FIELD OF THE INVENTION

The present invention relates to the field of wastewater treatment, and more particularly to a method and an arrangement for wastewater sludge treatment in a wastewater sludge handling process.

BACKGROUND OF THE INVENTION

Wastewater or sewage water is created by residential, institutional, commercial and industrial establishments and includes household wastewater from toilets, sinks etc. and may also include rainwater runoff as well as liquid waste from industry and commerce. Wastewater is typically collected and transported via a network of pipes and pump stations to a centralized wastewater treatment plant.

As the wastewater comes in to a wastewater treatment plant, first in a wastewater pre-treatment easily collected materials, such as trash, tree limbs, leaves, etc. are removed from the raw wastewater before they damage or clog the pumps and skimmers of primary treatment clarifiers. Wastewater pre-treatment typically also includes screening of the influent sewage water to remove all gross solids and large objects like cans, clothes, sticks, plastic packets etc. carried in the sewage stream. Furthermore, wastewater pre- treatment may include a sand removal process, i.e. a sand or grit channel or chamber where the velocity of the incoming wastewater is adjusted to allow the settlement of sand, grit, stones, and broken glass. Furthermore, wastewater pre-treatment may include fat and grease removal and/or preaeration treat- ment where the wastewater is passed through a tank where skimmers collect the fat floating on the surface. Air blowers in the base of the tank may also be used to help recover the fat.

In the wastewater treatment plant the pretreated wastewater is then passed into primary sedimentation tanks for primary settling. In the primary sedimentation tanks the wastewater is settled and sludge is removed from the tanks to sludge treatment facilities. In the primary sedimentation tanks also grease and oils rise to the surface and are skimmed off. In some plants there is also an intermediate settling applied after the primary settling.

In the wastewater treatment plant after the primary settling the wastewater is passed into secondary sedimentation tanks for secondary set- tling. Secondary settling is designed to substantially degrade the biological content of the sewage. The majority of wastewater treatment plants treat the settled sewage liquor using aerobic biological processes. Secondary settling systems are typically classified as fixed-film systems or suspended-growth sys- terns. Fixed-film wastewater treatment systems include trickling filters and/or rotating biological contractors, where the biomass grows on media and the sewage passes over its surface. Suspended-growth systems include activated sludge, where the biomass is mixed with the wastewater.

A filter removes a small percentage of the suspended organic mat- ter, while the majority of the organic matter undergoes a change of character, only due to the biological oxidation and nitrification taking place in the filter. With this aerobic oxidation and nitrification, the organic solids are converted into coagulated suspended mass, which is heavier and bulkier, and can settle to the bottom of a sedimentation tank. The final step in the secondary settling of the wastewater is to settle out the biological sludge and/or filter material through secondary sedimentation tanks and remove sludge to sludge treatment facilities and to produce sewage water containing low levels of organic material and suspended matter.

During the wastewater treatment process in the wastewater treat- ment plant there is wastewater sludge retrieved from the primary settling process and from the secondary settling process, and in some cases from the intermediate settling process. The retrieved wastewater sludge is then typically forwarded into a digestion process for wastewater sludge handling. In some cases there can be an additional wastewater sludge thickening process ap- plied before forwarding the wastewater sludge handling into the digestion process.

Wastewater sludge also referred to as Waste Activated Sludge (WAS), Thickened Waste Activated Sludge (TWAS) or Surplus Activated Sludge (SAS), is mainly composed of dead bacteria cells and it is a particularly difficult sludge for anaerobic digestion due to large sludge particle size. In order to enhance the digestion process of the wastewater sludge there may be an ultrasonic treatment of the sludge applied. Ultrasonic sludge treatment is typically carried out with the help of ultrasonic rods utilizing ultrasonic transducers, such as piezoceramic ultrasonic transducers.

Ultrasonic transducers compose ultrasonic sound from longitudinal waves comprising rarefactions i.e. negative pressures and compressions i.e. positive compressions. It is these alternating cycles of compression and rarefaction that, in high power ultrasonic applications, can produce a phenomenon referred to as "cavitation". The cavitation bubbles expand, oscillate and implode. The physical action produced by the cavitation bubbles reduces the sludge particle size distribution thereby increasing the number of sites available for microbial action thus helping the digestion process of the wastewater sludge. Typically ultrasonic rods used for distributing of the cavitation phenomenon are in the form of ultrasonic block horns or ultrasonic radial horns.

In the following, the prior art will be described with reference to the accompanying drawings of Figures 1 to 4, of which:

Figure 1 shows a wastewater sludge treatment arrangement according to the prior art;

Figure 2 shows an alternative wastewater sludge treatment arrangement according to the prior art;

Figure 3 shows a vibration element unit of a wastewater sludge treatment arrangement according to the prior art; and

Figure 4 shows a vibration pattern of a vibration element unit according to the prior art.

Figure 1 presents a wastewater sludge treatment arrangement ac- cording to the prior art. The wastewater sludge treatment arrangement according to the prior art comprises a wastewater sludge pipe channel 1 and an ultrasonic rod 2. In the wastewater sludge treatment arrangement according to the prior art the wastewater sludge is lead through the pipe channel 1 . The wastewater sludge pipe channel 1 comprises a block horn type ultrasonic rod 2, which ultrasonic rod 2 generates microscopic cavitation bubbles. The cavitation bubbles produced by the ultrasonic rod 2 reduce the particle size distribution in the bypassing wastewater sludge.

Figure 2 presents an alternative wastewater sludge treatment arrangement according to the prior art. The alternative wastewater sludge treat- ment arrangement according to the prior art comprises a wastewater sludge pipe channel 3 and ultrasonic rods 4, 5. In the alternative wastewater sludge treatment arrangement according to the prior art the wastewater sludge is lead through the pipe channel 3. The wastewater sludge pipe channel 3 comprises radial horn type ultrasonic rods 4, 5, which ultrasonic rods 4, 5 generate micro- scopic cavitation bubbles. The cavitation bubbles produced by the ultrasonic rods 4, 5 reduce the particle size distribution in the bypassing wastewater sludge.

There are a lot of deficiencies in the current wastewater sludge treatment arrangements according to the prior art. In the current prior art wastewater sludge treatment arrangements the energy transfer of the cavitation systems is somewhat poor and inefficient. Furthermore, the power penetration of the prior art cavitation arrangements is often poor and insufficient, especially with sludge having high viscosity. Typically, in a wastewater sludge treatment arrangement according to the prior art, a large portion of the wastewater sludge on the outer rim of the pipe channel 1 flows through untreated.

Furthermore, one typical problem with the current wastewater sludge treatment arrangements according to the prior art is too frequently required maintenance. In the current prior art wastewater sludge treatment ar- rangements the surface material of the ultrasonic rods 2, 4, 5 corrodes easily due to extensive power levels at the surface of the ultrasonic rods 2, 4, 5, this causing a need for maintenance. In addition, the ultrasonic rods 2, 4, 5 may sometimes also cause cavitation erosion of the pipe channel 1 , this also requiring maintenance.

In wastewater sludge treatment arrangements according to the prior art the vibration elements, e.g. ultrasonic transducers are typically arranged in vibration element units. Typical prior art the vibration element units are described for example in patent documents GB 2,419877A, CN 101391822A and US 6361747B1 .

Figure 3 shows a vibration element unit of a wastewater sludge treatment arrangement according to the prior art. The wastewater sludge treatment arrangement according to the prior art comprises vibration elements 6-9, e.g. ultrasonic transducers 6-9. The vibration elements 6-9 pass the vibration through the duct wall on to the sludge flowing in the duct.

Figure 4 shows a vibration pattern of a vibration element unit according to the prior art. A typical vibration pattern of a prior art vibration element is marked with number 10. The typical vibration pattern 10 of a prior art vibration element comprises a lot of peaks and dips, which tells that the vibration is not effective in penetrating in the sludge, especially in sludge having high viscosity. In today's demanding wastewater treatment plant environment, the user is constantly demanding more efficient processes and longer life time of use. There is a clear demand in the market for a method and for an arrangement for wastewater sludge treatment in a wastewater sludge handling pro- cess that would be better and more efficient than the current prior art solutions.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is thus to provide a method and an arrangement for implementing the method so as to overcome the above problems and to alleviate the above disadvantages.

The objects of the invention are achieved by a method for wastewater sludge treatment in a wastewater sludge handling process, the method comprising the step of collecting wastewater sludge from a primary settling unit and/or a secondary settling unit, which method further comprises the step of directing a specified portion of the wastewater sludge to a cavitation treatment in one or more cavitation treatment units, in which cavitation treatment the wastewater sludge is lead through a pipe channel having a cross section of an essentially regular polygon, the pipe channel having vibration elements arranged essentially side-by-side and uniformly bonded to the wall on each side of the pipe channel, said vibration elements providing a mode of vi- bration, where essentially all of the side wall of the pipe channel vibrate with a uniform forth and back movement.

Preferably, in the step of directing, a portion of 5-95% of the wastewater sludge in a wastewater sludge mainstream pipeline is directed to the cavitation treatment. Alternatively, in step of directing, a portion of 20-45% of the wastewater sludge in a wastewater sludge mainstream pipeline is directed to the cavitation treatment.

Preferably, the cavitation treated sludge is returned back to the wastewater sludge mainstream pipeline directly from the cavitation treatment units. Alternatively, the cavitation treated sludge is returned back to the wastewater sludge mainstream pipeline from the cavitation treatment units via a collector tank.

Preferably, the specified portion of the wastewater sludge is directed to the cavitation treatment directly from the primary settling unit. Alternatively, the specified portion of the wastewater sludge is directed to the cavitation treatment directly from the secondary settling unit. Preferably, there is an addi- tional wastewater sludge thickening process applied before directing the wastewater sludge to the cavitation treatment.

Furthermore, the objects of the invention are achieved by an arrangement for wastewater sludge treatment in a wastewater sludge handling process, which arrangement has a pipe channel, through which pipe channel a specified portion of the wastewater sludge is lead through, which pipe channel has a cross section of an essentially regular polygon, and which pipe channel (1 1 ) has vibration elements (12-17), (18-21 ) arranged essentially side-by-side and uniformly bonded to the wall on each side of the pipe channel (1 1 ), said vibration elements (12-17), (18-21 ) providing a mode of vibration, where essentially all of the side wall of the pipe channel vibrate with a uniform forth and back movement.

Preferably, the cross section of the pipe channel is hexagon. Preferably, the vibration elements are piezoceramic ultrasonic transducers. Prefer- ably, the arrangement has and an ultrasonic generator, an vibration reactor comprising the vibration elements and an impedance adapter for adapting and optimizing the impedances of the ultrasonic generator output and vibration reactor input in commissioning stage, said impedance adapter being connected between said ultrasonic generator and said vibration reactor. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a wastewater sludge treatment arrangement according to the prior art;

Figure 2 shows an alternative wastewater sludge treatment arrangement according to the prior art;

Figure 3 shows a vibration element unit of a wastewater sludge treatment arrangement according to the prior art; and

Figure 4 shows a vibration pattern of a vibration element unit according to the prior art.

Figure 5 shows one embodiment of a wastewater sludge treatment arrangement according to the present invention;

Figure 6 shows a vibration element unit of a wastewater sludge treatment arrangement according to the present invention; and

Figure 7 shows a vibration pattern of a vibration element unit according to the present invention. Figure 8 shows a block diagram of one embodiment of a wastewater sludge treatment arrangement according to the present invention; and

Figure 9 shows a block diagram of one embodiment for implementing a wastewater sludge treatment arrangement according to the present in- vention in a wastewater treatment plant.

The prior art drawings of Figure 1 to 4 have been presented earlier. In the following, the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings of Figures 5 to 9. DETAILED DESCRIPTION OF THE INVENTION

Figure 5 shows one embodiment of a wastewater sludge treatment arrangement according to the present invention. The wastewater sludge treatment arrangement according to the present invention comprises a wastewater sludge pipe channel 1 1 and several ultrasonic transducers 12-17. In the wastewater sludge treatment arrangement according to the present invention the wastewater sludge is lead through the pipe channel 1 1 . The cross section of the pipe channel 1 1 is an essentially regular polygon. In Figure 5 there is presented a pipe channel 1 1 having a cross section of a hexagon. On each side of the pipe channel 1 1 there is an ultrasonic transducer 12-17 attached to the wall of the pipe channel 1 1 . The ultrasonic transducers 12-17 may be for example piezoceramic ultrasonic transducers 12-17.

The wastewater sludge pipe channel 1 1 of the wastewater sludge treatment arrangement according to the present invention comprises an ultrasonic transducer 12-17 on each side wall of the pipe channel 1 1 , which ultra- sonic transducers 12-17 generate microscopic cavitation bubbles. The cavitation bubbles produced by the ultrasonic transducers 12-17 reduce the particle size distribution in the bypassing wastewater sludge.

In the wastewater sludge treatment arrangement according to the present invention the ultrasonic transducers 12-17 on each side wall of the pipe channel 1 1 cause good energy transfer from the side walls to the wastewater sludge. The produced cavitation generates microscopic but very strong impacts at solid surfaces of the particles in the bypassing wastewater sludge. The pressures in the microscopic impacts may reach over 1000 bar and the temperatures in the microscopic impacts may reach several thousands of degrees Celsius. The produced cavitation effectively disintegrates flocks, cells and micro-organisms in the bypassing wastewater sludge. Furthermore, the very strong overpressure/under pressure vibration caused by the cavitation generates microscopic but aggressive turbulent streaming at the solid surfaces of the particles in the bypassing wastewater sludge. The strong pressure vibra- tion caused by the cavitation effect solid materials also direct. The mechanical forces caused by the pressure vibration expand and compress material on microscopic level resulting e.g. opening of pores on the particles in the bypassing wastewater sludge.

In addition to the better and more even energy transfer from the side walls to the wastewater sludge the ultrasonic transducers 12-17 of the wastewater sludge treatment arrangement according to the present invention have more effective power penetration in sludge having high viscosity. Furthermore, due to better and more even energy transfer to the wastewater sludge, the wastewater sludge treatment arrangement according to the present invention has also lower cavitation erosion and a longer life time of use.

Figure 6 shows a vibration element unit of a wastewater sludge treatment arrangement according to the present invention. The wastewater sludge treatment arrangement according to the present invention comprises vibration elements 18-21 , e.g. ultrasonic transducers 18-21 or piezoceramic (PZT) vibration elements 18-21 .

The wastewater sludge treatment arrangement according to the present invention has a vibration element unit arranged on each side wall of the pipe channel. The vibration elements 18-21 of the vibration element unit pass the vibration through the side wall of the pipe channel on to the sludge flowing in the pipe channel.

Each of the vibration element units are equipped with vibration elements 18-21 , e.g. piezoceramic (PZT) vibration elements 18-21 having a special construction. This special construction of the vibration elements 18-21 provides a mode of vibration, where all the side wall of the pipe channel vibrate with a uniform forth and back movement. The vibrating wall surface does not vibrate with numerous peaks or maximum vibration points and "dead" points - as is usual with conventional vibrating surface equipped with several PZT- elements.

In a vibration element unit of a wastewater sludge treatment ar- rangement according to the present invention the vibration elements 18-21 are arranged essentially side-by-side and are uniformly bonded to the vibrating wall surface i.e. the side wall of the pipe channel. The vibration elements 18-21 can be identical. The vibration elements 18-21 may be preheated prior bonding. There may be pressure applied to the vibration elements 18-21 when bonding.

Figure 7 shows a vibration pattern of a vibration element unit according to the present invention. The vibration pattern of a present invention vibration element is marked with number 22. The vibration elements 18-21 according to the present invention provide a mode of vibration, where all the side wall of the pipe channel vibrate with a uniform forth and back movement. This means, that the vibration pattern 22 passing through the vibrating wall surface is very effective in penetrating in the sludge, even in sludge having high viscosity. The mode of vibration according to the present invention improves the energy transfer from vibrating surface to the highly viscoid sludge generating therefore better cavitation in sludge.

The mode of vibration according to the present invention reduces dramatically cavitation erosion of the vibrating surface, e.g. the side wall of the pipe channel, which erosion is the main mechanism destroying the vibrating surface after certain period of time. This means longer life time for the vibration element units.

The arrangement for wastewater sludge treatment in a wastewater sludge handling process according to the present invention may also have an impedance adapter connected between the ultrasonic generator and the vibration reactor. The impedance adapter according to the present invention is used in commissioning stage. The impedance adapter adapts and optimizes the im- pedances of the ultrasonic generator output and vibration reactor input. When these two impedances are matched and essentially parallel, the energy transfer from generator to reactor is best possible.

The impedance adapter according to the present invention gives better efficiency to the whole wastewater sludge treatment arrangement. When the reactive power with good impedance match is close to zero, the generator, connecting cables and reactor are not heated by reactive power, which means better durability and fewer failures.

Figure 8 shows a block diagram of one embodiment of a wastewater sludge treatment arrangement according to the present invention. The wastewater sludge treatment arrangement according to the present invention comprises a mainstream pipeline 23 for the wastewater sludge, also some- times referred to as Waste Activated Sludge (WAS), Thickened Waste Activated Sludge (TWAS) or Surplus Activated Sludge (SAS). In the wastewater sludge treatment arrangement according to the present invention a certain specified portion of the wastewater sludge in the wastewater sludge main- stream pipeline 23 is directed to cavitation treatment pipeline 24. The portion of the wastewater sludge directed to cavitation treatment pipeline 24 may be 5- 95%, typically 20-45% of the wastewater sludge in the wastewater sludge mainstream pipeline 23. The cavitation treatment pipeline 24 leads the wastewater sludge to one or more cavitation treatment units 25, 26. In the cavi- tation treatment units 25, 26 the wastewater sludge is subjected to cavitation produced by the ultrasonic transducers of the wastewater sludge treatment arrangement according to the present invention. The cavitation treated sludge is returned back to the wastewater sludge mainstream pipeline 23 either directly from the cavitation treatment units 25, 26 or via a collector tank 27 as shown in the Figure 8.

In the wastewater sludge treatment arrangement according to the present invention the cavitation produced by the ultrasonic transducers breaks the microscopic organisms in the bypassing wastewater sludge. The broken microscopic organisms release nutrients to the wastewater sludge activating the anaerobic bacteria. The activated bacteria population generates a secondary process boosting the whole digestion process. Due to the effectiveness of the cavitation treatment process of the wastewater sludge treatment arrangement according to the present invention it is sufficient only to have a portion of the wastewater sludge directed to cavitation treatment, this portion being 5- 95%, typically 20-45% of the mainstream wastewater sludge. The cavitation treated portion of the wastewater sludge then boosts the digestion for the total mainstream wastewater sludge.

Figure 9 shows a block diagram of one embodiment for implementing a wastewater sludge treatment arrangement according to the present in- vention in a wastewater treatment plant. As the Figure 9 shows, in a simplified wastewater treatment plant the wastewater is then passed through a primary settling unit 28 and thereafter passed through a secondary settling unit 29. The wastewater sludge collected from the primary settling unit 28 and the secondary settling unit 29 is lead into a wastewater sludge mainstream pipeline 30.

In the wastewater sludge treatment arrangement according to the present invention a certain specified portion of the wastewater sludge in the wastewater sludge mainstream pipeline 30 is directed to a cavitation treatment pipeline 31 . Alternatively, a certain specified portion of the wastewater sludge can be directed to the cavitation treatment pipeline 31 directly from the primary settling unit 28 or from the secondary settling unit 29. In some cases there can be an additional wastewater sludge thickening process applied before directing the wastewater sludge to the cavitation treatment pipeline 31 . The cavitation treatment pipeline 31 leads the directed portion of the wastewater sludge to the wastewater sludge cavitation treatment unit 32 according to the present invention. After the cavitation treatment in the wastewater sludge cavitation treat- ment unit 32 the cavitation treated sludge is returned back to the wastewater sludge mainstream pipeline 30. Thereafter the wastewater sludge mainstream pipeline 30 forwards the wastewater sludge to the digestion units 33, 34 of the wastewater treatment plant. As an output the digestion units 33, 34 of the wastewater treatment plant produce biogas 35 and stabilized sludge 36. The biogas 35 may be utilized for production of heat and/or electricity as well as production of transport fuel through further refinement. The stabilized sludge 36 may be utilized e.g. in agriculture as soil conditioner.

The implementing of the wastewater sludge treatment arrangement according to the present invention in the wastewater treatment plant boosts the biogas production of the digestion units 33, 34 up to 40 %. Furthermore, the wastewater sludge treatment arrangement according to the present invention reduces the volume of the residual sludge by approximately 20 %.

The wastewater sludge treatment arrangement according to the present invention produces better and more even energy transfer from the side walls to the wastewater sludge. The wastewater sludge treatment arrangement according to the present invention has more effective power penetration in sludge having high viscosity. Furthermore, the wastewater sludge treatment arrangement according to the present invention has also lower cavitation erosion and a longer life time of use. With the help of the effectiveness of the wastewater sludge treatment arrangement according to the present invention only a portion of the total wastewater sludge needs to be directed to the cavitation treatment. When implemented in a wastewater treatment plant the wastewater sludge treatment arrangement according to the present invention boosts the biogas production and reduces the volume of the residual sludge.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The in- vention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.