Apparatus for Thickening Sludge

THIS INVENTION concerns apparatus for thickening sludge particularly, though by no means exclusively, activated sludge.

In many cases, activated sludge is thickened by passage through a settling tank such that the residence time in the tank is of the order of 24 hours. This technique, besides requiring much time and large equipment, has the disadvantage that the thickened sludge will have a solids concentration of about 1.5 to 2% by weight.

With a view to obtaining higher levels of solids in the concentrated sludge, various techniques have been developed utilising sophisticated equipment such as flotation apparatus where suspended solids contained within the sludge are carried to the surface by small bubbles of gas, produced by electrolytic action.

Such flotation apparatus is effective in removing water from the sludge so that the concentrated sludge will have perhaps 5% solids by weight, or thereabouts. This technique has a number of disadvantages in that the equipment used is costly to purchase and maintain and that the processing costs are high.

An object of the present invention is to provide a system for thickening sludge to obtain a concentration of solids in the order of 5% by weight, with simple equipment and in an economic manner.

According to the present invention there is provided apparatus for thickening sludge, comprising a driven foraminous belt having a conveying run arranged at least substantially horizontally, a chute for feeding a mat of sludge onto the belt at one end of the conveying run, and means for causing the sludge to pass onto the belt in one or more zones whose width is substantially less than that of the belt, thus to leave one or more other zones of the belt free of sludge to permit drainage of water therefrom through the belt.

Preferably, said means comprises a plurality of flexible strips attached to and extending from the output end of the feed chute in the direction of travel of the belt, part way along the latter, the said strips being spaced apart thus to cause sludge to be fed onto the belt in a number of discrete parallel paths.

The invention will be further apparent from the following description with reference to the accompanying drawings which show, by way of example only, one form of apparatus embodying the invention.

Of the drawings:

FIG. 1 shows a perspective view of sludge thickening apparatus made in accordance with the invention;

FIG. 2 shows a side elevation of the apparatus;

FIG. 3 shows a section taken along line III--III of FIG. 1;

FIG. 4 shows a section taken along line IV--IV of FIG. 1;

and FIG. 5 shows a section taken along line V--V of FIG. 1.

Referring now to the drawings, the apparatus generally comprises a mesh conveying belt 10 driven by or wrapped around rollers 11, 12 and 13 and having a conveying run between rollers 11 and 12 of approximately 3 meters. The sludge to be thickened is delivered onto one end of the belt by a chute 14, and water which drains from the sludge through the belt 10 is collected by guide plates 15 and 16 and delivered to a pump 17 which supplies at least some of the drained water to one or more spray nozzles 18 which serve to wash any residual solids from the belt for further collection by a trough 19 from which the liquid and any residual solids are removed via a drainpipe 20 for disposal.

Attached to and extending from the delivery end of the feed chute 14 are a plurality of spaced parallel flexible strips 21 of a material such as rubber, which extends from the end of the feed chute 14 for approximately 1 to 2 meters in the direction of travel of the belt 10.

As sludge S to be thickened is fed towards the belt 10 on the chute 14, those zones of the belt 10 which pass beneath the strips 21 remain free of sludge as can be seen from FIG. 4, the sludge passing along the strips tending to fall to each side thereof thus to be fed onto those zones of the belt where the sludge is delivered direct from the feed chute. The length of the strips 21 is selected to ensure that the sludge deposited onto the strips from the feed chute falls into the zones of the belt occupied by the sludge and does not pass into those zones left free. At a certain distance downstream of the end of the flexible strips 21, there are provided a number of ploughs 22 disposed above the surface of the belt and aligned with the paths of sludge. These ploughs serve to deflect the uppermost portion of each path of sludge lying on the belt, onto those parts left free by the strips 21. It will be appreciated that the ploughs 22 are located sufficiently far downstream of the delivery position of the sludge onto the belt, for the first flush of free water to be drained from the sludge before reaching the ploughs. Thus, for the remainder of the conveying run of the belt 10 the sludge is formed into a mat of substantially uniform thickness throughout the width of the belt to permit drainage of residual water through the sludge during the remainder of the conveying run. This condition is illustrated in FIG. 5.

With the arrangement described herein it has been found that a greater concentration of solids can be achieved in the final sludge, when compared with that achieved using systems where no portions of the belt are left free of sludge during the initial flush of water or where a series of ploughs in contact with the belt close to the feed chute are used to establish free areas. In the latter case the ploughs while preventing residence of sludge in certain areas of the belt creates a spreading action by virtue of their contact with the belt thus forcing solids into the interstices of the belt which of course reduces the ability of the belt readily to permit drainage.