Fig. 141.—Contact Bed Apparatus.
In order to ensure that the sewage is held up in each bed for a suitable period for contact, the outlet is provided with an automatic syphon, which is arranged in such a way that the filling of the bed alone can start it. When the bed is full, the sewage flows through a pipe with an adjustable orifice into the timing pit. When this pit is full, the compression of the air in a small dome placed in the pit, and connected to the syphon by means of an air-pipe, releases the air in the syphon and allows it to start at the end of the desired period of contact. A special feature of these syphons is that they are so arranged that after they have emptied the bed they continue in action as syphons, taking the drainings from the bed, however small in volume they may be, and stopping off only when the bed commences to fill again, thus ensuring a thorough draining of the material. Another special feature of this apparatus is that there are no moving parts, the whole operation depending upon the transfer of air by means of the head of liquid. It is important to note that by this apparatus the period of contact in any bed can be arranged for any particular length of time, quite irrespective of the rate of flow of the sewage to the works, and independent of the filling and emptying of the other beds in the series.
Fig. 142.—Contact Bed Apparatus.