Eriksson, Niclas (Arsenalsgatan 8, Göteborg, S-411 20, SE)
Svenningsson, Lars (Eklandsgatan 10, Göteborg, S-412 55, SE)
Eriksson, Niclas (Arsenalsgatan 8, Göteborg, S-411 20, SE)
Svenningsson, Lars (Eklandsgatan 10, Göteborg, S-412 55, SE)
Technical Field The present invention concerns an arrangement to in- crease the concentration of solids in a solution. Even though the invention is specially developed for dewatering of sludge, it may be used generally to separate a solid soluble substance from a solvent, which do not have to be water.
Prior Art Sludge from a digester in a sewage treatment plant has often a content of dry matter (substances) of about 2 %. In order to increase the content of dry matter differ- ent flocculants are normally added, whereby polymers often are used. The flocked sludge is then today often separated in a means using centrifugation in order to increase the content of dry matter to about 20 %. However, polymers in dried sludge give environmental problems concerning deposi- tion of the treated sludge. By not using polymers or any other flocculant no "lumps" are formed. Such "lumps" are hard to dry and thereby to sanitize, furthermore, they may include a lot of moisture and thereby pathogenic bacteria and mould spores that may start to multiply.
Summary of the Invention One purpose of the present invention is to be able to increase the concentration of substances in a solution, without the need of adding any flocculant. Even if the in- vention mainly is developed for treatment of sewage sludge, a person skilled in the art realizes that the principle of the invention is useful in many circumstances where an in¬ crease of the dry matter of a solution is wanted. According to the invention a solution having solid, dissolved substances is placed on a semi-permeable membrane with a layer of air on the other side of the membrane. The membrane should only let the solvent and air through, but not the dissolved, solid substances, whereby the content of the dissolved substances will increase in the solution, in that the solvent "migrates" through the membrane. The mem- brane is designed to form channels letting moisture (water) and air pass. Thus, the invention may be used in order to increase the concentration of substances in a solution. One theory behind this is the well known strive of nature to level out differences in concentration. The possible theory of the invention will now be ex¬ plained further based on digested sludge from a sewage treatment plant as an example. When it comes to digested sludge from a sewage treatment plant it normally has a con¬ tent of dry matter of about 2 %. Simply put the sludge hav- ing a content of dry matter of about 2 %, i.e. sludge dis¬ solved in water, is a solution. Another solution is water in the membrane where the content of dry matter is 0 %. A solution having higher concentration has a lower partial pressure than solutions having a lower concentration. In this case the sludge dissolved in water has a lower concen¬ tration of water than the solution in the membrane and thereby a higher partial pressure. The difference in par¬ tial pressure means that the water will go into the mem¬ brane and that the content of dry matter of the sludge will increase. In order for this to work the membrane must be kept moist all the time and thereby moist enough in order to give the effect of an accumulation of solution. As the concentration of water in the membrane increases the water will fall out of the membrane by gravity. Water that fall out of the membrane is preferably collected and some of it is used to moisten the membrane. At an arrangement according to the present invention the molecules of the solvent (water) will penetrate into the more concentrated solution striving to level out the difference in concentration, i.e. the difference in partial pressure. In an arrangement of this kind the general gas law pV=nRT for diluted solutions applies. In that the the¬ ory is generally applicable it means that the present in- vention may be used for in principle any solution and for any solid, dissolved substances. The present invention concerns an arrangement of in¬ creasing the concentration of solid, dissolved substances in a solution. Hereby the difference in partial pressure of a solution with or without solid, dissolved substances is used. It is done in that the solution is placed on a semi¬ permeable membrane, which membrane is moistened with the solvent of the solution. The membrane holding the solution will then pass an area with an air layer on the other side of the membrane. Due to the different partial pressures for the solutions with and without solid, dissolved substances, the solvent of the solution will migrate through the mem¬ brane from the side where the solution contains solid, dis¬ solved substances to the other side. The arrangement according to the present invention is well suited for use in an automated process, in which the arrangement often is only a part of a larger process. Further objects and advantages of the present inven¬ tion will be obvious for a person skilled in the art when reading the detailed description below.
Brief Description of the Drawings The invention is more closely described below with reference to an example of an embodiment shown in the en- closed drawings. In the drawings: Fig. 1 is a principle sketch in the form of a sec¬ tional view of an arrangement using the principles of the present invention, Fig. 2 is a sectional view of a conveyor of the ar- rangement of Fig. 1, Fig. 3 is an enlarged sectional view of a membrane according to the present invention, Fig. 4 is a principle sketch in end view and in sec¬ tion of another arrangement using the principles of the present invention, Fig. 5 is a side view of the arrangement of Fig. 4, Fig. 6 is a sectional view of a part of the arrange¬ ment of Figs. 4 and 5, and Fig. 7 is a principle sketch of a part of the ar- rangement of Figs. 4 to 6.
Detailed Description of Preferred Embodiments As used in the description the expressions "upper", "lower" and similar expressions are in reference to the en- closed drawings. In the embodiment of Figs 1 to 3 one example of an arrangement according to the present invention is shown, which arrangement is used for dewatering of digested sludge. The arrangement is enclosed in a case or house 1. The house 1 has an inlet 2 to receive non-dewatered sludge, e.g. digested sludge from the digester of a sewage treat¬ ment plant. Furthermore, there are two outlets, one outlet 3 for water and one outlet 4 for dewatered sludge. The out¬ let 4 for dewatered sludge leads to a drying apparatus 5 for further dewatering of the sludge. As the design of the drying apparatus 5 is of no importance for the present in¬ vention it will not be described further here. In the house 1 also a conveyor β is arranged, on which the sludge 8 feed through the inlet 2 is received. In the shown example the conveyor 6 has a number of rollers 9 carrying a wire cloth 10 in a trough-like shape. A person skilled in the art realizes that the exact design of the conveyor may vary, as long as it fulfils the most elemen¬ tary demands for the invention to work. These demands are that the water should be let through, that the sludge 8 should be received in a secure way etc. It is e.g. possible to have a flat conveyor with fixed sides. According to the invention a semi-permeable membrane 11 is placed on top of the wire cloth 10 of the conveyor 6, i.e. closest to the sludge 8. A layer of air is formed be¬ neath the wire cloth 10 and, thus, the membrane 11. The wire cloth 10 is a support for the semi-permeable membrane 11, but if the membrane 11 has enough bearing strength on its own the wire cloth 10 may be taken away. A person skilled in the art realizes that the part carrying the mem¬ brane 11 does not have to be a wire cloth, and that it may be of any material letting fluid through. The membrane 11 is formed of a number of layers hav¬ ing alternating convex threads 12 and concave threads 13. In each layer the convex and concave threads 12, 13 are ar¬ ranged alternating, giving cells of varying shape between the convex and concave threads 12, 13. The cells are indi¬ cated in Fig. 3. The surfaces of the cells are closely ar¬ ranged to give a relatively large total surface. By the ar- rangement of alternating convex and concave threads 12, 13 a long useful life is given to the membrane 11. Practical tests also show that this improves the result. The membrane 11 may be made of any suitable material, such as e.g. nylon fibres. Nylon fibres are suitable e.g. for dewatering of sludge from digesters of sewage treatment plants. Depending on the solutions and solvents used the cells of the mem¬ brane 11 may be formed of many different materials. By con¬ trolling the size of the threads 12, 13 and, thus, the cells formed the size of the particles that may be feed out with the separated water is determined Suitable materials for the membrane are commercially available. In an arrangement of this kind, where the cells are arranged in a certain structure, the solvent, in this case water, is driven from cell to cell through the membrane 11 towards the higher concentration of solution. Thereafter the excess water (solvent) will fall off of the membrane 11 by gravity. In the lower part of the house 1 water separated from the membrane 11 is accumulated at a water level 7 between the upper and lower parts of the conveyor 6. The membrane 11 is hereby moistened continuously, at the same time as there is an air gap or air layer between the upper part of the conveyor 6 and the water level 7. Thus, the membrane 11 is normally rinsed during half of its turn, which is enough to keep up the moisture content. A person skilled in the art realizes that the membrane 11 may also be moistened in other ways, e.g. in that a water jet moistens the membrane 11 just before the sludge 8 is received. The lower part of the house 2 has two sections sepa¬ rated by a partition 15. In the bottom of one of the sec¬ tions the water outlet 3 is arranged, while the outlet 4 for dewatered sludge is arranged in the bottom of the other section. Thus, the partition 15 keeps separated water in the space above the water outlet 3, without any risk of it to spill over into the other section. In the area of the end of the conveyor 6 where the dewatered sludge is feed off, a scraper 14 is arranged to scrape off possible remaining material on the conveyor. In the shown example the scraper 14 is arranged on the parti¬ tion 15 between the two lower sections of the house 1. In one embodiment the size of the water outlet 3 may be controlled in order to keep the water level 7 at a suit¬ able height. In one example this is done by means of a switch closing and opening the outlet 3 in such a way that the water level 7 is placed between maximal and minimal al¬ lowed levels. Due to the air gap between the water level 7 and the upper part of the conveyor 6 excess water will rapidly fall out of the membrane 11 and down into the collection of wa¬ ter in the lower part of the house 1. In a practical example at a sewage treatment plant the membrane 11 has a cell size of about 1 μm while the wire cloth has a cell size of about 50 μm, which has proven to function well. As one example the wire cloth may be made of stainless steel. By varying the speed of the conveyor 6, by means of a suitable regulator (not shown) , the amount of dewatering may be controlled. In a working test digested sludge 8 hav- ing a content of dry matter of 2 % was feed in an amount of 10,000 kg/h through the sludge inlet 2, which gave 9,000 kg/h water at the water outlet 3 and 1,000 kg/h sludge hav¬ ing a content of dry matter of 20 % at the sludge outlet 4, by suitable control of the speed of the conveyor. The embodiment of Figs. 4 to 7 will now be described as an example with digested sludge from a sewage treatment plant. This embodiment comprises a drum 16, having two op¬ posite end walls and a wall forming the cylinder of the drum. The drum 16 is submerged into a receptacle 17 receiv- ing sludge 32 from an inlet 18. In the shown embodiment the receptacle 17 is part of a housing in which the arrangement is received. A semi-permeable membrane 28 is placed on the outside of the cylinder wall of the drum 16, letting water through but not solid, dissolved substances of the sludge 32. The membrane 28 of this embodiment corresponds to the membrane 11 of the previously described embodiment. Thus, the membrane is formed of a number of layers having alter¬ nating convex and concave threads 12, 13. The semi-perme¬ able membrane 28 is carried on a sheet metal 26, forming the cylinder of the drum 16. The sheet metal 26 of the drum 16 is furnished with a number of openings 27. Sludge is lifted from the receptacle 17 on the outer side of the drum 16, which is rotated by a motor 30. Dewatered sludge is feed out through an outlet 19. The end walls of the drum 16 are placed adjacent walls of the housing. Inside the drum 16 one or more vanes 20 are received on a hollow shaft 21. The vanes 20 extend over the total width of the drum 16, with a small gap to the end walls of the drum 16. The number of vanes 20 inside the drum 16 may vary. In one example 12 vanes 20 are received with a spac¬ ing of 30° on the hollow shaft 21 inside the drum 16. One end of each vane 20 abuts the inside of the drum 16. In the shown embodiment the vanes 20 have a bent outer part at the end abutting the inside of the drum 16. In the hollow shaft 21 a slit 24 is arranged in connection with each vane 20. Each slit 24 opens into a cavity 25 inside the hollow shaft 21. By means of the vanes 20 and the slits 24, water is lead into the cavity 25 of the hollow shaft 21. The cavity 25 ends in a water outlet 22. Often a vacuum pump 31 is at- tached to the water outlet 22 to increase the dewatering effect of the arrangement. The vanes 20 may be replaced with rows of suction tubes for transport of the water to the cavity 25 of the hollow shaft 21. As indicated above the drum 16 is rotated by a motor 30, which is connected to the hollow shaft 21. The hollow shaft 21 going through the drum 16 is received in bearings placed in opposite walls of the house receiving the drum 16. The end walls of the drum 16 and the vanes 20 are fixed to the hollow shaft 21. Thus, the drum 16 and the vanes 20 will rotate with the hollow shaft 21, but there will be no mutual movement between the drum 16 and the vanes 20. The motor 30 is arranged on the opposite side of the hollow shaft 21 to the water outlet 22 and the vacuum pump 31. In one embodiment the drum 16 was rotated at a speed of 6 rpm and the vacuum pump 31 reduced the pressure at the water outlet 22 to about 50 kPa. Preferably the pressure given by the vacuum pump 31 should not exceed about 80 kPa. By vary¬ ing the rotational speed of the drum 16 the amount of dewa- tering may be controlled. With an arrangement of this kind in one practical test the content of dry matter of the sludge feed out at the outlet 19 has reached about 30 %. By further fine-tuning of the process it is envisaged that the content of dry matter may be increased even further. In connection with the sludge outlet 19 a scraper 23 is arranged, to scrap dewatered sludge from the drum 16. Thus, the scraper 23 is to abut the outer surface of the drum 16, or more precisely the membrane 28 received on the outside of the drum 16. The position of the scraper 23 may be adjusted to control the contact pressure against the drum 16. The scraper 23 extends the total length of the drum 16. As stated above sludge will be lifted from the recep¬ tacle 17 by the rotation of the drum 16. The membrane 28 on the drum 16 receiving the sludge is moistened by a collec- tion of water 29 at the bottom of the drum 16, which col¬ lection of water 29 is formed in the drum 16 during opera¬ tion. At start up the drum 16 may have to make some revolu¬ tions before the membrane 28 is moistened enough. There is a small distance between the vanes 20 and the end walls of the drum 16, which helps in forming the collection of water 29 at the bottom of the drum 16. As the sludge is lifted from the receptacle 17 water of the sludge will go into the membrane 28 in the same way as referred to above for the embodiment of Figs. 1 to 3. Excess water of the membrane 28 will fall into the inside of the drum 16. The water is then collected by the vanes 20 and will follow the vanes down to the hollow shaft 21 and through the slit 24 of the hollow shaft 21 at each vane 20. The water is finally feed out through the cavity 25 of the hollow shaft 21 to the water outlet 22. By connecting a vacuum pump 31 to the cavity 25 the dewatering effect of the arrangement increases. How¬ ever, it should be noted that the arrangement as such would also work without a vacuum pump. The dewatered sludge is scraped off from the drum by means of the scraper 23 and will fall into the sludge outlet 19. The dewatered sludge may then be treated further in any suitable way. Also in this embodiment sludge is received on a mem¬ brane 28 that is moistened and as the membrane 28 passes a part with air on the side of the membrane 28 not receiving the sludge, excess water will fall out of the membrane 28. Thus, this embodiment has a sludge collection zone and a dewatering zone. The sludge collection zone being the part of the drum 16 submerged into the sludge and the dewatering zone being the part of the drum above the level of the sludge in the receptacle 17.