Original Sludge Research Articles

Na4P2O7-Modified Biochar Derived from Sewage Sludge: Effective Cu(II)-Adsorption Removal from Aqueous Solution

With the rapid development of industrialization, the amount of copper-containing wastewater is increasing, thereby posing a threat to the aquatic ecological environment and human health. Sludge biochar has received extensive concern in recent years due to its advantages of low cost and sustainability for the treatment of heavy-metal-containing wastewater. However, the heavy-metal-adsorption capacity of sludge biochar is limited. This study prepared a sodium pyrophosphate- (Na4P2O7-) modified municipal sludge-based biochar (SP-SBC) and evaluated its adsorption performance for Cu(II). Results showed that SP-SBC had higher yield, ash content, pH, Na and P content, and surface roughness than original sewage sludge biochar (SBC). The Cu(II)-adsorption capacity of SP-SBC was 4.55 times than that of SBC at room temperature. For Cu(II) adsorption by SP-SBC, the kinetics and isotherms conformed to the pseudo-second-order model and the Langmuir–Freundlich model, respectively. The maximum adsorption capacity of SP-SBC was 38.49 mg·g−1 at 35°C. Cu(II) adsorption by SP-SBC primarily involved ion exchange, electrostatic attraction, and precipitation. The desired adsorption performance for Cu(II) in the fixed-bed column experiment indicated that SP-SBC can be reused and had good application potential to treat copper-containing wastewater. Overall, this study provided a desirable sorbent (SP-SBC) for Cu(II) removal, as well as a new simple chemical-modification method for SBC to enhance Cu(II)-adsorption capacity.

Formation of Recalcitrant Compounds during Anaerobic Digestion of Thermally Pre-Treated Sludge: A Critical Macromolecular and Structural Study.

Thermal hydrolysis, when used as pre-treatment, enhances the anaerobic digestion of sewage sludge; moreover, due to the high temperature normally applied, undesirable recalcitrant compounds via Maillard reactions may also be formed. However, although the appearance of these recalcitrant compounds is widely reported, more information on the formation, structure, and fate of these compounds is still needed. This study was focused on understanding the amount and whereabouts of such compounds during the anaerobic digestion process with thermal pre-treatment in soluble and total phase and advance in its structural identification by analyzing their infrared (IR) spectra. It was found that, even with the improved methane production and COD degradation, at 165 °C for 30 min, humic-like compounds are formed which could not be degraded at the anaerobic digestion step. These compounds account for 25% of the original sludge. Infrared spectroscopy proved to be a powerful technique, permitting their differentiation from the natural humic-like compounds. This research provides new information about the structure of melanoidins at every stage of the thermal hydrolysis pre-treatment and how they contribute to the dissolved organic nitrogen.

Anti-clogging mechanism of freeze-thaw combined with step vacuum preloading in treating landfill sludge

Globally, landfill sludge (LS) has accumulated in large quantities, and its reduction and dewatering are urgently needed. To address pollution problems and clogging of drainage boards caused by chemical conditioning combined with traditional vacuum preloading (TVP), a freeze-thaw combined with step vacuum preloading (F/T-SVP) method is proposed. A comparative experimental study was carried out between TVP and SVP to explore the anti-clogging mechanism of F/T-SVP in treating LS. As a result, the water discharge for the original sludge (OS) is 1840 ml, the water discharge for TVP is 8830 ml and for SVP is 10,010 ml; The total settlement of SVP is 16% higher than that of TVP; TVP has a volume reduction ratio of 57.6%, while SVP has 66.8%; the OS's water content was 86%, which was reduced to 57.6% by F/T-SVP; The center of the drainage board of TVP is seriously clogged, while the particles of SVP are evenly distributed; The tendency for small particles to undergo transport is relatively low at the beginning of SVP, which can effectively reduce clogging; TVP mainly focuses on the compression of large pores into small pores, and SVP mainly focuses on the compression of large into small pores and micropores. In SVP, there is more consolidation and a more compact structure. When F/T-SVP is used to treat LS, the pores are gradually penetrated, effectively avoiding the generation of clogging and improving LS's drainage and consolidation.

Biodegradable surfactants from side streams of the vegetable oils production in technical systems

This work reports the structure of surfactants synthesized from by-products of the vegetable oil production. These are raw materials that do not compete with food products; they are low-cost phosphatidic sludge that can be used directly for chemical transformation. Fatty acid monoetanolamides derived from side streams of the vegetable oils production do not have residues of the original phosphatides or acylglycerols as determined by spectral methods. There are clearly reflected cross-peaks between the amide group and the adjacent methylene group. This indicates a high conversion of substrate and further confirms the amidation reaction. The main acyl residue of the synthesized surfactants are hydrocarbon chain of linoleic acid however, while the detection of methylene groups near double bonds indicates the presence of other fatty acids (oleic, linolenic and gadolein), this corresponds to the fatty acid composition of the original phosphatidic sludge. Synthesized surfactants are effective emulsifiers-stabilizers for dispersed systems. The use of environmentally friendly components in the composition of surfactants improves their biodegradability to 83–86%.

Heat treatment-enhanced bioleaching of new electroplating sludge containing high concentration of CuS and its mechanisms

Electroplating sludge by CaO/Ca(OH)2 precipitation craft represents a typical toxic heavy metals-bearing hazardous waste worldwide. Recently, sulfide electroplating sludge is growingly produced as a substitute for Ca(OH)2-based hydroxides sludge since sulfide precipitation process sharply reduces the production of sludge, posing an urgent challenge to the traditional treatment ways suitable for hydroxides sludge. In this work, bioleaching was firstly utilized to extract Cu from the new sulfide electroplating sludge containing high concentration of CuS, and a thermal pre-treatment was used to promote the bioleaching performance. The results showed that a low Cu dissolution of 53% from the original sulfide sludge by bioleaching was gained because of the refractory nature of sulfides. However, a thermal treatment evidently enhanced the bioleaching efficiency of sulfide electroplating sludge. The Cu release efficiency sharply rose from 59% to 100% when the roast temperature lifted from 200 °C to 600 °C. However, a higher temperature of 800 °C strongly injured the bioleaching, gaining the lowest Cu release of 46%. A full conversion of refractory CuS into highly water soluble CuSO4 occurred with the 600℃-roast, concomitant with the formation of a finer particle which witnessed a great rise in the specific surface area from 3.64 to 12.06 m2/g, accounting for 100% Cu liberation by bioleaching. In contrast, the calcination at 800 °C prompted the decomposition of CuSO4 to form CaSO4 which not only covered the sludge as coating layer but also induced sludge aggregation to form a larger particle with a smaller specific surface area of 1.22 m2/g, inhibiting the bioleaching. This present works revealed the internal mechanisms for a perfect combined process of the thermal treatment and bioleaching for tackling the sulfide electroplating sludge.

Comprehensive effects of grain-size modification of electrolytic manganese residue on deep dehydration performance and microstructure of sludge

As the by-product accompanied by sewage treatment, sludge has complex composition and high moisture content, therefore, its reutilization and disposal are still a challenge. In this paper, five kinds of quartz sand conditioners with different particle sizes (denoted as QS1, QS2, QS3, QS4 and QS5, respectively) were used to explore the effect of particle size distribution of conditioners on sludge dewatering performance. The moisture content, capillary suction time (CST), time to filter (TTF), specific resistance of filtration (SRF), particle size distribution curve, pore distribution law, scanning electron microscopy, isothermal adsorption-desorption curve and extracellular polymeric substances distribution were employed to characterize the modified sludge and explore the improvement mechanism. The results show that the particle size distribution of the conditioner significantly affects the efficiency of sludge dewatering. The wt% of sludge regulated with QS1 (QS1-S) could be reduced to 52%, and its CST value, TTF value and SRF value is 57.93 s, 278 s and 1.84 × 108 s2 g−1, respectively. The conjecture about the effect of difference of particle size distribution on sludge dewatering performance was verified with the original Electrolytic Manganese Residue (EMR) and the grain-size modified Electrolytic Manganese Residue (EMR6). Compared with those of the EMR-conditioned sludge, the CST, TTF and SRF of EMR6-conditioned sludge was decreased by 8.7%, 22.3% and 11.2%, respectively. According to analysis of surface microstructure, the surface of the sludge cake modified with QS1 is rough and sparse with rich pore structure. Compared with those of the undisturbed sludge (A0), the pore volume and specific surface area of the sludge modified with QS1 was increased by 61.65% and 38.62%, respectively. After grain-size modification, the dehydration effect of EMR6 (D10 4.25 μm, D50 19.65 μm, D90 73.26 μm) was significantly enhanced, and the D10, D50 and D90 value was close to that of QS1. It can be concluded that the particle size of QS1 (D10 3.27 μm, D50 15.66 μm, and D90 62.23 μm) can improve the dewatering performance of sludge by shearing the sludge particles to change the original sludge particle size distribution and improving the blockage of sludge dewatering channels.

Enhancement of hydrolysis efficiency and biogas production by treatment of secondary sludge with bacteriophage lysozymes

Anaerobic digestion of secondary sludge from wastewater treatment plants can degrade sludge while producing methane that can be burned as bioenergy. Hydrolysis of secondary sludge is the first step in anaerobic digestion, but is the rate-limiting step. In this study, to speed hydrolysis of secondary sludge, lysozymes from bacteriophage species T4, T7 and λ were applied to batch tests. The volatile suspended solids were removed by lysozymes at efficiencies of 23.6–50.1 %. Use of the lysozymes increased the reaction coefficient of biogasification of volatile suspended solids by 0.4–1.0 d−1, regardless of the origin of sludge, or sources of lysozyme. The removal efficiency of volatile suspended solids and methane yield had significantly positive correlation (p < 0.001). Relative abundances of genera Simplicispira (10.3 → 5.2 %), Dokdonella (4.9 → 3.9 %), and Thermomonas (2.7 → 1.2 %) were high in original secondary sludge, and maintained relative abundance over other genera even after lysozyme treatment. To increase the ratio of microbes to organics, sludge samples from various sources were aerated and cultivated with 40 g/L glucose. In experiments on this cultivated sludge, Klebsiella (3.7 → 23.0 %) Kosakonia (1.8 → 19.0 %) and Dysgonomonas (0.0 → 15.3 %) sharply increased after lysozyme treatment, due to their fast growth. Treatment with bacteriophage lysozymes can significantly increase sludge reduction.

Thermophilic dark fermentation fast start-up of hydrogen production with substrate concentration regulation and moderate pretreatment inoculum

Accelerating start-up of thermophilic dark fermentation would improve hydrogen production efficiency was investigated. To find an efficient method for accelerating start-up and explore the effect of inoculum type for hydrogen production system, three thermophilic hydrogen production EGSB (expanded granular sludge bed) reactors equipped with flocculent or granular sludge using different startup modes were constructed in this study. Among three reactors under butyric acid type of fermentation, EGSB3 (startup mode: pretreated granular sludge as inoculum and regulated substrate concentration) was the most efficient one with shortest start-up time (13 days) and maximum hydrogen production (1.64 mol H2/mol glucose). EGSB1 (startup mode: pretreated flocculent sludge as inoculum and regulated substrate concentration) was started up in 27 days at 1.27 mol H2/mol glucose in maximum. While EGSB2 (startup mode: original granular sludge as inoculum and without regulating substrate concentration) took 46 days for start-up and the 1.6 mol H2/mol glucose hydrogen producing rate was obtained. After start-up, the dominant hydrogen production bacteria were Thermoanaerobacterium thermosaccharolyticum strain TG57 in three reactors. EGSB3 reactor owned the most proportion with 53.21 % than others. In all, mild pretreatment and regulating substrate concentration for granular sludge would be benefit for fast startup of thermophilic dark fermentation hydrogen production leading to shortest startup time, high hydrogen production efficiency and most functional microorganism.

Iron valence state evolution and hydrochar properties under hydrothermal carbonization of dyeing sludge

Iron (Fe) migration mechanisms and hydrochar properties in dyeing sludge hydrothermal carbonization (HTC) are important topics in wastewater treatment. HTC treatment of sludge produces wastewater containing Fe so it is necessary to study the migration behavior of Fe during HTC treatment. This study investigated the basic properties and Fe migration behavior of hydrochar during HTC treatment supplemented with nitric acid (HNO3). The results showed that the carbonization degree and yield of hydrochar treated with the HNO3 solution (HHC) were much lower than those of hydrochar treated with ultrapure water (WHC). The variation of total Fe (TF) concentration indicated that the decomposition of organic material and dissolution of minerals in the aqueous release of Fe during the liquid phase, led to much lower TF concentrations compared to the original dyeing sludge. Fe release was further enhanced with the addition of HNO3 and increase of temperature, rendering a much lower TF concentration of the HHC compared to the WHC. The variations of Fe3+ and Fe2+ concentrations indicated that the HTC-treated hydrochar contained more Fe2+, caused by Fe3+ reduction with hydroxyl methyl-furfural and glucose in the liquid and subsequent Fe2+/Fe3+ transferral to the solid hydrochar phase. X-ray diffraction (XRD) showed that the main Fe content in WHC was FeO(OH), while HHC contained mainly Fe(SO4)(OH)•2H2O and Fe3O4. XPS and XRF showed that Fe could more easily enter the internal pores of the hydrochar instead of being deposited on the surface. This study provided more insights on Fe migration behavior during HTC treatment.

Effects of primary curing conditions and subsequent crumbling on properties of compacted soils treated with paper sludge ash-based stabilizers

Soil stabilization using paper sludge ash-based stabilizers (PSASs) has been developed as a technique for using the sustainable materials generated from industrial processes in construction projects. PSASs can be produced by insolubilizing the heavy metals in original paper sludge (PS) ash particles, i.e., the waste generated by the incineration of the PS discharged from paper mills. The aim of this study was to investigate the effects of the primary curing conditions and subsequent crumbling on the physical, compaction, and strength characteristics of PSAS-treated soils, using two types of PSASs with different water absorption and retention performances. For comparison, the same investigation was conducted on soils treated with blast furnace cement type B (BFCB). The experimental results revealed that, after crumbling, the PSAS-treated samples produced sand and gravel-like granules, regardless of the sealed or air primary curing conditions. In addition, the lower the water content at crumbling, the smaller the particle size. This was also true for the BFCB-treated samples. Consequently, the compaction test results demonstrated that, for one of the two types of PSASs, the dry densities of the PSAS-treated samples with sealed primary curing were almost the same as those without primary curing. The same trend was observed for the BFCB-treated samples. However, for the other type of PSAS, the dry densities of the PSAS-treated samples with sealed primary curing were lower than those without primary curing. This could be due to the difference in the degrees of disturbance caused by crumbling, depending on the type of PSAS. The rapid formation of hydrates in one type of PSAS may have significantly disturbed the treated samples owing to crumbling, resulting in a decrease in dry density. Finally, after secondary curing in a soaked environment, cone index tests were conducted on the PSAS- and BFCB-treated samples. The results indicated that the cone indices of the PSAS-treated samples with primary curing were higher or lower than those of the samples without primary curing, depending on the primary curing environment, number of curing days, and type of PSAS. The different trends, depending on the conditions, were considered to be caused by the combined effects of the “strength reduction owing to crumbling,” and “strength increase owing to water content reduction at compaction.” These mechanisms suggest that, for PSAS-treated soils with early strength development, the strength reduction caused by crumbling must be considered. However, for PSAS-treated soils with slow strength development, adjusting the water content of the treated soils through primary curing before compaction is an effective approach. Moreover, it is suggested that the curing conditions used for the laboratory mixtures be designed and set to reflect the field conditions and to minimize any discrepancies between the field and laboratory observations for the PSAS treatment.

Saline short-term shock and rapid recovery on anammox performance

Anaerobic ammonia oxidation (anammox) is an environmental-friendly biological nitrogen removal process, which has been developed as a promising technology in industrial wastewater treatment. However, anammox nitrogen removal under high saline conditions still faces many challenges. This study investigated the performance of anammox sludge under saline short-term shock and the strategy of rapid recovery. Salinity concentration, saline exposure time, and NaCl/Na2SO4 ratio were selected as three critical factors for short-term shock. The activity inhibition of anammox sludge were tested by using response surface methodology (RSM). Our results showed that, compared with the NaCl/Na2SO4 ratio, the salinity concentration and saline exposure time were the significant factor causing the anammox inhibition. The addition of glycine betaine (GB) in moderate amounts (0.1–5 mM) was found to help anammox to resist in relative low saline shock intensities (e.g., IC25 and IC50), with the activity retention rate of 94.7%. However, glycine betaine was not worked effectively under relatively high saline shock intensities (e.g., complete inhibition condition). Microbial community analysis revealed that Brocadiaceae accounted for only about 7.6%–13.2% at inhibited conditions. Interestingly, 16S rRNA analysis showed that the abundance of activated Brocadiaceae remarkably decreased with time after high-level saline shock. This tendency was consistent with the results of qPCR targeted hzsA gene. Finally, based on quorum sensing, the anammox activity was recovered to 93.5% of original sludge by adding 30% original sludge. The study realized the rapid recovery of anammox activity under complete inhibition, promoting the development and operation of salt-tolerant anammox process.

Reaction mechanism of Ca-reduction diffusion process used for sustainable recycling Nd-Fe-B sludge

The reduction diffusion (RD) process for Nd-Fe-B sludge has been reported as a green and efficient method for synthesizing recycled Nd2Fe14B magnetic powders. In this work, recycled Nd-Fe-B powders with low impurity content and uniform particle size distribution were successfully prepared. M3 T increased to 146.25 emu/g, which was about 17% and 32% higher than the purged and original sludge, respectively. Then, the recycled Nd-Fe-B sintered magnets with properties of Br = 12.14 kG, Hcj = 12.00 kOe, and (BH)m = 35.05 MGOe were successfully prepared by doping with 37.70 wt% Nd4Fe14B powders. Based on experimental studies of calcium thermal reduction of Nd-Fe-B sludge and thermo-kinetic analysis, the reaction mechanism of the RD method for sludge waste recovery was clarified. In the RD process, the oxides in the sludge were selectively reduced by Ca at 600–950 °C and the reduced Nd diffused to the surface of the Fe core, forming the Nd-Fe-B alloy, where the formation process included the nucleation stage, the alloy layer high-speed growth stage, and the completion stage. And the relationship between the alloy layer thickness and time during the high-speed growth period of 1–3 h was more in line with linear law (d = 5.41 t–4.31). The RD reaction conformed to the unreacted shrinking core model, and its reaction rate control step consisted of a one-way diffusion process for the diffusion of Nd into the Fe core.

Impact of acceleration of agricultural and industrial waste conditioning on thermal drying of sewage sludge.

Improving the dewatering performance of sludge is a necessary advancement for collaborative sludge disposal and energy-efficient utilization in power plants. Herein, the effects of temperature changes and the addition of drying accelerators derived from agricultural and industrial waste on the drying characteristics of sewage sludge (SS) were investigated from the perspectives of drying kinetics and micromorphology. According to the results, the drying time for sludge significantly showed a clear downward trend via medium-low temperature (160°C) thermal drying, which subsequently reduced the energy input substantially. When the drying temperature was 160°C, the ideal addition proportions of rice husk (RH), sludge ash (SA), and coal ash (CA) comprised 15%. The addition of SA prominently boosted the drying rate constant of the original sludge by 48%. Additionally, SEM images along with the spectral dimension obtained via fractal theory explicitly clearly demonstrated that SS + SA had more loose pore channels than other mixed samples. This provided a convenient approach to ensure the evaporation and migration of moisture and thereby shorten the drying time effectively. Consequently, the addition of sludge ash as a drying accelerator can promote the deep dewatering of sludge.

Solid waste material reuse analysis: filling the road subgrade with riverway silt and sediment.

This study proposes to explore solid waste material (SWM) reuse of the riverway silt and sediment, and examines the impacts of chemical composition on road construction through sensitivity analysis. Considering the characteristics of silt mixture, it is necessary to investigate the modified materials to improve the mechanical feasibility for subgrade filling. In this study, the water content of riverway silt and sediments was found to be important to determine the selection and content of modified materials. Specifically, the riverway silt and sediment with low water content could be improved effectively with 6 to 8% lime. Compared to the original sludge, the improved mixture had better particle size and permeability, and the carrying capacity also grew 2 to 3 times. On the other hand, the reuse of riverway silt and sediment with high water content over 40% was provided with multiple schemes. Among them, the modification scheme of construction waste or garbage slag showed well mechanical properties and environmental benefits in the sensitivity analysis, especially for the high water content sludge modified by the mixture of garbage slag and lime. The California bearing ratio (CBR2.5) was 2 to 5 times higher than the original silt, which would promote the reuse of multiple solid wastes in road construction. Finally, this study puts forward engineering measures to prevent heavy metals from polluting the water and soil environment by silt-improved soil roadbeds, and the improved riverway silt and sediment roadbeds were proved to be safe and reliable for the environment during service.

Development and Application of a High-Volume Recycled Powder Solidifying Material for Waterworks Sludge

Recycled powder (RP) is produced as a by-product during the process of recycling construction and demolition (C&D) wastes, presenting a low additional value. Using RP-based solidifying material can not only improve its utilization efficiency, but also reduce the cost of commercial solidifying materials. To date, this is the best solidifying material utilized to dispose the original waterworks sludge (OWS) with high moisture contents (60%), and the product could be used to fabricate non-fired bricks. This has become a new environment-friendly technology of “using waste to treat waste”. In this paper, the influence of different particle sizes and dosages of RP on the prepared solidifying material was studied. Besides, unconfined compression strength (UCS), volume stability, chemical composition, and heat of hydration, pore structure of the solidifying material were characterized. Then, non-fired bricks were prepared by using the solidifying material, recycled aggregate, and original waterworks sludge. The UCS and softing coefficient (SC) of the non-fired bricks were evaluated. As a result, the 28-day UCS of the solidifying material with optimal (M30) was 35.40 MPa, which could reach 84.37% of Portland cement (PC). The addition of RP increased the volume stability of the solidifying material. The addition of a large amount of RP reduced the heat flux and cumulative heat release of the solidifying material, while its porosity increased. The UCS of non-fired brick (NF20) in 28 days was 15.19 MPa and the SC after 28 days was 78.35%. In conclusion, the preparation of solidifying material using RP could be a promising approach and has a great potential in disposal of original waterworks sludge.

Recycling of chromium sludge to promote aggregation and growth of metal during self-reduction of nickel laterite ore

Addition of stainless steel pickling sludge, which contains hazardous hexavalent chromium, to low-grade nickel laterite ore has the potential to eliminate the hexavalent chromium and promote the aggregation and growth of iron-nickel particles during reduction with anthracite. The effect of chromium sludge on the aggregation and growth of metal particles during a self-reduction process of nickel laterite ore has been studied. The detoxification effect of hexavalent chromium in the sludge was evaluated using the standard TRGS 613 leaching test. Results showed that with increasing chromium sludge addition, the size of metal particles increased. With the addition of 13.94% by mass chromium sludge the average metal particle size in the reduce ore increased from 0.42 µm to 1.38 µm while the recovery of Fe, Ni and Cr increased from 53.73%, 19.39% and 33.36% to 78.33%, 69.22% and 58.26%, respectively. It is proposed that a low melting point eutectic phase of FeS formed in the reduced products, significantly promoted the migration and aggregation of metal particles. Leaching tests indicated that the original chromium sludge released &gt; 600 mg/kg Cr(VI) while the slag from the combined laterite and chromium sludge released less than the EPA limit of 2 mg/kg Cr(VI).

Deep dewatering of activated sludge using composite conditioners of surfactant, acid and flocculant: The mechanism and dosage model

To address the problem of difficult disposal caused by poor dewaterability of high-organic sludge in wastewater treatment plant, this study developed a sludge composite conditioner (SCC) consisting of sodium dodecyl sulfate (SDS), HCl and FeCl3. It has the potential to significantly improve the dewaterability of the high-organic sludge with the VSS/MLSS of 80%. The moisture content (MC) and bound water content of sludge were reduced from 98.00 to 59.65% and from 3.42 to 0.91 g/g dry sludge (DS) after being conditioned, respectively. The surfactant (SDS) promoted the dissolution of extracellular polymeric substances (EPS). The acid (HCl) enhanced the decomposition of the sludge flocs, making the insoluble EPS peel off and turn into the liquid phase. As a result, total EPS decreased by 52.70% compared to the original sludge. In addition, due to the neutralization effect of protons and FeCl3, the Zeta potential increased remarkably from −13.80 mV to −1.72 mV and the dispersed sludge particles formed during EPS dissolution process were re-flocculated, which increased the average size of the sludge particles. The ratio of proteins (PN)/polysaccharides (PS) also increased from 1.69 to 3.81. And a quantitative model of optimum dosage of SCC agents based on the influence of the sludge PS, PN and EPS content has been established, aiming to determine the dosage of each conditioner according to the properties of target sludge. In general, the SCC provided an effective pathway for sludge deep dewatering.

Nd-Fe-B: From sludge waste to powders via purification and modified Ca-reduction reaction process

The oil-based Nd-Fe-B sludge waste was directly recycled to single-phase Nd2Fe14B powders via the combination of purification and modified Ca-reduction reaction method. The impurities and organics in the purified Nd-Fe-B sludge waste were greatly reduced, reducing calcium consumption in the subsequent reduction and diffusion (RD) process, thereby reducing the cost. By further optimizing the Ca-reduction diffusion parameters, especially the mass ratio of CaCl2 and KCl, where the liquid CaCl2, along with the gaseous KCl provide conditions for sufficient and uniform Ca-reduction reaction, high-property Nd-Fe-B magnetic powders with good dispersion, uniform particle sizes, and excellent orientation, were successfully obtained. Using the mixed diffusion medium of CaCl2 and KCl with a mass ratio of 1:1, the room-temperature magnetization of the recycled Nd-Fe-B powders was increased to 157 emu/g at 3 T-magnetic field, which was about 28 % higher than that of the original sludge. The contents of carbon, hydrogen, and oxygen in the recycled magnetic powders were significantly reduced from 6.8, 1.8, and 5.9 wt% to 0.1, 0.19, and 0.56 wt%, respectively. In addition, the uniform grain size with X50 = 3.6 μm and good distribution greatly improved the orientation, which can be beneficial to the preparation of the bonded or sintered Nd-Fe-B-based magnets. The reasons that CaCl2-KCl mixed diffusion medium can lead to well-dispersed, uniform recycled powder with high magnetization arise from a combination of factors for improving the liquid phase reaction environment and the gaseous KCl to prevent agglomeration and inhibit particles merging or growth.

Effect of dewatering conditioners on pollutants with nitrogen, sulfur, and chlorine releasing characteristics during sewage sludge pyrolysis

Sewage sludge is a harmful solid by-product generated during sewage treatment, and pyrolysis is a technology adopted for its disposal. To reduce the energy consumption during pyrolysis, the sludge is subjected to pretreatments, such as conditioning and dewatering, to reduce the water content. However, the mechanism by which these conditioners affect the pyrolysis products during the pretreatment process has not yet been elucidated. In this study, X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical state changes of N, S, and Cl pollutants in the sludge samples with and without the conditioner before and after pyrolysis. Thermogravimetric-mass spectrometry (TG-MS) and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) were used to clarify the release characteristics of condensable and non-condensable gases during the pyrolysis of the sludge samples. The results showed that Fe produced more free radicals of •H and •CH3 to combined with •S• and •SH to form H2S and CH3SH, and promoted the decomposition of macromolecular substances. Moreover, different anion atmospheres caused various pollutant emission effects. The Cl− from the conditioner promoted the release of micromolecular N/Cl-containing pollutants. The SO42− enhanced the release of CH3SH and SO2 but inhibited the emission of N-containing pollutants. Several reaction kinetic models fitted by the Coats-Redfern method showed that the Fe-containing sludge exhibited lower activation energy than the original sludge. A comprehensive analysis of the response of conditioners in the sludge pyrolysis process can provide a sustainable and improved understanding of the integrated treatment and disposal of sludge via conditioning, dewatering, and pyrolysis.

Efficient reclaiming phosphate from aqueous solution using waste limestone modified sludge biochar: Mechanism and application as soil amendments

A novel limestone-modified biochar derived from sewage sludge was prepared to reclaim phosphorus (P) from aqueous solution, and the potential application of P-laden biochar as soil amendments was also investigated. The limestone-modified biochar demonstrated excellent performance on phosphate recovery from aqueous solution in a wide range of pH (2.0–11.0), with maximum adsorption capacity of the biochar (Limestone/sludge mass ratio of 3:1) up to 231.28 mg P/g, which was 10.7 times that of the original sludge biochar. The adsorption was well described by the pseudo second-order model and Langmuir isotherm model. According to the adsorption thermodynamic parameters, the phosphate adsorption was spontaneous (ΔG0 < 0) and endothermic (ΔH0 > 0) so that increasing the temperature was beneficial to adsorption. Characterization analysis by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscope-energy dispersive spectrometer (SEM-EDS) proved that electrostatic attraction, surface complexation and brushite (CaHPO4.2H2O) precipitation were the dominant mechanism. The P-laden biochar exhibited an excellent ability to be reused as a new slow-release P fertilizer for soil. Pot experiment results showed that the treatment of P-laden LB 3:1 (P content of 22.8%) addition (1 wt%) significantly promoted Indian Lettuce germination (increasing by 14.4%), plant height (increasing by 18.6%), and dry biomass (53.0%) compared with the control, though it underperformed compared to commercial fertilizer.