Waste Activated Sludge Solubilization Research Articles

Electro-assisted anaerobic digestion of waste activated sludge for biomethane production enhanced by hyperosmosis of sodium chloride: Characteristics and microbial mechanisms

This first-attempted study explored the hyperosmosis of sodium chloride (NaCl) for enhancing methane production from waste activated sludge (WAS) via electro-assisted anaerobic digestion. The 0.6–0.9 g/g TS levels of sodium chloride caused an obvious WAS solubilization with ca. 104.7–110.3 mg/L of soluble chemical oxygen demand (SCODMn) release within 96-h (including 132.56–158.34 mg/L of soluble protein and 55.67–66.36 mg/L of soluble carbohydrates). The NaCl addition increased conductivity and current density positively, but decreased microbial biodiversity, community evenness and archaeal metabolic activities negatively in the electro-assisted anaerobic digestion process. The proper use of NaCl for enhancing methanogenesis of WAS should seek a balance between such positive and negative impacts, which was at ca. 0.6 g/g TS adding level and maintained a good-cooperation between acidogens and methanogens. Acidogens of Acrobacter, Mesotoga, Mesoaciditoga, Candidatus-cloacamonas, Bacteroidales, Bacteroidetes, Anaerolineaceae, Treponema and Fusibacter contributed to VFAs generation, while Methanobacterium, Methanomassiliicoccus and Methanothrix were responsible for methanogenesis positively under NaCl-hyperosmosis circumstances. Finally, a mechanistic illustration for depicting biomethane conversion in electro-assisted anaerobic digestion with sodium chloride addition was proposed. The results obtained might shed lights on this efficient and low-cost strategy for WAS treatment and resource recovery.

Syntrophic Jiont of Sulfate-Reducing Bacteria and Hydrogen-Producing Acetogen Stimulated Methane Production from Waste Activated Sludge Digestion

Anaerobic digestion of waste-activated sludge (WAS) towards biogas recovery is constrained by the limited hydrolysis and inhibited acetogenesis steps that hinder subsequent energy recovery. This study employed Fe(VI)/S(IV) oxidation to enhance the WAS solubilization and coupled it with the syntrophic interaction of hydrogen-producing acetogen (HPA) and sulfate-reducing bacteria (SRB) to stimulate the successive procedure towards methane production. Results showed that the dosage ratio of HPA-SRB to WAS (H-S-W) with 1:1:50 outperformed with the highest methane production potential (11.63 ± 1.87 mL CH4/(g VSS·d). Meanwhile, the efficient and sequential process from acetogenesis to methanogenesis stimulated by HPA-SRB was evidenced by a significant decrease of 30.2% in the acetate concentration. The microbial community structure further manifested the crucial role of HPA-SRB with increased abundance of Desulfobulbus (2.07%), Syntrophomonas (1.24%) and Smithella (1.63%), which stimulated acetophilic methanogen boost with Methanobacterium dominating with 77.51% in H-S-W100. Furthermore, the positive syntrophic relationships among HPA-SRB and acetophilic methanogens towards methane production were confirmed via molecular ecological network and canonical correspondence analysis. This study highlighted the syntrophic cooperation of the mixed consortia of HPA and SRB on methane production based on Fe(VI)/S(IV) pretreatment and provided the theoretical and technical basis for the potential implementation of novel methanogenesis technology for WAS treatment.

Aluminum chloride enhances the production of short-chain fatty acids from waste activated sludge: Insights to performance, mechanism, and implications

Waste activated sludge (WAS), the major by-products during wastewater treatment, is hugely produced, which poses great environment and operation pressures. This work investigated feasibility, mechanism, and implications of aluminum chloride for carbon recovery enhancement from WAS. Results showed that aluminum chloride enhanced short-chain fatty acids (SCFAs) production potential, and the enhancement potentials were dosage dependent. The optimal dosage was 10 mg Al/g total suspended solids (TSS), with the maximum SCFAs production potential of 140 mg chemical oxygen demand/g volatile suspended solids, which was 52.2 % higher compared to that of control. Meanwhile, the compositions of SCFAs were regulated, suggesting that the recovered SCFAs were potential carbon source. Mechanism analysis showed that aluminum chloride increased the release of polysaccharides and proteins by 23.6 % and 71.7 %, respectively, compared to that in control, as well as the removal of extracellular polymeric substances to 27.5 % and the contribution of intracellular release to 38.0 %. This situation facilitated the solubilization of WAS and improved the biochemical degradability of organic matters. In addition, aluminum chloride promoted hydrolysis and acidification stages through affecting the activities of hydrolase, enriching acid-producing microorganisms, and inhibiting the activities of methanogens. This work provided some novel solutions for sludge treatment and carbon recovery, driving wastewater treatment plants to achieve carbon-neutral operation.

Improved anaerobic sludge fermentation mediated by a tryptophan-degrading consortium: Effectiveness assessment and mechanism deciphering

The hydrolysis of extracellular polymeric substances (EPS) represents a critical bottleneck in the anaerobic fermentation of waste activated sludge (WAS), while tryptophan is identified as an underestimated constituent of EPS. Herein, we harnessed a tryptophan-degrading microbial consortium (TDC) to enhance the hydrolysis efficiency of WAS. At TDC dosages of 5%, 10%, and 20%, a notable increase in SCOD was observed by factors of 1.13, 1.39, and 1.88, respectively. The introduction of TDC improved both the yield and quality of short chain fatty acids (SCFAs), the maximum SCFA yield increased from 590.6 to 1820.2, 1957.9 and 2194.9 mg COD/L, whilst the acetate ratio within SCFAs was raised from 34.1% to 61.2–70.9%. Furthermore, as TDC dosage increased, the relative activity of protease exhibited significant increments, reaching 116.3%, 168.0%, and 266.1%, respectively. This enhancement facilitated WAS solubilization and the release of organic substances from bound EPS into soluble EPS. Microbial analysis identified Tetrasphaera and Soehngenia as key participants in WAS solubilization and the breakdown of protein fraction. Metabolic analysis revealed that TDC triggered the secretion of enzymes associated with amino acid metabolism and fatty acid biosynthesis, thereby fostering the decomposition of proteins and production of SCFAs.

Revealing methane production potential from waste activated sludge through citric acid-assisted hydrogen corrosion of zero valent iron

Anaerobic digestion of waste activated sludge (WAS) for biofuel recovery is one key step for developing energy-self-sufficient and carbon–neutral wastewater treatment plants. Although zero valent iron (ZVI) is an alternative additive to promote methane production from WAS, low hydrogen corrosion of ZVI is one important limitation. This study explored whether methane production potential could be further enhanced while using citric acid (CA) to accelerate hydrogen corrosion of ZVI. Results showed that the maximum methane production reached 92.77 mL/g volatile suspended solid (VSS) with ZVI of 30 mM and CA of 6 mM, which increased by 63.8% compared to that with solo ZVI. Mechanism analysis demonstrated that the proposed strategy could accelerate the solubilization of WAS, increase the accumulation of short-chain fatty acids (SCFAs), and optimize the composition of SCFAs. Meanwhile, both the efficiencies of hydrogen yield and production rate obtained with ZVI and CA were the highest. The accumulation of hydrogen with ZVI and CA reached 5 mL/g VSS during the 9-hour period, which was 2.42 times higher than that with solo ZVI, and the hourly hydrogen production rate was 2.47 mL/g VSS−1h−1, which was higher than those with either solo CA or ZVI. Thus, the effectiveness of 1 + 1＞2 was obtained. Correspondingly, the concentration of ferrous ion with ZVI and CA increased to 2.32 times higher than that with solo ZVI. In addition, the microbial community analysis indicated that hydrogenotrophic methanogens were predominated with ZVI and CA. These findings may provide some new solutions for promoting the role of nano-ZVI in enhancing methane production from anaerobic digestion.

Natural zeolite enhances anaerobic digestion of waste activated sludge: Insights into the performance and the role of biofilm

Anaerobic digestion is widely employed for the treatment of waste activated sludge (WAS) due to its advantages like simultaneous energy recovery and sludge stabilization, promoting carbon-neutral operation of wastewater treatment plants. Natural zeolite, a low-cost and eco-friendly additive, has the potential to improve methane production from anaerobic digestion. This study investigated the effects of natural zeolite on anaerobic digestion when the substrate was WAS. It was found that methane production potential in response to natural zeolite was dosage-dependent. The optimal dosage was 0.1 g zeolite/g volatile suspended solids (VSS), with a methane yield of 181.89 ± 6.75 mL/g VSS, which increased by 20.1% compared to that of the control. Although the methane yields with other dosages of natural zeolite were higher than that of control, they were lesser than that with 0.1 g zeolite/g VSS. Natural zeolite affected transfer and conversion of proteins much more than polysaccharides in liquid phase and extracellular polymeric substances. In anaerobic digestion, natural zeolite had with little effects on WAS solubilization, while it improved hydrolysis, acidification, and methanogenesis. The dosages of natural zeolite did have significant effects on bacterial communities in biofilm rather than suspension, while the archaeal communities in biofilm and suspension were all greatly related to natural zeolite dosages. The developed biofilms promoted richness and functionality of microbial communities. The syntrophic metabolism relationships between methanogens and bacteria were improved, which was proved by selective enrichment of Methanosarcina, Syntrophomonas, and Petrimonas. The findings of this work provided some new solutions for promoting methane production from WAS, and the roles of natural zeolite in anaerobic digestion.

Enhanced methane yield in anaerobic digestion of waste activated sludge by combined pretreatment with fungal mash and free nitrous acid

This study explores a novel approach for enhancing anaerobic digestion of waste activated sludge (WAS) through the combined pretreatment of fungal mash and free nitrous acid (FNA). Aspergillus PAD-2, a fungal strain with superior hydrolase secretion, was isolated from WAS and cultivated in-situ on food waste to produce fungal mash. The solubilization of WAS by fungal mash achieved a high soluble chemical oxygen demand release rate of 548 mg L−1 h−1 within first 3 h. The combined pretreatment of fungal mash and FNA further improved the sludge solubilization by 2-fold and resulted in a doubled methane production rate of 416±11 mL CH4 g−1 volatile solids. The Gompertz model analysis revealed a higher maximum specific methane production rate and shortened lag time by the combined pretreatment. These results demonstrate that the combined fungal mash and FNA pretreatment offers a promising alternative for fast anaerobic digestion of WAS.

Advanced thermal hydrolysis for biopolymer production from waste activated sludge: Kinetics and fingerprints

Waste activated sludge (WAS) is the main residue of wastewater treatment plants, which can be considered an environmental problem of prime concern due to its increasing generation. In this study, a non-energetic approach was evaluated in order to use WAS as a renewable resource of high value-added products. For this reason, WAS was treated by thermal hydrolysis, H2O2 oxidation and advanced thermal hydrolysis (ATH) promoted by H2O2. The influence of temperature, H2O2 concentration and dosing strategy on biomolecule production (proteins and carbohydrates), size distribution (fingerprints) and various physico-chemical parameters (VSS, total and soluble COD, soluble TOC, pH and colour) was studied. The results revealed a synergistic effect between TH and H2O2 oxidation, which led to a significant increase in the production of both proteins and carbohydrates. In this sense, the concentration of proteins and carbohydrates obtained during TH at 85 °C for120 min was found to be 1376 ± 9 mg/L (121 mg/gVSSo) and 208 ± 4 mg/L (18 mg/gVSSo), respectively. However, in the presence of 4.5 mM H2O2/gVSSo under the same process conditions, the concentrations of proteins and carbohydrates exhibited a significant increase of 1.9-fold and 3.1-fold, respectively. Besides, the addition of H2O2 promoted the transformation of hydrophobic compounds, such as proteins and or lipids, into hydrophilic compounds, which presented low and medium sizes. An increase in temperature improved the solubilization rate and the yield of biomolecules significantly. Besides, the analysis of the kinetics related to the dosing strategy of H2O2 suggested the existence of two fractions during WAS solubilization, one of them being easily oxidizable, whereas the other one was more refractory to oxidation. Thus, the value of kH2O2 for the first addition of 1 mM H2O2/g VSSo was 0.020 L0.4 mgH2O2−0.4 min−1, while it was 4.3 and 8 times lower for the second and third additions, respectively.

Choline chloride pretreatment on volatile fatty acids promotion from sludge anaerobic fermentation: In-situ deep eutectic solvents-like formation for EPS disintegration and associated microbial functional profiles upregulation

Slow solubilization/hydrolysis efficiency has been widely recognized as a limiting factor for volatile fatty acids (VFAs) production from waste activated sludge (WAS) via anaerobic fermentation. This study developed an innovative choline chloride (ChCl)-based approach for WAS treatment. Accordingly, the VFAs yield was increased by 2.5–623.3%, which was attributed to the concurrent acceleration of WAS solubilization, hydrolysis, and acidification. Fourier transform infrared spectrometer (FTIR) and molecular docking analysis showed that ChCl can attack and disrupt extracellular polymeric substances (EPS) by forming hydrogen bonds with organics occurred in WAS (i.e. proteins). More importantly, the generated VFAs in turn interacted with ChCl for in-situ deep eutectic solvents (DES)-like formation to significantly improve the organics dissolution of WAS for more bioavailable substrates associated with VFAs biosynthesis, and such effects were intensified with the VFAs level. Besides, ChCl exhibited “screening-effects” to enrich the hydrolytic and acidogenic anaerobes in WAS fermentation systems, while reduced VFAs-consumers. Meanwhile, the microbial metabolic profiles shift to favor the protein metabolism, and the genetic traits associated with extracellular and intracellular substrates metabolism, membrane transport, and fatty acid biosynthesis were all upregulated. Noteworthily, the functional hydrolytic and acidogenic species maintained their microbial activity for efficient VFAs production via quorum sensing (QS) and two-component systems (TCS) to counteract ChCl stress. The ranking efficiency product (REP) indices revealed that ChCl pretreatment showed a higher sustainable footprint (84.0%) over other traditional pretreatment approaches for WAS fermentation in view of both economic and environmental benefits. This work provides an innovative niche for efficient VFAs production from WAS with the sustainable application.

Fe-loaded alginate hydrogel beads activating peroxymonosulfate for enhancing anaerobic fermentation of waste activated sludge: Performance and potential mechanism

The recovery of volatile fatty acids (VFAs) through anaerobic fermentation (AF) is usually restricted by the poor biodegradability of waste activated sludge (WAS). This study proposed a novel strategy, i.e. peroxymonosulfate (PMS) activated by Fe-loaded sodium alginate hydrogel beads (Fe-SA), to enhance AF performance. Experimental results demonstrated that the as-synthesized Fe-SA and PMS co-pretreatment synergistically enhanced WAS solubilization and VFAs production. The maximal VFAs yield of 2013 mg COD/L was achieved at the Fe-SA dosage of 4.0 mM/g TSS, which was 93.7% higher than that with sole PMS addition and 8.82 times higher than that of the control. Mechanistic studies elucidated that the generation of reactive radicals such as SO4•− and •OH from PMS was greatly induced by Fe-SA, which contributed to WAS disintegration and degradation of refractory compounds. Additionally, analysis of the key enzyme activities indicated that the Fe-SA could strengthen biological hydrolysis and acidogenesis of sludge during AF. Microbial analysis illustrated that Fe-SA evidently improved the abundances of fermentative microorganisms as well as functional gene expression via creating a favorable environment for microbial growth. This study demonstrated the applicable potential of Fe-SA hydrogel beads activating PMS for VFAs production and provides an important reference for developing advanced oxidation processes-based application in AF.

Insights into the Effects of CeO2 Nanoparticles on Medium-Chain Carboxylates Production from Waste Activated Sludge

The synthesis of medium-chain carboxylates (MCCs) from waste-activated sludge (WAS) upgrading has received considerable attention. However, limited research has been conducted on the effects of CeO2 nanoparticles (NPs) on this process. This study showed that 1 mg/g−TS of CeO2 NPs improved the solubilization of WAS, resulting in higher production of MCCs. At 5 mg/g−TS, CeO2 NPs weakly inhibited 3 biological steps. Despite this, there was an enhancement in WAS solubilization, thus the overall production of MCCs was similar to the control. However, doses of CeO2 NPs ranging from 25–100 mg/g−TS were unable to offset biological inhibition, leading to a decrease in MCC production. The toxic mechanisms involved were not the generation of reactive oxygen species or Ce ions from CeO2 NPs to anaerobic sludge, but instead the decline of extracellular polymeric substance (EPS) and destruction of the cell membrane through physical penetration. Microbial community analysis confirmed that 1 mg/g−TS of CeO2 NPs increased the relative abundance of key bacteria involved in the anaerobic fermentation of WAS. The MCC microbe Clostridium sensu stricto was enriched in the control group, while the relative abundance of this genus was significantly reduced with 100 mg/g−TS CeO2 NPs.

Enhanced hydrolysis/acidogenesis and potential mechanism in thermal-alkali-biofilm synergistic pretreatment of high-solid and low-organic-content sludge

Improving the anaerobic digestion (AD) of high-solid and low-organic-content sludge is imperative for sustainable waste activated sludge (WAS) management. Here, a thermal-alkali-biofilm pretreatment (TAB) was established to treat high-solid and low-organic-content sludge and compared with thermal and thermal-alkali methods. The results showed that TAB drastically improved WAS reduction, hydrolysis/acidogenesis efficiency, and biochemical methane potential. TAB possessed the lowest sludge particle size and the highest surface charge due to the stimulated proteolysis and WAS solubilization, supported by the protease activity test and secondary substrate identification. In addition, the biofilm assistance noticeably accelerated the elimination of autochthonous bacteria in WAS (e.g., Proteobacteria) and facilitated the enrichment of specialized fermentative microorganisms (e.g., Firmicutes) along with relevant functional genes, lying molecular foundation for the enhanced hydrolysis/acidogenesis in TAB. These findings could expand the application of biofilm in the AD of WAS and provide new insight into the pretreatment strategy of high-solid and low-organic-content sludge.

Effects of free nitrous acid combined with alkyl polyglucoside on short-chain fatty acids production from waste activated sludge anaerobic fermentation and fermentation liquor for polyhydroxyalkanoates synthesis

Free nitrous acid (FNA) pretreatment and alkyl polyglucoside (APG) addition are environmentally-friendly methods for enhancing waste activated sludge (WAS) anaerobic fermentation (AF) performances. The short-chain fatty acids (SCFAs) produced are available substrates for polyhydroxyalkanoates (PHAs) synthesis to reduce the C-source cost. This study aims to investigate the optimal fermentation conditions of FNA + APG treatment and the performance of PHAs synthesis by the SCFAs from AF system. Through investigating the FNA + APG combined treatment effects, key enzyme activities, and microbial community variation, the AFL from optimal fermentation condition was utilized as a C-source for PHAs synthesis. Experimental results indicated that 0.653 mg/L FNA + 0.075 g/g VSS APG treatment could synergistically achieve the maximum total SCFAs yield of 324.94 mg COD/g VSS at day 5 of AF, which was much more than those of sole FNA or APG treatment. Mechanism analysis indicated that FNA + APG treatment could promote WAS solubilization, hydrolysis and acidogenesis, while severely inhibiting methanogenesis. Microbial community analysis results demonstrated that SCFA producers, including Enterococcus and Clostridium, became the dominant genera in the FNA + APG reactor. Finally, AF liquor after ammonia removal was applied for batch-mode PHAs synthesis, and PHA yields increased to a maximum of 44.44 % of biomass (w/w) after five operation periods.

Response mechanisms of anaerobic fermentative sludge to zinc oxide nanoparticles during medium-chain carboxylates production from waste activated sludge

The conversion of waste activated sludge (WAS) into medium chain carboxylates (MCCs) has attracted much attention, while investigations of the impacts of ZnO nanoparticles (NPs) on this process are sparse. The present study showed that 8 mg/g-TSS of ZnO NPs have little effects on all key steps and the activity of anaerobes, and finally leading to unchanged MCCs production. Although 30 mg/g-TSS of ZnO NPs weakly inhibited the hydrolysis, acidogenesis, and chain elongation process, WAS solubilization was enhanced, thus, the improvement was enough to offset inhibition, also resulting in an insignificant impact on overall MCCs production. However, the improvement with ZnO NPs dosages above 100 mg/g-TSS was not sufficient to offset the biological inhibition, thus inducing negative impact on overall MCCs production. The decline of EPS induced by Zn2+ and generation of excessive reactive oxygen species (ROS) were the main factors responsible for the inhibitory effects of ZnO NPs on lower activity of anaerobes. For chain elongation process, the discriminative Clostridium IV (as MCCs-forming bacteria) with a strong adaptation to ZnO NPs (300 mg/g-TSS) was observed. The present study provided a deep understanding related to the effects of ZnO NPs on the production of MCCs production from WAS and identified a zinc resistance anaerobe, which would be significant for the evaluation of influence and alleviation of inhibition induced by ZnO NPs on the carbon cycle of organic wastes.

Sulfite altered permanganate effects on acetate-enriched short-chain fatty acids production during sludge anaerobic fermentation

Anaerobic fermentation is a promising method for waste activated sludge (WAS) treatment, but ineffective solubilization and hydrolysis limit its application. The current study examined the function of sodium sulfite (SDS) in potassium permanganate (PP)-conditioned WAS fermentation for short-chain fatty acids (SCFAs) biosynthesis. The presence of SDS in the PP system (PP/SDS) reduced the positive effects of PP on total SCFAs yield (2755 versus 3471 mg COD/L), while effectively increasing the proportion of acetate (from 41 to 81 %). Not only did SDS decrease the promoting effects of PP on WAS solubilization and hydrolysis efficiency by 5–42 %, it also shifted microbial metabolic pathways to favor acetate production. In addition, the amino acid metabolism with acetate as end product was enhanced. Moreover, PP/SDS inhibited methanogenesis, resulting in an accumulation of acetate in high quantities. Thus, the current study a provided insight and direction for effective WAS treatment with acetate-enriched SCFAs production.

Nonmonotonic effect of CuO nanoparticles on medium-chain carboxylates production from waste activated sludge

The growing applications of CuO nanoparticles (NPs) in industrial and agriculture has increased their concentrations in wastewater and subsequently accumulated in waste activated sludge (WAS), raising concerns about their impact on reutilization of WAS, especially on the medium-chain carboxylates (MCCs) production from anaerobic fermentation of WAS. Here we showed that CuO NPs at 10–50 mg/g-TS can significantly inhibit MCCs production, and reactive oxygen species generation was revealed to be the key factor linked to the phenomena. At lower CuO NPs concentrations (0.5–2.5 mg/g-TS), however, MCCs production was enhanced, with a maximum level of 37% compared to the control. The combination of molecular approaches and metaproteomic analysis revealed that although low dosage CuO NPs (2.5 mg/g-TS) weakly inhibited chain elongation process, they displayed contributive characteristics both in WAS solubilization and transport/metabolism of carbohydrate. These results demonstrated that the complex microbial processes for MCCs production in the anaerobic fermentation of WAS can be affected by CuO NPs in a dosage-dependent manner via regulating microbial protein expression level. Our findings can provide new insights into the influence of CuO NPs on anaerobic fermentation process and shed light on the treatment option for the resource utilization of CuO NPs polluted WAS.

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.

Sulfite altered permanganate pretreatment effects on the volatile fatty acid production during sludge anaerobic fermentation

Anaerobic fermentation is a low-carbon strategy for waste activated sludge (WAS) treatment with resource recovery. This study compared the effects of potassium permanganate (PM) and calcium sulfite-assisted PM (PM/CS) pretreatment on volatile fatty acids (VFAs) production during WAS fermentation. The effects of combined PM/CS on the total VFAs production were observed to be inferior to sole PM (2689 versus 3507 mg COD/L), but PM/CS pretreatment resulted in acetate-enriched VFAs (88.2% ratio) with improved sludge dewatering. The scavenging effects of interfering compounds on free radicals, which are previously speculated for extracellular polymeric substances (EPS) disruption, and the EPS-binding effects of Ca2+ in PM/CS reactors might both restrain the effective WAS solubilization and hydrolysis efficiencies in comparison with that of PM, and thus resulted in the reduced VFAs production. Moreover, both PM and PM/CS pretreatment altered the microbial community structure and metabolic traits, which were beneficial to the VFAs generation. However, the PM/CS pretreatment was more conducive to the enrichment of acetate-producing bacteria (e.g., Proteiniclasticum and Christensenellaceae_R_7_group) and acetate-forming metabolic functions (e.g., pyruvate metabolism and propanoate and butanoate degradation). Also, methanogenesis, which is an acetate-consumption process, was remarkably inhibited in PM/CS reactor, thereby promoting acetic acid accumulation effectively. This study demonstrated the distinct effects of different PM-based pretreatment on WAS fermentation and provided guidance on the appropriate pretreatment selection for WAS disposal.

Imidazolium ionic liquids as potential persistent pollutants promote the conversion of waste activated sludge to volatile fatty acids in anaerobic fermentation without promoting hydrogen production

In this study, [C8mim]PF6, or 3-methyl-1-octylimidazolium hexafluorophosphate was used to fill the gap in the knowledge regarding the effect of ionic liquids (ILs) on anaerobic hydrogen production of waste activated sludge (WAS). The 800 mg/L [C8mim]PF6 solution reduced hydrogen production by 66.26% and 45.78% but increased volatile fatty acids (VFAs) production by 140.15% and 133.75% at mesophilic and thermophilic conditions, respectively. Dynamics of the dissolved organic matter indicated that [C8mim]PF6 enhanced WAS solubilization by inducing cell rupture and enhancing complexation. In situ monitoring of the liquid aliquots revealed that the altered hydrogenase activity shifted the fermentation pathway shifted in a direction unfavorable to hydrogen production. Thermophilic fermentation mitigated the toxicity of [C8mim]PF6 via a more stable hydrogenase activity and fermentation pathway. Amplicon sequence variant analysis illustrated the evolution of microbial alliances and resistant strains that shared ecological niches. ILs leaking into WAS are not conducive to biohydrogen production. With the development of recovery technology, ionic IL may serve as potential substances facilitating the conversion of WAS to VFAs. Compared with thermophilic fermentation, mesophilic fermentation offers the advantages of higher VFA yield and lower cost, reflecting the cleanliness of the process. Our observations can serve as a reference for IL-rich wastewater treatment.

Improved methane production from the two-phase anaerobic digestion and dewaterability of anaerobically digested sludge by β-cyclodextrin pretreatment

Cyclodextrin-based technologies have drawn worldwide attention for wastewater and sludge treatment due to its extraordinary solubilization of hydrophobic organics. β-cyclodextrin (β-CD) has been applied for enhancing sludge solubilization for production of volatile fatty acids. However, the impact of β-CD on methane production in the anaerobic digestion seemingly was unnoticed in the past, which was likely due to its inhibition effect on the methanogenesis in single-phase digestion system. Herein, a new pretreatment strategy using β-CD was firstly explored to enhance methane yield in two-phase anaerobic digestion of waste activated sludge (WAS) and simultaneously improve anaerobically digested sludge (ADS) dewaterability. Results illustrated that the total methane yield in the two phase increased from 101.43 to 277.67 mL/g VS with β-CD level raising from 0 to 0.3 g/g TS. A better ADS dewaterability was also obtained (24.14% capillary suction time increase). Mechanism investigations suggested that WAS solubilization and acidification were significantly enhanced in the first phase (i.e., acidogenic phase), with 8.97-fold soluble chemical oxygen demand (SCOD) and 17.37-fold short-chain fatty acids (SCFAs) increases. This implied that more available substrates after β-CD pretreatment could be used for methanogenesis. Further calculation indicated the utilization of β-CD was not the main reason for the enhanced methane. Model-based explorations revealed hydrolysis rate and methane potential were improved by 153.17% and 175.36% after β-CD pretreatment. The activities of enzymes associated with hydrolysis/acidification/methanogenesis in methanogenic phase were improved even if those were suppressed in acidogenic phase. Moreover, β-CD increased the abundance of key hydrolysis-acidification microbes and strengthened both the hydrogenotrophic and aceticlastic methanogenesis pathways. Finally, a “multifaceted benefits” operation concept based on β-CD treatment was proposed, providing an implication for wastewater treatment and sludge disposal.