The present invention relates to an apparatus for the dewatering of sludge, in particular waste water sludge, which apparatus comprises a sieve drum rotatable around a substantially horizontal axis, said drum being closed at the one end and open at the other end, a sludge feed pipe extending from the open end of the sieve drum to an area in the vicinity of the inner side of the sieve drum at its closed end, means for collecting partially dewatered sludge passing out of the sieve drum at its open end and means for supplying rinsing water to the upper surface of the sieve drum to remove sludge residues therefrom.

Ordinarily, waste water sludge has a content of solids of between 0.5 and 2% by weight. A portion of the solids is typically of a colloid character and hence it is difficult to precipitate.

It is known to concentrate waste water sludge by feeding it to a concentration tank, the upper end of which is provided with a decantation means for discharging and disposal of liquid supernatant.

Such a concentration tank makes it possible to obtain such an increase in the content of sludge solids that the concentrated sludge can be subjected to a dewatering proper in conventional sludge dewatering apparatuses, e.g. a centrifuge, a belt screen press or a chamber filter press. As a result, the content of solids can be increased to 18-35% by weight and hence the amount of sludge is significantly reduced and simultaneously it becomes possible to remove the sludge in open tanks.

It is known to replace or supplement the treatment of waste water sludge in a concentration tank with a treatment of the sludge in a sludge dewatering apparatus of the type mentioned above.

In using the prior art dewatering apparatus a flocculant, e.g. an electrolyte, is ordinarily added to the waste water sludge in an amount of from 0.2 to 0.5% of the total amount of sludge before continously feeding the sludge through the sludge feed pipe

onto the inner side of the rotatable sieve drum, the sieve surface of which consists of a nylon or metal wire screen, and a radially inwards extending helical flange is mounted on the inner side of the sieve drum, the twist direction of said flange being so that during operation of the sieve drum sludge is conveyed in the direction towards the discharge end of the sieve drum.

During operation of the prior art sludge dewatering apparatus the sieve drum typically rotates at a rate of between 1.5 and 9 rpm and consequently the sludge will be conveyed slowly from the closed end of the sieve drum towards its open end. During this transportation the sludge will be partially dewatered as a portion of the water contained in the sludge passes through the sieve surface of the sieve drum and further down onto a collecting tray located beneath the sieve drum.

In order to remove sludge residues deposited on the sieve surface during operation, which would gradually reduce or completely stop the dewatering of the sludge, rinsing water is supplied continuously or intermittently onto the upper surface of the sieve drum by means of a row of nozzles located a short distance above the upper side of the drum along its entire length.

In connection with such rinsing of sludge residues from the sieve surface, the rinsing water flows from the upper portion of the sieve drum and either direcly down into the sludge or down into a separate collecting tray conducting the rinsing water to the open end of the drum, where it is supplied to the partially dewatered sludge. Thus, both methods counteract the dewatering of the sludge.

The object of the present invention is to avoid this drawback.

According to the invention this object is obtained with a sludge feed pipe which is open upwards and is located in the zone below the means for supplying rinsing water.

By using such an upwards open pipe it is possible to catch the major portion of the rinsing water, and hence the caught rinsing water will be conducted to the closed end of the sieve drum together with

the sludge to be dewatered. Thus, rewetting of that portion of the sludge which has already been partially dewatered, is avoided.

In a preferred embodiment of the apparatus of the invention the upwards open pipe has a U shaped sectional profile. Such a shape allows substantially all the rinsing water to be caught.

The means for supplying rinsing water to the upper surface of the sieve drum preferably comprise a row of closely spaced nozzles arranged on a common supply pipe located vertically above the rotation axis of the drum.

The invention will now be described in further details with reference to the drawings, in which

Fig. 1 is a perspective view of a sludge dewatering apparatus according to the invention, wherein portions of the casing and the sieve surface of the sieve drum are removed,

Fig. 2 is a side view partially in section of the apparatus of Fig. 1 without casing, and

Fig. 3 is a vertical section through the sieve drum, casing and support of an apparatus according to Figs. 1 and 2.

The apparatus shown in Figs 1, 2 and 3 comprises a casing 1 mounted on a frame 2. The casing 1 has a tray shaped bottom 3 with a discharge pipe 4. In the casing 1 a rotatable sieve drum 5 is mounted around a substantially horizontal axis and on the inside of the drum helical flange segments 6 are located. At its open end the casing 1 has an opening 7 located directly above a funnel 8 having a discharge pipe 9.

The apparatus shown further comprises a supply pipe 11 for sludge to be dewatered. Said supply pipe 11 is connected with an upwards open U shaped feed pipe 12 debouching near the opposite end of the sieve drum 5. The apparatus further comprises a row of nozzles 13 arranged on a pipe 14 extending over the entire length of the sieve drum and connected to a water supply pipe 15 having a valve 16.

Finally, the apparatus shown comprises supporting rollers 17 located at the open end of the sieve drum and drive means (not shown) for rotating the same.

In using the sludge dewatering apparatus shown sludge to be dewatered is fed to the inner side of the sieve drum 5 at its closed end, the sludge being supplied through the supply pipe 11 and the upwards open feed pipe 12.

During the continuous supply of sludge to be treated and the simultaneous rotation of the sieve drum 5, the sludge will move towards the opening 7 in a slow stream and simultaneously be dewatered gradually as water passes through the wall of the sieve drum and is collected on the tray shaped bottom 3 of the casing 1, from which it is removed through the dicharge pipe 4. Having reached the opening 7 the partially dewatered sludge falls down into the funnel 8, from which it is discharged through the discharge pipe 9.

The movement of the sludge towards the opening 7 is checked by the helical flange segments 6 conducting a portion of the sludge in the opposite direction of the rotation of the sieve drum 5 shown by the arrow 18 during the rotation of the sieve drum. Consequently, a powerful movement of the sludge is generated in relation to the sieve wall of the drum and hence an increased dewatering is obtained.

In order to remove sludge deposited on or in the sieve wall of the sieve drum 5, the sieve drum is sprayed continuously or intermittently with rinsing water supplied through the nozzles 13. A major portion of the rinsing water falling down together with the removed sludge residues from the upper portion of the drum towards its bottom, will be caught by the upwards open feed pipe 12. As a result, it is avoided that the rinsing water wets the sludge which has already been dewatered.