Regulating the hydrolysis of organic wastes by micro-aeration and effluent recirculation

Enhancement of methane production from anaerobic digestion of Erigeron canadensis via O2-nanobubble water supplementation

Malignant invasive Erigeron canadensis, as a typical lignocellulosic biomass, is a formidable challenge for sustainable and efficient resource utilization, however nanobubble water (NBW) coupled with anaerobic digestion furnishes a prospective strategy with superior environmental and economic effectiveness. In this study, influence mechanism of various O2-NBW addition times on methanogenic performance of E. canadensis during anaerobic digestion were performed to achieve the optimal pollution-free energy conversion. Results showed that supplementation of O2-NBW in digestion system could significantly enhance the methane production by 10.70-16.17%, while the maximum cumulative methane production reached 343.18 mL g-1 VS in the case of one-time O2-NBW addition on day 0. Furthermore, addition of O2-NBW was conducive to an increase of 2-90% in the activities of dehydrogenase, α-glucosidase and coenzyme F420. Simultaneously, both facultative bacteria and methanogenic archaea were enriched as well, further indicating that O2-NBW might be responsible for facilitating hydrolytic acidification and methanogenesis. Based on Kyoto Encyclopedia of Genes and Genomes (KEGG) cluster analysis, provision of O2-NBW enhanced the metabolism of carbohydrate and amino acid, translation as well as membrane transport of bacteria and archaea. This study might offer the theoretical guidance and novel insights for efficient recovery of energy from lignocellulosic biomass on account of O2-NBW adhibition in anaerobic digestion system, progressing tenor of carbon-neutral vision.

Reversing the damage: ecological restoration of polluted water bodies affected by pollutants due to anthropogenic activities

Aquatic ecosystems provide a large number of cultural, regulating, and supporting services to humans and play a pivotal role in sustaining freshwater-dependent ecosystems. However, an increase in human population coupled with economic growth in the last few decades has severely affected their functioning and ecological health. This has led to an increase in concentrations of pollutants originating from anthropogenic activities such as heavy metals, plastics, semi-volatile organic compounds, and endocrine disruptors. These pollutants provoke deleterious impacts on aquatic biodiversity and affect the water quality and functioning. In this paper, we discuss the sources and impacts of such pollutants as well as restoration techniques for reducing their impact on aquatic ecosystems. Several physical and chemical ecological restoration techniques, such as dredging, sediment capping, water diversion, adsorption, aeration, and flushing, can be employed to improve the water quality of water bodies. Additionally, biological techniques such as phytoremediation, phycoremediation, the use of biomembranes, and the construction of ecological floating beds can be employed to increase the population of aquatic organisms and improve the overall ecological health of aquatic ecosystems. Restoration techniques can effectively reduce the concentrations of suspended solids and dissolved phosphorus and increase the levels of dissolved oxygen. The restoration techniques for improving the ecological health of water bodies should not be limited to simply improving the water quality but should also focus on improving the biological processes and ecosystem functioning since it is essential to mitigate the adverse effects of pollutants and restore the vital ecosystem services provided by water bodies for future generations.

Mediation of Bacterial Interactions via a Novel Membrane-Based Segregator to Enhance Biological Nitrogen Removal

The regulation of microbial subpopulations in wastewater treatment plants (WWTPs) with desired functions can guarantee nutrient removal. In nature, "good fences make good neighbors," which can be applied to engineering microbial consortia. Herein, a membrane-based segregator (MBSR) was proposed, where porous membranes not only promote the diffusion of metabolic products but also isolate incompatible microbes. The MBSR was integrated with an anoxic/aerobic membrane bioreactor (i.e., an experimental MBR). The long-term operation showed that the experimental MBR exhibited higher nitrogen removal (10.45 ± 2.73 mg/L total nitrogen) than the control MBR (21.68 ± 4.23 mg/L) in the effluent. The MBSR resulted in much lower oxygen reduction potential in the anoxic tank of the experimental MBR (-82.00 mV) compared to that of the control MBR (83.25 mV). The lower oxygen reduction potential can inevitably aid in the occurrence of denitrification. The 16S rRNA sequencing showed that the MBSR significantly enriched acidogenic consortia, which yielded considerable volatile fatty acids by fermenting the added carbon sources and allowed efficient transfer of these small molecules to the denitrifying community. Moreover, the sludge communities of the experimental MBR harbored a higher abundance of denitrifying bacteria than those of the control MBR. Metagenomic analysis further corroborated these sequencing results. The spatially structured microbial communities in the experimental MBR system demonstrate the practicability of the MBSR, achieving nitrogen removal efficiency superior to that of mixed populations. Our study provides an engineering method for modulating the assembly and metabolic division of labor of subpopulations in WWTPs. IMPORTANCE This study provides an innovative and applicable method for regulating subpopulations (activated sludge and acidogenic consortia), which contributes to the precise control of the metabolic division of labor in biological wastewater treatment processes.

Microaerobic condition as pretreatment for improving anaerobic digestion: A review

Pretreatment of waste before anaerobic digestion (AD) has been extensively studied during the last decades. One of the biological pretreatments studied is the microaeration. This review examines this process, including parameters and applications to different substrates at the lab, pilot and industrial scales, to guide further improvement in large-scale applications. The underlying mechanisms of accelerating hydrolysis and its effects on microbial diversity and enzymatic production were reviewed. In addition, modelling of the process and energetic and financial analysis is presented, showing that microaerobic pretreatment is commercially attractive under certain conditions. Finally, challenges and future perspectives were also highlighted to promote the development of microaeration as a pretreatment before AD.

A comprehensive review on anaerobic fungi applications in biofuels production

Anaerobic fungi (Neocallimastigomycota) are promising lignocellulose-degrading microorganisms that can be exploited by the biofuel industry. While natural production of ethanol by these microorganisms is very low, there is a greater potential for their use in the biogas industry. More specifically, anaerobic fungi can contribute to biogas production by either releasing holocellulose or reducing sugars from lignocelluloses that can be used as a substrate by bacteria and methanogens involved in the anaerobic digestion (AD) process or by metabolizing acetate and formate that can be directly consumed by methanogens. Despite their great potential, the appropriate tools for engineering anaerobic fungi have not been established yet. The first section of this review justifies how the biofuel industry can benefit from using anaerobic fungi and is followed by their taxonomy. In the third section, the possibility of using anaerobic fungi for the consolidated production of bioethanol is briefly discussed. Nevertheless, the main focus of this review is on the upstream and mainstream effects of bioaugmentation with anaerobic fungi on the AD process. The present review also scrutinizes the constraints on the way of efficient engineering of anaerobic rumen fungi. By providing this knowledge, this review aims to help research in this field with identifying the challenges that must be addressed by future experiments to achieve the full potentials of these promising microorganisms. To sum up, the pretreatment of lignocelluloses by anaerobic fungi can prevent carbohydrate loss due to respiration (compared to white-rot fungi). Following fungal mixed acid fermentation, the obtained slurry containing sugars and more susceptible holocellulose can be directly consumed by AD microorganisms (bacteria, methanogens). The bioaugmentation of anaerobic fungi into the AD process can increase methane biosynthesis by >3.3 times. Despite this, for the commercial AD process, novel genetic engineering techniques and kits must be developed to efficiently improve anaerobic fungi viability throughout the AD process.

Intermittent Microaeration Technology to Enhance the Carbon Source Release of Particulate Organic Matter in Domestic Sewage

Domestic sewage treatment plants often have insufficient carbon sources in the influent water. To solve this problem, the commonly used technical means include an additional carbon source, primary sludge fermentation, and excess sludge fermentation, but these methods are uneconomical, unsustainable, and not applicable to small-scale wastewater treatment plants. Intermittent microaeration technology has the advantages of low energy-consumption, ease of application, and low cost, and can effectively promote anaerobic digestion of municipal sludge; however little research has been reported on its use to enhance the carbon sources release of particulate organic matter (POM) from domestic wastewater. Therefore, the effect of intermittent microaeration on the carbon source release of POM was evaluated in this study, with POM as the control test. The results showed that the release concentration of soluble chemical oxygen demand (SCOD) was the highest on day 4 under microaerobic conditions, and the concentrations of SCOD, NH4+-N, and PO43−-P in the liquid phase were 1153, 137.1, and 13 mg/L, respectively. Compared with the control group, the SCOD concentration increased by 34.2%, and the NH4+-N and PO43−-P concentrations decreased by 18.65% and 17.09%, respectively. Intermittent microaeration can effectively promote the growth of Paludibacter, Actinomyces, and Trichococcus hydrolytic fermentation functional bacteria. Their relative abundances increased by 282.83%, 21.77%, and 23.47%, respectively, compared with the control group. It can simultaneously inhibit the growth of acetate-type methanogenic archaea, Methanosaeta and Methanosarcina, with a decrease in relative abundances of 16.81% and 6.63%, respectively. The aforementioned data show that intermittent microaeration can not only promote the hydrolysis of POM, but can also reduce the loss of acetic acid carbon source, which is a cost-effective technical way to enhance the release of a carbon source of particulate organic matter in domestic sewage.

The Measurement, Application and Effect of Oxygen in Microbial Fermentations: Focusing on Methane and Carboxylate Production

Oxygen is considered detrimental to anaerobic fermentation processes by many practitioners. However, deliberate oxygen sparging has been used successfully for decades to remove H2S in anaerobic digestion (AD) systems. Moreover, microaeration techniques during AD have shown that small doses of oxygen may enhance process performance and promote the in situ degradation of recalcitrant compounds. However, existing oxygen dosing techniques are imprecise, which has led to inconsistent results between studies. At the same time, real-time oxygen fluxes cannot be reliably quantified due to the complexity of most bioreactor systems. Thus, there is a pressing need for robust monitoring and process control in applications where oxygen serves as an operating parameter or an experimental variable. This review summarizes and evaluates the available methodologies for oxygen measurement and dosing as they pertain to anaerobic microbiomes. The historical use of (micro-)aeration in anaerobic digestion and its potential role in other anaerobic fermentation processes are critiqued in detail. This critique also provides insights into the effects of oxygen on these microbiomes. Our assessment suggests that oxygen dosing, when implemented in a controlled and quantifiable manner, could serve as an effective tool for bioprocess engineers to further manipulate anaerobic microbiomes for either bioenergy or biochemical production.

Evolution of biochemical processes in coking wastewater treatment: A combined evaluation of material and energy efficiencies and secondary pollution

The application of advanced biological treatment technology results in improved coking wastewater (CW) effluent quality at lower material and energy input practiced by wastewater treatment plants. In wastewater treatment, the diversity of biological processes combinations affects the variety of microorganisms and biochemical reactions resulting in effluent quality. Four full-scale CW processes, anaerobic-anoxic-oxic (A/A/O), anoxic-oxic-hydrolytic-oxic (A/O/H/O), anoxic-oxic-oxic (A/O/O), and oxic-hydrolytic-oxic (O/H/O) were compared for their consumption of chemicals and energy, emissions of greenhouse gases, and excess sludge production. A new performance indicator combining the above mentioned parameters was proposed to comprehensively evaluate processes in capacity to CW. The O/H/O process showed stable and reliable operation with minimum chemicals cost and the average energy consumption, whereas A/A/O at its good performance in TN removal required a large amount of alkaline chemicals to maintain stability. Besides, a substantial addition of chemicals in A/A/O results in larger average amounts of inorganic sludge. Also, the A/A/O process with a single aerobic unit appeared to be incapable of energy saving when dealing with CW rich in nitrogen and poor in phosphorus. The process with dual aerobic units can achieve more complete carbon and nitrogen removal, which is related to the sequence of biochemical reactions. Diverse sequence combinations can create variation in HRT and DO, whereby contaminants proceed through distinct channels of degradation. In the comparative analysis of CWPIs, it could be seen that O/H/O is the biological treatment process with the least equivalent energy consumption input at present thus exhibiting promising application in CW treatment. The A/O/O and A/O/H/O combinations are good attempts of development; however, more energy-efficient operation modes have to be further investigated.

Genomic driven factors enhance biocatalyst-related cellulolysis potential in anaerobic digestion

Anaerobic digestion (AD) is a promising technology to recover bioenergy from biodegradable biomass, including cellulosic wastes. Through a few fractionation/separation techniques, cellulose has demonstrated its potential in AD, but the performance of the process is rather substrate-specific, as cellulolysis bacteria are sensitive to the enzyme-substrate interactions. Cellulosome is a self-assembled enzyme complex with many functionalized modules in the bacteria which has been gradually studied, however the genomic fingerprints of the culture-specific cellulosome in AD are relatively unclear especially under processing conditions. To clarify the key factors affecting the cellulosome induced cellulolysis, this review summarized the most recent publications of AD regarding the fates of cellulose, sources and functional genes of cellulosome, and omics methods for functional analyses. Different processes for organic treatment including applying food grinds in sewer, biomass valorization, cellulose fractionation, microaeration, and enzymatic hydrolysis enhanced fermentation, were highlighted to support the sustainable development of AD technology.

Effects of micro-aeration on microbial niches and antimicrobial resistances in blackwater anaerobic digesters

Anaerobic digestion (AD) of source-diverted blackwater (toilet flush) at ambient room temperature presents challenges for fast hydrolysis of particulate matters. This study investigated the effect of different micro-aeration dosages for blackwater AD. Sequencing batch reactors were operated at ambient room temperature (22 ± 1°C) with micro-aeration (0, 5, 10, 50, and 150 mg O₂ g^-1 COD_feed per cycle) and gradually reduced hydraulic retention times from 5 d to 2 d. The methanogenesis efficiencies were greater at low oxygen dosages (i.e., 0, 5, 10) while the volatile fatty acids (VFAs) accumulated more at high oxygen dosages (i.e., 50, 150). Microbial communities were significantly different under different oxygen dosages (p<0.05), with segregation of microbial ecological niches in low and high oxygen dosage communities. The low-oxygen-dosage niche (0, 5, and 10 mg g^-1 COD_feed) was inhabited by fermenting and syntrophic bacteria (e.g., Cytophaga, Syntrophomonas) and methanogens (e.g., Methanobacterium, Methanolinea, Methanosaeta). The high-oxygen-dosage niche (50 and 150 mg g^-1 COD_feed) had significantly (p<0.05) more facultative anaerobic bacteria (Ignavibacteriales and Cloacamonales), and aerobic bacteria (Rhodocyclales). Moreover, blackwater can be a source of antimicrobial resistance genes (ARGs), which are affected by different oxygen dosages. The ARG variation correlated with the microbial community composition (p<0.05). Low-oxygen-dosage communities contained a higher prevalence of mobile gene elements (intI1 and korB) and tetM, ermB, sul1, sul2, and blaCTX-M than the high-oxygen-dosage communities, indicating that oxygen dosage influenced the prevalence of populations carrying ARGs. These findings suggest that application of micro-aeration to AD can be used to control ARG profiles.

Rapid Two Stage Anaerobic Digestion of Nejayote through Microaeration and Direct Interspecies Electron Transfer

Corn is one of the main food products in Mexico. The elaboration of corn-derived products generates wastewater with a high organic load (nejayote). Anaerobic digestion is an indicated treatment for wastewater with high organic loads. The results of this study show that the application of microaeration in the hydrolysis-fermentative reactor increased the percentage of volatile fatty acids (VFA) available in the medium by 62%. The addition of a conductive material, such as granulated activated carbon (GAC), promotes DIET (Direct interspecies electrons transfer) in the methanogenic UASB reactor increasing the methane yield by 55%. Likewise, a great diversity of exoelectrogenic bacteria, with the ability to donate electrons DIET mechanisms, were developed in the GAC biofilm, though interestingly, Peptoclostridium and Clostridium (17.3% and 12.75%, respectively) were detected with a great abundance in the GAC biofilm. Peptoclostridium has not been previously reported as a participant in DIET process.

Different micro-aeration rates facilitate production of different end-products from source-diverted blackwater

The effects of micro-aeration on the performance of anaerobic sequencing batch reactors (ASBR) for blackwater treatment were investigated in this study. Different micro-aeration rates, 0, 5, 10, 50, and 150 mg O₂/L-reactor/cycle, and their effect on the hydrolysis, acidogenesis, and methanogenesis of blackwater were evaluated and compared at ambient temperature. Source-diverted blackwater (toilet water) contains high organic contents which can be recovered as biogas. Previous studies have found that anaerobic digestion of blackwater without micro-aeration can only recover upwards of less than 40% of chemical oxygen demand (COD) to methane at room temperature due to the low hydrolysis rate of biomass content in blackwater. This study achieved increases in blackwater hydrolysis (from 34.7% to 48.7%) and methane production (from 39.6% to 50.7%) with controlled micro-aeration (5 mg O₂/L-reactor/cycle). The microbial analysis results showed that hydrolytic/fermentative bacteria and acetoclastic methanogens (e.g. Methanosaeta) were in higher abundances in low-dose micro-aeration reactors (5 and 10 mg O₂/L-reactor/cycle), which facilitated syntrophic interactions between microorganisms. The relative abundance of oxygen-tolerant methanogen such as Methanosarcina greatly increased (from 1.5% to 11.4%) after oxygen injection. High oxygen dosages (50 and 150 mg O₂/L-reactor/cycle) led to reduced methane production and higher accumulation of volatile fatty acids, largely due to the oxygen inhibition on methanogens and degradation of organic matters by aerobic growth and respiration, as indicated by the predicted metagenome functions. By combining reactor performance results and microbial community analyses, this study demonstrated that low-dose micro-aeration improves blackwater biomethane recovery by enhancing hydrolysis efficiency and promoting the development of a functional microbial population, while medium to high-dose micro-aeration reduced the activities of certain anaerobes. It was also observed that medium-dose micro-aeration maximizes VFA accumulation, which may be used in two-stage anaerobic digesters.

Influence of temperature on enhancement of volatile fatty acids fermentation from organic fraction of municipal solid waste: Synergism between food and paper components

This study explores individual contributions and synergistic effects of food and paper, main components of organic fraction of municipal solid waste (OFMSW) towards volatile fatty acids (VFA) fermentation under different temperatures (25, 37, 42 and 52 °C). Thanks to the synergism of food and paper component (FC & PC), the results revealed that OFMSW is suitable for VFA production. Maximum VFA production was noticed to be 21.5 mg/L at 42 °C, ~2.1, and 1.42 times higher than fermentation of PC and FC. Enhanced hydrolysis of PC occurred at >37 °C, increasing alkalinity in leachate to 6.7 g/L at 42 °C, thus maintaining a stable pH (5.4-5.6) during acidogenic fermentation. Additionally, 74% of COD is hydrolyzed, of which 79% is converted to VFA based on biodegradable carbon at 42 °C. It is suggested that co-existence of FC and PC can enhance VFA production of OFMSW, and targeted VFA production can be maximized through process optimization.

Review on microaeration-based anaerobic digestion: State of the art, challenges, and prospectives

Microaeration (dosing small quantities of air or oxygen) is an effective approach to facilitate anaerobic digestion (AD) process and has gained increased attention in recent years. The underlying mechanisms of the facilitation effect of microaeration on AD process were reviewed in terms of accelerating hydrolysis, scavenging hydrogen sulfide, and affecting microbial diversity. Process parameters and control strategies were summarized to reveal considerable factors in implementing microaeration-based AD process. In addition, current applications, including lab-, pilot- and full-scale level cases, were summarized to provide guidance for further improvement in large-scale applications. The challenges and future perspectives were also highlighted to promote the development of AD process associated with microaeration.

Two Phase Anaerobic Digestion System of Municipal Solid Waste by Utilizing Microaeration and Granular Activated Carbon

In an anaerobic digestion (AD) process, the hydrolysis phase is often limited when substrates with high concentrations of solids are used. We hypothesized that applying micro-aeration in the hydrolysis phase and the application of granular activated carbon (GAC) in the methanogenesis phase could make the AD process more efficient. A packed bed reactor (PBR) coupled with an up-flow anaerobic sludge blanket (UASB) was conducted, and its effects on methane generation were evaluated. The micro-aeration rate applied in PBR was 254 L-air/kg-Total solids (TS)-d was compared with a control reactor. Micro-aeration showed that it reduced the hydrolysis time and increased the organic matter solubilization as chemical oxygen demand (COD) increasing 200%, with a volatile fatty acids (VFAs) increment higher than 300%, compared to the control reactor (without aeration). Our findings revealed that the implementations of microaeration and GAC in the two-phase AD system could enhance methane production by reducing hydrolysis time, increasing solid waste solubilization.

Intermittent micro-aeration: New strategy to control volatile fatty acid accumulation in high organic loading anaerobic digestion

This study developed an intermittent oxidation-reduction potential (ORP)-controlled micro-aeration system for high solids anaerobic digestion (AD) of lignocellulosic biomass without volatile fatty acids (VFA) accumulation at high organic loading rate (OLR). Traditional AD of Napier grass, a model lignocellulosic biomass, at an OLR of 5 g volatile solids (VS)/L/day resulted in an accumulation of total VFA concentration up to 9.2 g/L as acetic acid (HAc) equivalent, causing rapid drops in pH and methane yield, and driving the digester to the verge of failure. Once intermittent (every 24 h) ORP-controlled micro-aeration (at ORP of -470 mV) was initiated, the total VFA concentration rapidly decreased to 3.0 g HAc/L and the methane yield improved, resulting in stable digester performance without the need for alkalinity supplementation or OLR reduction. By combining reactor performance results, mass balance analyses, microbial community characterization data, and a bioenergetic evaluation, this study suggested that rapid VFA conversion and CH₄ production were carried out by facultative anaerobes and hydrogenotrophic methanogens under micro-aerobic conditions. This novel operating approach can be applied as an effective control strategy for high OLR AD processes especially in the event of VFA accumulation.

Enhancing methane production of anaerobic sludge digestion by microaeration: Enzyme activity stimulation, semi-continuous reactor validation and microbial community analysis

Effects of microaeration pretreatment on sludge hydrolysis, biogas production and microbial community structure in anaerobic digestion (AD) were investigated by bench-scale tests and semi-continuous experiments. Bench tests showed that microaeration led to the release of dissolved organic matters, generation of volatile fatty acids and stimulation of enzyme activity. Correlation analysis showed that methane production was significantly correlated with the activity of α-glucosidase at 0.01 level, and with protease activity, released polysaccharides and VFAs at 0.05 level. Semi-continuous experiments showed that microaeration accelerated the utilization of organic matters, increased biogas production by 16.4%, enhanced methane content in biogas, and improved sludge dewaterability. Microbial community structure analysis showed that microaeration promoted enrichment of hydrolytic and fermentative bacteria in AD reactor rather than methanogenic bacteria, and aceticlastic methanogenesis was the main methanogenic pathway for methane production.

Aeration to Improve Biogas Production by Recalcitrant Feedstock

Digestion of wastes to produce biogas is complicated by poor degradation of feedstocks. Research has shown that waste digestion can be enhanced by the addition of low levels of aeration without harming the microbes responsible for methane production. This research has been done at small scales and without provision to retain the aeration in the digestate. In this paper, low levels of aeration were provided to poultry litter slurry through a sub-surface manifold that retained air in the sludge. Digestate (133 L) was supplied 0, 200, 800, or 2000 mL/day air in 200 mL increments throughout the day via a manifold with a volume of 380 mL. Digesters were fed 400 g of poultry litter once weekly until day 84 and then 600 g thereafter. Aeration at 200 and 800 mL/day increased biogas production by 14 and 73% compared to anaerobic digestion while aeration at 2000 mL/day decreased biogas production by 19%. Biogas quality was similar in all digesters albeit carbon dioxide and methane were lowest in the 2000 mL/day treatment. Increasing feed to 600 g/week decreased gas production without affecting biogas quality. Degradation of wood disks placed within the digesters was enhanced by aeration.

Enhanced volatile fatty acids production during anaerobic digestion of lignocellulosic biomass via micro-oxygenation

A series of batch experiments were conducted to investigate the effects of inoculum type, oxygen (O₂) dosage, and incubation time on volatile fatty acids (VFAs) production during anaerobic digestion (AD) of Napier grass (Pennisetum purpureum), a high yielding energy crop. The results showed that anaerobically digested cattle manure (ADCM) as an inoculum generated significantly higher VFAs compared to that of anaerobically digested waste activated sludge (ADWAS) as an inoculum. Additionally, the incubation time of 3days and O₂ dosage of 15mL/g volatile solids_added showed the highest VFAs production when ADCM was used as an inoculum. Moreover, the VFAs production had a quadratic correlation with O₂ dosage with R² of 0.86. The Scanning Electron Microscopy (SEM) images of the digested fiber showed rough and crumbled surface structures as opposed to that of the undigested fiber, which was further confirmed by changes in structural composition of the digested fiber.

Effect of micro-aeration and inoculum type on the biodegradation of lignocellulosic substrate

The effect of various micro-aeration strategies on the anaerobic digestion (AD) of wheat straw was thoroughly examined using a mixture of inocula, containing compost and well digested sludge from biogas plant. The aim was to determine the most efficient oxygen load, pulse repetition and treatment duration, resulting in the highest methane production. The oxygen load had the largest impact on the biodegradability of straw, among the examined variables. More specifically, a micro-aeration intensity of 10mLO₂/gVS was identified as the critical threshold above which the AD performance was more susceptible to instability. The highest enhancement in biogas production was achieved by injecting 5mLO₂/gVS for a consecutive 3-day treatment period, presenting a 7.2% increase compared to the untreated wheat straw. Nevertheless, the results from optimisation case study indicated a higher increase of 9% by injecting 7.3mLO₂/gVS, distributed in 2 pulses during a slightly shorter treatment period (i.e. 47h).

Effect of aerobic pre-treatment on hydrogen and methane production in a two-stage anaerobic digestion process using food waste with different compositions

Aerobic pre-treatment was applied prior to two-stage anaerobic digestion process. Three different food wastes samples, namely carbohydrate rich, protein rich and lipid rich, were prepared as substrates. Effect of aerobic pre-treatment on hydrogen and methane production was studied. Pre-aeration of substrates showed no positive impact on hydrogen production in the first stage. All three categories of pre-aerated food wastes produced less hydrogen compared to samples without pre-aeration. In the second stage, methane production increased for aerated protein rich and carbohydrate rich samples. In addition, the lag phase for carbohydrate rich substrate was shorter for aerated samples. Aerated protein rich substrate yielded the best results among substrates for methane production, with a cumulative production of approximately 351ml/gVS. With regard to non-aerated substrates, lipid rich was the best substrate for CH₄ production (263ml/gVS). Pre-aerated P substrate was the best in terms of total energy generation which amounted to 9.64kJ/gVS. This study revealed aerobic pre-treatment to be a promising option for use in achieving enhanced substrate conversion efficiencies and CH₄ production in a two-stage AD process, particularly when the substrate contains high amounts of proteins.

Performance evaluation of micro-aerobic hydrolysis of mixed sludge: Optimum aeration and effect on its biochemical methane potential

This study evaluated the performance of a micro-aerobic hydrolysis of mixed sludge and its influence as a pretreatment of this waste for its subsequent anaerobic digestion. Three experimental series were carried out to evaluate the optimum micro-aeration levels in the range from 0.1 to 0.5 air volume/min.reactor volume (vvm) and operation times within the range of 24-60 h. The maximum methane yield [35 mL CH₄/g volatile suspended solids (VSS) added] was obtained for an aeration level of 0.35 vvm. This methane yield value increased 114% with respect to that obtained with the non-aerated sludge. In the micro-aeration process carried out at an aeration level of 0.35 vvm, increases in soluble proteins and total sugars concentrations of 185% and 192% with respect to their initial values were found, respectively, after 48 h of aeration. At the above micro-aerobic conditions, soluble chemical oxygen demand (COD_S) augmented 150%, whereas VSS content decreased until 40% of their initial respective values. Higher COD increases and VSS decreases were found at 60 h of micro-aeration, but the above parameters did not vary significantly with respect to the values found at 48 h.

Oxygen tolerance capacity of upflow anaerobic solid-state (UASS) with anaerobic filter (AF) system

In order to investigate the oxygen tolerance capacity of upflow anaerobic solid-state (UASS) with anaerobic filter (AF) system, the effect of microaeration on thermophilic anaerobic digestion of maize straw was investigated under batch conditions and in the UASS with AF system. Aeration intensities of 0-431mL O2/gvs were conducted as pretreatment under batch conditions. Aeration pretreatment obviously enhanced anaerobic digestion and an aeration intensity of 431mL O2/gvs increased the methane yield by 82.2%. Aeration intensities of 0-355mL O2/gvs were conducted in the process liquor circulation of the UASS with AF system. Dissolved oxygen (DO) of UASS and AF reactors kept around 1.39±0.27 and 0.99±0.38mg/L, respectively. pH was relatively stable around 7.11±0.04. Volatile fatty acids and soluble chemical oxygen demand concentration in UASS reactor were higher than those in AF reactor. Methane yield of the whole system was almost stable at 85±7mL/gvs as aeration intensity increased step by step. The UASS with AF system showed good oxygen tolerance capacity.

Effect of oxygen on the microbial activities of thermophilic anaerobic biomass

Low oxygen levels (μgO2L(-1)) in anaerobic reactors are quite common and no relevant consequences are expected. On the contrary, higher concentrations could affect the process. This work aimed to study the influence of oxygen (4.3 and 8.8mgO2L(-1), respectively) on the different microbial activities (hydrolytic, acidogenic and methanogenic) of thermophilic anaerobic biomass and on the methanogenic community structure. Batch tests in presence of oxygen were conducted using specific substrates for each biological activity and a blank (with minimum oxygen) was included. No effect of oxygen was observed on the hydrolytic and acidogenic activities. In contrast, the methane production rate decreased by 40% in all oxygenated batches and the development of active archaeal community was slower in presence of 8.8mgO2L(-1). However, despite this sensitivity of methanogens to oxygen at saturation levels, the inhibition was reversible.

Review of feedstock pretreatment strategies for improved anaerobic digestion: From lab-scale research to full-scale application

When properly designed, pretreatments may enhance the methane potential and/or anaerobic digestion rate, improving digester performance. This paper aims at providing some guidelines on the most appropriate pretreatments for the main feedstocks of biogas plants. Waste activated sludge was firstly investigated and implemented at full-scale, its thermal pretreatment with steam explosion being most recommended as it increases the methane potential and digestion rate, ensures sludge sanitation and the heat needed is produced on-site. Regarding fatty residues, saponification is preferred for enhancing their solubilisation and bioavailability. In the case of animal by-products, this pretreatment can be optimised to ensure sterilisation, solubilisation and to reduce inhibition linked to long chain fatty acids. With regards to lignocellulosic biomass, the first goal should be delignification, followed by hemicellulose and cellulose hydrolysis, alkali or biological (fungi) pretreatments being most promising. As far as microalgae are concerned, thermal pretreatment seems the most promising technique so far.

Micro-aerobic digestion of high-solid anaerobically digested sludge: further stabilization, microbial dynamics and phytotoxicity reduction

Micro-aerobic digestion was firstly applied for further stabilization and phytotoxicity reduction of high-solid anaerobically digested sludge (ADS) in room temperature, mesophilic and thermophilic conditions.

The chemical properties and microbial community characterization of the thermophilic microaerobic pretreatment process

Thermophilic microaerobic pretreatment (TMP) was recently reported as an efficient pretreatment method of anaerobic digestion (AD). In this study, the chemical properties and microbial community were characterized to reveal how TMP working. Compared with thermophilic treatment under anaerobic condition (TMP0), cellulase activity obviously improved under microaerobic condition (TMP1), which was 10.9-49.0% higher than that of TMP0. Reducing sugar, SCOD and VFAs concentrations of TMP1 were 2.6-8.9%, 1.8-4.8% and 13.8-24% higher than those of TMP0, respectively. TMP gave obvious rise to phylum Firmicutes, which associated with extracellular enzymes production. The proportion of class Bacilli (belongs to phylum Firmicutes and mainly acts during hydrolysis) in TMP1 was 124.89% higher than that of TMP0, which reflected the greater hydrolytic ability under microaerobic condition. The improved abundance of phylum Firmicutes (especially class Bacilli, order Bacillales) under microaerobic condition could be the fundamental reason for the improved AD performance of thermophilic microaerobic pretreated corn straw.

Treatment of petrochemical wastewater by microaerobic hydrolysis and anoxic/oxic processes and analysis of bacterial diversity

Microaerobic hydrolysis-acidification (MHA)-anoxic-oxic (A/O) processes were developed to treat actual petrochemical wastewater. The results showed that the overall COD removal efficiency was 72-79% at HRT=20h, and MHA accounted for 33-42% of COD removal, exhibiting good efficiency of acidogenic fermentation. Ammonium removal was more than 94%. The main pollutants in the influent were identified to be benzene, ketone, alcohols, amine, nitrile and phenols by GC-MS, and the majority of pollutants could be removed by MHA-A/O treatment. Proteobacteria was the most dominant bacteria in the system, accounting for more than 55% of the reads. The predominant genera in MHA, anoxic and oxic reactors were Anaerolineaceae and Sulfuritalea, Lactococcus and Blastocatella, and Saprospiraceae uncultured and Nitrosomonadaceae, respectively. This treatment system exhibited good performance in degrading the complex compounds in the petrochemical wastewater.

The thermophilic (55°C) microaerobic pretreatment of corn straw for anaerobic digestion

Microaerobic process has been proven to be an alternative pretreatment for the anaerobic digestion (AD) process in several studies. In this study, the effect of thermophilic microaerobic pretreatment (TMP) on the AD of corn straw was investigated. Results indicated that TMP process obviously improved the methane yield. The maximum methane yield was obtained at the oxygen loads of 5ml/g VSsubstrate, which was 16.24% higher than that of untreated group. The modified first order equation analysis showed the TMP process not only accelerated the hydrolysis rates but also reduced the lag-phase time of AD process. The structural characterization analysis showed cellulosic structures of corn straw were partly disrupted during TMP process. The crystallinity indexes were also decreased. In addition, large or destroyed pores and substantial structural disruption were observed after pretreatment. The results showed that TMP is an efficient pretreatment method for the AD of corn straw.

Microaeration for hydrogen sulfide removal in UASB reactor

The removal of hydrogen sulfide from biogas by microaeration was studied in Up-flow Anaerobic Sludge Blanket (UASB) reactors treating synthetic brewery wastewater. A fully anaerobic UASB reactor served as a control while air was dosed into a microaerobic UASB reactor (UMSB). After a year of operation, sulfur balance was described in both reactors. In UASB, sulfur was mainly presented in the effluent as sulfide (49%) and in biogas as hydrogen sulfide (34%). In UMSB, 74% of sulfur was detected in the effluent (41% being sulfide and 33% being elemental sulfur), 10% accumulated in headspace as elemental sulfur and 9% escaped in biogas as hydrogen sulfide. The efficiency of hydrogen sulfide removal in UMSB was on average 73%. Microaeration did not cause any decrease in COD removal or methanogenic activity in UMSB and the elemental sulfur produced by microaeration did not accumulate in granular sludge.

Microbial community distribution and extracellular enzyme activities in leach bed reactor treating food waste: effect of different leachate recirculation practices

This study aimed at understanding the relationship between microbial community and extracellular enzyme activities of leach bed reactor (LBR) treating food waste under different leachate recirculation practices (once per day and continuous) and liquid to solid (L/S) ratios (1:1 and 0.5:1). Microbial community analysis using PCR-DGGE revealed that Lactobacillus sp., Bifidobacter sp., and Proteobacteria were the most abundant species. Number of phylotypes was higher in LBRs with intermittent recirculation; whereas, lower number of phylotypes dominated by the key players of degradation was observed with continuous recirculation. The L/S ratio of 1:1 significantly enhanced the volatile solids removal compared with 0.5:1; however, this effect was insignificant under once a day leachate recirculation. Continuous leachate recirculation with 1:1 L/S ratio significantly improved the organic leaching (240 g COD/kgvolatile solid) and showed distinct extracellular enzyme activities suitable for food waste acidogenesis.

Single-phase and two-phase anaerobic digestion of fruit and vegetable waste: comparison of start-up, reactor stability and process performance

Single-phase and two-phase digestion of fruit and vegetable waste were studied to compare reactor start-up, reactor stability and performance (methane yield, volatile solids reduction and energy yield). The single-phase reactor (SPR) was a conventional reactor operated at a low loading rate (maximum of 3.5 kgVS/m3 d), while the two-phase system consisted of an acidification reactor (TPAR) and a methanogenic reactor (TPMR). The TPAR was inoculated with methanogenic sludge similar to the SPR, but was operated with step-wise increase in the loading rate and with total recirculation of reactor solids to convert it into acidification sludge. Before each feeding, part of the sludge from TPAR was centrifuged, the centrifuge liquid (solubilized products) was fed to the TPMR and centrifuged solids were recycled back to the reactor. Single-phase digestion produced a methane yield of 0.45 m3 CH4/kg VS fed and VS removal of 83%. The TPAR shifted to acidification mode at an OLR of 10.0 kgVS/m3 d and then achieved stable performance at 7.0 kgVS/m3 d and pH 5.5-6.2, with very high substrate solubilization rate and a methane yield of 0.30 m3 CH4/kg COD fed. The two-phase process was capable of high VS reduction, but material and energy balance showed that the single-phase process was superior in terms of volumetric methane production and energy yield by 33%. The lower energy yield of the two-phase system was due to the loss of energy during hydrolysis in the TPAR and the deficit in methane production in the TPMR attributed to COD loss due to biomass synthesis and adsorption of hard COD onto the flocs. These results including the complicated operational procedure of the two-phase process and the economic factors suggested that the single-phase process could be the preferred system for FVW.

Optimization of micro-aeration intensity in acidogenic reactor of a two-phase anaerobic digester treating food waste

Micro-aeration is known to promote the activities of hydrolytic exo-enzymes and used as a strategy to improve the hydrolysis of particulate substrate. The effect of different micro-aeration rates, 0, 129, 258, and 387 L-air/kg TS/d (denoted as LBR-AN, LBR-6h, LBR-3h and LBR-2h, respectively) on the solubilization of food waste was evaluated at 35°C in four leach bed reactors (LBR) coupled with methanogenic upflow anaerobic sludge blanket (UASB) reactor. Results indicate that the intensity of micro-aeration influenced the hydrolysis and methane yield. Adequate micro-aeration intensity in LBR-3h and LBR-2h significantly enhanced the carbohydrate and protein hydrolysis by 21-27% and 38-64% respectively. Due to the accelerated acidogenesis, more than 3-fold of acetic acid and butyric acid were produced in LBR-3h as compared to the anaerobic treatment LBR-AN resulting in the maximum methane yield of 0.27 L CH4/g VS(added) in the UASB. The performance of LBR-6h with inadequate aeration was similar to that of LBR-AN with a comparable hydrolysis degree. Nevertheless, higher aeration intensity in LBR-2h was also unfavorable for methane yield due to significant biomass generation and CO2 respiration of up to 18.5% and 32.8% of the total soluble hydrolysate, respectively. To conclude, appropriate micro-aeration rate can promote the hydrolysis of solid organic waste and methane yield without undesirable carbon loss and an aeration intensity of 258 L-air/kg TS/d is recommended for acidogenic LBR treating food waste.

The potential of oxygen to improve the stability of anaerobic reactors during unbalanced conditions: results from a pilot-scale digester treating sewage sludge

A well-functioning pilot reactor treating sewage sludge at approximately 4.4 NL/m(3)/d of oxygen supply and 18d of hydraulic retention time (HRT) was subjected to a hydraulic overload to investigate whether oxygen benefits successful operation in stressful circumstances. Only a mild imbalance was caused, which was overcome without deterioration in the digestion performance. Volatile solids (VS) removal was 45% and 43% at 18 and 14 d of HRT, respectively. Biogas productivity remained around 546 NmL/gVS, but it was slightly higher during the period of imbalance. Thereafter, similar performances were achieved. Under anaerobic conditions, VS removal and biogas productivity were respectively 41% and 525 NmL/gVS, hydrogen partial pressure rose, and acetic acid formation became less favourable. Oxygen seemed to form a more stable digestion system, which meant increased ability to deal successfully with overloads. Additionally, it improved the biogas quality; methane concentration was negligibly lower, while hydrogen sulphide and oxygen remained around 0.02 and 0.03%v/v, respectively.

Enhanced hydrolysis and methane yield by applying microaeration pretreatment to the anaerobic co-digestion of brown water and food waste

Microaeration has been used conventionally for the desulphurization of biogas, and recently it was shown to be an alternative pretreatment to enhance hydrolysis of the anaerobic digestion (AD) process. Previous studies on microaeration pretreatment were limited to the study of substrates with complex organic matter, while little has been reported on its effect on substrates with higher biodegradability such as brown water and food waste. Due to the lack of consistent microaeration intensities, previous studies were not comparable and thus inconclusive in proving the effectiveness of microaeration to the overall AD process. In this study, the role of microaeration pretreatment in the anaerobic co-digestion of brown water and food waste was evaluated in batch-tests. After a 4-day pretreatment with 37.5 mL-O2/L(R)-d added to the liquid phase of the reactor, the methane production of substrates were monitored in anaerobic conditions over the next 40 days. The added oxygen was consumed fully by facultative microorganisms and a reducing environment for organic matter degradation was maintained. Other than higher COD solubilization, microaeration pretreatment led to greater VFA accumulation and the conversion of other short chain fatty acids to acetate. This could be due to enhanced activities of hydrolytic and acidogenic bacteria and the degradation of slowly biodegradable compounds under microaerobic conditions. This study also found that the nature of inoculum influenced the effects of microaeration as a 21% and 10% increase in methane yield was observed when pretreatment was applied to inoculated substrates, and substrates without inoculum, respectively.

Microbial electrolysis cell scale-up for combined wastewater treatment and hydrogen production

This study demonstrates microbial electrolysis cell (MEC) scale-up from a 50mL to a 10L cell. Initially, a 50mL membraneless MEC with a gas diffusion cathode was operated on synthetic wastewater at different organic loads. It was concluded that process scale-up might be best accomplished using a "reactor-in-series" concept. Consequently, 855mL and 10L MECs were built and operated. By optimizing the hydraulic retention time (HRT) of the 855mL MEC and individually controlling the applied voltages of three anodic compartments with a real-time optimization algorithm, a COD removal of 5.7g L(R)(-1)d(-1) and a hydrogen production of 1.0-2.6L L(R)(-1)d(-1) was achieved. Furthermore, a two MECs in series 10L setup was constructed and operated on municipal wastewater. This test showed a COD removal rate of 0.5g L(R)(-1)d(-1), a removal efficiency of 60-76%, and an energy consumption of 0.9Whperg of COD removed.

Anammox: an option for ammonium removal in bioreactor landfills

Experiments carried out in bioreactor landfill simulators demonstrated that more than 40% of the total N was transferred into the liquid and gas phases during the incubation period of 380 days. Ammonium, an end product of protein degradation and important parameter to consider during landfill closure, tends to accumulate up to inhibitory levels in the leachate of landfills especially in landfills with leachate recirculation. Most efforts to remove ammonium from leachate have been focused on ex situ and partial in situ methods such as nitrification, denitrification and chemical precipitation. Besides minimal contributions from other N-removal processes, Anammox (Anaerobic Ammonium Oxidation) bacteria were found to be active within the simulators. Anammox is considered to be an important contributor to remove N from the solid matrix. However, it was unclear how the necessary nitrite for Anammox metabolism was produced. Moreover, little is known about the nature of residual nitrogen in the waste mass and possible mechanisms to remove it. Intrusion of small quantities of O2 is not only beneficial for the degradation process of municipal solid waste (MSW) in bioreactor landfills but also significant for the development of the Anammox bacteria that contributed to the removal of ammonium. Volatilisation and Anammox activity were the main N removal mechanisms in these pilot-scale simulators. The results of these experiments bring new insights on the behaviour, evolution and fate of nitrogen from solid waste and present the first evidence of the existence of Anammox activity in bioreactor landfill simulators.

Effect of microaerobic conditions on the degradation kinetics of cellulose

Limited oxygen supply to sludge digesters has shown to be an effective method to eliminate hydrogen sulfide from the biogas produced during anaerobic digestion but uneven results have been found in terms of the effect on the degradation of complex organic matter. In this study, the effect that the limited oxygen supply provoked on the "anaerobic" degradation of cellulose was evaluated in batch-tests. The microaerobic assays showed to reach a similar maximum production of methane than the anaerobic ones after 19 d and a similar hydrolytic activity (considering a first order rate constant); however, the microaerobic assays presented a shorter lag-phase time than the anaerobic test resulting in faster production of methane during the first steps of the degradation; specifically, the maximum methane production found in the anaerobic test in 19 d was found in the microaerobic test before the day 15.

Electrolysis-enhanced anaerobic digestion of wastewater

This study demonstrates enhanced methane production from wastewater in laboratory-scale anaerobic reactors equipped with electrodes for water electrolysis. The electrodes were installed in the reactor sludge bed and a voltage of 2.8-3.5 V was applied resulting in a continuous supply of oxygen and hydrogen. The oxygen created micro-aerobic conditions, which facilitated hydrolysis of synthetic wastewater and reduced the release of hydrogen sulfide to the biogas. A portion of the hydrogen produced electrolytically escaped to the biogas improving its combustion properties, while another part was converted to methane by hydrogenotrophic methanogens, increasing the net methane production. The presence of oxygen in the biogas was minimized by limiting the applied voltage. At a volumetric energy consumption of 0.2-0.3 Wh/L(R), successful treatment of both low and high strength synthetic wastewaters was demonstrated. Methane production was increased by 10-25% and reactor stability was improved in comparison to a conventional anaerobic reactor.

Anaerobic storage as a pretreatment for enhanced biodegradability of dewatered sewage sludge

Dewatered sewage sludge is often stored still before further processing and final disposal. This study showed that anaerobic storage of dewatered sewage sludge could hydrolyze organic matter from the sludge matrix, and increase soluble organic acid content from 90 to 2400 mg/L and soluble organic carbon content from 220 to 1650 mg/L. Correspondingly, the contents of proteins, celluloses and hemicelluloses were reduced by 2-9%. Applying anaerobic storage markedly enhanced the efficiency of the subsequent bio-drying process on stored sludge. Correspondingly, biogas and odor gas were produced immediately after commencing the sludge storage. Anaerobic storage with odor control can be applied as a pretreatment process for dewatered sewage sludge in wastewater treatment plants.