Sludge Anaerobic Fermentation Research Articles

Effects of sulfite and freezing co-pretreatment on the volatile fatty acids production of waste activated sludge in anaerobic fermentation

Volatile fatty acids (VFAs) produced during the anaerobic fermentation of waste activated sludge (WAS) are renewable energy sources. However, the biodegradability of untreated WAS is significantly low, thereby imposing a substantial constraint on the generation of VFAs during anaerobic fermentation of WAS. In this study, the experimental results showed that freezing and sulfite co-pretreatment had a synergism on the disintegration of WAS and the production of VFAs. The optimal freezing temperature for VFAs production were −20 °C, 8 h and sulfite dosage of 1400 mg/L resulted in the VFAs reaching a highest concentration of 627.58 mg/L, which was 17.41 times higher than that of the blank group. In addition, the disintegration effect of WAS has been improved, the concentration of soluble chemical oxygen demand (SCOD), proteins and polysaccharides were 7.21, 5.56 and 2.58 times than the blank group, respectively. The microbial community analysis revealed an increase of the OUT of co-pretreatment. Also showed that co-pretreatment could increase the abundance of Levilinea, Unclassified_Anaerolineaceae and Blvii28_wastewater_sludge_group, which might increase VFAs production. Furthermore, compared to sulfite pretreatment or freezing pretreatment, the co-pretreatment significantly enhanced the disintegration efficiency of WAS; shortened the hydrolysis required time for WAS; provided the higher concentration of substrates for VFAs production; and improved the overall efficiency of sludge anaerobic fermentation.

Ionic liquid coupled plasma promotes acetic acid production during anaerobic fermentation of waste activated sludge: Breaking the restrictions of low bioavailable substrates and altering the metabolic activities of anaerobes

This study explored the potential application of plasma coupling ionic liquid on disintegration of waste activated sludge and enhanced production of short-chain fatty acids (SCFAs) in anaerobic fermentation. Under optimal conditions (dosage of ionic liquid [Emim]OTf = 0.1 g/g VSS (volatile suspended solids) and discharge power of dielectric barrier discharge plasma (DBD) = 75.2 W), the [Emim]OTf/DBD pretreatment increased SCFA production by 302 % and acetic acid ratio by 53 % compared to the control. Mechanistic investigations revealed that the [Emim]OTf/DBD combination motivated the generation of various reactive species (such as H2O2, O3, •OH, 1O2, ONOO−, and •O2−) and enhanced the utilization of physical energies (such as heat). The coupling effects of [Emim]OTf/DBD synergistically improved the disintegration of sludge and biodegradability of dissolved organic matter, promoting the sludge anaerobic fermentation process. Moreover, the [Emim]OTf/DBD pretreatment enriched hydrolysis and SCFAs-forming bacteria while inhibiting SCFAs-consuming bacteria. The net effect was pronounced expression of genes encoding key enzymes (such as alpha-glucosidase, endoglucanase, beta-glucosidase, l-lactate/D-lactate dehydrogenase, and butyrate kinase) involved in the SCFA-producing pathway, enhancing the production of SCFAs from sludge anaerobic fermentation. In addition, [Emim]OTf/DBD pretreatment facilitated sludge dewatering and heavy metal removal. Therefore, [Emim]OTf/DBD pretreatment is a promising approach to advancing sludge reduction, recyclability, and valuable resource recovery.

Unraveling the effects and mechanisms of microplastics on anaerobic fermentation: Exploring microbial communities and metabolic pathways

To investigate the effects of microplastics (MPs) on hydrolysis, acidification and microbial characteristics during waste activated sludge (WAS) anaerobic fermentation process, five different kinds of MPs were added into the WAS fermentation system and results indicated that, compared to the control group, the addition of polyvinyl chloride (PVC)-MPs exhibited the least inhibition on volatile fatty acids (VFAs), reducing them by 13.49 %. Conversely, polyethylene (PE)-MPs resulted in the greatest inhibition, with a reduction of 29.57 %. MPs, while accelerated the dissolution of WAS that evidenced by an increase of lactate dehydrogenase (LDH) release, concurrently inhibited the activities of relevant hydrolytic enzymes (α-Glucosidase, protease). For microbial mechanisms, MPs addition affected the proliferation of key microorganisms (norank_f_Bacteroidetes_vadinHA17, Ottowia, and Propioniclava) and reduced the abundance of genes associated with hydrolysis and acidification (pfkb, gpmI, ilvE, and aces). Additionally, MPs decreased the levels of key hydrolytic and acidogenic enzymes to inhibit hydrolysis and acidification processes. This research provides a basis for understanding and unveils impact mechanisms of the impact of MPs on sludge anaerobic fermentation.

Tannic Acid Modulation of Substrate Utilization, Microbial Community, and Metabolic Traits in Sludge Anaerobic Fermentation for Volatile Fatty Acid Promotion.

Anaerobic fermentation is a crucial route to realize effective waste activated sludge (WAS) resource recovery and utilization, while the overall efficiency is commonly restrained by undesirable disruptors (i.e., chemical dewatering agents). This work unveiled the unexpectedly positive effects of biodewatering tannic acid (TA) on the volatile fatty acids (VFAs) biosynthesis during WAS anaerobic fermentation. The total VFAs yield was remarkably increased by 15.6 folds with enriched acetate and butyrate in TA-occurred systems. TA was capable to disintegrate extracellular polymeric substances to promote the overall organics release. However, TA further modulated the soluble proteins structure by hydrogen bonding and hydrophobic interactions, resulting in the decrease of proteins bioavailability and consequential alteration of metabolic substrate feature. These changes reshaped the microbial community and stimulated adaptive regulatory systems in hydrolytic-acidogenic bacteria. The keystone species for carbohydrate metabolism (i.e., Solobacterium and Erysipelotrichaceae) were preferentially enriched. Also, the typical quorum sensing (i.e., enhancing substrate transport) and two-component systems (i.e., sustaining high metabolic activity) were activated to promote the microbial networks connectivity and ecological cooperative behaviors in response to TA stress. Additionally, the metabolic functions responsible for carbohydrate hydrolysis, transmembrane transport, and intracellular metabolism as well as VFA biosynthesis showed increased relative abundance, which maintained high microbial activities for VFAs biosynthesis. This study underscored the advantages of biodewatering TA for WAS treatment in the context of resource recovery and deciphered the interactive mechanisms.

Non-targeted facilitation of primary sludge anaerobic fermentation by micro-aeration and the simultaneous nutrients transformations

Anaerobic fermentation is a widely used technology for resource recovery from municipal sludge (e.g., primary sludge). Micro-aeration is an emerging strategy which can be incorporated into this technology. However, the effect of micro-aeration on the anaerobic fermentation performance and the nutrients transformations are not well known. This study investigated the impact of micro-aeration on the production of volatile fatty acids (VFAs) from primary sludge as well as the simultaneous nitrogen (N) and phosphorus (P) transformations in the system. The obtained results indicated that application of micro-aeration improved the microbial activities and facilitated all the hydrolysis, acetogenesis and methanogenesis processes. Hydrogenotrophic methanogen was enriched by micro-aeration while the acetoclastic methanogenesis was still the dominant CH4 production pathway at gene level. The concentration of VFAs was lower with micro-aeration than in the control system, which could be attributed to the enhanced consumption of VFAs. The simultaneous transformations of N and P were observed with different patterns. Micro-aeration led to removal of up to 131.6 mg/L of N, with occurrence of nitrification and denitrification found to be responsible for the N removal. In contrast, the total P content remained unchanged in the reactor. However, micro-aeration was found to inhibit both extracellular organic-P mineralization and intracellular poly-P degradation, resulting in lower soluble PO43−-P and more P distributed in the organic-P fraction in solid phase. This study provides novel insights into the microscopic mechanisms of the micro-aeration on anaerobic fermentation and the simultaneous regulation of N and P release.

Facilitating effects of the synergy with zero-valent iron and peroxysulfate on the sludge anaerobic fermentation system: Combined biological enzyme, microbial community and fermentation mechanism assessment

This study evaluated a synergetic waste activated sludge treatment strategy with environmentally friendly zero-valent iron nanoparticles (Fe0) and peroxysulfate. To verify the feasibility of the synergistic treatment, Fe0, peroxysulfate, and the mixture of peroxysulfate and Fe0 (synergy treatment) were added to different sludge fermentation systems. The study demonstrated that the synergy treatment fermentation system displayed remarkable hydrolysis performance with 435.50 mg COD/L of protein and 197.67 mg COD/L of polysaccharide, which increased 1.13–2.85 times (protein) and 1.12–1.49 times (polysaccharide) for other three fermentation system. Additionally, the synergy treatment fermentation system (754.52 mg COD/L) exhibited a well acidification performance which was 1.35–41.73 times for other systems (18.08–557.27 mg COD/L). The synergy treatment fermentation system had a facilitating effect on the activity of protease, dehydrogenase, and alkaline phosphatase, which guaranteed the transformation of organic matter. Results also indicated that Comamonas, Soehngenia, Pseudomonas, and Fusibacter were enriched in synergy treatment, which was beneficial to produce SCFAs. The activation of Fe0 on peroxysulfate promoting electron transfer, improving the active groups, and increasing the enrichment of functional microorganisms showed the advanced nature of synergy treatment. These results proved the feasibility of synergy treatment with Fe0 and peroxysulfate to enhance waste activated sludge anaerobic fermentation.

Separation of nutrients from SCFAs with a dynamic membrane in a sludge anaerobic fermenter

The complexity and high cost to separate and recover short chain fatty acids (SCFAs), ammonium ions, and phosphates in the sludge fermentation liquid hinder the application of sludge anaerobic fermentation. In this study, an interesting phenomenon was found in a sludge anaerobic fermenter with a dynamic membrane (DM) which could not only enhance SCFAs production but also retain most SCFAs in fermenter. The separation factor of DM for NH3-N/SCFAs and PO43−/SCFAs throughout the DM development were about 40 and 80, respectively. Analysis reveals that rejection of SCFAs by DM could not be simply correlated to molecular weight or membrane pore size. The rejection mechanisms might be dominated by Donnan rejection. In addition, biodegradation in the DM may also have contribution. Findings of this study suggest the potential of DM as an economical technology for nutrients and SCFAs recover.

Peroxydisulfate activation and versatility of defective Fe3O4@MOF-808 for enhanced carbon and phosphorus recovery from sludge anaerobic fermentation

Although being viewed as a promising technology for reclamation of carbon and phosphorus from excess sludge, anaerobic fermentation (AF) grapples with issues such as a low yield of volatile fatty acids (VFAs) and high phosphorus recovery costs. In this study, we synthesized Fe3O4@MOF-808 (FeM) with abundant defects and employed it to simultaneously enhance VFAs and phosphorus recovery during sludge anaerobic fermentation. Through pre-oxidization of sludge catalyzed by FeM-induced peroxydisulfate, the soluble organic matter increased by 2.54 times, thus providing ample substrate for VFAs production. Subsequent AF revealed a remarkable 732.73 % increase in VFAs and a 1592.95 % increase in phosphate. Factors contributing to the high VFAs yield include the non-biological catalysis of unsaturated Zr active sites in defective FeM, enhancing protein hydrolysis, and the inhibition of methanogenesis due to electron competition arising from the transformation between Fe(III) and Fe(II) under Zr influence. Remarkably, FeM exhibited an adsorption capacity of up to 92.64 % for dissolved phosphate through ligand exchange and electrostatic attractions. Furthermore, FeM demonstrated magnetic separation capability from the fermentation broth, coupled with excellent stability and reusability in both catalysis and adsorption processes.

Enhancement of medium-chain fatty acids production from sludge anaerobic fermentation liquid under moderate sulfate reduction

In recent years, there has been a marked increase in the production of excess sludge. Chain-elongation (CE) fermentation presents a promising approach for carbon resource recovery from sludge, enabling the transformation of carbon into medium-chain fatty acids (MCFAs). However, the impact of sulfate, commonly presents in sludge, on the CE process remains largely unexplored. In this study, batch tests for CE process of sludge anaerobic fermentation liquid (SAFL) under different SCOD/SO42− ratios were performed. The moderate sulfate reduction under the optimum SCOD/SO42− of 20:1 enhanced the n-caproate production, giving the maximum n-caproate concentration, selectivity and production rate of 5.49 g COD/L, 21.4% and 4.87 g COD/L/d, respectively. The excessive sulfate reduction under SCOD/SO42− ≤ 5 completely inhibited the CE process, resulting in almost no n-caproate generation. The variations in n-caproate production under different conditions of SCOD/SO42− were all well fitted with the modified Gompertz kinetic model. Alcaligenes and Ruminococcaceae_UCG-014 were the dominant genus-level biomarkers under moderate sulfate reduction (SCOD/SO42− = 20), which enhanced the n-caproate production by increasing the generation of acetyl-CoA and the hydrolysis of difficult biodegradable substances in SAFL. The findings presented in this work elucidate a strategy and provide a theoretical framework for the further enhancement of MCFAs production from excess sludge.

Dynamic microbiome disassembly and evolution induced by antimicrobial methylisothiazolinone in sludge anaerobic fermentation for volatile fatty acids generation

In the post-COVID-19 pandemic era, various antimicrobials have emerged and concentrated in waste-activated sludge (WAS), affecting the biological treatment of WAS. However, there is still a knowledge gap in the dynamic response and adaptive mechanism of anaerobic microbiome under exogenous antimicrobial stress. This study found that methylisothiazolinone (MIT, as a typic antimicrobial) caused an interesting lag effect on the volatile fatty acids (VFAs) promotion in the WAS anaerobic fermentation process. MIT was effective to disintegrate the extracellular polymeric substances (EPS), and those functional anaerobic microorganisms were easily exposed and negatively impacted by the MIT interference after the loss of protective barriers. Correspondingly, the ecological interactions and microbial metabolic functions related to VFA biosynthesis (e.g., pyruvate metabolism) were downregulated at the initial stage. The syntrophic consortia gradually adapted to the interference and attenuated the MIT stress by activating chemotaxis and resistance genes (e.g., excreting, binding, and inactivating). Due to the increased bioavailable substrates in the fermentation systems, the dominant microorganisms (i.e., Clostridium and Caloramator) with both VFAs production and MIT-tolerance functions have been domesticated. Moreover, MIT disrupted the syntrophic interaction between acetogens and methanogens and totally suppressed methanogens' metabolic activities. The VFA production derived from WAS anaerobic fermentation was therefore enhanced due to the interference of antimicrobial MIT stress. This work deciphered dynamic changes and adaptive evolution of anaerobic syntrophic consortia in response to antimicrobial stress and provided guidance on the evaluation and control of the ecological risks of exogenous pollutants in WAS treatment.

Benzethonium chloride affects short chain fatty acids produced from anaerobic fermentation of waste activated sludge: Performance, biodegradation and mechanisms

Benzethonium chloride (BZC) is viewed as a promising disinfectant and widely applied in daily life. While studies related to its effect on waste activated sludge (WAS) anaerobic fermentation (AF) were seldom mentioned before. To understand how BZC affects AF of WAS, production of short chain fatty acids (SCFAs), characteristics of WAS as well as microbial community were evaluated during AF. Results manifested a dose-specific relationship of dosages between BZC and SCFAs and the optimum yield arrived at 2441.01 mg COD/L with the addition of 0.030 g/g TSS BZC. Spectral results and protein secondary structure variation indicated that BZC denatured proteins in the solid phase into smaller proteins or amino acids with unstable structures. It was also found that BZC could stimulate the extracellular polymeric substances secretion and reduce the surface tension of WAS, leading to the enhancement of solubilization. Beside, BZC promoted the hydrolysis stage (increased by 7.09 % to 0.030 g/g TSS BZC), but inhibited acetogenesis and methanogenesis stages (decreased by 6.85 % and 14.75 % to 0.030 g/g TSS BZC). The microbial community was also regulated by BZC to facilitate the enrichment of hydrolytic and acidizing microorganisms (i.e. Firmicutes). All these variations caused by BZC were conducive to the accumulation of SCFAs. The findings contributed to investigating the effect of BZC on AF of WAS and provided a new idea for the future study of AF mechanism.

Activation of peroxymonosulfate by sewage sludge-derived biochar to promote short-chain fatty acid production during sludge anaerobic fermentation

This study reports on the potential use of sewage sludge-derived biochar (SBC) as an activator for peroxymonosulfate (PMS) to improve the solubilization of waste activated sludge (WAS) and the production of short-chain fatty acids (SCFAs) during anaerobic digestion. The SBC produced by pyrolysis of WAS has active sites consisting of metals species and C = O groups, which can effectively activate PMS. SBC-activated PMS (PMS/SBC) can destroy cellular structures in WAS and release large amounts of organic matter, increasing the rate of hydrolysis and promoting subsequent SCFA production. Compared with the control, PMS/SBC treatment increased SCFA production by 801 % at a PMS dose of 0.14 g/g VSS (volatile suspended solids) and SBC dose of 0.24 g/g VSS. Moreover, PMS/SBC could degrade refractory organic pollutants in WAS (sulfamethoxazole removal efficiency >63 %), improving SCFA quality. Mechanistic investigations revealed that SBC activates PMS to produce •SO4−, 1O2, and •OH. The contributions of these species to sludge solubilization were 1O2 > •OH > •SO4−, with surface-bound radicals on SBC playing major roles. In addition, PMS/SBC treatment enriched hydrolysis and SCFA-forming bacteria and increased the expression of key genes for SCFA production during sludge anaerobic fermentation, which also contributed to SCFA accumulation. This work provides new insights into the mechanism of SBC activation of PMS and its potential applications in WAS resource utilization.

Peracetic acid (PAA)-based pretreatment effectively improves medium-chain fatty acids (MCFAs) production from sewage sludge

Peracetic acid (PAA), known for its environmentally friendly properties as a oxidant and bactericide, is gaining prominence in decontamination and disinfection applications. The primary product of PAA oxidation is acetate that can serve as an electron acceptor (EA) for the biosynthesis of medium-chain fatty acids (MCFAs) via chain elongation (CE) reactions. Hence, PAA-based pretreatment is supposed to be beneficial for MCFAs production from anaerobic sludge fermentation, as it could enhance organic matter availability, suppress competing microorganisms and furnish EA by providing acetate. However, such a hypothesis has rarely been proved. Here we reveal that PAA-based pretreatment leads to significant exfoliation of extracellular polymeric substances (EPS) from sludge flocs and disruption of proteinic secondary structures, through inducing highly active free radicals and singlet oxygen. The production of MCFAs increases substantially to 11,265.6 mg COD L−1, while the undesired byproducts, specifically long-chain alcohols (LCAs), decrease to 723.5 mg COD L−1. Microbial activity tests further demonstrate that PAA pretreatment stimulates the CE process, attributed to the up-regulation of functional genes involved in fatty acid biosynthesis pathway. These comprehensive findings provide insights into the effectiveness and mechanisms behind enhanced MCFAs production through PAA-based technology, advancing our understanding of sustainable resource recovery from sewage sludge.

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.

Ultrasound-enhanced peracetic acid oxidation improves short-chain fatty acids production from waste activated sludge

This study proposed a new technology, i.e., ultrasound (US)-enhanced peracetic acid (PAA) pretreatment, to enhance resource recovery from anaerobic sludge fermentation. Results show that the optimum operation condition was achieved with US-enhanced 10 mg PAA/g TSS (total suspended solids), leading to a remarkable 31.72-fold increase in short-chain fatty acids (SCFAs) production compared to the control group (with PAA contribution in SCFAs deducted). Mechanism explorations prove that US-enhanced PAA pretreatment significantly promoted sludge disruption and organic matter release, creating a substantial amount of soluble substances (i.e., tyrosine-like and tryptophan-like substances) for subsequent fermentation. Fourier transform infrared (FTIR) analysis indicates that US-enhanced PAA pretreatment induced alterations in the protein secondary structure and facilitated its transformation. Quenching experiments imply that the contribution of reactive species during sludge oxidation followed the order: 1O2 >OH > CH3C(O)O/CH3C(O)OO. These reactive species played essential roles in the inherent mechanisms responsible for sludge disintegration and organic conversion. On the other hand, the analysis of key enzyme activities reveals a severe inhibition of the enzymes associated with SCFAs consumption. Further microbial community investigation confirms the enrichment of acidogenic microorganisms, such as Clostridium_sensu_stricto_10 sp., norank_f__norank_o__Bacteroidales sp., etc. Overall, US-enhanced PAA pretreatment can facilitate energy recovery and enhance the efficiency of anaerobic fermentation, offering significant benefits in sustainable sludge management.

Dual oxidant strategy involving calcium peroxide and peroxydisulfate for waste activated sludge recycling: Removal of sulfamethoxazole and hydrocortisone

In this study, the removal of sulfamethoxazole (SMX) and hydrocortisone (HC) from waste activated sludge (WAS) was explored using a dual oxidant strategy involving calcium peroxide (CaO2) and peroxydisulfate (PDS). Synergistic effects were observed between CaO2 and PDS at 0.1 g/g TS (total solids), leading to 89 % and 54 % removal of SMX and HC, respectively, in 8 min in sludge. Ca2+, Ca(OH)2, H2O2, and SO42– from CaO2 and PDS hydrolysis promoted pollutant removal. SMX removal was mainly driven by 1O2, which had a 78 % contribution to the removal rate, while HC removal was mainly due to OH and SO4–, with an overall 79 % contribution. Furthermore, removal pathways for SMX and HC during CaO2/PDS were proposed. SMX, HC, and their intermediates with high toxicity transformed into less toxic or nontoxic products during CaO2/PDS oxidation. Moreover, the dewatering ability and solubility of sludge increased after CaO2/PDS treatment. Furthermore, the production of volatile fatty acids during anaerobic fermentation of sludge pretreated with CaO2/PDS increased by 1.8 times relative to the control, with a 13 % increase in acetic acid content. The altered relative abundance of Firmicutes and Proteobacteria played a key role in promoting the recovery of carbon sources from the sludge.

Distinct effects of chemical- and bio- flocculants on the sludge acidogenic fermentation for volatile fatty acids production by affecting the acidogenic steps, microbial community structure and metabolic functions

Flocculants play crucial roles in sludge treatment, while the specific impact of chemical and bio-flocculants on sludge anaerobic fermentation was unknown. This study unveiled the contrasting effects of chitosan (CTS) and poly‑aluminum chloride (PAC) on volatile fatty acids (VFAs) generation during sludge fermentation. CTS supplementation resulted in 17.2 % increase in VFAs production, while PAC exposure led to 7.6 % reduction compared to the control. Further investigation revealed that CTS facilitated sludge solubilization and hydrolysis, thus providing sufficient organic substrates for VFAs generation. Additionally, environmental-friendly CTS exposure positively influenced the abundance and activity of functional anaerobes, as well as the expression of genes associated with VFAs biosynthesis. In contrast, PAC exposure resulted in the formation of larger sludge flocs, which hindered WAS solubilization and hydrolysis. Meanwhile, its potential microbial toxicity also impeded the microbial metabolic activity (i.e., genetic expressions), resulting in unsatisfactory VFAs production.

Surfactant and antibiotic co-occurrence reshaped the acidogenic process for volatile fatty acids production during sludge anaerobic fermentation

The overuse of surfactants and antibiotics has led to their high concentration in waste activated sludge (WAS), and these exogenous pollutants have been shown to pose various influences on the subsequent anaerobic treatment process. Previous works have primarily concerned the impacts of individual pollutants on WAS anaerobic fermentation process. This work revealed the synergetic effects of sodium dodecyl benzene sulfonate (SDBS) and sulfadiazine (SDZ) co-occurrence in WAS on the biosynthesis of volatile fatty acids (VFAs). The addition of SDBS in the SDZ reactor significantly increased VFAs generation, and this increase was correlated with the concentration of SDZ. The VFAs production exhibited a 200.0–211.9 % and 5.9–20.4 % increase in comparison with the sole SDZ and SDBS reactor, respectively. The SDBS and SDZ co-occurrence facilitated the solubilization, hydrolysis, and acidification stages of WAS fermentation synchronously. SDBS was effectively to disintegrate the cemented structure of extracellular polymeric substances and meanwhile improve the SDZ solubilization, which increase the SDZ bioavailability as well as biotoxicity to the anaerobic species. Herein, the anaerobic consortia structure was evidently reshaped, and the keystone microbes Acetoanaerobium and Fususibacter, as well-tolerated hydrolytic-acidogenic bacteria, were greatly enriched. Furthermore, the functional microbial metabolic traits responsible for the substrate extracellular hydrolysis (e.g., glsA and MAN2C1), intracellular metabolism (e.g., ALDO and asdA), and fatty acid generation (e.g., aarC) were all upregulated in the SDBS/SDZ co-occurrence reactor.

Research on the Promotion of Sludge Anaerobic Fermentation with Sodium Citrate under Low Concentrations of Polyaluminum Chloride

Polyaluminum chloride (PAC) is used widely and increasingly in wastewater treatment plants, resulting in its inevitably high production in sludge. Previous studies have indicated that the production of short-chain fatty acids (SCFAs) is inhibited by the existence of PAC in sludge anaerobic fermentation, so it is necessary to study how to promote sludge anaerobic fermentation under low concentrations of PAC. In this study, sodium citrate (SC) was first used to improve the efficiency of anaerobic fermentation under low concentrations of PAC. The results showed that the production of SCFAs increased with SC, especially when the ratio of PAC to SC was 1:2, and the maximum production of SCFAs reached 2890 mg/L, which is 2.5 times more than when PAC only exists. The mechanism studies showed that SC could remove the Al3+ in the sludge floc, which led to deflocculation of the sludge floc, accelerated the destruction of extracellular polymers (EPS), and released soluble substances in the sludge. At the same time, the key enzymes that were bound and hidden originally in the sludge were also released, which promoted the further degradation of organic matter and shortened the anaerobic fermentation period. However, the higher concentration of SC was not conducive to sludge dehydration. Therefore, the optimal distribution ratio of PAC to SC should be 1:1. This study provides a new idea for the research and practical application of sludge anaerobic fermentation.

Performance and mechanism of calcium peroxide reducing H2S production from sludge anaerobic fermentation

Calcium peroxide (CaO2) addition is widely regarded as a high-efficient sludge treatment method to degrade pollutants and enhance resource recovery. In this study, it was further found that CaO2 addition changed sulfur distribution during sludge anaerobic treatment, which promoted sulfur flow from sludge solid to liquid phase but significantly inhibited toxic hydrogen sulfide (H2S) gas production. With CaO2 addition increased from 0 to 0.25 g/g VSS, the accumulated H2S yield decreased from 314.6 × 10−4 to 61.3 × 10−4 mg/g VSS while the accumulated SCFAs and hydrogen yields increased from 46.2 to 266.5 mg COD/g VSS, and 0.9 to 10.2 mL/g VSS. Mechanism analysis showed that the released OH– and oxidizing substances (e.g., ·OH and ·O2–) firstly destroyed functional groups and hydrogen bonding networks contained in sludge EPS, with the contents of C=O group, β-sheet and α-Helix being reduced by 24%, 19.7%, and 23.2%, respectively. This caused the cracking of sludge EPS and releases of soluble metal ions (e.g., Al, Mg, and Fe). The released metal ions in alkaline environment promoted the transformation of H2S gas and dissolved sulfide to metal sulfide, which was one important reason to reduce H2S gas release. Then, the free radicals further attacked sludge cells without EPS protection and caused cell lysis. Flow cytometry analysis showed that 0.25 g/g VSS CaO2 reduced the rate of living cells from 90.7% to 81.4%. Microbial analysis unveiled that CaO2 reduced the content of S2− producers (e.g., Desulfobulbus and Desulfovibrio) and the expression of related genes (e.g., PepD and Sat). Moreover, the activities of S2− producers, e.g., methionine lyase and adenosine-5′-phosphosulfate reductase also reduced by 41.3% and 64.8% with 0.25 g/g VSS CaO2 addition. These findings provide new insights into CaO2-based sludge treatment and might have significant implications for sludge treatment.