Improving volatile fatty acids production by exploiting the residual substrates in post-fermented sludge: Protease catalysis of refractory protein

A novel approach to enhance methane production during anaerobic digestion of waste activated sludge by combined addition of trypsin, nano-zero-valent iron and activated carbon

A novel approach with a combination of trypsin, nano-zero-valent iron (NZVI) and activated carbon (AC) was conducted to promote the methane production of waste activated sludge (WAS) during the anaerobic digestion (AD) processes. Results showed that the combined addition of trypsin-NZVI-AC exhibited the synergistic effect during different AD stages. Trypsin mainly facilitated the hydrolysis process and the acetic acid conversion, while NZVI-AC enhanced the substrate metabolism and the electronic transfer to subsequently produce methane. A dose of 1000 mg/L trypsin was optimal to maximize this synergistic effect. Metagenomic analysis showed that trypsin-NZVI-AC addition effectively improved the relative abundance of acetyl-CoA carboxylase, and then strengthened both acetoclastic methanogenesis (M00357) and hydrogenotrophic methanogenesis (M00567). Hydrogenotrophic methanogens such as Methanobacterium, Methanoculleus, and Methanosarcina were greatly enriched with trypsin-NZVI-AC compared with trypsin or NZVI-AC addition. Moreover, electroactive bacteria G. sulfurreducens and G. metallireducens were also enriched by this method to conduct direct interspecies electron transfer among methanogens, leading to the better improvement of methane production. These findings supply a promising way to optimize the enzyme pretreatment technology and elevate the methanogenic efficiency of WAS.

Enhanced acidogenic fermentation from Al-rich waste activated sludge by combining lysozyme and sodium citrate pretreatment: Perspectives of Al stabilization and enzyme activity

The accumulation of poly aluminum chloride (PAC) in dewatered waste activated sludge (WAS) can cause severe Al pollution and significantly reduce the production of volatile fatty acids (VFAs) from anaerobic fermentation. Herein, the combination of lysozyme and sodium citrate pretreatment was applied to stabilize the aluminum and enhance the VFAs production via anaerobic fermentation. The complexation and stabilization of aluminum by the citrate was efficient, which is conducive to relieving the inhibition of aluminum on lysozymes and other extracellular hydrolases. Compared with the control group, the lysozyme, protease and α-glucosidase activities were obtained at 1.86, 1.72, and 1.15 times, respectively, following the pretreatment. 129.71 mg/g volatile suspended solids (VSS) of soluble proteins and 26.3 mg/g VSS of polysaccharides were obtained within 4 h, together with the degradation of 124 % more proteins and 75 % more polysaccharides within three days. This provided a sufficient number of substrates for VFA production. 588.4 mg COD/g VSS of total VFAs were obtained after the six-day anaerobic fermentation from Al-rich WAS following the combination of lysozyme and sodium citrate pretreatment, which was 7.3 times higher than that of the control group. This study presents a novel approach for enhancing VFA production in anaerobic fermentation as well as reducing risk of Al hazards from Al-rich WAS.

Long-term monitoring of a thermal hydrolysis-anaerobic co-digestion plant treating high-strength organic wastes: Process performance and microbial community dynamics

Two parallel anaerobic digesters (8500 m³ capacity each), combined with thermal hydrolysis (TH) pretreatment, co-digesting dewatered sewage sludge, dewatered human feces, and food wastewater were monitored over a 12-month period from start-up to explore the feasibility of field application of the combined process. The waste mixtures before and after pretreatment and the feed and digestate of each digester were taken semimonthly (i.e., 48 samples in total) for analysis of the feed characteristics, process parameters, and digester microbial community structure. The TH pretreatment proved effective in improving the bioavailability of the waste mixture. The solubilization efficiency tended to increase with the particulate organic fraction in the raw mixture. Although fluctuations in the feed characteristics and loading significantly influenced the process and microbial behaviors, the digesters maintained stable performance during the study period. Our results demonstrate that the TH-anaerobic digestion process can achieve an effective and robust treatment of the waste mixture.

Enhanced hydrogen production from food waste dark fermentation by potassium ferrate pretreatment

Hydrogen generation from food waste anaerobic dark fermentation is identified as a promising strategy for resource recovery. In this work, an innovative strategy of using potassium ferrate (PF), a strong oxidant, to promote anaerobic dark fermentation of food waste to produce hydrogen has been reported. The experimental results revealed that PF enhanced the hydrogen production from food waste, the maximal hydrogen yield was 173.5 mL/g, and the optimal PF dosage was 0.4 g/g total suspended solids. PF shortened the lag phase for hydrogen generation from 120 to 96 h. Mechanisms investigation revealed that PF accelerated the disintegration of organic compounds and increased the soluble organic matter in the liquid phase. The strong oxidation of PF inhibited the processes of hydrolysis, acidification, acetogenesis, homoacetogenesis, and methanogenesis by using synthetic wastewater in the fermentation process. The inhibition of PF on these processes was further verified by the enzyme activity analysis. Economic analysis indicated that 0.1 g/g PF was the optimal dosage. PF treatment is a promising strategy to enhance the production of hydrogen from food waste dark fermentation.

In-situ ammonia stripping with alkaline fermentation of waste activated sludge to improve short-chain fatty acids production and carbon source availability

Alkaline fermentation of waste activated sludge (WAS) to produce short-chain fatty acids (SCFAs) has been proved to be promising to develop internal carbon source for denitrifying processes in municipal wastewater treatment plants. However, a large amount of ammoniacal nitrogen also releases during fermentation, resulting in inhibition of acidogenic process and reduction of carbon source availability. Alkaline fermentation of WAS combined with in-situ ammonia stripping was proposed to improve SCFAs production and carbon source availability simultaneously. The results showed that a maximal SCFAs production of 308.7 ± 4.8 mg chemical oxygen demand/g volatile suspended solid was achieved under the pH = 10 + In-situ ammonia stripping, which was 21.7% and 141.5% higher than that of the pH = 10 and the control, respectively. Most of the ammoniacal nitrogen was stripped out, and the WAS-derived SCFAs availability as carbon source increased by 103.2%. These findings indicate in-situ ammonia stripping could make alkaline fermentation of WAS more practical.

Advances in enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge

Low acid production and acid-forming process instability are becoming the major issues to limit the popularization of anaerobic fermentation to produce volatile fatty acid. Considerable research efforts have been made to address these problems, from studying the microorganisms that are primarily responsible for or detrimental to this process, to determining their biochemical pathways and developing mathematical models that facilitate better prediction of process performance to identify the mechanism and optimization of process control. A limited understanding of the complex microbiology and biochemistry of anaerobic fermentation is the primary cause of acid production upset or failure. This review critically assesses the recent advances in enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge from micro to macro scale, particularly relating to the microbiology, biochemistry, impact factors, and enhancement methods. Previous results suggest that further studies are necessary to substantially promote the efficiency and stability of acid production. One of the promising directions appears to be integrating the existing and growing pretreatment technologies and fermentation processes to enhance metabolic pathways of acetogens but inhibit activities of methanogens, which this study hopes to partially achieve.

Three-ways changed in headspace air on anaerobic fermentation

Different headspace condition has a great influence on fermentation process. In this study, whey protein was used as substrate, and the headspace air was changed in three different ways (H₂ mixed N₂, H₂ mixed CO₂, N₂ with different sparging rates) to explore the effects of these three methods on products. The result showed that H₂ mixed with CO₂ is more conducive to acid production. Homoacetogenesis played a central role in fermentation process. There is a turning point in the role of hydrogen and Homoacetogenesis, which is when the partial pressure of hydrogen is 0.268 atm. In the first two conditions, the acid concentration increased with the increase of hydrogen percentage. Nitrogen sparging way is adverse to acid production, but conducive to gas production.

A metabolic study to decipher amino acid catabolism-directed biofuel synthesis in Acetoanaerobium sticklandii DSM 519

Acetoanaerobium sticklandii DSM 519 is a hyper-ammonia-producing anaerobe. It has the ability to produce organic solvents and acids from protein catabolism through Stickland reactions and specialized pathways. Nevertheless, its protein catabolism-directed biofuel production has not yet been understood. The present study aimed to decipher such growth-associated metabolic potential of this organism at different growth phases using metabolic profiling. A seed culture of this organism was grown separately in metabolic assay media supplemented with gelatin and or a mixture of amino acids. The extracellular metabolites produced by this organism were qualitatively analyzed by gas chromatography-mass spectrometry platform. The residual amino acids after protein degradation and amino acids assimilation were identified and quantitatively measured by high-performance liquid chromatography (HPLC). Organic solvents and acids produced by this organism were detected and the quantity of them determined with HPLC. Metabolic profiling data confirmed the presence of amino acid catabolic products including tyramine, cadaverine, methylamine, and putrescine in fermented broth. It also found products including short-chain fatty acids and organic solvents of the Stickland reactions. It reported that amino acids were more appropriate for its growth yield compared to gelatin. Results of quantitative analysis of amino acids indicated that many amino acids either from gelatin or amino acid mixture were catabolised at a log-growth phase. Glycine and proline were poorly consumed in all growth phases. This study revealed that apart from Stickland reactions, a specialized system was established in A. sticklandii for protein catabolism-directed biofuel production. Acetone-butanol-ethanol (ABE), acetic acid, and butyric acid were the most important biofuel components produced by this organism. The production of these components was achieved much more on gelatin than amino acids. Thus, A. sticklandii is suggested herein as a potential organism to produce butyric acid along with ABE from protein-based wastes (gelatin) in bio-energy sectors.

Enhancement of Volatile Fatty Acids Production from Food Waste by Mature Compost Addition

Food waste (FW) collected from a university canteen was treated in acidogenic fermenters to produce volatile fatty acids (VFA) under biological pretreatment with mature compost. Batch assays working at pH 6 revealed an increment of 9.0%, 7.9%, and 4.1% (on COD basis) of VFA concentration when adding 2.5%, 3.5%, and 4.5% w/w of mature compost, respectively, even though the volatile solids (VS) concentration of food waste was lower in the tests with increasing doses of mature compost. For batch tests at pH 7, this VFA generation improvement was lower, even though enhanced COD solubilization was recorded. Operating in semi-continuous conditions at 35 °C, pH of 6, and hydraulic retention time (HRT) of 3.5 days, the addition of 2.5% w/w of mature compost led to a VFA concentration up to 51.2 ± 12.3% more (on VS basis) when compared to a reference reactor without compost addition. Moreover, the percentage of butyric acid on VS basis in the fermentation broth working at a pH of 6 increased from up to 12.2 ± 1.9% (0% compost addition) to up to 23.5 ± 2.7% (2.5% compost addition). The VFA production was not improved when a higher percentage of mature compost was used (3.5% instead of 2.5% w/w), and it slightly decreased when mature compost addition was lowered to 1.5% w/w. When working at a pH of 7 in the semi-continuous fermenters with the addition of 2.5% w/w mature compost at an HRT of 3.5 days, an improvement of 79% and 104% of the VFA concentration (on VS basis) were recorded as compared to fermenters working at a pH of 6 with 2.5% and 0% w/w of mature compost addition, respectively. At a pH of 7, higher production of propionic and valeric acids was found with respect to the reactor working at a pH of 6. The effect of pH on VFA generation was estimated to have greater contribution than that of only biological pretreatment using mature compost. At a pH of 7, the VFA yield was higher for the fermenter working with 2.5% w/w mature compost but at a pH of 7 and HRT of 5 days, the effect of mature compost on VFA production improvement was lower than that obtained at a pH of 6. Moreover, higher solubilization in terms of soluble chemical oxygen demand and total ammonium was detected when biological pretreatment using mature compost was applied at both a pH of 6 and a pH of 7, which indicates enhanced hydrolysis in both conditions.

Upgrading volatile fatty acids production through anaerobic co-fermentation of mushroom residue and sewage sludge: Performance evaluation and kinetic analysis

Due to complex inherent structure of lignocellulosic biomass, inefficient hydrolysis and acidification limits fermentative volatile fatty acids (VFA) production of mushroom residues. Meanwhile, the mushroom residues present insufficient nutrient with a high C/N ratio. To solve this issue, anaerobic co-fermentation of cellulose-rich Oyster champost and sewage sludge was tested to enhance the VFA production, and the effect of proportion of mixed substrate was investigated in this study. The results indicated that the sewage sludge yielded higher VFAs than the Oyster champost in single-substrate fermentative system. The maximal VFA yield of 595 mgCOD/gVS_added was achieved when the proportion of sewage sludge increased to 50% in the mixed substrate. In the co-fermentation system, the optimal C/N ratio and features of mixed substrate contributed to the enhancement in hydrolysis and acidification in terms of organic solubilization and VFA production, respectively. But the co-fermentation could not increase VFA/SCOD ratio, probably due to the existence of refractory products such as humic-like and protein-like materials. Besides, this co-fermentation system had strong buffer capacity and it was not necessary to dose chemicals to control the system pH for stable VFA production. Acetate was the dominant VFA product in co-fermentation systems. A modified-Logistic model fitted co-fermentation of sludge and Oyster champost well, and presented a faster rate and higher efficiency of VFA production.

Effects of substrate concentration, hydraulic retention time and headspace pressure on acid production of protein by anaerobic fermentation

Protein is a potential resource for fermentation, which is one of the rate limiting factors of fermentation. This study investigated some of the regularities of protein on dark fermentation for acid production, whey (82% protein) liquid was used to study the effect of varying headspace pressure (HP; 0.02-0.10 MPa), protein concentration (6.25-18.75 g/L) and hydraulic retention time (HRT; 24-48 h) on VFA production. The main fermentation product is acetate and butyrate. Low HP, low substrate concentration and short HRT were favorable for the generation of acid. When the protein concentration was 12.5 g/L and HRT was 24 h, the VFA concentration was up to 8924.190 mg/L at 0.02 MPa, compared to a 61.55% increase under 0.10 MPa. While the HRT was 48 h or the protein concentration was 18.75 g/L, VFA concentration was low and no more n-butyric acid or valeric acid. After variance analysis, we observed a significant effect of the three factors on acid.

High rejection rate of polysaccharides by microfiltration benefits Christensenella minuta and acetic acid production in an anaerobic membrane bioreactor for sludge fermentation

Substrates, the microbial community, and volatile fatty acids (VFAs) are interacting factors in anaerobic fermentation. In this paper, a microfiltration membrane was installed in a conventional reactor for sludge anaerobic fermentation, and changes in the VFA composition and the microbial community structure after the application were monitored. The membrane rejected much more polysaccharide than protein. VFA production, mainly acetic acid, was greatly enhanced by the membrane separation. The glycolytic species Christensenella minuta was significantly enriched. The explanation was inferred to be the higher retention rate of polysaccharide compared with proteins by the microfiltration. However, surprisingly, another polysaccharide using genus, Parabacteroides, did not benefit from the application of the membrane separation, suggesting more complicated mechanisms might be involved.

A viable approach for commercial VFAs production from sludge: Liquid fermentation in anaerobic dynamic membrane reactor

A novel strategy of liquid fermentation using anaerobic dynamic membrane reactor (AnDMBR) was proposed to enhance volatile fatty acids (VFAs) production from sewage sludge. Results indicated that liquid sludge fermentation in AnDMBR had the potential in commercial VFAs production. VFAs productivity and concentration as well as substrate conversion rate could reach as high as 7.8 kg VFA-COD/m³ d, 60 g/L and 0.38 kg VFA-COD/kg VS, respectively. Moreover, dynamic membrane was stably operated for approximately 70 days. During the operational period, membrane flux was increased from 6.25 to 25 L/m² d and only once online membrane cleaning was implemented. Results of microbial analyses showed bacterial richness and evenness in AnDMBR were increased by membrane separation and organic loading rate (OLR) increase, but reduced by excessive OLR, which should led the variations in the performances of AnDMBR. Furthermore, the necessity of liquid sludge fermentation for VFAs production was further confirmed by economic assessment and the bioavailability analysis of the residual solids in pretreated sludge. The residual solid was proved to be not conducive to enhance VFAs yield. Conversely, the energy consumption for VFAs production could be reduced from over 100 to below 20 kwh/kg VFAs by avoiding the "useless" residual solids entering into fermenters.

Full-scale production of VFAs from sewage sludge by anaerobic alkaline fermentation to improve biological nutrients removal in domestic wastewater

A full-scale project of thermal-alkaline pretreatment and alkaline fermentation of sewage sludge was built to produce volatile fatty acids (VFAs) which was then used as external carbon source for improving biological nitrogen and phosphorus removals (BNPR) in wastewater plant. Results showed this project had efficient and stable performances in VFA production, sludge reduce and BNPR. Hydrolysis rate in pretreatment, VFAs yield in fermentation and total VS reduction reached 68.7%, 261.32 mg COD/g VSS and 54.19%, respectively. Moreover, fermentation liquid with VFA presented similar efficiency as acetic acid in enhancing BNPR, obtaining removal efficiencies of nitrogen and phosphorus up to 72.39% and 89.65%, respectively. Finally, the project also presented greater economic advantage than traditional processes, and the net profits for VFAs and biogas productions are 9.12 and 3.71 USD/m³ sludge, respectively. Long-term operation indicated that anaerobic alkaline fermentation for VFAs production is technically and economically feasible for sludge carbon recovery.

Deep exploitation of refractory organics in anaerobic dynamic membrane bioreactor for volatile fatty acids production from sludge fermentation: Performance and effect of protease catalysis

Volatile fatty acids (VFAs) production from waste activated sludge fermentation could be improved in anaerobic dynamic membrane bioreactor (ADMBR) by retaining residual organics within the reactor and prolonging their reaction time. However, the accumulation of refractory organics made it operate unstably. Therefore, protease catalysis was adopted to deeply exploit those refractory organics in sludge. By combining with dynamic membrane retention, protease catalysis indeed presented a good performance. VFAs yield was further enhanced by over 40% in ADMBR. Membrane fouling was slightly relieved due to protein and polysaccharide degradations in the sludge of dynamic membrane. It was also interestingly found that not only protease activity of sludge was improved from 5 to 21 U/ml, but also β-GLC activity was enhanced from 13 to 20 μmoL/L/h. Microbial community analysis showed protease addition could reduce bacterial richness and evenness in sludge, and accelerate the growth of polysaccharides-hydrolyzing bacteria, as well as inhibit some proteolytic bacteria.

Amino acid catabolism-directed biofuel production in Clostridium sticklandii: An insight into model-driven systems engineering

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Model-driven systems engineering has been more fascinating process for microbial biofuel production.

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Clostridium sticklandii is a potential strain for the solventogenesis and acidogenesis.

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The present review provides an insight for the protein catabolism-directed biofuel production.

Model-driven systems engineering has been more fascinating process for the microbial production of biofuel and bio-refineries in chemical and pharmaceutical industries. Genome-scale modeling and simulations have been guided for metabolic engineering of Clostridium species for the production of organic solvents and organic acids. Among them, Clostridium sticklandii is one of the potential organisms to be exploited as a microbial cell factory for biofuel production. It is a hyper-ammonia producing bacterium and is able to catabolize amino acids as important carbon and energy sources via Stickland reactions and the development of the specific pathways. Current genomic and metabolic aspects of this bacterium are comprehensively reviewed herein, which provided information for learning about protein catabolism-directed biofuel production. It has a metabolic potential to drive energy and direct solventogenesis as well as acidogenesis from protein catabolism. It produces by-products such as ethanol, acetate, n-butanol, n-butyrate and hydrogen from amino acid catabolism. Model-driven systems engineering of this organism would improve the performance of the industrial sectors and enhance the industrial economy by using protein-based waste in environment-friendly ways.

Acidogenic fermentation characteristics of different types of protein-rich substrates in food waste to produce volatile fatty acids

In this study, tofu and egg white, representing typical protein-rich substrates in food waste based on vegetable and animal protein, respectively, were investigated for producing volatile fatty acids (VFAs) by acidogenic fermentation. VFA production, composition, conversion pathways and microbial communities in acidogenesis from tofu and egg white with and without hydrothermal (HT) pretreatment were compared. The results showed HT pretreatment could improve the VFA production of tofu but not for egg white. The optimum VFA yields were 0.46g/gVS (tofu with HT) and 0.26g/gVS (egg white without HT), respectively. Tofu could directly produce VFAs through the Stickland reaction, while egg white was converted to lactate and VFAs simultaneously. About 30-40% of total protein remained in all groups after fermentation. Up to 50% of the unconverted soluble protein in the HT groups was protease. More lactate-producing bacteria, mainly Leuconostoc and Lactobacillus, were present during egg white fermentation.

Improving volatile fatty acid yield from sludge anaerobic fermentation through self-forming dynamic membrane separation

Self-forming dynamic membrane (SFDM) separation was applied to the conventional sludge fermenter for improving VFA yields. Results indicated SFDM presented good performance in transferring products, retaining substrates, and enriching useful bacteria. The retention ratios of suspended solids, soluble COD, proteins, and polysaccharides reached 99%, 30%, 70%, and 40%, respectively, and more than 90% of the VFAs and ammonia could be transferred in a timely manner. The structure of the microbial community was optimized, which led to enhanced releases of hydrolytic enzymes and accelerated enrichments of functional bacteria. Protease and β-glucosidase activities increased from 1.0 to 5.0U/mL and 15.0 to 23.0μmol/L·h, respectively. VFA yield and sludge conversion ratio increased by 233.3% and 227.9%, respectively. Moreover, SFDM had good operation stability, including a short formation time, a long operation period, and a low transmembrane pressure. These results show VFA yield from sludge fermentation can be greatly improved by SFDM separation.

The role of methanogens in acetic acid production under different salinity conditions

In this study, a fed-batch acidogenic reactor was operated at a 3 d hydraulic retention time (HRT) and fed with alkaline pre-treated sludge to investigate salinity effects on methanogens' abundance, activities and their consumption of produced acetic acid (HAc) and total volatile fatty acids (VFAs). The salinity concentration was increased step-wise by adding sodium chloride. At 3‰ (parts per thousand) salinity, the average produced volatile fatty acids (VFAs) concentration was 2410.16 ± 637.62 mg COD L(-1) and 2.70 ± 0.36 L methane was produced daily in the acidogenic reactor. Further batch tests indicated methanogens showed a HAc degradation rate of 3.81 mg COD g(-1) VSS h(-1) at initial HAc concentration of 1150 mg COD L(-1), and showed tolerance up to 16‰ salinity (3.76 g Na(+) L(-1)) as indicated by a constant HAc degradation rate. The microbiological study indicated this can be related to the predominance of acetate-utilizing Methanosarcinaceae and Methanomicrobiales in the reactor. However, with salinity increased to 20‰ and 40‰, increases in VFAs and HAc production and decreases in methane production, methanogens population, acidogenic bacteria population and acidification extent were observed. This study demonstrated presence of acetate-utilizing methanogens in an acidogenic reactor and their high tolerance to salinity, as well as their negative impacts on net VFAs production. The results would suggest the presence of methanogens in the acidogenic reactor should not be ignored and the recovery of methane from the acidogenic reactor needs to be considered to avoid carbon loss.

Comparing the value of bioproducts from different stages of anaerobic membrane bioreactors

The anaerobic digestion process in anaerobic membrane bioreactors is an effective way for waste management, energy sustainability and pollution control in the environment. This digestion process basically involves the production of volatile fatty acids and biohydrogen as intermediate products and methane as a final product. This paper compares the value of bioproducts from different stages of anaerobic membrane bioreactors through a thorough assessment. The value was assessed in terms of technical feasibility, economic assessment, environmental impact and impact on society. Even though the current research objective is more inclined to optimize the production of methane, the intermediate products could also be considered as economically attractive and environment friendly options. Hence, this is the first review study to correlate the idea into an anaerobic membrane bioreactor which is expected to guide future research pathways regarding anaerobic process and its bioproducts.