DEWATERING OF SLUDGES

Abstract: Dewatering of an aqueous sludge such as a sewage sludge is effected by mixing fibrous material (such as shredded cellulosic material) into the sludge and simultaneously applying (a) vacuum or pressure to the mixture when supported on a filter and (b) an electric field to cause liquid to be drawn through the filter.

Description: Dewatering of Sludges The present invention relates to removal of liquid phase from aqueous sludges (i.e. liquid-solid phase mixtures) and is particularly applicable to sewage sludge or other sludges having similar physical characteristics. In this specification for convenience reference will be made to dewatering of sludges, but it is to be understood that the invention extends to the removal of any aqueous liquid phase from a sludge. The aqueous phase could be a solution. For various reasons it is frequently desired to dewater to a commercially economic extent sludges to facilitate transportation, handling, storage, disposal, or re-use. One conventional technique is to apply pressure or vacuum to the sludge over a filter to remove liquid such as water. However it has for a long time been desirable to devise improved techniques applicable to a solid/liquid separation phase. The invention will be illustrated with reference to the processing of sewage sludge but the invention is not restricted to such sludges. It is to be understood that other applications of a similar character may exist. For example it is envisaged that sludges resulting from the treatment of drinking water can usefully be treated in embodiments of the invention as well as mineral sludges such as coal tailings, other fine mineral tailings typically having particle diameters less than 0.1 mm and, in addition, sludges from the food and chemical industries. A particularly significant area of application of the invention is to sludges which contain inorganic matter and comprise material which will sustain and pass an electric current. Prior published proposals for dewatering of sludges include electrodewatering (ED ) . However despite such techniques having been proposed in the literature for a very long time, to the knowledge of the inventor no significant applications of EDW have been reported as applied to the treatment of sludges such as sewage sludge. EDW is a technique for enhancing filtration and comprises passing a current through the sludge during the formation of the sludge into a cake and during cake desaturation. It is thought that an electric field provides an extra force for dewatering over and above that achieved by applied pressure so that both the extent and rate of dewatering should be improved. The mechanism of EDW is not well understood but is thought to rely upon the electrokinetic phenomena comprising electrophoresis (which involves the migration of charged particles in suspension to oppositely charged electrodes) and electro- osmosis (which involves the migration of charged ions, which compensate the charges in the particles, to oppositely charged electrodes) . However the cost of electric power to improve the extent and rate of dewatering appears to be very significant, especially for sludges of a type that are difficult to dewater. Examples of prior publications discussing EDW include: - Bendit et al "Improving the Dewatering of Fine Coal and Tailings", 12th International Coal Preparation of Congress, May 23-27 1994 Cracow, Poland; Lockhart N C "Advances in Solid Liquid Separation", (Editor H S Muralidhava) , Battelle Press, Columbus Ohio 1986, Chapter 10; Kondoh and Hiraoka, "Commercialization of Pressurized Electroosmotic Dehydrator (PED)", Wat, Sci. Tech. Vol. 22, No. 12, pp 259-268, 1990. Bendit et al discuss exclusively dewatering of fine coal and mineral tailings and report that a demonstration scale vacuum filter of the horizontal belt-type was successful modified with the provision of an electric field as an additional driving force. Dewatering was enhanced significantly for fine and ultra fine coal as well as tailings. The authors recognise the possibility that electrically assisted methods may be useful for difficult colloidal suspensions generally. Lockhart discusses broadly electrodewatering and discusses experimental results from work with tailings from various mineral processing plants (coal washing, sand washing, mineral processing, fine coal products, brown coals etc) and also work on model clay suspensions. Lockhart discusses the fundamentals of electrodewatering and discusses some aspects including the significance of electrolysis in the mechanism, but concluding that it is obscure how electrolysis is involved and electrolysis considerations alone do not provide explanation for the volume of water transported per ion. Kondoh and Hiraoka discuss the need to decrease the water content in biological activated sludge arising from sewage treatment plants, but recognise that studies using an electro-osmotic process have demonstrated problems which have prevented any successful commercialisation. The authors discuss the costs of chemical treatment procedures contrasted to the cost of electrical power for aiding water removal with an electro-osmosis technique. Results in enhancing dewatering speed with the addition of an electrolyte in combination with a suitable polymer in a pressurised electro-osmotic dehydrated system are reported. Notwithstanding these proposals, the present inventor is not aware of any commercial scale plant being developed, established or proven as economically feasible where electrodewatering techniques are used. Plainly, commercial application of any process depends upon the process being attractive from a capital cost point of view and effective in terms of operating costs. In one aspect, the present invention consists in a method of reducing the liquid phase in an aqueous sludge comprising mixing fibrous material into the sludge and applying simultaneously (a) pressure or vacuum in a filtration method having a filter and (b) an electric field to the sludge supported on the filter, whereby the liquid phase is drawn through the filter. The invention is especially applicable to a method of processing a sludge which comprises, at least as a major component, organic material and wherein finely divided fibrous material is mixed into the sludge in an amount in the range of about 5-15% by wt (as dried solids) . However, more generally, a wider range of amount of fibrous material could be used especially where the aqueous sludge is derived from a sewage treatment plant. There is considerable variation in the types of sludges required to be treated in sewage treatment plants. Some are more difficult to process than others. An embodiment of the invention has been found to have encouraging results with sludges which are recognised as difficult to treat such as those which result from a biological nutrient removal plant. Particle sizes in such sludges are very difficult to measure, but typically will be in the range of 0.1-100 microns with the majority of the particles in the lower ' region of this range. Preferably the invention is implemented in a method in which, during the filtration stage, separated liquid is pumped out and, if the process is operated as a batch process, then periodically dewatered solid material is removed. Methods according to the invention can use a variety of fibrous materials such as kaowool, shredded paper or other cellulosic fibres such as coconut fibre and bagasse (which is the fibrous material discharged from sugar cane crushing operations) . The method can include in addition some non-fibrous additives to enhance dewatering. For example, beneficial results may be achievable by the addition of salts such as calcium chloride. Embodiments of the invention are believed to operate on a wide range of fibrous materials and fibre sizes. It is accepted that it is very difficult to characterise fibre sizes, but as a general indication in the case of shredded paper and sewage sludge, very finely shredded news print has been used and this may be conveniently classified as having fibre sizes of length of around 0.05 to 0.5 mm. Reference now will be made to the accompanying drawings of which: Figure 1 schematically illustrates known principles of assisted filtration techniques; Figure 2 illustrates a generally accepted explanation of the mechanistic steps in filtration/dewatering of fine suspensions as discussed by Bendit et al; Figure 3 illustrates the results of using an embodiment of the invention with shredded newsprint as an additive; Figure 4 illustrates the power consumption of the embodiment reported in Figure 3; Figure 5 illustrates comparative data for embodiments using a range of fibrous additives; Figure 6 shows contrasting data of simply using non- fibrous additives as opposed to fibrous additives; Figure 7 is another comparative graph showing electrodewatering results using pressure in an electric field, but without any additives; and Figure 8 is a diagram of a laboratory scale filter used to derive the results of Figures 3-7. Referring first to Figure 1, Figure IA illustrates the principles of filtering using only an electric field, but without pressure and it will be seen that the particulate matter migrates preferentially under the electric field to the upper anode. Figure IB illustrates known pressure filtering without electric field in which the solid particles migrate to the filter F shown here at the bottom of the cell. Figure IC illustrates the mechanism for electrically assisted pressure techniques in which a combination of features occur and it is believed that the better separation characteristics are obtained because, in crude terms, there is a reduction in the rate of compaction of a filter cake due to some solids moving towards the upper anode. Referring now to Figure 2, there is illustrated a conventional view of the formation of a filter cake in filtration/dewatering of fine suspensions. Figure 2A is a view on a greatly enlarged scale of a fragment of the structure showing schematically a filter 10 having filter material 11 and pores 12 supporting initially an aqueous mass of solid particles 14 and liquid 15. Figure 2B illustrates the initial suspension and as liquid phase is removed and pumped from the cell the volume of retained material progressively decreases and Figures IC to ID respectively show initial bridging across the filter pores, cake build up, full cake formation and completion of the filtration stage followed, in Figure IE, with cake compaction with water expression, at which point desaturation of the compacted cake can be caused by forced air or steam penetration. Finally in Figure 2F there is illustrated air breakthrough due to cake cracking. Referring now to Figures 3 and 4, an initial mixture was formed of 10% by weight of finely shredded newsprint along with a sewage sludge and 0.8 kg/tonne of flocculent (Zetag 92) which was subjected to pressure of 10 bar and simultaneously an electric field of 100 amps/sq.m. The apparatus used is illustrated in Figure 8 in which the filter apparatus comprises a cylindrical body 20 having a cylindrical main bore 21 and a conical counter-bore 22 leading to an axial drain tube 23. In practice a vacuum is applied to the drain tube 23 to draw off water from a sludge, which is placed in central zone 24 of the apparatus. A cylindrical plunger 25 is installed in the bore 21 and is adapted to be pressed downwardly by an applied pressure P. A disc-shaped centred metal cathode 26 is mounted at the bottom of the bore 21 and this cathode is perforated and supports a filter cloth 27A. The cathode 26 is connected by conductor 28 to a negative DC potential. A sintered metal perforated anode 29 is mounted in a disc-shaped cavity in the end of the plunger 25 and also is covered by a filter cloth 27B, a disc-shaped fluid extraction cavity 30 being located above and behind the anode 29 and connected to a discharge duct 31 which leads to a pipe 32 connected to a vacuum and adapted to draw up water which is removed from the sludge through the filter cloth 27B and anode 29. In use the volume of liquid expelled as a function of time was monitored and filtration was performed for 30 minutes. The data in Figures 3 and 4 show progress of dewatering net of the effect of the added solids. A significant acceleration of filtration rate and reduction of power requirements are achieved by embodiments of the invention where an additive is used, such as fibrous newsprint in contrast to a corresponding set of data where there is no additive. A range of fibrous additives have been tested as shown in the data of Figure 5. In these examples 25%-30% by weight additive of fibrous material was incorporated in the sludge before commencement of the treatment process. Bagasse and coconut fibre proved faster. The contrasting data of Figure 6 relates to powder additives with the same sludge feedstock and the same effective dose as the data reported and shown in Figure 5. Thus the fibrous additives had a marked beneficial effect compared with powder additives. Figure 7 shows the results of an experiment performed under similar conditions, but in the absence of any additive. As before pressure was applied by vacuum at 10 bar with an electric field of 100 amps/sq.m was applied. It should be noted that in Figures 5 and 6 the total solids in the filter cake are referred to and therefore the final value of sludge solids in filter cake must be reduced in value by 10% weight. Thus on average in the sludges referred to, the final sludge solids should be reduced from about 40% weight to 30% weight (due to the inclusion in the initial material of 25%-30% by weight of the additive) . While not being bound to any particular theory, the inventor suggests that the inclusion of solids of a relatively coarse size compared to the size of sludge particles does increase the permeability of the filter cake. By the use of a fibrous additive the inventor suggests that it is possible the fibres form a continuous network throughout the filter cake and can thus provide an extra pathway (either outside or inside the fibre, if the fibre is porous) along which water can travel. It is thought the increase in permeability achieved and the extra pathway are major factors in improving dewatering.

Claims: CLAIMS ; 1. A method of processing an aqueous sludge to reduce liquid phase in the aqueous sludge, the method comprising mixing fibrous material into the sludge and applying simultaneously (a) pressure or vacuum in a filtration method having a filter and (b) an electric field to the sludge supported on the filter, whereby the liquid phase is drawn through the filter. 2. A method as claimed in claim 1 and comprising using a sludge principally of organic material and fibrous material which is finely divided. 3. A method as claimed in claim 2 and wherein the method comprises using fibrous material having a fibre length of less than 1 mm, and using sludge having an average particle size substantially less than 1 mm. . A method as claimed in any one of the preceding claims and mixing the fibrous material into the sludge to form a mixture containing about 5%-40% by weight of fibrous material (as dried solids) . 5. A method as claimed in claim 4 and wherein the mixing step forms a mixture wherein the fibrous material is in the range of 5%-15% by weight (as dried solids) of the mixture. 6. A method as claimed in any one of the preceding claims and including using as part of the fibrous material one or more of (a) kaowool, (b) shredded paper, (c) coconut fibre, (d) bagasse, (e) other shredded cellulosic fibre. 7. A method as claimed in any one of the preceding claims and further including mixing powdered mineral material into the sludge. 8. A method as claimed in claim 7, wherein the mineral material is calcium chloride. 9. A method as claimed in any one of the preceding claims and using as the sludge a sewage sludge. 10. A method of dewatering sewage sludge from a biological nutrient process, the method comprising (i) mixing into the sewage sludge short fibre length fibrous material having a fibre length mainly in the range 0.05 to 0.5 mm to produce a mixture, (ii) supporting the mixture on a filter, (iii) simultaneously applying to the mixture (a) pressure or vacuum and (b) an electric field to draw water from the mixture through the filter, and (iv) removing dewatered sludge from the filter. 11. A method as claimed in any one of the preceding claims and includes using an applied vacuum of about 10 bar and an applied electric field of about 100 amps/sq.m. 12. A method as claimed in claim 1 and substantially as described herein with reference to Figures 3, 4, 5 of the accompanying drawings and using any one of the additives described with reference to Figure 5 and either with or without the additional additives described with reference to Figure 6.

Inventors: VEAL CHRISTOPHER JOHN, VEAL, CHRISTOPHER, JOHN
Applicants: COMMW SCIENT IND RES ORG, VEAL CHRISTOPHER JOHN, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR, VEAL, CHRISTOPHER, JOHN
Application year: 1996
Main classification: C02F11
IPC codes: B01D29, B01D35, B01D57, B01D61, C02F11