WO2020044066A1

WO2020044066A1 - Wastewater sludge treatment system

Info

Publication number: WO2020044066A1
Application number: PCT/HU2018/000056
Authority: WO
Inventors: Iván RAISZ; lldikó SANDORNÉ RAISZ
Original assignee: Enviro-Pharm Kft.
Priority date: 2018-08-30
Filing date: 2018-12-12
Publication date: 2020-03-05
Also published as: DE212018000428U1; HU4957U

Abstract

The invention is a wastewater sludge treatment system comprising: - a pre-dryer (10) comprising a first sludge space (10A) receiving a sludge to be pre- dried and a first heating space (10B), wherein the first sludge space (10A) is connected to a first sludge conduit (A1) adapted for introducing the sludge to be pre-dried and to a second sludge conduit (A2) adapted for carrying off pre-dried sludge, the first sludge space (10A) being also connected to a first air inlet conduit (D1 ) adapted for introducing drying air and to a first air outlet conduit (E1) adapted for discharging the drying air, and wherein the first heating space (10B) is connected to a first flue gas inlet conduit (B1) adapted for introducing flue gas and to a first flue gas outlet conduit (C1) adapted for discharging the flue gas, - a post-dryer (11) comprising a second sludge space (11A) receiving a sludge to be post-dried and a second heating space (11B), wherein the second sludge space (11A) is connected to a third sludge conduit (A3) adapted for introducing the sludge to be post-dried and to a fourth sludge conduit (A4) adapted for carrying off post-dried sludge, the second sludge space (10B) being also connected to a second air inlet conduit (D2) adapted for introducing drying air and to a second air outlet conduit (E2) adapted for discharging the drying air, and wherein the second heating space (11B) is connected to a second flue gas inlet conduit (B2) adapted for introducing flue gas and a second flue gas outlet conduit (C2) adapted for discharging the flue gas, - a furnace (13), wherein the fourth sludge conduit (A4) is introduced into a combustion chamber (14) thereof. The system is characterised by - further comprising a diffuser (12) arranged between the pre-dryer (10) and the post- dryer (1 1), wherein the diffuser (12) is connected to the first sludge space (10A) via the second sludge conduit (A2) and to the second sludge space (1 1A) via the third sludge conduit (A3), and wherein the diffuser (12) is connected to a third air inlet conduit (D3) adapted for introducing drying air and to a third air outlet conduit (E3) adapted for discharging the drying air, and - the first air outlet conduit (E1) and the third air outlet conduit (E3) are introduced into the combustion chamber (14) of the furnace (13).

Description

WASTEWATER SLUDGE TREATMENT SYSTEM

TECHNICAL FIELD

The invention is related to a system for wastewater sludge treatment suited for disposing, recovering and utilizing municipal wastewater sludge.

BACKGROUND ART

The prior art includes a number of different methods and apparatuses suited for treating, disposing, recovering and utilizing municipal wastewater.

A multistage sludge drying apparatus is disclosed in US 6,588,349 B1 , wherein efforts are made to provide for air treatment in the dryer, aiming at reducing the volatile material content of the air. Condensate produced during the drying process is also treated by passing it through a separator. At the end of the drying process the moisture content of biomass stays at a value around 15-20%, which can result in toxic combustion products being generated during combustion because high moisture content allows only for reaching lower temperatures in the combustion chamber. External fuel is also required for carrying out the drying process.

In the apparatus according to the Chinese patent application CN 107200458 A rotary furnaces fitted with a shroud are applied for the drying and pyrolysis of sludge. A guide shaft can be disposed in the centre portion of the furnaces. Pyrolysis gases are passed through a dedusting cyclone, with vapours containing combustible material produced during the drying process being introduced into a combustion chamber. By introducing them into the shroud of the drying furnace, flue gases produced during combustion are utilized for heating and drying the sludge. Fans adapted for generating air flow are also applied in the system. As with the previous one, this technical solution also has the disadvantage that it requires complementary heating.

As with the above mentioned technical solutions, in the document KR101588386 B1 the combustion heat of sludge is applied for sludge drying. This technical solution has the disadvantage that the moisture content of the sludge can be reduced prior to incineration only by a small extent, and therefore the slag produced by the combustion process requires further treatment, for example needs to be melted in a sintering furnace. The vitrified slag produced at the end of the process is not suitable for application as a soil conditioner.

According to the document CH 540858 A, vapours produced during the sludge drying process are utilized - after admixing fresh air to them - as combustion air in the combustion chamber, and therefore drying air is circulated in a closed system. The apparatus also comprises a cyclone for cleaning (dedusting) air. This technical solution has the drawback that the process is not self-sustaining after startup, and so, in addition to the combustion heat of the sludge, it requires additional heating during the entire course of the process.

The document CN 207330683 U discloses a sludge drying apparatus comprising a rotary shaft. A fluid is introduced in counterflow into the shroud of the dryer apparatus for heating the sludge.

In the document US5273556 A a sludge heating apparatus comprising a shroud is also applied, wherein the sludge is urged forward by rotating helical plates disposed on a motor-driven shaft. The sludge is heated up by steam introduced in counterflow into the shroud of the apparatus, the steam being subsequently released from the apparatus as condensate. During the process, carbon-based waste material is admixed to the sludge to be treated, and the resulting mixture is incinerated.

Hungarian patent with registration number HU 229745 discloses a method wherein electric energy is generated by drying and subsequently incinerating municipal wastewater sludge. In the course of the process, the sludge is dried in multiple stages, in a pre-heater comprising a shaftless transport screw, a pre-dryer and a dryer, the energy generated by combustion of the sludge thus produced (having a dry matter content of 95-99%) being utilized, on the one hand, for sludge drying, and on the other hand, for electric energy generation. The drawback of the method is that it has poor value for money, because the implementation of electric energy generation involves high investment costs, and can be expediently applied only above a population equivalent of 100,000. A further drawback is that drying air introduced in the same direction as the sludge passes through the shaftless screw with a poor component- and heat transfer rate, so the system has reduced energy efficiency. DESCRIPTION OF THE INVENTION

In light of the known solutions the need has arisen for providing a system for wastewater sludge treatment that is self-sustaining after startup, can be applied for treating municipal wastewater sludge with reduced environmental impact, can be operated more efficiently compared to earlier technical solutions, and the residual products discharged from the system can be utilized in agriculture.

The primary objective of the invention is to provide a wastewater sludge treatment system that is adapted for self-sustained operation after being initially started up, i.e. the system utilizes the heat generated by sludge incineration for drying and heating sludge, while the residual heat of the system is also recovered to the greatest possible extent.

A further objective of the invention is to provide a system with minimal air pollution, where the byproducts produced in the system are harmless to the environment, and can even be utilized in agriculture.

By utilizing a diffuser in the drying process, the wastewater sludge treatment system according to the invention allows self-sustained processing of the wastewater sludge produced in wastewater treatment plants even with a capacity of a population equivalent of 10.000-30.000. We have recognised that by including a slow stirring stage, i.e. a so- called diffuser adapted for intensifying diffusion processes going on in the sludge the energy demand of the drying process can be drastically reduced, because the diffuser allows moisture contained inside the sludge particles to rise to the surface of the particles, from where it can be removed more efficiently, with a lower energy input. The advantages associated with the addition of the diffuser far exceed the drawbacks associated with a more complex system; utilizing a diffuser the system does not require any additional fuel after startup as it generates all the heat required for sludge drying by incinerating the dried sludge. It is also advantageous that the sludge can be processed at the location where it is produced, thus completely eliminating transport needs and the associated environmental impact.

The wastewater sludge treatment system according to the invention does not release any harmful materials or environmental pollutants during its operation, since during the sludge drying process drying air introduced together with the sludge is either combusted or utilized later for drying. Residual combustion products of wastewater sludge incineration and ash can be put to agricultural use as soil improvers or soil conditioners.

The objectives according to the invention have been fulfilled by providing a wastewater sludge treatment system defined in Claim 1 . Preferred embodiments of the invention are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below by way of example with reference to the following drawings, where

Fig. 1 shows the technical block diagram of an embodiment of the wastewater sludge treatment system according to the invention,

Fig. 2 illustrates the subsystem related to solid material flow of the embodiment according to Fig. 1 ,

Fig. 3 illustrates the subsystem related to the flow of drying air in the embodiment according to Fig. 1 ,

Fig. 4 shows a sectional drawing of the pre/post-dryer of the embodiment according to Fig. 1 , and

Fig. 5 is a diagram showing, as a function of temperature, the equilibrium moisture content of each kilogram of drying air applied for sludge drying.

MODES FOR CARRYING OUT THE INVENTION

In Fig. 1 there can be seen the technical block diagram of a preferred embodiment of the wastewater sludge treatment system according to the invention. Wastewater sludge is conveyed forward in the system for wastewater sludge treatment in multiple stages, applying a pre-dryer 10, a post-dryer 11 , and a diffuser 12 arranged between them.

The pre-dryer 10 comprises a first sludge space 10A and a first heating space 10B; preferably the first heating space 10B encompasses the first sludge space 10A like an outer shell. The first sludge space 10A is connected to a first sludge conduit A1 adapted for introducing sludge to be pre-dried, and to a second sludge conduit A2 adapted for discharging pre-dried sludge. To provide for efficient drying, drying air is made to flow through the first sludge space 10A in the same direction as the sludge, and therefore a first air inlet conduit D1 and a first air outlet conduit E1 is connected to the first sludge space 10A, respectively for introducing and discharging the drying air. The first heating space 10B is connected to a first flue gas inlet conduit B1 and to a first flue gas outlet conduit C1 , through which a heating medium, preferably hot flue gas produced by incinerating the dried sludge can be made to flow such that sludge and the drying air can be heated inside the first sludge space 10A by the heating medium. To increase drying efficiency, the direction of flow of the heating medium in the first heating space 10B is opposite the flow direction of the sludge in the first sludge space 10A. The heating medium that cools off and partially condenses due to the heat exchange is discharged from the first heating space 10B via the first flue gas outlet conduit C1.

The post-dryer 1 1 comprises a second sludge space 1 1A and a second heating space 1 1 B and has the same configuration as the pre-dryer 10, i.e. the second heating space 11 B encompasses the second sludge space 11A as an outer shell. The second sludge space 11A is connected to a third sludge conduit A3 adapted for introducing sludge to be post-dried, and to a fourth sludge conduit A4 adapted for discharging post-dried sludge. To provide for efficient drying, the drying air is made to flow in the same direction as the sludge also in the second sludge space 1 1A, wherein a second air inlet conduit D2 and a second air outlet conduit E2, adapted respectively for introducing and discharging the drying air, is connected to the second sludge space 11 A. In the post- dryer 11 the second heating space 11 B is connected to a second flue gas inlet conduit B2 and a second flue gas outlet conduit C2, through which a heating medium, preferably hot flue gas produced by incinerating the dried sludge can be made to flow in the second heating space 1 1 B such that sludge can be dried to the desired final extent inside the second sludge space 11 A by the heating medium. Also, in the post-dryer 1 1 , the direction of flow of the heating medium in the second heating space 1 1 B is opposite the flow direction of the sludge in the second sludge space 11 A. The heating medium that cools off and partially condenses due to the heat exchange is discharged from the second heating space 11 B via the second flue gas outlet conduit C2.

Stirring screws 16A and 16B are arranged in the sludge space 10A of the pre-dryer 10 and in the sludge space 1 1A of the post-dryer 1 1A, respectively, with a first stirring screw 16A being arranged in the first sludge space 10A, and a second stirring screw 16B being arranged in the second sludge space 11A. It is important that the stirring screws 16A and 16B have shafts in order to prevent drying air from flowing at high velocity along the geometric axis of the pre-dryer 10 and the post-dryer 1 1. Sludge and drying air are agitated intensively by the stirring screws 16A and 16B both radially and in the longitudinal direction. The first stirring screw 16A and the second stirring screw 16B provide that in the pre-dryer 10 and the post-dryer 1 1 drying air comes into contact as effectively as possible with the heated walls of the first heating space 10B and the second heating space 1 1 B. The stirring screws 16A and 16B have variable rotational speed, which allows that the mass flow of the conveyed sludge can be adjusted to match current parameters, for example the current quantity of sludge to be dried, or the drying rate, etc.

Between the pre-dryer 10 and the post-dryer 11 a diffuser 12 is arranged, wherein the diffuser 12 is connected to the first sludge space 10A via the second sludge conduit A2 adapted for introducing pre-dried sludge, and to the second sludge space 1 1A via the third sludge conduit A3 adapted for discharging sludge to be post-dried. In the diffuser 12 the introduced pre-dried sludge is dried further by the circulation of drying air. The diffuser 12 is connected to a third air inlet conduit D3 adapted for introducing the drying air, and to a third air outlet conduit E3 adapted for discharging the drying air. In the diffuser 12 the drying air flows in a direction opposite the flow direction of the sludge, the flow rate of the drying air preferably being around 1-5 m³/h, more preferably around 2 m³/h. In the diffuser 12 the drying air is also applied for preventing the pre-dried sludge from agglomeration.

The diffuser 12 is preferably a vertically oriented cylindrical space with approximately identical diameter and height. A stirrer 28, preferably a stirrer 28 with its shaft set vertically, is arranged in the diffuser 12 with the purpose of slowly stirring the sludge that has been introduced into the diffuser 12 from the pre-dryer 10 from the top, wherein the sludge flows under the effect of gravity. Along the shaft of the stirrer 28 there are arranged horizontal support beams in 1-4 rows, preferably in 2 rows, with vertically arranged inclined stirring plates adapted for stirring the sludge in the diffuser 12 being arranged on them. The diffuser 12 allows that moisture still held inside sludge particles after moisture has already evaporated in the pre-dryer 10 from the capillaries located between the sludge particles and from the surface of the sludge particles can diffuse to the surface or near the surface. The diffuser 12 provides that enough time, preferably approximately 20-80 minutes, more preferably 60 minutes, is allowed for diffusion processes to complete. The rotation speed of the stirrer 28 has to be adjusted according to the time required by the diffusion processes. The rotation speed of the stirrer 28 is preferably 0.5-4 rotations per minute, more preferably 1 rotation per minute. The application of the diffuser 12 equipped with a stirrer 28 facilitates energy-efficient drying in the post-dryer 11 because it allows for drying the sludge to a given dry matter content also utilizing a post-dryer 1 1 that is shorter than the pre-dryer 10.

The dried sludge is incinerated in a furnace 13 comprising a combustion chamber 14 and a stirring space 15. The furnace 13 is preferably a dust furnace. The post-dried sludge (that preferably has a dry matter content of 95-99%) can be introduced into the furnace 13 via a fourth sludge conduit A4 interconnecting the post-dryer 1 1 and the furnace 13. The air required for combustion is provided, on the one hand, as fresh air through a third fresh air conduit F3, and, on the other hand, as drying air that is discharged from the sludge space 10A of the pre-dryer 10 and from the diffuser 12 and contains water vapour and optionally combustible and/or volatile materials. The drying air is fed to the furnace 13 from the pre-dryer 10 through the first air outlet conduit E1 , and from the diffuser 12 through a third air outlet conduit E3. In the stirring space 15 of the furnace 13 the flue gas produced by combustion (having a temperature of around 850 °C), can mix with the drying air and fresh air introduced therein, so before being discharged from the furnace 13, the hot flue gas can cool down to a temperature of preferably around 400 °C, while its flow rate increases. When the system is started up, a combustible gas adapted for facilitating combustion can also be introduced into the furnace 13 via a first auxiliary conduit G1 , by way of example from a gas supply network or a PB gas bottle connected to the furnace 13; this gas is however not required any more after the system has been started because self-supporting operation can be provided utilizing wastewater sludge only. The furnace 13 is optionally also provided with an internal cyclone 17A adapted for separating dust and ash produced during the combustion process that is conveyed from the furnace 13 into the ash and dust storage unit 18 through a first ash conduit H1.

Hot flue gas (cooled down to a temperature of 400°C) discharged from the furnace 13 is utilized in the system during the sludge drying process for heating the heating space 10B of the pre-dryer 10 and the heating space 1 1 B of the post-dryer 1 1. For discharging flue gas, a first flue gas inlet conduit B1 is included between the furnace 13 and the pre-dryer 10, and a second flue gas inlet conduit B2 is included between the furnace 13 and the post-dryer 11. Preferably, a dedusting external cyclone 17B is arranged in the common section of the flue gas inlet conduits B1 and B2 for filtering out the residual solid material content of the flue gas, in order to protect the components of the system - including, by way of example, the flue gas inlet conduits B1 and B2 and the heating spaces 10B and 1 1 B - from damage. Solid materials are conveyed from the external cyclone 17B to the ash and dust storage unit 18 via a second ash conduit H2.

For pre-heating the drying air conveyed through the pre-dryer 10, the post-dryer 1 1 and the diffuser 12 arranged between them, the system preferably comprises a first calorifier 19A and a second calorifier 19B. Because the cooled-down flue gas exiting the post dryer 1 1 still has sufficient heat content for heating the drying air, a second flue gas outlet conduit C2 is introduced into a heat transfer space of the first calorifier 19A. The flue gas cooled further down due to heat exchange is discharged from the heat transfer space of the first calorifier 19A via the third flue gas outlet conduit C3. A first fresh air conduit F1 adapted for introducing the drying air to be heated is connected to a heat absorption space of the first calorifier 19A, the heated-up drying air being conveyed from the heat absorption space to the first sludge space 10A via the first air inlet conduit D1. The first calorifier 19A is therefore adapted for heating fresh air recovering the residual heat of the heating space 1 1 B of the post-dryer 1 1 , which air is then utilized as drying air in the sludge space 10A of the pre-dryer 10.

The second calorifier 19B is adapted for recovering the heat of the drying air of the post dryer 1 1 and thereby heating fresh air for producing the drying air for the post-dryer 1 1 and the diffuser 12. A heat transfer space of the second calorifier 19B is connected to the second sludge space 1 1A - where the heated drying air utilized by the calorifier originates - via a second air outlet conduit E2. The drying air cooled down due to heat exchange in the second calorifier 19B is discharged from the heat transfer space of the second calorifier 19B via the fourth air outlet conduit E4. A second fresh air conduit F2 adapted for introducing drying air to be heated is connected to a heat absorption space of the second calorifier 19B, the heated drying air being conveyed from the heat absorption space to the second sludge space 1 1 A via the second air inlet conduit D2 and to the diffuser 12 via the third air inlet conduit D3.

The conduits exiting the heat transfer spaces of the calorifiers 19A and 19B, i.e. the third flue gas outlet conduit C3 and the fourth air outlet conduit E4, respectively, are connected to the first flue gas outlet conduit C1 exiting the first heating space 10B of the pre-dryer 10, and in the embodiment illustrated in the figure are jointly connected to a separator 20. Water produced in the system by condensing water vapour is separated in the separator 20 and is discharged from the system via a second auxiliary conduit G2.

For extracting gaseous materials from the heat transfer spaces of the calorifiers 19A and 19B the system preferably comprises a first fan 21A that can be included downstream of the separator 20, and causes the gaseous materials discharged from the system to flow through a third auxiliary conduit G3 towards a chimney. By preventing liquid from entering the first fan 21A through separating it from the gas flow, the separator 20 protects the first fan 21A from damage resulting from hydraulic shock. Optionally, a dust separator, preferably a filter, adapted for filtering dust from the gaseous materials discharged from the system can also be included in the third auxiliary conduit G3.

For increasing the efficiency of circulating the drying air of the pre-dryer 10 and the post- dryer 1 1 , the system preferably comprises a second fan 21 B and a third fan 21 C that are adapted for sucking the drying air through the first sludge space 10A and the second sludge space 1 1 A, respectively. The second fan 21 B is arranged in the first air outlet conduit E1 , while the third fan 21 C is arranged in the second air outlet conduit E2. The second fan 21 B and the third fan 21 C are adapted for maintaining a slightly reduced pressure in the first sludge space 10A and in the second sludge space 11 A, respectively, and thereby prevent the contaminant components and the volatile material content of the drying air and the sludge from escaping from the system through gaps potentially present at pipe joints.

In the embodiment illustrated in the figure, a dust separator 22 that is preferably made of filter fabric and is adapted for preventing the post-dried sludge from being carried forward into the second air outlet conduit E2 is also included in the second air outlet conduit E2 exiting the second sludge space 1 1A.

For status monitoring purposes the system for wastewater sludge treatment preferably comprises at least one temperature sensor 25. Optionally, a first temperature sensor 25A is connected to the combustion chamber 14 of the furnace 13, and/or a second temperature sensor 25B is connected to the external cyclone 17B, and/or a third temperature sensor 25C is connected to the first flue gas outlet conduit C1 exiting the heating space 10B of the pre-dryer 10, and/or a fourth temperature sensor 25D is connected to the second flue gas outlet conduit C2 exiting the heating space 1 1 B of the post-dryer 1 1 , and/or a fifth temperature sensor 25E is connected to the first air inlet conduit D1 entering the sludge space 10A of the pre-dryer 10, and/or a sixth temperature sensor 25F is connected to the second air inlet conduit D2 entering the sludge space 1 1 A of the post-dryer 11 , and/or a seventh temperature sensor 25G is connected to the dust separator 22. The first temperature sensor 25A and the second temperature sensor 25B are preferably thermocouples, more preferably thermocouples with a measurement range up to 1000 °C. The temperature sensors 25C-25G are preferably resistance thermometers, more preferably Pt1000 resistance thermometers.

Fig. 2 illustrates the subsystem related to solid material flow of the embodiment of the wastewater sludge treatment system shown in Fig. 1. The pre-dryer 10 is adapted for receiving raw, sedimented and/or pretreated sludge via the first sludge conduit A1 that comprises a transport screw. The mass flow rate of the sludge to be pre-dried is preferably about 150 kg/h. The sludge to be pre-dried can also be pretreated, preferably by a pre-dehydration unit 23 that is connected upstream of the pre-dryer 10 and can be applied for increasing the dry matter content of the sludge to be pre-dried, preferably to a dry matter content of at least 20-25%. The pre-dehydration unit 23 can be implemented, by way of example, as a filter, pressure filter, or centrifuge. The pre-dried sludge is conveyed, via a second sludge conduit A2, from the pre-dryer 10 into a diffuser 12 included between the pre-dryer 10 and a post-dryer 1 1 ; the second sludge conduit A2 also comprises a transport screw, and the mass flow rate of the pre-dried sludge conveyed via the second sludge conduit A2 is preferably about 120 kg/h. In the diffuser 12 the sludge is stirred by a vertical-shaft slow stirrer 28 such that there is sufficient time for moisture contained inside the sludge particles to rise to the surface of the particles. In the diffuser 12 the sludge preferably flows under gravity, with the diffuser 12 being preferably joined - at the bottom of the diffuser 12, at the centreline of the cylindrical body of the diffuser 12 - to the transport screw of the third sludge conduit A3 adapted for conveying the sludge to be post-dried into the post-dryer 11. Preferably, level sensors 27A and 27B are arranged in the third sludge conduit A3 for monitoring the quantity of sludge to be post-dried and for sending the measurement results to a control unit, by way of example a PLC. The transport screw arranged in the third sludge conduit A3 is started by the signal of the level sensors 27A and 27B after the sludge has been held in the diffuser 12 for a time period sufficient for carrying out the diffusion process. The slow stirring of the sludge carried out by the diffuser 12 allows moisture contained in the sludge particles to diffuse towards the surface, so the sludge to be post-dried can be dried to the required extent in the post-dryer 11 in a shorter time and with lower energy consumption, and therefore optionally a post-dryer 1 1 that is smaller (by way of example, shorter) than the pre-dryer 10 can be sufficient for performing its required function in the system. The post-dried sludge exiting the post-dryer 1 1 preferably at a mass flow rate of about 30 kg/h is conveyed to the furnace 13 by the transport screw of the fourth sludge conduit A4. The fourth sludge conduit A4 is preferably provided with level sensors 27C and 27D that are applied for monitoring the quantity of post-dried sludge and sending the measurement data to the control unit.

For adsorbing sulphur compounds produced during the combustion of sulphur compounds contained in the dry sludge, a limestone dust feeder 24 is optionally connected to the fourth sludge conduit A4 via a fourth auxiliary conduit G4. The post- dried sludge is mixed with the fine-grained limestone dust and is conveyed to the furnace 13 by the transport screw of the fourth sludge conduit A4. Sulphur oxides produced in the furnace 13 during combustion from the sulphur compounds of the wastewater sludge are adsorbed by the limestone dust, producing calcium sulphate that can be well utilized by plants. Combustion residues containing calcium sulphate and ash are separated by the dedusting internal cyclone 17A of the furnace 13, and, via the first ash conduit H1 , fed into the ash and dust storage unit 18 where they are collected. Ash and dust escaping from the furnace 13 together with the flue gas are separated by the dedusting external cyclone 17B, and, via the second ash conduit H2, also fed into the ash and dust storage unit 18 for collection. Thanks to its calcium sulphate content, the material held in the ash and dust storage unit 18 can be applied in agriculture as soil conditioner and soil improver.

Fig. 3 illustrates the subsystem related to drying air flow of the embodiment of the system for wastewater sludge treatment shown in Fig. 1 . The drying air fed into the pre-dryer 10 and into the post-dryer 1 1 is heated up by the first calorifier 19A and the second calorifier 19B, respectively.

Utilizing the residual heat of the flue gas that has cooled off during the heating of the sludge and has been discharged from the post-dryer 1 1 , the first calorifier 19A is applied for producing warm drying air from fresh air introduced via the first fresh air conduit F1 into the first calorifier 19A. Heated drying air is fed into the pre-dryer 10 via the first air inlet conduit D1 in order that it can partially absorb the moisture content of the sludge. The flow of heated drying air is maintained by a second fan 21 B included in the first air outlet conduit E1 , with the flue gas - further cooled down in the first calorifier 9A - is fed to the separator 20 via the third flue gas outlet conduit C3, where the moisture contained by the flue gas is separated.

Utilizing the heat of the drying air heated in the post-dryer 11 during the drying process, the second calorifier 19B produces fresh drying air for the post-dryer 11 from fresh air introduced into the second calorifier 19B via the second fresh air conduit F2. Heated-up drying air is brought from the second calorifier 19B into the post-dryer 11 via the second air inlet conduit D2, from where the heated drying air returns into the second calorifier 19B through the second air outlet conduit E2. For circulating the drying air of the post- dryer 1 1 a second fan 21 B is included in the second air outlet conduit E2, optionally complemented by a dust separator 22 adapted for separating contaminants contained in the drying air. The drying air cooled down due to heat exchange is discharged from the second calorifier 19B and is conveyed via the fourth air outlet conduit E4 to the separator 20. A first fan 21 A, disposed in the third auxiliary conduit G3 downstream of the separator 20, is applied for removing medium from the heat transfer space of the calorifiers 19A and 19B, and for subsequently discharging it from the system, i.e. for urging it towards a chimney. It is not shown in Fig. 3 that the drying air heated in the second calorifier 19B is introduced, via a third air inlet conduit D3, also into the diffuser 12.

The pressure of air, flue gas and vapours flowing in the system for wastewater sludge treatment is preferably monitored by at least one pressure sensor 26 connected to the system. Optionally, a first pressure sensor 26A is connected to the first air outlet conduit E1 and/or a second pressure sensor 26B is connected to the second flue gas outlet conduit C2 and/or a third pressure sensor 26C is connected to the second air outlet conduit E2 and/or a fourth pressure sensor 26D is connected to the third auxiliary conduit G3 exiting the separator 20.

During optimal operation of the system the pressure conditions in the subsystem according to Fig. 3 are the following: Both the second fan 21 B and the third fan 21 C are adapted for maintaining a pressure difference of preferably about D p = 2450 Pa between the input and delivery sides of the fans 21 B/21 C for circulating drying air, as a result of which a pressure difference of about D p = 1250 Pa occurs in the pre-dryer 10, with a pressure difference of about D p = 1050 Pa occurring in the post-dryer 1 1. In the heat absorption space of the calorifiers 19A/19B the pressure difference is approximately Dr = 80 Pa, while in the heat transfer space it is approximately Dr = 300 Pa. The pressure difference between the two sides of the dust separator 22 is approximately Dr = 250 Pa, while a pressure difference of approximately D p = 80 Pa occurs in the separator 20.

In Fig. 4 a sectional view of the pre-dryer 10 and post-dryer 1 1 is shown. As it was described in detail above in relation to Fig. 1 , the pre-dryer 10 and the post-dryer 1 1 have an identical configuration, having a length of about 2-4 m. Both the pre-dryer 10 and the post-dryer 1 consist of two spatial regions, one encompassed by the other, preferably of two concentric cylinders, where the outside-laying spatial region is the heating space 10B/1 1 B, and the inside one is the sludge space 10A/1 1 A. In the sludge spaces 10A/1 1 A there are arranged stirring screws 16A/16B adapted for urging forward the sludge; the rotational speed of the screws can be adjusted based on the quantity of the sludge to be processed, the moisture content of the sludge, and the targeted drying rate. The rotational speed is preferably 3-8 rpm; more preferably, in the case of a system with a processing capacity of 1000t/year the rotational speed required for processing sludge with a moisture content of approximately 20% it is 6 rotations per minute. The sludge is introduced into the pre-dryer 10 and into the post-dryer 1 1 via the first sludge conduit A1 and the third sludge conduit A3, respectively; and for discharging the sludge, a second sludge conduit A2 and a sludge conduit A4 is connected, respectively, to the sludge space 10A of the pre-dryer 10 and to the sludge space 1 1A of the post-dryer 1 1. The drying air is introduced into the pre-dryer 10 and into the post-dryer 1 1 (via the first air inlet conduit D1 and the second air inlet conduit D2, respectively) parallel, co-current to the sludge, but at a different velocity and flow rate, while it is discharged via the air outlet conduits E1/E2. Through the flue gas inlet conduits B1/B2 the heating spaces 10B/1 1 B receive the hot flue gas that was previously discharged from the furnace 13 and cooled preferably to a temperature of approximately 400 °C; in the heating spaces 10B/1 1 B the flue gas flows in the opposite direction relative to the flow direction of the sludge and the drying air in the sludge spaces 10A/1 1A. The flue gas introduced into the heating spaces 10B/1 1 B at a temperature of 400 °C has a high-velocity turbulent flow, and subsequently cools down to around 50-60 °C while undergoing partial condensation. In the sludge spaces 10A/1 1A, the flue gas introduced at a temperature of 400 °C transfers as much as 80% of its heat content to the moist sludge and drying air flowing in counter-current direction, thereby providing a sufficient heat exchange between the sludge spaces 10A/1 1A and the heating spaces 10B/1 1 B, while at this temperature pyrolysis of the drying sludge is not yet expected to occur. A helical guide member adapted for facilitating the flow of the flue gas may also be disposed in the heating spaces 10B/11 B. The sludge is urged forward and powerfully agitated by variable-speed stirring screws 16A/16B arranged in the sludge spaces 10A/11A, while drying air flow is provided applying fans 21 B/21 C (Fig. 1 ) disposed in the air outlet conduits E1/E2. Depending on the moisture content of the sludge and the flow rate of the drying air, the space filling ratio in the sludge spaces 10A/11A varies between 1/8 and 1/4, more preferably, the space filling ratio is approximately around 1/6. Powerful agitation of the sludge and drying air allows for sufficient heat exchange along the boundary surface of the sludge spaces 10A/1 1A and the heating spaces 10B/1 1 B.

In the sludge spaces 10A/1 1 A the temperature of both the drying air and the sludge rises in the direction of flow, however, due to the different heat capacity of the flowing mediums, the temperature of the drying air rises at a higher rate relative to the temperature of the sludge.

The hot flue gas may still contain fine-grained dust even after multiple filtering (see the dedusting cyclones 17A and 17B in relation to Fig. 1 ), which may get deposited on the heat transfer surfaces, i.e. the boundary surface between the sludge spaces 10A/1 1A and the heating spaces 10B/1 1 B, deteriorating heat transfer efficiency. To prevent this, the housing of the heating spaces 10B/1 1 B and/or a portion thereof can be opened and/or comprises a door through which the heat transfer surface can be cleaned.

The temperature conditions of the drying process are presented by way of example in relation to the pre-dryer 10. In the pre-dryer 10 the transport screw of the first sludge conduit A1 introduces municipal wastewater sludge into the first sludge space 10A, the sludge having high moisture content (containing even 75-80% of moisture) and a maximum temperature of around 20 °C, the mass flow rate thereof being 50-150 kg/h, preferably 150 kg/h. The drying air having a temperature of preferably 75 °C is introduced into the first sludge space 10A via the first air inlet conduit D1 parallel, co-current to the sludge, the drying air cooling off upon contacting the sludge. The minimum temperature - approximately 42 °C - of the drying air is reached at 2/5 of the length of the first sludge space 10A. After that, the air drying heats up, under the effect of the flue gas introduced into the heating space 10B of the pre-dryer 10 at a temperature of approximately 400 °C in a direction opposite the flow direction of the sludge and the drying air, to a temperature of approximately 80 °C before reaching the first air outlet conduit E1 exiting the first sludge space 10A, at which point it contains as much as 30% of water vapour. The temperature of pre-dried sludge exiting the pre-dryer 10 is approximately 70 °C.

Fig. 5 shows, as a function of temperature, the amount of water taken in by 1 kg of air at equilibrium in the pre-dryer 10 and post-dryer 11 of the system according to the invention. Numeric values of the data points visualised in the diagram are included in Table 1 , where temperature values are shown in column 1 ( t , in °C units); column 2 showing the equilibrium pressure of water vapour (p_v, in kPa); column 3 showing the pressure difference between water vapour pressure and the equilibrium water vapour pressure at a given temperature (D p, in kPa), i.e. the driving force of moisture removal; and finally column 4 including the amount of water (m_v, in kg) that can be taken up by 1 kg of air under equilibrium conditions. The moisture content of sludge enters the drying air as water vapour, the water vapour intake of drying air being determined by the equilibrium water vapour pressure of the drying air at the local temperature, and by the water vapour pressure determined by the local temperature of the sludge. In pre-dryer 10 and post-dryer 11 of the system the temperature of the sludge (and also the temperature of drying air flowing in the same direction) rises in the direction of flow of the sludge, the air containing moisture at the equilibrium concentration corresponding to the rising temperature, so a significant amount of moisture can be removed from the sludge even applying lower drying air flow rates. Deviation from the equilibrium vapour pressure increases the driving force of moisture and component transfer. Transport of water vapour molecules from the sludge to the drying air is facilitated by the temperature of the sludge to be pre- or post-dried. Table 1 The table shows the temperature dependence of the equilibrium water vapour pressure, pressure difference (driving force), and the amount of water that can be taken up by 1 kg of air.

In the following, two examples illustrating the operation of the system for wastewater sludge treatment according to Fig. 1 will be presented for processing sludge introduced at respective mass flow rates of 50 kg/h and 100 kg/h.

Example 1 Example 1 relates to the processing of sludge introduced at a mass flow rate of 50 kg/h; the temperature and mass flow rate values, as well as the dry matter content of the post- dried sludge introduced into the furnace 13 are included in Table 2, with the results of individual measurements being included in separate rows of the table.

Data common to all measurements related to Example 1 are the following: Sludge feed rate through the first sludge conduit A1 , i.e. the frequency of the transport screw of the first sludge conduit A1 , f = 25.3 Hz.

The gas introduced into the furnace 13 via the first auxiliary conduit G1 upon the startup of the system is PB gas, having a mass flow of 1.5 Nm³/h, where Nm³/h refers to flow rate (m³/h) at standard conditions.

The flow rate of the flue gas (having a temperature of 400 °C) flowing in the first flue gas inlet conduit B1 and in the second flue gas inlet conduit B2 is 950 m³/h.

The flow rate of the drying air is 280 m³/h.

Table 2 A cumulative table of the measurement data of Example 1.

Example 2

Example 2 relates to the processing of sludge introduced at a mass flow rate of 100 kg/h; the temperature and mass flow rate values, as well as the dry matter content of the post- dried sludge introduced into the furnace 13 are included in Table 3, with the results of individual measurements being included in separate rows of the table.

Data common to all measurements related to Example 2 are the following: Sludge feed rate through the first sludge conduit A1 , i.e. the frequency of the transport screw of the first sludge conduit A1 , = 40 Hz.

The gas introduced into the furnace 13 via the first auxiliary conduit G1 upon the startup of the system is PB gas having a mass flow of 2.7 Nm³/h. The flow rate of the flue gas (having a temperature of 400 °C) flowing in the first flue gas inlet conduit B1 and in the second flue gas inlet conduit B2 is 2050 m³/h.

The flow rate of the drying air is 280 m³/h.

Table 3 A cumulative table of the measurement data of Example 2.

The mode of industrial application of the invention according to the above description follows from the features of the invention. As can be seen from the description above, the invention fulfills its objectives in an extremely preferable manner compared to the prior art. Legends

10 pre-dryer

10A (first) sludge space

10B (first) heating space

11 post-dryer

11 A (second) sludge space

11 B (second) heating space

12 diffuser

13 furnace

14 combustion space

15 stirring space

16A (first) stirring screw

16B (second) stirring screw

17A (internal) cyclone

17B (external) cyclone

18 ash and dust storage unit

19A (first) calorifier

19B (second) calorifier

20 separator

21 A (first) fan

21 B (second) fan

21 C (third) fan

22 dust separator

23 pre-dehydration unit

24 limestone dust feeder

25A-G temperature sensors

26A-D pressure sensors

27A-D level sensors

28 stirrer

A1 (first) sludge conduit

A2 (second) sludge conduit

A3 (third) sludge conduit A4 (fourth) sludge conduit

B1 (first) flue gas inlet conduit B2 (second) flue gas inlet conduit C1 (first) flue gas outlet conduit C2 (second) flue gas outlet conduit C3 (third) flue gas outlet conduit

D1 (first) air inlet conduit

D2 (second) air inlet conduit

D3 (third) air inlet conduit

E1 (first) air outlet conduit

E2 (second) air outlet conduit

E3 (third) air outlet conduit

E4 (fourth) air outlet conduit

F1 (first) fresh air conduit

F2 (second) fresh air conduit

F3 (third) fresh air conduit

G1 (first) auxiliary conduit

G2 (second) auxiliary conduit

G3 (third) auxiliary conduit

G4 (fourth) auxiliary conduit

H1 (first) ash conduit

H2 (second) ash conduit

Claims

1 . A wastewater sludge treatment system comprising:

- a pre-dryer (10) comprising a first sludge space (10A) receiving a sludge to be pre- dried and a first heating space (10B), wherein the first sludge space (10A) is connected to a first sludge conduit (A1) adapted for introducing the sludge to be pre-dried and to a second sludge conduit (A2) adapted for carrying off pre-dried sludge, the first sludge space (10A) being also connected to a first air inlet conduit (D1 ) adapted for introducing drying air and to a first air outlet conduit (E1) adapted for discharging the drying air, and wherein the first heating space (10B) is connected to a first flue gas inlet conduit (B1) adapted for introducing flue gas and to a first flue gas outlet conduit (C1 ) adapted for discharging the flue gas,

- a post-dryer (1 1) comprising a second sludge space (1 1A) receiving a sludge to be post-dried and a second heating space (11 B), wherein the second sludge space (11 A) is connected to a third sludge conduit (A3) adapted for introducing the sludge to be post-dried and to a fourth sludge conduit (A4) adapted for carrying off post-dried sludge, the second sludge space (10B) being also connected to a second air inlet conduit (D2) adapted for introducing drying air and to a second air outlet conduit (E2) adapted for discharging the drying air, and wherein the second heating space (11 B) is connected to a second flue gas inlet conduit (B2) adapted for introducing flue gas and a second flue gas outlet conduit (C2) adapted for discharging the flue gas,

- a furnace (13), wherein the fourth sludge conduit (A4) is introduced into a combustion chamber (14) thereof,

characterised by

- further comprising a diffuser (12) arranged between the pre-dryer (10) and the post dryer (1 1 ), wherein the diffuser (12) is connected to the first sludge space (10A) via the second sludge conduit (A2) and to the second sludge space (1 1A) via the third sludge conduit (A3), and wherein the diffuser (12) is connected to a third air inlet conduit (D3) adapted for introducing drying air and to a third air outlet conduit (E3) adapted for discharging the drying air, and

- the first air outlet conduit (E1) and the third air outlet conduit (E3) are introduced into the combustion chamber (14) of the furnace (13).

2. The system according to claim 1 , characterised in that a dedusting external cyclone (17B) is connected to the combustion chamber (14) of the furnace (13), and the external cyclone (17B) is connected to the first heating space (10B) of the pre-dryer (10) via the first flue gas inlet conduit (B1) and to the second heating space (1 1 B) of the post-dryer (1 1) via the second inlet flue gas inlet conduit (B2).

3. The system according to claim 2, characterised in that an ash and dust storage unit (18) is connected to the furnace (13) and/or to the external cyclone (17B) via an ash conduit (H1 , H2).

4. The system according to any of claims 1 to 3, characterised by comprising a first calorifier (19A) and a second calorifier (19B) that are adapted for pre-heating the drying air,

- the second flue gas outlet conduit (C2) is introduced into a heat transfer space of the first calorifier (19A), wherein the heat transfer space is connected to a separator (20) via a third flue gas outlet conduit (C3), and

- the first calorifier (19A) is connected to a first fresh air conduit (F1) adapted for introducing drying air to be heated to a heat absorption space of the first calorifier (19A), wherein heated drying air is conveyed from the heat absorption space to the first sludge space (10A) via the first air inlet conduit (D1);

- the second air outlet conduit (E2) is introduced into a heat transfer space of the second calorifier (19B), wherein the heat transfer space is connected to the separator (20) via a fourth air outlet conduit (E4), and

- the second calorifier (19B) is connected to a second fresh air conduit (F2) adapted for introducing drying air to be heated to a heat absorption space of the second calorifier (19B), wherein the heated drying air is conveyed form the heat absorption space to the second sludge space (11A) and to the diffuser (12) via the second air inlet conduit (D2) and the third air inlet conduit (D3), respectively.

5. The system according to claim 4, characterised by comprising a first fan (21A), wherein the first fan (21 A) is attached to the separator (20).

6. The system according to any of claims 1 to 5, characterised by comprising a second fan (21 B) and a third fan (21 C), wherein

- the second fan (21 B) is arranged in the first air outlet conduit (E1 ), and - the third fan (21 C) is arranged in the second air outlet conduit (E2).

7. The system according to claim 6, characterised in that a dust separator (22) is arranged in the second air outlet conduit (E2) between the second sludge space (11 A) and the third fan (21 C).

8. The system according to any of claims 1 to 7, characterised by comprising a pre dehydration unit (23) that is connected to the first sludge space (10A) via the first sludge conduit (A1).

9. The system according to any of claims 1 to 8, characterised in that a limestone dust feeder (24) is connected to the fourth sludge conduit (A4).

10. The system according to any of claims 1 to 9, characterised in that a first stirring screw (16A) is arranged in the first sludge space (10A), a second stirring screw (16B) is arranged in the second sludge space (11 A), and a stirrer (28) is arranged in the diffuser (12).