WO2017055623A9

WO2017055623A9 - A method and a system for hygienization of sewage sludge

Info

Publication number: WO2017055623A9
Application number: PCT/EP2016/073550
Authority: WO
Inventors: Pawel Potocki; Jaroslaw TUREK
Original assignee: Pawel Potocki; Turek Jaroslaw
Priority date: 2015-10-03
Filing date: 2016-10-03
Publication date: 2017-09-08
Also published as: PL414238A1; WO2017055623A1

Abstract

A method for hygienization of a sewage sludge with a burnt lime (CaO), comprising: providing a dewatered sewage sludge with a dry matter content from 15% to 35% by weight; performing a preliminary hydration process (13) in a granulator (40) with a shoveling system comprising two shafts (46) with shoveling arms (47) rotating in opposite directions, by mixing the granulated dewatered sewage sludge with the burnt lime (CaO) in a ratio from 0,7 to 1,8 kg of CaO per 1 kg of the dry matter in the dewatered sewage sludge while maintaining the sewage sludge in a granular form to prevent the sludge sewage to homogenize; and next performing a main hydration process (15) in a belt reactor (50) to allow slaking of the burnt lime (CaO).

Description

A METHOD AND A SYSTEM FOR HYGIENIZATION OF SEWAGE SLUDGE

TECHNICAL FIELD

The present disclosure relates to a method for hygienization of sewage sludge, in particular via a unique method of granulation, and a system for hygienization of sewage sludge, in particular using active quicklime (CaO).

BACKGROUND

Waste sewage sludge, which is a by-product of sewage purification, may be contaminated by various pathogens such as: bacteria, viruses or endospore forms of some parasites of gastrointestinal tract. The pathogens contained in raw sewage waste may, in soil conditions, retain activity for a long time, for a few years.

One known method of killing pathogens contained in sewage sludge is hygienization, which involves application of strongly acidifying or alkalizing substances to kill pathogenic microorganisms, parasites and various endospore forms. This is due to ionization - in conditions of a high or low pH, various protein components, in particular of carboxylic groups - which influences the change of protein structure, and in consequence declines the enzymatic activity of microbes.

The chemicals currently used in hygienization process are: chlorine and its compounds, including chlorinated lime, lime, burnt lime: CaO (also called quicklime), which as a result of a strongly exothermic reaction with water convert into slaked lime (Ca(OH)₂ - calcium hydroxide) with an increase in ambient temperature to about 50 - 60 °C. Limewater is a by-product of a process of lime slaking, which, although short- living, shows a strongly toxic activity with respect to various microorganisms.

The drawback of utilization of the burnt lime in hygienization process is a lack of possibility to control the process conditions: CaO has a clumped form and is hardly soluble in water, which precludes maintaining of a constant pH level in the whole volume of the sanitized sewage sludge. In order to complete sewage sludge disinfection, it is important to keep accurate parameters of the hygienization process, namely: maintaining a high temperature of the sludge-lime mixture in the range from 55 to 70 °C, for at least 24 hours, maintaining a high pH of at least 12,5 in the initial process phase, providing thermal insulation of the devices, maintaining an intensive mixing of the sewage sludge with lime, supplying a properly dehydrated sludge and selecting an appropriate quality and dosage of burnt lime.

Hygienization process using burnt lime typically involves concentration and dehydration of sewage sludge by means of mechanical devices and subsequent mixing of the sludge with CaO, in order to carry out the burnt lime slaking process. A final product, i.e. the hygienized sewage sludge is, typically, used as an agricultural fertilizer.

The amount of the burnt lime and water necessary to increase the pH value and temperature in the slaking reaction, in order to heat the sewage sludge, evaporate moisture and obtain a desired amount of burnt lime, may be selected according to charts available in the literature or it may be calculated from known equations. Furthermore, the technical literature describes that the dosages of burnt lime (CaO) depend on: required hygienization level, sewage sludge type, concentration of dry matter in the sludge and sludge alkalinity.

It should be noted that slaking process including the reaction of burnt lime hydration is a strongly exothermic process, which can result in a significant increase of a temperature of the reaction mixture, even over 100 °C. The slaking process requires providing a water:lime ratio of 3:1 - so as to obtain a product having a consistency of a dough.

A lime hydration process is a process in which the reaction of lime does not occur in the whole volume of the reactants mass, which can be seen as a slight increase of the reactants temperature. The hydration process is relatively short and it occurs only in conditions of significant excess of water, wherein the ratio of water:reactant should be higher than 3:1 . A visible result of the hydration process is a product containing lime in a white powder form.

In the known sewage sludge hygienization processes, only the slaking process is carried out. The hydration process is not conducted.

There are known various methods of sewage sludge hygienization.

A Polish patent PL207095 describes a method for hygienization of municipal sewage and coke sewage, wherein a sewage sludge with a hydration of 97% and a temperature of 25 °C is intensively blended with an ammoniac gas liquor containing sodium phenolates, including sodium cresolates, ammonium chloride and aqueous solution of a polymer, and next the obtained mixture is subject to sedimentation. Subsequently, the obtained sludge is dehydrated, while the liquid which constitutes a waste product of the process is subject to a biological purification process together with sullages.

A Polish patent PL213040 describes a method for hygienization of sewage sludge, wherein sewage sludge containing 35% by weight of dry matter is mixed with a burnt lime and a mixture of natural sorbents obtained from basaltic saprolite in a two-stage digestion process, using sodium hydroxide.

A Polish patent application PL290773 describes a method for hygienization of sludge, wherein dehydrated sewage sludge with a hydration of 65-85% is mixed with a powdered burnt lime in ratio of 0,2 - 1 ,5 kg CaO per 1 kg of sludge dry matter, then it is kept for 12 to 16 hours, and then the mixture is dried for 3-7 days. A system for sewage sludge hygienization comprises a mixing container connected with a drainage device and lime container and a reactor.

The known hygienization methods based on the process of burnt lime slaking do not provide full control of the technological parameters of the hygienization, including the control of the pH value and the temperature in the whole volume of the sewage sludge, which can lead to incomplete sanitization of the sewage sludge.

Therefore, there is a need to modify sewage sludge hygienization process that involves burnt lime slaking reaction, to provide improved control of parameters of the hygienization process, including the pH and the temperature, in the whole volume of the hygienized sewage sludge. There is also a need to provide a system for sewage sludge hygienization which will facilitate the hygienization process and enable to control the process conditions.

SUMMARY

There is disclosed a method for hygienization of a sewage sludge with a burnt lime (CaO), comprising: providing a dewatered sewage sludge with a dry matter content from 15% to 35% by weight; performing a preliminary hydration process in a granulator with a shoveling system comprising two shafts with shoveling arms rotating in opposite directions, by mixing the granulated dewatered sewage sludge with the burnt lime (CaO) in a ratio from 0,7 to 1 ,8 kg of CaO per 1 kg of the dry matter in the dewatered sewage sludge while maintaining the sewage sludge in a granular form to prevent the sludge sewage to homogenize; and next performing a main hydration process in a belt reactor to allow slaking of the burnt lime (CaO). The shafts of the granulator may rotate with a speed of 30 to 80 rpm.

The method may comprise performing the preliminary hydration process in batches, until the temperature of the mixture increases to not more than 31 °C.

The method may comprise performing the preliminary hydration process continuously.

The method may comprise performing the preliminary hydration process for 60 to 240 seconds.

There is also disclosed a system for hygienization of a sewage sludge with a burnt lime (CaO), the system comprising: a granulator with a shoveling system comprising two shafts with shoveling arms rotatable in opposite directions, configured to, when the system is in use, perform a preliminary hydration process by mixing the granulated dewatered sewage sludge with the burnt lime (CaO) in a ratio from 0,7 to 1 ,8 kg of CaO per 1 kg of the dry matter in the dewatered sewage sludge while maintaining the sewage sludge in a granular form to prevent the sludge sewage to homogenize; a belt reactor configured to, when the system is in use, perform a main hydration process to allow slaking of the burnt lime (CaO).

The shafts of the granulator can be rotatable with a speed of 30 to 80 rpm.

The granulator may comprise a reaction chamber two reactant outlets (41 , 45), wherein first reactant outlet is located at an upper section of the reaction chamber and the second reactant outlet is located at the bottom of the reaction chamber and can be opened and closed.

The belt reactor may comprise a reaction chamber with a transport belt of a controllable speed, wherein the transport belt is inclined at an angle of up to 45 degrees.

The granulator may comprise a vapor outlet for transportation of the vapors released during the preliminary hydration process from the reaction chamber of the granulator to a water-acid scrubber.

The belt reactor may comprise a ventilation system for removing vapors released during the slaking process.

The elements of the system can be mounted in movable containers.

BRIEF DESCRIPTION OF DRAWINGS

The present disclosure is presented by way of examples on a drawing, in which: Fig. 1 presents a flowchart of a sewage sludge hygienization process;

Fig. 2 presents schematically a system for sewage sludge hygienization;

Fig. 3 presents a chart showing dependency between the excess of the water content in the sewage sludge, and the volume and of amount of burnt lime slaking;

Fig. 4A presents schematically a transport feeder in an isometric view;

Fig. 4B presents schematically a sewage sludge transport feeder in an isometric view;

Fig. 5A presents schematically a granulator in a longitudinal cross-section;

Fig. 5B presents schematically the granulator in an isometric view;

Fig. 5C presents schematically the granulator in an isometric top view;

Fig. 5D presents schematically an isometric view of the reaction chamber of the granulator;

Fig. 5E presents schematically a top view of the reaction chamber of the granulator;

Fig. 6A presents schematically a weighing transporter in an isometric view;

Fig. 6B presents schematically a weighing transporter, in two views;

Fig. 7A presents schematically a belt reactor in an isometric view;

Fig. 7B presents schematically a belt reactor in three views;

Fig. 8A presents schematically a water-acid scrubber in an isometric view;

Fig. 8B presents schematically a water-acid scrubber in two views;

Figs. 9A, 9B present schematically a ventilation system for hygienization in two isometric views.

The method presented herein provides hygienization of sewage sludge, i.e. the material which constitutes the waste after the processes of sewage purification, such as the process of waste water treatment.

The system for hygienization presented herein may be located in a direct vicinity of sewage purification plant, in order to reduce the distance and, thus, to decrease the cost of transport of the sewage sludge.

Fig. 1 presents a method for sewage sludge hygienization, and Fig. 2 presents schematically a system for sewage sludge hygienization.

The process for sewage sludge hygienization includes sewage sludge dehydration in step 1 1 , introduction of a sewage sludge and burnt lime (CaO) into a granulator in step 12, preliminary hydration of a burnt lime in the granulator in step 13. The preliminary hydration of the burnt lime is a result of mixing the burnt lime with the water that is contained in the sewage sludge. Next, in step 14, the reactants are conveyed to a belt reactor, in which the hydration process (slaking process) of burnt lime is carried out in step 15. The water necessary for slaking is the water contained in the sewage sludge. The steam produced during the slaking process is drawn from a reaction chamber of a belt reactor. Next, the obtained sludge is subject to maturation, in step 16. During the maturation process, the sludge is arranged in layers on a belt conveyor. The steam, removed from the hydration process (slaking process) that is carried out in the chamber of the belt reactor, may be further subject to condensation, purification and finally to deodorization, in step 17, for example by means of a water-acid scrubber. The latter additionally leads to decrease of the amount of the waste generated in the process, including heat derived from steam and alkaline impurities.

The sewage sludge dehydration 1 1 may be carried out in a conventional centrifuge 22, into which sewage sludge of a high water content may be supplied through a conventional piping system 21 equipped with a pump. During the centrifugation process, the sewage sludge is dehydrated in step 1 1 to obtain dry matter content in the sludge at the level of 15-25% by weight. After leaving the centrifuge 22, the sludge, having a consistency similar to a dough, is transported to the granulator by means of a transporting and dosing feeder 20, which may be a conventional screw conveyor, a pump, or a double-screw conveyor, for example having a structure as schematically depicted in Fig. 4B. Preferably, the transporting and dosing feeder 20 is equipped with a strain gauge system in a form of three strain gauge bridges, installed on the feeder supports and cooperating with an automation system, which provides a precise measurement of the mass of the raw materials so as to control the weight of the sewage sludge during its introduction into the chamber of the granulator 40. The use of the transporter allows an infinitely variable control of the stream flow of the feedstock.

The burnt lime may be supplied to the granulator from a CaO container 31 by mans of at least one transporting and dosing feeder 30, and more preferably by means of two transporting and dosing feeders: 30a, 30b, each equipped with a screw conveyor such as a shaftless screw conveyor with an infinitely variable control of rotation, and consequently also an infinitely variable control of mass flow. At least one (30b) of the conveyors 30a, 30b may be equipped with a strain gauge system enabling a precise measurement of the conveyed burnt lime (CaO). More preferably, the last conveyor 30b arranged within the technological line is equipped with a strain gauge system and a screw with a controlled drive. Fig. 4A presents schematically a screw conveyor 30 for conveying and dosing the CaO into the granulator 40 that is equipped with a housing in a form of a pipe 32. The conveyor may be equipped with a chopper allowing the burnt lime to be fed into the transport chamber of the conveyor 20, for example, made of stainless steel type SS2333 (AISI3094), a shaftless screw conveyor, for example made of a special steel resistant to abrasion, wherein the inside of the transport chamber may be coated with a plastic sheet resistant for an abrasion, with a thickness of 5 mm. The burnt lime can be transported and dosed into the granulator 40, for example, by a screw conveyor. Introduction of a strain gauge measurement of a mass as well as a screw with a controlled drive enables a precise control and precise dosing of accurate quantity of the CaO into the chamber of the granulator 40.

The substrates (i.e. the dehydrated sewage sludge and the burnt lime), dosed with the transporting and dosing conveyors 20, 30, are mixed in the granulator 40. As a result of the mixing process, which involves the contact of the burnt lime and the water that is contained within the sludge, the burnt lime undergoes a preliminary hydration 13. Figs. 5A-5E present schematically the granulator, wherein Fig. 5A shows the granulator in a longitudinal cross-section through a reaction chamber, with exemplary dimensions of the granulator, Fig. 5B shows the granulator overview, Fig. 5C shows the top view of the granulator, Fig. 5D shows an isometric view of the reaction chamber of the granulator, and Fig. 5E shows a top view of the reaction chamber of the granulator. The granulator 40 comprises a reaction chamber 42 to carry out the preliminary hydration process, an inlet 43 and outlets 41 , 45 of the reaction chamber, wherein the outlet 41 is provided in the upper part of the reaction chamber of the granulator, and the outlet 45 is provided in the bottom of the reaction chamber of the granulator 40. The granulator further comprises an outlet 44 for gaseous substances which are generated during the mixing the sewage sludge and the burnt lime. The outlet 44 for gaseous substances may be, for example, in a form of a conventional pipe.

The mixing of the sewage sludge and the burnt lime is carried out by a shoveling system of the granulator provided within the reaction chamber 42 and shown in details in Figs. 5D and 5E. The shoveling system comprises two shafts 46, each with a series of shoveling arms 47 arranged along the shaft and having shovels 48 at their ends, arranged along the shafts. The shovels 48 may have a form of flat rectangular (square) plates. The shovels 48 are positioned at an angle selected for the particular granulator dimensions, optimized for maximum efficiency during granulation. When the shafts 46 rotate, the shovels 48 lift the sewage sludge upwardly from the bottom, and next the sludge falls back under gravity to the bottom. The shafts 46 may rotate in opposite directions so that the sewage sludge is transported along the axes of the shafts from one side of the reaction chamber to another, as shown by the arrows in Fig. 5D.

The rotational speed of the shafts of the shoveling system can be regulated, preferably in the range of 30 to 80 rpm. The shoveling system can work in a continuous mode or in a batch mode.

The granulator 40, due to its structure, can work in a continuous or in a batch mode, depending of the process needs. In the continuous mode, the granulator 40 may be provided with an overflow feedstock release from the reaction chamber. In the batch mode, the granulator 40 may be provided with a feedstock drop system comprising an additional drain outlet 45, provided in the bottom of the reaction chamber of the granulator.

For example, in a case of low installation load, the preliminary hydration process may be carried out in a batch mode (i.e. periodically), with the drop of the feedstock through the drain outlet 45, after obtaining a desired degree of feedstock mixing. For example, the drain outlet 45 may be opened and closed by means of a pneumatic servomotor. Nonetheless, in case of heavy installation load, the mixing process carried out in the granulator 40 may be carried out in a continuous mode: with a continuous dosage of the sludge and the burnt lime and a continuous removal of the gases from the reaction chamber of the granulator though the outlet 44.

During the mixing process of the feedstock in the reaction chamber of the granulator 40, large sewage sludge conglomerates are broken to obtain fine sludge granules. However, the granulated structure of the sewage is still maintained, thereby preventing the sludge to homogenize. The granular structure of the sludge is maintained in the reaction chamber of the granulator as a result of the implemented shoveling system that provides proper shoveling of the sewage sludge during its mixing with the burnt lime. Because the mixing process is carried out in the aforementioned conditions, the outer surface of sludge granules is being surrounded by CaO particles, thereupon a burnt lime hydration reaction occurs only on the surface of the sewage sludge granules, and not within the whole volume of the granules. This leads to a prolongation of time of the hydration reaction. The degree of hydration of the burnt lime, in such conditions, depends on the degree of granulation of sewage sludge, wherein the bigger the sewage sludge granules are, the smaller the hydration level is. For this reason, the hydration process of the burnt lime carried out in the granulator 40, by means of the shoveling system, constitutes the preliminary hydration process - because the hydration reaction does not occur within the entire volume of the sludge granules, but only on the surface of the granules.

In order to keep a proper course of the reaction of hydration, it is important to keep the following parameters of the mixing process:

- keeping a weight ratio of the burnt lime to the sewage sludge dry matter (d.m.s.) in the range of 0,7 to 1 ,8 kg of CaO per 1 kg of sewage sludge dry matter (d.m.s.)

- maintaining the content of the dry matter in the sludge at the granulator inlet in the range from 15% to 35% by weight of dry matter (d.m.), and more preferably from 17% to 22% by weight d.m.

- not allowing the sludge sewage to homogenize by selecting a proper mixing time, proper mixing parameters (shear force value) and providing the shoveling system inside the reaction chamber of the granulator.

The process of preliminary hydration is carried out appropriately when the reaction time is prolonged and the temperature increase is small, during the process, wherein the temperature of the treated feedstock should increase to about 30 - 31 °C. The product of the preliminary hydration process should have a form of a white powder.

Thus, the preliminary hydration process 13, carried out in the granulator 40, does not involve the main slaking process of the burnt lime. The main slaking process requires keeping the ratio of the water to the burnt lime at the level of 3:1 . Moreover, the main slaking process features a high temperature increase, up to 50 - 60 °C, and provides a final process product having a dough consistency, and not a powder consistency. Therefore, in the preliminary hydration process as described herein, the main slaking process does not occur. This was achieved by use of the shoveling system installed in the reaction chamber of the granulator, that does not allow the sludge sewage to homogenize, during the mixing process of the sludge and the CaO, in the granulator (the hydration process does not occur in the whole volume of the sludge granules. The preliminary hydration process occurs only on the surface of the sludge granules), and by an increase of the sludge water content (the dry mass content in the sludge is of 17 - 22%) - this further prolongs the time of preliminary hydration reaction.

The process of preliminary hydration of the sewage sludge as described herein is therefore extensively controlled and it is carried out with a small temperature increase (up to about 30 - 31 °C), and only on the surface of the sludge granules. Moreover, due to the substantially low process temperature (max. 30 - 31 °C), the preliminary hydration process does not involve generation of the gaseous reaction products requiring additional collection and purification.

The time of mixing of the sewage sludge should be adjusted individually - depending on the degree of disintegration of the sewage sludge at the inlet of the granulator 40, so as to prevent the sewage sludge from homogenization. For example, the hydration time, depending on the granulation parameters, may be from 60 to 240 seconds.

Fig 3. presents a chart which represents dependency of the temperature and time of the reaction of slaking of the burnt lime on the amount of the make-up water expressed in [%]. Based on the chart analysis, one may observe that the temperature of the slaking reaction (hydration of the burnt lime) decreases whilst the water content in the sludge increases. For example, for the sludge, being the reaction substrate, with the water-up content of 400%, the maximum reaction temperature is 70 °C for the reaction time of 100 minutes, whilst, for the sludge with the water-up content of 300%, the reaction temperature is 80 °C, for the same reaction time, i.e. of 100 minutes.

After the preliminary hydration process 13, the mixture of the sewage sludge enters the belt reactor 50 in which the process of main hydration (slaking process) 14 is carried out. During the main hydration process, the main hydration reaction occurs, i.e. the slaking process. At the outlet, the reaction feedstock (comprising the sewage sludge, the hydrated lime and the reacted burnt lime) can have a temperature exceeding 100 °C (in the case of continuous working mode of the granulator). This indicates the beginning of the slaking process, i.e. the hydration reaction that occurs within the whole volume of the sewage sludge. Such a drastic temperature increase also indicates the occurrence of the reaction of the burnt lime with the water molecules contained within the interior of the sewage granules. Therefore, the process of the main hygienization of the sewage sludge occurs inside the belt reactor 50.

Figs. 7A-7B show schematically the reactor 50 of the sewage sludge hygienization system, wherein Fig. 7A presents the reactor in a general isometric view, and Fig. 7B presents the reactor in three views: front view, side view and top view. The reactor 50 has a form of a transport belt 51 which is enclosed in a tunnel, which serves as a reaction chamber 52 and it is equipped with a ventilation system 53 for collecting and removing a steam released during the exothermic reaction of hydration of CaO, i.e. the main slaking process. The transport belt 51 conveys the feedstock comprising the sewage sludge, the slaked lime and the burnt lime though the reaction chamber. On the transport belt 51 the main hydration process of the burnt lime (slaking reaction) 14 occurs. The transport belt 51 is inclined, with respect to the ground, at an angle of up to 45°. The average temperature of the feedstock in the reaction chamber 52 is about 30 - 120 °C. During the carrying of the feedstock through the reaction chamber 52, a part of the water, contained in the sewage sludge, evaporates, due to the temperature increase of the feedstock (which is the consequence of the exothermic hydration reaction that occurs within the whole mass of the sludge). The evaporated water (steam) is collected and removed from the reaction chamber 52 by means of the ventilation system 53. This prevents condensation of the steam and provides a progressive reduction of the water content in the feedstock conveyed through the reaction chamber 52. Moreover, the progressive reduction of the water content in the feedstock provides an increase of its temperature, as shown in Fig. 3. This proves the increase of the efficiency of the exothermic reaction of hydration of CaO. Preferably, the feedstock stays in the reaction chamber 52 for 5 - 25 minutes. For example, the belt reactor 50 can have a length of 1000 cm, with the nominal linear speed of the transportation belt of v=0,012 m/s, and with a throughput of about 2 m³ of feedstock per hour. As show in Figs. 7A- 7B, the reaction chamber 52 may have a rectangular cross-section of dimensions of 1500x1200 m, and it can be made of a stainless steel of type 304. The transport belt may be, for example, made of rubber-coated fabric. Preferably, the belt reactor may be further equipped with an inverter for controlling the linear speed of the transport belt. The reactor may be further provided with a system which facilitates setting of the reaction feedstock thickness, wherein the reaction feedstock thickness on the belt, preferably, may be of 10 to 70 mm. The belt reactor may be additionally equipped with a heating mat which prevents the device from freezing. Furthermore, the reactor may have reactor walls equipped with additional isolation, for example made of a mineral wool, such as a mineral wool having a heat-transfer coefficient of a=0,35 and isolation thickness of 100mm. The thermal isolation of the walls of reaction chamber 52 restricts the condensation of the steam that is generated during the slaking process within the reaction chamber. This further limits the phenomena of a secondary moistening of the feedstock being conveyed through the reaction chamber.

Figs. 9A-9B presents an example of the ventilation system 53 of the belt reactor 50 for collecting and removing the steam from the reaction chamber 52, wherein Fig. 9A presents the ventilation system in a top isometric view, and Fig. 9B presents the ventilation system in a side isometric view. The ventilation system may, for example, consist of three hood structures 531 connected to a common pipeline 532, and thus constituting a ventilation manifold system. Depending on the process needs and the length of the chamber of the belt reactor 50, the number of the hood structures 531 may be more or less than three. Each hood structure 531 may be additionally equipped with an entry guide provided with the measurement of a flow rate and temperature of a medium, i.e. the impure steam, transported in the ventilation system. This provides an additional control of the process of steam collecting and removing from each zone of the reaction chamber 52. Moreover, in order to prevent condensation of the steam, inside the ventilation system 53, as well as to prevent from gravitational re-passing the condensed steam back to the reaction chamber 52, the ventilation system may be provided with walls isolation structure, for example, a tube-in-tube isolation structure, provided with an isolation material, such as mineral wood, embedded between the two tubes. For example, an isolation material, which may be used in the tube-in-tube isolation structure, is a mineral wool having a heat-transfer coefficient of a=0,35. A longitudinal axis of the ventilation manifold system 532 is movable with respect to the longitudinal axes of the hood structures. This prevents from gravitational re-passing of the condensed steam back to the reaction chamber, in case the steam condenses inside the manifold of the ventilation system 532. The ventilation system 53 is equipped with a forced air circulation system, in which the air circulation may be forced by means of a radial fan. The process vapors, collected and removed from the reaction chamber 52 by the ventilation system 53, may be subsequently transferred into an absorber, such as for example, water-acid scrubber 70. In the scrubber 70, the following processes are carried out: alkaline vapors neutralization, steam condensation and deodorization process. The water-acid scrubber 70 facilitates reduction of the amount of environmentally harmful waste contained in the steam released from the sewage sludge. The water, purified in the water-acid scrubber 70, comprises a small amount of ions which are harmless for the water organisms and it has a pH value close to 7. The purified water may be, thus, transported to the sewage disposal system or it may be used as process water in various technological processes. The vapors that do not condense in the water-acid scrubber constitute mainly air, and therefore, they can be released to the environment.

Fig. 8A presents an exemplary embodiment of the water-acid scrubber 70 for purification of the vapors released in the main hydration process 15. The scrubber 70 is provided with a countercurrent spray chamber which is equipped with scrubber sprinklers connected to a piping for supplying the process water 72 and for supplying an acid solution 73 to neutralize an alkaline pH of the post-reaction vapors. The spray chamber 71 has an outlet for purified air 74, located in the upper part of the chamber, an outlet for process vapors 76, in the bottom part of the chamber 71 and an outlet 77 through which the purified water can be transported to a sewage disposal system.

After the sewage sludge is removed from the chamber of the belt reactor 50, the sewage sludge is supplied to a weighing transporter 60, where the sludge, in a form of thin layer, matures. The maturation process of the sludge involves the processes of evaporation, cooling and drying of the sludge. Additionally, the sewage sludge is weighed on the weighing transponder 60. The weighing transporter 60 may be arranged in a technological line next to the belt reactor 50, close to the outlet of sewage sludge of the belt reactor, enabling the sludge to be gravitationally transported from the transport belt of the reactor 50 onto the transport belt of the weighing transporter 60. The thickness of the sludge layer, on the weighing transponder 60, may be regulated by changing the linear speed of the transporting belt 61 of the weighing transporter 60 and/or by changing linear speed of the transporting belt 51 of the belt reactor 50.

Figs. 6A-6B present an example embodiment of the weighing transporter 60, wherein Fig. 6A depicts the weighing transponder 60 schematically in a general isometric view, and Fig. 6B presents the weighing transponder 60 in a side and a top view. The weighing transporter 60 may comprise a transport belt 61 and a chamber 62 constituting a housing for covering the transported sewage sludge. The weighing transporter 60 may be equipped with a strain gauge system for measuring the weight of the transported sewage sludge stream. The strain gauge system may comprise, for example, strain gauge bridges installed inside the supports of the weighing transporter 60. The strain gauge bridges may cooperate with an apparatus facilitating an online reading of a weight of the transported sewage sludge stream. For example, the online reading system may sample a signal from the strain gauges, with a proper high frequency, and then convert the sampled signal to obtain values of the mass flow of the sewage sludge. During the transport of the sewage sludge on the weighing transponder 60, the sewage sludge is dried up, in order to obtain the sewage sludge humidity of 10%. The sewage sludge drying process may be carried out by means of an air circulation system, installed inside the chamber 62 of the transporting belt 61 . The time of transportation and drying of the sewage sludge within the chamber 61 of the transporting belt 61 is selected so as to obtain the desired humidity of the sewage sludge. For example, the line speed of the transporter belt of the length of 3500 mm, may be of v=1 m/min.

From the weighing transporter 60, the hygienized sewage sludge of a desired humidity (of about 10%) may be stored under a cover or it may be directly transported to the customers.

The system for hygienization of the sewage sludge may be further equipped with an electronic system to control the stream flow of the reactants during each step of the hygienization process. For example, the hygienization system may be equipped with a system for measurement of the mass of the burnt lime, measurement of the temperature of the vapors in the collection and removal zone of the belt reactor 50, measurement of the temperature within the reaction chamber of the granulator 40, measurement of the temperature at the outlet of the water-acid scrubber, measurement of the temperature within the ventilation manifold system. Moreover the system may be provided with various meters, for instance, for measurement of the amount of the burnt lime in the burnt lime silo, for measurement of the pressure value in the belt reactor 50 chamber, for measurement of the pressure value in the vapor collecting system installed at the vapors outlet of the water-acid scrubber 70, and for measurement of the pH value of the water at the outlet of the water-acid scrubber.

The hygienized sludge that is obtained by the process disclosed herein, is completely hygienized, pasteurized, mineralized, stabilized it and may be safely used as a substance for enriching the soil. For example the hygienized sludge may be used as a fertilizer.

The process of the preliminary hydration of the burnt lime provides extensive control of the process parameters, including pH and temperature of the treated sewage sludge. The extensive control of the process parameters is possible due to the reaction of preliminary hydration of the burnt lime, which occurs only at the surface of the sewage sludge granules. This further provides the reduction of the temperature of the preliminary hydration process to 30 °C. The process of the main hydration is carried out after the process of the preliminary hydration, and the extensive hygienization occurs during the main hydration process.

Therefore, the presented method for hygienization involves the additional process of the preliminary hydration, wherein the preliminary hydration process is carried out in the conditions of water excess: the sewage sludge at this stage of hygienization process shows the dry mass content of 15 - 25% by weight, and more preferably the sewage sludge shows the dry mass content of 17 - 22% by weight (water constitutes the rest percentage). Moreover, in this preliminary hydration process, the hydration reaction occurs only on the surface of the sewage sludge granules, and not within the whole volume of the sewage sludge granules. Thus, the preliminary hydration process features slow process course, which can be observed as a substantially small temperature increase, up to about 30 - 31 °C, and this enables the extensive control of the process parameters.

Moreover, during the stage of preliminary hydration, the water content in the sewage sludge is substantially constant, which also improves the control of the hygienization parameters.

The preliminary hydration process involves the reaction of the highly active CaO and the water contained in sewage sludge - at the initial phase of the slaking reaction. Therefore, it is important to maintain a short reaction time, in order to prevent for main slaking, i.e. the slaking within the whole volume of granules of the sewage sludge.

Moreover, it turned out that the introduction of the process of the preliminary hydration also improves the parameters of the main hydration process that is carried out subsequently. In the main hydration process carried out in the belt reactor 40, the decrease of the pH value within the whole volume of the slaked mass is not observed. Additionally, the substantial temperature changes, that would indicate an uneven reaction progress, are not observed as well. The main slaking process that follows the preliminary hydration process shows an even reaction progress, which further leads to proper hygienization of the sewage sludge.

Moreover, the system for hygienization of the sewage sludge as disclosed herein, due to a simple structure and easy assembly method, can be also in a mobile form, and thus may be moved to various locations. For example, the system may consist of two 40-feet long containers and a mobile burnt lime container. The simple structure of the hygienization system enables the structure to be transported from one location to another.

Additionally, the measurement system that is included in the hygienization system allows the process parameters to be modified, and thus more precisely controlled, even during the hydration reaction. The measurement system facilitates the change of the following: the driving force of the ventilation system, the rotational speed of the granulator and the amount of the burnt lime introduced to the hydration process.

Furthermore, the granulator is equipped with a shoveling system comprising two shovel shafts that rotate in two opposing directions and it can work in batch or continuous mode, at various rotational speed in the range of 30 - 80Hz.

While the system and method presented herein has been depicted, described, and has been defined with reference to particular preferred embodiments, such references and examples of implementation in the foregoing specification do not imply any limitation on the invention. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader scope of the technical concept. The presented preferred embodiments are exemplary only, and are not exhaustive of the scope of the technical concept presented herein. Accordingly, the scope of protection is not linnited to the preferred embodiments described in the specification, but is only limited by the claims that follow.

Claims

1 . A method for hygienization of a sewage sludge with a burnt lime (CaO), comprising:

- providing a dewatered sewage sludge with a dry matter content from 15% to 35% by weight;

- performing a preliminary hydration process (13) in a granulator (40) with a shoveling system comprising two shafts (46) with shoveling arms (47) rotating in opposite directions, by mixing the granulated dewatered sewage sludge with the burnt lime (CaO) in a ratio from 0,7 to 1 ,8 kg of CaO per 1 kg of the dry matter in the dewatered sewage sludge while maintaining the sewage sludge in a granular form to prevent the sludge sewage to homogenize;

- and next performing a main hydration process (15) in a belt reactor (50) to allow slaking of the burnt lime (CaO).

2. The method according to claim 1 , wherein the shafts of the granulator (40) rotate with a speed of 30 to 80 rpm.

3. The method according to any of claims 1 -2, comprising performing the preliminary hydration process (13) in batches, until the temperature of the mixture increases to not more than 31 °C.

4. The method according to any of claims 1 -2, comprising performing the preliminary hydration process (13) continuously.

5. The method according to any of claims 1 -2, comprising performing the preliminary hydration process (13) for 60 to 240 seconds.

6. A system for hygienization of a sewage sludge with a burnt lime (CaO), the system comprising:

- a granulator (40) with a shoveling system comprising two shafts (46) with shoveling arms (47) rotatable in opposite directions, configured to, when the system is in use, perform a preliminary hydration process (13) by mixing the granulated dewatered sewage sludge with the burnt lime (CaO) in a ratio from 0,7 to 1 ,8 kg of CaO per 1 kg of the dry matter in the dewatered sewage sludge while maintaining the sewage sludge in a granular form to prevent the sludge sewage to homogenize;

- a belt reactor (50) configured to, when the system is in use, perform a main hydration process (15) to allow slaking of the burnt lime (CaO).

7. The system according to claim 6, wherein the shafts of the granulator (40) are rotatable with a speed of 30 to 80 rpm.

8. The system according to any of claims 6-7, wherein the granulator (40) comprises a reaction chamber (42) two reactant outlets (41 , 45), wherein first reactant outlet (41 ) is located at an upper section of the reaction chamber (42) and the second reactant outlet (45) is located at the bottom of the reaction chamber (42) and can be opened and closed.

9. The system according to any of claims 6-8, wherein the belt reactor (50) comprises a reaction chamber (52) with a transport belt (51 ) of a controllable speed, wherein the transport belt (51 ) is inclined at an angle of up to 45 degrees.

10. The system according to any of claims 6-9, wherein the granulator (40) comprises a vapor outlet for transportation of the vapors released during the preliminary hydration process from the reaction chamber (42) of the granulator (40) to a water-acid scrubber (70).

1 1 . The system according to any of claims 6-10, wherein the belt reactor (50) comprises a ventilation system (53) for removing vapors released during the slaking process.

12. The system according to any of claims 6-1 1 , wherein the elements of the system are mounted in movable containers.