GB2296493A

GB2296493A - Composting sludge

Info

Publication number: GB2296493A
Application number: GB9524389A
Authority: GB
Inventors: Wolfgang Mayer
Original assignee: SEVAR ENTSORGUNG; SEVAR Entsorgungsanlagen GmbH
Current assignee: SEVAR ENTSORGUNG; SEVAR Entsorgungsanlagen GmbH
Priority date: 1994-12-24
Filing date: 1995-11-29
Publication date: 1996-07-03
Anticipated expiration: 2015-11-29
Also published as: GB9524389D0; GB2296493B; DE4446739A1

Description

-I- Method for composting clarification sludge 2296493 The invention

concerns a method for composting clarification sludge in which dewatered clarification sludge, if necessary mixed with additive substances which increase the carbon component, is rotted, following drying, in a heap which is permeable to air.

The term dewatered clarification sludge refers to clarification sludge which has been dewatered, by means of a dewatering device, to a dry matter content of about 22 to 35 weight W. The clarification sludge is typically dewatered by means of a mechanical dewatering device.

Composting is the conversion of dead organic matter into a friable, soillike substance which is rich in nutrients. This compost soil contains nutrients in a form which can be taken up by plants as required and whose constituents are liable to little or no elution by water. This conversion of the organic waste substances is effected by, amongst other agents, microorganisms which, in order to flourish, require atmospheric oxygen and moisture.

The conversion of clarification sludges into odourless solids which are easily handled and constitute a valuable organic fertilizer is an environmental protection objective and a way of appropriately reutilizing clarification sludge that is produced. Considerable efforts are therefore made to develop suitable methods and facilities to achieve this object with a reasonable expenditure.

It is known that, for the purpose of composting clarification sludge, it is advantageous for the dry matter content to be 409k to 50% and that the carbon/nitrogen ratio of the solid matter component of the clarification sludge has a determining effect on the development of the composting process. The process used hitherto in the composting of clarification sludge, according to prior art, has been that whereby dry additive substances such as wood shavings, straw or recycled

2 dry material are admixed with the dewatered clarification sludge and the mixture - having first been shaped, if necessary, into pressings in a pressing device - is rotted in a heap which is permeable to air.

A disadvantage of the known methods is the increase in mass due to the substantial admixture of additive substances which also have to be composted. It is frequently difficult to maintain the conditions for a self-sustaining rotting process. In DE-OS 21 62 161, it has been proposed to effect the rotting process in a friable, heterogenous mixture containing an evenly distributed, microporous air volume. Known from DEOS 23 16 476 is the practice of effecting the rotting process in thermally insulated reactors with forced ventilation. It has further been proposed, in DE-PS 27 05 098, to form pressings from the digestible clarification sludge and to give these pressings a determined volume and a tubular structure.

In particular, the composting of mechanically dewatered clarification sludge with a carbon/nitrogen ratio of less than 11:1 has hitherto not been possible or not satisfactory with the known methods.

The object of the invention is to create a method for composting clarification sludge in which there is no undesirably large increase in bulk due to additive substances, the carbon/nitrogen ratio of the solid matter component of the clarification sludge is not adversely affected and a selfsustaining rotting process is possible without major technical elaboration.

The solution according to the invention consists in the combination of the following measures. The dewatered clarification sludge is structured into pieces in which the ratio of the square root of the enveloping surface to the cubic root of the volume is set to a value of greater than 3 1.0, and preferably between 2.0 and 3.5. The structured pieces are dried, by means of a water extraction device, to a dry matter content of 40 to 50 weight %. The dried, structured pieces are piled up as a loose, homogenous, evenly distributed and microporous heap and the heap is composted.

Two types of clarification sludge are distinguished, namely, aerobically and anaerobically stabilized clarification sludge. The aerobically stabilized clarification sludge only maintains its stable character for a short time, whereas the anaerobically stabilized clarification sludge is odour-stable for a longer period. The anaerobically stabilized clarification sludge is also referred to as digested clarification sludge. The organic dry matter content of the aerobic clarification sludge is about 20% higher than that of the digested clarification sludge. For this reason, the two clarification sludges have different carbon/nitrogen ratios. In the case of the aerobic clarification sludge, this is between 11:1 and 18:1 and in the case of the anaerobic clarification sludge it is between 6:1 and 11:1.

In the course of the invention it has become apparent, surprisingly, that clarification sludges with carbon/nitrogen ratios of less than 11:1 can be successfully composted by the measures according to the invention. According to the invention, additive substances are admixed only in cases where this is necessary. This may be necessary, for example, if the carbon/nitrogen ratio of the solid matter component of the clarification sludge is less than 11:1. Whether admixture is necessary in these cases for values up to 6:1 also depends on other conditions and characteristics of the clarification sludge and can be ascertained by the specialist by simple, common practical tests and experiments. Due to the fact that, normally, the composition of a clarification sludge from a given clarification plant exhibits only slight variation over time, it is generally only necessary for these tests to be carried out once for each of the places of origin of the 4 clarification sludge. It can thus be decided in an individual case whether the admixture of an additive substance is necessary for carbon/nitrogen ratios between 6:1 and 11:1. The admixture of an additive substance is not generally necessary in the case of ratios greater than 11:1 although, in a particular instance, it may be necessary to mix in a small quantity of an additive substance even in such cases for the purpose of improving the rotting process.

According to a further development of the invention, it is common to all applications that the quantity of carbon contained in the clarification sludge is not increased by more than 5 weight % by the admixture of the additive substance. This means an extremely small increase in the mass of the clarification sludge and consequently a reduction of the equipment requirement. The expensive delivery of the additive substances is also substantially reduced. For this reason, it is advantageous if the admixture of additive substances to the dewatered clarification sludge is less than 5 weight relative to its dry matter content.

The method according to the invention is preferably executed so that no additive substances are admixed with the clarification sludge which increase the carbon component, since no increase in mass is obtained in this case. In cases in which an additive substance is to be added nevertheless, it is advantageous if a liquid carbon carrier is used for this purpose. In particular, an alcohol, preferably methanol, is considered for this purpose, since this has a high carbon component without adding greatly to the increase in mass.

It was found, in the course of the invention, that it is necessary to enlarge the surface/volume ratio of the solid matter component of the dewatered clarification sludge by structuring, for example by structuring the dewatered clarification sludge into essentially homogenously shaped pieces with a defined geometric three-dimensional form. This forming of the solid matter component produces a structured material, for example a granulate, which has several advantageous characteristics. Firstly, it has a large surface area which promotes extraction of water with, for example, subsequent drying by the application of heat. Secondly, the moisture present in the solid matter component has a shorter migration path to the surface of the structured material. Furthermore, by this means it is possible to give the clarification sludge a structure which enables it to be stacked in a heap or pile without it deliquescing back to a homogenous mass, while at the same time containing sufficient moisture for the subsequent composting process. By this means, it is also possible to achieve a maximum surface area or porosity within the material for the supply of oxygen.

It has proved to be particularly advantageous, within the scope of the invention, if, in order to enlarge the surface/volume ratio of the solid matter component, the dewatered clarification sludge is shaped, by means of a pressing device, in the form of thin, solid cylinders, to produce a material with a spaghetti-type form. In the case of spaghetti-type pressings, the ratio specified in the main claim generally lies within the range of between 2.2 and 3.5. The method according to the invention is not limited to a specific conformation of the structured clarification sludge. On the contrary, it is also possible to use any other conformations in which the solid matter component is structured so as to achieve a large surface/volume ratio. Appropriate forms and possibilities for producing them are described, for example, in the publication by R. Schilp, "Zur Technologie der Pastengranulierung" ("Concerning the Technology of Paste Granulation"), separate print of ChemieIngenieur- Technik, issue 5/1977, page 374. It is also possible to obtain ratios substantially greater than 3 using structures which are more complex than the spaghetti-type structure. According to the current level of knowledge, an 6 upper limit which can be reasonably achieved in practical terms is about 5.

According to the invention, the structured clarification sludge is dewatered, by means of a water extraction device, to a dry matter content of 40 weight % to 50 weight %. In principle, it is possible to use all water extraction methods, particularly drying methods, whose end product exhibits the required structure, i.e., with which it is possible to form a stable heap, which has a looseness and porosity that allow for a sufficient oxygen content for the subsequent rotting and still contains sufficient water for the composting process. Proposed as a preferred water extraction device, within the scope of the invention is a drier, particularly a belt drier, in which the dewatered clarification sludge is dried with hot drying air. Such a belt drier is described, for example, in EP-PS 0 225 351.

The selected drying temperature should be as low as possible, in order that the biogenic component in the material to be fed is not reduced prior to composting. The aim is to achieve a gentle drying. It is advantageous if the temperature of the drying air is less than 150 OC. Gentle drying is generally achieved if the temperature of the clarification sludge during drying, or the extraction of water, is less than 75 OC.

It has proved to be particularly advantageous if the clarification sludge is dried in such a way that the outer surfaces of the structured, dried pieces have a hard, dry crust surrounding a softer core zone with a higher moisture content. In this case, the dried pieces have a very stable structure, rendering possible an ideal heap and favourable air conduction for the subsequent composting process.

The structured and dried clarification sludge can be piled up directly, to form an inherently stable heap, without further process treatment of its structure, and the heap composted.

7 Expedient provision may be made whereby the heap is precomposted in an intensive rotting process and the compost leaving the intensive rotting process is finally composted in an after-rotting stage.

An intensive rotting process, which is also called a hot phase, is characterized by the rapid breakdown of sugars and fats, the decomposition of protein chains and a rapid heating of the material to up to 75 OC. In this process, the raw material is rendered hygienic and large volumes of water are evaporated. The air drawn off is highly odorous. The intensive rotting process lasts for about two to three weeks, depending on the supply of nutrients.

The after-rotting stage comprises a fungal phase and a mineralization phase. The fungal phase is a strictly aerobic process in the mesophilic temperature range, in which there is intensive mycelium formation of certain fungi. During this phase, the biogenic material should be vigorously mixed in order to allow a sufficient provision of atmospheric oxygen. At the same time, the water balance in the material is to be monitored and the material watered if necessary. Both overwetting and drying out should be avoided. In the mineralization phase, mixing or moistening are generally no longer necessary. This phase serves to stabilize the material and assure the quality of the compost.

In the intensive rotting process, it is advantageous if the heap is mechanically loosened. The intensive rotting process can be effected, for example, in a rotting box equipped with spiked rollers, such as that described in the German patent application P 43 42 915.7. For the afterrotting stage, it is advantageous if a roofed rotting area is used, for example in the form of a hall, open at the side, in which the stacks are piled up.

8 It is proposed, according to another advantageous characteristic, that the heap should have a density of between 400 and 500 kg/m3.

The advantages of the method for composting dewatered clarification sludge according to this invention compared with the prior art consist in the fact that, with this method, it has become possible to compost even clarification sludges which have a carbon/nitrogen ratio of less than 11:1 and which hitherto have not been capable of being composted. Further advantages are that this is possible without a significant increase in bulk and in the form of a self-sustaining rotting process, without the necessity of a high level of technical elaboration.

The following embodiment example of the invention discloses further advantageous characteristics and peculiarities, which are described and explained more fully below with reference to the representation in the drawing.

Fig. 1 shows a schematic flow diagram of a method according to the invention. The dewatered clarification sludge 1 is delivered with a dry matter content of about 25%, i.e., compact. The dry matter content should generally be greater than 24%. Provided that the carbon/nitrogen ratio of the dewatered clarification sludge 1 is greater than 6:1 but less than 11:1, an additive substance 2 is admixed with it which increases the carbon component of the clarification sludge. This additive substance 2 may, advantageously, be a liquid carbon carrier, for example methanol. The dewatered clarification sludge 1 is then fed to a belt drier 6 which effects continuous drying. The clarification sludge 1 which is to be dried is fed continuously, in the intake section, as a uniformly shaped material, in the form of structured pieces. A feed device is used for this purpose, serving both to continuously dose the material and also to produce solid pieces having a spaghetti-type shape by means of a hydraulic 9 pressing device. The spaghetti-type strands are solid and typically have lengths of 30 mm to 100 mm and diameters of 6 mm to 10 mm. This form increases the surface for the delivery of heat necessary for drying and improves the extraction of moisture from the material. By this means, a high specific drying efficiency is achieved.

The structured, granulated material is transported through the belt drier 6 on a conveyor belt. The conveyor belt has a longitudinal slot-type perforation which promotes the aeration of the material. The structured pieces are disposed on the conveyor belt in such a way that they move as little as possible, thereby minimizing the friction between the pieces. This prevents the formation of dust. The drying zone of the belt drier 6 is divided into individual drying chambers in which hot drying gases are passed through the material to be dried. The drying chambers are divided into a product compartment, within which the conveyor belt is passed through with the clarification sludge, and an air circulation compartment, in which the thermal energy is supplied. A heat exchanger is located there for indirect heating, in addition to a ventilator for generating the circulating air current. The individual drying chambers are sealed off from each other so as to prevent mutual influencing of the circulating air currents. The transport air necessary for conveying the vapours is delivered and discharged through lateral openings in the drying chambers. This transport air is passed, as axial displacement air within the drier, in a current which is counter to the direction of transportation of the clarification sludge. This, in combination with the circulating air, results in a cross-countercurrent. Due to this cross- countercurrent, the wet clarification sludge meets drying air which already contains moisture. This prevents the pores of the granulate from closing and the drying being impeded as a result. The drying of the clarification sludge is further intensified through the process of the shrinking of the granulates and by the presence of non-homogeneities in the sludge, which results in the surface of the clarification sludge particles spreading out in a dendritic form.

In the outlet section of the belt drier 6, the dried pieces 3 are removed from the conveyor belt and conveyed further by means of a first conveyor device 11. The dry matter content of the dried pieces 3 is 40 weight % to 50 weight % and is therefore still sufficiently high to assure composting. on the other hand, the dried pieces 3 possess such a looseness and porosity that they can be piled up to form a stable heap 4 and composted in an intensive rotting process 7. The dried pieces 3 have a hard surface. A hard crust has formed, which renders possible the formation of a stable heap. The dried pieces 3 have a softer core zone which has a higher moisture content than the dry crust.

The dried pieces 3 can be composted by means of a static, dynamic or quasi-dynamic method, without further additive substances or structural change being necessary. The closed composting system depicted can be used to execute a quasidynamic method in an intensive rotting process 7 and a subsequent after-rotting stage 8. For this purpose, the dried pieces 3 are delivered to a rotting box 9, which is a rotting tower. A distribution system controls the exact distribution of the raw compost to the individual rotting towers. The distribution system selects first the rotting tower which contains the least compost and, by measurement of the mass, introduces only so much raw compost that the heap is not compressed by its own weight. A typical heap is about 2.50 metres in height. Uniform conditions are thus created in each zone. Following the first filling the rotting process progresses continuously.

The rotting towers are divided into two intensive rotting zones. A base with spiked roller 10 is provided for this purpose. The upper spiked rollers 10 divide the tower into an upper and a lower zone and perform the functions of loosening 11 and homogenizing. The raw compost is transferred into the lower zone and can be aerated and watered as required. The temperature, which has risen to about 70 OC, should be maintained for at least two days. After a holding period of about 5 to 6 days, the raw compost is transferred to the lower zone. In the lower zone, it is reheated to over 70 OC.

The spiked roller base allows the raw compost to be conveyed in an absolutely uniform manner, without bridging or uncontrolled settling. Using this method, it is possible to operate a continuous intensive rotting process. This ensures that the level of digestion of the compost leaving the rotting box 9 is constantly uniform. After a further 5 to 6 days, the compost is withdrawn with the lower base and transferred to a scraper base installed directly underneath the rotting tower. The scraper base transfers the fresh compost to a second conveyor device 12 which passes the compost into a ventilated mixer, where residual odorous matter is removed and the fresh compost is prepared for the after-rotting stage.

Intensive rotting in a rotting box 9 can be effected within 24 hours. Drums or tunnels can also be used for intensive rotting, as alternatives to the rotting tower depicted.

The after-rotting stage 8 is effected in a hall which is open at the side, in a so-called rotting stack 13, in which the fresh compost is piled up. The stacks have a base width of about 4.5 metres and are aerated and homogenized twice a week using a rotation vehicle. The hall includes a watering installation. Watering is effected on the basis of the dry matter content, which is ascertained daily. The determining factor is the greatest permissible dry matter content of 70 weight %, which must not be exceeded at any point in the total rotting period. Ideally, dry matter content values of about 60% are to be maintained. Watering is effected in the form of small droplets which are dripped in the form of a mist 12 on to the stacks 13 from nozzles. compost without runoff water.

This effects wetting of the After a holding time of about 6 to 10 weeks, or following completion of the fungal phase, the compost will exhibit a digestion level of about IV. Following a final loosening, the material does not undergo any further spontaneous heating in excess of 25 OC, and assumes ambient temperature. At this point in time, the finished compost undergoes a ripening process. It is piled in flat stacks of about 4.5 metres in height, there being no further need for aeration, turning or watering. It may be fermented further, depending on the degree of mineralization required.

Instead of being composted in an intensive rotting process 7, the dried pieces 3 can also be passed, by means of the first conveyor device 11, to a static composting system, for example by being piled up to form a heap 4 in the form of a stack 13. This is possible due to the fact that, following drying, the dried pieces 3 can be piled up to form a loosely heaped structure, so that there is a sufficient volume of air available in its interior for the commencing composting process.

Additive substances 2 which increase the carbon/nitrogen ratio of the solid matter component of the clarification sludge can be added prior to drying by admixing with the dewatered clarification sludge 1 or with the structured pieces or, following drying, by being added to the dried pieces 3 or to the compost leaving the intensive rotting process 7.

The ideal compost that can be produced from dewatered clarification sludge 1 using the method according to the invention has a residual moisture of abut 40 weight %, a high inner surface area and a humus content of about 150 g/kg to 200 g/kg dry matter.

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Claims

Claims

Method for composting clarification sludge in which dewatered clarification sludge (1), if necessary mixed with additive substances which increase the carbon component, is rotted, following drying, in a heap which is permeable to air, characterized in that the dewatered clarification sludge (1) is structured into pieces in which the ratio of the square root of the enveloping surface to the cubic root of the volume is set to a value of greater than 1.0, and preferably between 2.0 and 3.5, the structured pieces are dried, by means of a water extraction device, to a dry matter content of 40 to 50 weight %, the dried pieces (3) are piled up as a loose, homogenous, evenly distributed and microporous heap (4) and the heap (4) is composted.
2. Method according to Claim 1, characterized in that, for the purpose of enlarging the surface/volume ratio of the solid matter component of the dewatered clarification sludge (1), the dewatered clarification sludge (1) is structured into essentially homogenously shaped pieces with a defined geometric three-dimensional form.
3. Method according to Claim 2, characterized in that the clarification sludge is shaped in the form of thin, solid, spaghetti-type cylinders.

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4. Method according to Claim 1, characterized in that the structured pieces are dried in a drier with hot drying air.
5. Method according to Claim 4, characterized in that a belt drier (6) is used.
Method according to Claim 1, characterized in that the structured pieces are dried with drying air having a temperature of less than 150 OC.
Method according to Claim 1, characterized in that the temperature of the clarification sludge in the water extraction device is below 75 OC.
8. Method according to Claim 1, characterized in that the structured pieces are dried in such a way that the outer surfaces of the dried pieces (3) have a hard, dry crust surrounding a softer core zone with a higher moisture content.
9. Method according to Claim 1, characterized in that the heap (4) is precomposted in an intensive rotting process (7) and the compost leaving the intensive rotting process (7) is finally composted in an afterrotting stage (8).
10. Method according to Claim 9, characterized in that, in the intensive rotting process (7), the heap (4) is mechanically loosened.
11. Method according to Claim 10, characterized in that the intensive rotting process (7) is effected in a rotting box (9) equipped with spiked rollers (10).
12. Method according to Claim 9, characterized in that the after-rotting stage (8) is effected in stacks (13) on a roofed rotting area.

is
13. Method according to Claim 1, characterized in that the density of the heap (4) is between 400 and 500 kg /M3.
14. Method according to either of Claims 1 or 9, characterized in that the carbon/nitrogen ratio of the solid matter component of the dewatered clarification sludge (1), that of the structured pieces, the dried pieces (3) or of the compost leaving the intensive rotting process (7) is set to a value greater than 11:1 by the addition of additive substances (2).
15. Method according to Claim 14, characterized in that additive substances (2) are admixed only in such a quantity that does not increase the amount of carbon present in the clarification sludge by more than 5 weight %.
16. Method according to either of Claims 14 or 15, characterized in that a liquid carbon carrier is admixed as an additive substance (2).
17. Method according to Claim 16, characterized in that an alcohol, particularly methanol, or an alcohol derivative is admixed as an additive substance (2).
18. Method for composting clarification sludge substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.