Sludge dewatering method based on modified coal cinder coupling multivalent iron salt

Abstract

The present application relates to sludge treatment technical field, provide a kind of sludge dewatering method based on modified coal cinder coupling polyvalent iron salt.The sludge is very difficult to dewater due to its complexity of composition, single conditioner encounters many problems in the application process of sludge dewatering, the present application uses filter aid and conditioner combined way to dewater sludge with municipal sludge as research object, make both play their respective advantages by a best way, the feasibility of modified coal cinder, ferrate, iron salt synergistic conditioning to improve sludge dewatering performance, give full play to the strong oxidizing property of ferrate and the enhanced dewatering capacity of modified coal cinder.The present application investigates the feasibility of modified coal cinder, ferrate, iron salt synergistic conditioning to improve sludge dewatering performance, give full play to the strong oxidizing property of ferrate and the enhanced dewatering capacity of modified coal cinder.The sludge dewatering performance indexes (SRF, CST, settling property) are investigated respectively, and the best way of combined conditioning is obtained.

Description

Technical Field

[0001] This invention relates to the field of sludge dewatering technology, specifically to a sludge dewatering method based on modified coal slag coupled with multivalent iron salts. Background Technology

[0002] In recent years, the composition and volume of sludge have become increasingly complex, while the effective disposal rate remains unsatisfactory, leading to increasingly prominent negative impacts on the ecological environment and attracting widespread social attention. Sludge dewatering is a crucial step in the sludge treatment and disposal process. It not only reduces sludge volume but also saves on subsequent transportation and disposal costs, thus holding significant importance. Several common sludge dewatering technologies in China include flocculation dewatering, mechanical dewatering, drying dewatering, ultrasonic dewatering, and electroosmotic dewatering.

[0003] Due to its complex composition, sludge is extremely difficult to dewater. Single conditioners encounter numerous problems in sludge dewatering applications. For example, even after conditioning with FeCl3 alone, the sludge still has a high moisture content, and the dosage is often too large. While modified coal ash can effectively reduce the moisture content of sludge, excessive dosage increases the difficulty of subsequent sludge cake treatment. Ferrate preconditioning can effectively break down EPS (excessive flocculation) in sludge, but insufficient dosage weakens flocculation, while increasing the dosage raises treatment costs. This is because different conditioners have different structures and mechanisms of action, and using them alone is increasingly insufficient to meet the needs of actual production. Using multiple conditioners in combination can leverage the advantages of each to a certain extent. Therefore, the combined use of these agents has attracted widespread attention from researchers.

[0004] Ferrate is a green oxidant with high oxidizing power, and the resulting nascent hydrated iron oxide has excellent flocculation effects, showing great potential in sludge conditioning. Modified coal slag products can also be used for sludge conditioning as low-cost conditioning agents. The combined application of both offers both environmental and economic benefits. Summary of the Invention

[0005] Based on the above situation, the present invention provides a sludge dewatering method based on modified coal slag coupled with multivalent iron salts. The modified products of ferrate and coal slag are used as sludge dewatering conditioners to achieve deep sludge dewatering. This method has a good dewatering effect on the sludge to be treated, and is simple to operate, low in cost, and environmentally friendly.

[0006] To achieve the above objectives, the present invention provides a sludge dewatering method based on modified coal slag coupled with multivalent iron salts, comprising the following steps: Fe(VI) is first added to the sludge to be treated for preconditioning, followed by the addition of Fe(III), and then modified coal slag for combined conditioning. The modified coal slag is acid-modified or alkali-modified. The Fe(VI) is used to oxidize and break down the extracellular polymeric substances (EPS) in the sludge, releasing interstitial water and some bound water. The modified coal slag is used to adsorb the soluble EPS released after Fe(VI) oxidation and form skeletal channels that facilitate water migration, thereby reducing the sludge's specific resistance and capillary water absorption time and improving its settling performance. The Fe(VI) used is ferrate. Further, the modified coal slag is obtained through acid modification or alkali modification. The acid modification conditions are: acid concentration 4 mol / L, acid-to-ash ratio 3:1 mL / g, and acid leaching time 3 h; or the alkali modification conditions are: alkali concentration 4 mol / L, alkali-to-ash ratio 3:1 mL / g, and alkali leaching time 3 h. The modified coal slag is further described as acid-modified coal slag, with specific surface areas of 16.7 m² / g, 58.9 m² / g, and 119.7 m² / g for the original coal slag, alkali-modified coal slag, and acid-modified coal slag, respectively. The suspended solids concentration (SS) of the sludge to be treated is further described as 12.5 g / L. The dosage of Fe(VI) is further described as 1 mg Fe / g SS, and the reaction time is 60 min. The dosage of the modified coal slag is further described as 15 g / L or 30 g / L. Attached Figure Description

[0007] Figure 1 This describes the effect of coal slag and acid / alkali modified coal slag from Example 1 of the present invention on the specific resistance of sludge (SS=12.5g / L). Figure 2 This describes the effect of coal slag and acid / alkali modified coal slag from Example 1 of the present invention on the moisture content of sludge (SS=12.5g / L). Figure 3 This describes the effect of raw coal slag on sludge settling performance in Example 1 of the present invention (SS=12.5g / L). Figure 4 This describes the effect of alkali-modified coal slag on sludge settling performance in Example 1 of the present invention (SS=12.5g / L). Figure 5 This describes the effect of acid-modified coal slag in Example 1 of the present invention on sludge settling performance (SS=12.5g / L). Figure 6 This describes the effect of different types of coal slag on sludge settling performance in Example 1 of the present invention (SS=12.5g / L; coal slag dosage=24g / L). Figure 7 This describes the effect of different dosing methods on sludge SRF (SS=12.5g / L) in Example 2 of the present invention. Figure 8 This describes the effect of different dosing methods on the sludge moisture content (SS=12.5g / L) in Example 2 of the present invention. Figure 9 This describes the effect of different dosing methods on sludge settling performance in Example 2 of the present invention (SS=12.5g / L). Figure 10 This describes the effect of different dosing methods on sludge CST (SS=12.5g / L) in Example 2 of the present invention. Figure 11 This describes the effect of different addition sequences on sludge SRF (SS=12.5g / L) in Example 3 of the present invention. Figure 12 This describes the effect of different addition sequences on sludge SCOD (SS=12.5g / L) in Example 3 of the present invention. Figure 13 This describes the effect of different dosages of the present invention on the sludge SRF (SS=12.5g / L) in Example 3 of the present invention. Detailed Implementation

[0008] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0009] Example 1

[0010] (1) Using modified coal ash to condition sludge: Coal ash was added to the sludge of the sewage treatment plant to condition it. The optimal modification conditions of coal ash were determined by orthogonal experiment. The modified coal ash was used to conduct sludge settling and filtration dewatering experiments to investigate the effects of coal ash on sludge dewatering and settling performance under different addition conditions. The optimal addition amount of acid- and alkali-modified coal ash for sludge conditioning was determined to verify its feasibility as a sludge conditioner.

[0011] Table 1

[0012] The nine sets of experimental data obtained from the orthogonal experiment on acid-modified coal slag are listed in Table 1. The experimental conditions of the fifth set were: acid concentration of 4 mol / L, acid-ash ratio of 3:1 ml / g, and acid leaching time of 3 h. The coal slag prepared under these conditions was used to condition and dewater sludge, resulting in the sludge with the lowest specific resistance, 0.16 × 10⁻⁶. 13 The m / kg value indicates that this is the set of data closest to the optimal modification conditions. Analysis of the range R-values ​​shows that the order of influence of each factor on the specific resistance (SRF) from smallest to largest is: acid-ash ratio < acid leaching time < acid concentration. The R-values ​​for acid concentration and acid leaching time are significantly higher than the R-value for acid-ash ratio.

[0013] Table 2

[0014] Table 2 lists the orthogonal experimental data of the alkali-modified coal slag. It can be seen that among the nine sets of experimental data obtained from the orthogonal experiments on the alkali-modified coal slag, the fifth set of experimental conditions—alkali concentration of 4 mol / L, alkali-to-ash ratio of 3:1 ml / g, and alkali leaching time of 3 h—resulted in the coal slag used for sludge conditioning and dewatering, producing the sludge with the lowest specific resistance, 0.21 × 10⁻⁶. 13 m / kg, this is the optimal modification condition. According to the analysis of the range R value of alkali-modified coal slag, it can be seen that the order of influence of each factor on the resistivity (SRF) from smallest to largest is: alkali-ash ratio < alkali leaching time < alkali concentration, among which the R values ​​of alkali concentration and alkali leaching time are much higher than the R value of alkali-ash ratio.

[0015] Therefore, acid and alkali concentrations and immersion times are important factors affecting the modification of coal slag. Through orthogonal experiments on coal slag modification, the optimal conditions for both acid and alkali modification were selected as follows: modifying solution concentration = 4 mol / L; modifying solution to ash ratio = 3:1 ml / g; and immersion time = 3 h. Subsequent sludge dewatering experiments will investigate the impact of modified coal slag on the actual sludge dewatering effect under the above modification conditions, thereby selecting the optimal modification method from the acid and alkali modification approaches.

[0016] (2) The changes in the properties of coal slag after acid or alkali modification are mainly due to changes in the surface of the coal slag particles. Unmodified coal slag particles have smooth surfaces, while those modified with alkali become rough. After acid modification, the glassy body between the coal slag particles is dissolved by the acid, the originally connected particles are opened up, the number of pores between the particles increases, the surface becomes rougher, the specific surface area increases, and the adsorption performance is also improved. The specific surface area of ​​these three types of coal slag (raw coal slag, alkali-modified coal slag, and acid-modified coal slag) was measured to be 16.7 m², respectively. 2 / g, 58.9m 2 / g and 119.7m 2 / g. This indicates that acid-modified coal slag can better enhance its adsorption capacity.

[0017] After being conditioned with modified coal slag, the sludge mixture was dewatered by constant pressure filtration under a vacuum of 0.03 MPa. The relevant parameters are shown in Table 3.

[0018] Table 3

[0019] Table 3 shows that with the increase of different coal slag dosages, the time for the filter cake to crack after filtration of sludge with the three types of coal slag continuously decreased. This time is positively correlated with the specific resistance of the sludge; the longer the dewatering time, the greater the specific resistance of the sludge, and vice versa. At a smaller dosage of 8 g / L, the cracking time of the raw coal slag, alkali-modified coal slag, and acid-modified coal slag decreased from 2650 s to 767 s, 512 s, and 468 s, respectively. When the dosage was further increased to 16 g / L, the cracking time decreased to 323 s, 145 s, and 131 s, respectively. Simultaneously, the volume of the filtrate after sludge dewatering also continuously decreased, and the filtrate volume is directly proportional to the sludge dewatering time; that is, the longer the dewatering time, the larger the filtrate volume, and vice versa. When the amount of coal slag added is 8 g / L, the filtrate volume of raw coal slag, alkali-modified coal slag, and acid-modified coal slag changes from 91 mL to 91 mL, 89 mL, and 85 mL, respectively. When the amount added is 16 g / L, the filtrate volume changes to 89 mL, 88 mL, and 83 mL, respectively. It can be seen that if the same filtrate volume is achieved, the time required from smallest to largest is: acid-modified coal slag < alkali-modified coal slag < raw coal slag.

[0020] like Figure 1 After adding raw coal slag, acid-modified coal slag, and alkali-modified coal slag, the specific resistance of the sludge decreased significantly, and the sludge dewatering performance was significantly improved. With increasing dosage, the rate of decrease in specific resistance was initially rapid and then slowed down. Although all three types of coal slag significantly improved sludge dewatering, under the same dosage conditions, acid-modified coal slag showed the best effect, followed by alkali-modified coal slag, with raw coal slag showing the least effect. When the coal slag dosage was 8 g / L, the specific resistance of the sludge from raw coal slag, alkali-modified coal slag, and acid-modified coal slag decreased from the initial 0.78 × 10⁻⁶ g / L. 13 m / kg decreased to 0.7×10 13 m / kg, 0.56×10 13 m / kg, 0.43×10 13 When the dosage was increased to 24 g / L, the sludge specific resistance changed to 0.52 × 10⁻⁶ m / kg. 13 m / kg, 0.32×10 13 m / kg, 0.24×10 13With increasing dosage of coal ash and modified coal ash, the sludge specific resistance initially decreased sharply. This is because the strength of newly formed flocs in the sludge increased after conditioning. During mechanical dewatering, the number of flocs destroyed by shear force decreased, reducing the clogging of filter pores by fine particles in the sludge, thus decreasing filtration resistance and improving filtration performance. Furthermore, conditioning sludge with modified coal ash was more effective than conditioning with direct coal ash; the sludge specific resistance decreased faster with modified coal ash, indicating that modified coal ash has superior enhanced dewatering performance compared to raw coal ash. When the coal ash dosage increased from 24 g / L to 56 g / L, the sludge specific resistance decreased to 0.45 × 10⁻⁶. 13 m / kg, 0.21×10 13 m / kg, 0.21×10 13 m / kg. With increasing modified coal ash dosage, the decreasing trend of sludge resistivity slowed down. This may be because the change in resistance during sludge filtration exists within a certain range. Beyond this range, even with increased dosage, the change in sludge resistivity is no longer significant, indicating that there is an optimal dosage for conditioning sludge with modified coal ash. Based on the change in sludge resistivity, a modified coal ash dosage of 24~32 g / L was selected as the optimal dosage, at which point the sludge resistivity significantly decreased. Therefore, coal ash can be considered an effective sludge conditioner, and acid-modified coal ash exhibits better conditioning and sludge dewatering promotion capabilities.

[0021] (3) such as Figure 2The moisture content of the filter cake after filtration and dewatering of sludge treated with modified coal ash at different dosages was observed. As the dosage of modified coal ash increased, the moisture content of the filter cake gradually decreased. When the dosage was 8 g / L, the moisture content of the filter cakes from the original coal ash, alkali-modified coal ash, and acid-modified coal ash decreased from the initial 85% to 82%, 79%, and 76%, respectively. When the dosage increased to 24 g / L, the moisture content changed to 72%, 67%, and 60%, respectively. Furthermore, when the dosage increased to 56 g / L, the moisture content decreased to 61%, 57%, and 52%, respectively. This is because the sludge contains a large number of tiny particles, and the filter cake formed by these tiny particles has poor liquid permeability. During filtration, these tiny particles may deform, resulting in a gradual decrease in the porosity of the filter cake. Modified coal ash particles are hard and have a rough surface, exhibiting strong adsorption properties. In sludge, they can neutralize charges and act as adsorption bridging agents, disrupting the stability of the sludge colloidal dispersion system and causing small dispersed particles to aggregate into larger ones. Simultaneously, the spherical structure of the coal ash increases the porosity of the filter layer, reducing the resistance to water removal from the sludge cake. Furthermore, because the particle size of the coal ash is smaller than that of the sludge, some coal ash particles enter the pores between the sludge particles, releasing some interstitial water and thus improving the dewatering performance of the sludge. Adding coal ash to the sludge also creates voids and channels between the coal ash particles and sludge particles, leading to a decrease in specific resistance and a faster filtration rate.

[0022] (4) Figure 3 The diagram shows the changes in the settling performance of sludge after conditioning with raw coal ash. Sludge settling performance is commonly used as an indicator of sludge floc structure and sludge-water separation. Improving sludge settling performance helps to better remove interstitial water from the sludge, increase sludge solids content, and improve the efficiency of subsequent mechanical dewatering. It can be seen that as the sludge settling time increases, the sludge height gradually decreases, the volume of the sludge supernatant increases, and the water content removed by sludge concentration increases. When no coal ash is added, the sludge settling performance is poor, with almost no clear settling interface. When the coal ash dosage is 24 g / L, the sludge height percentage is 86.5% after 50 minutes of settling. To achieve the same sludge height percentage, it would take approximately 70 minutes with a coal ash dosage of 16 g / L, and approximately 40 minutes with a dosage of 32 g / L. Therefore, it can be seen that as the amount of coal ash added increases, the sludge settling speed increases, and the settling performance gradually improves. Within 30 minutes of the start of sludge settling, the sludge height percentage decreased from an initial 96.2% to 83.4% corresponding to a slag dosage of 32 g / L, and finally to 78.1%, at which point the slag dosage was 56 g / L. The modified slag showed an increased specific surface area and correspondingly increased adsorption energy. Figure 4The changes in the settling performance of sludge after conditioning with alkali-modified coal slag are shown. It can be seen that the settling speed of the alkali-modified sludge is significantly accelerated. When the dosage of alkali-modified coal slag is 24 g / L, the sludge height ratio is 84.1% after 50 minutes of settling. To achieve the same sludge height ratio, it would take approximately 60 minutes with a coal slag dosage of 16 g / L, and nearly 35 minutes with a dosage of 32 g / L. Comparatively, the enhancing effect of alkali-modified coal slag on sludge settling performance is superior to that of raw coal slag. This is because the surface of the alkali-modified coal slag is rougher, and the particles themselves are smaller, making it easier to become nuclei for large flocs. This allows the small sludge flocs to be adsorbed and aggregated, forming large, easily settling sludge flocs. Figure 5 The changes in settling performance of sludge after conditioning with acid-modified coal slag are shown. It can be seen that the settling performance of sludge conditioned with acid-modified coal slag is improved compared to that of sludge conditioned with alkali-modified coal slag, and the trend of the settling curve is similar to that of raw coal slag and alkali-modified coal slag. When the dosage of acid-modified coal slag is 24 g / L, the sludge height ratio is 82.5% after 50 minutes of settling. To achieve the same sludge height ratio, it would take less than 60 minutes with a coal slag dosage of 16 g / L, and nearly 35 minutes with a coal slag dosage of 32 g / L. Comparatively, the influence on sludge settling effect, from largest to smallest, is: acid-modified coal slag > alkali-modified coal slag > raw coal slag. Figure 3 , Figure 4 , Figure 5 It can be seen that the settling speed of sludge varies under different coal slag and different dosages. However, after a period of settling, the sludge height ratio never drops below 50%. This is because sludge thickening and free settling can only remove water in the sludge in a free form. If more water needs to be removed, mechanical dewatering is required. Figure 6This study compares the changes in settling performance of sludge treated with coal ash, alkali-modified coal ash, and acid-modified coal ash at a dosage of 24 g / L. It shows that the addition of appropriate amounts of different types of coal ash can increase the settling velocity, thus shortening the sludge settling time. After 30 minutes of settling, the sludge height percentages of raw coal ash, alkali-modified coal ash, and acid-modified coal ash decreased to 93.3%, 91.6%, and 90.1%, respectively. After 60 minutes, the corresponding sludge height percentages decreased to 85.2%, 82.4%, and 79.1%, respectively. In the final 90 minutes, the corresponding sludge height percentages decreased to 77.4%, 72.7%, and 67.4%. This indicates that coal ash has a porous structure with a porosity generally between 60% and 75%, possessing a large specific surface area and strong adsorption capacity. However, due to limitations in its physicochemical properties, its adsorption performance is not ideal. After acid and alkali modification, the SiO2 and Al2O3 on the surface covered by impurities dissolve, increasing the number of broken bonds. This not only enhances the chemical properties of the coal slag but also increases its specific surface area and reaction contact area, significantly improving its adsorption performance. Modified coal slag exhibits a certain adsorption effect on soluble organic matter in the sludge system. Through adsorption, soluble organic matter in the sludge system detaches from the surface of the sludge flocs, disrupting the previously relatively stable sludge floc structure and releasing free water, thereby improving the settling and flocculation performance of the sludge. Based on the above analysis, acid-modified coal slag is superior to alkali-modified coal slag for sludge dewatering; therefore, acid modification is chosen.

[0023] Example 2 (1) After the sludge was treated by four different methods, namely Fe(III), Fe(III)+Fe(VI), Fe(III)+modified coal slag, and Fe(III)+Fe(VI)+modified coal slag, the sludge mixture was dewatered by constant pressure filtration under a vacuum of 0.08 MPa. The relevant parameters are shown in Table 4.

[0024] Table 4

[0025] As can be seen from Table 4, with the increase of the dosage of iron salt, the time for the sludge treated by four different conditioning methods to be filtered until the filter cake cracks continuously decreases. This time has a positive correlation with the sludge specific resistance. The longer the cracking time required, the greater the sludge specific resistance, and vice versa. When the dosage of Fe(III) is 20 mgFe / gSS, the cracking times of Fe(III) alone, Fe(III)+Fe(VI), Fe(III)+modified cinder, and Fe(III)+Fe(VI)+modified cinder decrease from 2650 s, 1820 s, 75 s, and 71 s to 1860 s, 1260 s, 60 s, and 58 s respectively. When the dosage is further increased to 60 mgFe / gSS, the cracking times decrease to 875 s, 434 s, 57 s, and 57 s respectively. At the same time, the volume of the filtrate after sludge dewatering also continuously decreases, and the volume of the filtrate is proportional to the sludge dewatering time, that is, the longer the dewatering time, the greater the volume of the filtrate, and vice versa. When the dosage of Fe(III) is 20 mgFe / gSS, the volumes of the filtrate of the four methods of Fe(III) alone, Fe(III)+Fe(VI), Fe(III)+modified cinder, and Fe(III)+Fe(VI)+modified cinder change from 91 mL, 89 mL, 85 mL, and 83 mL to 90 mL, 88 mL, 80 mL, and 77 mL respectively. When the dosage of Fe(III) is 60 mgFe / gSS, the volumes of the filtrate change to 87 mL, 86 mL, 79 mL, and 75 mL respectively. It can be seen that if the same volume of filtrate is to be achieved, the order of the time required from small to large is: Fe(III)+Fe(VI)+modified cinder < Fe(III)+modified cinder < Fe(III)+Fe(VI) < Fe(III). It can be seen that under the same conditions, the enhanced dewatering effect of modified cinder is better than that of ferrate. This is because the strong oxidizing property of ferrate causes the sludge flocs to disintegrate. Although the bound water is released, it also increases the dewatering difficulty to a certain extent, resulting in a lower dewatering effect than that of modified cinder. The composite conditioning of modified cinder and ferrate can achieve the best effect. This dosing method can improve the deficiency of the dewatering ability of ferrate and significantly increase the sludge dewatering effect.

[0026] (2) Figure 7This study investigated the effects of four different conditioning methods—Fe(III) alone, Fe(III) + Fe(VI), Fe(III) + modified coal ash, and Fe(III) + Fe(VI) + modified coal ash—on the specific resistance of sludge. It can be seen that the specific resistance of the sludge decreased after all four conditioning methods, indicating a significant improvement in sludge dewatering performance. When the Fe(III) dosage was 0, the Fe(VI), modified coal ash, and Fe(VI) + modified coal ash methods all reduced the sludge specific resistance, while subsequent additions of Fe(III) had varying degrees of impact on these three methods. When the Fe(III) dosage was 40 mg Fe / gSS, the four methods—Fe(III) alone, Fe(III) + Fe(VI), Fe(III) + modified coal ash, and Fe(III) + Fe(VI) + modified coal ash—each reduced the initial sludge specific resistance by 8.06 × 10⁻⁶. 12 m / kg, 5.61×10 12 m / kg, 2.23×10 12 m / kg and 0.78×10 12 m / kg decreased to 7.21×10 12 m / kg, 4.41×10 12 m / kg, 1.82×10 12 m / kg and 0.51×10 12 The specific resistance of the sludge decreased by 10%, 22%, 20%, and 36% respectively, with the Fe(III) + Fe(VI) + modified coal slag showing the greatest impact. When the Fe(III) dosage was 100 mg Fe / gSS, the specific resistance of the sludge after conditioning with Fe(III) alone, Fe(III) + Fe(VI), Fe(III) + modified coal slag, and Fe(III) + Fe(VI) + modified coal slag were 5.31 × 10⁻⁶ m / kg, respectively. 12 m / kg, 3.03×10 12 m / kg, 1.49×10 12 m / kg and 0.31×10 12 m / kg decreased by 34%, 47%, 56%, and 62%, respectively.

[0027] With the increase of Fe(III) dosage, Fe(III)+Fe(VI)+modified coal slag has the best and most significant effect on improving the specific resistance of sludge, followed by Fe(III)+modified coal slag, and Fe(III)+Fe(VI) is slightly worse. This is because in the Fe(III) + Fe(VI) system, ferrates oxidize and break down the extracellular polymeric substances (EPS), weakening the EPS's ability to interfere with sludge dewatering and thus promoting sludge dewatering. In the Fe(III) + modified coal ash system, the flocculation properties of Fe(III) and the coagulant properties of modified coal ash are fully utilized, increasing the strength of newly formed sludge flocs and creating new water passages around the modified coal ash. This reduces the number of flocs destroyed by shear force during mechanical dewatering, decreasing the blockage of filter pores by fine particles in the sludge, thereby reducing filtration resistance and improving filtration performance. The Fe(III) + Fe(VI) + modified coal ash system combines reducing the impact of EPS with strengthening sludge flocs, not only reducing the influence of organic matter on sludge dewatering but also achieving sludge reduction to a certain extent, resulting in better sludge dewatering. Therefore, the Fe(III) + Fe(VI) + modified coal ash system can be considered an effective sludge conditioner combination.

[0028] After sludge conditioning using different dosing methods and subsequent filtration and dewatering, the moisture content of the filter cake is as follows: Figure 8With increasing Fe(III) dosage, the moisture content of the sludge cake decreased after treatment using four different methods: Fe(III), Fe(III) + Fe(VI), Fe(III) + modified coal ash, and Fe(III) + Fe(VI) + modified coal ash. When the Fe(III) dosage was 40 mg Fe / gSS, the initial sludge moisture content of 86%, 83%, 62%, and 58% was reduced to 82%, 75%, 58%, and 53% respectively using Fe(III) alone, Fe(III) + Fe(VI), Fe(III) + modified coal ash, and Fe(III) + Fe(VI) + modified coal ash. When the Fe(III) dosage increased to 100 mg Fe / gSS, the sludge moisture content decreased to 76%, 71%, 53.8%, and 47% respectively after conditioning with Fe(III) alone, Fe(III) + Fe(VI), Fe(III) + modified coal slag, and Fe(III) + Fe(VI) + modified coal slag. It can be seen that the decrease in sludge moisture content with Fe(III) alone was not particularly significant. This may be because the iron salt dosage was too small, making it difficult to form good flocs and hindering filtration and dewatering. When sludge was conditioned with Fe(III) + Fe(VI), the sludge moisture content decreased compared to Fe(III) alone. This is because ferrates can oxidize extracellular polymeric substances (EPS), releasing interstitial water while reducing the impact of EPS on sludge dewatering. Furthermore, the newly generated hydrated iron oxide also has a good flocculation effect, which implicitly increases the coagulation strength and enhances the dewatering effect. In the Fe(III)+ modified coal slag system, the addition of modified coal slag significantly reduced the moisture content of the sludge cake. This is because after the modified coal slag neutralizes the charge of the sludge particles, it more easily interacts with the polymers resulting from Fe(III) hydrolysis, forming an adsorption and bridging effect that disrupts the stability of the sludge colloidal dispersion. This causes small dispersed particles in the sludge to aggregate into larger particles. Simultaneously, the spherical structure of the coal slag increases the porosity of the filter layer, reducing the resistance to water removal from the sludge cake. In the Fe(III)+Fe(VI)+ modified coal slag system, the addition of ferrate breaks down EPS (extracellular polymeric substances), reducing the size of large sludge flocs. The rough surface of the modified coal slag, with its strong adsorption capacity, allows it to penetrate these small flocs, releasing some interstitial water. Furthermore, the reduced resistance of EPS to sludge dewatering leads to a more significant dewatering effect. Therefore, the Fe(III)+Fe(VI)+ modified coal slag system can effectively enhance sludge dewatering; however, the dosage is often too high, requiring further optimization considering multiple parameters.

[0029] (3) The settling effect of sludge can determine the size of sludge floc structure and whether the coagulant is within the effective addition range. Enhancing the settling effect of sludge can reduce the difficulty of mechanical dewatering and effectively reduce the water content of sludge. In (1), the effect of Fe(III) alone on sludge dewatering is not particularly obvious. Therefore, the dosage of Fe(III) should be appropriately increased, and compared with the combined addition when the dosage of Fe(III) is lower, the settling properties of the conditioned sludge and the change of sludge height within 100 min of settling time should be examined. At the same time, two systems with different combined methods should be selected for comparison.

[0030] Figure 9The diagram illustrates the effects of four different conditioning methods—Fe(III) alone, Fe(III) + Fe(VI), Fe(III) + modified coal slag, and Fe(III) + Fe(VI) + modified coal slag—on sludge settling performance. It can be seen that as the sludge settling time increases, the sludge height gradually decreases, while the volume of the sludge supernatant increases, and the water content from sludge concentration increases. When sludge is conditioned with Fe(III) alone at a dosage of 60 mg Fe / gSS, the sludge height reaches 95.2% after 30 minutes of settling. However, when the time is extended to 100 minutes, the sludge height percentage decreases to 85.9%, and the decrease in sludge height percentage slows down with increasing settling time. When Fe(VI) was added to condition the sludge together with Fe(III), the dosages of Fe(III) and Fe(VI) were 60 mg Fe / gSS and 3 mg Fe / gSS, respectively. After 30 minutes of settling, the sludge height accounted for 93.8%. However, when the time was extended to 100 minutes, the sludge height accounted for 79%, indicating that the sludge settling effect was enhanced. However, the settling interface was not particularly clear. This was because ferrates broke down the EPS in the sludge, reducing the impact of EPS on sludge settling. At the same time, some sludge floc particles were also broken down, forming a turbid settling interface. In the Fe(III)+ modified coal slag system, previous studies have shown that modified coal slag has a significant impact on sludge settling properties and can effectively improve sludge settling performance. Therefore, the dosage of Fe(III) and modified coal slag was set at 20 mg Fe / gSS and 30 g / L, respectively. After 30 min of settling, the sludge height accounted for 89%, while after 100 min, the sludge height percentage decreased to 64%. With the extension of settling time, the sludge height percentage decreased rapidly, and the settling effect at this point was stronger than that of Fe(III)+ modified coal slag. The sedimentation effect when 60 mg Fe / g SS is added (e(III)) is due to the increased adsorption energy of the modified coal slag, which generates an adsorption bridging effect in the sludge mixture. This promotes the sludge particles to flocculate and bridge together after colliding with each other after being conditioned by Fe(III). At the same time, the modified coal slag can destroy the stable double electric layer structure of the sludge colloidal particles, making the cohesion of sludge particles dominant. The fine sludge particles and the iron salt hydrolysis polymer flocculate into flocs with larger particle sizes, which are easy to settle and improve the sedimentation performance of sludge.

[0031] When Fe(III), Fe(VI), and modified coal ash were used to condition sludge simultaneously, with dosages of 20 mg Fe / gSS, 3 mg Fe / gSS, and 30 g / L, respectively, the sludge height reached 77.5% after 30 minutes of settling. This settling effect was equivalent to that of the Fe(III) + modified coal ash system after 60 minutes of settling, while the Fe(III) + Fe(VI) system required 100 minutes. When the time was extended to 60 minutes, the sludge height decreased to 65%, equivalent to that of the Fe(III) + modified coal ash system after 90 minutes of settling. When the time was extended to 100 minutes, the sludge height reached 54%, and the sludge settling efficiency slowed down in the subsequent stages. In the Fe(III)+Fe(VI)+ modified coal slag system, the pre-oxidation of ferrates breaks down the extracellular polymeric substances (EPS) in the sludge, resulting in a significant cell-dissolving effect and an increase in soluble organic matter within the system. Meanwhile, the modified coal slag exhibits enhanced chemical properties, with a larger specific surface area and reaction contact area, improving its adsorption capacity. It adsorbs soluble organic matter from the sludge flocs, reducing their impact on sludge settling. Furthermore, the high density of coal slag, when encapsulated within the flocs, increases the floc density and particle settling velocity, resulting in a significant settling effect. The combination of nascent hydrated iron oxide generated from ferrates, the polymer generated from the hydrolysis of ferric salts, and the modified coal slag with its large specific surface area effectively strengthens the aggregation of sludge floc particles and improves sludge settling performance.

[0032] (4) Capillary absorption time (CST) of sludge refers to the time required for sludge water to penetrate a certain distance on absorbent filter paper. It is a method to determine the filtration performance and state of sludge. The filtration performance of sludge determines the effect of different types of dewatering equipment. The higher the CST value, the worse the dewatering performance of sludge, and vice versa. The CST value of sludge can intuitively reflect the destabilization of sludge after conditioning, and can judge the flocculation superiority of flocculants, thereby selecting the optimal flocculant and its concentration. The main focus is on the changes in the CST value of sludge after preconditioning using four methods: Fe(III) alone, Fe(III) + Fe(VI), Fe(III) + modified coal slag, and Fe(III) + Fe(VI) + modified coal slag. Figure 10It can be seen that the influence of four different addition methods—Fe(III) alone, Fe(III) + Fe(VI), Fe(III) + modified coal ash, and Fe(III) + Fe(VI) + modified coal ash—on the sludge CST value shows a similar trend to the sludge specific resistance (SRF), decreasing with increasing Fe(III) dosage. When the Fe(III) dosage is 0, the combined addition of Fe(VI) and modified coal ash can further reduce the sludge CST value, from the initial 80s to 43s. At this time, the sludge CST values ​​after conditioning with Fe(VI) alone and modified coal ash alone are 46s and 60s, respectively. Ferrate has a better effect on reducing the sludge CST value than modified coal ash. When the Fe(III) dosage was 40 mg Fe / gSS, the CST values ​​of the sludge after conditioning with four different dosage methods—Fe(III) alone, Fe(III) + Fe(VI), Fe(III) + modified coal slag, and Fe(III) + Fe(VI) + modified coal slag—decreased from the initial 80 s, 46 s, 60 s, and 43 s to 73 s, 41 s, 49 s, and 36 s, respectively. However, as the Fe(III) dosage increased, the decreasing trend of the CST values ​​of the sludge after conditioning with the Fe(III) + Fe(VI), Fe(III) + modified coal slag, and Fe(III) + Fe(VI) + modified coal slag methods slowed down. At this point, with a Fe(III) dosage of 80 mg Fe / gSS, the corresponding sludge CST values ​​decreased to 67 s, 38 s, 45 s, and 32 s.

[0033] When Fe(III) is used alone to condition sludge, the CST value of the sludge decreases less and more slowly. This indicates that the amount of flocculant added is too small to improve the dewatering performance of the sludge. Furthermore, the hydrolysis products of iron salts cannot use sludge particles as crystal nuclei, and the presence of extracellular polymeric substances (EPS) prevents them from bridging with other small particles, thus failing to achieve the function of sweeping and trapping. They remain suspended in the liquid, affecting the coagulation effect and consequently the dewatering performance of the sludge. When Fe(VI) is added to condition sludge together with Fe(III), Fe(VI) can affect the particle size and structure of sludge due to its strong oxidizing properties. This causes the EPS to break down, reducing the dewatering resistance caused by the EPS encapsulating the sludge floc particles, releasing interstitial water in the sludge flocs, and enhancing the dewatering effect. At the same time, the ferrate preconditioning is reduced to nascent hydrated iron oxide, which has a high specific surface area and can promote the aggregation of floc particles, thereby reducing the CST value of the sludge. When modified coal ash is added to sludge to condition it together with Fe(III), the positively charged silicon-aluminum active sites of the modified coal ash, which are wrapped by impurities, are exposed on the particle surface. This can neutralize the negative charge on the sludge floc particles, causing the sludge flocs to destabilize. The chances of small floc particles colliding with each other and with the modified coal ash increase, forming large floc particles that are easy to settle. However, the modified coal ash has a limited impact on EPS in the sludge, so the sludge settling effect is weaker than when Fe(VI) is added.

[0034] When the Fe(III)+Fe(VI)+ modified coal slag system is used to condition sludge, the ferrate oxidizes and breaks down EPS, which weakens the effect of EPS on modified coal slag. Furthermore, the modified coal slag has a certain adsorption effect on the polysaccharides and proteins generated after EPS breakdown, which reduces the adverse factors of increased sludge viscosity caused by excessive soluble protein and polysaccharide content. The sludge after comprehensive conditioning is easier to dewater.

[0035] Example 3 The inventors have discovered that using the multivalent iron salt composite modified coal slag sludge conditioning and dewatering method provided in this invention can further improve the sludge dewatering effect.

[0036] (1) Since different sludge conditioners have different structures and modes of action, different order of addition may lead to different conditioning results when used in combination. Therefore, the influence of the order of addition was mainly investigated when ferrate and modified coal slag were used in combination. There are three different addition methods when ferrate and modified coal slag are used in combination: 1. First add modified coal slag and stir to mix well, then add ferrate, and finally add Fe(III) and stir to mix well. 2. Ferrate and modified coal slag are added to the sludge at the same time, and Fe(III) is added before stirring and mixing. 3. First, add ferrate and mix well, then add Fe(III), and then add modified coal slag and stir to mix well.

[0037] from Figure 11 As can be seen, the changes in specific resistance after sludge conditioning differ under the three different addition methods. When Fe(III) is not added, the specific resistance of the sludge after conditioning using methods 1, 2, and 3 are 2.15 × 10⁻⁶, respectively. 12 m / kg, 1.53×10 12 m / kg, 0.77×10 12 For ferrate, adding it first results in the greatest reduction in sludge SRF, followed by simultaneous addition, with modified coal slag being added last. When Fe(III) is added, all three addition methods reduce the sludge SRF to some extent. When the Fe(III) dosage is 40 mg Fe / gSS, the sludge SRF after treatment using methods 1, 2, and 3 is reduced to 1.76 × 10⁻⁶. 12 m / kg, 1.01×10 12 m / kg, 0.5×10 12 With an average dosage of m / kg, the specific resistance of the sludge decreased. However, when the Fe(III) dosage increased to 100 mg Fe / gSS, the sludge SRF after conditioning using methods 1, 2, and 3 decreased to 1.01 × 10⁻⁶ m / kg. 12 m / kg, 0.43×10 12 m / kg, 0.2×10 12 At m / kg, under the influence of Fe(III), the specific resistance of the sludge further decreased. Adding ferrate first allows for sufficient contact and oxidation with the extracellular polymeric substances (EPS) in the sludge, causing them to break down and release bound water, reducing their impact on subsequent Fe(III) hydrolysis. Modified coal ash, added to the sludge system at this time, effectively destabilizes some of the broken-down sludge flocs, allowing them to settle again, fully utilizing the oxidizing effect of ferrate and the electrostatic neutralization effect of modified coal ash. When modified coal ash is added first, although the sludge has already begun to destabilize and aggregate around the modified coal ash core, the subsequent addition of ferrate disrupts this trend. While it has some effect, it is not as effective as when ferrate is added first. Therefore, when ferrate is added first, the sludge after conditioning has the lowest specific resistance, the fastest filtration speed, and the shortest filtration time.

[0038] Figure 12The advantages of different addition sequences were examined by analyzing the changes in sludge SCOD, where ΔSCOD = SCODmeasured - SCODinitial. It can be seen that under the three different addition methods, the change in sludge SCOD gradually decreases with increasing Fe(III) addition. When Fe(III) is not added, the changes in SCOD after conditioning using methods 1, 2, and 3 are 1.72 g / L, 2.31 g / L, and 2.72 g / L, respectively. Adding ferrate first results in the largest change in sludge SCOD, followed by simultaneous addition, with modified coal slag added last. This is because when ferrate is added first, it comes into contact with the flocculent particles in the sludge, which can fully and effectively exert its cytolytic effect and release SCOD, thus increasing the SCOD content in the sludge supernatant. Although the modified coal slag added later has a certain adsorption and removal effect on organic matter in the sludge supernatant, its effect is very limited compared to the amount released after oxidation by ferrate. When modified coal slag is added first, it causes the sludge flocs to aggregate and settle, and the contact opportunity between the ferrate added later and the sludge flocs is relatively reduced, and the oxidation effect is also weakened accordingly. This makes the change in SCOD in the sludge supernatant less than the change in SCOD when ferrate is added first.

[0039] As the dosage of Fe(III) in the sludge system increased, the change in SCOD in the sludge supernatant gradually decreased. This is because the conditioning effect of Fe(III) also has a certain removal effect on SCOD in the sludge system. When the Fe(III) dosage was 40 mg Fe / gSS, the change in SCOD of the sludge after conditioning with methods 1, 2, and 3 decreased from the initial 1.72 g / L, 2.31 g / L, and 2.72 g / L to 1.56 g / L, 1.74 g / L, and 2.15 g / L, respectively. However, when the Fe(III) dosage increased to 100 mg Fe / gSS, the change in SCOD of the sludge after conditioning with methods 1, 2, and 3 decreased by 1.23 g / L, 1.42 g / L, and 1.74 g / L, respectively. It can be seen that Fe(III) has a certain removal effect on SCOD in sludge supernatant. The polymer formed after Fe(III) hydrolysis can adsorb organic matter and wrap with sludge floc particles, which reduces the SCOD content in the supernatant. The change in ΔSCOD is the smallest when ferrate is added last. This is because the SCOD content released by ferrate is relatively reduced when it is added later, which reduces the base amount of removable SCOD in sludge supernatant, thus weakening the effect of Fe(III) coagulation removal.

[0040] As can be seen from the above, preferential addition of ferrate not only enhances the reduction effect on sludge specific resistance, but also strengthens the release of SCOD after sludge breakdown, thereby affecting the impact of EPS in the flocs on sludge dewatering. Therefore, following the addition method of Method 3, adding ferrate first and mixing well, then adding Fe(III), and finally adding modified coal slag and mixing well, this method is selected as the optimal addition sequence.

[0041] (2) The combined effect of ferrate and modified coal slag can effectively promote the dewatering performance of sludge. At this time, the dosage of ferrate and modified coal slag is 0.4 mg Fe / gSS and 30 g / L, respectively. These dosages are the optimal dosages for sludge conditioning with ferrate alone and modified coal slag alone, respectively. At this time, the dosage of modified coal slag is the determining factor for the final sludge cake weight. Therefore, when ferrate and modified coal slag are added together, the dosage of modified coal slag should be appropriately reduced while ensuring a certain dewatering effect. Adjusting the dosage of ferrate and iron salt can reduce the weight of the sludge cake and provide a convenient prerequisite for subsequent sludge treatment and disposal. Now, we will examine the influence of the addition of iron salt on the sludge dewatering performance under different dosages of ferrate and modified coal slag. The dosage of ferrate was selected as 1.5 mg Fe / gSS and 3 mg Fe / gSS, respectively, while the dosage of modified coal slag was selected as 15 g / L and 30 g / L, respectively. The changes in the specific resistance of the sludge after conditioning were investigated under these two dosages.

[0042] Figure 13 This indicates the changes in sludge specific resistance after conditioning under different combinations of ferrate and modified coal slag additions. When Fe(III) is not added, the different combinations of ferrate and modified coal slag dosages have varying degrees of influence on sludge specific resistance. Fe(VI)1.5 + modified coal slag 15 and Fe(VI)1.5 + modified coal slag 30 respectively reduced the initial sludge specific resistance by 8 × 10⁻⁶. 12 m / kg decreased to 4.12×10 12 m / kg and 1.25×10 12 m / kg, while Fe(VI)3+ modified coal slag 15 and Fe(VI)3+ modified coal slag 30 reduced the specific resistance of the sludge to 2.37×10. 12 m / kg and 0.78×10 12m / kg. It can be seen that changes in the dosage of modified coal ash have a significant impact on the specific resistance of the sludge. This is because although ferrate can reduce the influence of extracellular polymeric substances (EPS), the proteins and polysaccharides generated after floc oxidation and breakdown dissolve in the solution, increasing the viscosity of the sludge solution and creating resistance to sludge dewatering. Modified coal ash, when conditioning sludge, can effectively reduce the specific resistance of the sludge through the combined effects of adsorbing soluble organic matter, electrostatic neutralization, adsorption bridging, and constructing a permeable framework. However, the dosage will increase accordingly, resulting in a larger sludge cake weight.

[0043] When Fe(III) is added to the sludge system alone, the specific resistance of the conditioned sludge gradually decreases with increasing dosage. It also has a promoting effect when combined with different dosages of ferrate and modified coal slag to condition the sludge. When the Fe(III) dosage is 60 mg Fe / gSS, the specific resistance of the conditioned sludge under the four dosage combinations—Fe(III) + Fe(VI) 1.5 + modified coal slag 15, Fe(III) + Fe(VI) 1.5 + modified coal slag 30, Fe(III) + Fe(VI) 3 + modified coal slag 15, and Fe(III) + Fe(VI) 3 + modified coal slag 30—is 2.32 × 10⁻⁶. 12 m / kg, 0.68×10 12 m / kg, 0.98×10 12 m / kg and 0.42×10 12 The specific resistance of the sludge decreased by 13% and the dosage of modified coal slag decreased by 50% compared to Fe(VI)3 + modified coal slag 10, with the Fe(III) + Fe(VI)3 + modified coal slag 15 dosage being 60 mg Fe / gSS. The effect of Fe(III) dosage on the sludge cake weight was far less than that of modified coal slag. With further increases in Fe(III) dosage, the specific resistance of the sludge after conditioning with the three dosage combinations of Fe(III) + Fe(VI)1.5 + modified coal slag 30, Fe(III) + Fe(VI)3 + modified coal slag 15, and Fe(III) + Fe(VI)3 + modified coal slag 30 was not significantly different, indicating a relatively small influence of Fe(III) dosage. It can be seen that the appropriate addition of Fe(III) can reduce the amount of modified coal slag required. To a certain extent, the polymer after Fe(III) hydrolysis plays a bridging and co-precipitation role, resulting in larger sludge floc particles. The nascent hydrated iron oxide generated after Fe(VI) oxidation can also work with the polymer, reducing the specific resistance of the sludge and improving dewatering performance. Therefore, the composite technology of multivalent iron salts combined with modified coal slag, compared with the combined addition of ferrates and modified coal slag, has a better promoting effect on the specific resistance and settling properties of sludge.

[0044] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combining the various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A sludge dewatering method based on modified coal slag coupled with multivalent iron salts, characterized in that, The process includes the following steps: first, Fe(VI) is added to the sludge to be treated for preconditioning, then Fe(III) is added, followed by the addition of modified coal slag for combined conditioning; wherein, the modified coal slag is acid-modified or alkali-modified coal slag; the Fe(VI) is used to oxidize and break down the extracellular polymeric substances (EPS) in the sludge and release interstitial water and some bound water, and the modified coal slag is used to adsorb the soluble EPS released after Fe(VI) oxidation and form a skeletal channel that facilitates water migration, thereby reducing the sludge specific resistance and capillary water absorption time and improving the sludge settling performance, and the Fe(VI) is ferrate.

2. The sludge dewatering method based on modified coal slag coupled with multivalent iron salts according to claim 1, characterized in that, The modified coal slag is obtained by acid modification or alkali modification; wherein, the acid modification conditions are acid concentration of 4 mol / L, acid-ash ratio of 3:1 mL / g, and acid leaching time of 3 h, or the alkali modification conditions are alkali concentration of 4 mol / L, alkali-ash ratio of 3:1 mL / g, and alkali leaching time of 3 h.

3. A sludge dewatering method based on modified coal slag coupled with multivalent iron salts according to claim 1 or 2, characterized in that, The modified coal slag is acid-modified coal slag, and the specific surface areas of the original coal slag, alkali-modified coal slag, and acid-modified coal slag are 16.7 m² / g, 58.9 m² / g, and 119.7 m² / g, respectively.

4. The sludge dewatering method based on modified coal slag coupled with multivalent iron salts according to claim 3, characterized in that, The suspended solids concentration (SS) of the sludge to be treated is 12.5 g / L.

5. The sludge dewatering method based on modified coal slag coupled with multivalent iron salts according to claim 4, characterized in that, The dosage of Fe(VI) was 1 mg Fe / gSS, and the reaction time was 60 min.

6. The sludge dewatering method based on modified coal slag coupled with multivalent iron salts according to claim 5, characterized in that, The modified coal slag is added at a rate of 15 g / L or 30 g / L.

Images