A method is provided for maintaining a biomass disposed on a substrate in a bioreactor tank. The bioreactor tank has an inlet and an outlet in fluid communication with a waste treatment system whereby an influent fluid may be selectively introduced through the inlet into the bioreactor tank for removal of waste material therefrom. This produces an effluent fluid which can be returned through the outlet to the waste treatment system. The method comprises processing a quantity of influent fluid through the bioreactor tank to produce a quantity of effluent fluid and removing the effluent fluid from the bioreactor tank through the outlet. The method further comprises isolating the bioreactor tank from the waste treatment system and establishing a set of dry cycle conditions in the bioreactor tank. The dry cycle conditions are selected to effect endogenous growth within at least a portion of the biomass whereby an average age of the at least a portion of the biomass may be tailored. The method also comprises restoring the bioreactor tank connection to the waste treatment system.