WO1993021287A1

WO1993021287A1 - Process for converting sludge into a fuel or a soil conditioner

Info

Publication number: WO1993021287A1
Application number: PCT/US1993/003455
Authority: WO
Inventors: Reiner Kruse
Original assignee: Sjoberg Partners, Inc.
Priority date: 1992-04-13
Filing date: 1993-04-13
Publication date: 1993-10-28
Also published as: AU3977693A

Abstract

A source of sludge (10) is adjusted with solid contents (38) in an adjuster (12) and passed to a premixer (14) wherein comminuted material (16) is added and passed to primary mixer (18) and then delivered by a pump (20) to press (22) where water (24) is expelled and the mixture is pressed into blocks and passed to pallet (26) and placed in storage (28) wherein gases (30) are expelled. The blocks (32) from storage are passed directly or comminuted in a comminuter (36) and part of the comminuted material is used as fuel (42) in burner (34) to produce heat (44) and an ash residue (46). The balance of the comminuted material from comminuted (36) is used as a soil conditioner or other uses (40) or passed as solid content (38) to adjuster (12).

Description

CONDITIONER Field of the Invention

This invention relates to a process for converting sludge into a combustible solid material which can be used as a fuel or a soil conditioner. Background of the Invention

Historically, wastewater or sewage was disposed by whatever techniques municipalities deemed cost effective or convenient, such as discharging untreated sewage into rivers, streams, or lakes. The Clean Air Act of 1972 mandated minimum standards for municipal wastewater treatment. The mandated treatment of wastewater created a situation where almost all municipalities have constructed wastewater treatment plants. Dependent upon the treatment technique and the composition of the wastewater, the treatment plants generate a sludge which may consist of the material filtered from the wastewater, activated sludge that contains microbic elements from the biological treatment process, and heavy metals from chemical precipitation.

The environmentally safe disposal of sludge is complicated by the fact that most sludges contain 93 to 98% wastewater. Presently, approximately 6.5 million "dry" tons of sludge is generated annually which is in the order of 130 million "wet" tons.

Innumerable schemes have been proposed for disposing of these materials. One of the common disposal techniques involves partially dewatering the sludge by pressing in a filter press, or kiln drying to bring the sludge to a state where it can be used as a solid fuel or a soil conditioner. These dewatering processes

- 1 - energy conservation.

For example, U.S. Patent Nos. 4,185,680 and 5,009,672 describe systems which generate a residual sludge which, after processing, gives off a combustible gas. The residual sludge is then compacted to a briquette or cake which is dried and used for manufacturing other products or burnable as a fuel. An alternative use for the dried sludge is as a soil conditioner.

U.S. Patent Nos. 4,405,332, 4,762,527, and 4,828,577 disclose mixing sludge with an additional combustible component to increase the overall heat content per unit volume. The intent is to provide a comminuted fuel which can be burned to dispose of the unwanted sludge.

U.S. Patent Nos. 4,043,764, 4,420,320, and 4,552,566 disclose a briquetting procedure for the solid waste or sludge, with or without a calorie enhancing additive to the potential fuel.

The burning the sludge to generate heat introduces technical problems such as insuring that the material to be burned does not interfere with the normal operation of the smoke stack and insuring that discharge and ash remaining from the combustion do not create environmental problems of a greater magnitude than the sludge itself. Further, the sludge of concern has a relatively low heat value upon combustion. Accordingly, most combustion schemes suggest combustion of sludge with oil, coal or the like to generate heat in commercial volumes.

Many patents concerned with the disposal of sludge from waste treatment systems by means other than combustion suggest that one of the uses of the resulting treated product is as a soil conditioner and no doubt that is true. However, the residue of

- 2 - makes the treated residue unsatisfactory as a soil conditioner. Spreading the treated product on lawns and other non-food producing soils may be a satisfactory disposal technique, provided the rain or irrigation run off from the treated soil does not discharge into a municipal water supply or other water source for human or farm animal consumption.

In addition to the problem of disposal of sludges, municipalities are also faced with the problem of disposal or recycling solid municipal waste. This includes a high percentage of paper and ligneous materials which are not suitable for conversion to paper products.

While the processes disclosed in the above-mentioned patents can be made to work, all are highly energy intensive.

fimπm--ιτv of the Invention

The present invention provides a solution to the two major problems municipalities now face, namely, the disposal of wastewater treatment sludges and the disposal of paper or paper like materials. In accordance with the present invention, the sludge and paper components are combined in a specified manner and formed into blocks so as to cause anaerobic fermentation and evolve heat sufficiently to achieve the desired degree of dewatering of the blocks, with no external fuel being required. A stable, environmentally safe product is the result which can be used as a fuel, soil conditioner or other applications meeting the goals of full re-use of materials which municipalities now dispose.

- 3 - combustible material. The proportions of ingredients directed into the premixer are not critical and may be varied depending upon the solids content of the sludge and the type of solid material, but the proportions are generally in the range of about 2/3 sludge and about 1/3 solid combustible comminuted material. Preferably, the solids content of the mixture is adjusted to about 25-35% solids.

After the premixing step the mixture is conveyed to a second or primary mixer which comprises, preferably, a multi-shaft screw mixer where the fully wet mixture is mixed for a second time until a substantially uniform consistency is achieved.

The apparatus used in the system provides a continuous process from the introduction of sludge into the system to the storage of the processed material. A homogenous mixture exits the primary mixer ready to be formed into block sized units for storage. The sludge-comminuted material mixture is discharged in a stream from the primary mixer onto a conveyor belt which delivers it to a press. The press may squeeze some moisture out of the mass and forms said flowing mass into a series of blocks of sufficiently rigid condition as to allow stacking on a pallet. The blocks are preferably formed having dimensions of about 20 x 20 x 40 centimeters and are stacked four high on pallets in a stack of about 80 x 80 x 80 centimeters. Individual blocks are spaced apart to allow exposure to air on essentially all sides of the formed block.

The pallet of stacked blocks is transported to a covered location for storage. It is intended that the loaded pallets be retained in storage for a period from 4 to 8 weeks, out of direct

- 4 - anaerobic fermentation takes place during which time the temperature rises in the block to a range between about 60' and about 70" C. Volatile gases resulting from the digestion process evolve and in the process drive off moisture as well. These other gases generated and the water vapor driven off reduce the weight of the blocks by 30 - 40%. This results in the moisture content of the block at the end of eight weeks being at about 20% or less by weight. The block is then ready for subsequent use which may be in comminuted condition and recycled as a part of the sludge feedstock, in unitary or comminuted condition as a fuel to generate heat, in comminuted condition as a soil conditioner or comminuted and pelletized to serve as a cat litter. In the case of a soil conditioner, it is preferred that the sludge is a sludge which has been treated to remove heavy metals.

Objects of the invention not clear from the above will be understood more fully by a reading of the description of the preferred embodiments and a review of the drawings.

Brief Description of the Drawings

The drawing is a diagrammatic view of the procedural steps of this invention.

Description of the Preferred -^gmhr-rHπι---τ-t-

The present invention utilizes a technical phenomenon which is quite different from the conventional approaches. The present invention is based on the surprising discovery that, under certain conditions, thermal energy evolved from anaerobic fermentation of sludge can be sustained at a level which is sufficiently intense to be effectively used as the primary energy source for the reduction of the water content of the sludge. More specifically.

- 5 - forming the product into blocks having a certain inherent nutrient and moisture content, the microorganisms present in the interior of the blocks ferment the sludge anaerobically with the simultaneous evolution of heat. It is a feature of the invention that the blocks are sufficiently large to effectively exclude air and at the same time retain heat to the point that the temperature in the blocks rises to about 60^* - 70" C (i.e., the heat is not efficiently radiated by the large blocks) . As fermentation raises the core temperature in the blocks, the fermentation tends to become more efficient since the microorganisms tend to be thermophilic. As a result, not only is sufficient thermal energy evolved to drive moisture out of the blocks, but the thermal energy released by fermentation over a sufficiently long period of time that the moisture content of the blocks is reduced to a level that the final product has a high caloric value (i.e., can be burned) and can be easily comminuted.

More specifically, anaerobic digestion of organic sludges generates heat. The organisms which work to decompose the organic materials operate in temperature, pH, and nutrient concentration ranges which are well known in the industry and need not be discussed here.

Accordingly, the present invention is concerned with the problem that in sludge resulting from waste water treatment and the like, water must be driven off to achieve a desired dry residue. While the conventional approaches have involved energy intensive application of mechanical dewatering techniques or application of external thermal energy, the present invention

- 6 - fermentation as the primary energy source for dewatering.

Basically, sludge is mixed with a combustible comminuted material, preferably in a first mixing machine for achieving a wetting of the solid combustible comminuted material and in a second mixing machine for achieving a homogenous mixture preferably having a solids content of about 30% or less. This mixture is delivered to a press. The homogenous mixture is compressed into solid blocks and stored to allow natural fermentation to take place, generating heat and volatile gases which escape with water vapor to reduce the weight of the block to about 60-70% of its original stored weight. The block of material having the reduced water content is higher in caloric value and suitable for use as a fuel, a soil conditioner, kitty litter or as a feedstock to be combined with sludge entering the system.

The present invention, by the novel utilization of the heat evolved from fermentation as the energy source for reduction of moisture content, is able to operate independently of weather, in a relatively small, preferably enclosed area, and without the cost of fossil fuel and expensive drying equipment. The present invention thus represents a significant contribution to the art. As can be seen in the figure, a source 10 of sludge is provided to a premixer 14, preferably in the proportion of about two parts sludge to one part comminuted material, by weight. The sludge may be of essentially any source including, but not limited to, waste from municipal sewage treatment plants, breweries, paperwood pulp factories, and mixtures thereof. Sludge may be obtained from a municipal sewage treatment plant. Sewage is generally referred to as "raw sewage," which is fed into a

- 7 - , , the waste from a municipal sewage plant. The present invention is mainly concerned with the "digested" form rather than "raw" sewage form.

Traditional sludge sources provide an aqueous mixture of solids and liquids. Solids conventionally comprise from about 2-

15% of the sludge by weight. The sludge is delivered to a premixer 14 and mixed with some fibrous solid comminuted material

16 in order to permit wetting of the solid material. The solid combustible material may be shredded straw, shredded waste paper, comminuted lumber waste materials including bark, sawdust, wood chips, wood bark or the like having a fibrous structure and suitable for burning without damage to the environment. The addition of fibrous material has the added benefit of raising the calorific value up to about 1,800 K cal/kg.

A preferred premixer 14 as shown in the illustrative embodiment is known as a conventional agricultural muck spreader.

However, many mixers may be appropriate. The function of the premixer is to churn the sludge with the dry fibrous material to make sure the dry fibers are wet. Wet fibers do not clump up and as a result the downstream pumping system does not jam.

During the experimental phase of this invention, it was noted that jamming sometimes tended to occur when the solids content was increased significantly above 30% because the sludge pumping system tended to clog up after a short period of time. It was determined that the simultaneous feeding of sludge and dry fibrous material into a single mixer caused the clogging because the fiber tended to clump and not be sufficiently wet as to permit mixing easily with the sludge. By using a premixer and using the

- 8 - disclosed, the pumping system tended to operate satisfactorily even where the solids contents of sludges rose above 30%. In other words, it was discovered that a churning premixer wet the feedstock to a degree that allowed its delivery to a second mixer without problems even though the discharge from the premixer was not homogenous. By using two mixers in series, the process could be operated continuously without problems.

The primary mixer moves the mass along its length in tumbling-turning fashion to and delivers a wet homogenous mass to a discharge pump.

In the illustrative embodiment of the invention, the mixture is wetted in churning premixer 14 and the mixture is discharged by gravity either directly into one end of an elongated primary mixer, or onto a conveyor belt and transported by the conveyor belt to an elongated primary mixer 18. Here the mixture is agitated, preferably by a twin shaft paddle mixer, to a uniform consistency and then delivered by a pump 20 to a press 22. The press compresses the mixed material into blocks of about

20 x 20 x 40 centimeters, usually with the expulsion of water 24.

In the case that microbes are seeded into the system to facilitate the digestion process, the seeding is preferably done in the second mixer 18.

As indicated above, the combination of (1) a premixer 14, (2) a primary mixer 18 and (3) the proper moisture and solids ratio of the sludge-comminuted material mixture overcomes the problem of pump 20 which tends to clog up without the specified two mixing steps and the proper solids content of the mixture.

- 9 - - a greater solids percentage than the mixture being delivered by pump 20 because of the water expelled at the press. Preferably the moisture content of the blocks exiting press 22 is about 40- 60% by weight and more preferably not substantially greater than about 50% by weight. The blocks are stacked on pallets about four blocks high and about four blocks wide with spaces between the blocks to allow air circulation around all sides. The spacing facilitates the dissipation of evolving gases and water vapor. The pallet 26 is easily conveyed by a forklift to a storage site 28 which may be completely enclosed or at least shaded from above to keep the stored blocks of material out of direct contact with rain, hail, snow and the like. The blocks are stored for several days, preferably 4 to 8 weeks, and anaerobic fermentation of the materials takes place causing the temperature in the blocks to rise to about 60" - 70" C. Carbon dioxide, methane, and other gases are generated 30 due to the digestion process and the heat generated tends to vaporize some of the residual water in the block. The generated gases and water vapor are allowed to escape to the atmosphere and constitute up to 30% to 40% by weight of the block as originally stored. Over time the digestion process slows and temperature drops.

Preferably the residual moisture content after the digestion period is about 20 - 25% at the time the stored blocks return to essentially ambient temperature. At this time the blocks are in condition for subsequent processing or use as needed. One use is as a fuel block 32 which may be delivered directly to a furnace or burner 34. After burning, the ash or residual material is

- 10 - a soil conditioner.

Alternatively, the blocks may be delivered to a comminuter 36 which shreds, hammers or otherwise breaks the dried block to particulate matter of a suitable size for whatever condition is desired.

One use for the comminuted blocks is identified by return line 38. Line 38 supplies comminuted material to the premixer for adjusting the solids content of the sludge.

Another use for the comminuted block is to deliver it in suitable size and condition as a soil conditioner 40. The size of particles delivered from comminuter 36 may or may not be the same as in recycling line 38. If desired, lime, fertilizer and/or other suitable agents are added to the comminuted block material for use as a soil conditioner for various reasons which are well known in the industry. Comminuting the partially digested block and moving the particles to a holding location prior to use as a soil conditioner results in a second composting where the comminuted materials again produce heat from exothermic chemical reactions and the mass rises in temperature to about 60^* C and slowly returns to ambient temperature (about 4 weeks).

Alternatively, after the second composting the material may be pelletized and packaged for sale as cat litter.

Additionally, the comminuted material from comminuter 36 may be used as a fuel 42 fed to furnace 34 either alone or in combination with blocks of fuel 32.

The furnace or burner 34 is intended to generate heat 44 for conventional purposes and a smoke stack (not shown) may be used to exhaust smoke and other gases from the furnace. Sludge blocks or

- 11 - supplement with oil or coal in order to raise the total caloric value, if desired. The residue of ash 46 may be disposed of in conventional fashion. The ash deposited in the furnace and discharged 46 is of a fluffy, nonvitrified, consistency allowing the discharge facility to operate on a continuous basis. Applicant theorizes that the satisfactory burning and lack of slag may be attributabe to the employment of two mixing units which provide a more homogenous product for burning, so that clumps of unburned material do not remain.

One purpose of this process is to evaporate water from the digested sludge without the use of outside energy. Accordingly, where there is a risk of rain, it is necessary to store the blocks in a covered area during the digestion phase to prevent a re- deposit of water. A roof is required and sidewalls are desirable but sheets of plastic may be used to screen the blocks from the environment.

Having thus described the invention in its preferred embodiment, it will be clear to those having ordinary skill in the art that modifications may be made in the apparatus and procedural steps without departing from the spirit of the invention. Accordingly, it is not intended that the drawing, nor the language used herein to describe the invention, be limiting on the invention itself. Rather, it is intended that the invention be lim ted only by the scope of the appended claims.

- 12

Claims

said process comprising

(a) providing to a premixer

(i) a source of sludge, said sludge being selected from a group comprising digested sewage, waste from breweries, sawmills, wood pulp processing factories, paper factories and mixtures thereof, and

(ii) a solid combustible comminuted material,

(b) mixing said sludge with said solid combustible comminuted material in said premixer in proportions of about 2/3 sludge to about 1/3 comminuted material by weight for a time sufficient to wet said solid combustible comminuted material and in proportions sufficient to adjust the solids content of the mixture to within the range of about 25-35% by weight,

(c) conveying said sludge and comminuted material mixture to a primary mixer and therein mixing the same for a time until a substantially uniform consistency is achieved,

(d) removing said mixture from said primary mixer and forming the same into blocks having a moisture content in the range of about 40-60% by weight, and

(e) placing said blocks in a covered environment for a period of at least several days while (1) the temperature of said blocks rises to a temperature in the range of about 60^* - 70^* C. by natural fermentation of said sludge and comminuted material mixture and (2) the residual moisture in said blocks is reduced to about 20-25% by weight.

2. The process of claim 1, wherein said solid combustible comminuted material is selected from the group consisting of

- 13 , , material.

3. The process of claim 1, wherein the solids content of said sludge and comminuted material mixture is not above 30% by weight at the exit of said primary mixer.

4. The process of claim 1, further comprising comminuting some of the blocks produced in step (e) and adding said comminuted material into said premixer in step (b) to adjust the solids content of the mixture.

5. The process of claim 1 further comprising comminuting said blocks produced in step (e) and applying the product to soil as a soil conditioner.

6. The process of claim 5, further comprising adding fertilizer into said premixer in step (b) .

7. The process of claim 1, further comprising a step of seeding microbes in the primary mixer, said microbes being capable of anaerobic fermentation of said sewage.

8. The process of claim 1, comprising a further step of comminuting the block produced in step (e) and exposing the comminuted material to the atmosphere for a period sufficient to allow for a further composting accompanied by a raise in temperature to about 60" C.

9. The process of claim 1, wherein said sludge provided to said premixer in step (a) has a solids content in the range of about 2-15% by weight.

10. The process of claim 1, wherein said blocks formed in step (d) have a size of about 20 x 20 x 40 cm.

11. A process for drying blocks comprising sludge, said process comprising

- 14 - the group consisting of digested sewage, waste from breweries, sawmills, wood pulp processing factories, paper factories and mixtures thereof with a solid combustible comminuted material in proportions of about 2/3 sludge to about 1/3 comminuted material by weight,

(b) forming the mixture obtained in step (a) into blocks having a moisture content in the range of about 40-60% by weight, and

(c) placing said blocks in a covered environment for a period of at least several days while (1) the temperature of said blocks rises to a temperature in the range of about 60" - 70^* C. by natural fermentation of said sludge and comminuted material mixture and (2) the residual moisture in said blocks is reduced to about 20-25% by weight.

12. The process of claim 11, wherein said solid combustible comminuted material is selected from the group consisting of shredded paper, shredded straw, and comminuted lumber waste material.

13. The process of claim 11, wherein the solids content of said sludge and comminuted material mixture is not above 30% by weight.

14. The process of claim 11, further comprising comminuting some of the blocks produced in step (c) and adding said comminuted material into said mixing step (a) to adjust the solids content of the mixture.

15. The process of claim 11 further comprising comminuting said blocks produced in step (c) and applying the product to soil as a soil conditioner.

- 15 - . , fertilizer into said mixing step (a) .

17. The process of claim 11, further comprising a step of seeding microbes in said mixing step (a), said microbes being capable of anaerobic fermentation of said sewage.

18. The process of claim 11, comprising a further step of comminuting the block produced in step (c) and exposing the comminuted material to the atmosphere for a period sufficient to allow for a further composting accompanied by a raise in temperature to about 60" C.

19. The process of claim 11, wherein said sludge provided to said mixing step (a) has a solids content in the range of about 2- 15% by weight.

20. A process for drying blocks comprising sludge, said process comprising

(a) mixing to a uniform consistency a sludge selected from the group consisting of digested sewage, waste from breweries, sawmills, wood pulp processing factories, paper factories and mixtures thereof with a solid combustible comminuted material as necessary to adjust the solids content of the mixture to within the range of about 25-35% by weight,

(b) pressing the mixture obtained in step (a) into blocks having a moisture content in the range of about 40-60% by weight, and

(c) storing said blocks for a period of at least several days while (1) the temperature of said blocks rises to a temperature in the range of about 60^* - 70^* C. by natural fermentation of said sludge and comminuted material mixture and

- 16 - 5% by weight.

- 17 -