The effects of pH on the production of volatile fatty acids and microbial dynamics in long-term reactor operation

Characteristics of volatile fatty acids production and microbial succession under acid fermentation via anaerobic membrane bioreactor treating kitchen waste slurry

In this study, two anaerobic membrane bioreactors (AnMBRs) were proceeded to produce volatile fatty acids (VFAs) from kitchen waste slurry under acidic conditions of pH 5 and 6. Higher fermentation potential and VFA quality were obtained at pH 6, with VFA production, yield and CODVFAs/CODeffluent ratio of 42.0 g/L, 0.4 g/g-CODinfluent and 80 % respectively. The alkali dosages during the stable operation at pH 5 and 6 were 0.03 g-NaOH/g-VFAs and 0.08 g-NaOH/g-VFAs respectively, far lower than that at pH 9 (0.4 g-NaOH/g-VFAs). The microbial community presented marked differences between pH 5 and 6. In addition, chemical cleaning and sludge discharge alleviated membrane fouling and ensured the stable membrane operation at pH 6, while these control strategies had no obvious improvement for fouling at pH 5 due to higher H+ toxicity. In short, pH 6 was more suitable for VFA production than pH 5 with AnMBR treating kitchen waste slurry.

Soil pH and total phosphorus regulate bacterial community assembly in slope restoration areas of the Tibetan Plateau's metal mining areas

Microbial community development is a crucial aspect of soil restoration. The employment of frame beams in conjunction with external soil has demonstrated efficacy in the rehabilitation of degraded roadside ecosystems within mining regions. Nonetheless, the effects of frame beams on the composition and stability of soil bacterial communities remain inadequately comprehended. We conducted a one-time soil sampling on a three-year restored slope in a large-scale metal mining area on the Tibetan Plateau, providing a snapshot of the current conditions and evaluating the restoration progress. Frame beams with external soil covers were applied at three different altitudes: A1 (4800-5000 m), A2 (4500-4700 m), and A3 (4200-4400 m). Restoration significantly altered bacterial community composition compared with controls. Proteobacteria had a higher relative abundance in the restoration area (average: 31.16 %), whereas Acidobacteriota were more abundant in the control area (average: 24.68 %). In the restoration area, soil bacterial α-diversity increased as elevation decreased, with the Shannon index rising from 5.34 (A1) to 5.82 (A3), suggesting that bacterial communities at higher altitudes are more sensitive to environmental conditions. Species turnover was the primary driving factor of β-diversity, accounting for 96.26 % under A1, 94.71 % under A2, and 91.94 % under A3, respectively. The nearest taxon index of bacterial communities shifted from negative to positive along the elevation gradient (-0.25 to 1.14), indicating an increasing trend toward community clustering. Within the bacterial co-occurrence network, soil pH and total phosphorus contribute significantly to network strength, closeness, and betweenness. Concluding, soil pH and total phosphorus were identified as key factors shaping bacterial diversity and assembly mechanisms. Our research contributes to the development of effective soil restoration strategies for alpine mining regions, providing insights into microbial community assembly and stability mechanisms.

Insights into nutrients recovery from food waste liquid Digestate: A critical review and systematic analysis

This review paper presents a critical analysis of global research on the liquid fraction of food waste (FW) digestate. The study found that research on FW liquid fraction management accounted for 43% of the literature, followed by treatment methods (26%) and physical-chemical characterization (22%). By 2023, China led in scientific production on FW liquid fraction, contributing 46%, followed by Poland with 10% and the USA with 8%. The review emphasizes current technologies for nutrient recovery from the liquid fraction, as well as practical implications and limitations, identifying gaps in the literature. The most used methods for nutrient recovery were biofertilizer production from microalgae and membrane technologies. However, there is a need for further research on nutrient value, circular economy integration, the impact of food additives, ecological problems associated with FW decomposition, pathogen breeding, harmonized legislation to support recovered fertilizer commercialization and innovative nutrient recovery technologies. This approach provides valuable insights for stakeholders, enabling the creation of effective strategies that promote sustainable agricultural practices and circular economy initiatives through nutrient recovery from FW digestate.

A review on waste activated sludge pretreatment for improved volatile fatty acids production and their upcycling into polyhydroxyalkanoates

Waste activated sludge (WAS), a byproduct of wastewater treatment (WWTPs) facilities is challenging to manage because of its high organic content. Most of WAS is managed via anaerobic digestion (AD) to produce biogas, which is not deemed economically viable. The AD of WAS into volatile fatty acids (VFA) and their subsequent upcycling into polyhydroxyalkanoates (PHA) is gaining popularity due to their high value and uses. However, the fundamental issue with WAS is its low solubility, and pretreatment is required to increase it. Pretreatment disintegrates sludge floc and enhances its solubility, supports acetogens, and inhibits methanogens, leading to increased VFA synthesis in the AD process. The key factors influencing VFA yield include the size of the sludge granules, the mixing rate, and the presence of resistant organic components. Fermented broth containing VFA from AD can be utilized directly as a feedstock for microbial fermentation to produce PHA using both pure as well as mixed cultures. Utilisation of mixed cultures is useful since they are robust, able to consume a wide range of substrates, and do not require sterility. In addition, the VFA, which is made up of various organic acids, impacts the structure, productivity, characteristics, and type of PHA produced by microbial communities. Considering the importance of WAS management through VFA production and its integration with PHA production process this review article discusses the WAS pretreatment strategies, various factors that influence the AD process, trends in VFA to PHA production technologies with challenges, and possible solutions for integrated process development.

Beyond guaiacol and halophenols: Unravelling isobutyric and isovaleric acids as new culprits in off-flavour spoilage by Alicyclobacillus spp

Industries that produce or use fruit-based products have faced several spoilage events, resulting in economic losses caused by product recalls and loss of consumer confidence. Some of these events correlate to the presence of Alicyclobacillus (ACB) in food products since they can produce off-flavours and odours in the final products. Guaiacol (2-methoxyphenol) and halophenols (2,6-dichlorophenol and 2,6-dibromophenol) have been widely explored as the primary culprits of off-flavour spoilage by ACB. However, different compounds might be correlated with these spoilage events. In this work, volatile metabolites produced by distinct ACB species (Alicyclobacillus acidoterrestris, Alicyclobacillus acidocaldarius, and Alicyclobacillus cycloheptanicus) in laboratory medium and fruit juices were identified by HS-SPME-GC-MS and investigated as potential spoilage-related compounds. Isobutyric acid (2-methylpropanoic acid) and isovaleric acid (3-methylbutanoic acid) were revealed to be produced by all three ACB species at concentrations that surpass the odour threshold. These cheesy, sweaty, and sour compounds were responsible for dissonant odours in peach, orange, and tomato juice, harming fruit-based products' quality. More importantly, this work suggests that ACB species previously identified as non-spoilage bacteria, based on a lack of ability to produce guaiacol and halophenols, can also threaten the juice, beverage, and dairy industries. As such, identification methods currently used in industries for ACB control in final products should be revised.

Simultaneous removal of nitrate, manganese, zinc, and bisphenol a by a biofilm reactor with β-CD modified corn stover biochar and PU sponges: Performance and microbial community response

In the present study, a biofilm reactor with manganese (Mn) redox cycling was established to remove nitrate (NO3--N), bisphenol A (BPA), zinc (Zn(II)), and Mn(II) using β-cyclodextrin (β-CD) modified corn stover biochar (BC) and polyurethane sponges loaded with Cupriavidus sp. HY129 and Pantoea sp. MFG10. At C/N = 2.0, HRT = 6 h, Mn(II) = 10.0 mg L-1, and BPA and Zn(II) concentrations = 1.0 mg L-1, the removal efficiencies of NO3--N, Zn(II), BPA, and Mn(II) were 81.5%, 86.5%, 87.9%, and 75.5%, respectively. The outcomes demonstrated the success that the addition of β-CD could accelerate electron transfer activity and the denitrification process. The remediation of BPA and Zn(II) was mainly through the adsorption of bioprecipitation generated by reactor operation. The bioreactor could preserve the stability of the biological community and the expression of pertinent functional genes under the coercion of BPA and Zn(II).

Critical impact of pressure regulation on carbon dioxide biosynthesis

Carbon dioxide (CO2) biosynthesis is a promising alternative to traditional chemical synthesis. However, its application in engineering is hampered by poor gas mass transfer rates. Pressurization is an effective method to enhance mass transfer and increase synthesis yield, although the underlying mechanisms remain unclear. This review examines the effects of high pressure on CO2 biosynthesis, elucidating the mechanisms behind yield enhancement from three perspectives: microbial physiological traits, gas mass transfer and synthetic pathways. The critical role of pressurization in improving microbial activity and gas transfer efficiency is emphasized, with particular attention to maintaining pressure within microbial tolerance limits to maximize yield without compromising cell structure integrity.

Combination of magnetite and sodium percarbonate to enhance acetate-enriched short-chain fatty acids production during sludge anaerobic fermentation

Large amounts of waste activated sludge are generated daily worldwide, posing significant environmental challenges. Anaerobic fermentation is a promising method for sludge disposal, but it has two technical bottlenecks: the availability of short-chain fatty acids (SCFAs)-producing substrates and SCFAs consumption by methanogenesis. This study proposes a pretreatment strategy combining sodium percarbonate (SPC) and magnetite (Fe3O4) to address these issues. Under optimized conditions (20 mg Fe3O4/g TSS and 15 mg SPC/g TSS), SCFAs production increased to 3244.10 ± 216.31 mg COD/L, about 3.06 times the control (1057.29 ± 35.06 mg COD/L) and surpassing reported treatments. The combined pretreatment enhanced the disruption of extracellular polymeric substances, increased the release of biodegradable matters, improved acidogenesis enzyme activities, and inhibited methanogenesis. Additionally, it increased NH4+-N release in favor of the recovery of phosphorus from sludge residual. This study demonstrates an efficient pretreatment for high SCFAs production and resource recovery from WAS.

The effects of headspace volume reactor on the microbial community structure during fermentation processes for volatile fatty acid production

The transition from traditional wastewater treatment plants to biorefineries represents an environmentally and economically sustainable approach to extracting valuable compounds from waste. Sewage sludge produced from wastewater treatment is incinerated or disposed of in specific landfills. Repurposing this waste material to recover valuable resources could help lower disposal costs and reduce environmental impact by producing other beneficial polymers. Microorganisms present in the sewage sludge can ferment organic pollutants, producing volatile fatty acids (VFA), precursors for biopolymers that could be used as an alternative to petroleum-derived plastics. To boost VFA production during sewage sludge fermentation, it is necessary to understand the operating microbial community and its metabolic capacities in anaerobic conditions. This study presents the influence of the headspace volume on the microbial community and the VFA production to define the best operational parameters in a 225 L pilot plant fermenter. The wasted sewage sludge was withdrawn from an oxic-settling-anaerobic plant that collected wastewater from the canteen and dormitory of the UNIPA Campus (Palermo University, Italy) and incubated using a 40% and a 60% headspace volume. The microbial community was analysed before and after the fermentation process through metataxonomic analysis, and VFA yields were determined by gas chromatography analysis. Our results showed that the 40% headspace volume induced a tenfold higher VFA production than the 60% headspace volume, modulating the microbial community's efforts to establish a VFA-producing factory. Notably, at 40% headspace, the relative abundance of bacteria, like Proteobacteria, Firmicutes, Actinobacteria, and Chloroflexi, significantly increased, as well as the relative abundance of Bacteroidetes and Verrucomicrobia decreased during the fermentation process. This result is consistent with the selection of efficient VFA-producing bacteria that lead to increased VFA yields that are not obtained at 60% headspace. Thus, reducing headspace is a promising strategy that can be implemented, even in full-scale plants, to optimise the wastewater reuse process and maximise VFA production to produce bioplastics, like polyhydroxyalkanoate, for the transition from linear wastewater treatment plants to circular biorefineries.

Methods for studying microbial acid stress responses: from molecules to populations

The study of how micro-organisms detect and respond to different stresses has a long history of producing fundamental biological insights while being simultaneously of significance in many applied microbiological fields including infection, food and drink manufacture, and industrial and environmental biotechnology. This is well-illustrated by the large body of work on acid stress. Numerous different methods have been used to understand the impacts of low pH on growth and survival of micro-organisms, ranging from studies of single cells to large and heterogeneous populations, from the molecular or biophysical to the computational, and from well-understood model organisms to poorly defined and complex microbial consortia. Much is to be gained from an increased general awareness of these methods, and so the present review looks at examples of the different methods that have been used to study acid resistance, acid tolerance, and acid stress responses, and the insights they can lead to, as well as some of the problems involved in using them. We hope this will be of interest both within and well beyond the acid stress research community.

Bacterial networks and enzyme genes in bacterial floccules from hydrolysis and aeration reactors in a dairy wastewater treatment system

The dairy industry generates substantial wastewater, which is commonly treated using integrated anaerobic hydrolysis and aerated biofilm reactors. However, the bacterial composition and functional differences within the generated floccules remain unclear. In this study, we employed 16S rRNA and metagenomic sequencing to compare bacterial communities and enzyme gene profiles between suspended floccules from the hydrolysis ponds and the aeration ponds. Results revealed that the bacterial phyla Firmicutes, Proteobacteria, and Bacteroidetes dominated the wastewater treatment system and the relative abundance of these bacterial phyla varied in each pond. Additionally, the aeration ponds exhibited higher bacterial operational taxonomic units and enzyme gene abundance. Network analysis demonstrated a more complex bacterial network structure in the hydrolysis ponds compared to the aeration ponds. Furthermore, enzyme gene abundance revealed higher metabolic enzyme genes in the hydrolysis ponds, while signal transduction enzyme genes were more abundant in the aeration ponds. Notably, the top 10 bacterial genera, primarily Hydromonas in the hydrolysis ponds and Ferruginibacter in the aeration ponds, exhibited distinct contributions to signal transduction enzyme genes. Hydromonas dominated the metabolic enzyme genes in both ponds. These findings provide crucial insights for optimizing dairy wastewater treatment technologies.

Biobased short chain fatty acid production - Exploring microbial community dynamics and metabolic networks through kinetic and microbial modeling approaches

In recent years, there has been growing interest in harnessing anaerobic digestion technology for resource recovery from waste streams. This approach has evolved beyond its traditional role in energy generation to encompass the production of valuable carboxylic acids, especially volatile fatty acids (VFAs) like acetic acid, propionic acid, and butyric acid. VFAs hold great potential for various industries and biobased applications due to their versatile properties. Despite increasing global demand, over 90% of VFAs are currently produced synthetically from petrochemicals. Realizing the potential of large-scale biobased VFA production from waste streams offers significant eco-friendly opportunities but comes with several key challenges. These include low VFA production yields, unstable acid compositions, complex and expensive purification methods, and post-processing needs. Among these, production yield and acid composition stand out as the most critical obstacles impacting economic viability and competitiveness. This paper seeks to offer a comprehensive view of combining complementary modeling approaches, including kinetic and microbial modeling, to understand the workings of microbial communities and metabolic pathways in VFA production, enhance production efficiency, and regulate acid profiles through the integration of omics and bioreactor data.

Integrated Biorefinery for a Next-Generation Methanization Process Focusing on Volatile Fatty Acid Valorization: A Critical Review

This review addresses the critical issue of a rapidly increasing worldwide waste stream and the need for sustainable management. The paper proposes an integrated transformation toward a next-generation methanization process, which leads not only to treating waste but also to converting it into higher value compounds and greener energy. Although the current and commonly used anaerobic digestion process is useful for biogas production, it presents limitations of resource exploitation and some negative environmental impacts. Focusing on the acidogenic stage in waste stream processing, the paper discusses the recent strategies to enhance the recovery of volatile fatty acids (VFAs). These acids serve as precursors for synthesizing a variety of biochemicals and biofuels, offering higher value products than solely energy recovery and soil fertilizers. Additionally, the importance of recycling the fermentation residues back into the biorefinery process is highlighted. This recycling not only generates additional VFAs but also contributes to generating clean energy, thereby enhancing the overall sustainability and efficiency of the waste management system. Moreover, the review discusses the necessity to integrate life cycle assessment (LCA) and techno-economic analysis (TEA) to evaluate the environmental impacts, sustainability, and processing costs of the proposed biorefinery.

Anaerobic acidification membrane bioreactor operating at acidic condition for treating concentrated municipal wastewater: Performance and implication

To address the low-carbon treatment requirements for municipal wastewater, a novel anaerobic acidification membrane bioreactor (AAMBR) was developed for recovering organic matter in terms of volatile fatty acids (VFAs). While the AAMBR successfully generated VFAs from municipal wastewater through forward osmosis (FO) membrane concentration, its operation was limited to a single pH value of 10.0. Here, performance of the AAMBR operating at acidic condition was evaluated and compared with that at alkaline condition. The findings revealed that the AAMBR with pH 5.0 efficiently transformed organic matter into acetic acid, propionic acid, and butyric acid, resulting in a VFAs yield of 0.48 g/g-CODfeed. In comparison with the AAMBR at pH 10.0, this study achieved a similar VFAs yield, a lower fouling tendency, a lower loss of nutrients and a lower controlling cost. In conclusion, this study demonstrated that a pH of 5.0 is optimal for the AAMBR treating municipal wastewater.

Exploitation of microbial activities at low pH to enhance planetary health

Awareness is growing that human health cannot be considered in isolation but is inextricably woven with the health of the environment in which we live. It is, however, under-recognized that the sustainability of human activities strongly relies on preserving the equilibrium of the microbial communities living in/on/around us. Microbial metabolic activities are instrumental for production, functionalization, processing, and preservation of food. For circular economy, microbial metabolism would be exploited to produce building blocks for the chemical industry, to achieve effective crop protection, agri-food waste revalorization, or biofuel production, as well as in bioremediation and bioaugmentation of contaminated areas. Low pH is undoubtedly a key physical-chemical parameter that needs to be considered for exploiting the powerful microbial metabolic arsenal. Deviation from optimal pH conditions has profound effects on shaping the microbial communities responsible for carrying out essential processes. Furthermore, novel strategies to combat contaminations and infections by pathogens rely on microbial-derived acidic molecules that suppress/inhibit their growth. Herein, we present the state-of-the-art of the knowledge on the impact of acidic pH in many applied areas and how this knowledge can guide us to use the immense arsenal of microbial metabolic activities for their more impactful exploitation in a Planetary Health perspective.

Tracking the extracellular and intracellular antibiotic resistance genes across whole year in wastewater of intensive dairy farm

Monitoring the annual variation of antibiotic resistance genes (ARGs) in livestock wastewater is important for determining the high-risk period of transfer and spread of animal-derived antibiotic resistance into the environment. However, the knowledge regarding the variation patterns of ARGs, especially intracellular ARGs (iARGs) and extracellular ARGs (eARGs), over time in livestock wastewater is still unclear. Herein, we conducted a year-round study to trace the profiles of ARGs at a Chinese-intensive dairy farm, focusing on the shifts observed in different months. The results showed significant differences in the composition and variation between iARGs and eARGs. Tetracycline, sulfonamide, and macrolide resistance genes were the major types of iARGs, while cfr was the major type of eARG. The environmental adaptations of the host bacteria determine whether ARGs appear as intracellular or extracellular forms. The total abundance of ARGs was higher from April to September, which can be attributed to the favorable climatic conditions for bacterial colonization and increased antibiotic administration during this period. Integron was found to be highly correlated with most iARGs, potentially playing a role in the presence of these genes within cells and their similar transmission patterns in wastewater. The intracellular and extracellular bacterial communities were significantly different, primarily because of variations in bacterial adaptability to the high salt and anaerobic environment. The intracellular co-occurrence network indicated that some dominant genera in wastewater, such as Turicibacter, Clostridium IV, Cloacibacillus, Subdivision5_genera_incertae_sedis, Saccharibacteria_genera_incertae_sedis and Halomonas, were potential hosts for many ARGs. To the best of our knowledge, this study demonstrates, for the first time, the annual variation of ARGs at critical points in the reuse of dairy farm wastewater. It also offers valuable insights into the prevention and control of ARGs derived from animals.

Ecological distribution of anaerobic granular sludge towards efficient anaerobic reactor

Anaerobic reactors often underperform compared to expectations. To identify the key factors, an ecological anaerobic reactor (EAR) with vertical partitions was developed and compared to a physical anaerobic reactor (PAR) as the control. It was observed that EAR achieved a much higher organic loading rate (OLR) compared to PAR (>100 vs 45 kg/m3·d). The different vertical distribution characteristics of anaerobic granular sludge could be ascribed to two vertical distribution patterns dominated in EAR and PAR, i.e., ecological and physical distributions. It was revealed that ecological distribution was formed by the habitat selection, resulting in promoted substrate availability and higher OLR. While physical distribution was mainly affected by hydraulic selection via granule settleability, causing declined substrate availability and lower OLR. Consequently, the promoted ecological distribution and weakened hydraulic selection in EAR contributed to its good performance. Overall, these findings could offer novel concepts for the development of reactors towards high performance.

Current Status and Prospects of Valorizing Organic Waste via Arrested Anaerobic Digestion: Production and Separation of Volatile Fatty Acids

Volatile fatty acids (VFA) are intermediary degradation products during anaerobic digestion (AD) that are subsequently converted to methanogenic substrates, such as hydrogen (H2), carbon dioxide (CO2), and acetic acid (CH3COOH). The final step of AD is the conversion of these methanogenic substrates into biogas, a mixture of methane (CH4) and CO2. In arrested AD (AAD), the methanogenic step is suppressed to inhibit VFA conversion to biogas, making VFA the main product of AAD, with CO2 and H2. VFA recovered from the AAD fermentation can be further converted to sustainable biofuels and bioproducts. Although this concept is known, commercialization of the AAD concept has been hindered by low VFA titers and productivity and lack of cost-effective separation methods for recovering VFA. This article reviews the different techniques used to rewire AD to AAD and the current state of the art of VFA production with AAD, emphasizing recent developments made for increasing the production and separation of VFA from complex organic materials. Finally, this paper discusses VFA production by AAD could play a pivotal role in producing sustainable jet fuels from agricultural biomass and wet organic waste materials.