A simple way to view SRT is the time needed to completely remove the biomass at steady state (assuming no regeneration). In an MBR, where SRT is controlled separately from HRT, the calculation is relatively straightforward. You may use volume removed (assuming good mixing) to calculate SRT. For example, 10 days SRT may be achieved by removing 10% MLSS, 20 days by 5% removal, and so on.
P. Praveen, W. Xiao, B. Lamba, K.-C. Loh, Low-retention operation to enhance biomass productivity in an algal membrane photobioreactor, Algal Research, 40 (2019) 101487.
P. Praveen, Y. Guo, H. Kang, C. Lefebvre, K.-C. Loh, Enhancing microalgae cultivation in anaerobic digestate through nitrification, Chem. Eng. J., 354 (2018) 905-912.
@praveen sir can you please provide the full file for "Low-retention operation to enhance biomass productivity in an algal membrane photobioreactor". I don't found the full text of the same. Also in second file, Formula used is not mentioned