Isolation, identification and utilization of thermophilic strains in aerobic digestion of sewage sludge

Effect of rhamnolipid on the performance of compound thermophilic bacteria agent pretreatment system for waste sludge hydrolysis

This study innovatively introduced rhamnolipid (RL) to compound thermophilic bacteria (TB) agent pretreatment system for further accelerating the waste sludge hydrolysis and substrates transformation. The results showed that combined pretreatment was beneficial for the sludge extracellular polymers (EPS) rupture and dissolved organic matters (DOM) release. In the optimal dosage of 40 mg/g SS RL, the activities of protease and α-glucosidase increased by 20.7 % and 33.3 % than that without RL addition, respectively. The addition of RL enhanced efficient contacts between hydrolases and organic substrates, and excitation emission matrix (EEM) spectrum revealed that combined pretreatment with 40 mg/g SS RL could achieve higher soluble microbial by-products occupancy (54 %) and lower fulvic acid-like substances (6 %) occupancy in DOM, promoting the waste sludge biodegradability. High organics availability conducted to more shifts in microbial community structure, compared with TB agent pretreatment, the relative abundance of genus Geobacillus and norank_f__Synergistaceae were enhanced by 29.08 and 0.33 times in combined pretreatment system, respectively, which was conducive to sludge hydrolysis and subsequent anaerobic fermentation process.

Combination of sequencing batch reactor activated sludge process with sludge lysis using thermophilic bacterial community for minimizing excess sludge

Sludge reduction is a major challenge in biological wastewater treatment. Hydrolytic enzymes secreted by thermophilic bacteria can lyse sludge and thus achieve sludge reduction, and the indigenous thermophilic community in sludge can lyse sludge more effectively. In this study, the feasibility of combining a sludge lysis reactor based on thermophilic bacteria community (LTBC reactor, 75 °C) with a conventional sequencing batch activated sludge reactor (SBR) for sludge reduction (i.e., LTBC-SBR process) was systematically investigated first time. The effect of lysed sludge returning to the biochemical tank on pollutant removal efficiency, sludge flocculation, sludge settling, and microbial community and function of the LTBC-SBR process was studied. In the LTBC1-SBR process, a sludge growth rate of 0.71 g TSS/day was observed when the lysed sludge reflux ratio (LRR) was 1, and the sludge generation was reduced by 81.5% compared to the conventional SBR reactor. In the LTBC1-SBR process, the removal efficiencies of chemical oxygen demand and total nitrogen were 94.0% and 80.5%, respectively. There was no significant difference in the sludge volume index from the SBR to the LTBC1-SBR stage, however, the effluent suspended solids concentration increased from 35.2 ± 2.1 mg/L to 80.1 ± 5.3 mg/L. This was attributed to the reflux of sludge lysate. In addition, the changes in extracellular polymers content and composition resulted in poor sludge flocculation performance. Heterotrophic bacteria associated with Actinobacteria and Patescibacteria enriched in LTBC1-SBR with relative abundance of 28.51 ± 1.25% and 20.01 ± 1.21%, respectively, which decomposed the macromolecules in the refluxed lysed sludge and contributed to the sludge reduction. Furthermore, due to the inhibition of nitrite-oxidizing bacteria, the nitrite concentration in the effluent of the LTBC1-SBR system reached 4.7 ± 1.1 mg/L, and part of the denitrification process was achieved by short-cut nitrification and simultaneous denitrification. These results indicate that in-situ sludge reduction technology based on lyse sludge lysing by thermophilic community has considerable potential to be widely used in wastewater treatment.

Calcium peroxide significantly enhances volatile solids destruction in aerobic sludge digestion through improving sludge biodegradability

This work put up a novel strategy of applying calcium peroxide (CaO₂) in aerobic sludge digestion and provided insights into such system. The degradation percentage of sludge and total inorganic nitrogen production in the digesters with CaO₂ at 0.02 g/g-VS-WAS increased by 25.8% and 18.8% of control. CaO₂ addition allowed various key microbes related to organics degradation to accumulate in the system. Moreover, the modelling and chemical (i.e., excitation emission matrix (EEM) fluorescence and fourier transformation spectroscopy (FTIR)) analyses revealed that CaO₂ addition enhanced sludge biodegradability with more release of biodegradable organics and increased degradation of recalcitrant organics, which can be transformed into biodegradable organics with the action of CaO₂. Subsequent transformation test indicated that CaO₂ enabled to promote hydrolysis and catabolism of biodegradable substrates in sludge. Further investigations on function mechanism suggested that CaO₂ carried on positive action for sludge aerobic digestion mainly through the enhancement by ·OH.

Enhanced sludge reduction during swine wastewater treatment by the dominant sludge-degrading strains Chryseobacterium sp. B4 and Serratia sp. H1

Sludge reduction is considered a main target for sludge treatment and an urgent issue for wastewater treatment. In this study, two dominant sludge-degrading strains, identified as Chryseobacterium sp. B4 and Serratia sp. H1, were used for inoculation in swine wastewater treatment to investigate the enhancement of sludge reduction. The results showed the volatile suspended solid (VSS) removal rate in experimental groups inoculated with Chryseobacterium sp. B4, Serratia sp. H1, and a combination of the two strains improved by 49.4%, 11.0%, and 30.5%, compared with the control with no inoculation. Furthermore, microbial community structure and functional prediction analyses indicated Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria could play an essential role in sludge reduction, and the dominant sludge-degrading strains B4 and H1 enhanced sludge reduction by strengthening carbohydrate, nucleotide, amino acid, and lipid metabolism and membrane transport functions. This study provides new insights into sludge reduction during wastewater treatment with dominant sludge-degrading strains.

The entering of polyethylene terephthalate microplastics into biological wastewater treatment system affects aerobic sludge digestion differently from their direct entering into sludge treatment system

The entering of the widespread polyethylene terephthalate (PET) microplastics into biological wastewater treatment system results in their retention in sewage sludge, which inevitably enters the sludge treatment system. However, all previous studies regarding the impact of microplastics on sludge treatment system were conducted by directly adding microplastics to system and focusing on anaerobic sludge digestion, although PET microplastics commonly enter into the biological wastewater treatment system first before sludge being subsequently treated. The potential impact of the microplastics on waste activated sludge (WAS) aerobic digestion is also completely missing. Therefore, herein the influences of PET microplastics with different entry paths on WAS aerobic digestion as well as the key mechanisms involved was firstly explored. Experimental results demonstrated that compared to the control test, the entering of PET microplastics to biological wastewater treatment system inhibited WAS aerobic digestion by 10.9 ± 0.1% through the decreased hydrolysis, although WAS solubilization during aerobic digestion was improved due to the change of generated WAS characteristics. In contrast, when PET microplastics was directly added to the sludge aerobic digester, there was little impact on solubilization, while the hydrolysis were inhibited seriously, thereby suppressing WAS aerobic digestion more severely by 28.9 ± 0.1%. Further investigation revealed that PET microplastics reduced the populations of key bacteria (e.g., Saprospiraceae, Chitinophagaceae and Xanthomonadaceae) involved in aerobic digestion via induced oxidative stress or/and releasing toxic chemical. This study provided a more accurate approach to assessing the real situation regarding the influences of PET microplastics on aerobic sludge digestion.

Enhancing swine wastewater hydrolysis with thermophilic bacteria and assisted pretreatments

Slow degradation rate of swine wastewater, which is mainly caused by particulate and refractory organic matters, is the main drawback of anaerobic digestion. Therefore, it is necessary to improve the hydrolysis of swine wastewater. In this study, different pretreatments were used to hydrolyze swine wastewater, including thermophilic bacteria (TB), alkali, acid, ultrasound (UL), and ultrasonic-combined thermophilic bacteria (UL-TB) pretreatment. The hydrolysis effect was investigated by analyzing the changes of pretreated soluble chemical oxygen demand (SCOD), soluble protein, and carbohydrate. The experimental results showed that effect of different pretreatments on swine wastewater hydrolysis had the following order: TB = alkali>UL-TB > UL>acid. Alkali pretreatment was effective for the release of protein from swine wastewater, and TB pretreatment was advantageous for carbohydrate release during hydrolysis. The results could provide valuable information for the disposition of swine wastewater as well as the application of TB-related pretreatments. PRACTITIONER POINTS: TB and alkali pretreatment exhibited the highest hydrolysis ability. The release of carbohydrate by TB was higher than other pretreatments. Ultrasonic assistance generated inhibition on the hydrolysis of TB.

Alkaline aided thermophiles pretreatment of waste activated sludge to increase short chain fatty acids production: Microbial community evolution by alkaline on hydrolysis and fermentation

Adding alkaline into an anaerobic waste activated sludge (WAS) fermentation with thermophilic bacteria pretreatment could efficiently improve short-chain fatty acids (SCFAs) accumulation to 3550 ± 120 mg COD/L. The acidification rate in combined test was 21.2%, while that was 15.6% and 10.7% in sole thermophilic bacteria pretreatment and control tests respectively. Four distinct groups of microbes could be identified with noticeable shifts using the combined pretreatments, and tremendous effects were analyzed on organic content especially of the soluble proteins and SCFAs concentrations. Particularly, alkaline addition would significantly change the functional microbial structures, including the decrease of Caloramator with the function of thermophilic proteolytic and the increase of Acidobacteria TM7 and Petrimonas sp. The results above suggested that alkaline addition could decrease the hydrolytic substances consume by thermotolerance bacteria and final improve SCFAs accumulation in fermentation process.

Effects of reactive oxygen species scavengers on thermophilic micro-aerobic digestion for sludge stabilization

In thermophilic digestion systems, sludge stabilization may be adversely impacted by high concentrations of ammonia nitrogen or the rapid accumulation of fatty acids; however, few studies have focused on the mitigation of the inhibition of reactive oxygen species (ROS). In this study, fulvic acids or tea polyphenols were introduced to a thermophilic digestion system and the effects of ROS scavengers on sludge stabilization were investigated. As fulvic acids or tea polyphenols were added to sludge, they reacted with metal cations, such as Cu²⁺ and Zn²⁺, to form stable complexes that enabled active metal ions to be transported into cells to enhance the oxidase activities. Therefore, the digestion systems presented a lower O₂^•- content compared with that of a control group with no additive. Both fulvic acids and tea polyphenols mitigated the adverse effects of the ROS and enhanced the reduction of volatile solids (VS), however, fulvic acids better facilitated the sludge stabilization. The optimal dosage was 0.3% of the total solids as fulvic acids were added to the sludge every 48 h. The VS reduction in the digester reached 38.2% at 14 d, which was clearly higher than that in the control group. The pathway of ROS scavengers affecting sludge stabilization was proposed, and it may be helpful to gain deeper insight into the characteristics of thermophilic digestion processes as well as the mechanism of sludge stabilization.

A novel acetogenic bacteria isolated from waste activated sludge and its potential application for enhancing anaerobic digestion performance

The development of anaerobic digestion (AD) for volatile fatty acids (VFAs) production from waste activated sludge (WAS) is arrested due to low hydrolysis and acidification efficiency. This study proposed to enhance WAS reduction and VFAs accumulation during AD process via bioaugmentation of acetate-producing bacteria. Four acetogens were firstly isolated from a temperature-phased anaerobic digestion (TPAD) system. The acetate production efficiency of different isolates ranged from 15.8 to 73.7 mg acetate/g TOC, in which the bacterial strain NJUST19 was found to be the most effective strain. The results of morphological, biochemical characteristics as well as phylogenetic analysis showed that the isolate NJUST19 was Gram-positive and rod-shaped, catalase-negative, nitrate reduction-positive, methyl red-negative and capable of starch and gelatin hydrolysis, for which the name of Clostridium sp. NJUST19 was proposed. The optimal culture conditions (i.e. initial pH and temperature) were evaluated for their effects on microbe growth of selected NJUST19, and the maximum acetate production was observed at pH 9.0 and temperature of 40 °C. In the case of modified TPAD system inoculated with Clostridium sp. NJUST19, total suspended solids (TSS) removal rate and maximum VFAs accumulation increasing to 35.3% and 4200 mg/L, respectively, which was much higher than that of control (21.9% and 2894 mg/L). These results indicated that Clostridium sp. NJUST 19 is capable of enhancing digestion efficiency with a great benefit for VFAs production, offering potential prospects for bioaugmentation of WAS anaerobic digestion.

Impacts of ammonia nitrogen on autothermal thermophilic micro-aerobic digestion for sewage sludge treatment

The concentration of ammonia nitrogen is relatively high during autothermal thermophilic micro-aerobic digestion (ATMAD), which could significantly affect the sludge stabilization. This paper aims to investigate the impacts of ammonia nitrogen on ATMAD process, batch experiments were carried out with dosage of certain amount of NH₄HCO₃ into digestion system. The total ammonia nitrogen (TAN) was considered as a suitable indicator to characterize the ammonia inhibition. As the TAN reached to approximately 1000 mg L^-1, the sludge digester presented a relatively low removal of volatile solids, due to adverse effects of ammonia nitrogen on sludge digestion. Three pathways that closely related to ammonia inhibition were investigated in this research. Digestion system could be inhibited by high TAN due to K⁺ deficiency of the cells and the decline of Mg²⁺ in the cytoplasm, and the accumulations of reactive oxygen species lead to oxidative stress for the microbes. Ammonia inhibition can be mitigated by the increase of oxidative enzyme.

Evaluation of Autothermal Thermophilic Aerobic Digester Performance for the Stabilization of Municipal Wastewater Sludge

BACKGROUND AND OBJECTIVE

Sludge stabilization process in terms of operational, environmental and economic indexes is the most important stage of treatment and its disposal. This study was aimed to determine the performance of Autothermal Thermophilic Aerobic Digestion (ATAD) system as one of the low-cost and biocompatible methods of sludge treatment.

MATERIALS AND METHODS

This study has been done using a laboratory scale Autothermal Thermophilic Aerobic Digestion (ATAD). The reactor was consisted of two polyethylene tanks with a final capacity of 100 L for each tank. Both tanks with all fittings were installed on a metal frame. The variables of study were temperature, dissolved oxygen, pH, volatile organic compounds, total solids, COD and the number of Ascaris eggs and fecal coliforms per gram of dry matter of the sludge. The temperature was measured hourly and the pH and dissolved oxygen were measured and controlled twice per day. One-way ANNOVA was applied to analyze reasults.

RESULTS

According to the results, the temperature of sludge increased from 11.7-61.2°C by biological reactions. Pathogen organisms were reduced from 80×106 to 503 in number during 72 h. After 6 days pathogen organisms and Ascaris eggs were removed completely. Volatile organic compounds and COD were reduced 42 and 38.3% respectively during the 6 days.

CONCLUSION

It is concluded that the performance of ATAD in removing organic compounds from wastewater sludge were desirable. Resulted sludge from stabilization process were appropriate for use in agriculture as a soil supplement and met the indexes of class A sludge according to EPA's standards (CFR 40 Part 503).

Strengthen effects of dominant strains on aerobic digestion and stabilization of the residual sludge

In order to strengthen the aerobic digestion of residual sludge, shorten the time of sludge stabilization and further reduce operating costs, 3 dominant strains identified as Pseudomonas sp. L3, Acinetobacter sp. L16 and Bacillus sp. L19 were isolated from long-term aerobic digestion sludge. Results showed that the sludge stabilization time were reduced by 3-4days compared with the control when the dominant strains were added to the process of sludge aerobic digestion. The addition of dominant strains accelerated the accumulation of TOC, nitrate nitrogen and ammonia nitrogen in the digestive solution at different levels, and it was beneficial to the dissolution of phosphorus. Controlling DO 3-5mg/L, pH 6.5, the strains of Pseudomonas sp. L3 and Bacillus sp. L19 were combined dosing with the dosage of 2% in the process of sludge aerobic digestion, compared with the control, digestion rates of TOC and MLSS were increased about 19% and 16%, respectively.

Selective simplification and reinforcement of microbial community in autothermal thermophilic aerobic digestion to enhancing stabilization process of sewage sludge by conditioning with ferric nitrate

The effect of ferric nitrate on microbial community and enhancement of stabilization process for sewage sludge was investigated in autothermal thermophilic aerobic digestion. The disinhibition of volatile fatty acids (VFA) was obtained with alteration of individual VFA concentration order. Bacterial taxonomic identification by 454 high-throughput pyrosequencing found the dominant phylum Proteobacteria in non-dosing group was converted to phylum Firmicutes in dosing group after ferric nitrate added and simplification of bacteria phylotypes was achieved. The preponderant Tepidiphilus sp. vanished, and Symbiobacterium sp. and Tepidimicrobium sp. were the most advantageous phylotypes with conditioning of ferric nitrate. Consequently, biodegradable substances in dissolved organic matters increased, which contributed to the favorable environment for microbial metabolism and resulted in acceleration of sludge stabilization. Ultimately, higher stabilization level was achieved as ratio of soluble chemical oxygen demand to total chemical oxygen demand (TCOD) decreased while TCOD reduced as well in dosing group comparing to non-dosing group.

Effects of ultrasonic-assisted thermophilic bacteria pretreatment on hydrolysis, acidification, and microbial communities in waste-activated sludge fermentation process

A novel pretreatment method combining ultrasonic with thermophilic bacteria (Geobacillus sp. G1) was employed to pretreat waste-activated sludge (WAS) for enhancing the WAS hydrolysis and subsequent volatile fatty acids (VFAs) production. The soluble protein and carbohydrate were mostly released from intracellular ultrasonic-assisted Geobacillus sp. G1 pretreatment, and accumulated to 917 ± 70 and 772 ± 89 mg COD/L, respectively, which were 2.53- and 2.62-fold higher than that obtained in control test. Excitation emission matrix (EEM) fluorescence spectroscopy revealed the highest fluorescence intensity (FI) of protein-like substances, indicating the synergistic effect of ultrasonic and Geobacillus sp. G1 pretreatments on WAS hydrolysis. The maximum VFAs accumulation was 4437 ± 15 mg COD/L obtained in ultrasonic-assisted Geobacillus sp. G1 pretreatment test. High-throughput pyrosequencing analysis investigated that the microbial communities were substantial determined by the pretreatment used. The hydrolysis enhancement was caused by an increase in extracellular enzymes, which was produced by one of dominant species Caloramator sp. The positive effect was well explained to the enhancement of WAS hydrolysis and final VFAs accumulation.

Disinhibition of excessive volatile fatty acids to improve the efficiency of autothermal thermophilic aerobic sludge digestion by chemical approach

In this study, we explored a chemical approach to eliminate inhibition of excessive volatile fatty acids (VFAs) in autothermal thermophilic aerobic digestion (ATAD). Ferric nitrate, ferric chloride, potassium nitrate and potassium chloride were employed to demonstrate the combined action of ferric ion and nitrate ion. Supplementation of ferric nitrate in the sludge digestion system resulted in reducing the concentration of Total VFAs (TVFA) by round 5000mg/L and more than 2000mg/L of acetic acid at the end of digestion. Lower TVFA concentration contributed to faster sludge stabilization rate and the VS removal of ferric nitrate dosed digester achieved 38.18% after 12days digestion which was 9days in advance compared with the stabilization time of sludge in digester without chemicals addition. Lower concentrations of NH4(+)-N and SCOD in supernatant while higher content of TP in digestion sludge were obtained in digester with ferric nitrate added.

Utilization of alkali-tolerant strains in fermentation of excess sludge

This study aimed at exploring a new approach for producing volatile fatty acids (VFAs) from excess sludge (ES). Two representative alkali-tolerant bacteria (HIT-01 and HIT-02) were isolated from ES, and inoculated separately or jointly into ES to investigate their effects on VFAs accumulation, soluble organic compounds concentrations, and bacterial community structure of ES under alkaline anaerobic conditions (pH 10.0). Four fermentation conditions were examined for 20days. Joint-inoculation with the two strains achieved the highest VFAs concentration (3139mg/L). Acetic and propionic acids were the dominant acid species. On the 9th day, all the inoculated ES had the higher concentrations of soluble organic compounds than the un-inoculated control ES. Denaturing gradient gel electrophoresis (DGGE) analysis revealed that these two strains (HIT-01 and HIT-02) became predominant in the bacterial community during alkaline anaerobic fermentation. The results demonstrated that bioaugmentation might be useful for enhancing VFAs accumulation from ES.

Effect of dosing time on the ammonium nitrogen disinhibition in autothermal thermophilic aerobic digestion for sewage sludge by chemical precipitation

Magnesium ammonium phosphate (MAP) precipitation was introduced to remove ammonium nitrogen (NH4(+)-N) in autothermal thermophilic aerobic digestion (ATAD) in this study. The dosing time of MgCl2 · 6H2O and NaH2PO4 · 2H2O will influence the removal efficiency of ammonium nitrogen greatly, and the time interval of 2nd, 7th, 12th day were chosen in ATAD process. The lowest NH4(+)-N concentration was found in the 2nd day dosing digester, and 38.37% of VS removal rate was obtained after 12 days digestion, which achieved stabilization 9 days earlier than the non-dosing digester. It revealed that removal of ammonium nitrogen could accelerate the sludge stabilization process. Meanwhile, 49.30% of VS removal rate was found in the 2nd day dosing digester in the 21st day, much higher than that in the non-dosing digester, the 7th day dosing digester, and the 12th day dosing digester, with the corresponding value of 38.37%, 38.38% and 37.04%, respectively.

Effects of organic loading rates on reactor performance and microbial community changes during thermophilic aerobic digestion process of high-strength food wastewater

To evaluate the applicability of single-stage thermophilic aerobic digestion (TAD) process treating high-strength food wastewater (FWW), TAD process was operated at four organic loading rates (OLRs) from 9.2 to 37.2 kg COD/m(3)d. The effects of OLRs on microbial community changes were also examined. The highest volumetric removal rate (13.3 kg COD/m(3)d) and the highest thermo-stable protease activity (0.95 unit/mL) were detected at OLR=18.6 kg COD/m(3)d. Denaturing gradient gel electrophoresis (DGGE) profiles and quantitative PCR (qPCR) results showed significant microbial community shifts in response to changes in OLR. In particular, DGGE and phylogenetic analysis demonstrate that the presence of Bacillus sp. (phylum of Firmicutes) was strongly correlated with efficient removal of organic particulates from high-strength food wastewater.

Effect of S-TE (solubilization by thermophilic enzyme) digestion conditions on hydrogen production from waste sludge

S-TE (solubilization by thermophilic enzyme) digestion was always used for waste sludge solubilization and reduction with the theory of lysis enzyme secreted by thermophilic bacteria. The effects of pretreated times, pH and sludge concentrations on S-TE digestion were investigated to determine the optimum S-TE conditions on hydrogen fermentation from waste sludge. The experimental results showed that the protein, carbohydrate and SCOD (soluble chemical oxygen demand) were released after S-TE pretreatment, and the optimal conditions on sludge solubilization were 12-16 h, pH 7-9 and 4.31 g/L TSS, respectively. The optimal S-TE conditions on hydrogen yields were 8h, pH 6 and 6.83 g/L TSS (total suspended solid), and the maximum hydrogen yields were 30.80 ml/g VSS (volatile suspended solid), 33.75 ml/g VSS and 42.90 ml H2/g VSS, respectively. It was concluded that the optimal S-TE digestion conditions on sludge solubilization were different from that on hydrogen production.

Induction of hexanol dehydrogenase in Geotrichum spp. by the addition of hexanol

Excessive hexanol content distorts the flavor of foods and is harmful to human health. Previously, two strains of fungi were found capable of producing hexanol-degrading enzymes. The current study identified these strains as Galactomyces geotrichum according to the gene sequence of the 26 S rDNA D1/D2 region (strain S12) and genus Geotrichum according to the gene sequence of ITS region (strain S13). Parallel analysis of extracellular and intracellular enzyme activities showed that the enzymes mainly accumulated intracellularly. Native polyacrylamide gel electrophoresis with reactive dyes showed the enzymes were alcohol dehydrogenases induced by the addition of hexanol. Hexanol was catalyzed into hexanoic acid and hexanal by strain S12 and into hexanoic acid by strain S13. The optimum conditions for the induction of enzymes were determined to be 6-9 h in the presence of 0.7 g/l hexanol. The identification of two strains capable of enzymatically degrading hexanol and optimum conditions for their induction will facilitate their use in industrial applications.

The one-stage autothermal thermophilic aerobic digestion for sewage sludge treatment: stabilization process and mechanism

Batch experiment was carried out in a simulated thermophilic aerobic digester to investigate the digestion process of one-stage autothermal thermophilic aerobic digester and to explore the sludge stabilization mechanism. Volatile solids removal was 38.4% at 408 h and 45.0% at 552 h. Chemical oxidation demand, total nitrogen, and ammonia nitrogen in supernatant increased rapidly up to 168 h, and all of them fluctuated moderately after 360 h. Volatile fatty acid (VFA) accumulated rapidly up to 24 to 168 h, then declined sharply, reaching a low concentration after 312 h. Propionic, iso-valeric, and iso-butyric acids, in addition to acetic acids, were also the major components of VFA. As the biochemical metabolic process was inhibited under oxygen-deficiency condition, the digestion system can produce acetic, propionic, butyric acids and other VFA constituents to meet the demand for NAD(+) and maximize ATP generation. The ORP affected the VFA production and depletion as well as sulfate levels.