Insights into microbial interaction profiles contributing to volatile fatty acids production via acidogenic fermentation of waste activated sludge assisted by calcium oxide pretreatment

Performance and mechanism of a novel hydrolytic bacteria pretreatment to boost waste activated sludge disintegration and volatile fatty acids production during acidogenic fermentation

In this study, an innovative mixed hydrolytic bacteria culture (HB) (the main dominant bacterial species: Lactobacillus acetotolerans), as an environmentally friendly pretreatment technique, was developed to enhance the volatile fatty acids (VFAs) production from waste-activated sludge (WAS). The highest VFAs production of 517 and 518 mg/g VSS were achieved with HB 8% and HB 8%-35 °C pretreatments, which were almost 3.6 folds compared to the control (143 mg/g VSS), respectively. The mechanism analysis revealed that HB boosted the bioavailability of organics released from WAS and significantly accelerated sludge solubilization. Protease and α-glucosidase enzymatic activity were improved and associated with hydrolysis and acidogenesis. Furthermore, the microbial community analysis showed that HB pretreatment significantly increased the hydrolytic and acidifying bacteria proportions (e.g., Veillonella, Macellibacteroides sp., Clostridium_sensu_stricto_1 and Bacteroides sp., etc.). This study provides a promising, low-cost, and eco-friendly approach for recovering resources from WAS and transforming them into high-value products.

Enhanced performance and economic feasibility of sewage sludge digestion using a two-stage anaerobic digestion with a dynamic membrane and alkaline-thermal pretreatment

This study suggests a high-rate sewage sludge anaerobic digestion (AD) process. An alkaline-thermal pretreatment and a dynamic membrane (DM) were used to enhance AD efficiency and economic feasibility in a two-stage system. The effect of pretreatment on volatile fatty acid (VFA) production in the acidogenic phase was investigated at various hydraulic retention times (HRT). After optimizing the acidogenic phase condition (HRT of 3 days), single- and two-stage AD processes with DM modules were operated simultaneously to compare performance. The highest methane production rates of 0.69 L/L/d for single-stage AD and 1.10 L/L/d for two-stage AD were observed at a total HRT of 12 days. Phase separation enhanced the growth of acetoclastic methanogens. A techno-economic analysis showed that the two-stage AD system would achieve a positive net present value within 2 years. This study demonstrated the feasibility of high-rate AD systems for sewage sludge using DM, alkaline-thermal pretreatment, and phase separation.

Efficient caproate production from lignocellulose via single-step electro-fermentation platform without organic electron donor

Caproate production by microbial fermentation gained the advantages of sustainability and eco-friendliness, but challenged by sterile fermentation environment, necessity of organic electron donors. Here, a single-step electro-fermentation (EF) process of mixed culture was proposed for caprate production from rice straw. At the optimal potential of -0.8 V, caproate concentration, yield and selectivity in the neutral red (NR)-mediated EF system were 2.4 g/L, 0.2 g/g and 26.6%. Long-term operation accumulated 5.3 g/L caproate with the yield and selectivity of 0.2 g/g and 34.2% in the EF+NR system. Bioaugmentation by dosing chain-elongation microbial consortium further improved the caproate production, yield and selectivity to 9.1 g/L, 0.3 g/g and 41.5%, respectively. The improved caproate production in the bioaugmented EF+NR system was likely due to the enhanced interspecies electron transfer, reconstructed microbial community, multiple electron donors and suitable pH environment. Present study offers a feasible strategy for cost-effective caprate production directly from waste biomass.

Enhanced hydrolysis of waste activated sludge and recovery of volatile fatty acids: Performance and mechanistic analysis of synergistic treatment with sodium citrate and calcium oxide

Anaerobic fermentation has emerged as a promising method of transforming waste activated sludge into high-value products (e.g., volatile fatty acids (VFAs)). This work developed sodium citrate (SC)-calcium oxide (CaO) pretreatment to accelerate the production of VFAs by enhancing sludge solubilization and disintegration of extracellular polymeric substances. The results showed that co-pretreatment with 0.25 g/g TSS of SC and 0.05 g/g TSS of CaO effectively boosted VFAs accumulation (5823.3 mg COD/L), which was 12.2 times higher than the Control group. SC-CaO pretreatment enhanced hydrolysis and acidogenesis by providing ample organic substrates, thereby promoting the growth of hydrolytic and acidogenic bacteria. Additionally, the fermentation broth resulting from co-pretreatment exhibited lower phosphorus concentration and higher biodegradability. Economic analysis confirmed that the combined pretreatment is cost-effective. This work provides a viable strategy for enhancing high-value product recovery from sludge.

Calcium oxide enhances the anaerobic co-digestion of excess sludge and plant waste: performance and mechanism

The study investigates the effect of the oxidant calcium oxide (CaO) on the codigestion of excess sludge (ES) and plant waste (PW) under mesophilic anaerobic conditions to enhance methane production. The findings indicate that CaO significantly elevated methane yield in the codigestion system, with an optimum CaO addition of 6% resulting in a maximum methane production of 461 mL/g volatile solids, which is approximately 1.3 times that of the control group. Mechanistic exploration revealed that CaO facilitated the disintegration of organic matter, enhanced the release of soluble chemical oxygen demand, and increased the concentrations of soluble proteins and polysaccharides within the codigestion substrate. The presence of CaO was conducive to the generation and biological transformation of volatile fatty acids, with a notable accumulation of acetic acid, a smaller carboxylic acid within the VFAs. The proportion of acetate in the CaO-amended group increased to 32.6-36.9%. Enzymatic analysis disclosed that CaO enhanced the activity of hydrolytic and acidogenic enzymes associated with the ES and PW codigestion process but suppressed the activity of coenzyme F420. Moreover, CaO augmented the nutrient load in the fermentation liquid. The study provides an alternative scheme for the efficient resource utilization of ES and PW.

A novel microbial community restructuring strategy for enhanced hydrogen production using multiple pretreatments and CSTR operation

To achieve rapid enrichment of the targeted hydrogen-producing bacterial population and reconstruction of the microbial community in the biological hydrogen-producing reactor, the activated sludge underwent multiple pretreatments using micro-aeration, alkaline treatment, and heat treatment. The activated sludge obtained from the multiple pretreatments was inoculated into the continuous stirred tank reactor (CSTR) for continuous operations. The community structure alteration and hydrogen-producing capability of the activated sludge were analyzed throughout the operation of the reactor. We found that the primary phyla in the activated sludge population shifted to Proteobacteria, Firmicutes, and Bacteroidetes, which collectively accounted for 96.69% after undergoing several pretreatments. This suggests that the multiple pretreatments facilitated in achieving the selective enrichment of the fermentation hydrogen-producing microorganisms in the activated sludge. The CSTR start-up and continuous operation of the biological hydrogen production reactor resulted in the reactor entering a highly efficient hydrogen production stage at influent COD concentrations of 4000 mg/L and 5000 mg/L, with the highest hydrogen production rate reaching 8.19 L/d and 9.33 L/d, respectively. The main genus present during the efficient hydrogen production stage in the reactor was Ethanoligenens, accounting for up to 33% of the total population. Ethanoligenens exhibited autoaggregation capabilities and a superior capacity for hydrogen production, leading to its prevalence in the reactor and contribution to efficient hydrogen production. During high-efficiency hydrogen production, flora associated with hydrogen production exhibited up to 46.95% total relative abundance. In addition, redundancy analysis (RDA) indicated that effluent pH and COD influenced the distribution of the primary hydrogen-producing bacteria, including Ethanoligenens, Raoultella, and Pectinatus, as well as other low abundant hydrogen-producing bacteria in the activated sludge. The data indicates that the multiple pretreatments and reactor's operation has successfully enriched the hydrogen-producing genera and changed the community structure of microbial hydrogen production.

Metagenomic approach reveals the mechanism of calcium oxide improving kitchen waste dry anaerobic digestion

In light of the characteristics of excessive acidification and low biogas yield during kitchen waste (KW) dry digestion, the impact of the calcium oxide (CaO) on KW mesophilic dry digestion was investigated, and the enhanced mechanism was revealed through metagenomic approach. The results showed that CaO increased the biogas production, when the CaO dosage was 0.07 g/g (based on total solid), the biogas production reached 656.84 mL/g suspended solids (VS), approximately 8.38 times of that in the control. CaO promoted the leaching and hydrolysis of key organic matter in KW. CaO effectively promoted the conversion of volatile fatty acid (VFA) and mitigated over-acidification. Macrogenome analysis revealed that CaO increased the microbial diversity in KW dry digestion and upregulated the abundance of genes related to amino acid and carbohydrates metabolism. This study provides an effective strategy with potential economic benefits to improve the bioconversion efficiency of organic matter in KW.

Enhanced 4-bromophenol anaerobic biodegradation in electricity-stimulated anaerobic system: The key role of humic acid in reshaping microbial eco-interrelations and functions

Electricity-stimulated anaerobic system (ESAS) has shown great potential for halogenated organic pollutants removal. Exogenous redox mediators can improve electron transfer efficiency to enhance pollutants removal in ESAS. In this study, humic acid (HA), a low-cost electron mediator, was added into ESAS to enhance the simultaneous reductive debromination and mineralization of 4-bromophenol (4-BP). Results showed that the highest 4-BP removal efficiency at 48 h was 95.43 % with HA dosage of 30 mg/L at - 700 mV, which was 34.67 % higher than that without HA. The addition of HA decreased the requirement for electron donors and enriched Petrimonas and Rhodococcus for humus respiratory. HA addition regulated microbial interactions, and enhanced species cooperation between Petrimonas and dehalogenation species (Thauera and Desulfovibrio), phenol degradation-related species (Rhodococcus) as well as fermentative species (Desulfobulbus). Functional genes related to 4-BP degradation (dhaA/hemE/xylC/chnB/dmpN) and electron transfer (etfB/nuoA/qor/ccoN/coxA) were increased in abundance by HA addition. The enhanced microbial functions, as well as species cooperation and facilitation, all contributed to the improved 4-BP biodegradation in HA-added ESAS. This study provided a deep insight into microbial mechanism driven by HA and offered a promising strategy for improving halogenated organic pollutants removal from wastewater.

Response characteristics of indigenous microbial community in polycyclic aromatic hydrocarbons (PAHs) contaminated aquifers under polyethylene microplastics stress: A microcosmic experimental study

To understand the response characteristics of indigenous microbial community in PAH-contaminated aquifers to the coexistence of microplastics. In this paper, we constructed a groundwater microecosystem using lithologic media collected from the field and subjected it to the stress of a polyethylene microplastics (PE-MPs) concentration gradient. By conducting adsorption experiments and 16S rRNA sequencing, we revealed the growth, structure, metabolism, and resistance mechanisms of the indigenous microbial community in the aquifer lithologic media exposed to varying levels of co-stress from PE-MPs and phenanthrene. Our findings suggest that the adsorption capacity of aquifer lithologic media for phenanthrene is significantly weaker than that of PE-MPs. Additionally, our observations indicated that small particle lithologic media had a greater adsorption capacity for phenanthrene than large particle lithologic media. The presence of PE-MPs was found to increase both the abundance and diversity of microbial communities, although the relationship was not linear with the content of PE-MPs. When exposed to the combined stress of PE-MPs and phenanthrene, the relative abundance of Proteobacteria decreased while that of Bacteroidetes increased. Several genera belonging to Proteobacteria (Aeromonas, Desulfovibrio, Klebsiella, Pantoea, and Microvirgula) and Bacteroidetes (Macellibacteroides and Bacteroides) occupied a central position in the microbial community interaction network and showed significant correlations with other genera. Furthermore, an increase in the proportion of genera capable of degrading various refractory organics was observed. The presence of PE-MPs increased the phenanthrene content in the aquifer lithologic media, thereby intensifying the inhibitory effect on indigenous microbial community in this environment. Despite an increase in the phenanthrene content of aquifer lithologic media due to the presence of PE-MPs, indigenous microbial community in this environment exhibited resistance to the combined inhibition of PE-MPs and phenanthrene through a series of resistance mechanisms. These mechanisms included strengthening the N-cycle process, enhancing metabolic capacity for phenanthrene, improving perception, response, and adaptation to changes in the external environment or intracellular state, modifying the transmembrane transport of the cell membrane to the substrate, and regulating life processes.

Bacterial antioxidant mechanism in calcium peroxide aided sludge anaerobic fermentation

Although calcium peroxide (CaO₂) can enhance the short-chain fatty acids (SCFAs) production in sludge anaerobic fermentation, the microbiological mechanisms underlying this process remain unclear. In this study, it is aimed to elucidate the bacterial protective mechanisms in response to the oxidative stress induced by CaO₂. Results show that extracellular polymeric substance (EPS) and anti-oxidant enzymes play vital roles in protecting bacterial cells from CaO₂. The addition of CaO₂ resulted in increased relative abundances of genes exoP and SRP54, which are associated with EPS secretion and transportation. Superoxide dismutase (SOD) played a crucial in alleviating oxidative stress. The dosage of CaO₂ significantly influences the succession of the bacterial community in the anaerobic fermentation system. With 0.3 g CaO₂/g VSS, the net income was approximately 4 USD/ton of sludge treated. The CaO₂-assisted anaerobic fermentation process has the potential to recover more resources from sludge and thus, benefit the environment.

Phenanthrene regulates metabolic pathways for hydrogen accumulation in sludge alkaline dark fermentation

The influence of phenanthrene (PHE), a general polycyclic aromatic hydrocarbon in waste activated sludge, on sludge alkaline dark fermentation for hydrogen accumulation was investigated prospectively. The yield of hydrogen was 16.2 mL/g TSS with 50 mg/kg TSS PHE, which was 1.3-fold greater than that of the control. Mechanism research demonstrated that hydrogen production and the abundance of functional microorganisms were facilitated, whereas those of homoacetogenesis were reduced. The activity of pyruvate ferredoxin oxidoreductase in the conversion of pyruvate to reduced ferredoxin for hydrogen production was promoted by 57.2%, and that of carbon monoxide dehydrogenase and formyltetrahydrofolate synthetase, closely associated with hydrogen consumption, was suppressed by 60.5% and 55.9%, respectively. Moreover, the encoding genes involved in pyruvate metabolism were significantly up-regulated, while genes related to consuming hydrogen to reduce carbon dioxide and produce 5-methyltetrahydrofolate were down-regulated. This study notably illustrates the effect of PHE on hydrogen accumulation from metabolic pathways.

Application and improvement methods of sludge alkaline fermentation liquid as a carbon source for biological nutrient removal: A review

Alkaline fermentation can reduce the amount of waste activated sludge and prepare sludge alkaline fermentation liquid (SAFL) rich in short-chain fatty acids (SCFAs), which can be used as a high-quality carbon source for the biological nutrient removal (BNR) process. This review compiles the production method of SAFL and the progress of its application as a BNR carbon source. Compared with traditional carbon sources, SAFL has the advantages of higher efficiency and economy, and different operating conditions can influence the yield and structure of SCFAs in SAFL. SAFL can significantly improve the nutrient removal efficiency of the BNR process. Taking SAFL as the internal carbon source of BNR can simultaneously solve the problem of carbon source shortage and sludge treatment difficulties in wastewater treatment plants, and further reduce the operating cost. However, the alkaline fermentation process results in many refractory organics, ammonia and phosphate in SAFL, which reduces the availability of SAFL as a carbon source. Purifying SCFAs by removing nitrogen and phosphorus, directly extracting SCFAs, or increasing the amount of SCFAs in SAFL by co-fermentation or combining with other pretreatment methods, etc., are effective measures to improve the availability of SAFL.

Production and use of Scenedesmus acuminatus biomass in synthetic municipal wastewater for integrated biorefineries

Bioethanol production from algal biomass is a promising alternative for sustainable biofuel production. Algae possess a high photosynthetic capacity and an adaptive ability to thrive under harsh environmental conditions. The potential properties of Scenedesmus acuminatus CCALA 436 were assessed in this research for its bioethanol efficiency, and the effects of growing the algae in wastewater and at different concentrations of mepiquat chloride were studied. Also, pre-treatment efficiencies of different concentrations of calcium oxide were carried out on microalgae biomass. Superoxide dismutase, catalase activity, glutathione, and malondialdehyde contents of microalgae were examined, and the changes in chlorophyll, photoprotective carotenoid contents, and protein concentrations were determined. The results revealed that the maximum sugar and ethanol contents of Scenedesmus acuminatus CCALA 436 were 44.7 ± 1.5% and 20.32 g/L, respectively, for 50% wastewater and mepiquat chloride (2.5 mg/L) after pre-treatment with calcium oxide (0.08%). Additionally, the levels of oxidative enzymes varied depending on the wastewater concentrations. These findings indicate Scenedesmus acuminatus CCALA 436 grown in wastewater and mepiquat chloride can be used for the treatment of wastewater and the production of ethanol and high-value products such as carotenoid.

Calcium peroxide and freezing co-pretreatment enhancing short-chain fatty acids production from waste activated sludge towards carbon-neutral sludge treatment

Short-chain fatty acids (SCFAs) recovery through anaerobic fermentation is a promising technology to achieve carbon-neutral in waste activated sludge (WAS) management. After 0.15 g CaO₂/g volatile suspended solids (VSS) addition and three-cycle freezing co-pretreatments, the maximal SCFAs production of 438.5 mg COD/g VSS was achieved within 4 days fermentation, and more than 70 % of SCFAs was composed of acetate and propionate, which achieved a higher level than reported in previous studies. Mechanism explorations elucidated that co-pretreatment triggered sludge solubilization, promoting the release of biodegradable organics, providing more biodegradable substrates for SCFAs generation. Further microbial community analysis indicated that the abundances of hydrolytic microorganisms and acidogens were enriched, whereas methanogens were inhibited. Besides, environmental analysis suggested that co-pretreatment could achieve carbon reduction benefits of 0.116-0.291 ton CO₂/ton WAS, demonstrating its huge carbon-neutral potential benefits.

Simultaneous recovery of short-chain fatty acids and phosphorus during lipid-rich anaerobic fermentation with sodium hydroxide conditioning

Anaerobic fermentation (AF) could achieve simultaneous recovery of short-chain fatty acids (SCFAs) and phosphorus (P) when waste activated sludge (WAS) and meat processing waste (MPW) act as co-substrate. However, long-chain fatty acids, the degradation intermediates of lipids, always inhibit anaerobic microbial activity. Therefore, sodium hydroxide (NaOH) conditioning was applied to improve the lipid-rich AF performance in this study. The results demonstrated that 96% WAS (v/v) with NaOH addition that remaining at pH 7.5 could achieve the maximum SCFAs yield (1180.05 mg/g VS_fed) at 12 d, and ortho-P content in the AF liquor (AFL) was much more than that of without NaOH addition. Anaerovibrio and Aminobacterium, one kind of lipolytic and proteolytic bacteria, respectively, became the major genus in the lipid-rich AF system. 86% of P in the AFL from 96% WAS + pH 7.5 reactor was recovered through vivianite crystallization method, with 91% of SCFAs remaining in the post-AFL. Meanwhile, analysis results verified vivianite formation in the P precipitate products. Overall, this study provided a new idea to achieve SCFAs and P simultaneous recovery from WAS and MPW through AF with NaOH conditioning and vivianite crystallization.

Insights into microbial interactive mechanism regulating dissimilatory nitrate reduction processes in riparian freshwater aquaculture sediments

Aquaculture can substantially alter the accumulation and cycling of nutrients in sediments. However, the microbial mechanisms mediating sediment dissimilatory nitrate (NO₃^-) reduction in freshwater aquaculture ponds are still unclear, which rule the removal and retention of N element. In the present study, three microbial NO₃^- reduction processes in riparian aquaculture pond sediments (i.e., crab, shrimp and fish ponds) and natural freshwater sediments (i.e., lakes and rivers) were investigated via isotopic tracing and molecular analyses. The potential rates of denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) significantly increased in the aquaculture ponds compared with the natural freshwaters. Denitrification contributed 90.40-94.22% to the total NO₃^- reduction (product as N₂), followed by 2.49-5.82% of anammox (product as N₂) and 2.09-5.18% of DRNA (product as NH₄⁺). The availability of C and N substrates, rather than functional gene abundance, regulated the activities of NO₃^- reductions and microbiome composition. Microbial mechanism based on network analysis indicated that heterotrophic denitrifiers and DNRA bacteria (e.g., Bacillus, Micromonospora, Mycobacterium and Brachybacterium) determined the community structure and function for N conversions in aquaculture ponds, whereas the such microbial network in natural freshwater sediments was manipulated by autotrophic denitrifiers (e.g., Desulfuromonas, Polaromonas, Solitalea). Collectively, this study provides an in-depth exploration of microbial nitrogen removal in freshwater aquaculture areas and supports management strategies for N pollution caused by reclamation for aquaculture in riparian zones.

Immobilization of lead (Pb) using ladle furnace slag and carbon dioxide

Global sustainable development faces challenges in greenhouse gas emissions, consumption of energy and non-renewable resources, environmental pollution, and waste landfilling. Current technologies for immobilization of heavy metals face similar challenges; for example, the use of cement, magnesia, lime, and other binders for immobilization of heavy metals is associated with carbon dioxide emission and consumption of limestone/magnesite and energy. In these contexts, this study introduced a novel and sustainable method for immobilization of lead (Pb) by using an industrial solid waste (ladle furnace slag, LFS) and a greenhouse gas (carbon dioxide). In this laboratory investigation, LFS was first mixed with the lead nitrate and then treated by conventional curing (without carbon dioxide) and carbonation curing (with carbon dioxide) for different periods. The treated LFS were then analyzed by various chemical analyses and microanalysis. The results showed that LFS with conventional curing is not effective in immobilization of lead, while LFS with carbonation curing can effectively immobilize lead. The leaching concentrations of Pb from carbonated LFS were four orders of magnitude lower than those with conventional curing. LFS can achieve carbon dioxide uptake of up to 8% of LFS mass. During the carbonation process, carbonates were produced and wrapped LFS particles to prevent the release of lead, lead nitrate was also carbonated into lead carbonate, and the pH of LFS was reduced to 9.36-9.58, close to the minimum solubility of lead carbonate; these are the main reasons for lead immobilization. In summary, the use of LFS with carbon dioxide for immobilization of lead can not only sequester carbon dioxide, but also reduce the cost of binders, non-renewable resource consumption, energy use, and LFS landfilling.

Effect of Different Acid and Base Potassium Ferrate Pretreatment on Organic Acid Recovery by Anaerobic Digestion of Sludge

Potassium ferrate has strong oxidation in both acid and alkali environments, which has attracted extensive attention. However, the impact of the pH environment on this coupling process with the goal of resource recovery has not received attention. Under the goal of the efficient recovery of organic acid, the changes of solid-liquid characteristics of sludge after acid and alkaline ferrate pretreatment and during anaerobic digestion were discussed. The results showed that compared with blank control groups, after alkaline ferrate pretreatment, the volatile suspended solids (VSSs) decreased the most, reaching 28.19%. After being pretreated with alkaline ferrate, the sludge showed the maximum VFA accumulation (408.21 COD/g VSS) on the third day of digestion, which was 1.34 times higher than that of the acid ferrate pretreatment. Especially in an alkaline environment, there is no need to add additional alkaline substances to adjust the pH value, and the effect of sludge reduction and acid production is the best.

Potential of eggshell waste derived calcium for sustainable production of biogas from cassava wastewater

Cassava is a staple crop that plays a significant role in the food security of many countries. However, its processing produces a liquid by-product known as cassava wastewater (CW), which can have adverse environmental consequences if discarded without treatment. Despite its cyanide content, CW has a high organic content and may be profitable when used to produce biogas. In this study, the influence of calcium particles from eggshell residues was investigated on the anaerobic digestion of CW. Moreover, the performance of the bioreactor was remotely monitored. Calcium particles from milled-calcined chicken eggshells were added to the bioreactor, and biogas production was investigated for 21 days. Adding 1 g/L and 3 g/L of calcium particles increased biogas (Bio H₂ + Bio CH₄) production by 195% and 338%, respectively. Finally, the requirement for digestate post-treatment before use in agriculture was observed after assessing its phytotoxicity through the germination and root growth of L. sativa seeds.

Synergic role of ferrate and nitrite for triggering waste activated sludge solubilisation and acidogenic fermentation: Effectiveness evaluation and mechanism elucidation

Enhancing anaerobic treatment efficiency of waste activated sludge (WAS) toward preferable resource recovery would be an important requirement for achieving carbon-emission reduction, biosolids minimization, stabilization and security concurrently. This study demonstrated the synergic effect of potassium ferrate (PF) and nitrite on prompting WAS solubilisation and acidogenic fermentation toward harvesting volatile fatty acids (VFAs). The results indicated the PF+NaNO₂ co-pretreatment boosted 7.44 times and 1.32 times higher WAS solubilisation [peak soluble chemical oxygen demand (SCOD) of 2680 ± 52 mg/L] than that by the single nitrite- and PF-pretreatment, respectively, while about 2.77 times and 2.11 times higher VFAs production were achieved (maximum VFAs accumulation of 3536.25 ± 115.24 mg COD/L) as compared with the single pretreatment (nitrite and PF)-fermentations. Afterwards the WAS dewaterability was improved simultaneously after acidogenic fermentation. Moreover, a schematic diagram was established for illustrating mechanisms of the co-pretreatment of PF and nitrite for enhancing the VFAs generation via increasing key hydrolytic enzymes, metabolic functional genes expression, shifting microbial biotransformation pathways and elevating abundances of key microbes in acidogenic fermentation. Furthermore, the mechanistic investigations suggested that the PF addition was conducive to form a relatively conductive fermentation environment for enhancing electron transfer (ET) efficiency, which contributed to the VFAs biotransformation positively. This study provided an effective strategy for enhancing the biodegradation/bioconversion efficiency of WAS organic matters with potential profitable economic returns.

Initial pH Conditions Shape the Microbial Community Structure of Sewage Sludge in Batch Fermentations for the Improvement of Volatile Fatty Acid Production

Conversion of wastewater treatment plants into biorefineries is a sustainable alternative for obtaining valuable compounds, thus reducing pollutants and costs and protecting the environment and human health. Under specific operating conditions, microbial fermentative products of sewage sludge are volatile fatty acids (VFA) that can be precursors of polyhydroxyalkanoate thermoplastic polyesters. The role of various operating parameters in VFA production has yet to be elucidated. This study aimed to correlate the levels of VFA yields with prokaryotic microbiota structures of sewage sludge in two sets of batch fermentations with an initial pH of 8 and 10. The sewage sludge used to inoculate the batch fermentations was collected from a Sicilian WWTP located in Marineo (Italy) as a case study. Gas chromatography analysis revealed that initial pH 10 stimulated chemical oxygen demands (sCOD) and VFA yields (2020 mg COD/L) in comparison with initial pH 8. Characterization of the sewage sludge prokaryotic community structures—analyzed by next-generation sequencing of 16S rRNA gene amplicons—demonstrated that the improved yield of VFA paralleled the increased abundance of fermenting bacteria belonging to Proteobacteria, Bacteroidetes, Chloroflexi, and Firmicutes phyla and, conversely, the reduced abundance of VFA-degrading strains, such as archaeal methanogens.

Hydrochar prepared from digestate improves anaerobic co-digestion of food waste and sewage sludge: Performance, mechanisms, and implication

This work reported a new waste functionalization and utilization method, which use digestate to prepare hydrochar to improve methane production from food waste (FW) and sewage sludge (SS). Experimental results presented that 10 g/L hydrochar obtained the cumulative methane production of 133.11 ± 1.18 mL/g volatile solids added, 26.99 % higher than that without hydrochar addition. By monitoring the conversion of model metabolic intermediates, 10 g/L hydrochar was determined to favor hydrolysis, acidogenesis and methonogenesis bio-processes involved in methane production, thus improving the degradation of solubilized organics and consumption of short-chain fatty acids (SCFAs) during the co-digestion. Microbial investigation revealed that 10 g/L hydrochar enriched the microbes relevant to methane production (e.g., Methanosaeta and Syntrophomonas), but reduced the abundances of hydrolysis- and acidogenesis-related microbes (e.g., Acinetobacter). This hydrochar-based preparation and utilization strategy might offer a novel paradigm for waste-control-waste, bringing economic and environmental benefits.

Enzymatic Potential of Filamentous Fungi as a Biological Pretreatment for Acidogenic Fermentation of Coffee Waste

Spent coffee grounds (SCGs) are a promising substrate that can be valorized by biotechnological processes, such as for short-chain organic acid (SCOA) production, but their complex structure implies the application of a pretreatment step to increase their biodegradability. Physicochemical pretreatments are widely studied but have multiple drawbacks. An alternative is the application of biological pretreatments that include using fungi Trametes versicolor and Paecilomyces variotii that naturally can degrade complex substrates such as SCGs. This study intended to compare acidic and basic hydrolysis and supercritical CO₂ extraction with the application of these fungi. The highest concentration of SCOAs, 2.52 gCOD/L, was achieved after the acidification of SCGs pretreated with acid hydrolysis, but a very similar result, 2.44 gCOD/L, was obtained after submerged fermentation of SCGs by T. versicolor. This pretreatment also resulted in the best acidification degree, 48%, a very promising result compared to the 13% obtained with the control, untreated SCGs, highlighting the potential of biological pretreatments.

Valorization of pretreated waste activated sludge to organic acids and biopolymer

Polyhydroxybutyrate (PHB) is a natural polyester that may be made by utilizing volatile fatty acids (VFAs) as a substrate. VFA generated by continuous anaerobic fermentation of waste activated sludge (WAS) was fed into bioreactors for PHB synthesis in this work. Series of optimization tests were conducted to increase the biodegradability and hydrolysis of waste activated sludge. It was found out that 0.05 g/g TS of SDBS (sodium dodecylbenzene sulfonate), 70 °C (heat treatment) and 2hr (time) as pretreatment condition would give the highest solubilization. Impact of pH adjustment on the acidogenesis of pretreated WAS was evaluated in batch experiments at varying initial pH (4-10). The result indicated that when operational pH was between 7.5 and 8, the VFA yield was increased by 5.3-18.1%. Continuous acidogenic operation validated the SDBS pretreatment and pH adjustment warranted stable VFA conversion from WAS at a yield of 47% in COD basis. Firmicutes, Actinobacteria and Proteobacteria were affiliated as dominant bacterial phyla in the continuous acidogenesis. The effluent of the continuous acidogenesis was converted to biopolymer with the average yields of 0.23 g PHB-COD/g VFA_added-COD in the feast mode and 0.34 g PHB-COD/g VFA_added-COD in the famine mode. In feast and famine cycle, the average VFA utilization was 55% and 60% respectively. The sequential SDBS pretreatment, acidogenesis and PHB production would produce 162 g of PHB from 1 kg of WAS as COD basis.

Polycyclic aromatic hydrocarbons stimulate acidogenesis, acetogenesis and methanogenesis during anaerobic co-digestion of waste activated sludge and food waste

Polycyclic aromatic hydrocarbons (PAHs) have been reported to influence acetic acid production during anaerobic treatment. However, investigations of the impacts of PAHs on the anaerobic co-digestion of waste activated sludge and food waste are limited. Therefore, the effects of PAHs on anaerobic co-digestion were explored in this study. Four kinds of PAHs all exhibited positive contributions to methane production, especially phenanthrene. Mechanism exploration revealed that acidogenesis, acetogenesis, and methanogenesis were improved in the presence of phenanthrene, and acetotrophic methanogenesis had the greatest improvement with 69.4%. Dominant bacteria and archaea related to acetic acid and methane accumulation were changed by phenanthrene. Moreover, extracellular polymeric substances, coenzyme F₄₂₀, and McrA gene copy number were promoted by phenanthrene, which was beneficial for the generation of acetic acid and methane. Overall, this study provides new insights into the role of organic pollutants in the anaerobic co-digestion of solid wastes.

Unraveling the behaviors of sulfonamide antibiotics on the production of short-chain fatty acids by anaerobic fermentation from waste activated sludge and the microbial ecological mechanism

Antibiotics in waste activated sludge (WAS) has drawn increasing attention because of their persistent and bioaccumulation characteristics. Most study illustrated the role of antibiotics in anaerobic fermentation from WAS, but lacking the analysis at microbial level as well as the possible interaction between them. This study investigated the effect of three sulfonamide antibiotics (sulfamethoxazole (SMX), sulfaquinoxaline (SQX), and sulfadiazine (SD)) on WAS fermentation and explored its microbiological mechanism. Results indicated that the production of short-chain fatty acids (SCFAs) was significantly improved by 1.9 folds with a peak value at 4626.1 mg COD L^-1 in the existence of SD. This was attributed to the promoted release of soluble proteins and polysaccharides with the existence of sulfonamide antibiotics (SAs) as revealed by the excitation-emission matrix (EEM) spectrum. Analysis of microbial community structure showed that the total abundance of the fermenters in groups with SAs was1.2-1.6 times of that in Control. Specifically, the acid-forming genus Tissierella in SMX and SQX increased by 12.1%-15.0% compared with the Control, while the proteolytic genus Proteinivorax dominated in SD with 39.5%. Molecular ecological networks (MENs) analysis further revealed the potential cooperative relationships among different fermenters. This study was anticipated to provide some valuable information for the behavior of antibiotics in WAS fermentation.

Insights into carbon recovery from excess sludge through enzyme-catalyzing hydrolysis strategy: Environmental benefits and carbon-emission reduction

This study introduced the excellent improvement of enzyme cocktail (lysozyme and protease) on hydrolysis efficiency and the role of reducing carbon emission as an alternative carbon source. The best dosing method after optimization was to add four parts of lysozyme at 0 h and one part of protease at 1 h. The extracellular proteins and polysaccharides increased by 118% and 64% respectively under the optimal dosing mode. Enzyme cocktails reduced more organic matters and extended the distribution of sludge particles in the small particle size part. The enzymatic-treated sludge could reduce 21.09 kg CO₂/t VSS if utilized to replace methanol for denitrification carbon source. Enzyme cocktails did better in enhancing both solubilization and hydrolysis than single enzymes under the optimal method. This study will provide a more integrated and comprehensive system for enzymatic pretreatment and new insight into the enzymatic pretreatment enhancing hydrolysis and reducing carbon emission.

Medium-chain carboxylates production by co-fermentation of sewage sludge and macroalgae

The co-fermentation of sewage sludge and macroalgae at different mixing ratios was performed for medium-chain carboxylates (MCCs) production. The results showed that MCCs production was enhanced in co-fermentation groups due to the abundant readily available organics supplied by macroalgae and the alkaline buffer capacity provided by sewage sludge. Highest MCCs concentration of 112.7 mmol C/L (25.5 mmol C/g VS_added) was obtained in the co-fermentation group with sludge/macroalgae ratio of 4:6, which was higher than MCCs produced from the mono-fermentation of sewage sludge (41.7 mmol C/L, 9.4 mmol C/g VS_added) or macroalgae (79.9 mmol C/L, 18.1 mmol C/g VS_added). Microbial analysis showed that species from genus Romboutsia, Terrisporobacter, Clostridium_sensu_stricto_12, Paraclostridium, unclassified_f_Peptostreptococcaceae and Caproiciproducens were significantly positively correlated with MCCs production. Metabolic pathways analysis demonstrated that the co-fermentation promoted the chain elongation process by stimulating the rate-limiting steps involved in the conversion of ethanol to Acetyl-CoA and circular fatty acid biosynthesis pathway.

Revealing the mechanisms of rhamnolipid enhanced hydrogen production from dark fermentation of waste activated sludge

Rhamnolipid (RL), as an environmentally compatible biosurfactant, has been used to enhance waste activated sludge (WAS) fermentation. However, the effect of RL on hydrogen accumulation in anaerobic fermentation remains unclear. Therefore, this work targets to investigate the mechanism of RL-based dark fermentation system on hydrogen production of WAS. It was found that the maximum yield of hydrogen increased from 1.76 ± 0.26 to 11.01 ± 0.30 mL/g VSS (volatile suspended solids), when RL concentration increased from 0 to 0.10 g/g TSS (total suspended solids). Further enhancement of RL level to 0.12 g/g TSS slightly reduced the production to 10.80 ± 0.28 mL/g VSS. Experimental findings revealed that although RL could be degraded to generate hydrogen, it did not play a major role in enhancing hydrogen accumulation. Mechanism analysis suggested that RL decreased the surface tension between sludge liquid and hydrophobic compounds, thus accelerating the solubilization of WAS, improving the proportion of biodegradable substances which could be used for subsequent hydrogen production. Regardless of the fact that adding RL suppressed all the fermentation processes, the inhibition effect of processes associated with hydrogen consumption was much severer than that of hydrogen production. Further investigations of microbial community revealed that RL enriched the relative abundance of hydrogen producers e.g., Romboutsia but reduced that of hydrogen consumers like Desulfobulbus and Caldisericum.

Intermittent pH control strategy in sludge anaerobic fermentation: Higher short-chain fatty acids production, lower alkali consumption, and simpler control

The pH control to promote short-chain fatty acids (SCFAs) production during anaerobic alkaline fermentation basically focused on constant pH control. In this study, a simple and consumption-reducing intermittent pH control strategy at moderate temperature (23 ± 2 °C) was investigated with adjusting pH to 10 when naturally reduced to 8. The intermittent pH control strategy could alleviate the inhibition of acid-producing bacteria by strong alkaline and high FA concentration. Meanwhile, microbial diversity promoted by 6% and 69% while the relative abundance of acid-producing bacteria increased by 36% and 61% compared to blank and constant pH fermenters. The relative genes abundance related to amino acid metabolism and fatty acid production were mostly promoted and led to enhanced SCFAs production. In the long-term fermenter, the intermittent pH control strategy could result in a 68% reduction in alkali consumption and a 37% increase in SCFAs production compared to that of the constant pH at 10.

Polysorbate-80 pretreatment contributing to volatile fatty acids production associated microbial interactions via acidogenic fermentation of waste activated sludge

Polyoxyethylene dehydration sorbitol monooleate (polysorbate-80) pretreatment enhanced volatile fatty acids (VFAs) production of waste activated sludge (WAS) in acidogenic fermentation. The results showed that polysorbate-80 ameliorated WAS solubilization obviously with a soluble chemical oxygen demand (SCOD) increasing to 1536 mg/L within 4 h. Within 2 days of acidogenic fermentation, the maximal VFAs arrived to 2958.35 mg COD/L via polysorbate-80-pretreatment. The polysorbate-80 pretreatment boosted microbial diversity and richness in fermentation process. The Clostridium, Macellibacteroides and Acidocella strengthened microbial cooperation for the metabolic functions enhancement (e.g. amino acid metabolism and carbohydrate metabolism) for VFAs generation from WAS organics. Overall, the polysorbate-80 could play positive roles on the transformation of organic matter from sludge solid matters to VFAs, which was turned out to become an effective enhancing strategy for future WAS treatment / bioresource recovery with relatively low cost.

New insight into enhanced short-chain fatty acids production from waste activated sludge through pretreatment of cation exchange resin coupled NaCl addition

In this study, a novel pretreatment of cation exchange resin (CER) coupled NaCl addition was proposed to enhance waste activated sludge (WAS) hydrolysis and promote short-chain fatty acids (SCFAs) production in the anaerobic fermentation process. At the optimal pretreatment condition of 3 g/g SS CER and 15 g/L NaCl, considerable SCOD (i.e. 5107 mg/L, 35.4% of TCOD) was released after 2-day coupled treatment, which provided sufficient organic substance for the subsequent SCFAs production. The sludge hydrolysis mechanism was illustrated, i.e. CER triggered extracellular polymeric substances (EPS) disruption and NaCl induced microbial cells lysis. The synergistic interaction between CER and NaCl pretreatment was investigated and application potential of fermentative liquid was evaluated after the coupled pretreatment-enhanced anaerobic fermentation. In the presence of abundant biodegradable substrates in the fermentative liquid, 4742 mg COD/L (i.e. 388 mg COD/g VSS) of SCFAs production was achieved within 6-day anaerobic fermentation, mainly composed of acetic and propionic acids (70.4% of total SCFAs).

Waste-to-energy: Cellulase induced waste activated sludge and paper waste co-fermentation for efficient volatile fatty acids production and underlying mechanisms

This study mainly investigated the feasibility of utilizing cellulase to enhance waste activated sludge (WAS) and paper waste (PW) co-fermentation for the generation of volatile fatty acids (VFAs). The introduction of cellulase effectively enhanced the co-fermentation efficiency, and the maximum VFAs generation reached 3014 mg COD/L with 60 mg cellulase/g TSS while it was 1512 mg COD/L in the control reactor. The presence of cellulase evidently improved the concentration of soluble bioavailable substrates (e.g., carbohydrates and proteins) via inducing the EPS disintegration and PW disruption. More importantly, the functional anaerobes (i.e., Macellibacteroides and Bacteroides) and the microbial activities (i.e., ATP, key acid-forming enzymes, and genetic expressions) that related with the VFAs biosynthesis were enriched and enhanced due to the stimulation of cellulase, contributing to the ultimate VFAs promotion. This study provided a novel strategy to recover valuable products from waste biomass with constructive mechanistic exploration.

Propionic acid-rich fermentation (PARF) production from organic wastes: A review

Nowadays, increasing attention has been drawn to biological valorization of organic wastes. Wherein, propionic acid-rich fermentation (PARF) has become a focal point of research. The objective of this review is to make a thorough investigation on the potential of PARF production and give future outlook. By discussing the key factors affecting PARF including substrate types, pH, temperature, retention time, etc., and various improving methods to enhance PARF including different pretreatments, inoculation optimization and immobilization, a comprehensive summary on how to achieve PARF from organic waste is presented. Then, current application of PARF liquid is concluded, which is found to play an essential role in the efficient denitrification and phosphorus removal of wastewater and preparation of microbial lipids. Finally, the environmental performance of PARF production is reviewed through life cycle assessment studies, and environmentally sensitive sectors are summarized for process optimization, providing a reference for waste management in low carbon scenarios.

Synergistic pretreatment of CaO and freezing/thawing to enhance volatile fatty acids recycling and dewaterability of waste activated sludge via anaerobic fermentation

To avoid the generally deteriorated dewaterability of sludge in waste activated sludge (WAS) anaerobic acidogenesis, the combination of varied calcium oxide (CaO) dosage (i.e., 0.01-0.07 g/g TS) and freezing/thawing pretreatment (5 F/T cycles) was investigated for concurrently improving the production of volatile fatty acids (VFAs) and dewatering performance of sludge. The highest release of soluble chemical oxygen demand (SCOD) (1836 ± 96 mg/L) and accumulation of VFAs (448.0 mg COD/g VS) were reached through the co-pretreatment of CaO (0.07 g/g TS) and F/T (50 h at -24 °C) (i.e., 0.07 CaO-F/T). Meanwhile, optimal dewaterability of sludge was also achieved in 0.07 CaO-F/T co-pretreated WAS fermentation, which was reflected by large particle size (98.32 μm), low capillary suction time (41.6 s), decreased specific resistance to filtration (SRF, reduced 47.5% against blank) and high zeta potential (-9.59 mV). Co-pretreatment of CaO and F/T reduced the species number of total microbial population, but improved the abundance of acid-producing bacteria. Increased abundance of Bacteroides, Macellibacteroides, Petrimonas, Prevotella, Clostridium, and Megasphaera was positively relevant to the high yields of VFAs. The economic analysis indicated that CaO-F/T was economically acceptable with considerable estimated net profits, which provided a feasible and efficient alternative for further WAS treatment at large scale.

Volatile Fatty Acid Production from Organic Waste with the Emphasis on Membrane-Based Recovery

In recent years, interest in the biorefinery concept has emerged in the utilization of volatile fatty acids (VFAs) produced by acidogenic fermentation as precursors for various biotechnological processes. This has attracted substantial attention to VFA production from low-cost substrates such as organic waste and membrane based VFA recovery techniques to achieve cost-effective and environmentally friendly processes. However, there are few reviews which emphasize the acidogenic fermentation of organic waste into VFAs, and VFA recovery. Therefore, this article comprehensively summarizes VFA production, the factors affecting VFA production, and VFA recovery strategies using membrane-based techniques. Additionally, the outlook for future research on VFA production is discussed.