JP2008149304A - Sludge drying method and dried sludge obtained after drying - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a drying method, a system and dried matter by drying sludge with supply of low thermal energy, in a drying process of sewage sludge generated in a sewage sludge treating process or sludge generated in the waste water treating process of a food factory (hereafter sludge), by a drying method not by vaporization but by a chemical reaction. <P>SOLUTION: In the sludge drying process, sludge is set in a high pressure and high temperature state to vaporize moisture contained in sludge to high pressure steam; sludge is subjected to hydrolysis using the steam; and oxidized for decomposition using humidified air to dry sludge using hydrolysis reaction heat and oxidizing decomposition reaction heat. By drying sludge by this process, hydrolyzed or oxidized and decomposed dried sludge is obtained. In this drying process, a large amount of wastewater and exhaust gas generated by decompressing after the completion of the drying process is introduce into a cleaning facility, then is reused in the dying process. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a drying method for drying sludge generated at the time of wastewater treatment at a water purification plant or a food factory with an efficient and low energy supply, and a sludge dried product obtained after drying.

  Conventionally, as a drying method of sludge represented by sewage sludge generated in a water purification plant (sludge in which 80% or more of the main component is organic matter), a method of using sunlight as a drying heat source has been proposed. In this method, a filter medium or sand is laid on the bottom of a drying tank and dried in the sun while separating moisture and sludge. However, this method requires a vast site and exhaust measures.

  Moreover, a centrifugal thin film dryer (for example, patent document 2) is mentioned as a method of reducing the supply energy required for drying sludge. In this method, sludge and moisture are separated by a centrifugal thin film machine. In this centrifugation, hot air is used to dry the sludge after centrifugation, and at the same time, the sludge-containing sludge generated by centrifugation is also dried. This method has high heat transfer efficiency and good sludge drying efficiency. However, the structure is complex and the system is large. In addition, the processing time is long and the processing capacity is low.

  Furthermore, a vacuum sludge drying method using a vacuum distillation kettle has been proposed. This method is a method of drying sludge with low supply energy using the boiling point lowering action by reducing the pressure. (For example, patent document 3) For this reason, quick drying is possible and an odor can also be prevented. However, since processing time is required, there is a drawback that the processing capacity is lowered.

JP 2006-239634 (6th page, FIG. 1) JP-A-8-318300 (6th page, FIG. 1) JP 2004-313918 (page 6, FIG. 1)

All the conventional sludge drying methods represented by the above methods are methods for evaporating the contained water. In order to evaporate the water, it is necessary to supply enormous heat energy commensurate with the latent heat of vaporization. In order to efficiently use the supplied energy, a large-scale apparatus or system such as a centrifugal thin film dryer or a vacuum dryer is required.
Therefore, we made it a subject to obtain a drying method and system for drying sludge with a low thermal energy supply and a dried product by a drying method by a chemical reaction that is not evaporation.

  In order to solve the above-mentioned problems, the present invention places the sludge under high pressure and high temperature in a step of drying sewage sludge generated in the sewage treatment process or sludge generated in the wastewater treatment process of the food factory (hereinafter referred to as sludge). The water contained in the sludge is vaporized at high pressure, the steam is used, the sludge is hydrolyzed, and oxygen is used for oxidative decomposition, thereby using the hydrolysis reaction heat and the oxidation decomposition reaction heat. A method for drying the sludge, and a dried product obtained by drying the sludge in the step.

  Further, in the drying process, a large amount of waste liquid and exhaust gas generated when the pressure is reduced after completion of the drying process is reused in the drying process after the introduction of purification equipment.

  The sludge in the present invention is 50% or more of the dry components thereof, and the sludge that has been disposed of through the steps of dehydration → drying → incineration is the target. Since the sludge has a water content of 70% or more and contains a lot of hydrophilic organic substances, it often contains water as bound water. For this reason, there exists a tendency for the energy supply more than an evaporation latent heat to be needed for drying.

  The high pressure conditions used in the present invention are 5 Mpa or more and 100 Mpa or less. This is because water vapor having hydrolysis reactivity at 5 Mpa or more can be obtained. Further, if it is 100 Mpa or more, water vapor reaches subcriticality and exhibits a solvent property stronger than hydrolyzability, which is not suitable.

  The high temperature used in the present invention is 150 ° C. or higher and 300 ° C. or lower. This is because the hydrolysis reaction is started at 150 ° C. or higher, and the water in the sludge is evaporated to reach a necessary pressure. Above 300 ° C., it becomes a critical condition of water and is not suitable because it has only solvent properties.

  In the present invention, the heat supply method for making the sludge high temperature and high pressure is heated air. It is preferable to raise the sludge to a predetermined temperature and pressure by introducing air heated by a burner or steam into a pressure-resistant container in which the sludge is enclosed. By using heated air, the moisture of sludge is effectively utilized for the hydrolysis reaction. It is also effective for unreacted water evaporation.

  The drying time in the present invention is preferably 30 minutes or more and 120 minutes or less as the hydrolysis reaction time, and 30 minutes or more as the hydrolysis product drying time. Both the hydrolysis reaction time and the drying time depend on the moisture content of the sludge. If the water content is high, both the reaction time and the drying time tend to be long.

  In the present invention, a large amount of water vapor is released in the process of depressurizing the pressure vessel after completion of the sludge hydrolysis reaction. The discharged water vapor is cooled, treated as waste water, and the treated water is reused as cooling water.

  The dried sludge generated in the present invention is hydrolyzed by water vapor and thermally decomposed by an oxidation reaction, and therefore has a lower molecular weight than the initial sludge. Moreover, since the molecular terminal is a hydroxyl group or a carboxylic acid group, high combustibility can be expected. Since it has a low moisture content and high combustibility, it can be used as a combustion aid.

  In the present invention, water in the sludge is incorporated into the sludge molecules by a hydrolysis reaction. That is, the water is not in the form of water, but is decomposed into hydrogen molecules and hydroxyl molecules and taken into the sludge molecules. In this state, no latent heat of evaporation is generated, and the sludge can be dried with a small amount of externally supplied energy.

  The heat source for sludge drying in the present invention is heated air supplied from the outside. At the same time, the heat of chemical reaction generated from the chemical reaction (hydrolysis reaction caused by reaction between sludge and moisture and oxidative decomposition reaction caused by reaction between sludge and oxygen) is also a heat source for drying. These reactions are basically exothermic reactions and may generate more heat than the supply heat source. By using these reaction heats for drying, it becomes possible to dry the sludge with the heat generated by the sludge itself. For this reason, the energy supplied for sludge drying from the outside can be saved greatly.

  As described above, the present invention is a completely different concept from the conventional concept of evaporating and drying moisture, that is, a method of drying sludge using chemical heat and reaction heat. . For this reason, it is a drying method that reduces a large amount of supply energy (evaporation latent heat) necessary for evaporating the water in the sludge and improves the combustibility of the obtained sludge dry matter. Furthermore, the sludge dried product obtained in the present invention is considered to play an important role as an alternative fuel in Japan, which depends on overseas energy fuel sources.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a flow in the implementation of the present invention.

  The dewatered sludge is stored in the sludge storage silo 1. A predetermined amount (for the processing capacity of the hydrolysis dryer 2) is injected into the hydrolysis dryer 2 from the silo with a screw pump. During the sludge injection, the sludge stirrer 2-2 is rotated so that the sludge is evenly injected into the entire hydrolysis dryer 2.

  After completion of the sludge injection, the pressure-resistant lid at the top of the sludge inlet 2-1 is closed. Steam supplied from the boiler 4 is introduced into the air heater 5 and the jacket of the hydrolysis dryer 2 and heated to a predetermined temperature (250 ° C.).

  Predetermined pressurized air (0.78 MPa) is introduced into the air heater 5 from the compressor 3. Heated (250 ° C.) air is introduced into the hydrolysis dryer 2.

  When the temperature in the hydrolysis dryer 2 reaches 250 ° C., the pressure of the pressure regulating valve 11 is adjusted to 3 Mpa. Next, the pressure of the pressure regulating valve 12 is adjusted so that the internal pressure of the hydrolysis dryer 2 is 2 Mpa and 250 ° C.

  The sludge injected into the hydrolysis dryer 2 is heated to 250 ° C., and moisture evaporation starts. Due to the water vapor, the pressure inside the hydrolysis dryer 2 rises to a predetermined pressure and temperature (2 Mpa, 250 ° C.). Excess water vapor that has risen above the predetermined pressure is discharged from the pressure regulating valve 12 to the steam cooler 6.

  While rotating the sludge stirrer 2-2, a predetermined pressure and temperature (2 Mpa, 250 ° C.) are maintained for 30 minutes to 1 hour. During this time, water vapor undergoes a hydrolysis reaction with sludge, and chemical reaction drying starts.

  After the chemical reaction drying is completed, the pressure of the pressure regulating valve 12 is decreased over 30 minutes, and excess water vapor is opened to the steam cooler 6. During the opening, the pressure regulating valve 11 is opened, and heated air is fed into the hydrolysis dryer 2. Thereby, the oxidation thermal decomposition reaction of sludge is caused. At the same time, the water in the sludge is evaporated and dried. During this time, since the sludge generates heat of chemical reaction inside the hydrolysis dryer 2, the supply of external energy necessary for maintaining a predetermined temperature is extremely low. For this reason, the operation rate of the boiler 4 and the compressor 3 becomes far lower than normal drying.

  By opening the pressure regulating valve 12, the pressure in the hydrolysis dryer 2 is brought to normal pressure. Furthermore, excess water is dried with heated air at normal pressure, and the chemical reaction drying step is completed. After the completion of the process, the sludge dried product outlet 2-3 of the hydrolysis dryer 2 is opened, and the sludge stirrer 2-2 is rotated. The sludge dried product is pushed out to the sludge agitator 2-2 and discharged out of the hydrolysis dryer 2 from the sludge dried product outlet 2-3.

  The water vapor introduced into the steam cooler 6 is cooled by the cooling water. The cooled water vapor aggregates and returns to water. The water generated at this time is mixed with water-soluble low molecules generated by the hydrolysis of sludge and sludge fine powder released by releasing the high pressure. For this reason, the water produced by cooling is a waste liquid mixed with impurities. Since this waste liquid is immediately after agglomeration, it is at a high temperature and contains various substances depending on the properties of the sludge before drying.

  The generated waste liquid is introduced into the waste liquid precipitation tank 7. Using a flocculant in the waste liquid precipitation tank 7, solid-liquid separation and secondary cooling of the waste liquid are performed. At this time, for the precipitated sludge, a small amount of ozone is dissolved in the waste liquid, and the volume of sludge is reduced by using the microorganism-containing activated sludge activated by ozone. In addition, the solid-liquid separated waste liquid is introduced into the waste liquid purification tank 8.

  The waste liquid introduced into the waste liquid septic tank 8 is treated by an optimum method according to the component in which the waste liquid is dissolved. Treated water that has been purified to a medium water level that can be used as cooling water or boiler water by combining methods such as MO membrane and coagulation sedimentation is stored in the treated water storage tank 9.

  The treated water stored in the treated water storage tank 9 is sent to the steam cooler 6 by a pump and reused as steam cooling water. It is also reused as heat exchange water for the boiler 4. Excess treated water is purified to the drainage standard as an overflow and discharged to the outside.

  The low molecular weight compound generated by the sludge hydrolysis or oxidative decomposition reaction dissolves in the water vapor discharged from the hydrolysis dryer 2 and causes odor. Sludge silos contain low-molecular compounds that are inherently contained in sludge, which causes odor. These odor-causing substances are collected from each source by piping and exhausted to the outdoors through the exhaust deodorizer 10.

  In the sludge drying process, the most odor-causing substances are generated in the sludge storage silo 1, the waste liquid sedimentation tank 7, and the waste liquid purification tank 8. Therefore, these places are connected by an exhaust pipe, sucked by a fan through the exhaust deodorizer 10, and exhausted outdoors.

  The dried sludge obtained in the above sludge drying process had a water content of around 20%. In addition, the burned calorie is as high as about 8,000 kcal / kg, and is considered suitable as an auxiliary fuel.

Flow chart of sludge drying example in the present invention

Explanation of symbols

1, sludge storage silo 2, hydrolysis dryer 2-1, sludge inlet 2-2, sludge stirrer 2-3, sludge dried product outlet 3, compressor 4, boiler 5, air heater 6, steam cooler 7 , Waste liquid sedimentation tank 8, waste liquid purification tank 9, treated water storage tank 10, exhaust deodorizer 11, pressure regulating valve 12, pressure regulating valve

Claims (5)

  In the process of drying sewage sludge generated in the sewage treatment process and sludge generated in the wastewater treatment process of food factories (hereinafter referred to as sludge), the sludge is kept under high pressure and high temperature, so that the moisture contained in the sludge is increased. A method of drying the sludge using the hydrolysis reaction heat and the oxidation decomposition reaction heat by evaporating, hydrolyzing the sludge using the vapor, and using oxygen to oxidatively decompose the sludge; A dried product obtained by drying in this step.   2. The method according to claim 1, wherein in the drying step, a large amount of drainage and exhaust gas generated when the pressure is reduced after completion of the drying step is reused in the drying step after the introduction of purification equipment.   The method according to claim 1, wherein the pressure in the hydrolysis is 5 Mpa to 100 Mpa, and the temperature is 150 ° C to 300 ° C.   The method of claim 1, wherein the method of heating the sludge uses heated and dry air.   2. The dried product according to claim 1, wherein the dried product has a water content of 25% or less and is hydrolyzed and oxidatively decomposed as compared with the sludge, thereby reducing the molecular weight.

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