Abstract: The invention concerns a heating arrangement for ambient air introduced into a biological filter intended for the treatment of waste waters. The arrangement comprises intake openings for outside air, a member for heating the outside air, a blower, for blowing the heated air through the filter material layer, as well as air exhaust openings. According to the invention, the said heating member comprises a heat exchanger fitted as operative between the intake openings and the blower, the warmer side of the said heat exchanger being in contact with the waste water flowing in the filter, whereby the outdoor air is heated by the effect of the quantity of heat transferred from the waste water. The heat exchanger preferably consists of a hose made of aluminium, which hose interconnects the intake openings and the blower and which is fitted so that it is at least partly immersed in the waste water flowing in the filter.