typically 20–30 mg/L).
Formulas
HRT (hrs) = Tank volume (m³) ÷ Flow (KLD) × 24
SRT (days) = Biomass in system ÷ Daily solids leaving system
= (V × MLSS) ÷ (Q_w × MLSS + Q_e × TSS_e)
Where: Q_w = wastage rate (m³/d), Q_e = effluent flow, TSS_e = effluent TSS
Why HRT and SRT Matter
HRT (Hydraulic Retention Time) is the average time that liquid stays in the bioreactor. Short HRT reduces capital cost (smaller tanks) but may not allow sufficient contact time for treatment. Typical HRT: 6–12 hrs (Conventional AS), 18–36 hrs (Extended Aeration).
SRT (Sludge Retention Time), also called Mean Cell Residence Time (MCRT) or Sludge Age, is the average time that microorganisms (sludge) stay in the system. SRT controls which microorganisms grow: short SRT (5–8 days) favours fast-growing heterotrophs for BOD removal; long SRT (≥10 days) is required to retain slower-growing nitrifying bacteria for ammonia removal.
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