The invention relates to a carrier element for an aerobic biological water treatment system. The carrier element has a first end and a second end at a distance from each other, the maximum diameter of the first end being larger than the maximum diameter of the second end. It also has biofilm growing surface structures that extend from the first end to the second end and from inner part of the element towards periphery of the element and at least two support structures that encircle the growing surface structures at the periphery of the element and connect the growing surface structures to each other. The support structures define the outer boundary surface of the carrier element, whereby the support structures are spaced from each other so that apertures allowing access to the biofilm growing surface structures are formed between the support structures. The invention relates also to a water treatment system employing the carrier elements.