Sludge bound water deep removal method based on supercritical CO2 acidification-seepage synergistic mechanism

Abstract

The invention discloses a sludge bound water deep removal method based on a supercritical CO2 acidification-seepage synergistic mechanism, and belongs to the technical field of sewage treatment and sludge recycling. The deep removal method of the sludge bound water comprises the following steps: pressurizing and liquefying CO2, and then heating and pressurizing to obtain supercritical CO2; the sludge is placed in the supercritical CO2 for at least one time of acidification-seepage synergistic reaction, and sludge bound water is removed. The method provided by the invention effectively solves the problem that bound water in sludge particles is not easy to release and remove, realizes reduction of the content of the bound water in the sludge and improvement of the heat value, and provides a new thought for low-carbon and efficient dehydration and resource utilization of the sludge.

Description

Technical Field

[0001] This invention relates to the fields of wastewater treatment and sludge resource utilization, and in particular to a method for deep removal of sludge bound water based on a supercritical CO2 acidification-seepage synergistic mechanism. Background Technology

[0002] Municipal sewage sludge production is enormous and increasing annually; improper disposal can cause serious environmental and social problems. Sludge dewatering is a crucial prerequisite and key step affecting the efficiency and effectiveness of sludge treatment and disposal. Efficient dewatering technology can significantly reduce sludge volume, lowering the environmental and economic burden of subsequent transportation and treatment. However, due to the high water content, complex colloidal structure, and high hydrophilicity of sludge produced by urban wastewater treatment plants, its dewatering performance is extremely poor. The water in sludge is divided into free water and bound water: free water is relatively easy to remove through concentration or mechanical dewatering, while bound water, because it is contained within microbial cells or tightly adsorbed by extracellular polymeric substances (EPS), has significantly different physicochemical properties from free water, becoming the core technical bottleneck for deep sludge dewatering.

[0003] Sludge dewatering is a crucial preliminary step in sludge treatment, disposal, and resource utilization. Currently, mainstream technologies include mechanical filter press, thermal drying, and chemical conditioning agent addition, all of which have significant drawbacks: mechanical filter press can only remove free water and cannot reach the bound water that is bonded to the sludge matrix through hydrogen bonds and van der Waals forces, resulting in insufficient dewatering depth; thermal drying has extremely high energy consumption and is prone to generating volatile pollutants that can cause secondary pollution; chemical conditioning agents can introduce exogenous impurities, reduce the calorific value of sludge, and hinder subsequent resource utilization.

[0004] To overcome the above-mentioned shortcomings, researchers have turned their attention to supercritical CO2 dehydration technology, which has shown certain potential due to its unique phase characteristics. However, the existing technology still has three major problems that seriously restrict its large-scale industrial application.

[0005] Firstly, the efficiency of bound water removal is limited, and the multiple characteristics of supercritical CO2 are not fully utilized. Existing technologies only use it as a simple dewatering medium, failing to synergistically leverage its advantages such as low surface tension, high diffusivity, solubility and extractability, and excellent mass transfer and permeability. Relying solely on single-phase action to remove water fails to penetrate deep into the sludge flocs to break the interaction between bound water and the matrix, resulting in poor dewatering performance and wasted supercritical CO2 capabilities.

[0006] Secondly, CO2 consumption is large, utilization rate is low, and operating costs are high. Existing technologies lack efficient recycling systems and have not optimized mass transfer efficiency. Supercritical CO2 is mostly a one-time unidirectional flow, with a large amount being discharged without participating in the reaction. At the same time, to ensure dehydration effect, CO2 needs to be continuously replenished, which exacerbates consumption and waste, making it difficult to promote industrialization.

[0007] Third, the reaction mechanism is singular, failing to achieve synergistic optimization of acidification and seepage migration. The existing system only focuses on phase dewatering, without constructing acidification reaction conditions and implementation pathways. It lacks the destructive and modifying effect of acidification on sludge flocs, and cannot achieve synergistic enhancement through acidification and seepage migration, further limiting dewatering efficiency and treatment effect.

[0008] In summary, all existing technologies have unavoidable drawbacks. Developing a deep sludge dewatering method that efficiently removes bound water from sludge, reduces CO2 consumption, achieves synergistic acidification and seepage, and is low-carbon and free of secondary pollution is an urgent problem to be solved in this field. Summary of the Invention

[0009] The purpose of this invention is to provide a method for deep removal of sludge bound water based on a supercritical CO2 acidification-percolation synergistic mechanism, so as to solve the problems existing in the prior art.

[0010] To achieve the above objectives, the present invention provides the following solution: One of the technical solutions of the present invention is a method for deep removal of sludge bound water based on a supercritical CO2 acidification-seepage synergistic mechanism, comprising the following steps: CO2 is liquefied under pressure, then heated and pressurized to obtain supercritical CO2; The sludge is placed in the supercritical CO2 to undergo at least one acidification-percolation synergistic reaction to remove the sludge bound water.

[0011] Preferably, the method for preparing supercritical CO2 includes the following steps: liquefying CO2 under pressure, then heating it to 60~65℃ and increasing the pressure to 15~35 MPa to obtain supercritical CO2.

[0012] Preferably, the sludge has a moisture content of ≤86%.

[0013] Preferably, the method for deep removal of bound water from sludge based on the supercritical CO2 acidification-percolation synergistic mechanism further includes a step of gradient pressure filtration when the sludge moisture content is higher than 86%.

[0014] Preferably, the gradient pressure filtration includes: first pressurizing to 0.2~0.4 MPa, holding the pressure for 8~12 min, then increasing to 0.4~0.6 MPa, and holding the pressure until no filtrate seeps out.

[0015] Preferably, the mass ratio of supercritical CO2 to sludge is 1:(10~17).

[0016] Preferably, the acidification-percolation synergistic reaction time is 90~120 min.

[0017] Preferably, after the acidification-percolation synergistic reaction, the pressure is further reduced to atmospheric pressure in a gradient manner, and CO2 is purified by adsorption using molecular sieves.

[0018] Preferably, the gradient decompression includes: first reducing the pressure to 8-12 MPa at a rate of 0.1-0.3 MPa / min, and then reducing it to atmospheric pressure at a rate of 0.08-0.12 MPa / min.

[0019] The core innovation of this invention lies in constructing a "supercritical CO2 acidification-percolation synergistic mechanism": CO2 molecules, under supercritical conditions, percolate into the interior of sludge and microbial cells through sludge pores, replacing bound water molecules; simultaneously, the acidic fluid (pH 3.5~5.5) generated by CO2 dissolving in water, on the one hand, causes the sludge flocs and microbial cells to disintegrate, and on the other hand, it reacts chemically with the minerals in the sludge, promoting the dissolution and migration of mineral elements, changing the microstructure of the sludge (increasing specific surface area and pore volume), providing more adsorption sites and diffusion channels for supercritical CO2, further enhancing the removal of bound water molecules, forming a virtuous cycle of "acidification breaking bonds - percolation migration - structural optimization - deep removal".

[0020] This invention achieves the dual goals of efficient removal of bound water and improvement of sludge calorific value by precisely controlling process parameters (temperature, pressure, reaction time) and adapting the design to supercritical CO2 preparation and recovery equipment, providing technical support for the low-carbon resource utilization of sludge.

[0021] The present invention discloses the following technical effects: (1) The method of the present invention effectively solves the problem that the bound water inside the sludge particles is not easy to release and remove, and realizes the reduction of the bound water content and the increase of the calorific value of the sludge, providing a new idea for the low-carbon, high-efficiency dewatering and resource utilization of sludge.

[0022] (2) The method of the present invention is environmentally friendly and has no secondary pollution. No flocculants, conditioners or other chemical agents are added throughout the process, which avoids problems such as increased dry sludge production and reduced calorific value caused by chemical agents. The migration rate of heavy metals in the sludge is ≤3%, and the resource utilization safety is high.

[0023] (3) The method of the present invention is low-carbon and energy-saving. According to the calculation, the dehydration energy consumption is only 2.73 GJ / t DS, which is 82.88% more energy-saving than the traditional thermal drying method. It can reduce operating costs and provide a new path for CO2 emission reduction, thus helping to achieve the "dual carbon" goal.

[0024] (4) The method of the present invention uses an acidification-permeation synergistic mechanism, combined with gradient pressurization and dynamic parameter matching (the combination mode of the above pressurization, acidification and permeation parameters is dynamically adjusted according to the differences in the initial water content of sludge and the binding strength of bound water, so as to achieve efficient dewatering and calorific value enrichment of different types of sludge), and the bound water removal rate is significantly improved and the calorific value enrichment effect of dewatered sludge is obvious.

[0025] (5) The method of the present invention has a high resource recycling rate, with a CO2 recycling rate of ≥95%. The dehydrated filtrate can be reused in sewage treatment plants after treatment, realizing a dual resource cycle of water and CO2, with significant economic and environmental benefits. Detailed Implementation

[0026] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0027] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0028] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0029] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This specification and embodiments are merely exemplary.

[0030] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0031] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0032] This invention discloses a method for deep removal of bound water from sludge based on a supercritical CO2 acidification-seepage synergistic mechanism, comprising the following steps: (1) Sludge pretreatment and characteristic prediction: Take concentrated sludge or dewatered sludge from the sewage treatment plant, analyze and determine its initial moisture content, bound water content and elemental composition, and store it in a 4℃ refrigerator in a sealed container to avoid changes in characteristics. If the initial moisture content of the sludge is >86%, gradient pressure filtration pretreatment is adopted to make the moisture content of the sludge ≤86% to obtain filter press sludge. (2) Precise preparation and system control of supercritical CO2: After pressurizing and liquefying room temperature CO2 (purity ≥99.0%, which can be recovered by flue gas capture), the pressure is increased to 15~35MPa in stages by CO2 injection pump (designed maximum pressure of 35 MPa), and heated to 60~65℃ by constant temperature water bath (i.e., program pressurization-heating coordinated control) to ensure that CO2 smoothly enters the supercritical state (critical temperature of 31.1℃ and critical pressure of 7.38 MPa) to obtain supercritical CO2 with purity ≥99.0%. (3) Acidification-permeation synergistic reaction: Sludge is added to a reactor containing supercritical CO2 (the inner wall is coated with polytetrafluoroethylene to avoid sludge adsorption and equipment corrosion), and the reaction is carried out for 90-120 min at a mass ratio of supercritical CO2 to sludge of 1: (10~17) to ensure that CO2 and sludge are in full contact.

[0033] When supercritical CO2 comes into contact with water on the surface and inside the sludge, it partially dissolves to form H2CO3, which lowers the pH of the sludge system to 3.5~5.5, breaks the hydrogen bonds and ionic bonds in the sludge floc structure, and promotes the removal of bound water. At the same time, the high-density fluid characteristics of supercritical CO2 are utilized to form a micro-pressure gradient in the reactor (the pressure fluctuation of the micro-pressure gradient is ≤±0.5 MPa), which drives the CO2 fluid that dissolves the bound water to seep and migrate along the sludge pores.

[0034] (4) Graded separation and resource recovery: The slow decompression-condensation coupling separation process is adopted. First, the pressure of the reactor is reduced to 8-12 MPa at a rate of 0.1-0.3 MPa / min, and then reduced to atmospheric pressure at a rate of 0.08-0.12 MPa / min. This avoids the sludge particles from adsorbing water again due to the sudden pressure drop, and dehydrated sludge is obtained. After the supercritical CO2 is restored to gaseous state in the decompression reactor, it is purified by molecular sieve adsorption to remove trace amounts of water. The purity of the purified CO2 is ≥99.0%, and the recycling rate is ≥95%. After the pH of the dehydrated filtrate collected in the reactor is adjusted to 6.5-7.5, it is reused for water replenishment in the aeration tank of the sewage treatment plant to realize water resource recycling.

[0035] In some embodiments of the present invention, step (1) of gradient pressure filtration pretreatment includes: first pressurizing to 0.2~0.4 MPa, holding the pressure for 8~12 min, then increasing to 0.4~0.6 MPa, and holding the pressure until no filtrate seeps out (total pressurization time ≤1 h, to avoid sludge particles compacting and clogging pores), to obtain press sludge; The initial moisture content of concentrated or dewatered sludge is 84-98%; The moisture content of the filter press sludge is 83-86%, and the bound water content is 1.2-1.8 kg H2O / kg DS. Based on the elemental analysis results, the higher heating value (HHV) is calculated to be 9-10 MJ / kg according to the standard formula of GB / T 30727-2014, which serves as the benchmark parameter for subsequent dewatering effect.

[0036] In some embodiments of the present invention, in step (3), the reaction time is dynamically adjusted according to the initial bound water content of the sludge: when the initial bound water content of the sludge is ≥1.5 kg H2O / kg DS, the reaction time is 90~105 min; when the initial bound water content of the sludge is <1.5 kg H2O / kg DS, the reaction time is 105~120 min; and the acidification-seepage synergistic reaction step can also be repeated.

[0037] In some embodiments of the present invention, the debound sludge obtained by the first acidification-seepage synergistic reaction in step (4) has a water content of 75-80%, a bound water content of 0.4-0.8 kg H2O / kg DS, and a higher heating value of 11-13 MJ / kg; the debound sludge obtained by the second acidification-seepage synergistic reaction can have a water content reduced to 44% or less, and a bound water content reduced to 0.25 kg H2O / kg DS or less. The method for deep removal of sludge bound water based on the supercritical CO2 acidification-percolation synergistic mechanism of the present invention does not require the addition of chemical agents, and the migration rate of heavy metals in sludge is ≤3%.

[0038] Example 1 A method for deep removal of bound water from sludge based on a synergistic mechanism of supercritical CO2 acidification and seepage: (1) Sludge pretreatment: The concentrated sludge (taken from the sludge storage tank of a sewage treatment plant in Tianjin, with a water content of 95.8%, a bound water content of 1.41 kg H2O / kg DS, and a higher calorific value of 9.83 MJ / kg, and stored in a sealed refrigerator at 4℃) was pretreated by gradient pressure filtration (first pressurized to 0.3 MPa, held for 10 min, then increased to 0.5 MPa, and continuously pressurized until no filtrate seeps out, with a total pressurization time ≤1 h) to obtain filter press sludge.

[0039] The filter press sludge has a moisture content of 86%, a bound water content of 1.76 kg H2O / kg DS, and a higher calorific value of 9.19 MJ / kg.

[0040] (2) Preparation of supercritical CO2: Room temperature CO2 (purchased from a chemical company in Tianjin, with a purity of 99.0%) was pressurized and liquefied. The pressure was then increased to 20 MPa in stages by a CO2 injection pump (designed maximum pressure of 35 MPa). The reactor was heated to 65°C by a constant temperature water bath to ensure that the CO2 smoothly entered the supercritical state (critical temperature of 31.1°C and critical pressure of 7.38 MPa) to obtain supercritical CO2 with a purity of 99.0%.

[0041] (3) Acidification-percolation synergistic reaction: 100 g of filter sludge was added to the 2L reactor (with polytetrafluoroethylene coating on the inner wall) used in step (2) to prepare supercritical CO2. According to calculation, under the conditions of step (2), the mass ratio of supercritical CO2 to sludge is 1:13.42. H2CO3 was generated, which reduced the pH of the sludge system to 3.5. At the same time, a micro-pressure gradient was formed. The pressure fluctuation of the micro-pressure gradient is ≤±0.5 MPa. The reaction time is 120 min.

[0042] (4) Graded separation and resource recovery: The slow decompression-condensation coupling separation process is adopted. First, the pressure of the reactor is reduced to 10 MPa at a rate of 0.2 MPa / min, and then reduced to atmospheric pressure at a rate of 0.1 MPa / min to obtain dehydrated sludge. After the supercritical CO2 is restored to gaseous state in the decompression reactor, it is purified by molecular sieve adsorption to remove trace moisture. The purity of the purified CO2 is 99.0%, which is recycled (utilization rate ≥ 95%). After the pH of the dehydrated filtrate collected in the reactor is adjusted to 7.0, it is reused for water replenishment in the aeration tank of the sewage treatment plant to realize water resource recycling.

[0043] The debound water sludge obtained in this embodiment has a water content of 77.19%, a bound water content of 0.49 kg H2O / kgDS, a higher heating value of 11.22 MJ / kg, and a bound water removal rate of 65.25%.

[0044] Example 2 Same as Example 1, except that the heating temperature for preparing supercritical CO2 in step (2) is 60°C.

[0045] The debound water sludge obtained in this embodiment has a water content of 78.81%, a bound water content of 0.55 kg H2O / kgDS, a higher heating value of 11.0 MJ / kg, and a bound water removal rate of 60.99%.

[0046] Example 3 Same as Example 1, except that step (3) is repeated. Specifically, acidification-percolation synergistic reaction: 100 g of filter press sludge (the mass ratio of supercritical CO2 to sludge is 1:13.42) is added to the 2L reactor (with polytetrafluoroethylene coating on the inner wall) in step (2) to prepare supercritical CO2. The supercritical CO2 prepared in step (2) generates H2CO3, which lowers the pH of the sludge system to 3.5 and forms a micro-pressure gradient. The pressure fluctuation of the micro-pressure gradient is ≤±0.5 MPa. The reaction is carried out for 120 min. After cooling to room temperature and pressure, step (3) is repeated.

[0047] The debound water sludge obtained in this embodiment has a water content of 44.00%, a bound water content of 0.25 kg H2O / kgDS, a higher heating value of 13.0 MJ / kg, and a bound water removal rate of 82.27%.

[0048] Comparative Example 1 Same as Example 1, except that the heating temperature for preparing supercritical CO2 in step (2) is 55°C.

[0049] The debound water sludge obtained in this embodiment has a water content of 80.21%, a bound water content of 0.79 kg H2O / kgDS, a higher heating value of 10.5 MJ / kg, and a bound water removal rate of 43.97%.

[0050] Comparative Example 2 Same as Example 1, except that the heating temperature for preparing supercritical CO2 in step (2) is 45°C.

[0051] The debound water sludge obtained in this comparative example had a water content of 82.93%, a bound water content of 1.20 kg H2O / kgDS, a higher heating value of 10.1 MJ / kg, and a bound water removal rate of 14.89%.

[0052] Comparative Example 3 Same as Example 1, except that the step-by-step pressurization of supercritical CO2 preparation in step (2) is 10 MPa.

[0053] The debound water sludge obtained in this comparative example had a water content of 81.47%, a bound water content of 0.98 kg H2O / kgDS, a higher heating value of 10.2 MJ / kg, and a bound water removal rate of 30.50%.

[0054] Comparative Example 4 Same as Example 1, except that step (1) gradient pressure filtering pretreatment step is omitted.

[0055] The debound water sludge obtained in this comparative example had a water content of 80.35%, a bound water content of 0.90 kg H2O / kgDS, a higher heating value of 10.2 MJ / kg, and a bound water removal rate of 25.62%.

[0056] Comparisons of Examples 1-3 and Comparative Examples 1-4 show that the bound water removal rate of the method of the present invention is ≥60.99%, with a maximum of 82.27%, significantly better than the comparative examples (maximum 43.97%). The moisture content of the debound sludge can be reduced to a minimum of 44.00%, and the bound water content can be reduced to a minimum of 0.25 kg H2O / kg DS, with the higher calorific value increased to 13.0 MJ / kg, meeting the resource utilization requirements for incineration power generation. The optimal process range is 60-65℃, 15-20 MPa, and 90-120 min. Gradient pressurization pretreatment and dynamic mass ratio adjustment can further improve the removal efficiency.

[0057] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for deep removal of bound water from sludge based on a supercritical CO2 acidification-seepage synergistic mechanism, characterized in that, Includes the following steps: CO2 is liquefied under pressure, then heated and pressurized to obtain supercritical CO2; The sludge is placed in the supercritical CO2 to undergo at least one acidification-percolation synergistic reaction to remove the sludge bound water.

2. The method for deep removal of bound water from sludge according to claim 1, characterized in that, The method for preparing supercritical CO2 includes the following steps: liquefying CO2 under pressure, then heating it to 60~65℃ and increasing the pressure to 15~35 MPa to obtain supercritical CO2.

3. The method for deep removal of bound water from sludge according to claim 1, characterized in that, The sludge has a moisture content of ≤86%.

4. The method for deep removal of bound water from sludge according to claim 1, characterized in that, It also includes a step of gradient pressure filtration when the sludge moisture content is higher than 86%.

5. The method for deep removal of bound water from sludge according to claim 4, characterized in that, The gradient pressure filtration process includes: first pressurizing to 0.2~0.4 MPa, holding the pressure for 8~12 min, then increasing the pressure to 0.4~0.6 MPa, and holding the pressure until no filtrate seeps out.

6. The method for deep removal of bound water from sludge according to claim 1, characterized in that, The mass ratio of supercritical CO2 to sludge is 1:(10~17).

7. The method for deep removal of bound water from sludge according to claim 1, characterized in that, The acidification-percolation synergistic reaction takes 90-120 minutes.

8. The method for deep removal of bound water from sludge according to claim 1, characterized in that, Following the acidification-percolation synergistic reaction, the process further includes reducing the pressure to atmospheric pressure using a gradient decompression method and purifying CO2 by adsorption using molecular sieves.

9. The method for deep removal of bound water from sludge according to claim 8, characterized in that, The gradient decompression includes: first reducing the pressure to 8-12 MPa at a rate of 0.1-0.3 MPa / min, and then reducing it to atmospheric pressure at a rate of 0.08-0.12 MPa / min.