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

A novel microbial community restructuring strategy for enhanced hydrogen production using multiple pretreatments and CSTR operation

To achieve rapid enrichment of the targeted hydrogen-producing bacterial population and reconstruction of the microbial community in the biological hydrogen-producing reactor, the activated sludge underwent multiple pretreatments using micro-aeration, alkaline treatment, and heat treatment. The activated sludge obtained from the multiple pretreatments was inoculated into the continuous stirred tank reactor (CSTR) for continuous operations. The community structure alteration and hydrogen-producing capability of the activated sludge were analyzed throughout the operation of the reactor. We found that the primary phyla in the activated sludge population shifted to Proteobacteria, Firmicutes, and Bacteroidetes, which collectively accounted for 96.69% after undergoing several pretreatments. This suggests that the multiple pretreatments facilitated in achieving the selective enrichment of the fermentation hydrogen-producing microorganisms in the activated sludge. The CSTR start-up and continuous operation of the biological hydrogen production reactor resulted in the reactor entering a highly efficient hydrogen production stage at influent COD concentrations of 4000 mg/L and 5000 mg/L, with the highest hydrogen production rate reaching 8.19 L/d and 9.33 L/d, respectively. The main genus present during the efficient hydrogen production stage in the reactor was Ethanoligenens, accounting for up to 33% of the total population. Ethanoligenens exhibited autoaggregation capabilities and a superior capacity for hydrogen production, leading to its prevalence in the reactor and contribution to efficient hydrogen production. During high-efficiency hydrogen production, flora associated with hydrogen production exhibited up to 46.95% total relative abundance. In addition, redundancy analysis (RDA) indicated that effluent pH and COD influenced the distribution of the primary hydrogen-producing bacteria, including Ethanoligenens, Raoultella, and Pectinatus, as well as other low abundant hydrogen-producing bacteria in the activated sludge. The data indicates that the multiple pretreatments and reactor's operation has successfully enriched the hydrogen-producing genera and changed the community structure of microbial hydrogen production.

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.

Synergic effects of free ammonia and sodium percarbonate for enhancing short-chain fatty acid production during sludge fermentation: Effectiveness assessment and mechanism elucidation

The production of short-chain fatty acids (SCFAs) from waste activated sludge (WAS) via anaerobic fermentation is typically restricted by poor sludge decomposition capacity and low substrate-availability. Therefore, the free ammonia (FA)‑sodium percarbonate (SPC) technology was presented to successfully overcome the limitation while addressing unsatisfactory acidogenic fermentation pretreated with sole FA or SPC. It revealed that FA + SPC co-pretreatment could boost the SCFA concentration to 347.1 mg COD/g VSS at 180 mg/L FA and 0.15 g/g TSS SPC. In-depth studies demonstrated that FA + SPC pretreatment greatly improved sludge disintegration, biodegradability of substrates and acidification of hydrolysis products. Furthermore, FA + SPC co-pretreatment stimulated the activity of hydrolytic and acidogenic enzymes but inhibited methanogenic enzymes while changing the microbial structure and promoting the enrichment of fermentation microorganisms. The synergistic effect of FA and SPC in this work improves the yield of SCFAs from WAS and facilitates the study of WAS carbon resource recovery.

Hydrogen production promotion and energy saving in anaerobic co-fermentation of heat-treated sludge and food waste

The objective of this paper is to gain insights into the synergistic advantage of anaerobic co-fermentation of heat-treated sludge (HS) with food waste (FW) and heat-treated food waste (HFW) for hydrogen production. The results showed that, compared with raw sludge (RS) mixed with FW (RS-FW), the co-substrate of HS mixed with either FW (HS-FW) or HFW (HS-HFW) effectively promoted hydrogen production, with HS-HFW promoted more than HS-FW. The maximum specific hydrogen production (MSHP) and the maximum hydrogen concentration (MHC) of HS-HFW were 40.53 mL H2/g dry weight and 57.22%, respectively, and 1.21- and 1.45-fold as high as those from HS-FW. The corresponding fermentation was ethanol type for HS-HFW and butyric acid type for HS-FW. The net energy production from RS-FW and HS-FW was both negative, but it was positive (2.57 MJ) from 40% HFW addition to HS-HFW. Anaerobic fermentation was more viable for HS-HFW.

Percarbonate-strengthened ferrate pretreatment for enhancing short-chain fatty acids production from sewage sludge

Sewage sludge management poses a pressing environmental challenge, demanding the implementation of sustainable solutions to facilitate resource recovery. Short-chain fatty acids (SCFAs) serve as valuable chemicals and renewable energy sources, underscoring the importance of maximizing their production to achieve sustainable waste management. Therefore, this study proposes a novel and green strategy, i.e., percarbonate-strengthened ferrate pretreatment to enhance SCFAs synthesis from sewage sludge, because percarbonate could activate ferrate oxidation through providing (bi) carbonate and hydrogen peroxide. Results show that percarbonate largely reduces the required ferrate dosage for fermentation improvement, and their combination exhibits obvious synergistic effects on SCFAs accumulation and sludge reduction. Under the optimal pretreatment conditions, SCFAs production is promoted to 3670.2 mg COD/L, representing a remarkable increase of 5512.4 %, 156.0 % or 395.1 % compared to the control, percarbonate alone or ferrate alone, respectively. Mechanism explorations demonstrate that percarbonate-strengthened ferrate pretreatment significantly enhances sludge solubilization, elevates substrate biodegradability, and alters the physiochemical properties of sludge to favor organics fermentation. The synergistic effects on solid organics release and sludge properties can be attributed to the combined mechanisms of enhanced oxidation and alkaline hydrolysis. Further investigations on metabolic pathways reveal that the combination substantially improves key enzyme activities associated with hydrolysis and SCFAs formation, while severely inhibits that of SCFAs consumption. These findings are further supported by the functional genes coding relevant enzymes. Moreover, the combination alters microbial structures and compositions, leading to the screening and enrichment of key microbes that facilitate SCFAs accumulation. This innovative strategy holds significant promise in advancing sewage sludge management towards a more circular and resource-efficient paradigm.

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.

Performance and mechanisms of citric acid improving biotransformation of waste activated sludge into short-chain fatty acids

Numerous effective chemical strategies have been explored for short-chain fatty acids (SCFAs) production from waste activated sludge (WAS), but many technologies have been questioned due to the chemical residues. This study proposed a citric acid (CA) treatment strategy for improving SCFAs production from WAS. The optimum SCFAs yield reached 384.4 mg COD/g VSS with 0.08 g CA/g TSS addition. Meanwhile, CA biodegradation occurred and its contribution to the yield of total SCFAs, especially acetic acid, cannot be ignored. Intensive exploration indicated the sludge decomposition, the biodegradability of fermentation substrates, as well as the abundance of fermenting microorganisms were definitely enhanced in the existence of CA. The optimization of SCFAs production techniques based on this study deserved further study. This study comprehensively revealed the performance and mechanisms of CA enhancing biotransformation of WAS into SCFAs and the findings promotes the research of carbon resource recovery from sludge.

Enhanced biohydrogen production of anaerobic fermentation by the Fe3O4 modified mycelial pellets-based anaerobic granular sludge

To improve the catalytic efficiency and stability of hydrogen-producing bacteria (HPB), the Fe₃O₄ nanoparticles modified Aspergillus tubingensis mycelial pellets (AT)-based anaerobic granular sludge (Fe₃O₄@AT-AGS) was developed. The Fe₃O₄@AT-AGS protected flora with abundant extracellular polymeric substances, which increased diversity and stability of flora in early and late stage. The porous structure enhanced mass transfer efficiency, thus promoted dominant flora transferred from lactate-producing bacteria (LPB) to HPB in middle stage. The Fe₃O₄ improved biomass of mycelial by 19.5 %. The enhancement of dehydrogenase and conductivity of Fe₃O₄ increased the HPB proportion, electron transfer, and butyrate fermentation in early and middle stage. The Fe₃O₄@AT-AGS enhanced HPB abundance, dehydrogenase activity and stability, and significantly inhibited propionate fermentation. The biohydrogen production and yield respectively reached 2792 mL/L and 2.56 mol/mol glucose. Clostridium sensu stricto 11 as dominant microbes reached 77.3 %. This provided strategy for alleviating inhibition of LPB and improving competitiveness of HPB during biohydrogen production.

Upgrading volatile fatty acids production from anaerobic co-fermentation of orange peel waste and sewage sludge: Critical roles of limonene on functional consortia and microbial metabolic traits

Orange peel waste (OPW) and sewage sludge (SS) valorization for volatile fatty acids (VFAs) production from anaerobic co-fermentation are attractive and feasible. The highest VFAs reached 11996.3 mg COD/L within 10 d at the mass ratio (TS/TS) of 1:1, which was approximately 30-fold of that in sole SS fermentation. The OPW provided plenty of organic substrates and facilitated the fermentation processes by disintegrating SS structure and inhibiting methanogenesis due to the abundant limonene. Also, the OPW feeds reshaped the microbial community and enriched fermentative bacteria, especially those saccharolytic ones (i.e. Prevotella-7). The key genes involved in membrane transport (i.e. ptsG), glycolysis (i.e. pgk), pyruvate metabolism (i.e. ace), and fatty acid biosynthesis (i.e. accA), which are associated with VFAs biosynthesis, were up-regulated in OPW/SS reactors. Overall, it was the increase in bioavailable organic matter and functional microorganisms, and the simultaneous enhancement of metabolic activity that improved the efficient VFAs production.

Enhanced methane yield through sludge two-phase anaerobic digestion process with the addition of calcium hypochlorite

This study investigated the effects of calcium hypochlorite (Ca(ClO)₂) on biomethane generation from sludge two-phase anaerobic digestion system. In first (acidogenic) phase, volatile fatty acids (VFAs) were largely generated when pretreated by Ca(ClO)₂, while the methane yield was severely inhibited. In second (methanogenic) phase, the methane yield was observably enhanced by Ca(ClO)₂. Further calculation showed that the total methane yield from the two phases was firstly promoted from 156.0 ± 4.5 to 269.9 ± 5.2 mL when Ca(ClO)₂ dosage enhanced from 0 to 1.6 g/L, which then reduced to 235.4 ± 5.5 mL when Ca(ClO)₂ content reached 2.0 g/L. Mechanism analysis showed that the suppression of Ca(ClO)₂ on coenzyme F₄₂₀ activity was relieved in methanogenic phase, and the abundances of functional microbes in methanogenic phase were enriched when added with Ca(ClO)₂. The Ca(ClO)₂-based method well realized the balance between efficacy and economy, possessing outstanding potential for large-scale applications.

Revealing the mechanisms of rhamnolipid enhanced hydrogen production from dark fermentation of waste activated sludge

Rhamnolipid (RL), as an environmentally compatible biosurfactant, has been used to enhance waste activated sludge (WAS) fermentation. However, the effect of RL on hydrogen accumulation in anaerobic fermentation remains unclear. Therefore, this work targets to investigate the mechanism of RL-based dark fermentation system on hydrogen production of WAS. It was found that the maximum yield of hydrogen increased from 1.76 ± 0.26 to 11.01 ± 0.30 mL/g VSS (volatile suspended solids), when RL concentration increased from 0 to 0.10 g/g TSS (total suspended solids). Further enhancement of RL level to 0.12 g/g TSS slightly reduced the production to 10.80 ± 0.28 mL/g VSS. Experimental findings revealed that although RL could be degraded to generate hydrogen, it did not play a major role in enhancing hydrogen accumulation. Mechanism analysis suggested that RL decreased the surface tension between sludge liquid and hydrophobic compounds, thus accelerating the solubilization of WAS, improving the proportion of biodegradable substances which could be used for subsequent hydrogen production. Regardless of the fact that adding RL suppressed all the fermentation processes, the inhibition effect of processes associated with hydrogen consumption was much severer than that of hydrogen production. Further investigations of microbial community revealed that RL enriched the relative abundance of hydrogen producers e.g., Romboutsia but reduced that of hydrogen consumers like Desulfobulbus and Caldisericum.

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.