EP0370019A1

EP0370019A1 - Preparation of free-flowing solids from aqueous waste

Info

Publication number: EP0370019A1
Application number: EP88904150A
Authority: EP
Inventors: Allan N. Cohan
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-05-21
Filing date: 1988-04-26
Publication date: 1990-05-30
Also published as: WO1988009316A1; JPH02504359A

Abstract

Procédé de production de matières solides coulant librement à partir d'une composition de déchets de boue aqueuse. Le procédé consiste à utiliser une composition sous la forme de boue (1) d'un déchet de boue aqueux et une matière filmogène. La composition aqueuse résultante est vaporisée (9) dans une zone d'évaporation (7) pour enlever suffisamment d'eau de manière à produire des solides coulant librement (19).A method of producing free flowing solids from an aqueous slurry composition. The method consists in using a composition in the form of mud (1) of an aqueous mud waste and a film-forming material. The resulting aqueous composition is vaporized (9) in an evaporation zone (7) to remove enough water to produce free flowing solids (19).

Description

PREPARATION OF FREE-FLOWING SOLIDS FROM AQUEOUS WASTE

DESCRIPTION

Technical Field

The present invention is concerned with a process for removing water from aqueous waste compositions to produce free-flowing solid materials.

The process of the present invention is especially advantageous in treating aqueous sludge waste to provide usable fertilizer compositions.

Background of Invention

A major waste management problem involves disposal of sewage sludge. The problems concerning disposal of sewage sludge have existed for a considerable period of time. Attempts to remove the water from sewage sludge have not been especially satisfactory. For instance, it is extremely difficult to reduce the water content of sewage sludge below about 50%, such as by mechanical methods such as centrifuges and screens. Moreover, methods that employ evaporation by heating have not been particularly attractive from a commercial viewpoint because of the relatively large energy requirements. Summary of the Invention

The present invention is concerned with providing a method useful for treating aqueous sludge waste that is relatively economical in its energy requirements. Moreover, the method of the present invention is relatively easy to implement on an industrial scale and is consistently reliable. The present invention provides a method for producing a free-flowing powder from the solids present in the sludge waste. A further advantage of the present invention is that free-flowing powders produced in accordance with the present invention find use in fertilizer compositions. The powders produced pursuant to the present invention are in the form of round, hollow beads that are non-dusting and relatively east to distribute uniformly over a large area. Moreover, the present invention makes it possible to prepare powders that are safe to handle.

In accordance with the present invention, a method is provided for producing free-flowing solid material from an aqueous sludge waste composition. The method includes providing a slurry of the aqueous sludge waste composition containing about 5% to about 38% by weight of solids; and adding a film-forming material. The film-forming material is added in an amount sufficient to encase and preferably enrobe the solids. The resulting composition is sprayed into an evaporation zone heated to a temperature of from about 350°F to about 750ºF to remove water from the sprayed droplets. Free-flowing solid particles are formed that contain the solids from the sludge encased and preferably enrobed in the film-forming material. The free-flowing solid particles are separated from the gases and water vapor.

The present invention provides an efficient method for separating solids from sewage sludge.

Furthermore, the process of the present invention produces particles that are hard, resist crushing, and are non-dusting. Moreover, the present invention provides solid materials that can be put to valuable use as fertilizers.

Brief Description of the Drawings

Figure 1 is a schematic diagram of apparatus suitable for handling sewage sludge compositions and includes a dryer, furnace, cyclone separator, and scrubber.

Figure 2 is a schematic elevated diagram of a bag collector that may be used as an alternative to the scrubber illustrated in Figure 1.

Best and Various Modes of Carrying out the Invention

The aqueous composition to be treated in accordance with the present invention is aqueous sewage sludge which may be a digested sludge or a crude undigested sludge. Such sewage sludge compositions normally contain about 15% to about 50% by weight of solids and more usually about 20% to about 35% by weight of solids. Sludge compositions depending upon the source and/or pretreatment prior to being obtained for treatment in accordance with the present invention can be in a form that is difficult to handle and further process. For instance, various sludge compositions that have previously been subjected to centrifuging to reduce the solids content to about 15% by weight and, therefore, in a compacted form were in a non-pumpable, difficult to handle, cake-like form.

The sewage sludge compositions processed according to the present invention must be a flowable and pumpable slurry containing about 5% by weight to about 38% by weight sludge solids and preferably about 10% to about 18% by weight sludge solids. When the sewage sludge received is not a pumpable slurry, it is necessary to convert it into the form of a slurry by diluting the sludge with a liquid in the required amount. The preferred liquid is water. However, other liquids compatible with the sludge and that are readily vaporizable at the temperatures to be employed can be used. Such other liquids include propylene glycol, ethanol, glycerol, and sorbitol.

In accordance with the invention the composition is provided with a film-forming agent which coats particles and/or provides "a continuous solid phase within which the solids are embedded after being subjected to the process. The words "encase" or "encasing" are used broadly to include a coating or an embedding of particles in a matrix. In accordance with preferred aspects of the present invention, the film-forming agent enrobes or totally surrounds the individual sludge particles.

Examples of typically useful film-forming materials are carbohydrates such as dextrins, starch, pectin, algin, methyl cellulose, carboxy methyl cellulose, carboxy methyl amylose, carboxy methyl amylopectin, dextrose, fructose, maltose, lactose, and dextrans; natural gums such as tragacanth, acacia, arabic, locust bean, caraya, and carrageen. Also included in the types of materials suitable for the present invention are cereal products such as finely-divided cereal material made from wheat, barley, rice, corn, and hydrolyzed cereal solids.

The film-forming agent must be present in a concentration which is sufficient to provide a substrate for embedding waste solids and/or to coat them; and preferably to enrobe or totally surround individual sludge particles. The concentration of film-forming material is usually at least about 1% by weight of the solids in the untreated composition and the concentration of film-forming materials is preferably about 1% to about 5% by weight of the solids in the sludge composition. Higher concentrations may be used, but there is no apparent advantage in exceeding a concentration of film-forming agents of 50% by weight of the solids to be encased or preferably enrobed.

The liquid, when employed for diluting the sludge, the film-forming material, and the sludge can be admixed with each other simultaneously or in any sequence. A preferred method when it is necessary to include additional liquid, for example the water, is to add the liquid to the film-forming material and then add the combination to the sludge.

After the composition of the film-forming agent and slurry are formed, the composition is sprayed or atomized employing any of the conventional types of atomizing devices used for spray drying, A description of some types of atomizing devices is contained on pages 838-848 of Chemical Engineers Handbook, John H. Perry, 3rd Edition, McGraw-Hill (1950). Such principal atomization devices include low-pressure, high-volume atomizers, high-pressure nozzles pressurized by 2,000- 6,000 psi pumps, and high-speed rotating discs. A typical spray nozzle suitable for carrying out the process of the present invention includes a spray pressure system such as is produced by Spray System, Inc. employing a core and spinner type of arrangement such as a Model 58 or 68 core, along with a Model 21 or 17 insert.

The size of the nozzles can vary over a wide range and is primarily dependent upon the volume of materials to be sprayed into the drying zone and the desired size of the ultimate particle. As the particle size is increased it not only is retained in the heated zone a shorter period of time, but a longer time is required to dry it. The nozzle size should not be so large that the resulting particles are too large to remain in the heated zone long enough to be dried. The composition to be sprayed will typically be at a temperature of about 70°F to about 190°F as it enters the atomizing device.

The sprayed composition is directed to a zone maintained at a high temperature in order to dry the beads by evaporation of the liquid from the surface thereof. The temperature is preferably kept below that at which the resulting dried particles will calcine, sinter, or burn and the inlet temperature will, therefore, preferably be below about 750°F and more preferably will be in the range of from about 350°F to 750°F. It is important that the dried particles are not heated to a temperature that would cause calcining, sintering, and/or burning, in order to provide additional assurance that small particles such as submicron particles are not formed and carried off with the gas.

The evaporation zone is preferably heated by introducing therein a hot gas which, for example, may be air or combustion products of a furnace. The temperature of the gases which leave the evaporating zone is preferably only slightly above 212°F as, for example, about 240°F to 250°F in order to transfer as much energy as possible from the incoming hot gases to the sprayed solution and the resulting beads being dried.

Generally, an individual sprayed particle will be in contact with the elevated temperatures for about 0.5 seconds to about 20 seconds and more usually about 0.5 seconds to about 10 seconds. The evaporation zone is preferably heated by injecting a heated gas such as hot air in the evaporation zone which also acts as a carrier to convey the particles to be cooled.

Suitable flow rates for the heated gas are from about 6,000 CFM to about 15,000 CFM for a 10-20 foot dryer. The flow rate can, of course, be scaled up or down depending upon the size of the evaporation.

In the preferred method of carrying out this invention the free-flowing powder is contacted with a cooling gas to cool the particles to about 150°F to about 225°F. The cooling zone is preferably provided by incorporating ambient air into the vicinity of the bottom of the dryer employed.

The preferred free-flowing particles obtained in accordance with the present invention are in the form of hollow free-flowing round spheres containing less than about 1% to about 6% and preferably about 5% by weight of water encased in and preferably enrobed by the film- forming material and are suitable as fertilizer.

Usually the minimum water content is 1% by weight or slightly less than 1% by weight. The particles include a small quantity of air space within the particles. The powders of the present invention are non-dusting and relatively easy to distribute uniformly over a large area. In view of their round shape the particles tend to roll over each other when being spread or distributed over the ground as a fertilizer. If desired, the composition to be sprayed pursuant to the present invention can contain other constituents such as other soil enrichment additives employed in fertilizers in amounts effective for their intended purposes such as phosphates, sulfates, and potassium. The compositions can be tailored in this manner to provide specialized fertilizers for specific crops such as corn, tomatoes, leafy vegetables, and citrus crops.

Pursuant to preferred aspects of the present invention, the sludge is pre-treated prior to admixture with the film-forming agent to destroy bacteria in the sludge. For instance, the sludge can be sterilized by subjecting it to a temperature of about 160°F for about 45 minutes, or to a flash pasteurization treatment. Also, it may be desirable to remove non-soluble materials from the sludge by filtration.

The process of the invention may be carried out in equipment as shown in Figure 1. The film-forming agent, additional liquid such as water, and auxiliary ingredients, if used, are premixed and then admixed with the sludge to be treated in feed zone (1) and introduced into pump (3) through feed line (2) where it may be mixed with recycled products from line (39). Pump (3) forces the feed solution into dryer (7) through line (5) and nozzle (9) where it is formed into a spray. A typical dryer suitable for the present invention is disclosed in U.S. Patent 4,180,593 to Cohan, disclosure of which is incorporated herein by reference. The sprayed droplets within the dryer (7) are contacted with hot air which is forced by pump (6) from inlet line (4) through furnace (10) and line (12) into the upper portion of dryer (7). The resulting dried droplets or beads, together with the water vapor, exit through dryer outlet (11) into line (13) which leads to cyclone (15). The beads may be carried through line (13) by the exiting gases and water vapor or a conveyor, such as a screw conveyor, may be used. Coolant gas such as ambient air can be introduced via vents and a venturi arrangement (not shown) located in the vicinity of elbow (60) to cool the powder. The ambient air can be pulled into the system by the pressure differential caused by exhaust fan (51) that puts the system under vacuum.

The cyclone (15) separates the beads from the water vapor, gases, and fines. The beads are removed from the bottom of cyclone (15) through valve (17) and line (19) which carries them to a further cooling zone, if desired (not shown).

The gases, water vapor, and fines are carried through line (21) and introduced into scrubber (23) where the fines are scrubbed from the incoming mixture by contacting with a spray from nozzles (47). Baffles (29) are provided in,the upper portion of scrubber (23) to reduce or eliminate carry-over of water droplets. The scrubbed gases and water vapor are carried to the atmosphere by exhaust fan (51) via duct (49).

The lower zone (31) of scrubber (23) contains water which falls from nozzles (47) and, thus, contains fines which were scrubbed from the gases and water vapor. Makeup water is introduced as needed through line (25) controlled by valve ( 27 ) . The water from zone ( 31 ) is pumped from exit line (33) by pump (35) through line (37) to either recycle to nozzles (47) via line (43) and valve (45) or mix with a portion of the feed by line (39) and valve (41) to pump (3).

The temperatures of the liquids in the scrubber (23) typically will be at least slightly greater than the condensation temperature of water at the pressures within the scrubber in order to prevent all the water vapor which is introduced by line (21) from condensing within scrubber (23). At start-up even cold makeup water may be introduced into scrubber (23) since sufficient heat will be transferred to the sprayed water from the hot gases and water vapor introduced from line (21). The temperature of the liquid in zone (31) is, therefore, ideal for use in spray nozzles (47) and is within a desirable range for transferring in line (39) for mixing with the feed to be pumped by pump (3) into dryer (7).

Figure 2 is a diagrammatic representation of a bag filter which may be used in carrying out the process of this invention in place of the scrubber shown in Figure 1. In using the bag filter of Figure 2, a mixture of hot gases, water vapor, and fines from the cyclone separation (15) is introduced into the lower portion of bag collector (63) through inlet line (61). The mixture is drawn upward by fan (71) via line (69) into particle collection bags (67) which are suspended at their closed end from upper header (68). The gases and water vapor pass through the bags which retain the fine solids which are removed through product valve (65).

The above system for carrying out the present invention is a closed system that precludes the release of noxious odors.

Claims

CLAIMSHaving thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A process for producing free-flowing solid material from an aqueous sludge waste composition comprising: (a) providing a flowable slurry of an aqueous sludge waste composition containing about 5% to about 38% by weight of sludge solids and a film-forming material in an amount sufficient to at least encase the sludge solids; (b) spraying the resulting flowable slurry into an evaporation zone heated to a temperature of about 350°F to about 750°F whereby water is removed as a vapor from the sprayed composition, and retaining the resulting sprayed droplets in said zone for a period long enough to form free-flowing solid material comprising solids encased in said film-forming material; and (d) separating said free-flowing solid materials from the gases and water vapor.

2. The process of claim 1 which further comprises providing said slurry by adding water to an aqueous sludge waste composition previously centrifuged that contains at least about 15% by weight of solids in an amount sufficient to render the sludge waste composition flowable.

3. The process of claim 1 wherein said slurry contains about 10% to about 18% by weight of sludge solids.

4. The process of claim 3 wherein said slurry is obtained by diluting a waste composition containing about 20% to 35% by weight of solids.

5. The process of claim 1 wherein fertilizer nutrients are added prior to spraying step (c).

6. The process of claim 1 wherein said free-flowing, solid material contains less than 1% to about 6% by weight of moisture.

7. The process of claim 1 wherein said film-forming agent is added in an amount sufficient to enrobe the sludge solids.

8. The process of claim 7 wherein said film-forming material is added in an amount up to about 50% by weight of the sludge solids.

9. The process of claim 7 wherein said film-forming material is added in an amount of about 1% to about 5% by weight of the sludge solids.

10. The process of claim 1 wherein said film-forming material is a carbohydrate.

11. The process of claim 9 wherein said evaporating zone is heated by introducing therein dry air.

12. The process of claim 10 wherein at least a portion of the aqueous solution which contains solids is recycled to form a portion αf feed to the evaporation zone.

13. The process of claim 1 wherein the solid material that has been separated from the gases and water vapor is cooled.

14. The process of claim 13 wherein said solid material is cooled to a temperature of about 150°F to about 225°F.

15. The free-flowing solid material obtained by the process of claim 1.

16. Free-flowing round, hollow sludge solids particles having a moisture content of about 1% to about 5% by weight and being enrobed in a film-forming material.

17. The particles of claim 16 wherein said film-forming material is present in an amount up to about 50% by weight of the sludge solids and are non-dusting.

18. The particles of claim 17 wherein said amount is about 1% to about 5% by weight of the sludge solids.