Sludge Solubilization Research Articles

Enhancing waste sludge solubilization through radio frequency treatment perforating bacterial cells

Sludge solubilization is known as a rate-limiting step of anaerobic digestion. Although radio frequency (RF) has been applied for sludge pretreatment due to its similar thermal effect as microwave, the potential non-thermal effects of RF treatment remain controversial. In this study, we demonstrate that RF pretreatment enhances the solubilization and lysis of sludge by 8.02%–19.69% through both thermal and non-thermal mechanisms with less energy input. Scanning electron microscope images provide direct evidence that RF-induced microcurrents penetrated bacterial cells, leading to the release of intracellular substances through formed pores. Additionally, the non-thermal effect of RF treatment which could weaken the cell protection and accelerate the lysis rate involves the disruption of binding forces between extracellular polymeric substances and microbial cells. On average, the utilization of RF at a frequency of 27.12 MHz demonstrates its efficacy as a sludge pretreatment technique, as evidenced by a 13.39% reduction in energy consumption and a 16.9% improvement in treatment performance compared to conductive heating (CH). The findings of this study elucidate the possible mechanism of RF treatment of sludge and could establish a theoretical basis for the practical application of RF treatment in sludge management.

Enhancing methane production in anaerobic digestion of waste activated sludge by combined thermal hydrolysis and photocatalysis pretreatment

Thermal hydrolysis (TH) is promising for sludge pretreatment, but the refractory substances generated at high temperatures inhibit anaerobic digestion. In this study, a novel combined TH and photocatalytic pretreatment method was proposed to improve the anaerobic digestion performance of waste activated sludge. The results showed that the combined pretreatment (170 °C, 0.5 g/L TiO2) increased methane yield by 66 % from 111 ± 5 m L/g VS to 185 ± 5 m L/g VS. After TH pretreatment, photocatalysis further promoted sludge solubilization by destroying extracellular polymeric substances, resulting in an increase in released soluble organic matter from 292 ± 16 mg/L to 4,091 ± 85 mg/L. In addition, photocatalysis improved the biodegradability of sludge by reducing the melanoidin and humic acid contents by 26 % and 20 %, respectively. The proposed novel pretreatment method effectively overcomes the bottleneck of TH technology and provides an alternative pretreatment technology for improving sludge resource recovery.

Effect of potassium persulphate-induced microwave pretreatment for cost-effective sludge solubilization and bioenergy recovery

The disintegration of municipal waste activated sludge (MWAS) by energy-intensive physical pretreatments is considered expensive. In the present study, an effort was undertaken to disintegrate the sludge by microwave assisted pretreatment (MAP) in an energy-efficient and economical way by combining the microwave with oxidant potassium persulphate. Besides, the sulphate radicals generated through activated persulfate can efficiently disintegrate and solubilize sludge. In this context, microwave pretreatment was combined with potassium persulfate to activate the generation of sulphate radicals for economically enhanced sludge solubilization. MAP was done by varying the microwave power levels (10 to 50 %) and pretreatment time (0 to 30 min). Microwave assisted potassium persulphate pretreatment (MAPPP) was done by varying the potassium per sulphate dosage (0.005 to 0.04 g/g Total solids (TS) and pretreatment time (0 to 14 min). Combining microwave with potassium persulfate brings about a synergistic effect, generating sulphate and hydroxyl radicals that can effectively disintegrate and solubilize biopolymers of sludge. A higher sludge lysis rate of 37.25 % was achieved by this microwave assisted potassium persulfate pretreatment (MAPPP) when compared to microwave assisted pretreatment (MAP) alone (26.27 % after thickening). A higher methane yield of 219.14 mL/g Volatile solids (VS) was obtained through MAPPP compared to MAP (154.22 mL/g VS). The detailed mass, energy, and economic assessment at a large-scale level for MAP and MAPPP revealed that MAPPP was energetically and economically feasible with an energy ratio of 1.22 and net profit of 42.03 USD/Ton compared to MAP. Thus, it can be confirmed that MAPPP was economically viable to implement on a large scale.

Optimization of the Solubilization of Faecal Sludge through Microwave Treatment

Optimization of the Solubilization of Faecal Sludge through Microwave Treatment

Enhanced hydrolysis of waste activated sludge and recovery of volatile fatty acids: Performance and mechanistic analysis of synergistic treatment with sodium citrate and calcium oxide

Anaerobic fermentation has emerged as a promising method of transforming waste activated sludge into high-value products (e.g., volatile fatty acids (VFAs)). This work developed sodium citrate (SC)-calcium oxide (CaO) pretreatment to accelerate the production of VFAs by enhancing sludge solubilization and disintegration of extracellular polymeric substances. The results showed that co-pretreatment with 0.25 g/g TSS of SC and 0.05 g/g TSS of CaO effectively boosted VFAs accumulation (5823.3 mg COD/L), which was 12.2 times higher than the Control group. SC-CaO pretreatment enhanced hydrolysis and acidogenesis by providing ample organic substrates, thereby promoting the growth of hydrolytic and acidogenic bacteria. Additionally, the fermentation broth resulting from co-pretreatment exhibited lower phosphorus concentration and higher biodegradability. Economic analysis confirmed that the combined pretreatment is cost-effective. This work provides a viable strategy for enhancing high-value product recovery from sludge.

Effect of low-temperature thermal-alkali pre-treatment on waste activated sludge solubilisation

The study was aimed to examine the effects of alkaline and thermal-alkaline pre-treatment on sewage sludge solubilisation with different alkali and its doses ranging from 40 mg alkali/g TS to 120 mg alkali/g TS. Initially, the effect of alkali pre-treatment was monitored with four alkali at room temperature, to further enhance soluble chemical oxygen demand (sCOD) and reduce treatment time, thermal-alkaline pre-treatment was performed with varying temperature from 40 to 70 °C. To explore and optimise the effects of temperature, alkali, and alkali doses on COD solubilisation, response surface methodology was employed. Statistical analysis of the variance was conducted to identify the relationship between experimental data and the proposed model. The thermal-alkaline process was optimised for maximum COD solubilisation with optimal conditions achieved at 65.61 °C of 66.57 mg alkali /g TS for NaOH and 27 °C of 20 mg/g TS for KOH. The maximum COD solubilisation achieved at these optimised conditions for NaOH and KOH was 21.28 % and 16.13 %, respectively. The results demonstrated a significant improvement in protein and carbohydrate concentration and a 16–20 % reduction in sludge solid content after thermal-alkali pre-treatment.

Sludge solubilization method using hydrodynamic cavitation

Abstract This study investigates the effects of hydrodynamic cavitation(HC) on sludge solubilization using an advanced rotation hydrodynamic cavitation reactor(ARHCR), varying rotation speeds of rotor. The effectiveness was analysed by measuring biogas yield during anaerobic digestion(AD). The results show that AD performance with treated sludge was improved by HC, indicating that sludge was effectively solubilized. However, the much higher temperature formed by HC leads to the formation of refractory materials that lower AD performance. Overall, this study suggests that HC can be a promising technique to improve sludge solubilization, however, the issue of refractory materials needs to be addressed for optimal performance.

Natural deep eutectic solvents pretreatments for sludge dewaterability improvement and subsequent volatile fatty acids production: Insights of sludge disintegration and metagenomic characterization

Higher economic returns and carbon–neutral benefits drive research shifting to volatile fatty acids (VFAs) recovery from sludge treatment. The novel natural deep eutectic solvents (NADES) based pretreatments of waste activated sludge (WAS) were proposed, focusing on sludge dewaterability, organics dissolution, and subsequent production of VFAs via anaerobic fermentation. Three different pretreatment methods, i.e., NADES treatment alone, NADES with 80 °C thermal treatment, and NADES with 180 °C hydrothermal treatment were systematically compared. The dewaterability of sludge significantly improved only when NADES assisted by thermal treatments, which was interpreted by a substantial decline in capillary suction time (CST) and water-holding capacity. All three pretreatments boosted the VFAs production from sludge (1.40–2.33 times compared with control), and increased the proportion of acetic acid by about 10%. Results showed that the highest VFAs yield of 2728.71 mg COD/L was obtained from NADES combined with 180 °C hydrothermal treatment, which was increased by 133 % compared with the control. Such performance is much higher than a surfactant assisted hydrothermal treatment recently reported in the literature (increased by 27 %). Mechanistic studies elucidated that NADES stimulated sludge solubilization, enhancing the release of organic substrates for VFAs production. Further metagenomic analysis demonstrated that the hydrolytic-acidogenic bacteria were enriched after NADES treatments. Moreover, glycosyl transferases (GTs) and carbohydrate esterases (CEs) were the primary carbohydrate-active enzymes (CAZymes) enhanced by NADES addition. This study provides new insights into green solvents simultaneously enhanced sludge dewatering and resource recovery.

Synergistic effects of peracetic acid and free ammonia pretreatment on anaerobic fermentation of waste activated sludge to promote short-chain fatty acid production for polyhydroxyalkanoate biosynthesis: Mechanisms and optimization

Peracetic acid (PAA) combined with free ammonia (FA) pretreatment can be utilized to promote anaerobic fermentation (AF) of waste activated sludge (WAS) to produce short-chain fatty acids (SCFAs), and the resulting SCFAs are desirable carbon sources (C-sources) for polyhydroxyalkanoate (PHA) biosynthesis. This work aimed to determine the optimum conditions for PAA + FA pretreatment of sludge AF and the feasibility of using anaerobic fermentation liquor (AFL) for PHA production. To reveal the mechanisms of integrated pretreatment, the impacts of PAA + FA pretreatment on different stages of sludge AF and changes in the microbial community structure were explored. The experimental results showed that the maximum SCFA yield reached 491.35 ± 6.02 mg COD/g VSS on day 5 after pretreatment with 0.1 g PAA/g VSS +70 mg FA/L, which was significantly greater than that resulting from PAA or FA pretreatment alone. The mechanism analysis showed that PAA + FA pretreatment promoted sludge solubilization but strongly inhibited methanogenesis. According to the analysis of the microbial community, PAA + FA pretreatment changed the microbial community structure and promoted the enrichment of bacteria related to hydrolysis and acidification, and Proteiniclasticum, Macellibacteroides and Petrimonas became the dominant hydrolytic and acidifying bacteria. Finally, after alkali treatment, the AFL was utilized for batch-mode PHA production, and a maximum PHA yield of 55.05 wt% was achieved after five operation periods.

Evaluation of the optimal sewage sludge pre-treatment technology through continuous reactor operation: Process performance and microbial community insights

This study investigated the impact of two low-temperature thermal pre-treatments on continuous anaerobic reactors’ performance, sequentially fed with sludge of different total solids content (∼3 % and ∼6 %) and subjected to progressively increasing Organic Loading Rates (OLR) from 1.0 to 2.5 g volatile solids/(LReactor⋅day). Assessing pre-treatments’ influence on influent sludge characteristics revealed enhanced organic matter hydrolysis, facilitating sludge solubilization and methanogenesis; volatile fatty acids concentration also increased, particularly in pre-treated sludge of ∼6 % total solids, indicating improved heating efficiency under increased solids content. The reactor fed with sludge pre-treated at 45 °C for 48 h and 55 °C for an extra 48 h exhibited the highest methane yield under all applied OLRs, peaking at 240 ± 3.0 mL/g volatile solids at the OLR of 2.5 g volatile solids/(LReactor⋅day). 16S rRNA gene sequencing demonstrated differences in the reactors’ microbiomes as evidence of sludge thickening and the different pre-treatments applied, which promoted the release of organic matter in diverse concentrations and compositions. Finally, the microbial analysis revealed that specific foam-related genera increased in abundance in the foam layer of reactors’ effluent bottles, dictating their association with the sludge foaming incidents that occurred inside the reactors during their operation at 2.0 g volatile solids/(LReactor⋅day).

Energy efficient biohydrogen production from waste activated sludge through a novel phase separated pretreatment

Disintegration of sludge by microwave irradiation is considered an energy-intensive pretreatment as it demands more significant energy input to achieve the desirable solubilization. Extracellular polymeric substance (EPS) present in the floc matrix of sludge limits the extent of sludge solubilization. Hence, this study attempted to disrupt the EPS with potassium peroxymonosulphate (PPMS) before microwave (MW) pretreatment to attain maximal solubilization in an energy-efficient way. A maximal EPS disruption was achieved at 0.009 g/g TS of PPMS dosage and treatment time of 36 min. The outcome of the phase-separated pretreatment shows that PPMS induced MW pretreatment (PPMS-MW) obtained a maximum chemical oxygen demand (COD) solubilization of 35.67 % at a specific energy input of 8795 kJ/kg TS. In contrast, the MW pretreatment achieved only 22.41 % at a specific energy input of 21973 kJ/kg TS. When comparing both PPMS-MW and MW pretreatments, an ample amount of energy was saved by PPMS-MW than MW. The biohydrogen production potential assay showed that PPMS-MW obtained an increased biohydrogen production of 79.76 mL/g VS compared to MW (50.10 mL/g VS). Thus, this phase-separated PPMS-MW pretreatment was considered efficient compared to MW pretreatment.

Deep eutectic solvents pretreatment enhances methane production from anaerobic digestion of waste activated sludge: Effectiveness evaluation and mechanism elucidation

Anaerobic digestion (AD) is a prevalent waste activated sludge (WAS) treatment, and optimizing methane production is a core focus of AD. Two DESs were developed in this study and significantly increased methane production, including choline chloride-urea (ChCl-Urea) 390% and chloride-ethylene glycol (ChCl-EG) 540%. Results showed that ChCl-Urea mainly disrupted extracellular polymeric substances (EPS) structures, aiding in initial sludge solubilization during pretreatment. ChCl-EG, instead, induced sludge self-driven organic solubilization and enhanced hydrolysis and acidification processes during AD process. Based on the extent to which the two DESs promoted AD for methane production, the AD process can be divided into stage Ⅰ and stage Ⅱ. In stage Ⅰ, ChCl-EG promoted methanogenesis more significantly, microbiological analysis showed both DESs enriched aceticlastic methanogens-Methanosarcina. Notably, ChCl-Urea particularly influenced polysaccharide-related metabolism, whereas ChCl-EG targeted protein-related metabolism. In stage Ⅱ, ChCl-Urea was more dominant than ChCl-EG, ChCl-Urea bolstered metabolism and ChCl-EG promoted genetic information processing in this stage. In essence, this study investigated the microbial mechanism of DES-enhanced sludge methanogenesis and provided a reference for future research.

Unravelling the role of coumarin on anaerobic digestion of waste activated sludge: Determination of operational dynamics and microbial community structure

Anaerobic digestion, as an effective method for sludge treatment, still has excellent potential for development in energy recovery. Therefore, this study proposed a method to increase methane production by adding coumarin (a green antibacterial agent). The results indicated that coumarin can significantly improve the methane production performance of sludge, but high coumarin concentrations delay methane production. Methane generation through the acetoclastic pathway is inhibited at high concentrations of coumarin and can be restored after bacterial adaptation or degradation of coumarin. Mechanism exploration revealed that coumarin enhanced the destruction of EPS and poorly tolerant bacterial cell membranes, thereby benefiting the solubilization of sludge. It was also found that coumarin promotes sludge hydrolysis, acidification, and methane production stages. Microbial analysis revealed that the abundance of functional microorganisms in the coumarin reactor was more advantageous to methane production. These findings are expected to provide a reference for enhancing the methane production performance of sludge.

A novel fluidized electrode electrolysis system activated ferrate (VI) oxidation promoting sludge solubilization: Performance and mechanism

Energy and resource recovery is a viable and sustainable solution for sludge treatment and disposal, while sludge solubilization is one of the most critical issues hindering sludge conversion to valuable products. Although ferrate (VI) was considered a green sludge pretreatment approach, the poor stability of ferrate (VI) hindered the effective utilization of its oxidative potential. This study demonstrated that an innovative electrochemical system with granular activated carbon (GAC) as the fluidized electrode activated potassium ferrate (PF) oxidation could effectively catalyze the destruction of sludge structure, thereby promoting the release of organic matter and improving sludge solubilization. The results showed that the electro- granular activated carbon- potassium ferrate (E-GAC-PF) process achieved a sludge solubilization of 30.9 % within 15 min, resulting in a reduction in sludge volume by 33.9 %. In comparison, the sludge solubilization rate under PF oxidation alone was 11.5 %, with corresponding in sludge volume reduction of 13.0 %. The radical scavenging experiment and probe compound (sulfoxide) oxidation experiment revealed that the E-GAC-PF system was responsible for the production of a higher quantity of Fe (V) and Fe (IV), which significantly contributed to the sludge decomposition. Additionally, XPS and XRD analysis discovered that Fe3O4 was present as a byproduct, which opened up a new route for iron recycling during the pretreatment. Hence, E-GAC-PF system held promise as a pioneering and efficient alternative for catalytically activating PF, thereby facilitating environmentally friendly and highly efficient pretreatment of sludge.

Evaluation of initial pH and urea hydrogen peroxide (UHP) co-pretreatment on waste-activated sludge

Wastewater treatment and conversion into renewable energy sources have been of great interest in recent times due to growing environmental pollution concerns and need for sustainable energy sources. Sewage sludge treatment can convert sludge into renewable energy. In this study, the impact of initial pH and urea hydrogen peroxide (UHP) co-pretreatment on sludge hydrolysis and anaerobic digestion was investigated. The pH of sludge was initially adjusted to 7, 9, and 11 before the addition of 8 mmol/g VS UHP. Under 24 h pretreatment, alkaline medium and UHP effectively enhanced sludge solubilization and hydrolysis. The combination of chemical, sonication, and centrifugation improved the extraction of extracellular polymerase substances released in soluble state. Secondly, anaerobic digestion was performed for 11 days to determine the influence of a lower mesophilic temperature (20 °C) and retention time on the pretreated sludge. The highest NH4+-N concentration of 5.32 g/L was recorded in pH 7+UHP. The most significant total VFA concentration of 13.1 g COD/L was observed in pH 7+UHP on day 9. Acetic acid, isovaleric acid and propionic acid accounted for 80%–83% of the total VFA composition in all pretreated reactors. Lower mesophilic temperature efficiently optimized UHP and VFA production in the pretreated reactors. Microbial metabolism was stabilized under a longer retention time. Alkaline pH and longer retention time elevated NH4+-N and VFA concentration. The results showed that initial pH and UHP co-pretreatment of waste activated sludge offer an alternative pathway for enhancing sludge hydrolysis and VFA production applicable in sludge treatment.

Machine learning assisted modelling of anaerobic digestion of waste activated sludge coupled with hydrothermal pre-treatment

This study utilizes decision-tree-based models, including Random Forest, XGBoost, artificial neural networks (ANNs), support vector machine regressors, and K nearest neighbors algorithms, to predict sludge solubilization and methane yield in hydrothermal pretreatment (HTP) coupled with anaerobic digestion (AD) processes. Analyzing two decades of published research, we find that ANN models exhibit superior fitting accuracy for solubilization prediction, while decision-tree models excel in methane yield prediction. Pretreatment temperature is identified as pivotal among various variables, and heating time surprisingly emerges as equally significant as holding time for solubilization and surpasses it for methane yield. Contrary to prior expectations, the HTP method's impact on sludge solubilization and AD performance is minimal. This study underscores data-driven models' potential as resource-efficient tools for optimizing advanced AD processes with HTP. Notably, our research spans nearly two decades of lab, pilot, and full-scale studies, offering novel insights not previously explored.

Enhanced sludge solubilization by a fluidized electrode: Granular activated carbon promoted electrochemical oxidation

Electrochemical sludge pretreatment is receiving increasing attention because of its small footprint and higher environmental compatibility. However, the limited effective area of electrode plates and the low conductivity of sludge hinder the widespread application of electrochemical pretreatment. In this study, granular activated carbon (GAC) was employed to construct a fluidized electrode electrochemical system (FEE) to promote electrochemical pretreatment. Under the optimal operating parameters, the FEE system could effectively facilitate sludge decomposition, indicated by 126% increase in soluble chemical oxygen demand (SCOD) and 23.1% reduction in sludge volume. Mechanism study revealed that the addition of GAC significantly enhanced the conductivity of sludge, thereby promoting the oxidation capacity of FEE system. Furthermore, continuously generated H2O2 in FEE further promoted sludge solubilization. GAC offered an effectively, green and sustainable enhancement approach for sludge electrochemical pretreatment.

Ozone combined with mechanical agitation to enhance sludge cell lysis and provide an alternative carbon source for nitrogen removal of bioreactors

The activated sludge method results in the production of significant amounts of excess sludge. Moreover, biological nitrogen removal requires an external carbon source, which presents environmental and economic challenges. To address these challenges, this study combined ozone with mechanical agitation to achieve sludge cell lysis. Through both experiments and COMSOL Multiphysics simulations, optimal conditions were identified. These conditions include a six-bladed disk turbine stirrer speed of 800 rpm, an ozone concentration of 40 mg/L, and an ozone dose of 100 mg·g−1 MLSS (Mixed Liquid Suspended Solids). Notably, the sludge solubilization efficiency improved by 20.1 % when compared to using ozone alone. After the sludge dissolution, this paper observed elevated concentrations of soluble chemical oxygen demand (SCOD), total nitrogen (TN) and total phosphorus (TP) in the supernatant, reaching 3126 mg/L, 144.3 mg/L, and 49.6 mg/L, respectively. This study also investigated the feasibility of using this supernatant as an external carbon source in a sequencing batch reactor. The results showed that while the supernatant had minimal impact on COD removal, it significantly enhanced nitrogen removal. Compared to using glucose as an external carbon source, the TN removal rate increased by 16.9 %. However, TP removal was inhibited.

Permanganate (PM) pretreatment improves medium-chain fatty acids production from sewage sludge: The role of PM oxidation and in-situ formed manganese dioxide

Medium-chain fatty acids (MCFAs) production from sewage sludge is mainly restricted by the complex substrate structure, competitive metabolism and low electron transfer rate. This study proposes a novel permanganate (PM)-based strategy to promote sludge degradation and MCFAs production. Results show that PM pretreatment significantly increases MCFAs production, i.e., attaining 12,036 mg COD/L, and decreases the carbon fluxes of electron acceptor (EA)/electron donor (ED) to byproducts. Further analysis reveals that PM oxidation enhances the release and biochemical conversion of organic components via disrupting extracellular polymers (EPS) structure and reducing viable cells ratio, providing directly available EA for chain elongation (CE). The microbial activity positively correlated with MCFAs generation are apparently heightened, while the competitive metabolism of CE (i.e., methanogensis) can be completely inhibited. Accordingly, the functional bacteria related to critical bio-steps and dissimilatory manganese reduction are largely enriched. Further mechanism exploration indicates that the main contributors for sludge solubilization are 1O2 (61.6 %) and reactive manganese species (RMnS), i.e., Mn(V)/Mn(VI) (22.3 %) and Mn(III) (∼16.1 %). As the main reducing product of PM reaction, manganese dioxide (MnO2) can enable the formation of microbial aggregates, and serve as electron shuttles to facilitate the carbon fluxes to MCFAs during CE process. Overall, this strategy can achieve simultaneous hydrogen recovery, weaken competitive metabolisms and provide electron transfer accelerator for CE reactions.

Enzyme-enhanced acidogenic fermentation of waste activated sludge: Insights from sludge structure, interfaces, and functional microflora

Anaerobic fermentation is widely installed to recovery valuable resources and energy as CH4 from waste activated sludge (WAS), and its implementation in developing countries is largely restricted by the slow hydrolysis, poor efficiency, and complicate inert components therein. In this study, enzyme-enhanced fermentation was conducted to improve sludge solubilization from 283 to 7728 mg COD/L and to enhance volatile fatty acids (VFAs) yield by 58.6 % as compared to the conventional fermentation. The rapid release of organic carbon species, especially for tryptophan- and tyrosine-like compounds, to outer layer of extracellular polymeric substance (EPS) occurred to reduce the structural complexity and improve the sludge biodegradability towards VFAs production. Besides, upon enzymatic pretreatment the simultaneous exposure of hydrophilic and hydrophobic groups on sludge surfaces increased the interfacial hydrophilicity. By quantitative analysis via interfacial thermodynamics and XDLVO theory, it was confirmed that the stronger hydrophilic repulsion and energy barriers in particle interface enhanced interfacial mass transfer and reactions involved in acidogenic fermentation. Meanwhile, these effects stimulate the fermentation functional microflora and predominant microorganism, and the enrichment of the hydrolytic and acid-producing bacteria in metaphase and the proliferation of acetogenic bacteria, e.g., Rubrivivax (+9.4 %), in anaphase also benefits VFAs formation. This study is practically valuable to recovery valuable VFAs as carbon sources and platform chemicals from WAS and agriculture wastes.