In biological biogas reactor, COD or organic fraction are convert in in gas and microbial biomass (solid). If you calculate a mass balance, with mass of solid growth (microbial) inside your reactor and flowrate of mass of solid outlet, you can evaluate a retention time of solid and or microbial inside your system (total weight of biomass inside the reactor divided by daily basis weight of biomass in the outlet= MRT or SRT....MRT if you are able to dissociate microbial with other inert solid). For a CSTR the concentration of solid outlet (microbial + inert solid) are equal of the inside reactor concentration. If there is no biomass recirculation, in this case HRT=SRT.
This is not the case of attached growth reactor where the main biomass keep fixed on the support inside the system. The outlet of the reactor is supposed to contain less active biomass than the CSTR reactor because the biomass remains attached on support.
Hope that will help you.
Sincerly.
Yann
Yes. I agree with Yann Le Bihan in attached growth system microbes stay in the reactor while water flows. So the retention time of microbes in the reactor is greater than water retention time in the reactor.
SRT depends on how much we circulate biomass into the reactor. In real case we circulate portion of the solids went out with water. So it is basically lower than MRT (since part of the microbes not retained).
