WO2003082753A1 - Method and device for destroying cellular structures in waste water and sludge in biological sewage works - Google Patents

Abstract

In order to destroy cellular structures in waste water and suspensions of micro-organisms, especially sludge in biological sewage works, the suspension is guided through a nozzle (4), optionally after homogenisation, with the aid of a cavitation, said nozzle having a narrow section and then a wider section being called a Lavel-nozzle. By increasing the flow speed in the narrow section (Q2) of the nozzle (4), the static pressure of the suspension is reduced to a level below the vapour pressure so that cavitation bubbles are formed and break down during subsequent pressure compensation in the wider section (Q3) of the nozzle. As a result, high energy transverse strain fields are induced, wherein the cells are broken up. The suspension can be guided through the same nozzle repeatedly, according to the desired degree of cell disaggregation, without the need for high energy expenditure.

Description

Description:

PROCESS AND DEVICE FOR DESTRUCTING CELLULAR STRUCTURES IN WASTEWATER AND SLUDGE OF BIOLOGICAL WASTEWATER PLANTS

The invention relates to a method and a device for destroying cellular structures in suspensions of microorganisms, in particular in sludges from biological sewage treatment plants.

Technical field:

In the treatment of wastewater in industrial and municipal biological sewage treatment plants, the metabolism of biodegradable substances by bacteria results in sewage sludge in the form of bacterial suspensions. Since this sewage sludge can only be landfilled, burned or used for agriculture to a limited extent due to laws and economic constraints, sewage sludge reduction or avoidance is becoming increasingly important.

For this purpose, it is known to use the process of disintegrating sewage sludge from aerobic or anaerobic degradation processes, in which the cell walls of the microorganisms in the sewage sludge are destroyed and the cell contents are released.

The digestion can take place, among other things, through biological and chemical reactions, through pressure and temperature changes or through kinetic energy.

Cell disruption has two main objectives. On the one hand, the anaerobic sludge treatment is to be improved by accelerated and increased degradation. The acceleration is based on the mechanical support of the hydrolysis, since cell disruption leads to the release of the easily degradable internal water in the cell. In addition, facultative anaerobic microorganisms are unlocked, which can otherwise partially survive the digestive process and are partly responsible for the residual organic matter in the digested sludge. The cell disruption makes them accessible for increased degradation. On the other hand, disintegration opens up the possibility of using internal Zeil water, which contains organic substances such as protein and polysaccharides, as an internal carbon source. This enables both a reduction in the amount of sludge and the digestion time and an increase in the amount of digestible gas that can be used for energy purposes. Other advantages include the destruction of floating sludge and thread bacteria and an improvement in the settling properties of the sludge.

State of the art:

An overview of the conventional mechanical disintegration processes can be found in N. Dichtl, J. Müller, E. Engelmann, F. Günthert, M. Oswald: Disintegration of sewage sludge - a current overview in: Correspondence sewage, (44) No. 10, pp. 1726-1738 (1997).

Disintegration devices suitable for large-scale use are, in particular, the agitator ball mill, the high-pressure homogenizer and the ultrasonic homogenizer.

While the cell disruption in the agitator ball mill is effected by the rotation of the balls in a cylindrical grinding chamber filled with grinding balls made of tempered glass or ceramic, cavitation processes are used to disintegrate the cells in the ultrasonic and high-pressure homogenizer.

Ultrasonic homogenizers consist of three main components. A generator generates a high frequency voltage in the range of 20 to 40 kHz. A ceramic crystal made of piezo-electric material converts the electrical into mechanical impulses and a sonotrode transfers them into the medium. The ultrasonic vibrations generate alternating high and low pressure in the liquid high-energy shear stress fields that cause cavitation. If the cohesive forces of the liquid molecules are overcome in the negative pressure phase of the vibration, cavitation nuclei, such as interfaces, air bubbles or particles, microbubbles, which can grow over several vibration cycles, preferably arise. If they exceed a critical size, they become unstable and implode. Pressure surges are generated that cause local temperatures of several thousand degrees Celsius and pressure peaks of 500 bar. The pressure waves overlap in such a way that fluid vortices are created in which shear stress fields develop and the cells are stressed on scissors.

The intensity of the cavitation increases with increasing power and amplitude and decreasing frequency of the sonotrodes. The parameters of the liquid, especially the vapor pressure, the surface tension, the viscosity and the number of cavitation nuclei, are also important.

High pressure homogenizers were developed for the milk processing industry. They consist of a multi-stage high pressure pump and a homogenizing valve. The high pressure pump compresses the suspension to pressures of several hundred bar. The suspension is then expanded to ambient pressure through a homogenizing gap, which is formed by a stationary valve seat and an adjustable valve body. In the case of a continuous suspension flow, the pressure drop during the expansion results in high flow rates. Therefore the static pressure in the suspension decreases until the vapor pressure of the liquid is reached. This creates vapor bubbles or cavitation bubbles, which lead to a further acceleration of the gas-liquid flow. The cavitation bubbles collapse and induce high-energy shear stress fields in which the cells are unlocked.

All known methods for mechanical disintegration have in common that the cost and energy expenditure for producing the cavitation processes, by which the forces for splitting up the cell walls of the microorganisms arise, are very high. This applies to the manufacture, but also to the operation and maintenance of the high pressure and ultrasonic homogenizers. While the high-pressure homogenizers have to generate very high pressures that require a high pump capacity, the ultrasonic processes require a large amount of electrical energy to feed the sonotrodes. A further disadvantage of utilizing the cavitation phenomena is that there are signs of detachment on the devices and materials, which is why expensive materials such as titanium have to be used especially for wear-intensive components such as the ultrasonic sonotrodes. From DE 34 28 353 AI a further device for treating sewage sludge by means of cavitation is known. The mixture of substances to be treated is discharged from an elevated tank, deflected via a pipe bend and fed to a down pipe. A cavitation zone is to be formed in some areas, in which the cell walls of the bacteria contained in the liquid are destroyed. Since the pressure in the pipeline essentially depends on the outflow rate of the mixture of substances from the tank and thus on the fill level of the tank, the results cannot be reproduced uniformly with the same pipe diameter. Furthermore, as with the other known methods, the space required is very large.

Presentation of the invention:

The object of the invention is to develop a method and a device with which the disadvantages mentioned can be avoided and a more efficient cell disruption in suspensions of microorganisms can be achieved with reduced expenditure on energy and equipment.

According to the invention, this object is achieved by a method having the features of claim 1 and by a device having the features of claim 5.

Advantageous further developments result from the subclaims.

The basic idea of the invention is to use ultrasound or high-pressure disintegrators that are not complex in terms of machine and energy technology to generate cavitation phenomena for the digestion of organic substances, but rather a so-called Laval nozzle with a cross section that initially narrows and then widens again. By reducing the cross-section, the flow velocity of the medium is increased so that the pressure drops below the vapor pressure, while cavitation bubbles are generated by pressure equalization when flowing through the subsequently widening cross-section.

In addition to the low primary costs for the digestion device compared to the known disintegration devices, the economic importance of the device according to the invention lies above all in the operating costs that must be taken into account on an ongoing basis. The energy required to generate the pressures in the pipelines of approximately 10 bar is far lower than in the known methods. The specially selected shape of the nozzle largely prevents the material from becoming detached and worn. As a result, the financial expenses for repair, maintenance and maintenance can be kept low.

The method according to the invention can be used in particular in the treatment of biological waste in sewage treatment plants. The process, which can be used at any point in wastewater and sludge treatment or the clarification process, not only represents a low-energy variant of sewage sludge minimization, but also leads to a significantly higher yield of fermentation gas and a reduction in the subsequent digestion of the sludge organic residual substance.

For industrial use, depending on the required flow rate, several of the devices according to the invention can be arranged in parallel. This has the advantage that the disintegration process is not completely interrupted even if a device fails.

Brief description of the drawings:

A device for carrying out the method according to the invention is explained in more detail with reference to the drawings in FIGS. 1 to 3.

1 shows a diagram of a two-stage disintegration system with a device according to the invention

FIG. 2 shows a section from FIG. 1

Fig. 3 shows a schematic diagram of Fig. 2 on a larger scale.

WAYS OF IMPLEMENTING THE INVENTION AND INDUSTRIAL APPLICABILITY:

In a particularly advantageous embodiment of the device 1 according to the invention, as shown in FIG. 1, it is used as part of a two-stage disintegration process. A suspension of microorganisms is in the direction of the arrow by the device 1 according to the invention Pipelines 2, a feed pump 3 and a disintegration device 4 according to the invention are promoted.

A homogenization device, a so-called rotary vortex disintegrator 5, is preferably used as the first stage of the disintegration process. The swirl disintegrator 5 is an aggregate of a fine shredder and a swirl machine. The suspension is first finely comminuted by means of a cutting roller mill and then conveyed into the rotary whirling machine in which there is a high-speed rotor which is designed as a hollow shaft. The suspension is conveyed into the hollow shaft via longitudinal slots. After a longitudinal pass through the hollow shaft, the sludges to be treated also exit the hollow shaft via longitudinal slots and pass through a swirl chamber equipped with baffle plates before they leave the swirl disintegrator 5 under a maximum pressure of up to 4 bar. The unit is provided with flange connections 6 and can therefore be integrated into any existing sewage or sludge line. Due to the pressure, shear and acceleration and impact forces that occur and their interactions, the grain size distribution in the suspension is evened out.

The suspension is then conveyed through the disintegration device 4 by means of the pump 3. There the desired destruction of the aggregates and the cell disruption is effected.

FIG. 2 shows the disintegration device 4 in the form of a Laval nozzle from FIG. 1 in longitudinal section. The Laval nozzle 4 is surrounded by a housing 7, in which sound insulation can be arranged. According to an advantageous embodiment of the invention, the Laval nozzle 4 is composed of several individual parts 8, 9, 10. As a result, the middle part 9 with the narrowest cross section can be replaced very quickly and, for example, adapted to changing pressure conditions or the composition of the suspension. Since this is the most stressed part, the entire nozzle does not need to be replaced if there are signs of wear. The individual parts 8, 9, 10 of the nozzle are connected to one another in a positive and non-positive manner. Threaded bolts 11 are provided as connecting means, with which a prestressing force can additionally be applied. For connection to the pipes 2 4 flange connections 12 are provided at the ends of the Laval nozzle. The processes in the Laval nozzle 4 can be described in detail using the schematic diagram in FIG. 3. In the flow direction, which is indicated by an arrow 13, the Laval nozzle 4 connects to the cross section of the pipeline 2 with the inside diameter Di with the initial cross section Qi. Since the cross section Qi of the Laval nozzle 4 continuously narrows down to the cross section Q ₂ over the length Li, the flow rate of the suspension increases continuously. A value of approximately 30 ° is advantageously chosen for the opening angle i of the narrowing nozzle.

At the same time as the flow rate increases, the static pressure of the suspension decreases. In the narrowest cross section Q _{2 of} the Laval nozzle 4, the static pressure drops to the vapor pressure due to the increase in the flow rate. This creates vapor bubbles within the suspension.

According to the length L _{2 /} on which the cross section Q _{2 of} the Laval nozzle 4 runs constantly, it widens again continuously. The flow rate of the suspension slows down and the resulting vapor bubbles collapse as the pressure rises again. The sudden change in volume of the bubbles creates high temperatures and pressures in this area, which cause the desired destruction of the cell walls. The opening angle _{3 of} the widening nozzle is preferably 10 °. This determines the length L ₃ , after which the final cross section Q _{3 of} the Laval nozzle 4 is reached and the suspension is again fed to the pipeline 2 with the inside diameter D ₃ .

In the majority of cases, a single treatment of a suspension in the manner according to the invention will suffice. However, there is certainly the possibility of carrying out the treatment several times in succession, either by returning the suspension in a nozzle after the treatment and conveying it again through the same nozzle or the same multi-stage disintegration system, or by using several nozzles, each with an intermediate connection of pumps are arranged one behind the other. In any case, the treatment can be continued until the desired degree of digestion of the suspension is reached.

In some cases, it can also make sense to treat only a partial flow of the suspension. Even by destroying cellular structures in a partial stream then when this partial flow is fed back to the untreated suspension, a greater degradation of biological structures can also be achieved in it.

In principle, the cross-sectional shape of the nozzle can also be arbitrary. A circular cross-section certainly makes sense, which will generally also correspond to the cross-section of the pipeline into which the nozzle is inserted. In principle, however, the nozzle can also have an oval or polygonal to rectangular cross section.

Claims

claims: 1. A method for destroying cellular structures in waste water and in suspensions of microorganisms, in particular sludges from biological sewage treatment plants, characterized in that the suspension is conveyed under pressure in such a way through a nozzle (4) with a cross-section that initially narrows and then widens again (Laval nozzle), that by reducing the static pressure of the suspension below the vapor pressure as a result of increasing the flow rate with subsequent pressure equalization, collapsing cavitation bubbles are generated. 2. The method according to claim 1, characterized in that the suspension is treated several times in succession. 3. The method according to claim 2, characterized in that the suspension is conveyed several times through the same nozzle. 4. The method according to claim 2, characterized in that the suspension is conveyed successively through a plurality of nozzles arranged one behind the other. 5. A device for performing the method according to claim 1, with a pipe (2) for conveying the suspension and a feed pump (3), characterized in that in the pipe (2) after the feed pump (3) with a nozzle (4) an initially narrowing and then widening cross section is arranged. 6. Device according to claim 5, characterized in that the cross section of the nozzle (4) narrows from a cross section Qi corresponding to the cross section of the pipeline (2) to a cross section Q _{2 which is} constant over a length L ₂ and then narrows again to a cross section Q ₃ expanded. 7. Device according to claim 6, characterized in that the transitions between the cross sections Q _x and Q ₂ and Q ₂ and Q _{3 are} rounded. 8. Device according to claim 6, characterized in that the cross section of the nozzle (4) from a cross section Qi corresponding to the cross section of the pipeline (2) over a length Li continuously narrows to a cross section Q _{2 which is} constant over a length L ₂ then continuously expanded over a length L ₃ to a cross section Q ₃ . 9. Device according to claim 8, characterized in that the length L _{3 of} the expansion of the cut is greater than the length Li of the narrowing. 10. Device according to claim 8 or 9, characterized in that the opening angle αj of the nozzle (4) in the inlet area over the length Li is approximately 20 ° to 40 °, preferably 30 °. 11. Device according to one of claims 8 to 10, characterized in that the opening angle _{3 of} the nozzle (4) in the outlet area over the length L _{3 is} approximately 5 ° to 20 °, preferably 10 °. 12. Device according to one of claims 5 to 11, characterized in that the pump (3) and the associated nozzle (4) is connected upstream of a homogenization device (5). 13. Device according to claim 12, characterized in that the homogenization device (5) is a rotary vortex disintegrator. 14. Device according to one of claims 5 to 13, characterized in that a plurality of pumps (3) and associated nozzles (4) are arranged parallel to one another. 15. Device according to one of claims 5 to 13, characterized in that a plurality of pumps (3) and associated nozzles (4) are arranged one behind the other. 16. Device according to one of claims 5 to 15, characterized in that the nozzle (4) has a circular cross section. 17. Device according to one of claims 5 to 16, characterized in that the nozzle (4) is composed in the axial direction of a plurality of interchangeable individual parts (8, 9, 10). 18. Device according to one of claims 5 to 17, characterized in that the individual parts (8, 9, 10) of the nozzle (4) are releasably connected to one another by axially parallel connecting means (11).