Please refer to the detailed process of domestication and enrichment culture of activated sludge in biological treatment of domestic sewage.
Generally speaking, if we do it in the laboratory, we will use glucose to prepare simulated domestic sewage with COD around 300, and then add the sludge from the sewage treatment plant into the reactor and start aeration. Ventilation should be appropriate to prevent sludge bulking. The simulated domestic sewage should be kept unblocked, and sludge will form in a week or so. It was a biofilm when I made it.