Anaerobic co-digestion of chicken manure and microalgae Chlorella sp.: Methane potential, microbial diversity and synergistic impact evaluation

Anaerobic co-digestion of microalgal biomass, sugarcane vinasse, and residual glycerol from biodiesel using simplex-centroid mixture design: methane potential, synergic effect, and microbial diversity

Microalgal biomass (MB) is a promising feedstock for bioenergy production. Nonetheless, the cell recalcitrance and the low C/N ratio limit the methane yield during anaerobic digestion. As an alternative to overcome these challenges, MB co-digestion with different feedstocks has been proposed. Thus, this study evaluated the anaerobic co-digestion (AcoD) of MB cultivated in wastewater with sugarcane vinasse (VIN) and residual glycerol from biodiesel production (GLY). Batch tests were conducted using augmented simplex-centroid mixture design to investigate the impact of AcoD on methane production (SMP), synergistic effects, and the influence on microbial community. When compared to MB digestion, 150 NmL CH4.g-1VS, binary and ternary AcoD achieved SMP increases from 120 to 337%. The combination of 16.7:16.7:66.7 (MB:VIN:GLY) showed the highest SMP for a ternary mixture (631 NmL CH4.g-1VS). Optimal synergies ranged from 1.3 to 1.4 and were primarily found for the MB:GLY AcoD. Acetoclastic Methanosaeta genus was predominant, regardless the combination between substrates. Despite the largest SMP obtained from the MB:GLY AcoD, other ternary mixtures were also highly synergetic and therefore had strong potential as a strategic renewable energy source.

Trace elements' deficiency in energy production through methanogenesis process: Focus on the characteristics of organic solid wastes

Excessive or insufficient supplementation of trace elements (TEs) limits the progression of anaerobic digestion. The main reason for this is the lack of sufficient understanding of digestion substrate characteristics, which significantly affects the demand for TEs. In this review, the relationship between TEs requirements and substrate characteristics is discussed. We mainly focus on three aspects. 1) The basis for TE optimization and existing problems: The optimization of TEs often based on the total solids (TS) or volatile solids (VS) of substrates, does not fully consider substrate characteristics. 2) TE deficiency mechanisms for different types of substrates: nitrogen-rich, sulfur-rich, TE-poor, and easily hydrolyzed substrates are the four main types of substrates. The mechanisms underlying TEs deficiency in the different substrates are investigated. 3) Regulation of TE bioavailability: characteristics of substrates affect digestion parameters, which disturb the bioavailability TE. Therefore, methods for regulating bioavailability of TEs are discussed.

The effect of calcium hydroxide addition on enhancing ammonia recovery during thermophilic composting in a self-heated pilot-scale reactor

A modified outdoor large-scale nutrient recycling system was developed to compost organic sludge and aimed to recover clean nitrogen for the cultivation of high-value-added microalgae. This study investigated the effect of calcium hydroxide addition on enhancing NH₃ recovery in a pilot-scale reactor self-heated by metabolic heat of microorganisms during thermophilic composting of dewatered cow dung. 350 kg-ww of compost was prepared at the ratio of 5: 14: 1 (dewatered cowdung: rice husk: compost-seed) in a 4 m³ cylindrical rotary drum composting reactor for 14 days of aerated composting. High compost temperature up to 67 °C was observed from day 1 of composting, proving that thermophilic composting was achieved through the self-heating process. The temperature of compost increases as microbial activity increases and temperature decreases as organic matter decreases. The high CO₂ evolution rate on day 0-2 (0.02-0.08 mol/min) indicated that microorganisms are most active in degrading organic matter. The increasing conversion of carbon demonstrated that organic carbon was degraded by microbial activity and emitted as CO₂. The nitrogen mass balance revealed that adding calcium hydroxide to the compost and increasing the aeration rate on day 3 volatilized 9.83 % of the remaining ammonium ions in the compost, thereby improving the ammonia recovery. Moreover, Geobacillus was found to be the most dominant bacteria under elevated temperature that functions in the hydrolysis of non-dissolved nitrogen for better NH₃ recovery. The presented results show that by thermophilic composting 1 ton-ds of dewatered cowdung for NH₃ recovery, up to 11.54 kg-ds of microalgae can be produced.

Anaerobic co-digestion of thermo-alkaline pretreated microalgae and sewage sludge: Methane potential and microbial community

To improve methane production from sewage sludge (SS), co-digestion of SS and microalgae (MA) was studied and the application of thermo-alkaline pretreatment to MA was evaluated. The results showed that thermo-alkaline pretreatment at 90°C for 120 min on MA was the optimum pretreatment condition. Furthermore, when the volatile solids (VS) ratio of SS and MA was 1:2, the methane yield reached maximum (368.94 mL/g VS). Fourier transform infrared (FT-IR) and thermogravimetric analysis confirmed the synergetic effects of thermo-alkaline pretreated MA on its co-digestion with SS. The analyses of microbial community indicated that Methanobacterium and Methanosarcina were the dominant methanogens during the co-digestion process. However, the relative abundance of Methanosarcina in thermo-alkaline pretreated groups was higher compared to unpretreated groups. The microbial community structure might be affected by thermo-alkaline pretreatment rather than by the MA dosage in the co-digestion.

Enhanced anaerobic digestion of post-hydrothermal liquefaction wastewater: Bio-methane production, carbon distribution and microbial metabolism

Hydrothermal liquefaction (HTL) is a cost-effective and environment-friendly technology for using biomass to produce bio-crude oil. The critical challenge of HTL is its complicated aqueous product containing high concentrations of organics and diverse toxicants. This paper reports the continuous anaerobic digestion of raw and zeolite-adsorbed Chlorella HTL wastewater using up-flow anaerobic sludge bed reactors. The bio-methane production capacity, total carbon distribution and microbial response were investigated. The anaerobic process was severely suppressed when more than 20% raw wastewater was fed; while it showed essentially improved performance till 60% pre-treated wastewater was added. Produced methane contained 17.3% of the total carbon in feedstock, which was comparable with the value (16.7%) when 25% of raw wastewater was added. The metagenomic analysis revealed distinct microbial community structures in different stages and feedstock shifts. The abundance of functional genes was consistent with anaerobic digester performance.

Semi-solid state promotes the methane production during anaerobic co-digestion of chicken manure with corn straw comparison to wet and high-solid state

Total solid content (TS) is an important factor for biogas production during anaerobic digestion. In this study, we explored the influence of different TS (5% wet, 15% semi-solid and 25% solid state) on the relative cumulative methane production (RCMP) during anaerobic co-digestion of chicken manure with corn straw. Results showed that total ammonium nitrogen and free ammonia nitrogen concentration increased with the increase of TS. Ammonium nitrogen in treatments at 15% TS was 2.25-2.76 times as high as that at 5% TS, which was below 3 times. The highest chemical oxygen demand removal and RCMP were obtained in the treatment of 15% TS with a ratio of 2:1 chicken manure: corn straw (based on TS). The RCMP in the treatments of 15% TS were 3.63-4.59 times higher than that of 5% TS based on the volume of substrates. The abundance of Caldicoprobacter improving the degradation of corn straw was significantly positively correlated with the RCMP, and the average abundance of Caldicoprobacter at 15% TS was 8.33 and 7.02 times higher than that at 5% and 25% TS, respectively. Structural equation models analysis suggested that TS significantly impacted the RCMP by indirectly impacting free ammonia nitrogen and microbial abundance. These findings indicated semi-solid state (15% TS) decreased ammonia nitrogen releasing and improved the abundance of Caldicoprobacter, and increased RCMP during anaerobic co-digestion of chicken manure with corn straw.

Anaerobic Digestion of Chicken Manure in the Presence of Magnetite, Granular Activated Carbon, and Biochar: Operation of Anaerobic Reactors and Microbial Community Structure

The influence of magnetite nanoparticles, granular activated carbon (GAC), and biochar, as well as their combinations on the anaerobic digestion of chicken manure and the structure of microbial communities was studied. The addition of magnetite, GAC, and biochar increased the rate of methane production and the total methane yield. It has been observed that these additives stimulated anaerobic microorganisms to reduce the concentration of accumulated volatile organic acids. Various bacterial species within the classes Bacteroidia and Clostridia were found at higher levels in the anaerobic reactors but in different proportions depending on the experiment. Members of the genera Methanosarcina, Methanobacterium, Methanothrix, and Methanoculleus were mainly detected within the archaeal communities in the anaerobic reactors. Compared to the 16S rRNA gene-based study, the mcrA gene approach allowed a higher level of Methanosarcina in the system with GAC + magnetite to be detected. Based on our findings, the combined use of granular activated carbon and magnetite at appropriate dosages will improve biomethane production.

Biochar assisted cellulose anaerobic digestion under the inhibition of dodecyl dimethyl benzyl ammonium chloride: Dose-response kinetic assays, performance variation, potential promotion mechanisms

This study evaluated the inhibitory effect and mitigation strategy of dodecyl dimethyl benzyl ammonium chloride (DDBAC) suppression on anaerobic digestion. With the 12 h-suppression, only 16.64% of anaerobes were alive, and acetotrophic methanogens were significantly inhibited. As for batch test, DDBAC suppression significantly prolonged the start-up of systems and decreased the biogas production. In cellulose semi-continuous digestion process, the DDBAC suppression induced volatile fatty acids accumulation and pH decrease. However, the biochar amended reactor effectively mitigated the DDBAC suppression and achieved 370.5 mL/d·g-chemical-oxygen-demand biogas production. Moreover, 17.8% more protein in extracellular polymeric substances was secreted as the bio-barrier to defense the DDBAC suppression. Furthermore, microbial analysis showed that biochar addition selectively enriched directed interspecies electron transfer (DIET) participant bacteria (Anaerolineaceae and Syntrophomonas) and methanogens (Methanosaeta and Methanobacterium). Meanwhile, the potential metabolic pathway analysis showed that the abundance of amino acids and energy metabolism were increased 28% and 8%, respectively. The abundance of encoding enzyme related to hydrogenotrophic and acetotrophic methanogenesis enriched 1.88 times and 1.48 times, respectively. These results showed the performance and mechanisms involved in DIET establishment with ethanol stimulation biochar addition.

Sustainable microalgal biomass valorization to bioenergy: Key challenges and future perspectives

The global trend is shifting toward circular economy systems. It is a sustainable environmental approach that sustains economic growth from the use of resources while minimizing environmental impacts. The multiple industrial use of microalgal biomass has received great attention due to its high content of essential nutrients and elements. Nevertheless, low biomass productivity, unbalanced carbon to nitrogen (C/N) ratio, resistant cellular constituents, and the high cost of microalgal harvesting represent the major obstacles for valorization of algal biomass. In recent years, microalgae biomass has been a candidate as a potential feedstock for different bioenergy generation processes with simultaneous treating wastewater and CO₂ capture. An overview of the appealing features and needed advancements is urgently essential for microalgae-derived bioenergy generation. The present review provides a timely outlook and evaluation of biomethane production from microalgal biomass and related challenges. Moreover, the biogas recovery potential from microalgal biomass through different pretreatments and synergistic anaerobic co-digestion (AcoD) with other biowastes are evaluated. In addition, the removal of micropollutants and heavy metals by microalgal cells via adsorption and bioaccumulation in their biomass is discussed. Herein, a comprehensive review is presented about a successive high-throughput for anaerobic digestion (AD) of the microalgal biomass in order to achieve for sustainable energy source. Lastly, the valorization of the digestate from AD of microalgae for agricultural reuse is highlighted.

Realising the Potential of Renewable Energy as a Tool for Energy Security in Small Island Developing States

Small Island Developing States (SIDS) are heavily dependent on the use of imported fossil fuels to address their energy needs. This has a negative impact on the environment, SIDS budgets, and energy security. Therefore, this study aimed to investigate the role of renewable energy (RE) as a tool for energy security in SIDS. In this regard, using VOSviewer, a widely known software tool, two bibliometric analyses were performed with a focus on the literature that explores two intertwined issues: (i) the links between RE and energy security; and (ii) the implications of RE and energy security in SIDS. The results from the study show that RE can help SIDS enhance their energy security and assure long-term energy sustainability. In addition, the results show that with the reduction in the cost of batteries in the future, they will eventually replace diesel generators. Moreover, the study showed that renewable energy may assist SIDS in their long-term efforts to achieve food security. The analysis discusses the major obstacles and the potential solutions for the integration of RES into the energy generation of SIDS. The paper concludes with useful recommendations to help island nations reduce their carbon footprint.

Microalgae-based livestock wastewater treatment (MbWT) as a circular bioeconomy approach: Enhancement of biomass productivity, pollutant removal and high-value compound production

The intensive livestock activities that are carried out worldwide to feed the growing human population have led to significant environmental problems, such as soil degradation, surface and groundwater pollution. Livestock wastewater (LW) contains high loads of organic matter, nitrogen (N) and phosphorus (P). These compounds can promote cultural eutrophication of water bodies and pose environmental and human hazards. Therefore, humanity faces an enormous challenge to adequately treat LW and avoid the overexploitation of natural resources. This can be accomplished through circular bioeconomy approaches, which aim to achieve sustainable production using biological resources, such as LW, as feedstock. Circular bioeconomy uses innovative processes to produce biomaterials and bioenergy, while lowering the consumption of virgin resources. Microalgae-based wastewater treatment (MbWT) has recently received special attention due to its low energy demand, the robust capacity of microalgae to grow under different environmental conditions and the possibility to recover and transform wastewater nutrients into highly valuable bioactive compounds. Some of the high-value products that may be obtained through MbWT are biomass and pigments for human food and animal feed, nutraceuticals, biofuels, polyunsaturated fatty acids, carotenoids, phycobiliproteins and fertilizers. This article reviews recent advances in MbWT of LW (including swine, cattle and poultry wastewater). Additionally, the most significant factors affecting nutrient removal and biomass productivity in MbWT are addressed, including: (1) microbiological aspects, such as the microalgae strain used for MbWT and the interactions between microbial populations; (2) physical parameters, such as temperature, light intensity and photoperiods; and (3) chemical parameters, such as the C/N ratio, pH and the presence of inhibitory compounds. Finally, different strategies to enhance nutrient removal and biomass productivity, such as acclimation, UV mutagenesis and multiple microalgae culture stages (including monocultures and multicultures) are discussed.

Influence of augmentation of biochar during anaerobic co-digestion of Chlorella vulgaris and cellulose

The anaerobic co-digestion (AcoD) of microalgae is a prospective option for generating biomethane from renewable sources. This study investigates the effects of inoculum-to-substrate ratio (ISR), C/N ratio and biochar (BC) load on the AcoD of Chlorella vulgaris and cellulose. An initial augmentation of BC at ISR 0.5-0.9 and C/N ratio 10-30 offered a pH buffering effect and resulted in biomethane yields of 233-241 mL CH₄/g VS, corresponding to 1.8-4.6 times the controls. BC addition ameliorated significantly AcoD, supporting the digestate stability at less favourable conditions. The effect of the process variables was further studied with a 2³ factorial design and response optimisation. Under the design conditions, the variables had less influence over methane production. Higher ISRs and C/N ratios favoured AcoD, whereas increasing amounts of BC reduced biomethane yield but enhanced production rate. The factorial design highlighted the importance of BC-load on AcoD, establishing an optimum of 0.58 % (w/v).

Protein biomethanation: insight into the microbial nexus

Biomethanation of carbohydrates (e.g., lignocellulosic biomass) and lipids (e.g., waste oils) has been well studied. However, investigations on the biomethanation of protein-rich biowastes (PRBs) and associated microbial communities have not been reported. This review summarizes the challenges in the metabolic process of anaerobic digestion of PRBs and the microbial instability associated with it. We discuss the diversity of bacterial and archaeal communities via metagenomics under PRB mono- and codigestion. A stable community structure with enhanced metabolic activity is a core factor in PRB biomethanation. The application of strategies such as codigestion of PRBs with carbon-rich biomass and microbial stimulation/augmentation would make PRB biomethanation more feasible.

A sustainable approach by using microalgae to minimize the eutrophication process of Mar Menor lagoon

The present study evaluates the removal capacity of microalgae photobioreactors of environmental pollutants present in wastewater from the dry riverbed El Albujón, as a way to minimize the eutrophication process of the Mar Menor. Particularly, the capacity of four autochthonous microalgae consortia collected from different locations of the salty lagoon to remove emerging contaminants (simazine, atrazine, terbuthylazine, adenosine and ibuprofen), nitrates, and phosphates, was evaluated. Among the four microalgae consortia, consortium 1 was the best in terms of biomass productivity (0.11 g L^-1 d^-1) and specific growth rate (0.14 d^-1), providing 100% removal of emerging contaminants (simazine, atrazine, terbuthylazine, adenosine and ibuprofen), and a maximal reduction and consumption of macronutrients, especially nitrates and phosphates, reaching levels below 28 mg L^-1, that is, a decrease of 89.90 and 99.70% of nitrates and phosphates, respectively. Therefore, this consortium (Monoraphidium sp., Desmodesmus subspicatus, Nannochloris sp.) could be selected as a green filter for successful large-scale applications. This study is the first one that combines the successful removal of herbicides, ibuprofen and adenosine as emerging contaminants, and nitrate removal.

Obstacles faced by methanogenic archaea originating from substrate-driven toxicants in anaerobic digestion

Anaerobic digestion (AD) is used to treat waste and produce bioenergy. However, toxicants, which originate from the substrate, can inhibit or damage the digestion process. Methanogenic archaea (MA), which are the executor in the methanogenesis stage, are more sensitive than bacteria to these toxicants. This review discusses the effects of substrate-driven toxicants, namely, antibiotics, H₂S and sulfate, heavy metals (HMs), long-chain fatty acids (LCFAs), and ammonia nitrogen, on the activity of MAs, methanogenic pathways, and the inter-genus succession of MAs. The adverse effects of these five toxicants on MA include effects on pH, damages to cell membranes, the prevention of protein synthesis, changes in hydrogen partial pressure, a reduction in the bioavailability of trace elements, and hindrance of mass transfer. These effects cause a reduction in MA activity and the succession of MAs and methanogenic pathways, which affect AD performance. Under the stress of these toxicants, succession occurs among HA (hydrogenotrophic methanogen), AA (acetoclastic methanogen), and MM (methylotrophic methanogen), especially HA gradually replaces AA as the dominant MA. Simultaneously, the dominant methanogenic pathway also changes from the aceticlastic pathway to other methanogenic pathways. A comprehensive understanding of the impact of toxicants on MA permits more specific targeting when developing strategies to mitigate or eliminate the effects of these toxicants.

Anaerobic co-digestion of chicken manure and cardboard waste: Focusing on methane production, microbial community analysis and energy evaluation

This study aimed to investigate the synergistic effect and microbial community changes between chicken manure (CM) and cardboard (CB) during anaerobic co-digestion. Meanwhile, the energy balance of biogas engineering was extrapolated based on the batch tests. In batch tests, co-digestion system achieved the highest improvement (14.2%) and produced 319.62 mL CH₄/gVS with a 65:35 ratio of CB: CM. More extracellular polymeric substance secretion promoted the electron transfer for acidogenesis and more hydrolase was provided with 31.6% improvement. The microbial analysis illustrated that higher acetoclastic Methanosaeta abundance was achieved, leading to 211% enhancement of acetoclastic pathway. Moreover, associated network illustrated that the higher methane production was mainly achieved through matching of hydrolytic bacteria and acidogenesis bacteria. As for energy balance, the synergistic effect increased the energy output by 38% and energy recovery to 46.4%.

Stimulating methane production from microalgae by alkaline pretreatment and co-digestion with sludge

Methane production through anaerobic digestion (AD) is a solution of energy recovery from microalgae, but some features of microalgae limit the efficiency of AD. In this study, alkaline pretreatment and co-digestion with sludge were both applied to enhance the methane production from microalgae in batch experiments. The results showed that alkaline pretreatment increased the disintegration degree of microalgae from 20% to 34% at maximum after 12-h treatment, but the specific methane production (SMP) only increased from 279 to 298 ml/g volatile solids (VS). Co-digestion with sludge stimulated methane production, and the best synergy with an SMP of 343 ml/g VS occurred when the ratio of microalgae to sludge reached 2:1 based on their VS. The yield was 12.4% and 20.0% higher than those from mono digestion of microalgae and sludge, respectively, and the synergy was evaluated at 14.8%. Therefore, co-digestion is a better choice for improving methane production from microalgae.

The mechanisms of biogenic methane metabolism by synergistic biodegradation of coal and corn straw

The mechanisms associated with the biomethane metabolism through the synergistic biodegradation of both coal and corn straw were explored to improve the utilization rate of corn straw. This applies to the filling of the goaf with corn straw and the production of biomethane using indigenous bacteria in the mine water with coal. The results showed that new macromolecular substances (e.g., Tetracosane and Pentacosane) were produced on the third day. A lower coal rank leads to a lower biodegradation rate of low-molecular-weight substances (e.g., butyric acid and valeric acid). Under the addition of coal samples, the biodegradation rate of cellulose, hemicellulose and lignin in corn straw could reached up to 29.82%, 35.79% and 6.16%, respectively. The addition of corn straw promoted the complementary advantages of archaeal genera (such as Methanosarina and Methanospirillum) and decreased the adverse bacterial genera (such as Desulfovibrio and Pseudomonas) in the fermentation system of single coal.

Unveiling microbial structures during raw microalgae digestion and co-digestion with primary sludge to produce biogas using semi-continuous AnMBR systems

Methane production from microalgae can be enhanced through anaerobic co-digestion with carbon-rich substrates and thus mitigate the inhibition risk associated with its low C:N ratio. Acclimated microbial communities for microalgae disruption can be used as a source of natural enzymes in bioenergy production. However, co-substrates with a certain microbial diversity such as primary sludge might shift the microbial structure. Substrates were generated in a Water Resource Recovery Facility (WRRF) and combined as follows: Scenedesmus or Chlorella digestion and microalgae co-digestion with primary sludge. The study was performed using two lab-scale Anaerobic Membrane Bioreactors (AnMBR). During three years, different feedstocks scenarios for methane production were evaluated with a special focus on the microbial diversity of the AnMBR. 57% of the population was shared between the different feedstock scenarios, revealing the importance of Anaerolineaceae members besides Smithella and Methanosaeta genera. The addition of primary sludge enhanced the microbial diversity of the system during both Chlorella and Scenedesmus co-digestion and promoted different microbial structures. Aceticlastic methanogen Methanosaeta was dominant in all the feedstock scenarios. A more remarkable role of syntrophic fatty acid degraders (Smithella, Syntrophobacteraceae) was observed during co-digestion when only microalgae were digested. However, no significant changes were observed in the microbial composition during anaerobic microalgae digestion when feeding only Chlorella or Scenedesmus. This is the first work revealing the composition of complex communities for semi-continuous bioenergy production from WRRF streams. The stability and maintenance of a microbial core over-time in semi-continuous AnMBRs is here shown supporting their future application in full-scale systems for raw microalgae digestion or co-digestion.

Co-digestion of microalgae with potato processing waste and glycerol: effect of glycerol addition on methane production and the microbial community

The production of methane-rich biogas from the anaerobic digestion (AD) of microalgae is limited by an unfavorable biomass carbon-to-nitrogen (C/N) ratio; however, this may be ameliorated using a co-digestion strategy with carbon-rich feedstocks. For reliable plant operation, and to improve the economics of the process, secure co-feedstock supply (ideally as a waste-stream) is important. To this end, this study investigated the feasibility of co-digesting microalgae (Chlorella vulgaris) with potato processing waste (potato discarded parts, PPW_dp; potato peel, PPW_p) and glycerol, while monitoring the response of the methanogenic community. In this semi-continuous study, glycerol (1 and 2% v/v) added to mixtures of C. vulgaris : PPW_dp enhanced the specific methane yields the most, by 53–128%, whilst co-digestion with mixtures of C. vulgaris : PPW_p enhanced the methane yields by 62–74%. The microbial communities diverged markedly over operational time, and to a lesser extent in response to glycerol addition. The acetoclast Methanosaeta was abundant in all treatments but was replaced by Methanosarcina in the potato peel with glycerol treatment due to volatile fatty acid (VFA) accumulation. Our findings demonstrate that the performance of microalgae co-digestion is substantially improved by the addition of glycerol as an additional co-feedstock. This should improve the economic case for anaerobically digesting microalgae as part of wastewater treatment processes and/or the terminal step of a microalgae biorefinery.

Glycerol as an additional co-substrate enhanced methane yields by up to 128% when co-digestion with microalgae and potato waste.

Hydrothermal treatment of Chlorella sp.: Influence on biochemical methane potential, microbial function and biochemical metabolism

This study focused on the effect of hydrothermal treatment (HTT) on biochemical methane potential (BMP) of Chlorella sp. The BMP was in the range of 119.16-485.90 mLCH₄/gVS, and increased by 80.31%-210.16% after HTT, while reduced 23.94% at hydrothermal treatment severity (HTS) 5.21. The cell wall was more greatly disrupted with increasing HTS, accompanied with the increase of volatile fatty acids (VFAs) and fermentation inhibitors (5-HMF and more complex chemical compositions) recoveries. The reducing sugar yields were 0.94-3.65% and obtained its maximum at a retention time of 30 min. Illumina MiSeq sequencing clarified that, the phylum Chloroflexi with functions of hydrolysis and acidogenesis, decreased with increasing HTS. The family Methanosaetaceae belonging to acetoclastic methanogens, had an unexpected decrease at HTS 5.21. As the response, VFAs concentration was less than 1 g/L after biochemical metabolism, while high concentrations of VFAs and inhibitors at HTS 5.21 led to the poor performance.

Revealing the correlation of biomethane generation, DOM fluorescence, and microbial community in the mesophilic co-digestion of chicken manure and sheep manure at different mixture ratio

Batch co-digestion tests of chicken manure (CM) and sheep manure (SM) at different ratio (R_s/c) were conducted under mesophilic condition (35 °C). Batch kinetic analysis of bioCH₄ production, excitation-emission matrix (EEM) fluorescence of dissolved organic matter (DOM), and microbial community were investigated. The well-fitted modified Gompertz model (R², 0.98-0.99) resulted that the co-digestion markedly improved the methane production rate and shortened the lag phase time. The highest bioCH₄ yield of 219.67 mL/gVS_add and maximum production rate of 0.378 mL/gVS_add/h were obtained at an optimum R_s/c of 0.4. Additionally, a significant variation of DOM was detected at the R_s/c of 0.4 with a consistent degradation of soluble microbial byproduct-like and protein-like organics. The positive synergy effects of co-digestion conspicuously enhanced the bioCH₄ production efficiency. FI₃₇₀ and NADH were significantly correlated to R_s/c (p < 0.05). Moreover, the correlations among process indicator, EEM-peaks and different environmental parameters were evaluated by Pearson correlation analysis. The high diversity of acetoclastic methanogens and hydrogenotrophic methanogens in the co-digestion improved the stability of process. Graphical Abstract.

Early warning indicators for mesophilic anaerobic digestion of corn stalk: a combined experimental and simulation approach

BACKGROUND

Monitoring and providing early warning are essential operations in the anaerobic digestion (AD) process. However, there are still several challenges for identifying the early warning indicators and their thresholds. One particular challenge is that proposed strategies are only valid under certain conditions. Another is the feasibility and universality of the detailed threshold values obtained from different AD systems. In this article, we report a novel strategy for identifying early warning indicators and defining threshold values via a combined experimental and simulation approach.

RESULTS

The AD of corn stalk (CS) was conducted using mesophilic, completely stirred anaerobic reactors. Two overload modes (organic and hydraulic) and overload types (sudden and gradual) were applied in order to identify early warning indicators of the process and determine their threshold values. To verify the selection of experimental indicators, a combined experimental and simulation approach was adopted, using a modified anaerobic bioconversion mathematical model (BioModel). Results revealed that the model simulations agreed well with the experimental data. Furthermore, the ratio of intermediate alkalinity to bicarbonate alkalinity (IA/BA) and volatile fatty acids (VFAs) were selected as the most potent early warning indicators, with warning times of 7 days and 5-8 days, respectively. In addition, IA, BA, and VFA/BA were identified as potential auxiliary indicators for diagnosing imbalances in the AD system. The relative variations for indicators based on that of steady state were observed instead of the absolute threshold values, which make the early warning more feasible and universal.

CONCLUSION

The strategy of a combined approach presented that the model is promising tool for selecting and monitoring early warning indicators in various corn stalk AD scenarios. This study may offer insight into industrial application of early warning in AD system with mathematical model.

Improved methane production and energy recovery of post-hydrothermal liquefaction waste water via integration of zeolite adsorption and anaerobic digestion

Hydrothermal liquefaction (HTL) is a promising technology for converting organic wastes into bio-crude oil, with organic-rich post-hydrothermal liquefaction wastewater (PHWW) as by-product. In this study, zeolite adsorption and anaerobic digestion (AD) were integrated to improve the methane production and energy recovery of PHWW from Chlorella 1067. A statistical design for maximum toxicants removal by zeolite was applied before AD process. Zeolite could mitigate the inhibition associated to compounds such as ammonia, N-heterocyclic compounds, etc. in PHWW and thereby shortening the lag phase and increasing methane production by 32-117% compared with that without zeolite adsorption. Zeolite adsorption also increased energy recovery efficiency (up to 70.5%) for this integrated system. Integration of HTL and AD brought higher energetic return from feedstock via oil and biomethane production, which may offer insight into industrial application of microalgae biomass in the circular economy. In addition, carbon and nitrogen flow for the integrated process was determined.

Identification of parameters needed for optimal anaerobic co-digestion of chicken manure and corn stover

While studies have shown that anaerobic co-digestion of chicken manure (CM) and corn stover (CS) is an efficient method to treat these agricultural wastes, the microbial ecology of these systems and optimal parameters for the digestion process are yet to be determined. In this study, the effects of different initial substrate concentrations and CS : CM mixture ratios on co-digestion and microbial community structure were evaluated. Results demonstrated that both the highest cumulative methane yields and methane production rates were obtained from reactors with a CS : CM ratio of 1 : 1 during hemi-solid-state anaerobic digestion (HSS-AD). Cumulative methane yields and methane production rates were 24.8% and 42% lower in solid-state anaerobic digestion (SS-AD) reactors using the same CS : CM ratios. Analysis of microbial community structures revealed that cellulolytic bacteria and a diversity of syntrophic microorganisms capable of direct interspecies electron transfer (DIET) and hydrogen interspecies transfer (HIT) were enriched in the best-performing reactors. Methanosarcina species also dominated during HSS-AD, and their presence was positively correlated with methane production in the reactors.

The effects of different initial substrate concentrations and CS : CM mixture ratios on co-digestion performance and microbial community structure were evaluated in this study.

The auto fluorescence characteristics, specific activity, and microbial community structure in batch tests of mono-chicken manure digestion

Batch tests inoculated with granular and suspended sludge of mono chicken manure (CM) digestion were conducted. Kinetic analysis showed a maximum bio-CH₄ generation (6 mL/gVS/d) at an optimal TS of 10-12%. At a TS of 25%, serious inhibition was found for granular sludge and even greater inhibition for the suspended sludge caused by free ammonia. The auto fluorescence of Excitation-mission matrix with parallel factor analysis (PARAFAC) showed that the dissolved organic matter (DOM) varied between the form C1, C2, C3 and C4. The split component of the SMP-like C2 and protein-like C4 significantly related to the bio-methane production in time series. The canonical correlation analysis (CCA) indicated that ammonia, pH, and TS influenced the PARAFAC component significantly. The aceticlastic methanogens of the genus Methanosaeta and acetogens of the genus Syntrophobacter predominated in the CM sludge. The methanogens and acetogens formed a metabolic cooperation, making the process a stable methane produced activity.

Biomethanation potential for co-digestion of municipal solid waste and rice straw: A batch study

Rice straw (RS) contains a high amount of lignocellulosic materials which are difficult to degrade without thermal pretreatment. In the present study, co-digestion of municipal solid waste (MSW) and RS was carried out in three different ratios i.e., 1:1, 2:1, and 3:1 to get the maximum biomethanation potential and methane generation rate constant (k). The biogas and methane (CH₄) potential increased by 60% and 57%, respectively for MSW and RS in the ratio 2:1 as compared to other combination. The values of k, biochemical methane potential (µ_b) and sludge activity were measured as 0.1 d^-1, 0.99 CH₄-COD/COD_fed and 0.50 g CH₄-COD/g VSS, respectively. The sludge activity was found to be 100% for 2:1 ratio. Co-digestion of RS with MSW can also optimize the C/N ratio which is an essential parameter in the anaerobic digestion process.

Co-digestion of chicken manure and microalgae Chlorella 1067 grown in the recycled digestate: Nutrients reuse and biogas enhancement

The present investigation targeted on a sustainable co-digestion system: microalgae Chlorella 1067 (Ch. 1067) was cultivated in chicken manure (CM) based digestate and then algae biomass was used as co-substrate for anaerobic digestion with CM. About 91% of the total nitrogen and 86% of the soluble organics in the digestate were recycled after the microalgae cultivation. The methane potential of co-digestion was evaluated by varying CM to Ch. 1067 ratios (0:10, 2:8, 4:6, 6:4, 8:2, 10:0 based on the volatile solids (VS)). All the co-digestion trials showed higher methane production than the calculated values, indicating synergy between the two substrates. Modified Gompertz model showed that co-digestion had more effective methane production rate and shorter lag phase. Co-digestion (8:2) achieved the highest methane production of 238.71mL⋅(g VS)^-1 and the most significant synergistic effect. The co-digestion (e.g. 8:2) presented higher and balanced content of dominant acidogenic bacteria (Firmicutes, Bacteroidetes, Proteobacterias and Spirochaetae). In addition, the archaea community Methanosaeta presented higher content than Methanosarcina, which accounted for the higher methane production. These findings indicated that the system could provide a practicable strategy for effectively recycling digestate and enhancing biogas production simultaneously.