Anaerobic co-digestion of biodiesel waste glycerin with municipal wastewater sludge: microbial community structure dynamics and reactor performance

Long-term impact of nano zero-valent iron on methanogenic activity, microbial community structure, and transcription activity in anaerobic wastewater treatment system

Nano zero-valent iron (NZVI) is commonly used in industrial wastewater treatment. However, its long-term impact mechanisms of metabolization in anaerobic systems are not well understood. This study investigated the effects of long-term and continuous addition of NZVI on methanogenic activity, microbial community, and transcription activity. The results demonstrated that low levels of NZVI (1000 mg/L) induced inhibition of methanogenesis after 80 days, while high levels of NZVI (5000 mg/L) immediately led to a sharp decrease of cumulative methane production and chemical oxygen demand removal, which arrived at a steady state (14.4 % of control and 17 %) after 30 days. NZVI adversely affected cell viability, adenosine triphosphate production, and fatty acid evolution of cell membranes played a crucial role in resisting chronic NZVI toxicity. Moreover, high NZVI levels hindered the transcription of key enzymes CoM and mcrA, while low NZVI levels maintained its high CoM and mcrA activity, but down-regulated the transcription of cdh and hdr. Besides, amino-utilizing bacteria was reduced under the high NZVI concentration, while low NZVI changed dominant genus with potential protein hydrolysis function from Candidatus Cloacamonas to Sedimentibacter. These results provide a guideline for proper NZVI utilization in wastewater treatment.

Two problems in one shot: Vinasse and glycerol co-digestion in a thermophilic high-rate reactor to improve process stability even at high sulfate concentrations

Anaerobic co-digestion (AcoD) of sugarcane vinasse and glycerol can be profitable because of the destination of two biofuel wastes produced in large quantities in Brazil (ethanol and biodiesel, respectively) and the complementary properties of these substrates. Thus, the objective of this study was to assess the effect of increasing the organic loading rate (OLR) from 2 to 20 kg COD m^-3 d^-1 on the AcoD of vinasse and glycerol (50 %:50 % on a COD basis) in a thermophilic (55 °C) anaerobic fluidized bed reactor (AFBR). The highest methane production rate was observed at 20 kg COD m^-3 d^-1 (8.83 L CH₄ d^-1 L^-1), while the methane yield remained stable at around 265 NmL CH₄ g^-1 COD_rem in all conditions, even when influent vinasse reached 1811 mg SO₄^2- L^-1 (10 kg COD m^-3 d^-1). Sulfate was not detected in the effluent. Bacterial genera related to sulfate removal, such as Desulfovibrio and Desulfomicrobium, were observed by means of shotgun metagenomic sequencing at 10 kg COD m^-3 d^-1, as well as the acetoclastic archaea Methanosaeta and prevalence of genes encoding enzymes related to acetoclastic methanogenesis. It was concluded that process efficiency and methane production occurred even in higher sulfate concentrations due to glycerol addition.

The Role of Slurry Reflux in a Corn Stalk Continuous Anaerobic Digestion System: Performance and Microbial Community

Slurry reflux is a low-cost slurry reduction technology, which can solve the problem that a large amount of slurry cannot be completely consumed in a biogas plant. Anaerobic digestion (AD) of corn stalks with slurry reflux and non-reflux was compared and evaluated in continuous anaerobic digestion to clarify the effects of slurry reflux on AD with organic loading rate (OLR) variation. It was found that slurry reflux increased cumulative methane production and improved system stability. The average methane yield of the slurry reflux group was 224.19 mL/gVS, which was 41.35% higher than that of the non-reflux group. High-throughput sequencing results showed that slurry reflux increased the microbial community richness. The dominant microorganisms in the reflux group were in phylum Bacteroidetes, which have the capacity to degrade polymers, and Methanothrix, which is an aceticlastic methanogen. The relative abundances of Bacteroidetes and Methanothrix were 32.41% and 41.75%, respectively. Clostridium III and Saccharofermentans, which are related to syntrophic acetate oxidation and hydrolysis, were increased in relative abundance in the slurry reflux system. The increase of the OLR altered the main methane-producing pathway from the acetoclastic methanogenic pathway to the hydrogenotrophic methanogenic pathway in the AD system, and the slurry reflux can delay this trend. This study provided an effective way for the reduction and utilization of slurry in a biogas plant.

A Review of Basic Bioinformatic Techniques for Microbial Community Analysis in an Anaerobic Digester

Biogas production involves various types of intricate microbial populations in an anaerobic digester (AD). To understand the anaerobic digestion system better, a broad-based study must be conducted on the microbial population. Deep understanding of the complete metagenomics including microbial structure, functional gene form, similarity/differences, and relationships between metabolic pathways and product formation, could aid in optimization and enhancement of AD processes. With advancements in technologies for metagenomic sequencing, for example, next generation sequencing and high-throughput sequencing, have revolutionized the study of microbial dynamics in anaerobic digestion. This review includes a brief introduction to the basic process of metagenomics research and includes a detailed summary of the various bioinformatics approaches, viz., total investigation of data obtained from microbial communities using bioinformatics methods to expose metagenomics characterization. This includes (1) methods of DNA isolation and sequencing, (2) investigation of anaerobic microbial communities using bioinformatics techniques, (3) application of the analysis of anaerobic microbial community and biogas production, and (4) restriction and prediction of bioinformatics analysis on microbial metagenomics. The review has been concluded, giving a summarized insight into bioinformatic tools and also promoting the future prospects of integrating humungous data with artificial intelligence and neural network software.

Microbiota profile in mesophilic biodigestion of sugarcane vinasse in batch reactors

The vinasse is a residue of ethanol production with the potential for methane production, requiring an allochthonous inoculum. Several microorganisms act in the different phases of anaerobic digestion, and the identification of these microbial communities is essential to optimize the process. The characterization of the microbiota involved in the biodigestion of vinasse was observed in the initial stage (IS), at the peak of methane production (MS) and the end of the process (FS) of the best performance assay by high-throughput sequencing. The highest methane production was 0.78 mmolCH₄.gVS.h^-1 at 243.7 h in the substrate/inoculum ratio of 1.7, with consumption partial of acetic, propionic and isobutyric acids and an 82% reduction of chemical oxygen demand. High microbial diversity was found. The genera Clostridium, Acinetobacter, Candidatus Cloacamonas, Bacteroides, Syntrophomonas, Kosmotoga, the family Porphyromonadaceae and the class Bacteroidia were the most abundant in the maximum methane production. Methane production was driven by Methanobacterium and Methanosaeta, suggesting the metabolic pathways used were hydrogenotrophic and acetoclastic.

Stress induced by crude glycerol in a thermophilic digester: microbial community divergence and resilience, but slow process recovery

Recovery from stress is an important property for anaerobic digestion (AD). Although AD is quite adaptable with regard to waste composition, new substrates added to stable systems may cause process decline. We tested whether crude glycerol would cause stress to a thermophilic AD microbiome previously stabilized long-term on a low C/N ratio feedstock. Three-percent (v/v) crude glycerol was added to the basal substrate (poultry litter) for two hydraulic retention time (HRT) periods. This caused stress where biogas volume and methane percentage dramatically decreased and VFA levels increased. When the basal substrate was resumed, secondary inhibition occurred, resulting in even greater stress (biogas production ceased, methane 3.6%). Unassisted recovery of system processes required eight HRT periods. In contrast, crude glycerol applied at a lower organic loading rate did not cause inhibition. Crude glycerol caused changes in dominance in the microbial community (16S rRNA pyrotags). Although process resilience was slow, the recovery of digester functions occurred in conjunction with the recovery of community structure, particularly putative syntrophic acetate-oxidizing bacteria. KEY POINTS: • Crude glycerol caused stress in thermophilic co-digestion with poultry litter. • Unassisted resilience of digester functions (methane) required 8 HRT. • Syntrophic acetate-oxidizing bacteria implicated for keystone resilience functions. Graphical abstract.

Anaerobic co-digestion of industrial landfill leachate and glycerin: methanogenic potential, organic matter removal and process optimization

The objective of this study was to evaluate the performance of the anaerobic co-digestion of different concentrations of industrial landfill leachate associated with crude residual glycerin, in relation to the methanogenic potential, COD removal, accumulated methane production, the effects of the factors (food/microorganism ratio and percentage of glycerin added to the leachate) and their interactions on kinetic parameters of methane production (CH₄) using the modified Gompertz model. Co-digestion tests were carried out in bench scale (400 mL of useful volume) under batch mode at 30 ± 1°C during 30-day incubation of anaerobic sewage sludge as inoculum. The parameters glycerin addition to the leachate (v/v) (0%, 1.5%, 5%, 8.5% and 10%) and F/M ratio (0.3, 0.5, 1, 1.5 and 1.7) were investigated using Central Composite Rotational Design method (CCRD). The results indicated significant effect to the response variables: methanogenic potential, COD removal, accumulated production of CH₄ and maximum estimated production of CH₄, considering a confidence interval of 95% (p < .05). The ideal mixture of 95.13% of leachate with 4.87% of raw glycerin was obtained by desirability test to F/M of 1.61 gCOD of substrate per gVSS (volatile suspended solids) of sludge. Methanogenic potential was 0.19 L_NCH₄ gTVS_rem ^-1, and the average removal of COD was 92%, resulting in accumulated production and maximum estimated production of CH₄ of 74 and 80 mL, respectively. It was noted that the process of co-digestion of the industrial landfill leachate with the crude residual glycerin is promising, due to is potential of complementing and balancing organic materials, nutrients and other components that influence the biological process. Abbreviations: AN: ammoniacal nitrogen; BMP: biochemical methane potential; CCRD: central composite rotational design; COD: chemical oxygen demand; F/M ratio: food/microorganism ratio; FSS: fixed suspended solids; NTP: normal temperature and pressure; TSS: total suspended solids; TA: total alkalinity; TFS: total fixed solids; TKN: total Kjeldahl nitrogen; TP: total phosphorus; TS: total solids; TVA: total volatile acids; TVS: total volatile solids; VSS: volatile suspended solids; WWTP: wastewater treatment plant.

Enhancing methane yield from crude glycerol anaerobic digestion by coupling with ultrasound or A. niger/E. coli biodegradation

Anaerobic digestion of crude glycerol from biodiesel production is a feasible way for methane production. However, crude glycerol (CG) contains impurities, such as long-chain fatty acids (LCFA) that can inhibit methanogenic microorganisms. Ultrasound promotes the hydrolysis of LCFA and deagglomerates the microorganisms in biological flocs. Furthermore, Aspergillus niger and Escherichia coli produce lipases capable of degrading LCFA. This study aims at improving the methane yield from anaerobic digestion by coupling with ultrasound or E. coli/A. niger biodegradation. The effect of the different treatments was first assessed in a perfectly mixed batch reactor (PMBR), using diluted CG at concentrations of 0.2%, 1.7%, and 3.2% (v/v). Later, the best conditions were replicated in an upflow anaerobic sludge blanket (UASB) reactor to simulate full-scale practical applications. Experiments in the PMBR showed that ultrasound or A. niger biodegradation steps improved methane yield up to 11% for 0.2% CG and 99% for 1.7% CG, respectively. CG biodegradation by E. coli inhibited the subsequent anaerobic digestion for all concentrations tested. Using a UASB digester, ultrasonic treatment of CG led to an average increase of 29% in methane production. The application of ultrasound led to a lower accumulation of propionic acid in the digested material and increased biogas production. On the other hand, an average 77% increase in methane production was achieved using a preliminary CG biodegradation step by A. niger, when operated at a loading rate of 2.9 kg COD m^-3 day^-1. Under these conditions, an energy gain of 0.48 kWh day^-1, with the production of the 0.434 m³ CH₄ kg^-1 COD_removal and 0.573 m³ CH₄ kg^-1 VS, and a biogas quality of 73% in methane were obtained. The digested material was analyzed for the detection and quantification of added-value by-products in order to obtain a broad assessment of the CG valorization through anaerobic digestion. In some experiments, propionic and oxalic acid were detected. However, the accumulation of propionic caused the inhibition of the acetogenic and methanogenic microorganisms.

Nitrite and free nitrous acid sludge pre-treatments to enhance methane production in continuous anaerobic digestion: Comparing process performance and associated costs

Secondary sludge pre-treatment with free nitrous acid (FNA) has been proven to enhance methane production during anaerobic digestion. However, it is still unclear if the same enhancement can be achieved only using nitrite, without sludge acidification. In this paper, secondary sludge was pre-treated during 5 h with nitrite within the range of 50-250 mg NO₂^--N/L at neutral pH (6.7). Results obtained from biochemical methane potential tests (BMPs) indicated that sludge pre-treatment at 150 mg NO₂^--N/L presented the best enhancement of methane production (24% as compared to the control). These conditions were used to pre-treat sludge added in a continuous lab-scale anaerobic digester that operated in parallel to another digester receiving sludge pre-treated with FNA (250 mg NO₂^--N/L at pH 5.5). Results showed a very similar performance in terms of methane enhancement in both reactors, indicating that sludge acidification is not needed to improve methane yield. A preliminary economic assessment also highlights the need for assessing real chemical costs and national power prices before the implementation of these pre-treatment steps as the associated benefits can significantly change depending on the country where the wastewater treatment plant is located.

Microbiome network analysis of co-occurrence patterns in anaerobic co-digestion of sewage sludge and food waste

Addition of food waste (FW) as a co-substrate in anaerobic digesters of wastewater treatment plants is a desirable strategy towards achievement of the potential of wastewater treatment plants to become energy-neutral, diverting at the same time organic waste from landfills. Because substrate type is a driver of variations in phylogenetic structure of digester microbiomes, it is critical to understand how microbial communities respond to changes in substrate composition and concentration. In this work, high throughput sequencing was used to monitor the dynamics of microbiome changes in four parallel laboratory-scale anaerobic digesters treating sewage sludge during acclimation to an increasing amount of food waste. A co-occurrence network was constructed using data from 49 metagenomes sampled over the 161 days of the digesters' operation. More than half of the nodes in the network were clustered in two major modules, i.e. groups of highly interconnected taxa that had much fewer connections with taxa outside the group. The dynamics of co-occurrence networks evidenced shifts that occurred within microbial communities due to the addition of food waste in the co-digestion process. A diverse and reproducible group of hydrolytic and fermentative bacteria, syntrophic bacteria and methanogenic archaea appeared to grow in a concerted fashion to allow stable performance of anaerobic co-digestion at high FW.

A Review on Anaerobic Co-Digestion with a Focus on the Microbial Populations and the Effect of Multi-Stage Digester Configuration

Recent studies have shown that anaerobic co-digestion (AnCoD) is superior to conventional anaerobic digestion (AD). The benefits of enhanced bioenergy production and solids reduction using co-substrates have attracted researchers to study the co-digestion technology and to better understand the effect of multi substrates on digester performance. This review will discuss the results of such studies with the main focus on: (1) generally the advantages of co-digestion over mono-digestion in terms of system stability, bioenergy, and solids reduction; (2) microbial consortia diversity and their synergistic impact on biogas improvement; (3) the effect of digester mode, i.e., multi-stage versus single stage digestion on AnCoD. It is essential to note that the studies reported improvement in the synergy and diverse microbial consortia when using co-digestion technologies, in addition to higher biomethane yield when using two-stage mode. A good example would be the co-digestion of biodiesel waste and glycerin with municipal waste sludge in a two-stage reactor resulting in 100% increase of biogas and 120% increase in the methane content of the produced biogas with microbial population dominated by Methanosaeta and Methanomicrobium.

Co-digestion of sewage sludge with crude or pretreated glycerol to increase biogas production

Anaerobic co-digestion of sewage sludge and glycerol from the biodiesel industry was evaluated in three experimental stages. In the first step, the addition of higher proportions of crude glycerol (5-20% v/v) to the sludge was evaluated, and the results showed a marked decrease in pH and inhibition of methane production. In the second step, co-digestion of sludge with either a lower proportion (1% v/v) of crude glycerol or glycerol pretreated to remove salinity resulted in volatile acid accumulation and low methane production. The accumulation of volatile acids due to the rapid degradation of glycerol in the mixture was more detrimental to methanogenesis than the salinity of the crude glycerol. In the third step, much lower amounts of crude glycerol were added to the sludge (0.3, 0.5, 0.7% v/v), resulting in buffering of the reaction medium and higher methane production than in the control (pure sludge). The best condition for co-digestion was with the addition of 0.5% (v/v) crude glycerol to the sewage sludge, which equals 0.6 g glycerol/g volatile solids applied. Under this condition, the specific methane production (mL CH₄/g volatile solids applied) was 1.7 times higher than in the control.

Improvement of carbon usage for phosphorus recovery in EBPR-r and the shift in microbial community

Enhanced biological phosphorus removal and recovery (EBPR-r) is a biofilm process that makes use of polyphosphate accumulating organisms (PAOs) to remove and recover phosphorus (P) from wastewater. The original process was inefficient, as indicated by the low P-release to carbon (C)-uptake (P_rel/C_upt) molar ratio of the biofilm. This study successfully validated a strategy to improve the P_rel/C_upt ratio by at least 3-fold. With an unchanged supply of carbon in the recovery stream, an increase in the hydraulic loading in stages I, II and III (7.2, 14.4 and 21.6 L, respectively) resulted in a 43% increase in the P_rel/C_upt ratio (0.069, 0.076 and 0.103, respectively). The ratio further increased by 150% (from 0.103 to 0.255) when the duration of the P uptake period was increased from 4 h (stage III) to 10 h (stage IV). Canonical correspondence analysis showed that, correlated to the 3-fold increase in the P_rel/C_upt ratio, there was an increase in the abundance of PAOs ("Candidatus Accumulibacter" Clade IIA) and a decrease in the occurrence of glycogen accumulating organisms (GAOs) (family Sinobacteraceae). However, the four stage operation impaired denitrification, resulting in a 5-fold reduction in the N_den/P_upt ratio. The decline in denitrification was consistent with a decrease in the abundance of denitrifiers including denitrifying PAOs (family Comamonadaceae and "Candidatus Accumulibacter" Clade IA). Overall, a strategy to facilitate more efficient use of carbon was validated, enabling a 3-fold carbon saving for P recovery. The new process enabled up to 80% of the wastewater P to be captured in a P-enriched stream (>90 mg/L) with a single uptake/release cycle of recovery.

Anaerobic digestion of food waste: A review focusing on process stability

Food waste (FW) is rich in biomass energy, and increasing numbers of national programs are being established to recover energy from FW using anaerobic digestion (AD). However process instability is a common operational issue for AD of FW. Process monitoring and control as well as microbial management can be used to control instability and increase the energy conversion efficiency of anaerobic digesters. Here, we review research progress related to these methods and identify existing limitations to efficient AD; recommendations for future research are also discussed. Process monitoring and control are suitable for evaluating the current operational status of digesters, whereas microbial management can facilitate early diagnosis and process optimization. Optimizing and combining these two methods are necessary to improve AD efficiency.

Seeking to enhance the bioenergy of municipal sludge: Effect of alkali pre-treatment and soluble organic matter supplementation

The aim of this research is to enhance the mesophilic anaerobic digestion of municipal sludge from Cadiz-San Fernando (Spain) wastewater treatment plant at 20days hydraulic retention time (HRT). Two different strategies were tested to improve the process: co-digestion with the addition of soluble organic matter (1% v/v); and alkali sludge pre-treatment (NaOH) prior to co-digestion with glycerine (1% v/v). Methane production (MP) was substantially enhanced (from 0.36±0.09 L CH₄ l/d to 0.85±0.16 L CH₄ l/d), as was specific methane production (SMP) (from 0.20±0.05 L CH₄/g VS to 0.49±0.09 L CH₄/g VS) when glycerine was added. The addition of glycerine does not seem to affect sludge stability, the quality of the effluent in terms of pH and organic matter content, i.e. volatile fatty acids (VFA), soluble organic matter and total volatile solid, or process stability (VFA/Alkalinity ratio<0.4). Alkali pre-treatment prior to co-digestion resulted in a high increase in soluble organic loading rates (more than 20%) and acidification yield (more than 50%). At 20days HRT, however, it led to overload of the system and total destabilization of the mesophilic anaerobic co-digestion of sewage sludge and glycerine.

The effect of mixing intensity on the performance and microbial dynamics of a single vertical reactor integrating acidogenic and methanogenic phases in lignocellulosic biomass digestion

The ready formation of scum in vertical reactors has been a bottleneck in the digestion of lignocellulosic materials for biogas production. This study describes a single vertical reactor that integrates the acidogenic and methanogenic phases of this process. The effects of two types of maize stover feedstock (fresh and silage) and two mixing intensities (20 and 70rpm) on methane yield were orthogonally determined. Fresh maize stover yielded approximately 14% more methane than silage maize stover. Mixing at 20rpm contributed to methane yield, while mixing at 70rpm blurred the phase boundary, resulting in accumulation of volatile fatty acids and loss of methanogens. The upper and lower phases clearly constituted a two-phase fermentation system. Clostridiales occupied the acidogenic phase, while the predominant bacteria in the methanogenic phase were Bacteroidetes, Chloroflexi, and Synergistetes. The absolute predominance of Methanosaetaceae clearly demonstrated that aceticlastic methanogenesis was the main route of methane production.

Comparative biogas generation from fruit peels of fluted pumpkin (Telfairia occidentalis) and its optimization

This study evaluated the potentials of fluted pumpkin fruit peels for biogas generation using three different pre-treatment methods (A, B, C) and the optimization of its process parameters. The physic-chemical characteristics of the substrates revealed it to be rich in nutrients and mineral elements needed by microorganisms. Gas chromatography analysis revealed the gas composition to be within the range of 58.5±2.5% Methane and 27±3% Carbon dioxide for all the three digestions. The study revealed that combination of three pre-treatment methods enhanced enormous biogas yield from the digested substrates as against the use of two methods and no pre-treatment experiment. Optimization of the generated biogas data revealed that RSM predicted higher gas yield than ANN, the latter gives higher accuracy and efficiency than the former. It is advocated that fluted pumpkin fruit peels be used for energy generation especially in the locations of its abundance.

Enhancing ethanol production from thermophilic and mesophilic solid digestate using ozone combined with aqueous ammonia pretreatment

Pretreatment with ozone combined with aqueous ammonia was used to recover residual organic carbon from recalcitrant solid digestate for ethanol production after anaerobic digestion (AD) of rice straw. Methane yield of AD at mesophilic and thermophilic conditions, and ethanol production of solid digestate were investigated. The results showed that the methane yield at thermophilic temperature was 72.2% higher than that at mesophilic temperature under the same conditions of 24days and 17% solid concentration. And also the ethanol production efficiency of solid digestate after thermophilic process was 24.3% higher than that of solid digestate after mesophilic process. In this study, the optimal conditions for integrated methane and ethanol processes were determined as 55°C, 17% solid concentration and 24days. 58.6% of glucose conversion, 142.8g/kg of methane yield and 65.2g/kg of ethanol yield were achieved, and the highest net energy balance was calculated as 6416kJ/kg.

Addition of crude glycerine as strategy to balance the C/N ratio on sewage sludge thermophilic and mesophilic anaerobic co-digestion

The effect of adding crude glycerine during continuous sewage sludge anaerobic digestion was investigated under thermophilic and mesophilic temperatures. Addition of CGY at thermophilic temperature range showed a negative impact on stability and performance of the process, even at low doses. The extreme pH values of CGY, together with the rapid release of VFA, causes SS alkalinity fail to control pH drop. On the contrary, at mesophilic temperature range the process performs steadily, with 148% increase in methane production when CGY represented 1% v/v of the influent (27% of influent COD). Further CGY percentages did not show any added improvement; the biomass shift, due to a high C/N ratio, could explain this behaviour. Results suggested that CGY can be used as co-substrate of SS anaerobic digestion though, depending on the characteristics of CGY, and on operational conditions, different parameters should be taken into account to achieve a steady and consistent operation.