Solids Residence Time (SRT)
11/20/2025
“How can we measure how long on average microorganisms spend in a wastewater reactor?”
Microorganisms are the backbone of wastewater reactors. These microscopic solids work hard to clean incoming wastewater to suitable environmental levels. But these pieces of biomass only have a limited amount of time in the reactor as solids are ejected from the treatment system. The average amount of time a microorganism spends in the reactor, known as the Solids Residence Time, is equal to the total biomass within the system divided by the exit flow rate of biomass. We can represent this relationship with the formula SRT = Total Volume of Biomass/(Waste rate of biomass + Effluent rate of biomass). A low SRT indicates a reactor is quickly cycling through biomass, which leads to a mostly young bacteria, while a high SRT has the opposite properties.
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