Hydraulic retention time (HRT) of each reactor is calculated as follow. For overall HRT, the sum of all reactor volumes (ΣV)is divided by influent flow rate (Q)

image004 (1)          ———————————–(1)

For municipal MBR, HRT typically ranges 2-4 hours and 1-2 hours in aeration and membrane tanks, where overall HRT in oxic condition spans at 3-6 hours. HRT of anoxic tank ranges 1-2 hours in general. Overall HRT ranges 5-8 hours for MLE process, but, if multiple anaerobic and anoxic reactors exist for nutrient removal as shown here, overall HRT can be even 12 hours.

The solids retention time (SRT), which is also called sludge age, is calculated as follow.

image006        ———————————–(2)

V= individual reactor volume (m3)
Xi  = MLSS in each reactor (mg/L)
QX = excess biosolids removal rate (m3/d)
XX = MLSS in the excess biosolids flow (mg/L)

The typical ranges of SRT is discussed here, but it has been known that the lower the SRT is the higher the membrane fouling is in general. While target SRT is typically 20 days or so, it can go down to 12-15 days. Membrane fouling may decrease to some extent at a SRT of 20+ days, but the higher the SRT is the higher the capital and operating costs are in general.

which is also called as mean cell residence time (MCRT)


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