GB2129326A

GB2129326A - Moving belt filter

Info

Publication number: GB2129326A
Application number: GB08326692A
Authority: GB
Inventors: William P Scott
Original assignee: Ashbrook Simon-Hartley Inc; Ashbrook Simon Hartley Inc
Current assignee: Ashbrook Simon-Hartley Inc; Ashbrook Simon Hartley Inc
Priority date: 1982-11-02
Filing date: 1983-10-05
Publication date: 1984-05-16
Also published as: GB2129326B; GB8326692D0

Abstract

A filter for dewatering sludge comprises a moving foraminous belt (16) onto which the sludge, preconditioned with polyelectrolyte from pump (32), is fed along chute (15). Staggered obstructions (23) divide the sludge into discrete streams between which zones of the belt are substantially free of sludge to permit drainage of water therefrom. The sludge is then compressed between belt (16) and a second belt (18) which are directed around a series of rollers. A pair of floats (25, 26) rest on the belt (16) or on the sludge thereon, in respective zones between and aligned with upstream obstructions (23), and their respective vertical positions are compared to derive a signal which controls the polyelectrolyte supply pump (32), thereby ensuring that the sludge is sufficiently stiff not to spread laterally across the belt downstream of an obstruction (23) without excessive consumption of polyelectrolyte. The control box (31) also produces signals indicating excessive or lack of sludge on the belt. <IMAGE>

Description

SPECIFICATION A sludge drainage system This invention concerns equipment for dewatering sludges wherein a feed of sludge is deposited onto a moving foraminous belt to permit water to drain through the belt, and particularly, though by no means exclusively, concerns a composite dewatering system which further incorporates a belt press wherein the initially dewatered sludge is sandwiched between a pair of belts which are pressed together to squeeze further water from the sludge, preferably by passage back and forth around a series of rollers in a serpentine formation.

In equipment of this kind, in the initial gravity drainage section, it has been proposed to mount above the belt a plurality of obstructions which plough or divert the oncoming sludge into parallel streams between which remain areas of the belt which are substantially free of sludge thus to permit released water to drain readily through the belt.

In this section, it has been found that streaming is enhanced by creating, a floc structure within the sludge and this is achieved by pre-conditioning the latter with a polyelectrolyte. Optimum use of polyelectrolyte is determined according to the minimum amount required to produce a floc of sufficient density, and it is an object of the present invention to provide a system for monitoring the floc structure of the sludge on the belt thus to enable the addition of polyelectrolyte to be optimised. If the floc structure is insufficient, then the low density sludge rapidly flows into the free areas of the belt thus impairing the passage of water therethrough.

According to the present invention, there is provided a sludge drainage system comprising a moving foraminous belt onto which a feed of sludge pre-conditioned with polyelectrolyte is deposited to permit water to drain therefrom through the belt during passage of the latter, at least one obstruction being disposed above the belt to divert the passage of sludge to create at least one continuous zone downstream of the obstruction wherein the belt surface remains substantially free of sludge and wherein freed water can readily pass through the belt, characterised by a pair of transducers capable of sensing the levels of sludge in said free area and elsewhere on said belt respectively, said transducers producing signals representative of said levels and means responsive to said signals to display, record or convert same to enable optimum levels to be achieved.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:~ Fig. 1 is a schematic illustration of a sludge drainage system incorporating a gravity drainage section and a belt press; Fig. 2 is a plan view of the gravity drainage section; Figs. 3a and 3b are side and end elevations respectively of part of the arrangement illustrated in Fig. 2; and Fig. 4 is a side elevation.

With further reference to Fig. 1, the system comprises a pre-conditioning and feed section generally indicated at 10, a gravity drainage system generally indicated at 11 and a belt press section generally indicated at 12. In the section 10 the sludge to be dewatered is conditioned by the addition of a polyelectrolyte, in a conditioning vessel 13. The conditioned sludge flows over a weir 14 and via a shallow chute 15 onto a moving foraminous, continuous belt 16 driven in the direction of arrow 17. The sludge after initial dewatering leaves the gravity drainage section 1 1 and is sandwiched between the belt 16 and a further belt 18 for passage around a series of rollers 19 in the press section 12, wherein the belts pass in a serpentine manner around the rollers. Dried sludge leaving the section 12 is collected in a chute 20.Water freed from the sludge and passing through the belts 16 and 18 is collected in a further chute (not shown) disposed below the sections 1 1 and 12, and sprays 21 and 22 are provided to wash residual sludge from the belts before they return to the sections 1 1 and 12 respectively.

Referring now to Fig. 2, there are provided 5 rows of ploughs 23 which are pivotally mounted freely on support shafts 24 extending across the machine above the belt 16. The ploughs 23 rest freely on the belt 16 thus to divert the oncoming sludge into a plurality of discrete streams leaving continuous zones of the belt between them substantially free of sludge. In the event that abnormal solid or fibrous sludge content builts up against the ploughs 23, they are arranged to pivot upwardly to allow such material to pass.

In the operation of the system, the addition of polyelectrolyte to the sludge to create a floc structure therein ensures that the streams created by the ploughs 23 do not readily flow into the freed areas of the belt. However, if the floc structure is insufficient then the resultant lower density sludge will rapidly fill these areas.

Therefore it is important to optimise the floc structure whilst minimising the use of polyelectrolyte which renders the process expensive. The establishment and maintenance of the floc structure is usually monitored "manually" by the operator who inspects the sludge and adjusts the rate of feed of polyelectrolyte accordingly. Now, in accordance with the invention, there is provided a system which automatically monitors flocculation, and comprises a pair of floats or "skis" 25 and 26 located, one in an area between a pair of ploughs 23 just downstream thereof, and the other aligned with one of said ploughs, just downstream thereof.

As can be seen from Figs. 3a and 3b, each plough is pivotally mounted by means of a bearing 27 on a support arm 28 which depends from one of a pair of shafts 29, 30 as extensions of a pair of potentiometers (not shown) within a control box 31 for a purpose to be described.

Thus when the system is in operation and optimum flocculation is present the float 26 should rest upon the belt whilst the float 25 rides at a higher level on the surface of the flocculated sludge. If flocculation is poor the float 25 will ride at a lower level, and, by virtue of sludge flowing behind the adjacent plough 23, the float 26 will rise.

Thus, signals derived from the potentiometers in the control box 31, when compared, will give an indication to the operator of the condition of the sludge. That is, if there is no sludge on the belt, then both floats are at their lowest positions. This condition may be identified by a signal emitted on line 33. If sludge is present but flocculation is poor then both floats may be at the same level above the belt. If both floats are at their maximum heights, then there is an overflow condition which may be identified by a signal emitted on line 34, and which must be corrected, whilst for optimum flocculation the vertical separation of the floats is at a maximum. The signals derived from the potentiometers can be fed to a pump 32 controlling the supply of polyelectrolyte to the sludge to adjust same thus to compensate for changes in solids content of the incoming sludge.

The establishment of a correct level of flocculation whilst ensuring optimum drainage also serves to present the gravity-drained sludge to the press section 12 in a satisfactory condition to prevent sludge from being squeezed from the edges of the belts.

The provision of a level-sensing device of the kind described herein is of considerable advantage to the operator who otherwise must frequently climb to the upper regions of the equipment which in practice are usually well above eye level to inspect the sludge, and the whole performance can be monitored and adjusted manually or automatically with greater convenience and precision.

Claims

1. A sludge drainage system comprising a moving foraminous belt onto which a feed of sludge preconditioned with polyelectrolyte is deposited to permit water to drain therefrom through the belt during passage of the latter, at least one obstruction being disposed above the belt to divert the passage of sludge to create at least one continuous zone downstream of the obstruction wherein the belt surface remains substantially free of sludge and wherein freed water can readily pass through the belt, characterised by a pair of transducers capable of sensing the levels of sludge in said zone and elsewhere on said belt respectively, said transducers producing signals representative of said levels, and means responsive to said signals to display, record or convert same to enable optimum levels to be achieved.

2. A sludge drainage system according to Claim 1, wherein said transducers are comprised by a pair of floats one of which is located in line with and downstream of said obstruction in the direction of travel of said belt, the other of said floats being disposed elsewhere on the belt, each said float being free to rest upon the belt or to be elevated from the latter to float upon the surface of sludge on the belt.

3. A sludge drainage system according to Claim 1 or Claim 2, wherein a plurality of obstructions are provided and disposed in at least one transverse row across the belt thus to divert sludge thereon into a number of discrete parallel streams and leaving continuous zones of the belt between said streams substantially free of sludge.

4. A sludge drainage system according to Claim 1, wherein said means responsive to said signals comprises at least one indicator displaying the height differential between the two transducers and thus representing the condition of sludge upon the belt.

5. A sludge drainage system according to Claim 1, wherein said means responsive to said signals includes further means to adjust the addition of polyelectrolyte to the sludge and/or sludge feed rate, thus to condition the sludge to achieve optimum flocculation.

6. A sludge drainage system according to Claim 1, including means whereby said signals provide an indication in the event of an overflow condition.

7. A sludge drainage system according to Claim 1, including means whereby said signals provide an indication in the event that no sludge is present upon the belt.

8. A sludge drainage system according to any preceding claim, including a further belt which downstream of said obstruction is brought into contact with said aforementioned belt such as to sandwich sludge between the belts, the belts together passing around a plurality of rollers to apply pressure thus to remove further water from the sludge.

9. A sludge drainage system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.