WO1996006805A1

WO1996006805A1 - Method of drying of sludge

Info

Publication number: WO1996006805A1
Application number: PCT/NO1995/000143
Authority: WO
Inventors: Sverre Golten
Original assignee: Stord International A/S
Priority date: 1994-08-30
Filing date: 1995-08-30
Publication date: 1996-03-07
Also published as: NO943207D0; NO943207L; AU3400695A

Abstract

Reference is made to a process for treating sludge from discharge fluid, especially watery sewage sludge, whereby the sludge undergoes a drying process in a drying apparatus where the fluid content of the sludge is substantially separated from the drystuff content of the sludge, in order to produce a product material out of quantities of drystuff of the sludge having lower moisture, and the process is characterised in that during that portion of the drying process where the sludge passes through a phase of higher viscosity, a so-called glue phase, bits of a solid substance, so-called pellets, are added to the sludge.

Description

METHOD FOR DRYING OF SLUDGE

The present invention relates to a process of treating discharge fluids, especially waste water, which exhibit the character of glue water, as is indicated in the introduction to the accompanying claim 1. The invention especially has to do with that part of a discharge fluid treatment where larger solid particles are already removed by mechanical purification so that one operates with a sludge of high fluid content, low fibre content and a portion of very fine solid particles. Such a sludge can have varying degrees of viscosity, that is to say it can be thinly liquid or viscous. As examples sludge can be mentioned from sewage purification plants, sludge residues from the extraction of palm oil, and from other industrial processes. The invention thus has to do with a process where sludge having the afore-mentioned properties is to be dried in order to separate agglomerated impurities in more or less dry form from the fluid portion. If desired the wholly or partially dry material can thereafter be used further in finely divided form or in pellet form, for example it can be deposited or employed as manure.

To-day it is generally undesirable that sewage sludge be released untreated out into oceans and watercourses, and for this reason a long series of proposals exist for the mechanical and chemical treatment of such sludges in order to separate the solid substances from the water portion. Preferably such solid substances are either deposited on land, can be composted for the production of earth improvement means, or are employed for the production of manure.

In the instances where the sludge from the waste water is to undergo a drying process, the sludge gradually passes through a so-called glue phase as the water content is reduced. During the glue phase the sludge acquires a very high viscosity, and forms larger coherent viscous cakes or lumps, and these readily stick fast to the drying apparatus itself and will make it difficult or even make it impossible to be able to carry out the drying in a practically feasible manner. In individual instances even wedging fast of movable parts of the drying apparatus in the viscous material takes place.

The handling of the sludge in this glue phase also causes a series of other problems such as falling drying capacity, the occurrence of high wear and tear of the individual parts of the drying apparatus plus the consumption of energy becoming extremely high.

When the sludge goes through the glue phase the high consumption of energy is illustrated by the full line curve of the enclosed Fig. 1 which illustrates the variation in the consumption of energy ( that is to say the shearing force, γ ) as a function of the moisture content in a sludge which undergoes a drying at a water content of from about 95 and down to about 35 weight %. According to the full line curve of Fig. 1 this glue phase arises when the sludge has a water content of between about 70 and 45%, and is reflected in a dramatic increase in the shearing force, that is to say the necessary energy which must be supplied in order to tumble the sludge phase in the drying apparatus. When the sludge is dried further to a fluid content lower than 35-45 weight %, the sludge gradually passes over into powder form which is easier to dry and transport.

From the two U.S. Patent Specification Nos. 4,761,893 and 5,318,184 a process is known for drying drained sewage sludge where dry substances are produced in the form of pellets and more finely divided material. According to these patents a portion of the already dried material, which have specific particles sizes, are to be recycled to the sludge. The said patent specifications do not mention however what effect this has on the drying process itself, but are more concentrated on the particle size of the dried sludge which is produced. However it is expected that the particles led back function as embryos for the growth of larger particles from dry substances of the sludge.

Besides there exist on the market a series of proposals for drying processes and drying apparatuses, but hitherto no process or apparatus has solved the problems by mixing and moving the material during the glue phase of the sludge.

There is consequently a need for a process which can totally or partially eliminate the afore-mentioned negative effects in the drying of sludge.

The process according to the present invention is characterised by the features which are evident from the following independent claim 1.

Further preferred embodiments of the process according to the present invention will be evident from the accompanying dependent patent claims. The novelty in the invention is consequently to mix bits of a solid substance (pellets) into the sludge at the entry to the drying process, whereby the solid bits will follow the sludge through the whole drying process and the movement of these will prevent lumping (running together) of the sludge and sticking to the drying apparatus when the sludge goes through the glue phase. After the drying the bits can be separated from the dried sludge and can be recycled, such as continuously, back to the entry of the drying process. None of the prior known drying processes supplies and recycles bits of a solid foreign material with the afore-mentioned intention. It will be evident that it is preferred to use a material in the form of pellets which is different from the raw material which is to be dried. Furthermore the pellets are not to follow with the dried material out of the drying process, but are recycled and are employed only as a catalyst to promote the drying of the sludge. The bits of foreign material supplied must be inert, so that they neither physically nor chemically, change their structure during contact with the sludge through the drier, and do not supply undesired substances to the sludge either.

The process according to the present invention can be used on any soft, fluid-containing substance where the fluid becomes partially or completely evaporated during a drying process, and where the dried substance is conveyed out of the drying apparatus for further treatment.

During the treatment the fluid-containing raw material which contains very fine solid particles, low in fibre content and dissolved substances, goes over into the afore- mentioned glue phase. The expression glue phase is employed because the raw material passes through a viscous consistency, and it feels soft and sticky when held in the hand. Furthermore the sludge has such a high fluid content in this phase that it can have an extremely soft or liquid consistency. In this specification the term "sludge" is used as a common designation for several types of raw material, that is to say raw materials from a series of processes where materials are to be stirred for drying, for example sludge from sewage purification plants. In the present invention the drying/drying process involves completely or partially removing water or another fluid from the sludge by supplying heat in an optional manner to evaporate the fluid. In the process according to the present invention a rotary drier is preferably used, and the outer drum, or the inner rotor, or both parts can rotate to achieve the effect of mixing and conveying the supplied sludge into the drying component, and where heat is supplied in an optional manner to the drier to evaporate the fluid in the sludge. Furthermore the drying process can be continuous, the wet sludge being fed the whole time into the drying apparatus and the dried sludge being continuously removed from the drying apparatus, after which the pellets are removed from the dry substances and are recycled to the entrance of the drier. The drying process can also be carried out charge-wise or discontinuously, for example by filling the whole drying apparatus with wet sludge/pellets in a charge and remaining in the drying apparatus during the complete drying period, after which the dried sludge is conveyed in a charge out of the drier. As mentioned the heat for carrying out the drying process can be supplied indirectly in the form of steam, hot water or heating oil, or be supplied directly to the drying drum as hot air or another hot gas. Other forms of heating can also be used, and particularly electrical resistance heating can be carried out.

The solid bits, particles in the form of pellets or briquettes can be used in the form of a sphere, ellipsoid, cylinder, cube or any other optional form. The pellets are not of the same material as the dry substance in the sludge. The pellets must further preserve their chemical and physical integrity. This means that they must be insoluble in the mixture with the sludge, and also they must not be disintegrated under that mechanical and chemical stress they are subjected to as a consequence of the tumbling together with that temperature they are exposed to. Consequently the principle is that the pellets remain in the drying process, that they do not become consumed and do not accompany the dried sludge out of the process.

The pellets used have a size which is favourable to the flow of the mixture of sludge and pellets through the drying process, and will normally have a size with a maximum cross- sectional diameter of 10-25 mm. The size is however not limited to the said range, and can thus be up to a size of 50 mm, since the size will obviously depend on the design of the drying drum and the characteristics of the sludge. The pellets can besides have the shape of spheres, ellipsoids, cylinders, cubes or other appropriate forms, and they are preferably made of nylon or of a metal. Furthermore it is an important parameter that the pellets can be readily separated from the dried sludge, and it is also important that they are not larger than enabling them to be readily advanced into the drying apparatus without wedging themselves fast or causing other problems, such as wear and tear on the apparatus parts and the like.

By recycling in connection with the present invention is meant that the particles supplied are fed into the drying process, and after drying ends the whole or a portion of the volume of the pellets is led back to the entry of the drying process to be employed over again. Continuous recycling involves the pellets being immediately and uninterruptedly conveyed back to the entry of the drying process. It is assumed that the pellets supplied involve that large accumulation in the form of a viscous glue phase-sludge cake being broken up during the tumbling by the pellets into a larger number of smaller particles which can be more easily moved by the element of rotation, and the material will dry more rapidly as a result of the increased surface since the number of sludge particles becomes greater, and the transfer of heat from the heating elements to the surfaces of the sludge particles is increased and facilitated considerably. A lump formation and the sticking to the drying apparatus is thereby avoided.

The process according to the present invention involves bits of a solid substance, such as pellets, being mixed together with wet sludge in the entrance to a drying apparatus and passing through the drying process functioning as an artificial, and preferably inert particle, the movement of which between themselves and between the sludge and the internal surface of the drying apparatus prevents the occurrence of a glue phase in a period during the drying where the sludge has an extremely high viscosity. After the drying the pellets and the dried material produced from the sludge are separated in a vibrating or rotating separator after which the dried sludge is conveyed for further treatment and the pellets are recycled back to the entrance of the drying apparatus for reuse.

The volume ratio between pellets and wet sludge, the quantity of wet sludge fed in and the degree of recycling of pellets in kilo per minute, plus the drying time and the speed of the drying process, which heat media and temperatures that are desired to be used can be regulated and adapted to the different types of sludge which it is appropriate to treat in this manner. As regards the necessary volume ratio between pellets and sludge practical experiments have shown that this will vary strongly all according to which origin the sludge has, and the ratio can be from higher than 0 and up to 10 volume parts of pellets per volume part of sludge, and it is especially preferred that the number of volume parts of pellets per volume part of sludge lies within the range of 5-10.

The extent of recycling the pellets together with the volume ratio between pellets and wet sludge must moreover be high enough to prevent formation of the glue phase or to limit its scope.

The invention will now be described further with reference to the accompanying Figures, in which:

Fig. 1 shows the development of the shearing force as a function of changes in the moisture content of a typical sludge from a sewage purification plant, with and without the use of the process according to the present invention.

Fig. 2 shows schematically a drying plant which the present invention can be used on.

The explanation of the full line curve of Fig. 1 is discussed earlier in this specification.

Fig. 2 shows an example of a drying apparatus which can be used in connection with the present process. However one shall not be limited to the construction of such an apparatus sketched below, since the method can be easily adapted to any type of apparatus without the latter having to be modified appreciably in order that solid pellet bits shall also be able to be advanced through the drier with the sludge. In Fig. 2 there is shown a drying apparatus 10 of the rotary type having an external stationary container 12 in which a rotary shaft 14 is installed having blades 16 mounted thereon. The rotary shaft is driven by means of a motor 18 including a gear.

By the reference numeral 20 an entrance 20 is shown for the sludge in the form of a pipe which is coupled to the drying drum (on the right side) . The pipe 20 comprises a screw conveyor 22 for the continuous supply of sludge. Alternatively equipment 24 can be mounted in the form of a pump which leads sludge through a pipe 26 into the drying drum 12. On the other side of the drying drum 12 an outlet 28 is arranged in the form of a pipe and having a screw conveyor 30 for the withdrawal of material from the drum 12. The outlet pipe 28 leads into a separator 32 for separating solid pellets from the dried material. From the separator 32 an outlet pipe 36 including a screw conveyor 38 leads forwards to a device 40 for further treatment of the dried sludge. Furthermore by means of a conduit 42 including a screw conveyor 44 the separator 32 is connected to the entrance side of the separator 12 for conducting pellets back to the process. In the top of the rotary drier a conduit 46 is arranged for diverting steam from the drying process. In the illustrated apparatus the heat supply is thought to be carried out by indirect heating in that the rotary shaft 14 and the stirring blades comprise hollow bodies through which heat medium can be circulated for further transfer by heat conduction and convection to the sludge material which is to be dried. Preferably the stirring blades 16 consist of hollow discs and on their outer peripheries scraping plates are mounted adjusted at an angle so that the material is gradually advanced through the drum. Furthermore there are arranged stationary scraping rods which project in between each disc. How the apparatus according to Fig. 2 can be used and run to dry a typical sludge from a communal and industrial sewage purification plant will be explained in the following description.

An allotted volume of pellets is supplied to the drying apparatus, after which the drying apparatus is activated by starting the rotor and the circulation of the heating medium. Furthermore the remaining apparatus components are started for supplying sludge, mixing the mixture, conducting out dried material, the screening operation and the recycling of pellets. The wet sludge is now fed continuously into the drying apparatus either through the entrance 20 or 26. As a result of the rotation of the stirring blades 16 the pellets supplied in advance to the drum and the sludge are well mixed. And the material in the form of the mixture of pellets/sludge is advanced slowly to the left forwards towards the outlet to the pipe 28. The whole plant is dimensioned and must be run so that the wet sludge material which is supplied at the right side gradually dries during the forward feeding towards the outlet. The evaporated water is withdrawn from the drying apparatus through the conduit 46. The mixture of dried sludge and pellets is conveyed out of the drier by means of the screw conveyor 30 to the separator 32 which can be of the vibrating or rotating type and the pellets are separated from the dried sludge. The separated pellets are conveyed and recycled continuously back to the entrance 48 of the drier 12 by means of the screw conveyor 44 to be mixed again with the wet sludge newly fed in and the drying cycle is repeated.

The final product in the form of dried sludge is conveyed through the pipe 36 by means of the screw conveyor 38 to the station 40 for further treatment. In the continuous operation of the apparatus, that is to say that the supply of sludge through the pipe 20 if desired 26 is constant the combined parameters such as the speed of forward feeding through the drum, the supply of heat, and the like must be regulated until a stationary operative relationship is achieved. Example A mixture of sewage sludge from a communal sewage purification plant and sludge from an industrial extraction process for plant oil were treated by the process according the present invention during a preliminary experiment where palm oil nuts of varying size were used as pellets. To a drying apparatus of the type which is described above there was added for continuous operation 300 kg./hour of sludge having a dry substance content of 20 weight per cent. During the drying heat was supplied via steam at a pressure of 7 bars and a temperature of about 165°C, and the steam was circulated through the rotating shaft and the disc-shaped stirring blades and removed as water condensate. The total heating surface of the apparatus was 31 m².

The drying was carried out by way of introduction without the addition of palm oil nuts. After operating for an hour it was noted that the sludge material passed over into a glue phase, where it lumped itself together into large cakes which gradually looked like stopping up the whole apparatus, and the energy supply to the stirring mechanism had to be increased to get this to rotate at all. Thereafter palm oil nuts were gradually added up to a quantity of 5 volume parts nuts per volume part sludge. It was quickly observed that the large cakes were now gradually broken up into smaller lumps which the apparatus could readily tumble, and the energy supply to the stirring mechanism could be reduced. At the outlet from the apparatus the product was in the form of a substantially dry powder removed in a mixture with the nut pellets. The nut pellets could be readily separated from the product with a vibrating sieve and recirculated. It was estimated that relative to the heat energy supplied by the heating oil the evaporation capacity of water from the sludge was about 10 kg./m².hour. The addition of palm oil nuts consequently had a big and surprising positive effect on the operation of the drying apparatus. This is also reflected in that the necessary stirring energy (shearing force) supplied fell significantly as is evident from the broken line of Fig. 1.

In further tests pellet spheres of plastic were employed under otherwise identical conditions to the experiment mentioned above, and it was found that the method functioned very well with up to 10 volume parts pellets per part sludge. In addition to a sludge of sewage and palm nut oil the method is also carried out with sludge from paper production where relatively few pellets are needed, while for sludge from fish processing waste a significantly higher proportion was needed, that is to say up to 10 volume parts pellets. For sludge from the purification plant for a brewery it was found that 5 volume parts pellets per part sludge was sufficient for the process to go satisfactorily.

Therefore that which is achieved in particular by the present invention relative to earlier is that the energy supply in the drying of the sludge in the glue phase is heavily reduced, both in being able to reduce the energy for stirring the material, and in utilising more effectively and rapidly the heat supplied for the evaporation of the water phase of the sludge. That is to say an improved specific evaporation of the water portion of the sludge reckoned in kilos of evaporated fluid per square metre heating surface and hour (kg./m² hour). When the sludge at the entrance to the drying apparatus has a high fluid content, this gives an extremely good heat transfer. Furthermore the power consumption achieved for carrying out the stirring remains low because the sludge does not pass through the afore- mentioned damaging glue phase with the high viscosity, but remains in a thinly movable and thinly fluid form during the advancement through the drying apparatus. In addition a good self-cleaning effect of the heating surfaces is obtained due to the pellets rubbing against these and preventing the sludge forming flakes, so-called scaling, which sticks on the wall surface of the drum with the result that the heat transfer is reduced.

When the sludge is to be dried to a moisture content of 5-15% the nuisance of dust and the danger of the explosion of dust in the present invention is extremely small. This is due to the outlet from the drying apparatus only having a short dust zone, and furthermore the speed of evaporation and the speed of the gas is low in this locality. Furthermore improvements in the starting up and stopping of the drying process are obtained because the drying drum can be started and heated without the supply of sludge with a total recycling of pellets after which wet sludge is gradually fed in to achieve balanced and stable operative conditions in the drying process. When the drying process is to be shut off the feeding in of wet sludge is halted and one waits until all the dried sludge has come out of the drying apparatus after which the process is stopped. The drying process according to the present invention will have the greatest significance for drying sludge from communal and industrial sewage purification plants. Sludges from such plants are considered to be extremely contaminating, and they can also be dangerous to environmental life since they contain bacteria, chemicals and heavy metals. The drying process according to the present invention can be used for the two main types which are used to-day in the treatment of sludge: - Partial drying where the sludge is dried to approximately 50% moisture where it becomes combustible and can be burned off in an incinerator.

- Total drying where the sludge is dried to 5 - 15% moisture and is used for land fills, manure, animal feed or other applications.

The process according to the present invention can also be used for drying other sludge substances as for example animal feed, food products, and various sludges from decanting centrifuges and chemical processes.

Claims

Patent Claims

1. Process for the treatment of sludge from discharge fluid, whereby the sludge undergoes a drying process in a drying apparatus where the fluid portion of the sludge is substantially separated from the drystuff content of the sludge, in order to produce a product material out of quantities of drystuff of the sludge having lower moisture, characterised in that during that portion of the drying process where the sludge passes through a phase of higher viscosity, a so-called glue phase, bits of a solid material, so-called pellets, are added to the sludge.

2. Process in accordance with claim 1, characterised in that the mixture of pellets and sludge are agitated during the drying process.

3. Process in accordance with claim 1 or 2, characterised in that heat is supplied during the drying process.

4. Process in accordance with claims 1 - 3, characterised in that the pellets used consist of a substance which is different from the sludge.

5. Process in accordance with one of the preceding claims, characterised in that the pellets have the form of spheres, ellipsoids, cylinders, cubes or another appropriate shape.

6. Process in accordance with one of the preceding claims, characterised in that the pellets have a diameter of up to 50 mm, and preferably have a size in the range of 10 -

25 mm.

7. Process in accordance with one of the preceding claims, characterised in that the pellets are separated from the dried sludge to be if desired recirculated.

8. Process in accordance with one of the preceding claims, characterised in that a drying apparatus is used in the form of a rotary drying apparatus having an internal rotor, which comprises hollow discs for the circulation of heat medium.

9. Process in accordance with one of the preceding claims, characterised in that sewage sludge is dried, and the pellets are supplied when the sludge is in a glue phase where the moisture content is 45 - 70 weight %.