GB2261612A - Separation of solids from liquids - Google Patents

Abstract

Grit is separated from sewage in two stages (Fig. 1), the first comprising a settling tank from whose sump a liquid mixture of settled grit and organic contaminants is pumped to the inlet end of a second stage, which comprises an elongate inclined trough in which the grit settles while the organics are carried along with the liquid flow. The trough 17 may be adjustably inclined, with flow uphill or downhill or horizontal. In Fig. 6 the liquid flows out of the trough to the left of baffle 28, but after some grit has accumulated the flow is stopped and the trough is tilted, pulling its outlet end to the right of baffle 28. The grit is then flushed or scraped out. In Figs. 7, 8 the trough is fixed and grit removed by blades on an endless conveyor or by a screw device lowered into the trough when required. in Fig. 9 grit is flushed from a tilted trough into a hopper with an inclined screw conveyor. The trough may be vibrated in use. <IMAGE>

Description

SEPARATION OF SOLIDS FROX LIQUIDS This invention relates to a method of and apparatus for separating from a liquid phase certain solid phase particles having a specific gravity greater than that of the liquid phase or other solid phase particles not required to be separated, particularly but not exclusively grit from an aqueous sewage material.

It is known to perform a primary separation operation on the aqueous flow of sewage material entering a sewage treatment plant to remove grit from the flow and there are a number of recognised primary separation devices including constant velocity grit channels which are fixed, elongate (for example 30 metres) channels of a predetermined cross-section along which the plant inlet flow passes and where the aim is that flow conditions are controlled such that the grit phase can settle in the base of the channel for subsequent mechanical removal.

Unfortunately it is found that known primary separators, particularly when operating on raw sewage, separate other particulate material from the flow in addition to the grit, for example organic solids, and thus the collected grit removed from the primary separator has with it undesirable less dense solids which must be removed before the grit can be utilized. In this respect it is known to pass the "contaminated" grit from the primary separator to a secondary grit classifier together with part of the original aqueous phase, or water from another source, with the aim of the classifier separating the grit from the undesirable less dense solids, delivering the grit in de-watered form to a collation point, and returning the aqueous phase, together with the less dense solids, to the primary separator.

Such systems can work well but frequently are found to retain a large proportion of the organic and less desirable solids within the grit discharge. Disposal of the product is difficult and can be extremely odorous.

While particular reference is made above to the treatment of waste water or sewage effluent in a sewage treatment plant, it is to be understood that the invention disclosed herein may be used in other applications where reliable and economic separation of solid phase particles from a liquid phase is called for.

In accordance with the first aspect of the present invention there is provided a method of separating liquid and solid phases from a mixture flow comprising subjecting the flow to primary separation, delivering the solid phase collected at the primary separator to a secondary separator together with part of the original liquid phase, or additional liquid, and performing secondary separation in the secondary separator, the secondary separator being a constant velocity separator.

The term "Constant Velocity Separator" is used herein to denote a separator consisting of a channel member having an inlet at one end thereof for the mixture to be separated, and an outlet at the opposite end, the cross sectional shape of the channel member, its orientation, and the rate of flow into the channel of mixture to be separated, being so arranged that the flow along the channel member achieves velocity conditions at which the required solid particles settle to the base of the channel and the liquid phase together with any less dense, undesirable, solids flow from the outlet end of the channel.The preferred constant velocity separator operates as a batch processing device in that a predetermined volume of mixture will be supplied to the device and after the predetermined volume has been treated separation will cease, any liquid phase remaining within the channel will be discharged, and the retained solid phase, which has settled to the base of the channel, will then be removed in any convenient manner, whereafter the separator can operate on a subsequent batch of mixture.

Conveniently said secondary separator is operated in batch treatment mode.

The invention also resides in apparatus for performing the above method comprising a primary separator, a secondary separator in the form of a constant velocity separator, and, means for delivering the solid phase collected at the primary separator together with part of the original liquid phase or additional liquid, to the secondary separator.

Desirably there is provided scraper means for scraping settled grit along the channel member of the secondary separator to be discharged at an outlet end of the channel member.

Conveniently there is provided movable outlet gate means at an outlet end of the channel member of the secondary separator movable to direct liquid phase flowing from the channel member outlet end and grit discharged from said outlet end to different collection systems.

In accordance with a further aspect of the present invention there is provided a constant velocity separator, comprising a channel member and means mounting the channel member for tilting movement whereby the inclination of the plane of the base of the channel of the channel member, relative to the horizontal in use, can be altered.

Conveniently the channel member is pivoted intermediate its ends for rocking movement about the pivot so that either the inlet end, or the outlet end of the channel member can be positioned above, or below, the horizontal in use.

Alternatively the channel member is pivoted adjacent its outlet end and means is provided for raising and lowering the remainder of the channel relative to the outlet end.

Conveniently the outlet end of the channel member is pivoted by being supported on a wheel or roller which can move bodily along the support surface as the remainder of the channel member is raised or lifted so that the tilting movement of the channel is accompanied by longitudinal movement of the channel.

Desirably associated with the channel member is a device for discharging separated solid phase material from the channel after separation has ceased, the discharge means being clear of the channel during separation.

Preferably the device includes a paddle shaped to match the cross-section of the channel member and means for engaging the paddle in the channel member and moving the paddle longitudinally relative thereto to sweep settled material to the outlet end of the channel member.

Desirably the separator further includes means for introducing clean water as the flow of effluent into the channel member ceases, whereby the last of the effluent and any light phase solids settling therefrom are flushed from the channel member.

The invention further resides in a grit removal system for use in sewage treatment comprising a primary separator, and, a secondary separator as specified above for performing secondary separation, on the solid phase collected after primary separation.

Conveniently said primary separator is a cylindrical grit trap.

One example of the invention is illustrated in the accompanying drawings wherein: Figure 1 is a diagrammatic representation of a grit removal system for a sewage treatment plant, Figure 2 is an enlargement of part of Figure 1, Figures 3, 4 and 5 are respectively views illustrating three alternative cross sectional shapes of the channel member of Figures 1 and 2, Figure 6 illustrates an alternative mounting for the channel member illustrated in Figures 1 and 2, Figure 7 is diagrammatic representation of a mechanism for discharging collected solid material from the channel member, Figure 8 is a view similar to Figure 7 of an alterative, and Figure 9 is a view of a constant velocity separator of the kind illustrated in Figure 3 having an inclined-screw conveyor associated therewith for dewatering and transporting the separated solid material.

An example of the invention, and various alternatives thereto will now be described with reference to the accompanying drawings which illustrate equipment for use in separating grit from aqueous sewage effluent in a sewage, or similar water-treatment plant. It is to be understood however that the equipment can find application in other areas for example the separation of grit from a water supply. destined to become cooling, or even drinking water, and the removal of dense solid particles from effluents, and other liquid mixtures in chemical processing.

Referring first to Figure 1 of the drawings there is illustrated a grit separation system comprising a primary separator 11 and a secondary separator 12. The primary separator is a cylindrical grit trap of the kind manufactured and marketed by the applicants under the trade name "JETA". Aqueous effluent carrying grit suspended by the turbulence of the flow enters the cylindrical chamber 13 of the separator and while flowing around the chamber 13 between a tangential inlet and a tangential outlet grit settles into a well 14 in the base of the chamber 13. A rotating impellor 13a in the chamber 13 provides an upward liquid flow whereby much of the lower density organic solids is caused to remain suspended in the aqueous phase rather than settling with the grit into the well 14.Thus much of the lower density solid phase remains with the aqueous flow and passes from the primary separator 11 by way of the outlet thereof for further treatment forming no part of the present invention. Inevitably however some of the less dense solids does settle with the grit in the well 14, and thus the grit in the well 14 is contaminated with undesirable solids which must be removed before the grit can be considered to be a useful product of the process.

Extending centrally of the chamber 13, and vertically down into the well 14 is a grit suction pipe 15 through which the content of the well 14 can be extracted, and transferred along a delivery pipe 16 to the secondary separator 12. Thus at predetermined periods of time, or when the grit settling in the well 14 has reached a predetermined level, a pumping apparatus is operated to pump material from the well 14 to the secondary separator 12. A high portion of the material pumped from the well 14 is grit, but of course since the well also contains the aqueous phase of the mixture being separated then part of the aqueous phase and entrained and entrapped lower density solids are also pumped along the pipe 16 to the secondary separator 12.

The secondary separator 12 comprises a rectilinear channel member 17 which may be from 2 to 7 metres in length, but conveniently of the order of 5 metres in length. The channel member is formed from resilient material, but is supported so that the base of the channel therein is planar. As illustrated in Figures 3, 4 and 5 the cross sectional shape of the channel member, in particular the profile of the channel therein, can take a number of different forms, the form chosen being determined by the nature of the application for which the separator is intended. A convenient construction of the channel member comprises a fabrication of plates of P.V.C. stiffened and reinforced with mild steel elements, the whole then being encapsulated in a glass reinforced plastics material to achieve a robust, lightweight, corrosion free construction.Moreover in such a construction the floor of the channel would have a surface roughness which would assist the initial capture of settled fine grit particles.

At one end (the right hand end in the drawings) the channel member 17 has an inlet 18 and the opposite end 19 of the channel defines an outlet. Although the inlet 18 is illustrated in the drawings as being in line with the length of the channel member 17, it is to be understood that an inlet union could be provided at right angles to the length of the channel, and entering the channel from above, below, the right, or the left. There is a baffle device in the channel adjacent the inlet 18 to control the flow of material into the channel and so assist in obtaining constant velocity flow in the channel. In the arrangement illustrated in Figures 1 and 2 the channel member 17 is supported intermediate its ends on a pivot, or fulcrum 21 so that the inclination of the channel member, relative to the horizontal, can #e adjusted.In particular it is of course the plane of the base of the channel of the channel member whose inclination relative to the horizontal is of importance in determining the operation of the separator. Primarily the adjustment of the position of the channel member 17 relative to the horizontal is determined when the apparatus is installed, and thereafter it is not intended to be altered unless the nature of the separation which the separator is called upon to perform changes. However, it would be possible to monitor the performance of the separator, and to adjust the inclination of the channel member in use in accordance with the monitored results. Monitoring and control apparatus could be provided for this purpose, although for many applications such an arrangement would be unduly complex.

There are many variables in the arrangement of the channel member which affect the flow and settlement rates in the channel member in use. The inclination of the member may be varied from a position in which the outlet end of the channel is lower than the inlet end, through a horizontal position to a position in which the outlet end is higher than the inlet end. The manner in which the inlet flow is controlled by means of a baffle can determine the energy with which material enters the channel and thus its influence upon settlement and the point along the channel at which settlement commences.

Fluming (flow restriction) can be provided at the outlet end of the channel to control flow rate and flow depth within the channel. The cross sectional shape of the channel can be selected to provide predetermined settlement conditions for a given flow characteristic, and in some circumstances it may be desirable to have a channel cross-section which is not constant throughout the length of the channel. The selection of these variables to suit particular applications'is within the competence of one skilled and experienced in hydrodynamics and given the relatively simple construction of the channel member and the possibility of adjusting many of the variables in situ then "fine tuning" can be achieved using trial and error if necessary.

The operation of the system illustrated in Figure 1 is as follows. Aqueous sewage effluent which has been subjected to screening or comminution flows into the chamber 13 of the primary separator through a tangential inlet. The aqueous flow carries high density mineral material (grit) and lower density comminuted organic material, the high density particles settling, as described above, in the well 14. Periodically the content of the well 14 is pumped into the channel member of the secondary separator 12 which, in the example shown, has been set-up so that its outlet end 19 is higher than its inlet end 18. The rate of flow of material through the pipe 16 into the channel member 17 is controlled in relation to the inclination of the channel member and its cross sectional shape such that the aqueous mixture flows at a constant velocity along the channel member. The flow rate is such that the high density particles are permitted to settle under gravity into the base of the channel member while the aqueous flow together with the lower density particles flow to the outlet end of the channel member where they are discharged for further treatment in the treatment plant.

The flow of liquid through the pipe 16 will be maintained either for a predetermined time, or until a predetermined volume of effluent from the well 14 has been treated, the time, or the volume, being predetermined on the basis of the anticipated quantity of high density solids contained by the flow, and the settlement rate within the channel member. Coincident with the flow ceasing a quantity of clean water is flushed through the channel member 17 so that the last of the effluent and any light phase solids setting therefrom as the flow ceases are flushed from the channel member. The water may be supplied from a reservoir 20 by way of a valve controlled supply pipe 20a as shown in Figure 1, or from any other convenient source.After the flow has ceased the channel member can be tilted so that its outlet end is lower than its inlet end thereby permitting the channel member and the settled grit to drain. Thereafter the grit can be removed from the channel member in any convenient manner, for example by flushing with clean water, or by scraping.

Figure 2 illustrates the channel member 17-to a larger scale, and shows the range of inclinations through which the channel member can be moved. Figures 3, 4 and 5 illustrate the preferred (Figure 3) and two alternative cross sectional shapes for the channel member, the shape chosen being determined by the desired constant velocity flow rate, and the nature of the material to be separated. Although a positive inclination (outlet above inlet) is shown in Figure 1 there will be applications where zero inclination or even a negative inclination (inlet above outlet) is appropriate for settlement of the higher density solid phase.

Figure 6 illustrates an alternative construction in which the channel member 17 has, adjacent its outlet end 19, a downwardly projecting bracket 22 pivotally supporting one or more coaxial wheels or rollers 23. The wheels or rollers 23 are rotatable about a horizontal axis, and ride on a horizontal platform 24. Intermediate its ends the channel member 17 is pivotally connected to the outer end of a piston 26 of a piston and cylinder arrangement, the lower end of the cylinder 25 of which is pivotally connected to an anchorage point 27.It will be recognised that extension of the piston 26 from a retracted position relative to the cylinder 25 lifts the inlet end of the channel member 17 relative to the outlet end 19 the channel member pivoting about the axis of rotation of the wheels or rollers 23, and at the same time moving bodily (to the right in Figure 6) as permitted by rolling of the wheel or roller 23 on the platform 24. As is apparent such movement of the channel member 17 bodily displaces the outlet end of the channel 19 from one side, to the opposite side of a collector member 28.The collector member 28 is of inverted Yshape in transverse cross section, and in the normal operating position of the channel member 17 (as shown in full line in Figure 6) the flow of water and light density solids passes to the left hand side of the collector 28 as indicated by arrow A whereas during subsequent removal of the settled grit the channel member 17 is lifted to the position shown in broken lines in Figure 6 so that the grit washed, or scraped from the base of the channel passes to the right hand side of the collector as indicated by the arrow B.

In an alternative to the construction shown in Figure 6 the roller or wheel support arrangement 23, 24 is replaced by a fixed pivot so that the channel member 17, upon operation of the piston and cylinder arrangement 25, 26, pivots but does not move longitudinally. Thus there is insufficient longitudinal movement of the outlet end 19 to effect the change of flow from one side to the other of the collector 28 and instead of the collector 28 shown in Figure 6 there is provided an arrangement in which a system of gates or valves, automatically, or manually actuated, are used to direct the liquid flow during the separation, and the grit discharged after separation.

Figure 7 illustrates a preferred method of discharging the grit 29 which has collected in the base of the channel member 17. Mounted above the open face of the channel member 17, but having no operative association therewith during the grit settlement phase of operation, is an elongate endless belt or chain 31 extending around a pair of drive wheels or pulleys 32 and carrying one or more outwardly projecting paddles 33. One or both of the pulleys 32 can be driven so causing the or each paddle 33 to be brought down into the channel of the channel member 17, and to traverse the length of the channel from the inlet end to the outlet end thus scraping the settled grit 29 to the outlet end 19 of the channel member and pushing it from the outlet end for collection.It will be understood that during the settlement phase of operation of the channel member 17 the or each paddle 33 is "parked" clear of the channel as shown in broken lines in Figure 7. The shaping of the or each paddle 33 of course matches the internal profile of the channel member 17. As mentioned in relation to Figure 6 a system of gates or valves is used to direct the liquid flow during separation and the grit discharged by the paddle or paddles, a preferred arrangement being a movable chute at the outlet end of the trough, movable to divert liquid into a drain channel and grit into a collection system.

Figure 8 shows a further alternative method of discharging settled grit from the channel member 17.

Positioned above the open face of the channel member 17, and movable vertically downwardly into the channel of the channel member 17 is an elongate, driven, helical screw auger 34, and when it is time to clear accumulated grit from the base of the channel member 17, the auger and its supporting frame is lowered into the channel member 17 and the drive motor 35 of the auger 34 is energised to rotate the auger. The direction of rotation of the auger is such that the helical solid flight of the auger carries the accumulated grit 29 to the outlet 19 of the channel at which point it is collected. 'After clearance of the grit from the channel the auger 34 is withdrawn from the channel, and takes no part in the separation phase of the operation of the channel member 17.

Figure 9 illustrates a channel member 17 used in conjunction with a screw conveyor 38 which serves to dewater and transport the grit discharged from the channel 17 after a separation phase of its operation.

Thus the channel member 17 has an inlet 18 through which effluent enters, and a second inlet 36 through which clean flushing water can be introduced during clearance of accumulated grit. The flow of aqueous effluent and low density solids from the outlet 19 during the separation phase is collected in a drain chamber 37 from where it is conducted by pipework to further processing in the sewage treatment plant. After the separation phase of the operation the channel member 17 is tilted upwardly by the mechanism shown in Figure 6 so that its discharge end 19 no longer communicates with the drain chamber 37, but instead communicates with an inlet hopper 39 of the inclined screw conveyor 38. Fresh water is fed through the inlet 36 to flush the accumulated grit from the channel member 17 into the hopper 39, and of course if desired the apparatus could be modified to incorporate the paddle assembly 33 or the screw auger 34 in place of, or in addition to, the flushing water. The inclined screw conveyor 38 is of known form, and operates in known manner to dewater the grit discharged into the inlet hopper 39 from the channel member 17. Dewatered grit 41 is discharged from a grit outlet 42 of the conveyor for collection.

It will be recognised that the channel member 17 can be utilized in a secondary separation capacity "downstream" of other forms of primary separator for example, fixed elongate constant velocity channels and cross-flow grit removers.

The foregoing disclosure is primarily concerned with a channel 17 operating in batch mode since this permits the flow rate of liquid in the channel to be controlled more accurately than would be the case if continuous treatment dependent upon the treatment plant primary inlet flow rates were performed. However, a pair of channels 17 can be operated together each in batch mode but out of phase such that while one channel is being emptied of settled grit the other is receiving effluent for treatment. In this way although each channel still operates in batch mode, the effect of the two together is to operate in continuous mode it being understood that suitable flow diverters or the like are associated with the supply of effluent to the channels to switch the flow between channels 17 at the appropriate time.Naturally more than two channels can be used in conjunction with one another and suitable flow diverters, to process larger flow rates. For example three 1200 out of phase channels could be employed or where four channels are used they could be 900 out of phase or operated as two 1800 out of phase pairs of channels.

The channel member 17 variants described above can be used to effect separation of grit at other stages in the sewage treatment process. For example such arrangements could be used in the separation of grit from an aqueous sludge, of a more viscous nature than that pumped from a primary separator, as is present during later stages of the treatment process or derived from other sewage treatment plants or septic tanks and transported to the channel 17 for subsequent grit separation. Sludge would be fed to the channel member 17 from a holding tank or the like by way of the pipe 16 and if necessary water will be added to provide the appropriate consistency for separation in the channel member 17. The gravity settlement process may be augmented by vibrating the channel member 17 and its contents. It is believed that vibration of the channel member 17 also may be advantageous in the separation process described above where less viscous mixtures are treated and to permit such vibration it will be advantageous to provide resilient mountings for the channel member. The vibration can be produced by a rotating, out of balance, mechanism associated with the channel member 17 or its mountings, but of course other vibration generators of a mechanical, electrical, or electronic nature are possible.

The variants described above can also be utilised for separation of mixtures in environments other than sewage treatment.

Claims (17)

CLIMBS

1. A method of separating liquid and solid phases from a mixture flow comprising subjecting the flow to primary separation, delivering the solid phase collected at the primary separator to a secondary separator together with part of the original liquid phase, or additional liquid, and performing secondary separation in the secondary separator, the secondary separator being a constant velocity separator.

2. A method as claimed in Claim 1, wherein said secondary separator is operated in batch treatment mode.

3. Apparatus for use in the method claimed in Claim 1 or Claim 2, comprising a primary separator, a secondary separator in the form of a constant velocity separator, and, means for delivering the solid phase collected at the primary separator together with part of the original liquid phase or additional liquid, to the secondary separator.

4. A method as claimed in Claim 1 or Claim 2 or apparatus as claimed in Claim 3, including scraper means for scraping settled grit along the channel member of the separator to be discharged at an outlet end of the channel member.

5. A method as claimed in any one of Claims 1, 2 and 4 or apparatus as claimed in Claim 3 or Claim 4, including movable outlet gate means at an outlet end of the channel member of the secondary separator movable to direct liquid phase flowing from the channel member outlet end and grit discharged from said outlet end to different collection systems.

6. A constant velocity separator, comprising a channel member and means mounting the channel member for tilting movement whereby the inclination of the plane of the base of the channel of the channel member, relative to the horizontal in use, can be altered.

7. A separator as claimed in Claim 6, wherein the channel member is pivoted intermediate its ends for rocking movement about the pivot so that either the inlet end, or the outlet end of the channel member can be positioned above, or below, the horizontal in use.

8. A separator as claimed in Claim 6, wherein the channel member is pivoted adjacent its outlet end and means is provided for raising and lowering the remainder of the channel relative to the outlet end.

9. A separator as claimed in Claim 8, wherein the outlet end of the channel member is pivoted by being supported on a wheel or roller which can move bodily along the support surface as the remainder of the channel member is raised or lifted so that the tilting movement of the channel is accompanied by longitudinal movement of the channel.

10. A separator as claimed in any one of Claims 6 to 9, wherein associated with the channel member is a device for discharging separated solid phase material from the channel after separation has ceased, the discharge means being clear of the channel during separation.

11. A separator as claimed in Claim 10, wherein said device includes a paddle shaped to match the crosssection of the channel member and means for engaging the paddle in the channel member and moving the paddle longitudinally relative thereto to sweep settled material to the outlet end of the channel member.

12. A separator as claimed in any one of Claims 6 to 11, further including means for introducing clean water as the flow of effluent into the channel member ceases, whereby the last of the effluent and any light phase solids settling therefrom are flushed from the channel member.

13. A separator as claimed in any one of Claims 6 to 16, having movable outlet gate means at the outlet end of the channel member movable to direct liquid phase outlet flow and grit discharged from said outlet end to different collection systems.

14. A constant velocity separator substantially as hereinbefore described with reference to any one of the accompanying drawings.

15. A grit removal system for use in sewage treatment comprising a primary separator, and, a secondary separator as claimed in any one of Claims 6 to 14 for performing a secondary separation on the solid phase collected after primary separation.

16. A system as claimed in Claim 15, wherein said primary separator is a cylindrical grit trap.

17. A method as claimed in Claim 1, wherein said secondary separator is a constant velocity separator as claimed in any one of Claims 6 to 14.