Accumulation Of Short-chain Fatty Acids Research Articles

Comparison of volatile metabolic profiles in fermented milk of Streptococcus thermophilus during the postripening period at different incubation temperatures

Streptococcus thermophilus has been extensively applied in fermented milk. This study used gas chromatography-ion mobility spectroscopy to determine and evaluate the volatile metabolites in raw milk, milk fermented at 37°C, and milk fermented at 42°C. Ten discriminatory volatile metabolites were identified at different incubation temperatures: acetone, 2-heptanone, 2-pentanone, 2-hexanone, butanal, hexanal, ethyl acetate, 3-methylbutanal, 3-methylbutanoic acid, and 2-methylpropanoic acid, indicating that fermentation temperature affected the spectrum of volatiles in milk fermented by different strains of S. thermophilus. Specifically, fermentation at 37°C led to accumulation of short-chain fatty acids, whereas fermentation at 42°C enriched ketones and other flavor substances in the fermented milk, enhancing the flavor of the product. This work examined the differences between the volatile metabolites produced by different S. thermophilus strains fermented at different temperatures to evaluate the effect of temperature on the metabolic pathways.

Enhancing short-chain fatty acids recovery through anaerobic fermentation of waste activated sludge with cation exchange resin assisted lysozyme pretreatment: Performance and mechanism

Short-chain fatty acids (SCFAs) biosynthesis through anaerobic fermentation has attracted increasing attention for achieving economic benefits and carbon recovery in waste activated sludge (WAS) management. Lysozyme has been reported as a promising biological pretreatment for WAS hydrolysis and SCFAs generation, whereas the low efficiency and high dosage limited its application. In this study, cation exchange resin (CER) was innovatively employed for extracellular polymer substances (EPS) disruption, expecting to promote the lysozyme-induced enzymolysis and solubilization of dissolved organic matters (DOMs). With 3 g/g SS CER and 0.10 g/g SS lysozyme, 5404 mg/L SCOD (i.e. 36.5% of TCOD) and 43.1% VSS reduction rate were realized in the coupled pretreatment, which were 2.63 and 2.09 times than those with sole lysozyme pretreatment. Large amounts of DOMs were released attributed to EPS disruption and cell lysis, leading to 3760 mg COD/L SCFAs accumulation mainly composed of acetic and propionic acids (i.e. occupying 59.7–65.3%). The coupled pretreatment boosted the enrichment of hydrolysis and SCFAs production bacteria, accompanied by the corresponding metabolic functions enhancement (i.e. organic matters metabolism, microbial proliferation and membrane transport). The new sight into the potential mechanism between lysozyme and CER was proposed, and the engineering implication was demonstrated. The findings provided an environmental and economical solution to realize carbon recovery in WAS management.

Novel Insights into the Mechanisms of Periodate-Based Pretreatment in Enhancing Short-Chain Fatty Acids from Waste Activated Sludge

Resource recovery in the form of short-chain fatty acids (SCFAs) from waste activated sludge (WAS) is usually limited due to the quantity and quality of SCFAs. To mitigate these restrictions, this study provides a novel method by using periodate (PI) pretreatment. With an increase in the PI concentration from 0 to 50 mg/g total solids (TS), the SCFA yield increased from 1486 to 3109 mg COD/L and acetic acid production was enhanced by more than twice. Further increase of PI had an adverse effect on SCFA production. Electron spin resonance analysis confirmed that hydroxyl radicals (•OH) and superoxide radicals (•O2–) might play crucial roles in the pretreatment process. Further mechanism studies revealed that PI pretreatment improved the disintegration of WAS and decomposition of humus as well as lignocellulose, thereby providing more substrates for SCFA production. The model-based synthetic wastewater tests combined with enzymatic analyses revealed that the stronger suppression of SCFA consumers compared to SCFA generators causes SCFA accumulation; specifically, the inhibitory order was methanogenesis > hydrolysis > acetogenesis > acidogenesis. The suppressed gene expression of acetate-dependent (such as cdh [EC: 2.1.1.245]) and CO2-type methanogenesis (such as fwd [EC: 2.3.1.101] and mch [EC: 1.5.98.1]) further triggered SCFA accumulation with PI addition. Moreover, PI pretreatment caused the microbial community to shift toward the favorable hydrolytic and acidification-associated (e.g., Longilinea, Acinetobacter, and Macellibacteroides) microorganisms. This study is expected to broaden an unknown application of PI and provide an alternative strategy for resource recovery in some specific circumstances.

The high dose of inulin exacerbated food allergy through the excess accumulation of short-chain fatty acids in a BABL/c mouse model

Inulin dietary supplement is conventionally beneficial to gut health and can potentially prevent food allergy (FA). This study aimed to determine how dietary inulin interventions at different doses affect the OVA-induced FA in a BALB/c mouse model. Although the middle dose of inulin (50 mg per mouse) showed the best therapeutic effect on FA, high-inulin supplementation (80 mg per mouse) provoked severe allergic and intestinal inflammatory responses, which were characterized by elevated serum allergic inflammation-related factor levels, dysfunctional gut barrier, unbalanced luminal pH value, decrease in intestinal antioxidant capacity, and disordered gut microecology. Moreover, profiling of SCFAs indicated that the high-inulin-induced excess accumulation of SCFAs in the colon was responsible for the gut immune disorders. Spearman correlation analysis unraveled that the featured bacterial taxa in the high-inulin-treated mice were Ruminococcaceae and Bifidobacterium, of which the relative abundance was negatively correlated with expression of tight junction proteins and improvement of T cell homeostasis, and positively correlated with levels of allergic inflammation-related indexes. Our work suggested that high-inulin dietary supplementation can be detrimental to allergic individuals and highlighted the importance for personalized use of inulin-type dietary supplements to safely improve human health.

Divalent cation chelation enhancing carbon migration and recovery from anaerobic fermentation of waste activated sludge

Recently, the short-chain fatty acids (SCFAs) production with carbon recovery from waste activated sludge has received increasing attention. Despite that numerous approaches have been investigated, the divalent cation chelation towards enhancing anaerobic fermentation and the carbon migration fates has been rarely reported. This work attempted to chelate structural divalent cations from sludge flocs by sodium pyrophosphate (SP) at the dosages of 0–0.6 g/g SS, facilitating sludge disintegration and hydrolysis with particulate organic matters solubilization. At the optimal SP dosage of 0.4 g/g SS, the chemical oxygen demand (COD) and carbon source content in sludge solid were reduced by 45.24 % and 35.50 % within 1-day treatment, leading to considerable sludge hydrolysis and carbon release performances with soluble COD/total COD of 23.21 % and carbon release rate of 26.94 % in 2-day anaerobic fermentation, respectively. Numerous SCFAs of 237.43 mg COD/g VSS were produced by 4-day anaerobic fermentation, with the overall carbon recovery rate of 21.19 %, which were dominantly composed by acetate and propionate. Meanwhile, the SP-inhibited biogas generation also contributed to SCFAs accumulation. The divalent cation chelation obviously improved rate constant and equilibrium performances of anaerobic fermentation process, which were 1.81–5.21 and 1.36–2.82 times higher than those without SP, respectively. The carbon migration pattern was proposed by interphase carbon source balance and the mechanism was also illustrated. The crucial novelty was to propose an innovative and economic sludge hydrolysis pattern by divalent cation chelation towards improving carbon recovery.

Enhanced short chain fatty acids production from anaerobic fermentation of primary sludge using free ammonia pretreatment

This study is the first to report a new free ammonia (FA) pretreatment technique to recover short-chain fatty acids (SCFAs) from primary sludge (PS). The results showed FA pretreatment (240 mg N/L) largely enhanced the SCFAs production from PS to 355.8 mg chemical oxygen demand (COD)/g VSS, which was about 4.5-fold that of control (i.e., 80 mg COD/g VSS). Mechanism investigations showed that FA pretreatment greatly accelerated the PS solubilization and destroyed the integrity of EPS, and the easily biodegradable substrates in the fermentation broth increased significantly, providing microorganisms with more organics to convert into SCFAs. Further analyses displayed that FA greatly inhibited the methane production during fermentation, resulting in further accumulation of SCFAs. Given that the NH4+-N released during the fermentation can be converted into FA in situ, this could facilitate the transition of wastewater treatment plants operation from linear to circular mode.

In vitro fermentation of fructooligosaccharide and galactooligosaccharide and their effects on gut microbiota and SCFAs in infants

• Gut microbes can degrade FOS and GOS to produce SCFAs and lactate. • The change of intestinal flora were the main factors affecting SCFAs accumulation. • The abundance of CAZymes was significantly higher in the GOS and FOS + GOS groups. • FOS and GOS may increase beneficial microbes and inhibit harmful microbes. Oligosaccharides are widely added to infant formula milk powders as prebiotics for intestinal health. Using in vitro fermentation conditions, this study investigated the fermentation characteristics of fructooligosaccharides (FOS) and galactooligosaccharides (GOS) with infant fecal inoculum. The changes in short-chain fatty acids (SCFAs) and lactate production were monitored by HPLC. The contents of SCFAs and lactate increased significantly in the GOS and FOS + GOS groups. Furthermore, the metagenomic analysis revealed that microbial communities were distinctively different. Most bacteria were significantly positively correlated with SCFAs production and negatively correlated with pH, particularly Bifidobacterium , Enterococcus , Weissella and Streptococcus . More importantly, the relative abundance of most carbohydrate-active enzymes (CAZymes, EC:2.3.1.54, EC:2.3.1.9 and EC:2.8.3.8, etc.) were significantly higher in the GOS and FOS + GOS groups. These results suggested that FOS and GOS had a significant regulatory effect on the composition and metabolism of the intestinal microbiota, further promoting the production of SCFAs and lactate.

Effects of modified biochars on the shifts of short-chain fatty acid profile, iron reduction, and bacterial community in paddy soil.

Biochar is well known as an effective means for soil amendment, and modification on biochar with different methods could improve the benefits for environmental remediation. In this study, two modified biochars were generated with nitric acid (NBC) and hydrogen peroxide (OBC) pretreatment, and a control biochar was produced after washing with deionized water (WBC). The dynamics of short-chain fatty acids (SCFAs), iron concentration and bacterial community in rice paddy soil amended with different biochars or without adding biochar (CK) were studied during 70 days of anaerobic incubation. Compared to CK treatment, the accumulation of SCFAs was largely inhibited by the amendment of biochars. Besides, OBC and WBC increased the accumulation of Fe(II) at the initial stage of incubation. Via 16S rRNA gene sequencing, modified biochars caused significant response of bacterial community in comparison to WBC at Day 0-1, and three biochars favored bacterial α-diversity in the paddy soil at the end of the incubation. Interestingly, positive and negative correlations between NBC and several bacteria taxa (e.g. Geobacter, Fonticella and Clostridium) were observed. The study revealed that modified biochars had significant effects on the shifts of SCFAs, Fe(III) reduction and bacterial diversity, which provides fundamental information for future application of modified biochars in rice cropping ecosystem.

Heat-assisted potassium ferrate pretreatment enhancing short-chain fatty acids production from waste activated sludge: Performance and mechanisms

This work presented an innovative strategy for short-chain fatty acids (SCFAs) production from anaerobic waste activated sludge (WAS) fermentation via heat assisted potassium ferrate (PF) pretreatment. Results showed that under optimal pretreatment condition (55 °C + 0.1 g PF/g TSS), SCFAs production achieved 4068.4 mg COD/L with acetic acid reaching 1766.1 mg COD/L, significantly higher than those of the control (77.39 mg COD/L, only acetic acid contained) as well as individual heat (2930.8 and 1203.5 mg COD/L) and individual 0.1 g PF/g TSS (1970.1 and 982.7 mg COD/L) pretreatment groups. Mechanism analysis revealed that heat-assisted PF pretreatment aggravated WAS disintegration by facilitating the disruption of EPS and cells, hence accelerating organics release from sludge flocs to the supernatant. Besides, the biodegradability of released substances was also improved thereby providing quantities of directly available substrates for hydrolysis and acidification process. Analysis of key enzymes involved in SCFAs accumulation showed that heat-assisted PF pretreatment promoted the relative bioactivities of protease, butyrate kinase (BK) and acetate kinase (AK) by 51.6%, 39.2% and 155.6% compared to the control respectively, while suppressing the bioactivity of coenzyme F420. Microbial community analysis demonstrated the enrichment of Proteobacteria and Firmicutes, which were related to SCFAs accumulation. Meanwhile, in genus level, hydrolytic bacteria (e.g., Acinetobacter sp.), acidification bacteria (e.g., Petrimonas sp.) and iron-reducing bacteria (e.g., Romboutsia sp.) were enriched while SCFAs consuming bacteria (e.g., Candidatus_Competibacter sp.) were inhibited.

Butyrate induces development-dependent necrotizing enterocolitis-like intestinal epithelial injury via necroptosis.

The accumulation of short-chain fatty acids (SCFAs) from bacterial fermentation may adversely affect the under-developed gut as observed in premature newborns at risk for necrotizing enterocolitis (NEC). This study explores the mechanism by which specific SCFA fermentation products may injure the premature newborn intestine mucosa leading to NEC-like intestinal cell injury. Intraluminal injections of sodium butyrate were administered to 14- and 28-day-old mice, whose small intestine and stool were harvested for analysis. Human intestinal epithelial stem cells (hIESCs) and differentiated enterocytes from preterm and term infants were treated with sodium butyrate at varying concentrations. Necrosulfonamide (NSA) and necrostatin-1 (Nec-1) were used to determine the protective effects of necroptosis inhibitors on butyrate-induced cell injury. The more severe intestinal epithelial injury was observed in younger mice upon exposure to butyrate (p = 0.02). Enterocytes from preterm newborns demonstrated a significant increase in sensitivity to butyrate-induced cell injury compared to term newborn enterocytes (p = 0.068, hIESCs; p = 0.038, differentiated cells). NSA and Nec-1 significantly inhibited the cell death induced by butyrate. Butyrate induces developmental stage-dependent intestinal injury that resembles NEC. A primary mechanism of cell injury in NEC is necroptosis. Necroptosis inhibition may represent a potential preventive or therapeutic strategy for NEC. Butyrate induces developmental stage-dependent intestinal injury that resembles NEC. A primary mechanism of cell injury caused by butyrate in NEC is necroptosis. Necroptosis inhibitors proved effective at significantly ameliorating the enteral toxicity of butyrate and thereby suggest a novel mechanism and approach to the prevention and treatment of NEC in premature newborns.

Capsaicin combined with dietary fiber prevents high-fat diet associated aberrant lipid metabolism by improving the structure of intestinal flora.

Capsaicin (CAP) and dietary fibers are natural active ingredients that given separately do positively affect obesity and metabolic diseases. However, it was unknown whether their combined administration might further improve blood lipids and gut flora composition. To test this hypothesis we administered capsaicin plus dietary fibers (CAP + DFs) to male rats on a high-fat diet and analyzed any changes in the intestinal microbiota make up, metabolites, and blood indexes. Our results showed that combining CAP with dietary fibers more intensely reduced total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C). CAP + DFs also increased gut bacteria variety, and the abundance of several beneficial bacterial strains, including Allobaculum and Akkermansia, while reducing harmful strains such as Desulfovibrio. Additionally, CAP + DFs significantly increased arginine levels and caused short-chain fatty acids accumulation in the contents of the cecal portion of rats' gut. In conclusion, notwithstanding the rats were kept on a high-fat diet, adding CAP + DFs to the chow further improved, as compared with CAP alone, the lipidemia and increased the gut beneficial bacterial strains, while reducing the harmful ones.

Synergistic improvement of short-chain fatty acid production from waste activated sludge via anaerobic fermentation by combined plasma-calcium peroxide process

In this study, the combination of dielectric barrier discharge plasma (DBD) with calcium peroxide (CaO2) achieved significant synergistic effects in promoting hydrolysis of waste activated sludge (WAS) and short-chain fatty acid (SCFA) production during anaerobic fermentation. Compared with the control, DBD/CaO2 pretreatment increased SCFA production by 116 %, acetic acid ratio by 39 %, and sludge reduction by 30 % under the optimal conditions (discharge power = 76.5 W, CaO2 dosage = 0.05 g/g VSS). Mechanism investigations elucidated that DBD/CaO2 enhanced the generation of •OH, 1O2, and •O2−, synergistically promoted decomposing extracellular polymeric substances (EPS), lysing cells, releasing biodegradable substances, and enhancing acetic acid-enriched SCFA accumulation from fermentation. Meanwhile, Illumina MiSeq sequencing analysis revealed that the enrichment of hydrolytic and SCFAs-forming bacteria and the decrease in SCFAs-consuming bacteria by DBD/CaO2 treatment also contributed. This work provides an effective method to boost the SCFA production from WAS fermentation.

Mechanism of dielectric barrier discharge plasma technology to improve the quantity of short-chain fatty acids in anaerobic fermentation of waste active sludge.

The mechanism of improving the anaerobic fermentation performance of waste active sludge by using dielectric barrier discharge (DBD) plasma pretreatment technology was investigated. The maximum accumulation of short-chain fatty acids (SCFAs) was observed on the 7th day of anaerobic fermentation when the DBD power was 76.50 W, which was 1726.70 mg COD/L, 1.50 times of the control group. The ratio of acetic acid in DBD group was 9.30% higher than that in the control. Further mechanism research indicated that DBD pretreatment can destroy the structure of extracellular polymer substances and release organic substances such as protein and polysaccharide. The dissolved organic matter analysis indicated that the DBD technique could increase the release of biodegradable organics (eg., tyrosine proteins, soluble microbial by-products), thus accelerate the biotransformation of organic substance. Bacterial community structure analysis showed that the increase in the abundance of Firmicutes and Bacteroidetes and the decrease in the abundance of Proteobacteria in DBD group were beneficial to the accumulation of SCFAs. Besides, further archaeal analysis indicated that the decrease of Methanosaeta sp. and Methanosarcina sp. abundance in the DBD group facilitate acetic acid accumulation. This study demonstrated that the DBD technique can be used as an effective and potential pretreatment method to improve sludge anaerobic fermentation performance.

Ferulic acid improves intestinal barrier function through altering gut microbiota composition in high-fat diet-induced mice.

A high-fat diet (HFD) induces gut microbiota (GM) disorders, leading to intestinal barrier dysfunction and inflammation. Ferulic acid (FA) has shown anti-obesity effects, e.g., reducing body weight and food intake. However, the mechanism linking the anti-obesity effects of FA and GM modulation remains obscure. The present study aimed to clarify the mechanism underlying the anti-obesity effects of FA and modulation of the GM. C57BL/6J mice were fed by a low-fat diet (LFD) and HFD with or without FA at a dose of 100mg/kg of body weight by oral gavage for 12weeks. Using high-throughput sequencing, gas chromatography, real-time fluorescence quantitative PCR and immunohistochemical staining, the attenuation of obesity by FA were assessed via intestinal barrier integrity, inflammation, and the GM. FA reduced weight gain, improved HFD-induced GM imbalance, significantly enhanced intestinal short-chain fatty acid (SCFA)-producing bacteria (e.g., Olsenella, Eisenbergiella, Dubosiella, Clostridiales_unclassified, and Faecalibaculum) along with SCFA accumulation and its receptors' expression, decreased endotoxin-producing bacteria or obesity-related bacterial genera, and serum endotoxin (lipopolysaccharides), and inhibited the colonic TLR4/NF-κB pathway. Thus, FA can mitigate colonic barrier dysfunction and intestinal inflammation, induce the production of SCFAs and inhibit endotoxins by modulating the GM. These results indicate that enhancement of intestinal barrier by altering the GM may be an anti-obesity target of FA and that FA can be used as a functional compound with great developmental values.

Unraveling the behavior of nitrite on promoting short-chain fatty acids accumulation from waste activated sludge by peracetic acid pretreatment: Extracellular polymeric substance decomposition and underlying mechanism

Peracetic acid (PAA) is an emerging oxidant for waste activated sludge (WAS) treatment due to its strong oxidization and few toxic byproducts. Nitrite which can be in-situ recovered from WAS fermentation liquor, its protonated form (free nitrous acid, FNA) is regarded as the cost-effective inactivator. The stubborn extracellular polymeric substance (EPS) is the rate-limiting step for energy/resource recovery from WAS. This work found that the co-pretreatment of PAA and FNA can effectively promote short-chain fatty acids (SCFAs) production during anaerobic fermentation. Higher PAA dosage (100 mg/g VSS, FP4WAS) in co-pretreatment was beneficial for organics release (1976.9 mg COD/L), remarkably increased by 10.3– 96.5 % than that of low PAA dosage (25– 75 mg/g VSS), and promoted by 105.1 % and 62.1 % than FNA (FWAS)/PAA (100 mg/g VSS, P4WAS)-pretreated WAS. Effective release of soluble organics contributed to the SCFAs accumulation (7679 ± 86 mg COD/L, 4 d), enhanced by 200.0 % and 19.0 % than FWAS and P4WAS, respectively. Acetic (HAc) and propionic acid (HPr) peaked at 6344.7 mg COD/L in FP4WAS (accounted for 82.6 %), which increased by 10.6– 899.0 % than other groups. Moreover, OH and O2- were detected in co-pretreatment, may play the synchronous effect with the crucial intermediates of NO, NO2 and ONOO−/ONOOH on EPS decomposition.

Performance and mechanism of sodium percarbonate (SPC) enhancing short-chain fatty acids production from anaerobic waste activated sludge fermentation

A novel pretreatment technique (i.e., using Sodium percarbonate, SPC) to improve the short-chain fatty acids (SCFA) production waste activated sludge (WAS) was proposed in this study. Results indicated that the maximum SCFA production of 1605.7 mg COD/L and acetic acid of 52.9% were attained at 0.2 g SPC/g TSS, being 8.4 and 2.8 times of the control (191.3 mg COD/L and 19%), respectively. Meanwhile, the optimal time for SCFA accumulation was decreased from 6d (control) to 4d (0.2 g/g TSS). Mechanism explorations unraveled that SPC largely accelerated WAS solubilization and enhanced the bioavailability of organics released from WAS. It improved enzymatic activities related to hydrolysis and acidogenesis, while suppressed the Coenzyme F420 responsible for SCFA consumption. Illumina MiSeq sequencing analysis showed that SPC substantially enhanced the relative abundances of hydrolytic and/or acid-forming microbes. Furthermore, CO3- and O2− were the key factors to production enhancement in SPC-involved sludge fermentation.

New insight into selective Na+ stress on acidogenic fermentation of waste activated sludge from microbial perspective: Hydrolase secretion, fermentative bacteria screening, and metabolism modification

The short-chain fatty acids (SCFAs) production was facilitated in Na+ assistant anaerobic fermentation of waste activated sludge, whereas the relevant micro-ecosystem characteristics were unclear. This work explored the microbial community and hydrolases shifts under Na+ stress. It demonstrated that the selective Na+ stress increased protease activities to 126–160% while inhibiting α-glucosidase activities to 83.1–91.3%. Correspondingly, the biodegradation rates of protein and glucose were improved from 25.6% and 45.8% to 39.0% and 55.2%, respectively. Furthermore, the microbial species richness and biodiversity gradually decreased, attributing to Na+-induced cell lysis. Selective Na+ stress was the prime inducement for microbial community evolution and promoted “SCFAs-producing” bacterial community formation by “bacteria-screening” roles. Overall, the hydrolytic bacteria and acidogens became dominant bacteria, which were resistive to Na+ stress, while the SCFAs consumers were restrained as they are salinity-sensitive. Such microbial community shift modified functional metabolisms for enhancing anaerobic fermentation performance. The pathways for metabolism and genetic information processing were improved, positively relating to the facilitated catabolism of fermentation substrates. Consequently, the mechanism of maximized SCFAs accumulation by selective Na+ stress was proposed, i.e. microbial modification of fermentation ecosystem for facilitating sludge hydrolysis and acidification with impeded methanogenesis.

Unraveling the Behavior Of Nitrite on Promoting Short-Chain Fatty Acids Accumulation from Waste Activated Sludge by Peracetic Acid Pretreatment: Extracellular Polymeric Substance Decomposition and Underlying Mechanism

Unraveling the Behavior Of Nitrite on Promoting Short-Chain Fatty Acids Accumulation from Waste Activated Sludge by Peracetic Acid Pretreatment: Extracellular Polymeric Substance Decomposition and Underlying Mechanism

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

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

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

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