GB1424261A - Clarification of liquid - Google Patents

Abstract

1424261 Settling tank for sewage sludge F M C CORP 12 Jan 1973 [12 Jan 1972 (2)] 1862/73 Heading B1D [Also in Division Cl] Settled sludge is withdrawn from the floor of a rectangular settling tank by suction nozzles suspended from a reciprocating bridge, the nozzles being of inverted T form (Fig. 3, not shown) and having horizontal sections with apertures 90 and blades 92 which direct sludge into the apertures. In Figs. 1 to 4, nozzles are arranged in pairs and one nozzle of each pair operates depending on the direction of bridge movement. Apertures 90 vary in diameter and/ or spacing with distance from the vertical parts 84 of the nozzles to compensate for variation in suction, as illustrated in Figs. 9, 10, 11, 15 (not shown). The suction is created by delivering compressed air to parts 84 of the nozzles which act as air lift pumps, and parts 84 discharge into a trough 44 on the bridge which discharges to a gutter on the tank wall. In Figs. 1 to 4, liquid flows along the tank in the direction of bridge movement, and overflow channels (26) for clean liquid extend lengthwise between the nozzles. The bridge is moved by a motor powered by a supply whose frequency is varied in steps, by a bridge extension engaging cams on the tank wall, to vary the bridge speed so that it moves more slowly over regions where most sludge will have accumulated since the previous traverse. Valves 96, 98 enable air supply to the nozzles to be adjusted to compensate for lateral variations in sludge thickness, and these arrangements ensure that sludge of uniform solids content is withdrawn from the tank, whence it may be recycled to an aeration stage. In Figs. 5 to 8, the bridge speed is controlled by probes (140) which depend from the bridge and vary the bridge motor frequency in response to variations which they detect in sludge thickness. Positive displacement or centrifugal pumps draw the sludge through the nozzles, and valves (136) enable control of pumping speed. In Fig. 5 the pair of nozzles of Fig. 4 replaced by one nozzle with two sets of apertures 106, 108, one of which is closed in each direction of movement by a pivoted blade 110 actuated through cables by a motor controlled by the bridge limit switches. Fig. 6 (not shown) illustrates a simpler construction wherein one set of apertures cooperates with a blade pivoted by end stops. In Figs. 12 to 15 (not shown) the liquid flows through the tank perpendicular to the bridge movement. The bridge speed is varied as in Figs. 1 to 4, and each nozzle has means monitoring the sludge concentration therein and continuously adjusting a valve as the bridge traverses so that the sludge concentration remains uniform. The valve may control the sludge flow directly through a centrifugal pump or control the air supply to an air lift nozzle. Nozzles are designed to compensate for some being located in regions where more sludge will settle than others. This embodiment includes a skimmer which pushes scum into a trough on one side of the tank and is lifted clear of the liquid when the bridge moves away from the trough.