An aerated biological filtration system for treating waste water includes a treatment vessel of elongated configuration which incorporates a circulating filter bed and a static filter bed. A liquid-phase growth apparatus suitable for practicing said liquid-phase growth process. The radius of the concave molded portion is greater than the diameter of a circular cross-sectional area of the intermediate element such that compression of the intermediate element caused particularly by the patient’s movement allows for a defined deformation. The fire stop disc seals an opening associated with a pipe penetration.