Using a strong chemical oxidant, potassium ferrate (K2FeO4), in waste activated sludge treatment: A review

Application of calcium peroxide in promoting resource recovery from municipal sludge: A review

The harmless disposal, resource recovery, and synergistic efficiency reduction of municipal sludge have been the research focuses for the last few years. Calcium peroxide (CaO2) is a multifunctional and safe peroxide that produces an alkaline oxidation environment to promote the fermentation of municipal sludge to produce hydrogen (H2) and volatile fatty acids (VFAs), thus realizing sludge resource recovery. This review outlines the research achievements of CaO2 in sludge resource recovery, improvement of sludge dewaterability, and removal of pollutants from sludge in recent years. Meanwhile, the mechanism of CaO2 and its influencing factors have also been comprehensively summarized. Finally, the future development direction of the application of CaO2 in municipal sludge is prospected. This review would provide theoretical reference for the potential engineering applications of CaO2 in improving sludge treatment in the future.

Research progress in improving sludge dewaterability: sludge characteristics, chemical conditioning and influencing factors

Sludge from wastewater treatment processes with high water content and large volume has become an inevitable issue in environmental management. Due to the challenging dewatering properties of sludge, current mechanical dewatering methods are no longer sufficient to meet the escalating water content standards of sludge. This paper summarizes the characteristics of various sludge and raises reasons for the their dewaterability differences. Affected by extracellular polymeric substances, biological sludge is hydrophilic and negatively charged, which limits the dewatering degree. The rheological properties, flocs, ionic composition, and solid phase concentration of the sludge also influence the dewatering to some extent. For these factors, the chemical conditioning measures with simple operation and excellent effect improve its dewaterability, which mainly include flocculation/coagulation, acid/alkali treatment, advanced oxidation, surfactant treatment and combined treatment. There is a growing necessity to explore the development of new chemical conditioning agents, even though traditional agents continue to remain widely used. However, the development of these new agents should prioritize finding a balance between various factors such as efficiency, effectiveness, ease of operation, environmental safety, and cost-effectiveness. Electrochemical dewatering enhances solid-liquid separation, and its coupling with chemical conditioning is also an excellent means to further reduce water content. In addition, the improvement of press filter is an effective way, which is influenced by pressure, processing time, sludge cake thickness and pore structure, filter media etc. In general, it is essential to develop new conditioning agents and enhance mechanical filtration press technology based on a thorough understanding of various sludge properties. Concurrently, an in-depth study of the principles of mechanical pressure filtration will contribute to establishing a theoretical foundation for effective deep sludge dewatering and propel further advancements in this field.

Enhanced dewaterability of sludge by Fe(II)-sludge biochar activate persulfate

Sludge biochar supported Fe(II) (Fe(II)-SBC) was successfully prepared using waste activated sludge as peroxydisulfate (PDS) activator to condition sludge for deep dewatering. The experimental results showed that Fe(II)-SBC with FeO on it could effectively active PDS to produce SO 4 - ⋅ and HO ⋅ . The radicals could destroy the structure of sludge cells and extracellular polymeric substance (EPS), transformed the hydrophilic and tightly bound EPS into soluble-EPS, degrade partial proteins and polysaccharides and released bound water. The negatively charged groups on sludge floc were dripped off by SO 4 - ⋅ /HO ⋅ or neutralized with Fe2+, Fe3+, H+, or Fe(II)-SBC, leading to an increase in zeta potential to -2.24 mV and sludge destabilization. The residual Fe(II)-SBC served as a skeleton builder that decreased the compression coefficient of the sludge cake to 0.75. Under the combined functions, the CST and SRF were reduced by 70% and 82.7%, respectively, and Wc was reduced to 72.4%. The byproducts of Fe3+ and SO42- finally remained in sludge cake in the form of NaFeSi2O6 and CaSO4.

Use of biochar and a post-coagulation effluent as an adsorbent of malachite green, beneficial bacteria carrier, and seedling substrate for plants belonging to the poaceae family

Wastewater treatment plants produce solid and semi-solid sludge, which treatment minimises secondary environmental pollution because of wastewater treatment and obtaining new bioproducts. For this reason, in this paper, the co-pyrolysis of biogenic biomasses recovered from a biological reactor with immobilised fungal and bacterial biomass and a tertiary reactor with Chlorella sp. used for dye-contaminated wastewater treatment was carried out. Biogenic biomasses mixed with pine bark allowed the production and characterisation of two types of biochar. The raw material and biochar were on the "in vitro" germination of Lolium sp. seeds, followed by adsorption studies for malachite green (MG) dye using the raw material and the biochar. Results showed that using 60 mg L-1 of a cationic coagulant at pH 6.5 allowed for the recovery of more than 90% of the microalgae after 50 min of processing. Two biochar resulted: BC300, at pH 5.08 ± 0.08 and BC500, at pH 6.78 ± 0.01. The raw material and both biochars were co-inoculated with growth-promoting bacteria; their viabilities ranged from 1.7 × 106 ± 1.0 × 101 to 7.5 × 108 ± 6.0 × 102 CFU g-1 for total heterotrophic, nitrogen-fixing and phosphate-solubilising bacteria. Re-use tests on Lolium sp. seed germination showed that with the post-coagulation effluent, the germination was 100%, while with the biochar, with and without beneficial bacteria, the germination was 98 and 99%, respectively. Finally, BC500 adsorbed the highest percentage of malachite green at pH 4.0, obtaining qecal values of 0.5249 mg g-1 (R2: 0.9875) with the pseudo-second-order model.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03766-x.

Enhancing Ferrate Oxidation of Micropollutants via Inducing Fe(V)/Fe(IV) Formation Needs Caution: Increased Conversion of Bromide to Bromate

This study explores the formation of bromate (BrO3-) in the copresence of Fe(VI) and bromide (Br-). It challenges previous beliefs about the role of Fe(VI) as a green oxidant and highlights the crucial role of intermediates Fe(V) and Fe(IV) in the conversion of Br- to BrO3-. The results show that the maximum concentration of BrO3- of 48.3 μg/L was obtained at 16 mg/L Br- and that the contribution of Fe(V)/Fe(IV) to the conversion was positively related to pH. The study suggests that a single-electron transfer from Br- to Fe(V)/Fe(IV) along with the generation of reactive bromine radicals is the first step of Br- conversion, followed by the formation of OBr- which was then oxidized to BrO3- by Fe(VI) and Fe(V)/Fe(IV). Some common background water constituents (e.g., DOM, HCO3-, and Cl-) significantly inhibited BrO3- formation by consuming Fe(V)/Fe(IV) and/or scavenging the reactive bromine species. While investigations proposing to promote Fe(V)/Fe(IV) formation in Fe(VI)-based oxidation to enhance its oxidation capacity have been rapidly accumulated recently, this work called attention to the considerable formation of BrO3- in this process.

Contribution identification of hydrolyzed products of potassium ferrate on promoting short-chain fatty acids production from waste activated sludge

Potassium ferrate (K2FeO4) has been extensively employed to promote short-chain fatty acids (SCFAs) production from anaerobic fermentation of waste activated sludge (WAS) because of its potent oxidizing property and formation of alkaline hydrolyzed products (potassium hydroxide, KOH and ferric hydroxide, Fe(OH)3). However, whether K2FeO4 actually works as dual functions of both an oxidizing agent and an alkalinity enhancer during the anaerobic fermentation process remains uncertain. This study aims to identify the contributions of hydrolyzed products of K2FeO4 on SCFAs production. The results showed that K2FeO4 did not execute dual functions of oxidization and alkalinity in promoting SCFAs production. The accumulation of SCFAs using K2FeO4 treatment (183 mg COD/g volatile suspended solids, VSS) was less than that using either KOH (192 mg COD/g VSS) or KOH & Fe(OH)3 (210 mg COD/g VSS). The mechanism analysis indicated that the synergistic effects caused by oxidization and alkalinity properties of K2FeO4 did not happen on solubilization, hydrolysis, and acidogenesis stages, and the inhibition effect caused by K2FeO4 on methanogenesis stage at the initial phase was more severe than that of its hydrolyzed products. It was also noted that the inhibition effects of K2FeO4 and its hydrolyzed products on the methanogenesis stage could be relieved during a longer sludge retention time, and the final methane yields using KOH or KOH & Fe(OH)3 treatment were higher than that using K2FeO4, further confirming that dual functions of K2FeO4 were not obtained. Therefore, K2FeO4 may not be an alternative strategy for enhancing the production of SCFAs from WAS compared to its alkaline hydrolyzed products. Regarding the strong oxidization property of K2FeO4, more attention could be turned to the fates of refractory organics in the anaerobic fermentation of WAS.

Role of potassium ferrate in anaerobic digestion of waste activated sludge: Phenotypes and genotypes

The specific effects of potassium ferrate (PF) on acid and methane production in anaerobic digestion need further exploration. This study comprehensively investigated the role of PF in organic matter conversion in waste activated sludge (WAS) digestion. Due to the high pH produced by PF self-decomposition, the hydrolysis of organic matter was promoted, whereas the methanogenesis was inhibited. PF could further directly oxidize protein and polysaccharides released by hydrolysis to produce volatile fatty acids (VFAs) and involve in the transformation of ammonia nitrogen. PF could induce the enrichment of functional genes related to fermentation pathways and lessen those related to methanogenesis, and the phylum resistant to PF oxidation and the strains capable of producing VFAs were enriched, resulting in VFAs accumulation. This study analyzed the participation way of PF in anaerobic digestion and provided a theoretical basis for the application of PF in promoting VFAs recovery from sludge digestion.

Effect of KMnO4/pH adjustment of extracellular polymeric substances under waste activated sludge on sludge dewatering

In this study, we examine the dewaterability of sludge after treatment by KMnO4 at various pH levels, with the goal of understanding the dewaterability of strong oxidizers to waste activated sludge. Good dewatering performance is observed, with capillary suction times (CST) reduced from 263.4 to 30.1 s, and specific resistance to filtration (SRF) falling by 19.6%. Proteins and polysaccharides in tightly bound extracellular polymeric substances (EPS) were also significantly reduced. Based on spectroscopic and electrochemical analysis, we propose mechanisms for the improved dewatering in terms of changes to the sludge's physicochemical properties and EPS. Under strong oxidation, the structure surrounding the bound water is oxidized and bound water is released, so the dewaterability of the sludge is improved.Weiliang Pan and Jiaoni Li contributed equally to this work.

Enhancing methane production from waste activated sludge with heat-assisted potassium ferrate (PF) pretreatment: Reaction kinetics and mechanisms

This work proposed a novel strategy via heat-assisted potassium ferrate (PF) pretreatment to enhance methane production from waste activated sludge (WAS) during anaerobic digestion. In this research, five dosages of PF (i.e., 0, 0.05, 0.1, 0.3 and 0.5 g/g VSS) at two temperatures (i.e., 25 °C and 55 °C) were explored. Biochemical methane potential experiments illustrated that heat-assisted PF pretreatment improved cumulative methane production with the maximum yield up to 163.93 mL CH₄/g VSS, 149.0 %, 119.6 % and 121.0 % of that in the control, individual 0.5 g PF/g VSS and individual heat (i.e., 55 °C) pretreatment digesters, respectively. The maximum methane potential (B₀) was promoted by 63.2 % with heat-assisted PF pretreatment compared to the control, while the hydrolysis rate (k) changed slightly. Mechanism analysis revealed that heat-assisted PF pretreatment accelerated WAS solubilization and enhanced the biodegradability of released substances, providing more available matrix for bacteria during the following anaerobic digestion processes. Microbial community analysis exhibited that several microbes such as Proteiniclasticum sp., Sedimentibacter sp. and Methanosaeta sp. associated with hydrolysis, acidification and methanogenesis respectively were improved after heat-assisted PF pretreatment. In addition, the relative bioactivities of protease, butyrate kinase and acetate kinase were also increased. Furthermore, variation of dominant genes associated with methane production indicated that acetate-dependent methanogenesis was the main pathway while CO₂-dependent methanogenesis pathway was inhibited by heat-assisted PF pretreatment.

Spatial distribution of fecal pollution indicators in sewage sludge flocs and their removal and inactivation as revealed by qPCR/viability-qPCR during potassium ferrate treatment

Sludge reuse and utilization is one of important routines of disseminating fecal pollution to surface water and groundwater. However, it remains unclear the spatial distribution of fecal pollution indicators in sludge flocs and their reductions during sludge treatment processes. In this study, the abundances of fecal pollution indicators including cross-assembly phage (crAssphage), JC and BK polyomavirus (JCPyV, BKPyV), human adenovirus (HAdV), the human-specific HF183 Bacteroides (HF183) and Escherichia coli (EC) in soluble extracellular polymeric substances (S-EPS), loosely-bound EPS (LB-EPS), tightly-bound EPS (TB-EPS), and pellets of sludge flocs were determined, and the effect of potassium ferrate (PF) treatment on their removal and inactivation was investigated by using both qPCR and viability-qPCR. Results showed that all investigated indicators were detected in each fraction of sludge flocs. The PF treatment led to a great migration of indicators from sludge pellets to sludge EPS and some extent of their inactivation in each fraction of sludge flocs. The overall reductions of human fecal indicators in sludge determined by qPCR were 0-1.30 logs, which were 0-2 orders of magnitude lower than those of 0.69-2.39 logs detected by viability-qPCR, implying their inactivation by PF treatment to potentially alleviate the associated human health risks.

Self-regulated immobilization behavior of multiple heavy metals via zeolitization towards a novel hydrothermal technology for soil remediation

More efficient soil remediation technologies are highly anticipated to treat large quantities of heavy metal-polluted urban sites nowadays. Herein, a novel hydrothermal technology of converting heavy metal-polluted soils into zeolites for in-situ immobilizing heavy metals was proposed. The zeolites (analcime and cancrinite) could be synthesized hydrothermally with certain Na/Si and Al/Si ratios. The formed zeolites could manage to change their species and structure during zeolitization to accommodate different heavy metals in soil according to their size and charge. Since smaller-size Cu²⁺ was introduced, analcime and some cancrinite possessing small cages could be formed adaptively to immobilize the Cu²⁺ by replacing Na⁺ and forming Cu²⁺-OH and Cu²⁺-O. Whereas, cancrinite with large channels managed to form to immobilize the larger-size Cd²⁺ by forming Cd²⁺-O. Interplanar spacing variation of zeolites also corresponded to their structural change for accommodating different heavy metals. Leaching results showed the amounts of Cu and Cd leached from the synthesized zeolites were reduced to 0.005% and 0.05% respectively, reflecting a more stable immobilization of smaller heavy metals by small cages, in agreement with the results of distribution coefficient (K_d). Negligible effect of pH environment on the leaching rates further confirmed the stable structural immobilization of heavy metals by zeolites.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Freezing method assists calcium hypochlorite for synergistically promoting methane production from sludge anaerobic digestion

Anaerobic digestion is widely considered to be a promising technology for waste activated sludge (WAS) treatment, by which sludge stabilization and resource recovery are simultaneously achieved. The poor reaction efficiency however hinders the large-scale applications of WAS anaerobic digestion technology. This study reported an efficient sludge pretreatment method by combining freezing with calcium hypochlorite (Ca(ClO)₂) for enhancing the anaerobic digestion efficiency. Experimental data showed that the optimal combination was freezing at -20 °C coupled with 0.075 g/g VSS (volatile suspended solids) Ca(ClO)₂, by which the maximum biomethane production of 274.4 ± 8.2 mL/g VSS was realized, 1.62 times higher than that of the control. Model-based analysis demonstrated that higher potential and rate for methane production were attained by the combined pretreatment. Mechanism analysis revealed that the extracellular polymeric substances (EPS) and microbial cells were both effectively destructed when treated by combined freezing and Ca(ClO)₂, and more dissolved organics were generated in consequence. Microbial analysis demonstrated that the co-treated reactor enriched more functional microbes (such as Methanosaeta, Methanosarcina and Candidatus_Methanofastidiosum) responsible for biomethane generation than that of the control. Furthermore, the number of fecal coliform was largely reduced in co-treated reactor. As the correlation between sludge anaerobic digestion performance and numerous pretreatment parameters was systematically revealed, this study can provide important references for engineers when applying the combined freezing and Ca(ClO)₂ technology in practical engineering.

Moderate potassium ferrate dosage enhances methane production from the anaerobic digestion of waste activated sludge

The annual increase of waste activated sludge (WAS) has become an urgent problem to be solved in sewage plants worldwide. Anaerobic digestion (AD) of WAS is an attractive choice to maximize the resource utilization rate. Nevertheless, the disintegration of sludge complex polymers is difficult, resulting in a low bioconversion rate. Potassium ferrate (PF), as a green oxidant with strong oxidizing property, has attracted great attention in WAS pretreatment recently. The effects of PF pretreatment on WAS hydrolysis and its dosage-response on methane production were investigated in the present study. Results show that as PF dosage raise from 0 to 50 g-K2FeO4/ kg-TS (total solids), the methane yield enhanced significantly by 40.3% from 0.083 to 0.12 L/g-VSadded (volatile solids). Nevertheless, the further increase in PF dosage resulted in decreased methane production. Especially with the PF dosage of 500 g-K2FeO4/ kg-TS, methane production is even slightly lower than the control reactor without PF oxidation. The mechanism analysis showed that although the dissolution of polysaccharides and proteins was enhanced with the high dosage of PF, the accompanying released humic-like substances and high concentration of ferric ions should be the main reasons inhibiting methane production.

Effect of ferrate pretreatment on anaerobic digestibility of primary sludge spiked with resin acids

Resin acids are mixtures of high molecular weight carboxylic acids found in tree resins. Due to higher hydrophobicity and low solubility, they tend to adsorb on the suspended solids in pulp and paper (P&P) mill wastewater and accumulate in primary sludge through settling. Anaerobic digestion (AD) is a common practice stabilizing sludge; however, high concentration of resin acids affects the AD process. The aim of this research was mainly to determine the impact of ferrate (Fe (VI)) oxidation on selected resin acids and anaerobic digestibility of ferrate-treated primary sludge (PS) spiked with the resin acids. First, batch control oxidation of model resin acids with Fe (VI) was conducted to identify an optimum dosage, pH and contact time using a Box-Behnken design approach. Thereafter, anaerobic treatability studies of primary sludge spiked with resin acids both under control condition and optimum ferrate pretreatment were conducted. Up to 97% oxidation of resin acids occurred in pure water, while only 44%-62% oxidation of resin acids occurred in PS with an increasing Fe (VI) dosage from 0.034 to 0.137 mg Fe (VI)/mg tCOD_fed. The pretreatment did not affect the anaerobic biodegradability of resin acids; however, it lowered their negative influences on the PS digestibility. About 0.076 mg Fe (VI) dosage/mg tCOD_fed solubilized the sludge increasing the methane production by 40% compared to the untreated digester. The potential benefits of ferrate pretreatment of P&P primary sludge include resin acids oxidation and subsequent toxicity reduction, higher sludge solubilization enhancing methane production and enabling anaerobic digestion at higher COD loading.

Effects of potassium ferrate-walnut shell pretreatment on dehydration performance of residual sludge

Sludge dehydration is not only the first process of sludge reduction but also difficult for sludge treatment and disposal, while the high moisture content of sludge would also limit its resource utilization. In this paper, the residual sludge was treated by the strong oxidizing property of potassium ferrate (PF), and walnut shells were used as a skeleton builder to obtain the recycling dehydrated sludge. It also provides a new solution for the poor stability of PF in the treatment of sludge and waste walnut shells. The experiment results showed that the optimum dosage of PF and walnut shell was 60 mg/gDS and 0.8 g/gDS, respectively, and the water content of the combined PF and walnut shell treatment decreased by 5.2% and 3.7% compared to that of PF conditioning alone and walnut shell conditioning alone, respectively. In addition, scanning electron microscopy and three-dimensional fluorescence spectroscopy revealed a large number of cracks on the sludge surface after the combined treatment, and the sludge floc structure became more loose and dispersed, and the hydrophobic substances humic acid and fulvic acid increased, indicating that the dewatering performance of the sludge was effectively improved, further confirming that the combined PF and walnut shells treatment provides a new idea and method for sludge dewatering.

Synergic role of ferrate and nitrite for triggering waste activated sludge solubilisation and acidogenic fermentation: Effectiveness evaluation and mechanism elucidation

Enhancing anaerobic treatment efficiency of waste activated sludge (WAS) toward preferable resource recovery would be an important requirement for achieving carbon-emission reduction, biosolids minimization, stabilization and security concurrently. This study demonstrated the synergic effect of potassium ferrate (PF) and nitrite on prompting WAS solubilisation and acidogenic fermentation toward harvesting volatile fatty acids (VFAs). The results indicated the PF+NaNO₂ co-pretreatment boosted 7.44 times and 1.32 times higher WAS solubilisation [peak soluble chemical oxygen demand (SCOD) of 2680 ± 52 mg/L] than that by the single nitrite- and PF-pretreatment, respectively, while about 2.77 times and 2.11 times higher VFAs production were achieved (maximum VFAs accumulation of 3536.25 ± 115.24 mg COD/L) as compared with the single pretreatment (nitrite and PF)-fermentations. Afterwards the WAS dewaterability was improved simultaneously after acidogenic fermentation. Moreover, a schematic diagram was established for illustrating mechanisms of the co-pretreatment of PF and nitrite for enhancing the VFAs generation via increasing key hydrolytic enzymes, metabolic functional genes expression, shifting microbial biotransformation pathways and elevating abundances of key microbes in acidogenic fermentation. Furthermore, the mechanistic investigations suggested that the PF addition was conducive to form a relatively conductive fermentation environment for enhancing electron transfer (ET) efficiency, which contributed to the VFAs biotransformation positively. This study provided an effective strategy for enhancing the biodegradation/bioconversion efficiency of WAS organic matters with potential profitable economic returns.

PPCPs and heavy metals from hydrothermal sewage sludge-derived biochar: migration in wheat and physiological response

Once the sludge was directly used in the farmland, it will have a negative impact on human health through the food chain because sludge contains pollutants. Sewage sludge pyrolysis into biochar is an effective way to realize sludge harmless and resourceful utilization. This research used hydrothermal carbonization method to convert sludge into sludge biochar (SLBC) to reduce the types and contents of pharmaceuticals and personal care products (PPCPs) and available heavy metals. Furthermore, migration of the residual caffeine (Caf), acetaminophen (Ace), and heavy metals (Cr, Pb, Cu, Zn) released from the SLBC in the wheat was assessed. The results showed that the levels of Caf, Ace, Pb, Cu, and Zn accumulated in the shoots were lower than the limit regulated by Drug and Food Additive Use Standard in China (Caf: 150 mg/kg; Ace: 2.5 ~ 5 mg/kg; Pb: 0.3 mg/kg; Cu: 10 mg/kg; Zn: 20 mg/kg). The migration of Cr from roots to shoots was also significantly controlled by SBLC. SBLC delayed the germination time of wheat seeds with increasing in hydrothermal temperature, the germination rate and root length showed a decreasing trend. Evans blue and O₂^- fluorescence staining of root tips also confirmed this conclusion. When the wheat was exposed to the low temperature and dose of SLBC, the chlorophyll contents and growth of wheat can be significantly increased; the oxidative damage of cell plasma membrane and net photosynthetic rate were reduced. However, 0.8 g/L of SLBC made plants suffer abiotic stress and caused oxidative damage to plants, and decreased membrane system stability. The study provides some parameters for sludge to realize resource utilization in the agricultural system.

Exploring the role of potassium ferrate and straw fiber in enhancing the strength of cement-based solidified municipal sludge

The effect of potassium ferrate (PF) and straw fiber (SF) on the strength of cement-based solidified municipal sludge, including the influence of reducing the organic matter in the sludge on the efficiency of the hydration of the cement, was studied. Single-factor tests, orthogonal tests, and linear weighted optimization methods were used to obtain suitable ratios to meet practical requirements, and then SEM and XRD analyses were used to explore the solidification mechanism. The results showed that PF and SF had significant influence on the strength, with SF having the greatest influence and the strength increasing with the amount of both admixtures, and cement had no significant influence on the strength. After linear weighting optimization, the ideal dosage was found to be 20% cement, 20% PF, and 5% SF, which produced a solidified sludge that had an strength of 126.87 kPa, far higher than the 50 kPa required to qualify for disposal in landfills. Analysis of the mineral content and microstructure showed that PF and SF could promote cement hydration and produce more hydration products, and the density of the optimized sample was much higher than that of the raw sludge and a sludge sample mixed with 20% cement alone.

Enhanced technology for sewage sludge advanced dewatering from an engineering practice perspective: A review

The increasing production of sludge poses significant environmental risks. Sludge disposal and transport are costly because of the high water content (WC). Reducing the WC of sludge is the most efficient strategy to decrease treatment costs. However, the sludge contains a large amount of hydrophilic organic matter, causing poor dewaterability. Therefore, research on preconditioning and mechanical dewatering has great significance for advanced sludge dewatering. In this study, the features of sludge, the advantages and disadvantages of preconditioning methods, and the action mechanisms (including physical, chemical, and biological preconditioning) are thoroughly described. In addition, the dewatering principle and engineering applications of mechanical dewatering techniques are introduced in this manuscript, especially the application of vacuum preloading as an in-situ dewatering technology in sludge. Finally, cost analysis of different conditioning and mechanical dewatering methods is conducted to explore their application feasibility. This manuscript provides new insights for engineering applications of preconditioning methods and mechanical dewatering technology.

pH regulated potassium ferrate oxidation promotes acetic acid yield and phosphorous recovery rate from waste activated sludge

To improve the dose efficiency of K₂FeO₄ in waste activated sludge (WAS) treatment, pH regulation on K₂FeO₄ pretreatment and acidogenic fermentation was investigated. Four pretreatments were compared, i.e. pH3 + 50 g/kg-TS, pH10 + 50 g/kg-TS, neutral pH + 50 g/kg-TS and neutral pH + 100 g/kg-TS (without pH adjustment). The higher short chain fatty acids (SCFAs) yield and phosphorous dissolution rate was found under the condition of pH 10.0. In pH10 + 50 g/kg-TS, the maximum concentration of SCFAs was 5591 mg-COD/L, which yield was 22.6 times higher than that of the neutral pH + 50 g/kg-TS (237 mg COD/L). The acidogenic fermentation period could be shortened to 5 days and acetic acid accounted for 70 % of SCFAs. Furthermore, PO₄^3--P in the hydrolysate (346.5 mg/L) accounted for 47.59 % of TP, which is easier to be recovered by chemical precipitation. Therefore, a more economical and feasible utilization mode of potassium ferrate was proposed.

An electro-peroxone oxidation-Fe(III) coagulation sequential conditioning process for the enhanced waste activated sludge dewatering: Bound water release and organics multivariate change

As a by-product of wastewater treatment, waste activated sludge (WAS) has complex composition, strong hydrophilic extracellular polymeric substance (EPS), which make it difficult to dewater. In this study, an electro-peroxone oxidation-Fe(III) coagulation (E-peroxone-Fe(III)) sequential conditioning approach was developed to improve WAS dewaterability. At E-peroxone oxidation stage, hydrogen peroxide was generated through 2-electron path on a carbon polytetrafluoroethylene cathode, and reacted with the sparged O₃ to produce hydroxyl radicals. At the subsequent coagulation stage, Fe(III) was dosed to coagulate the small WAS fragments and release water from WAS. Along E-peroxone-Fe(III) subsequent conditioning process, the physicochemical properties of WAS, main components, functional groups and evolution of protein secondary structure, and typical amino acids in EPS, as well as the type and semi-quantitative of elements in WAS, were investigated. The results indicated that under the optimal conditions, the reductions of specific resistance to filterability (SRF) and capillary suction time (CST) for WAS equalled 78.18% and 71.06%, respectively, and its bound water content decreased from 8.87 g/g TSS to 7.67 g/g TSS. After E-peroxone oxidation, part of protein and polysaccharide migrated outside from TB-EPS to slime, the ratio of α-helix/(β-sheet + random coil) declined, even some of organic-N disintegrated to inorganic-N. At Fe(III) coagulation stage, re-coagulation of the dispersed WAS fragments and easy extraction from inner EPS for protein and polysaccharide occurred. Furthermore, the protein secondary structure of β-sheet increased by 13.48%, the contents of hydrophobic and hydrophilic amino acids also increased. In addition, a strong negative correlation between the hydrophobic amino acid content of Met in slime and CST or SRF (R²_CST = -0.999, p < 0.05 or R²_SRF = -0.948, p < 0.05) occurred, while a strong positive correlation between the hydrophilic amino acid content of Cys in TB-EPS and CST or SRF (R²_CST = 0.992, p < 0.05 or R²_SRF = 0.921, p < 0.05) occurred, which could be related to the WAS dewaterability.

Stepwise freezing-thawing treatment promotes short-chain fatty acids production from waste activated sludge

Waste activated sludge (WAS), as the byproducts of wastewater treatment plants, has been greatly produced. With high cost and environmental risk of WAS disposal, to explore a low-cost and environment-friendly technology has been a great challenge. Considering that WAS is a collection of organic matters, anaerobic fermentation has been selected as a sustainable way to simultaneously recover resources and reduce environmental pollution. To recover short-chain fatty acids (SCFAs) has gained great concern because of the high value-added application and high-efficiency production process. Considering the temperature in some areas of the world can reach to below 0 °C, this study proposed an efficient strategy, i.e., stepwise freezing and thawing treatment, to promote SCFAs production. The maximal production of SCFAs, i.e., 246 mg COD/g volatile suspended solid, was obtained with the shortened retention time of five days. Mechanistic studies showed that the solubilization of both extracellular polymeric substances (EPSs) and microbial cells could be accelerated, with the EPSs removal of 58.3% for proteins and 59.0% for polysaccharides. Also, the hydrolysis process was promoted to provide more substrates for subsequent acidogenisis, and the functional microorganisms, such as Romboutsia, Paraclostridium, Macellibacteroides and Conexibacter, were greatly enriched, with a total abundance of 26.2%. Moreover, compared to control, methanogenesis was inhibited at a shortened sludge retention time (e.g., five days), which benefited to the accumulation of SCFAs, but the methane production was increased by 25.2% at a longer sludge retention time (e.g., ten days). Thus, these findings of this work may provide some new solutions for the enhanced resource recovery from WAS, and further for carbon-neutral operation of wastewater treatment plants.

Efficient and ecofriendly cellulose-supported MIL-100(Fe) for wastewater treatment

Due to their efficiency and accessibility, benzodiazepines are widely manufactured and consumed and as a result, they can be found in almost all wastewaters. Among the materials that were used for the removal of drug contaminants from wastewater, metal–organic frameworks (MOFs) demonstrated unique properties. In this regard, a composite of carboxymethylated cellulose (CMC) and MIL-100(Fe) was prepared via a sonochemical method and used for the removal of lorazepam from wastewater in various conditions. A maximum capacity of 811 mg g⁻¹ was achieved which is considered a great improvement compared to bare MIL-100(Fe) (150 mg g⁻¹) and other previously reported adsorbents. It is noteworthy that the efficiency of the adsorbent did not reduce in the second and third cycle of adsorption/desorption. Moreover, the effect of pH, dose of adsorbent, isotherms and the kinetics of this process were studied using UV-vis and HPLC analyses and the adsorbents were fully characterized with PXRD, TGA, BET, SEM, ZP and FT-IR techniques. Our findings demonstrate that this composite is clearly a green, recyclable and efficient adsorbent for the removal of lorazepam and opens our way to further potential applications in the removal of other active pharmaceutical ingredients.

In this manuscript, the successful utilization of an improved cellulose-supported MOF, MIL-100(Fe)@CMC, for the adsorption of lorazepam is reported.

Freezing-low temperature treatment facilitates short-chain fatty acids production from waste activated sludge with short-term fermentation

This study proposed a novel and high-efficiency strategy, i.e., freezing followed by low-temperature thermal treatment, to significantly promote short-chain fatty acids (SCFAs) production from waste activated sludge compared to traditional freezing/thawing treatment. The maximal production of SCFAs was 212 mg COD/g VSS with a shortened retention time of five days, and the potentially recovered carbon source, including SCFAs, soluble polysaccharides and proteins, reached 321 mg COD/g VSS, increased by 92.1 and 28.3% compared to sole freezing and thermal treatment. Both the solubilization and hydrolysis steps of WAS were accelerated, and the acid-producing microorganisms, such as Macellibacteroides, Romboutsia and Paraclostridium, were greatly enriched, with a total abundance of 13.9%, which was only 0.54% in control. Interestingly, the methane production was inhibited at a shortened retention time, resulting in SCFAs accumulation, whereas it was increased by 32.0% at a longer sludge retention time, providing a potential solution for energy recovery from WAS.

Adaptability of organic matter and solid content to Fe2+/persulfate and skeleton builder conditioner for waste activated sludge dewatering

Sludge conditioning is important for improved dewatering, with the sludge characteristics impacting the effect of conditioning. A composite conditioner, Fe2+-activated sodium persulfate (Fe2+/SPS) combined with phosphogypsum (PG), was used to examine its impact on sludges with different organic contents (34.6-43.8%) or different solid contents (2.8-5.9%). Response surface optimization analysis shows that when the best conditioning is achieved, the reduction of the specific resistance to filtration (SRF) is not sensitive to organic matter content, but the dewatering performance of the sludge is greatly affected by the solid content. The oxidation role of Fe2+/SPS and the skeleton builder role of PG together affect the conditioning, oxidation playing a major role in conditioning, especially for greater organic matter content. The organic content (maximum ηSOL value was 0.32) also affects the effectiveness of the skeleton builder more than the solid content (Maximum ηSOL value was 0.25). Changes in PG significantly impacts the optimal molar ratio and dosage of Fe2+/SPS. Sludge with greater solid content requires greater Fe2+/SPS dosage to provide stronger oxidation to destroy flocs, and the maximum Fe2+:SPS molar ratio was 1.14 with solid content of 5.9 wt%. The composite conditioning decreases the content of extracellular polymeric substances and proteins/polysaccharides. This study provides new insight into the relationship between the oxidation role of Fe2+/SPS and the skeleton builder role of PG for sludge conditioning strategies according to the optimal conditions.

Construction of high-performance g-C3N4-based photo-Fenton catalysts by ferrate-induced defect engineering

A highly-efficient Fe–g-C3N4 photo-Fenton catalyst was prepared using ferrate as a critical iron source and defect control additive. The complementary synergistic effects of Fe species and defect sites promote photoelectron transfer.

Enhanced methane production from waste activated sludge by potassium ferrate combined with ultrasound pretreatment

This study explored the effect of potassium ferrate (PF) combined with ultrasound (US) pretreatment on methane generation from sludge by a series of experiments and simulations. Batch experiments showed that the pretreatment of PF coupled with US exhibited positively synergy on the methane yield. And by the pretreatment of 0.05 g/g TSS (total suspended solids) PF cooperated with US (1 W/mL, 25 kHz, 15 min), the methane yield was enhanced from 180.62 ± 3.26 to 228.83 ± 4.76 mL/g VSS (volatile suspended solids). Mechanism studies confirmed that the co-pretreatment of PF and US efficiently promoted sludge disintegration, and the biodegradability of sludge organics was obviously enhanced. Microbial community analysis showed that the functional microorganisms participating in sludge anaerobic digestion were enriched by PF cooperated with US pretreatment, with the total abundance enhanced from 12.96% in the control to 17.96% in PF + US pretreated reactor.

Freezing pretreatment assists potassium ferrate to promote hydrogen production from anaerobic fermentation of waste activated sludge

Anaerobic fermentation is an eco-friendly technology for waste activated sludge (WAS) treatment, during which resource recycle can be achieved. However, traditional sludge anaerobic fermentation is limited by the poor efficiency. We herein reported a novel high-efficiency technology by combining freezing with potassium ferrate (PF) for sludge pretreatment to promote hydrogen production from anaerobic fermentation. Experimental results demonstrated that freezing coupled with PF pretreatment exerted positively synergetic effect on hydrogen production. The maximal hydrogen production of 12.50 mL/g VSS (volatile suspended solids) was detected in the fermenter pretreated by freezing (-12 °C for 24 h) coupled with PF at 0.15 g/g TSS (total suspended solids), which was 1.34, 2.33, and 7.91 times of that from the individual PF, individual freezing, and control fermenters, respectively. The simulation results based on the modified Gompertz model indicated that both the hydrogen production potential and rate were promoted by freezing coupled with 0.15 g/g TSS PF pretreatment, from 2.14 to 13.52 mL/g VSS and 0.012 to 0.163 mL/g VSS/h, respectively. Thorough mechanism investigations revealed that the sludge EPS (extracellular polymeric substances) and microbial cells were both effectively damaged by combined freezing and PF pretreatment, resulting in the acceleration of sludge disintegration. Further investigations demonstrated that except for the acidogenesis, the other biochemical processes were all inhibited by freezing coupled with PF pretreatment, but the inhibitory extent for hydrogen consuming processes was more serious than that responsible for its generation. Gene sequencing analysis illuminated that both of the hydrolytic and hydrogen generating bacteria were largely enriched in the combined pretreatment fermenter. Moreover, the dewatering performances of fermented sludge were found to be notably enhanced by freezing coupled with PF pretreatment.

Anaerobic digestion: An alternative resource treatment option for food waste in China

With the implementation of zero-waste city and waste classification in China, a large amount of food waste (FW) began to appear in concentration, and there was an urgent requirement for appropriate and efficient treatment technology. Traditional FW disposal methods (landfill and incineration) could cause several environmental problems, so resource recycling has become the main development trend of FW in China. In recent years, anaerobic digestion (AD) technology for FW resource treatment has attracted much attention due to its advantages such as the ability to obtain clean energy, low carbon emissions, and suitability for large-scale treatment compared with other recycling technologies (composting, feed, and breeding insects). Chinese policy is conducive to the development of AD for FW, which has the potential to produce methane and achieve economic and environmental benefits. This paper presents an overview of the researches, application situations, and perspectives for the AD of FW resource treatment in China. The bibliometric analysis shows that China has the most interest in the AD of FW compared to other countries, and the amount and characteristics analysis of FW indicates that FW is suitable for treatment by AD. At the same time, this review analyzes the influence factors, methods to promote AD, working mechanism, secondary pollution of AD. Besides, the article introduces and analyzes the current policies, application status, economic and environmental benefits, and problems of AD for FW resource treatment in China. AD is considered as an alternative resource treatment technology for FW, although there are still several problems such as odors, digestate, etc. In the future, China should focus on the reform of management policy, the implementation of the AD circular economy model, and the research of the biorefinery model based on AD technology.

Occurrence, effect, and fate of residual microplastics in anaerobic digestion of waste activated sludge: A state-of-the-art review

The plastic products have large consumption over last decades, resulting in a serious microplastics (MPs) pollution. Specially, the main removal way of MPs from wastewater is to transfer MPs from liquid to solid phase, leading to its enrichment in waste activated sludge (WAS). Anaerobic digestion has been served as the most potential technique to achieve both resource recovery and sludge reduction, herein this review provides current information on occurrence, effect, and fate of MPs in anaerobic digestion of WAS. The effects of MPs on WAS anaerobic digestion are greatly related to forms, particles sizes, contents, compositions and leachates of MPs. Also, the presence of MPs not only can change the effects of other pollutants on anaerobic digestion of WAS, but also can affect the fates of them. Besides, the future perspectives focused on the fate, effect and final removal of MPs during WAS anaerobic digestion process are outlined.

Potassium ferrate coupled with freezing method enhances methane production from sludge anaerobic digestion

This study proposed a novel sludge pretreatment technology by combining freezing with potassium ferrate (PF) for synergistically enhancing the methane yield from sludge anaerobic digestion. Experimental results showed that the methane production was promoted from 170.1 ± 5.6 to 223.8 ± 7.0 mL/g VSS (volatile suspended solids) when pretreated by freezing coupled with 0.05 g/g TSS (total suspended solids) PF, with 31.6% increase. Kinetic model analysis indicated that the methane production potential and hydrolysis rate of sludge after combined pretreatment were enhanced by 32.0% and 15.0%, respectively. Mechanism studies revealed that freezing coupled with PF pretreatment effectively disrupted both extracellular polymeric substances (EPS) and microbial cells in sludge, consequently resulted in violent sludge disintegration. All the microbes responsible for hydrolysis, acidification and methanogenesis were found to be enriched by co-treatment of freezing and PF. Moreover, the fecal coliform in pretreated sludge was largely inactivated after anaerobic digestion.

Removal of cyanobacteria and cyanotoxins by ferrate from polluted lake water

Freshwater cyanobacterial blooms are becoming increasingly problematic microbiological pollutants, especially for the water resource and surface natural lakes. Cyanobacterial blooms, which produce toxins and microcystins, negatively affect the quality of water, animal, and human health, and they have also negative impact on recreational activities. The effect of electrochemically prepared potassium ferrate (green oxidation agent) on the water polluted by cyanobacteria and cyanotoxins was studied. The two most frequently occurring cyanobacterial genus Microcystis and Anabaena and the most toxic and abundant microcystin MC-LR were successfully inactivated and treated by ferrate. Potassium ferrates were applied at different conditions, such as varied hydrodynamics flow of samples, pH, and Fe(VI) concentrations. High detected elimination efficiency was consequently tested on the real water matrix from microbiological polluted natural lake Šaštín-Gazárka in Slovakia. The ferrate application leads to the better chemical, biological, microbiological, and ecotoxicological outcomes.

An anaerobic dynamic membrane bioreactor for enhancing sludge digestion: Impact of solids retention time on digestion efficacy

An anaerobic dynamic membrane bioreactor (AnDMBR), which enabled the decoupling of hydraulic retention time (HRT) and solids retention time (SRT), was used for enhancing sludge digestion, with the associated mechanisms elucidated. With the increase of SRT, the biogas production and sludge reduction rate were both enhanced. The specific biogas production and volatile solids (VS) reduction rate were improved to 0.79 L/g VS and 55.9% under SRT 50 d, respectively. Microbial community analysis revealed that Chloroflexi, which is capable of degrading metabolites and dead cells, was enriched at longer SRT. Further analysis showed that both acetoclastic and hydrogenotrophic methanogenesis contributed to the enhanced biogas production under higher SRT compared to the dominance of acetoclastic methanogenesis under lower SRT. The enhanced utilization of organic matter and acetate at longer SRT further confirmed the mechanisms. The results highlighted the potential of AnDMBR for high-efficient sludge digestion.

Response of anaerobic digestion of waste activated sludge to residual ferric ions

This study was conducted to investigate the effects of residual ferric ions (FI), released from iron or its oxides for wastewater or waste activated sludge (WAS) treatment, on anaerobic digestion of WAS. Herein it was found that the anaerobic digestion process was greatly affected by FI dosages as well as FI distributions. The responses of performance and microorganism suggested that a low FI (e.g., 0.125 mmol/g volatile suspended solid (VSS)) enhanced methane production by 29.3%, and a medium FI (e.g., 0.3 mmol/g VSS) promoted short chain fatty acids accumulation to reach the maximum of 247 mg chemical oxygen demand /g VSS, conversely, a high FI (e.g., 0.9 mmol/g VSS) led to severe inhibition on acidogenesis and methanogenesis. The findings may provide some new insights for mechanism understanding on anaerobic digestion process influenced by iron or its oxides, as well as the disposal of WAS contained FI.

Chemical speciation and distribution of potentially toxic elements in soilless cultivation of cucumber with sewage sludge biochar addition

Potentially toxic elements in municipal sewage sludge can be effectively immobilized during biochar production via pyrolysis. However, the bioavailability of these elements when biochar is applied in soilless cultivation to improve substrate quality has yet to be sufficiently established. In this study, we investigated the chemical speciation and cucumber plant uptake of potentially toxic elements in soilless cultivation when the growth substrate was amended with sewage sludge biochar (0, 5, 10, 15, and 20 wt%). It was found that the addition of 10 wt% biochar was optimal with respect to obtaining a high cucumber biomass and achieving low environmental risk considering the occurrence of hormesis. When the substrate was amended with 10 wt% biochar, cucumber fruit contained lower concentrations of As, Cr, and Zn and smaller bioavailable fractions of As, Cd, Cr, Ni, Cu, and Zn compared with the fruit of control plants, thereby meeting national safety requirements (standard GB 2762-2012, China). Most of the As and Cd taken up by cucumbers accumulated in the leaves and fruit, whereas Cr was found primarily in the roots, and most Ni, Cu, and Zn was detected in the fruit. Importantly, only small proportions of the potentially toxic elements in biochar were taken up by cucumber plants (As: 0.0075%; Cd: 0.038%; Ni: 0.0064%; Cu: 0.0016%; and Zn: 0.0015%). Given that the As, Cd, Ni, and Zn speciation in sewage sludge biochar was effectively immobilized after cultivation, the findings of this study indicate that sewage sludge biochar is a suitable substrate amendment in terms of the risk posed by potentially toxic elements.

Revealing the mechanisms for potassium ferrate affecting methane production from anaerobic digestion of waste activated sludge

This study investigated the issue of potassium ferrate (PF) affecting anaerobic methane generation from sludge by a set of experimental and model analyses. Experimental results indicated that the methane production was significantly promoted from 164.7 to 204.1 mL/g VSS (volatile suspended solids) with PF dosage enhanced from 0 to 0.05 g/g TSS (total suspended solids). Further enhancement of PF dosage reduced methane production, which even decreased to 135.4 mL/g VSS when PF dosage increased to 0.1 g/g TSS. Model-based analysis showed that except for methane production potential, the methane production rate was also promoted by PF treatment, which was sufficiently enhanced from 8.80 to 11.88 mL/g VSS/d when PF dosage was 0.05 g/g TSS. Mechanism studies indicated that PF not only promoted sludge disintegration, but also enhanced the proportion of biodegradable organics in sludge liquor, and the digestion potential of the non-biodegradable humus and lignocellulose were promoted.

Efficient Removal 17-Estradiol by Graphene-Like Magnetic Sawdust Biochar: Preparation Condition and Adsorption Mechanism

The occurrence of environmental endocrine disrupting chemicals (EDCs) in aquatic environments has caused extensive concern. Graphene-like magnetic sawdust biochar was synthesized using potassium ferrate (K₂FeO₄) to make activated sawdust biochar and applied for the removal of 17-estradiol (E2). The characterization showed that the surface morphology of five graphene-like magnetic sawdust biochars prepared with different preparation conditions were quite different. The specific surface area and pore structure increased with the increment of K₂FeO₄ addition. The results have shown that graphene-like magnetic sawdust biochar (1:1/900 °C) had the best removal on E2. The experimental results indicated that pseudo-first-order kinetic model and the Langmuir model could describe the adsorption process well, in which the equilibrium adsorption capacity (q_e,1) of 1:1/900 °C were 59.18 mg·g⁻¹ obtained from pseudo-first-order kinetic model and the maximum adsorption capacity (q_max) of 1:1/900 °C were 133.45 mg·g⁻¹ obtained from Langmuir model at 298K. At the same time, lower temperatures, the presence of humic acid (HA), and the presence of NaCl could be regulated to change the adsorption reaction in order to remove E2. Adsorption capacity was decreased with the increase of solution pH because pH value not only changed the surface charge of graphene-like magnetic sawdust biochar, but also affected the E2 in the water. The possible adsorption mechanism for E2 adsorption on graphene-like magnetic sawdust biochar was multifaceted, involving chemical adsorption and physical absorption, such as H-bonding, π-π interactions, micropore filling effects, and electrostatic interaction. To sum up, graphene-like magnetic sawdust biochar was found to be a promising absorbent for E2 removal from water.