Title of Invention

"APPARATUS FOR SOLID-LIQUID GRAVITY SEPARATION"

Abstract

Apparatus for solid-liquid gravity separation, particularly of sludge from purification stations comprising a planar, fixed thickening or draining grate (2), to which the sludge is continuously supplied, said sludge being permanently scraped by a scraping system having flexible blades (4), wherein said grate is constituted by a plurality of bars (6, 6'), whose gaps vary continuously over their lengths between the inlet (1) and the outlet (3) of the apparatus, the value of the gaps between the bars of the grate (2), together with their cross-section being chosen so as to obtain a percentage vacuum exceeding 30% and which may reach 50%.

Full Text

The present invention relates to improvements made to a plant for thickening a solid-liquid suspension by draining. FR-A-2, 581,559 describes a plant for continuously thickening a solid-liquid suspension, this plant being placed on the delivery side of the sludge feed pump. This plant includes a flat and stationary, draining or thickening screen onto which the sludge to be thickened is continuously delivered, this sludge being permanently scraped by a scraping system having flexible blades. The concentration of the sludge increases gradually as it it advances over the screen area. This known plant, intended more particularly for thickening the sludge from purification plants, can be used either in final thickening, for example in the case of sludge for agricultural use, or in prethickening, upstream of a dehydrating plant. Although in general this type of plant is satisfactory, enabling a wide range of sludge concentrations to be thickened, it has the disadvantage, however, of operating at relatively low hydraulic flow rates, in particular in the case of solid-liquid suspensions of low concentration, i.e. having concentrations of less than 5 g/1. Moreover, it is for this reason that this known thickening plant is generally fed from an intermediate structure or device whose function is to raise the concentration of the solid-liquid suspension fed to the plant. Moreover, this known plant is equipped with a screen consisting of a large number of bars of large cross-section, the percentage of open space of which is about 25%. This characteristic contributes to the hydraulic limitation of this type of plant. Starting from this prior art, the present invention is intended to make improvements thereto, especially for the purpose of increasing, significantly, the acceptable hydraulic flow rates, this being so in particular for solid-liquid suspensions of low concentration. The invention also makes it possible to dispense with the intermediate structure or device intended, in plants according to the prior state of art to increase the concentration of the solid-liquid suspension fed to the plant. Likewise, the improved plant according to the invention makes it possible to gain freedom from variations in the quality and concentration of solid-liquid suspensions, caused by the intermediate structures and devices. The plant's operating parameters (supply of reactants, washing sequences, etc.) may be optimized, taking into account that it is fed with a solid-liquid suspension of stable concentration, for example a water-sludge suspension coming from an aeration tank. Consequently, the present invention relates to a plant intended for gravity solid-liquid separation, in particular of sludge from purification plants, which includes a flat and stationary, thickening or draining screen onto which the sludge is continuously delivered, this sludge being permanently scraped by a scraping system having flexible blades, this plant being characterized in that the said screen is formed by a large number of bars, the gaps between which vary along their lengths. According to the invention, the characteristics of the gaps, as well as their variations, may be defined based on the nature of the solid-liquid suspension to be thickened, on its concentration, on the rate of feed to the plant and also based on pilot trials. According to the invention, the gap between the bars of the screen may vary continuously or discon-tinuously between the inlet and the outlet of the plant. According to another characteristic of the invention, the thickening screen may be inclined, with a slope rising between the inlet and the outlet of the plant, it being possible for this slope to be between 1 and 40°, preferably between 2 and 15°. According to the invention it is also possible to use a screen whose bars have a small cross-section so as to further increase, significantly, the percentage of open space. This characteristic, combined with that relating to the choice of the value of the gap between the bars of the draining screen, makes it possible according to the invention to obtain a draining screen having a percentage of open space of greater than 30% and possible up to 50%. Other characteristics and advantages of the present invention will emerge from the description given below with reference to the appended drawings which show illustrative embodiments thereof, these embodiments being free of any limiting character. In the drawings: - Figure 1 is a plan view of the improved plant according a first embodiment of the invention; - Figure is a view similar to Figure 1 of another illustrative embodiment of the improved plant according to the invention; and - Figure 3 is a diagrammatic view of the plant according to the invention in longitudinal section. With regard to the general description of the thickening plant to which the present invention refers, the reader may refer to the description in Patent FR-A-2,581,559 on which the invention is an improvement. Referring firstly to Figures 1 and 3, these show diagrammatically a thickening plant which includes a sludge inlet 1, a flat and stationary draining or thickening screen 2 onto which the sludge is continuously spread, this sludge being permanently scraped by flexible blades 4, the thickened sludge being collected at 3 and the draining water being discharged at 5. In this illustrative embodiment, the gaps between the bars 6 making up the draining screen 2 increase from the inlet 1 to the outlet 3 of the plant. For this purpose, the screen 2 includes a first sheet 2a consisting of bars 6 having a small gap and a second sheet 2b consisting of bars 6' having a larger gap. In this way, the gaps between the ars of the screen 2 vary discontinuously between the inlet and outlet of the plant. Of course, provision may also be made for the gap between the bars to decrease from the inlet 1 to the outlet 2 of the plant, it being possible for this variation to be continuous or discontinuous, gradual or stepped. In the illustrative embodiment shown in Figure 2, the draining screen 2 is made so that the gap between the bars 6 making up the draining screen 2 increases continuously between the inlet 1 and the outlet 3 of the plant. As specified above, the characteristics of the gaps between the bars of the draining screen 2 as well as their variations, are defined based on the nature of the solid-liquid suspension to be thickened, its concentration, on the rate of feed to the plant and also on pilot trials. Referring to Figure 3, it can be seen that according to the present invention the draining screen 2 is inclined to the horizontal at an angle ot, the slope of the screen rising between the inlet 1 and the outlet 3 of the plant and the angle a being between 1 and 40°, preferably between 2 and 15° . This characteristic enables the retention time of the solid-liquid suspension on the draining screen 2 to be increased. Given below by way of nonlimiting example is a result of comparative trials between a plant according to the prior art mentioned above and the improved plant according to the invention. These trials, performed on a pilot plant, bring out clearly the performance characteristics provided by the invention. A) Characteristics of the solid-liquid suspe nsion : - lengthily aerated municipal sewage sludge - concentration: 3 to 5 g/1 - Mohlman index: 150 B) Characteristics of the screen according to the prior art (FR-A-2,581,559) - screen width: 500 mm - screen length: 1500 mm - slope: 0 degrees - gap: 600 microns - bar width: 1.8 mm C) Characteristics of the improved screen accor ding to the invention - screen width: 500 mm - screen length: 1500 mm - slope: 4 degrees - gaps: 350 microns over 750 mm, then 600 microns over 750 mm - bar width: 1 mm. D) Performance characteristics - Screen according to the prior art: hydraulic screen-feed flow rate: 4 to 6 m3/h dryness on leaving the screen: 6% - Improved screen according to the inven tion: hydraulic screen-feed flow rate: 15 to 22 m3/h. dryness on leaving the screen: 6% As can be seen, the improvements which form the subject of the invention enable the mass flow rate of the sludge treated on this type of equipment to be at least tripled. Of course, it remains the case that the present invention is not limited to the illustrative embodiments described and/or shown here but it encompasses all variants thereof which fall within the scope of the appended claims. CLAIMS 1. Plant intended for gravity solid-liquid sepa ration, in particular of sludge from purification plants, which includes a flat and stationary, thickening or draining screen (2) onto which the sludge is continuously delivered, this sludge being permanently scraped by a scraping system having flexible blades (4), characterized in that the said screen consists of a large number of bars (6,6'), the gaps between which vary along their lengths. 2. Plant according to Claim 1, characterized in that the characteristics of the gaps between the bars of the draining screen (2), as well as their variations, may be defined based on the nature of the solid-liquid suspen sion to be thickened, on its concentration, and on the rate of feed to the plant. 3. Plant according to either of the preceding claims, characterized in that gap between the bars of the screen (2) varies continuously between the inlet (1) and the outlet (3) of the plant. 4. Plant according to either of Claims 1 and 2, characterized in that the gap between the bars of the screen (2) varies discontinuously between the inlet (1) and the outlet (3) of the plant. 5. Plant according to any one of the preceding Claims 1, 2 and 4, characterized in that draining screen (2) consists of two or more sheets (2a,2b) of bars having different gaps, the gap between the bars increasing from the inlet (1) to the outlet (3) of the plant. 6. Plant according to any one of Claims 1, 2, and 4, characterized in that the screen (2) consists of two or more sheets of bars having different gaps, the gap between the bars decreasing from the inlet to the outlet of the plant. 7. Plant according to any one of the preceding claims, characterized in that the gaps between the bars of the screen, and their cross-section, are chosen so as to obtain a percentage of open space which may be greater than 30% and possibly be up to 50%. 8. Plant according to any one of the preceding claims, characterized in that the said draining or thickening screen (2) is inclined to the horizontal with a slope (a) rising between the inlet and the outlet of the plant, this slope being between 1 and 40°, preferably between 2 and 150. 9. Plant for gravity solid-liquid separation substantially as herein described with reference to the accompanying drawings.

Documents:

2648-del-1997-abstract.pdf

2648-del-1997-claims.pdf

2648-del-1997-correspondence-others.pdf

2648-del-1997-correspondence-po.pdf

2648-del-1997-description (complete).pdf

2648-del-1997-drawings.pdf

2648-del-1997-form-1.pdf

2648-del-1997-form-13.pdf

2648-del-1997-form-19.pdf

2648-del-1997-form-2.pdf

2648-del-1997-form-3.pdf

2648-del-1997-form-4.pdf

2648-del-1997-form-6.pdf

2648-del-1997-gpa.pdf

2648-del-1997-petition-137.pdf

2648-del-1997-petition-138.pdf