DE102012102473A1 - Treating sewage sludge, useful for improving its storage- and/or transport properties, comprises mechanically dewatering sludge, and mixing dewatered sludge with superabsorbent or hydrogel to obtain granules with specific moisture content - Google Patents

Abstract

Treating sewage sludge, comprises mechanically dewatering the sewage sludge, and mixing the dewatered sewage sludge with 0.01-10 wt.%, preferably 0.3-2 wt.% a superabsorbent or hydrogel to obtain granules exhibiting a moisture content of 40-90 wt.%, preferably 70-85 wt.%, and an average diameter of 0.1-20 mm, preferably 1-5 mm. An independent claim is also included for granules obtainable by the above method, comprising the sewage sludge and 0.01-10 wt.%, preferably 0.3-2 wt.% the superabsorbent or the hydrogel.

Description

Sewage sludge usually accumulates in sewage treatment as liquid sludge or after mechanical dewatering as a moist solid. The mechanical dewatering takes place for example by filter presses, centrifuges, belt presses or screw presses, after the liquid sludge flocculant, z. B. organic polymeric compounds were added.

A particular problem is the poor storage and transport properties of sewage sludge. Due to the high humidity and partly thixotropic behavior, caking is often formed.

In WO 2005 095 291 describes a process in which first a sewage sludge suspension is thickened with the aid of a flocculant in the form of an organic polymer material and then in the thickened suspension a second flocculant based on an organic polymer material is added and finally followed by a mechanical dewatering process.

After mechanical dehydration, the dry matter content increases to about 15 to 40%. For economically viable recycling, for example as a fuel, however, significantly higher dry matter contents are required. These higher dry matter contents can be achieved by drying. In addition to conventional drying methods, drying by means of solar energy has also been proposed. A disadvantage of the drying process is either the energy requirements or the area and the time required. In the case of solar drying, the dependence on solar radiation is added.

In EP 2 228 347 A method and apparatus for dewatering mechanically dewatered sewage sludge is described. The sewage sludge is conveyed by means of a screw located in a closed housing along the longitudinal axis of the screw conveyor in a conveying direction, being blown for further dewatering of sewage sludge air substantially perpendicular to the conveying direction in the closed housing and the sewage sludge is acted upon with the air. This has a high expenditure on equipment and energy costs result.

Disadvantages of the prior art:

• - Mechanical dewatering leads to dry matter contents of typically only 15 to 25 wt .-%, with elaborate high-pressure filter presses a maximum of 40 to 50 wt .-%.
• - Sludge are not easy to handle, especially disadvantageous is the tendency to caking during storage and transport
• - Thermal drying processes are complex and expensive and require appropriate equipment
• Air drying requires expensive equipment to provide a large contact surface; to a large residence time
• - Solar drying processes require large area, solar radiation and long residence time and often mechanical devices to improve the contact with the gas atmosphere

The object was to provide a method which makes it possible to improve the storage and / or transport properties of sewage sludge and to dry the sewage sludge, preferably completely or partially, in a particularly simple and energy-saving manner.

Furthermore, it was an object to provide granules of sewage sludge, which have good storage and transport properties.

The object is achieved by a method for the treatment of sewage sludge, wherein the sewage sludge is first dehydrated mechanically and then 0.01 to 10 wt .-%, preferably 0.05 to 3 wt .-%, particularly preferably 0.3 to 2 wt .-%, of a superabsorbent or hydrogel is mixed, so that in this case granules are obtained, which have a moisture content of 40 to 90 wt .-%, preferably 50 to 85 wt .-%, particularly preferably 70 to 85 wt .-% and an average diameter of 0.1 to 20 mm, preferably 0.5 to 10 mm, particularly preferably 1 to 5 mm.

The resulting granules form a non-adhesive, transportable, in particular pneumatically conveyable and not prone to caking bulk material. The morphology and particle size of the granules is dependent on the type and amount of the superabsorbent or hydrogel and the type and intensity of the mixture or the mixing unit. The composition of sewage sludge can also have an influence. Preferably, a suitable granule size is adjusted by suitable selection and metered amount of the superabsorbent or hydrogel. These relationships can be determined for the particular sewage sludge composition and quality by simple routine mixing experiments. The following applies: the higher the dosage, the finer the granules are obtained.

As a mixing unit, all mixers, stirrers, kneaders or other devices can be used, which allow a sufficiently large homogeneity of the mix in a reasonable time. Advantageously, devices with high-speed mixing tools. But even kneaders are basically suitable.

The term "superabsorbent" particularly includes those polymers which are able to take up by swelling a multiple of their own weight of water or other liquids and retain the absorbed amount of liquid even under a certain pressure. Superabsorbents can be of natural or synthetic origin. From the DE 20 2011 002 784 are known as examples of natural superabsorbent potato starch or polysaccharides of a general nature.

Superabsorbents are i. d. R. small, powdery particles with diameters ranging from the nanometer range up to several hundred micrometers, but can also have a size in the millimeter range.

In general, synthetic superabsorbents are prepared by polymerizing partially neutralized acrylic acids in aqueous solution in the presence of polyfunctional crosslinkers by free-radical reaction to give a gel which is dried, ground and classified into the desired particle size. Typical methods disclose the US 4286082 , the DE 2706135 as well as the US 4076663 ,

Another production method for synthetic superabsorbents consists of inverse suspension polymerization and emulsion polymerization. These methods are for example the US 4340706 , of the DE 3713601 as well as the DE 2840010 described. In both processes, one or more crosslinkers which have at least two intramolecular, unsaturated ethylenic double bonds are additionally added to the polymerization. This results in polymeric networks that meet the requirements of the superabsorbent, namely to be water-swellable, but not water-soluble.

An additional possibility of modifying the water-binding additives is to use co-monomers in the polymerization of acrylate resins. For this come as a co-monomer in the broadest sense, all chemical compounds having at least one ethylenic double bond in question. Various examples of such monomers are disclosed in U.S. Pat DE 10130427 , of the DE 4020780 , of the DE 10161496 or in the DE 10 2005 021 221 listed.

Superabsorbents are also known which are formed in the reaction of natural compounds with synthetic materials or monomers. Such products are described, for example, in R. Dhodapkar, NN Rao, SP Pande, SN Kaul, Biosource Technology, 97 (2006) 887-885 and R. Dhodapkar, NN Rao, SP Pande, T. Nandy, S. Devotta, Reactive and Functional Polymers, 67 (2007) 540-548 , These superabsorbents are more environmentally friendly with comparable properties than purely synthetic products and are generally better biodegradable.

It is further known that the superabsorbents which can be used for the purposes of the present invention can consist of polymeric beads which are increasingly crosslinked from the inside to the outside ( Ullmann's Ecyclopedia of Industrial Chemistry, 6th. Edition, Vol. 35 pp 73, 2003 and DE 20 2005 020 566 ). The advantage of this crosslinking gradient ("core-shell structure") is that the mentioned beads better hold the moisture under mechanical load (pressure).

Furthermore, hybrid materials consisting of polymeric superabsorbers in combination with organic and / or inorganic hollow spheres are known (US Pat. DE 10 2009 016 404 ). The advantage of combining with hollow spheres is that the superabsorbent hybrid materials absorb the moisture faster than the polymeric superabsorbents alone, without significantly reducing the water absorption capacity. In this respect, these hollow-sphere-containing materials in the context of the present invention also constitute a superabsorbent or a moisture-absorbing organic polymeric material.

A whole series of other superabsorbents are known, which form a mixture of polymeric superabsorbers and ground mineral additives ( DE 20 2006 020 295 . DE 10 2005 021 221 . DE 10 2009 034 137 . DE 20 2007 016 362 . DE 20 2011 002 784 . DE 20 2011 003 679 . WO 2006/119828 ). These mineral additives can either themselves have a water-absorbing effect (eg bentonites) or act indirectly, by improving the flow behavior of the superabsorber for liquids and thus favorably influence the rate of binding of moisture. A combination of absorbent polymers, mineral additives and alginate from seaweed, especially brown algae, is also known ( DE 29516675 ).

In superabsorber technology, a distinction is still made between superabsorbents and hydrogels. While superabsorbents are understood to mean the almost dry, water-pulling substances, hydrogels are already wholly or partly swollen superabsorbers. Thus, superabsorbent particles may be particles of superabsorbent polymer in the dry state, especially particles that either have no water at all or are up to be about 10 wt .-% water, be like this DE 60 2004 002 202 is disclosed. The terms superabsorbent gel, superabsorbent hydrogel, then refer to a superabsorbent polymer having a water content of at least about 10% by weight, typically to particles having at least their own weight and typically many times their own weight of water. The use of pre-soaked superabsorbent gels or hydrogels or superabsorbent hydrogels, all terms describing the same state, may be advantageous because hydrogels that are not yet saturated with liquid, in some cases absorb more fluid more quickly. The present invention thus also encompasses the use of hydrogels for (ver) mixing with sewage sludge.

• – „Free Swell Capacity” (FSC) ERT 440.2-02
• – Centrifuge Retention Capacity, (CRC) ERT 441.2-02
• – Absorption under Pressure, (AUP) ERT 442.2-02.

Of the "European Disposables and Nonwovens Association," Avenue Eugene Plasky, 157-1030 Brussels, Belgium (EDNA) exists a font "EDNA Recommended Test Methods" (short: ERT) from the year 2002 in which, inter alia, three methods are described which characterize the swelling behavior. These are:

• - Free Swell Capacity (FSC) ERT 440.2-02
• - Centrifuge Retention Capacity, (CRC) ERT 441.2-02
• - Absorption under Pressure, (AUP) ERT 442.2-02.

In particular, superabsorbents or hydrogels are understood to mean all superabsorbent compositions which comprise superabsorbents or hydrogels which are characterized in that they are in accordance with EDNA ERT 440.2-02 (or ISO 17190-5: 2001 ) have a swelling capacity (FSC value) of at least 4 g / g, preferably 10 g / g and particularly preferably more than 30 g / g.

The superabsorbents or hydrogels preferably comprise polymers of acrylic acid, preferably a copolymer of sodium acrylate and acrylamide, or are composed thereof. The presently specified superabsorbents or hydrogels or substances or materials containing them are thus part of the subject of the invention. It is particularly advantageous with the superabsorbers or hydrogels that only a small amount is required due to their high water binding potential. In this way, the total mass of sewage sludge is increased only slightly and the cost of mixing, packaging, storage and transport is limited.

It has been shown that the superabsorbents must absorb only a small part of the existing water in order to achieve a significant change in the consistency of the sewage sludge (suspension, sludge, filter cake or paste is converted into granules). Due to the small amount of superabsorbent added, the residual moisture content of the resulting granules is generally only slightly lower than the residual moisture of the sewage sludge prior to the addition of the superabsorbent. With the help of superabsorbents or hydrogels therefore stable granules can be obtained with a usually very high water content.

For example, Luquasorb 1161 from BASF or Favor from Ashland / Evonik can be used. With the same dosage or processing, Luquasorb 1161 gives slightly finer granules.

In contrast to the organic polymeric materials used as superabsorbents, which are insoluble in water, the organic polymeric materials used as flocculants for sewage sludge have a similar basic chemical structure but no crosslinking of the polymer chains and are water-soluble.

The purpose of the flocculants is to combine solid particles or microflakes into larger structures, while the purpose of the superabsorbents is to bind or absorb water.

It is also possible that in order to increase the stability of the granules additionally 0.1 to 20% by mass, preferably 0.1 to 5% by mass, particularly preferably 0.1 to 3% by mass, of materials containing water or moisture chemically Cement, in particular cement with a setting time of less than 1 hour, quicklime, fly ash, anhydride or salts, which are able to convert into a form rich in crystal water, preferably cement. In this way, the resulting granules are mechanically stabilized and improved in terms of their storage and transport properties and can also be used in the construction sector.

Preferably, the addition of the materials which can bind water or moisture chemically, following the addition of the superabsorbent or hydrogel and formation of the granules, since in this way a hardening and hardening of the entire mass is avoided during mixing.

Furthermore, in addition to the superabsorbent or the hydrogel to reduce the development of odors, such as iron sulfate, are added.

It is particularly advantageous with regard to an energetic utilization of the sewage sludge if, in addition to the superabsorber or the hydrogel, further materials are admixed to the sewage sludge, preferably combustible materials, particularly preferably finely divided, combustible materials. For example, for this purpose coal, coke, wood flour, flour or other combustible dusts in question or such substances containing the materials mentioned. In this way, favorable synergistic properties result: the required amount of superabsorbent for the formation of suitable granules is lower, and the combustible material has in addition to its function promoting the granulation additionally the positive effect of an energy carrier in a subsequent combustion of the granules.

Very particularly advantageous is a combination of a superabsorbent and a finely divided, combustible material as additives for sewage sludge when the finely divided, combustible, material is in a moist state and for the intended use as an energy source) is subjected to a drying step, such as wet coal, especially lignite , In this case, joint processing and drying is possible, resulting in significant cost savings.

In a further embodiment of the invention, however, it is also possible that, apart from a superabsorber or hydrogel, no further additives are added to the sewage sludge.

It is also possible to use so-called "off-spec" products as superabsorbent or hydrogel, i. H. Materials that have been manufactured or treated incorrectly. Due to the high quality requirements for substances that may come into contact with the human body, it may happen that superabsorbents or hydrogels are not (anymore) suitable for hygiene applications, eg. B. because of microbiological contaminants, but are easily used for sludge treatment. In this respect, the method according to the invention can represent a practicable and economically attractive exploitation possibility for such faulty batches. It is also possible to use superabsorbents or hydrogels which have already been used for another application but still have a part of their absorption capacity.

Preferably, the sewage sludge is mechanically dewatered before the addition of the superabsorbent or hydrogel, for example by means of a filter press or a centrifuge. In this case, preferably a dry matter content of 10 to 50 wt .-%, preferably 15 to 45 wt .-%, particularly preferably 20 to 35 wt .-%, can be achieved. After the mechanical dewatering, the sewage sludge is preferably present in particulate or pasty form and is then mixed with that of the superabsorbent or hydrogel and converted into a granulate. The mixing can be continuous or discontinuous. Continuous mixing can better prevent initial caking of sewage sludge.

The mechanical dehydration is advantageous because in this way only comparatively small amounts of superabsorbent or hydrogel are required to form the granules according to the invention.

The granules obtained according to the invention have good storage and transport properties and can be filled as bulk material in containers and removed from them again. In particular, the granules are characterized in that they are pneumatically conveyable.

In a preferred embodiment of the invention, the moisture content of the granules obtained in the mixture is reduced by contact with a gaseous medium.

The granules can be dried by adding a gaseous medium, e.g. As air is passed through the granules or past the granules. Due to the significantly larger compared to the liquid, pasty or lumpy starting material surface of the granules drying takes place very quickly.

Compared to a lumpy material with an assumed average diameter of 20 cm (a not untypical value for filter cake from a filter press), the geometric surface of granules with a diameter of 10 mm becomes larger by a factor of 20, with a diameter of 1 mm the factor the surface enlargement 200, with a diameter of 0.1 mm, the factor of the surface enlargement 2000.

In addition, the time it takes water to get from the inside of a particle to its surface is considerably smaller for smaller particles than for large ones. With good gas exchange at the surface of the particles, this may be the rate-determining step for drying.

As gaseous medium, air, preferably heated air having a temperature of less than 70 ° C, particularly preferably having a temperature of 40 to 60 ° C, can be used. It is particularly advantageous to add the moisture content of the granules obtained during the mixing with a gas dry, which contains waste heat that is otherwise unusable. Thus, the waste heat of an industrial combustion process can be used, which can no longer be used to generate electricity or steam. For example, the waste heat during the incineration of sewage sludge can be used to pre-dry supplied sewage sludge.

Due to the large geometric surface of the granules but can also with air at a temperature of less than 50 ° C, preferably less than 40 ° C, more preferably less than 30 ° C, dried. This has the advantage that either little or no energy costs for drying are required.

Due to the good storage and transportability of the granules, which are obtained when mixing sewage sludge with the superabsorber or Hydrogek the sewage sludge can be easily moved to a place where such waste heat is generated.

Also, a storage in silos or a transport in silo vehicles is made possible, so that odors are avoided. Furthermore, it is possible to carry out the drying of the granules in the silo vehicle by passing air through the silo.

It is also possible to use for drying a gas containing dust particles. In this way, in addition to the drying effect at the same time allows dedusting of the gas.

In one embodiment of the invention, the resulting granules are in the form of a bed in a bulk material container, preferably a silo, which is flowed through by a gas, is particularly preferably flows through from bottom to top. The gas velocity is preferably chosen so that the resulting granules are in the form of a fixed bed and have no macroscopic motion.

Alternatively, the granules obtained may be in the form of a flat bed, wherein the height of the bed is less than 0.5 m, preferably less than 0.3 m, more preferably less than 0.1 m. In this way, due to the large geometric surface of the granules an effective drying is achieved without complex mechanical methods for turning the granulate are required. The drying effect can be additionally accelerated by the granules are swept by a gas stream, which is preferably produced by fans.

Preferably, the moisture content of the granules obtained in the mixture and dried by contact with a gaseous medium may be less than 30% by weight, preferably less than 15% by weight, particularly preferably less than 7% by weight.

The moisture content or the moisture or the residual moisture (all these terms are used synonymously here) corresponds to the weight loss, which occurs at 105 ° C in a drying oven until the mass constancy is reached. Mass constancy is considered to be achieved if the mass change between 30-minute weighing intervals is less than 0.1% absolute. The dry matter content (or solids content) in% corresponds to 100% minus residual moisture content (in%).

The invention also granules, which are obtainable by one of the claimed methods.

Preferably, the invention comprises granules containing sewage sludge and 0.01 to 10 wt .-%, preferably 0.05 to 3 wt .-%, particularly preferably 0.3 to 2 wt .-%, of a superabsorbent or hydrogel, wherein the granules a Humidity of 40 to 90 wt .-%, preferably 50 to 85 wt .-%, particularly preferably 70 to 85 wt .-%, and have an average diameter of 0.1 to 20 mm, preferably 0.5 to 10 mm, more preferably 1 to 5 mm.

Preferably, the volume fraction of the voids in the granular bed is 2 to 35 wt .-%, preferably 5 to 25 wt .-%, particularly preferably 10 to 20%.

A further advantage is that storage in silos or transport in silo vehicles is made possible by the conditioning of the sewage sludge according to the invention, so that odor nuisance is avoided.

The treatment of sewage sludge according to the invention can be carried out jointly for the mineral fraction (primary sludge), the activated sludge fraction (also biological fraction) or both.

The optimum granule size results from the intended uses.

If the granules obtained are to be dried, it is advantageous to provide granules which, on the one hand, have the largest possible geometric surface area, but on the other hand are not blown away by the gas stream used for drying. Preference is therefore given to granules having an average diameter (volume average) of 0.1 to 20 mm, preferably 0.5 to 10 mm, more preferably 1 to 5 mm produced. The granule size is determined by (light microscopic) counting assuming a constant density and a spherical shape of the individual particles.

The higher the dosing amount of the superabsorbent or hydrogel is selected, the finer are usually the granules particles. If sufficient time and space is available for drying, the granules may also have an average diameter of more than 5 mm, but if the drying process is to be carried out very efficiently, smaller granules are desirable. Accordingly, the cost of the fine granule process is higher due to the cost of the additives.

In any case, it is advantageous to strive for a bed of granules which has the largest possible void volume and the lowest possible flow resistance for the gas used for drying. This is preferably achieved by not using too fine granules and a largely uniform granule size.

Preferably, the bulk density of the sewage sludge granules is at least 10%, more preferably more than 20% less than the density of the sewage sludge prior to the addition of the additives.

In summary, the invention shows the advantages that sewage sludge can be converted by simple addition of a superabsorbent or hydrogel in a granulate and thus in a manageable form, so that storage or transport as bulk material or pneumatic conveying is much easier. In addition, the granule structure allows a much easier evaporation or evaporation of moisture and thus efficient drying of sewage sludge, which was possible in the prior art only by means of complex mechanical support or with the aid of thermal energy.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2005095291 [0003]
• EP 2228347 [0005]
• DE 202011002784 [0011, 0019]
• US 4286082 [0013]
• DE 2706135 [0013]
• US 4076663 [0013]
• US 4340706 [0014]
• DE 3713601 [0014]
• DE 2840010 [0014]
• DE 10130427 [0015]
• DE 4020780 [0015]
• DE 10161496 [0015]
• DE 102005021221 [0015, 0019]
• DE 202005020566 [0017]
• DE 102009016404 [0018]
• DE 202006020295 [0019]
• DE 102009034137 [0019]
• DE 202007016362 [0019]
• DE 202011003679 [0019]
• WO 2006/119828 [0019]
• DE 29516675 [0019]
• DE 602004002202 [0020]

Cited non-patent literature

• R. Dhodapkar, NN Rao, SP Pande, SN Kaul, Biosource Technology, 97 (2006) 887-885 [0016]
• R. Dhodapkar, NN Rao, SP Pande, T. Nandy, S. Devotta, Reactive and Functional Polymers, 67 (2007) 540-548 [0016]
• Ullmann's Ecyclopedia of Industrial Chemistry, 6th. Edition, Vol. 35 pp 73, 2003 [0017]
• "European Disposables and Nonwovens Association", Avenue Eugene Plasky, 157-1030 Brussels, Belgium (EDNA), there is a font "EDNA Recommended Test Methods" (short: ERT) from the year 2002 [0021]
• ISO 17190-5: 2001 [0022]

Claims (23)

Process for the treatment of sewage sludge to improve the storage and / or transport properties, characterized in that the sewage sludge is first dehydrated mechanically and then 0.01 to 10 wt .-%, preferably 0.05 to 3 wt .-%, particularly preferably 0.3 to 2 wt .-%, of a superabsorbent or hydrogel is mixed, so that in this case granules are obtained, having a humidity of 40 to 90 wt .-%, preferably 50 to 85 wt .-%, particularly preferably 70 to 85 Wt .-%, and have an average diameter of 0.1 to 20 mm, preferably 0.5 to 10 mm, particularly preferably 1 to 5 mm. A method according to claim 1, characterized in that the sewage sludge in addition to the superabsorbent or the hydrogel further materials are mixed, preferably finely divided, combustible materials, particularly preferably finely divided, moist, combustible materials. A method according to claim 1 or 2, characterized in that in addition to the superabsorbent or the hydrogel the sewage sludge coal, coke, wood flour, flour or other combustible dusts are added. Method according to one or more of the preceding claims, characterized in that to increase the stability of the granules additionally 0.1 to 20% by mass, preferably 0.1 to 5% by mass, particularly preferably 0.1 to 3% by mass, Materials which can chemically bind water or moisture may be added, preferably cement, in particular cement with a setting time of less than 1 hour, quicklime, fly ash, anhydride or salts, which are able to convert to a form rich in crystal water. A method according to claim 4, characterized in that the addition of the materials which can chemically bind water or moisture, takes place after the addition of the other additives. Method according to one or more of the preceding claims, characterized in that in addition to the superabsorbent or hydrogel additives in addition to reduce the development of odors, such as iron sulfate, are added. Process according to Claim 1, characterized in that, apart from a superabsorber or hydrogel, no further additives are added to the sewage sludge. Method according to one or more of the preceding claims, characterized in that the sewage sludge is mechanically dehydrated before the addition of the superabsorbent or hydrogel, preferably by means of a filter press or a centrifuge. Method according to one or more of the preceding claims, characterized in that the sewage sludge after mechanical dewatering a dry matter content of 10 to 50 wt .-%, preferably 15 to 45 wt .-%, particularly preferably 20 to 35 wt .-%, having , Method according to one or more of the preceding claims, characterized in that the sewage sludge is in particulate or pasty form. Method according to one or more of the preceding claims, characterized in that the granules can be filled as bulk material in containers and removed from these again. Method according to one or more of the preceding claims, characterized in that the granules are pneumatically conveyed. Method according to one or more of the preceding claims, characterized in that the moisture content of the granules obtained in the mixture is reduced by contact with a gaseous medium. A method according to claim 12, characterized in that it is in the gaseous environment to air, preferably heated air having a temperature of less than 70 ° C, more preferably at a temperature of 40 to 60 ° C, is. A method according to claim 12, characterized in that it is in the gaseous environment to air at a temperature of less than 50 ° C, preferably less than 40 ° C, more preferably less than 30 ° C, is. Method according to one or more of the preceding claims, characterized in that the granules obtained in the form of a bed in a bulk material container in the present case, preferably a silo, which is traversed by a gas, particularly preferably flows through from bottom to top, are dried. Method according to one or more of the preceding claims, characterized in that the granules obtained are in the form of a bed, which has no macroscopic movement. Method according to one or more of the preceding claims, characterized in that the granules obtained are in the form of a flat bed spread, the height of the Bed less than 0.5 m, preferably less than 0.3 m, more preferably less than 0.1 m, is. A method according to claim 17, characterized in that the granules are swept by a gas stream, which is preferably generated by fans. Method according to one or more of the preceding claims, characterized in that the moisture content of the granules obtained in the mixture and dried by contact with a gaseous medium less than 30 wt .-%, preferably less than 15 wt .-%, more preferably less than 7 wt .-%, is. Granules obtainable according to one of the preceding claims. Granules containing sewage sludge and 0.01 to 10 wt .-%, preferably 0.05 to 3 wt .-%, particularly preferably 0.3 to 2 wt .-%, of a superabsorbent or hydrogel, wherein the granules have a humidity of 40 to 90 wt .-%, preferably 50 to 85 wt .-%, particularly preferably 70 to 85 wt .-%, and have an average diameter of 0.1 to 20 mm, preferably 0.5 to 10 mm, particularly preferably 1 to 5 mm. Granules according to claim 21, characterized in that the volume fraction of the voids in the granular bed 2 to 35 wt .-%, preferably 5 to 25 wt .-%, particularly preferably 10 to 20%, is.