Trace element supplementation in the biogas production from wheat stillage--optimization of metal dosing

Kinetic and elemental characterization of HAP-based high-rate partial nitritation/anammox system orienting stability and inorganic elemental requirements

Anammox-based processes are attractive for biological nitrogen removal, and the combination of anammox and hydroxyapatite (HAP) is promising for the simultaneous removal of nitrogen and phosphorus from wastewater. However, the kinetics of one-stage partial nitritation/anammox (PNA) in which ammonia-oxidizing bacteria (AOB) and anammox bacteria (AnAOB) exist in a reactor are poorly understood. Moreover, inorganic elements are required to promote microbial cell synthesis and growth; therefore, monitoring of elements to prevent the limitation and inhibition of the process is critical. The minimum amounts of inorganic elements required for a one-stage PNA process and the elemental flow remain unknown. Therefore, in this study, kinetics, stoichiometry, and element flow in the long-term, high-rate, continuous, one-stage HAP-PNA process with microaerobic granular sludge at 25 °C were determined using process modeling, parameter estimation, and mass balance. The biomass elemental composition was determined to be CH2.2O0.89N0.18S0.0091, and the biomass yield (Yobs) was calculated to be 0.0805 g/g NH4+-N. Therefore, a stoichiometric reaction equation for the one-stage HAP-PNA system was also proposed. The maximum specific growth rate (μm) of AnAOB and AOB were 0.0360 and 0.0982 d-1 with doubling times of 19 and 7.1 d, respectively. Finally, the elemental requirements for stable and high-rate performance were determined using element flow analysis. These findings are essential for developing the anammox-based process in a stable and resource-efficient manner and determining engineering applicability.

Coupling anaerobic co-digestion of winery waste and waste activated sludge with a microalgae process: Optimization of a semi-continuous system

Wine production represents one of the most important agro-industrial sectors in Italy. Wine lees are the most significant waste in the winery industry and have high disposal and storage costs and few applications within the circular economy. In this study, anaerobic digestion and a microalgae coupled process was studied in order to treat wine lees and waste activated sludge produced within the same facility, with the aim of producing energy and valuable microalgae biomass that could be processed to recover biofuel or biostimulant. Chlorella vulgaris was cultivated on liquid digestate in a semi-continuous system without biomass recirculation. The best growth and phytoremediation performance were achieved applying a hydraulic retention time (HRT) of 20 days with a stable dry weight, lipid and protein storage of 1.85 ± 0.02 g l-1, 33.48 ± 7.54 % and 57.85 ± 10.14 % respectively. Lipid characterization highlighted the potential use in high quality biodiesel production, according to EN14214 (<12 % v/v linolenic acid). The microalgae reactor's liquid output showed high removal of ammonia (95.72 ± 2.10 %), but low organic soluble matter reduction. Further semi-continuous process optimization was carried out by increasing the time between digestate feeding and biomass recovery at HRT 10. These operative changes avoided biomass wash-out and provided a stable phytoremediation of the digestate with 84.58 ± 4.02 % ammonia removal, 33.01 ± 1.44 % sCOD removal, 38.06 ± 2.65 % of polyphenols removal.

Selective trace elements significantly enhanced methane production in coal bed methane systems by stimulating microbial activity

Trace elements are associated with the microbial degradation of organic matter and methanogenesis, as enzymes in metabolic pathways often employ trace elements as essential cofactors. However, only a few studies investigated the effects of trace elements on the metabolic activity of microbial communities associated with biogenic coalbed methane production. We aimed to determine the effects of strategically selected trace elements on structure and function of active bacterial and methanogenic communities to stimulate methane production in subsurface coalbeds. Microcosms were established with produced water and coal from coalbed methane wells located in the Powder River Basin, Wyoming, USA. In initial pilot experiments with eight different trace elements, individual amendments of Co, Cu, and Mo lead to significantly higher methane production. Transcript levels of mcrA, the key marker gene for methanogenesis, positively correlated with increased methane production. Phylogenetic analysis of the mcrA cDNA library demonstrated compositional shifts of the active methanogenic community and increase of their diversity, particularly of hydrogenotrophic methanogens. High-throughput sequencing of cDNA obtained from 16S rRNA demonstrated active and abundant bacterial groups in response to trace element amendments. Active Acetobacterium members increased in response to Co, Cu, and Mo additions. The findings of this study yield new insights into the importance of essential trace elements on the metabolic activity of microbial communities involved in subsurface coalbed methane and provide a better understanding of how microbial community composition is shaped by trace elements.IMPORTANCEMicrobial life in the deep subsurface of coal beds is limited by nutrient replenishment. While coal bed microbial communities are surrounded by carbon sources, we hypothesized that other nutrients such as trace elements needed as cofactors for enzymes are missing. Amendment of selected trace elements resulted in compositional shifts of the active methanogenic and bacterial communities and correlated with higher transcript levels of mcrA. The findings of this study yield new insights to not only identify possible limitations of microbes by replenishment of trace elements within their specific hydrological placement but also into the importance of essential trace elements for the metabolic activity of microbial communities involved in subsurface coalbed methane production and provides a better understanding of how microbial community composition is shaped by trace elements. Furthermore, this finding might help to revive already spent coal bed methane well systems with the ultimate goal to stimulate methane production.

Anaerobic co-digestion of human excreta, food leftovers and kitchen residue: 1 ternary mixture design, synergistic effects and RSM approach

Anaerobic digestion of multiple substrates can generate more biogas while remaining stable, if positive synergistic effects are achieved. The type of co-digested substrates and the mixing ratio used, are the most important variables as each substrate has unique set of characteristics. Optimizing the volume ratios by testing various substrate mixing ratios is a popular method for determining the best-performing ratio of substrate mixture. The ternary mixture design has reportedly been found to quicken the process of testing different mixing ratios with high accuracy without running several experiments. Therefore, a ternary mixture design and a response surface approach are used in this work to ascertain the relationship between substrate mix and responses (biogas yield, methane yield, and synergy). The findings of the experiment revealed that R9 comprising 78.8 % human excreta, 11.8 % food leftovers and 9.4 % kitchen residue, had the highest methane production of 764.79 mLCH4/gVS and a synergistic index of 3.26. Additionally, the 3D response surface plots from the response surface model showed important and shared interactions between Human Excreta, (HE), Food Leftovers (FLO), and Kitchen Residue (KR). HE and KR had a similar positive synergistic effect on biogas yield, methane yield, and synergy, which was not the case for FLO. The response surface plots showed that the predicted responses (methane yield, biogas yield and synergy) increased with increasing HE and KR fractions and decreased with increasing FLO fractions in the substrate mixtures.

Emerging Strategies for Enhancing Propionate Conversion in Anaerobic Digestion: A Review

Anaerobic digestion (AD) is a triple-benefit biotechnology for organic waste treatment, renewable production, and carbon emission reduction. In the process of anaerobic digestion, pH, temperature, organic load, ammonia nitrogen, VFAs, and other factors affect fermentation efficiency and stability. The balance between the generation and consumption of volatile fatty acids (VFAs) in the anaerobic digestion process is the key to stable AD operation. However, the accumulation of VFAs frequently occurs, especially propionate, because its oxidation has the highest Gibbs free energy when compared to other VFAs. In order to solve this problem, some strategies, including buffering addition, suspension of feeding, decreased organic loading rate, and so on, have been proposed. Emerging methods, such as bioaugmentation, supplementary trace elements, the addition of electronic receptors, conductive materials, and the degasification of dissolved hydrogen, have been recently researched, presenting promising results. But the efficacy of these methods still requires further studies and tests regarding full-scale application. The main objective of this paper is to provide a comprehensive review of the mechanisms of propionate generation, the metabolic pathways and the influencing factors during the AD process, and the recent literature regarding the experimental research related to the efficacy of various strategies for enhancing propionate biodegradation. In addition, the issues that must be addressed in the future and the focus of future research are identified, and the potential directions for future development are predicted.

Enhancement of methanogenic activity by micronutrient control: Micronutrient availability in relation to sulfur transport

The main purpose of this research was to clarify the influence of the addition of iron (Fe) alone (0-100 mg/L) or 50 mg/L of Fe with 2 mg/L each of cobalt (Co), copper (Cu) and nickel (Ni) on the methanogenic activity of a mesophilic two-stage UASB system treating ethanol wastewater at a fixed chemical oxygen demand (COD) loading rate of 16 kg/m³/day under a continuous mode of operation and steady state condition. The addition of Fe provided the dual benefits of a reduction in both the dissolved sulfide and the hydrogen sulfide (H₂S) content in produced gas, resulting in marginally improved hydrogen (H₂) and methane (CH₄) productivities. When the Fe dosage was increased beyond the optimum value of 50 mg/L, the process performance drastically declined, as a consequence of the high total volatile fatty acid (VFA) concentrations that inhibited both the acidogens and methanogens predominantly present in the 1^st and 2^nd reactors, respectively. The chemical precipitation of iron sulfide was responsible for the reduction of produced H₂S in both the aqueous and gaseous phases as well as the minimization of added amounts of all other micronutrients to fulfil the sufficiency of all micronutrients for anaerobic digestion (AD). The addition of 2 mg/L each of Co, Cu and Ni together with 50 mg/L Fe resulted in the greatest enhancement in process performance, as indicated by the improved CH₄ yield (mL/g COD applied) to about 42.3%, compared to that without micronutrient supplement.

Integrated biorefineries for repurposing of food wastes into value-added products

Food waste (FW) generated through various scenarios from farm to fork causes serious environmental problems when either incinerated or disposed inappropriately. The presence of significant amounts of carbohydrates, proteins, and lipids enable FW to serve as sustainable and renewable feedstock for the biorefineries. Implementation of multiple substrates and product biorefinery as a platform could pursue an immense potential of reducing costs for bio-based process and improving its commercial viability. The review focuses on conversion of surplus FW into range of value-added products including biosurfactants, biopolymers, diols, and bioenergy. The review includes in-depth description of various types of FW, their chemical and nutrient compositions, current valorization techniques and regulations. Further, it describes limitations of FW as feedstock for biorefineries. In the end, review discuss future scope to provide a clear path for sustainable and net-zero carbon biorefineries.

Improvement of Biogas Quality and Quantity for Small-Scale Biogas-Electricity Generation Application in off-Grid Settings: A Field-Based Study

Small-scale electrical power generation (<100 kW) from biogas plants to provide off-grid electricity is of growing interest. Currently, gas engines are used to meet this demand. Alternatively, more efficient small-scale solid oxide fuel cells (SOFCs) can be used to enhance electricity generation from small-scale biogas plants. Most electricity generators require a constant gas supply and high gas quality in terms of absence of impurities like H2S. Therefore, to efficiently use the biogas from existing decentralized anaerobic digesters for electricity production, higher quality and stable biogas flow must be guaranteed. The installation of a biogas upgrading and buffer system could be considered; however, the cost implication could be high at a small scale as compared to locally available alternatives such as co-digestion and improved digester operation. Therefore, this study initially describes relevant literature related to feedstock pre-treatment, co-digestion and user operational practices of small-scale digesters, which theoretically could lead to major improvements of anaerobic digestion process efficiency. The theoretical preamble is then coupled to the results of a field study, which demonstrated that many locally available resources and user practices constitute frugal innovations with potential to improve biogas quality and digester performance in off-grid settings.

Cow manure stabilizes anaerobic digestion of cocoa waste

Anaerobic digestion of a mono-feedstock often causes low methane yields and process instability. An effective strategy to overcome these barriers is co-digestion with animal manure. The obtained process improvement is often attributed to buffer capacity, nutrients, vitamins and trace metals, and microorganisms present in manure, but it remains unknown which factor plays the key role in digester performance. Here, we investigated anaerobic digestion of cocoa waste in four different treatments: mono-digestion, addition of synthetic nutrients, co-digestion with sterile cow manure, and co-digestion with raw cow manure. Co-digestion with raw manure resulted in the highest methane yield of 181 ± 39 L kg^-1 VS (volatile solids), similar to the co-digestion with sterile manure, i.e., 162 ± 52 L kg^-1 VS. The supplementation of synthetic nutrients to the anaerobic digestion of cocoa waste only temporarily increased methane yield, indicating that this will tackle a lack of nutrients in the short term, but has a limited long-term contribution to the stabilization of the process. Hence, because of the inability of synthetic nutrients to stabilize the digestion process and the similarity between the digesters fed sterile and raw manure, both at the physico-chemical and microbial level, the key contribution of manure co-digestion with cocoa seems to be the provision of buffering capacity.

Multiple Effects of Different Nickel Concentrations on the Stability of Anaerobic Digestion of Molasses

Molasses is a highly thick by-product produced after sugarcane crystallization constitutes large amounts of biodegradable organics. These organic compounds can be converted to renewable products through anaerobic digestion. Nevertheless, its anaerobic digestion is limited due to its high chemical oxygen demand (COD) and ion concentration. The effects of nickel (Ni2+) on the stability of anaerobic digestion of molasses were established by studying the degradation of organic matter (COD removal rate), biogas yield, methane content in the biogas, pH, and alkalinity. The results showed that there were no significant effects on the stability of pH and alkalinity. Increased COD removal rate and higher methane content was observed by 2–3% in the digesters receiving 2 and 4 mg/L of Ni2+ in the first phase of the experiment. Ni2+ supplemented to reactors at concentration 2 mg/L enhanced biogas yield. Overall, it is suggested that the addition of Ni2+ has some effects on the enhancement of biogas yield and methane contents but has no obvious effects on the long-lasting stability of the molasses digestion.

Study on the biogas potential of anaerobic digestion of coffee husks wastes in Ethiopia

The poorly controlled discharge of coffee husks in Ethiopia causes severe environmental pollution and is a waste of resources. The volatile solid and carbon content in coffee husks waste indicates that it is rich in organic matter and has huge potential to produce biogas. This study investigated the feasibility of coffee husks to produce biomass through anaerobic digestion, based on temperature, initial pH, inoculum/substrate (I/S) ratio and carbon/nitrogen (C/N) ratio. The study demonstrated that the maximum production of biogas and methane reached 3359.6 ml and 2127.30 ml, respectively, under the conditions of mesophilic temperature (35±1°C), an initial pH of 7, an I/S ratio of 0.75 and a C/N ratio of 30. Based on this result, the effects of trace elements (Fe²⁺, Ni²⁺, Co²⁺) on biogas production and methane content were also explored. Compared with the group with no addition of trace elements, the experiment adding trace elements had significant enhancement effects on the production of biogas and methane, in which Fe²⁺ played a leading role (p<0.05). Fe²⁺ promoted the hydrolysis and acidification of coffee husks, resulting in the production of a series of intermediates such as volatile fatty acids and the other kinds of dissolved organic matter. Furthermore, the cooperation of Ni²⁺, Co²⁺ and Fe²⁺ enhanced the activity of the enzyme system in methanogens, promoting methane production. The results in this paper show that coffee husks have clear biogas potential through anaerobic digestion, and its effective utilization could fulfill the dual purpose of solid waste reclamation and local environmental protection in Ethiopia.

Nutrient augmentation enhances biogas production from sorghum mono-digestion

This study investigated the effects of the addition of micro- (Fe, Co, Ni, and Mo) and macro-(Sulfur) nutrients on mono-digestion of sorghum under mesophilic conditions. A continuous stirred-tank reactor was operated for more than 420 days under seven different experimental conditions. The experimental results showed poor performance for methane production and process stability without nutrient supplementation. Serious deficiencies in Co and S were confirmed by nutrient analysis of dry sorghum and digestate. Nutrient augmentation efficiently enhanced methane production and volatile fatty acid (VFA) removal. Methane production reached 223 mL-CH₄/g-VS, almost matching the yield predicted by biochemical methane potential (BMP) test. S was demonstrated to have a critical effect on metal availability in the digester. Consequently, to maintain stable methane fermentation, suitable supplementations of S and Co are recommended for anaerobic sorghum mono-digestion.

A brief review of anaerobic membrane bioreactors emphasizing recent advancements, fouling issues and future perspectives

This review summarizes the recent development and studies of anaerobic membrane bioreactor (AnMBR) to control fouling issues. AnMBR is an emerging waste water treatment technology mainly because of its low sludge residual, high volumetric organic removal rate, complete liquid-solid separation, better effluent quality, efficient resource recovery and the small footprint. This paper surveys the fundamental aspects of AnMBRs, including its applications, membrane configurations, and recent progress for enhanced reactor performance. Furthermore, the membrane fouling, a major restriction in the practical application of AnMBR, its mechanism and antifouling strategies like membrane cleaning, quorum quenching, ultrasonic treatment, membrane modifications, and antifouling agents are briefly discussed. Based on the review, the key issues that require urgent attention to facilitate large scale and integrated application of AnMBR technology are identified and future research perspectives relating to the prevalent issues are proposed.

Effect of Cobalt, Nickel, and Selenium/Tungsten Deficiency on Mesophilic Anaerobic Digestion of Chemically Defined Soluble Organic Compounds

Trace elements (TEs) are vital for anaerobic digestion (AD), due to their role as cofactors in many key enzymes. The aim of this study was to evaluate the effects of specific TE deficiencies on mixed microbial communities during AD of soluble polymer-free substrates, thus focusing on AD after hydrolysis. Three mesophilic (37 °C) continuous stirred-tank biogas reactors were depleted either of Co, Ni, or a combination of Se and W, respectively, by discontinuing their supplementation. Ni and Se/W depletion led to changes in methane kinetics, linked to progressive volatile fatty acid (VFA) accumulation, eventually resulting in process failure. No significant changes occurred in the Co-depleted reactor, indicating that the amount of Co present in the substrate in absence of supplementation was sufficient to maintain process stability. Archaeal communities remained fairly stable independent of TE concentrations, while bacterial communities gradually changed with VFA accumulation in Ni- and Se-/W-depleted reactors. Despite this, the communities remained relatively similar between these two reactors, suggesting that the major shifts in composition likely occurred due to the accumulating VFAs. Overall, the results indicate that Ni and Se/W depletion primarily lead to slower metabolic activities of methanogenic archaea and their syntrophic partners, which then has a ripple effect throughout the microbial community due to a gradual accumulation of intermediate fermentation products.

Effect of nickel, cobalt, and iron on methanogenesis from methanol and cometabolic conversion of 1,2-dichloroethene by Methanosarcina barkeri

Methanogens are responsible for the last step in anaerobic digestion (AD), in which methane (a biofuel) is produced. Some methanogens can cometabolize chlorinated pollutants, contributing for their removal during AD. Methanogenic cofactors involved in cometabolic reductive dechlorination, such as F₄₃₀ and cobalamin, contain metal ions (nickel, cobalt, iron) in their structure. We hypothesized that the supplementation of trace metals could improve methane production and the cometabolic dechlorination of 1,2‐dichloroethene (DCE) by pure cultures of Methanosarcina barkeri. Nickel, cobalt, and iron were added to cultures of M. barkeri growing on methanol and methanol plus DCE. Metal amendment improved DCE dechlorination to vinyl chloride (VC): assays with 20 µM of Fe³⁺ showed the highest final concentration of VC (5× higher than in controls without Fe³⁺), but also in assays with 5.5 µM of Co²⁺ and 5 µM of Ni²⁺ VC formation was improved (3.5–4× higher than in controls without the respective metals). Dosing of metals could be useful to improve anaerobic removal of chlorinated compounds, and more importantly decrease the detrimental effect of DCE on methane production in anaerobic digesters.

Food waste from restaurant sector - Characterization for biorefinery approach

The aim of this study is the analysis of food waste (FW) composition from local catering services to assess potential biorefinery development. Moisture content of different FW samples showed that 27-47% (w/w) was organic material. Main components were lipids (25.7-33.2, w/w), starch (16.2-29.4%, w/w) and proteins (23.5-18.3%, w/w) on a dry basis. A metal profile with Na and Mg as main components, followed by trace elements, i.e. Zn or Fe, was also found in food waste samples. Statistical tests in combination with principal component analysis provides an efficient methodology to establish specific composition variations between FW from different catering services, while relating them to FW typology. The combination of chemical characterization with statistical study constitutes a promising decision-making tool for FW processing and valorization. The innovative methodology presented in this study provides systematic evaluation of FW composition and variability to allow selection of the most appropriate valorization paths.

Anaerobic co-digestion of sewage sludge, food waste and yard waste: Synergistic enhancement on process stability and biogas production

Anaerobic co-digestion (co-AD) could be a more sustainable waste management solution by sharing the existed anaerobic digestion (AD) facilities and generating more biogas energy. In this study, a series of co-AD of different urban derived organic wastes (sewage sludge-SS, food waste-FW, yard waste-YW) were conducted in a semi-continuous mode, and the corresponding dynamic evolutions of microbial community structure were followed by using real-time quantitative polymerase chain reaction (qPCR). As for co-AD of two feedstocks, introduction of SS (25%, VS basis) in FW significantly improved the process stability and archaea/total microbe ratio (from 0.4% to 17.1%), which might be due to the regulating effect of abundant trace metals in SS; co-AD of SS (25%, VS basis) with YW improved the methane yield by 2.04 times than AD of YW only together with higher methane contents (57.4 ± 1.3% vs. 50.9 ± 2.2%); in co-AD of FW and YW, synergistic effects in terms of increased methane production (3.4-19.1%) were observed, which was correlated with more robust growth of both bacteria and archaea. As for co-AD of three feedstocks, high methane yields of 314.9 ± 17.1 mL/g VS were achieved with a reliable stability. These findings could provide some fundamental and technical information for the co-treatment of urban derived organic wastes in centralized AD facilities.

Chemically Enhanced Primary Sludge as an Anaerobic Co-Digestion Additive for Biogas Production from Food Waste

In order to overcome process instability and buffer deficiency in the anaerobic digestion of mono food waste (FW), chemically enhanced primary sludge (CEPS) was selected as a co-substrate for FW treatment. In this study, batch tests were conducted to study the effects of CEPS/FW ratios on anaerobic co-digestion (coAD) performances. Both soluble chemical oxygen demand (SCOD) and protease activity were decreased, with the CEPS/FW mass ratio increasing from 0:5 to 5:0. However, it was also found that the accumulation of volatile fatty acids (VFAs) was eliminated by increasing the CEPS/FW ratio, and that corresponding VFAs concentrations decreased from 13,872.97 to 1789.98 mg chemical oxygen demand per L (mg COD/L). In addition, the maximum value of cumulative biogas yield (446.39 mL per g volatile solids removal (mL/g VSsremoval)) was observed at a CEPS/FW ratio of 4:1, and that the tendency of coenzyme F420 activity was similar to biogas production. The mechanism analysis indicated that Fe-based CEPS relived the VFAs accumulation caused by FW, and Fe(III) induced by Fe-based CEPS enhanced the activity of F420. Therefore, the addition of Fe-based CEPS provided an alternative method for FW treatment.

Seasonal variation of the organic and inorganic composition of sugarcane vinasse: main implications for its environmental uses

Sugarcane vinasse is the main waste stream of the Brazilian agroindustry. The typical composition of sugarcane vinasse gives it a high polluting potential that implies the necessity to define sustainable strategies for managing this waste. Knowledge of the inorganic and organic composition of vinasse and its seasonal variation is extremely important to conduct scientific research to define alternative managements for vinasse disposal other than fertigation. This study evaluated the variability of vinasse composition throughout the same harvesting season and among three harvesting seasons of one Brazilian annexed biorefinery (2015-2017). The contents of chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), total solids (TS), suspended solids (SS), salinity (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^-, F^-), nutrients (N, P, S), trace metals (Al³⁺, As²⁺, Ba²⁺, Cd²⁺, Cr³⁺, Co²⁺, Cu²⁺, Fe²⁺, Pb²⁺, Mn²⁺, Hg²⁺, Mo²⁺, Ni²⁺, Se²⁺, Zn²⁺), and specific soluble organic compounds (sugars, alcohols, and organic acids), as well as pH and conductivity, were monitored in 13 samples. The results indicated that sugarcane vinasse is a suitable feedstock for biological treatments, such as anaerobic digestion processes for energy recovery, as well as substrate for biomass (e.g., microalgae, energy crops, lignocellulosic biomass) growth. The application of a previous treatment makes vinasse a more environmentally friendly natural fertilizer for land fertigation.

Synthetic Effect of EDTA and Ni2+ on Methane Production and Microbial Communities in Anaerobic Digestion Process of Kitchen Wastes

Batch tests were carried out to study the effect of simultaneous addition of ethylenediaminetetraacetic acid and Ni2+ (EDTA-Ni) on anaerobic digestion (AD) performances of kitchen wastes (KWs). The results indicated that the cumulative biogas yield and methane content were enhanced to 563.82 mL/gVS and 63.7% by adding EDTA-Ni, respectively, which were almost 1.15 and 1.07-fold of that in the R2 with Ni2+ addition alone. At the same time, an obvious decrease of propionic acid was observed after EDTA-Ni addition. The speciation analysis of Ni showed that the percentages of water-soluble and exchangeable Ni were increased to 38.8% and 36.3% due to EDTA-Ni addition, respectively. Also, the high-throughput sequencing analysis revealed that the EDTA-Ni promoted the growth and metabolism of Methanosarcina and Methanobacterium, which might be the major reason for propionic acid degradation and methane production.

Effects of barium on the pathways of anaerobic digestion

The sufficient presence of trace elements (TE) is essential for anaerobic digestion. Barium (Ba) is considered a non-essential trace element that can be collaterally added to digesters as part of low-cost trace element sources or because of its presence in some feedstocks, such as crude glycerol. In the present study, the impact of Ba supplementation (2-2000 mg/L) on each stage of the anaerobic digestion (AD) process was evaluated using pure substrates (i.e., cellulose, glucose, a mixture of volatile fatty acids, sodium acetate and hydrogen) as well as a complex substrate (i.e., dried green fodder). Hydrolytic activity was affected at dosages higher than 200 mg Ba/L, whereas cellulose degradation was completely inhibited at 2000 mg Ba/L. The negative effects of the addition of Ba to methane production were observed only in the hydrolytic activity, and no effects were detected at any barium dosage in the subsequent anaerobic steps. Because Ba does not have a reported role as a cofactor of enzymes, this response could have been due to a direct inhibitory effect, a variation in the bioavailability of other trace elements, or even the availability of CO₂/SO₄ through precipitation as Ba-carbonates and sulphates. The results showed that the addition of Ba modified the chemical equilibrium of the studied system by varying the soluble concentration of some TEs and therefore their bioavailability. The highest variation was detected in the soluble concentration of zinc, which increased as the amount of Ba increased. Although little research has shown that Ba has some utility in anaerobic processes, its addition must be carefully monitored to avoid an undesirable modification of the chemical equilibrium in the system.

The chemical and dynamic distribution characteristics of iron, cobalt and nickel in three different anaerobic digestates: Effect of pH and trace elements dosage

The enhancement of the bioavailability and process controllability of trace elements (TEs) addition is of significance to improve the anaerobic digestion (AD) performance. In order to understand the bioavailability of TEs, the chemical form distribution patterns of endogenous/exogenous Fe, Ni and Co with different dosage (Fe: 1, 10 and 100 mg/kg; Ni and Co: 0.1, 1 and 10 mg/kg) and different pH condition (6.5; 7.5 and 8.5) were investigated in three different anaerobic digestates. The results showed that the exogenous TEs dosage exerted no obvious effects on the TEs distribution patterns. The chemical fractionation of TEs was more affected by pH, redox potential, AD process and digestate resource. Under pH 6.5 and with redox potential rising, the acid-extractable and reducible fractions and of Fe, Ni and Co were increased while the oxidizable fractions were decreased, which enhanced the bioavailability of TEs.

Effect of trace element addition and increasing organic loading rates on the anaerobic digestion of cattle slaughterhouse wastewater

In this study, anaerobic digestion of slaughterhouse wastewater with the addition of trace elements was monitored for biogas quantity, quality and process stability using CSTR digesters operated at mesophilic temperature. The determination of trace element concentrations was shown to be deficient in Fe, Ni, Co, Mn and Mo compared to recommendations given in the literature. Addition of these trace elements resulted in enhanced degradation efficiency, higher biogas production and improved process stability. Higher organic loading rates and lower hydraulic retention times were achieved in comparison to the control digesters. A critical accumulation of volatile fatty acids was observed at an organic loading rate of 1.82 g L^-1 d^-1 in the control compared to 2.36 g L^-1 d^-1 in the digesters with trace element addition. The improved process stability was evident in the final weeks of experimentation, in which control reactors produced 84% less biogas per day compared to the reactors containing trace elements.

Microbial rRNA gene expression and co-occurrence profiles associate with biokinetics and elemental composition in full-scale anaerobic digesters

This study examined whether the abundance and expression of microbial 16S rRNA genes were associated with elemental concentrations and substrate conversion biokinetics in 20 full-scale anaerobic digesters, including seven municipal sewage sludge (SS) digesters and 13 industrial codigesters. SS digester contents had higher methane production rates from acetate, propionate and phenyl acetate compared to industrial codigesters. SS digesters and industrial codigesters were distinctly clustered based on their elemental concentrations, with higher concentrations of NH₃ -N, Cl, K and Na observed in codigesters. Amplicon sequencing of 16S rRNA genes and reverse-transcribed 16S rRNA revealed divergent grouping of microbial communities between mesophilic SS digesters, mesophilic codigesters and thermophilic digesters. Higher intradigester distances between Archaea 16S rRNA and rRNA gene profiles were observed in mesophilic codigesters, which also had the lowest acetate utilization biokinetics. Constrained ordination showed that microbial rRNA and rRNA gene profiles were significantly associated with maximum methane production rates from acetate, propionate, oleate and phenyl acetate, as well as concentrations of NH₃ -N, Fe, S, Mo and Ni. A co-occurrence network of rRNA gene expression confirmed the three main clusters of anaerobic digester communities based on active populations. Syntrophic and methanogenic taxa were highly represented within the subnetworks, indicating that obligate energy-sharing partnerships play critical roles in stabilizing the digester microbiome. Overall, these results provide new evidence showing that different feed substrates associate with different micronutrient compositions in anaerobic digesters, which in turn may influence microbial abundance, activity and function.

Anaerobic treatment of source-separated domestic bio-wastes with an improved upflow solid reactor at a short HRT

Anaerobic treatment is the core technology for resource and energy recovery from source-separated domestic bio-wastes. The higher efficiency of an improved upflow solid reactor (IUSR) designed in this study was demonstrated in the treatment of concentrated black water and kitchen waste. The highest methane production of 48L/person/day was achieved at the hydraulic retention time (HRT) of 7days, while the other measures of performance at the HRT of 8.3days were better than at the HRT of 7 or 10days, achieving a methane production of 43L/person/day, removal of total chemical oxygen demand (TCOD) of 89%, removal of soluble chemical oxygen demand (SCOD) of 92%, and conversion of chemical oxygen demand (COD) to methane of 71%. It is not recommended to decrease HRT lower than 7days due to the instability of the initial period. The concentrations of volatile fatty acids (VFAs) in the IUSR were less than 10mg/L, indicating that the anaerobic process was stable. Sludge bed development showed that sludge bed with high microbial activity was formed in the bottom and that the precipitation zone of effluents formed should preferably occupy 30% of the height of the IUSR. The effluents of the IUSR could be used for irrigation in agriculture in combination with a settling tank accompanied by disinfection to remove solids and pathogens.

Anaerobic digestion of Brewery Spent Grains: Trace elements addition requirement

The current study evaluates and compares the stability of anaerobic digestion of Brewery Spent Grains (BSG) with and without addition of nutrients. Based on the composition of the BSG two levels of nutrients addition were defined. Control reactor, without addition of nutrients, showed signs of instability after 3months of operation and collapsed. On the contrary, supplemented reactors led to a COD removal rate of 60% and a methane production of 280NLCH₄.kg^-1 VS_added. According to these results, it was possible to define an additive solution adapted to BSG degradation.

Optimization of semi-continuous anaerobic digestion of sugarcane straw co-digested with filter cake: Effects of macronutrients supplementation on conversion kinetics

Anaerobic digestion of sugarcane straw co-digested with sugarcane filter cake was investigated with a special focus on macronutrients supplementation for an optimized conversion process. Experimental data from batch tests and a semi-continuous experiment operated in different supplementation phases were used for modeling the conversion kinetics based on continuous stirred-tank reactors. The semi-continuous experiment showed an overall decrease in the performance along the inoculum washout from the reactors. By supplementing nitrogen alone or in combination to phosphorus and sulfur the specific methane production significantly increased (P<0.05) by 17% and 44%, respectively. Although the two-pool one-step model has fitted well to the batch experimental data (R²>0.99), the use of the depicted kinetics did not provide a good estimation for process simulation of the semi-continuous process (in any supplementation phase), possibly due to the different feeding modes and inoculum source, activity and adaptation.

Improved methane production from sugarcane vinasse with filter cake in thermophilic UASB reactors, with predominance of Methanothermobacter and Methanosarcina archaea and Thermotogae bacteria

Biogas production from sugarcane vinasse has enormous economic, energy, and environmental management potential. However, methane production stability and biodigested vinasse quality remain key issues, requiring better nutrient and alkalinity availability, operational strategies, and knowledge of reactor microbiota. This study demonstrates increased methane production from vinasse through the use of sugarcane filter cake and improved effluent recirculation, with elevated organic loading rates (OLR) and good reactor stability. We used UASB reactors in a two-stage configuration, with OLRs up to 45gCODL^-1d^-1, and obtained methane production as high as 3LL^-1d^-1. Quantitative PCR indicated balanced amounts of bacteria and archaea in the sludge (10⁹-10¹⁰copiesg^-1VS), and of the predominant archaea orders, Methanobacteriales and Methanosarcinales (10⁶-10⁸copiesg^-1VS). 16S rDNA sequencing also indicated the thermophilic Thermotogae as the most abundant class of bacteria in the sludge.

Effects of trace metal deficiency and supplementation on a submerged anaerobic membrane bioreactor

This study examined the effects of a deficiency in trace metals (TMs) on the performance of a submerged anaerobic membrane bioreactor (SAMBR). When trace metals were excluded from the feed to the SAMBR, COD removal and methane yield reduced while VFAs in the effluent increased. A reduction of up to 37.48% in the total metal content in the reactor was observed, while the less bioavailable fractions increased up to 13.29%. Pulse addition of trace metals for 7 days at 5-times the daily metal loading was effective in improving the performance of the SAMBR by increasing the amount of trace metals in the bioavailable fractions from 2.12% to 11.92%, with up to 87.7% of added metals retained in the reactor within 24h. However, the second and third pulse at 5 and 10-times daily metal loading did not result in similar changes in metal speciation and might have inhibited the methanogens.

Effects of adding trace elements on rice straw anaerobic mono-digestion: Focus on changes in microbial communities using high-throughput sequencing

Although trace elements are known to aid anaerobic digestion, their mechanism of action is still unclear. High-throughput sequencing was used to reveal the mechanism by which adding trace elements affects microbial communities and their action. The results showed that the highest methane yields, with addition of Fe, Mo, Se and Mn were 289.2, 289.6, 285.3, 293.0mL/g volatile solids (VS), respectively. The addition of Fe, Mo, Se and Mn significantly (P<0.05) reduced the level of volatile fatty acids (VFAs). The dominant bacteria and archaea were Bacteroidetes and Methanosaeta, respectively. Compared with the proportion of Methanosaeta in the control group, treatment with added trace elements increased Methanosaeta by as much as 12.4%. Microbial community analysis indicated that adding trace elements changed the composition and diversity of archaea and bacteria. Methane yield was positively correlated with bacterial diversity and negatively correlated with archaeal diversity for most treatments.

Analysing the mechanisms of sludge digestion enhanced by iron

Carbon-neutral operation of wastewater treatment plants (WWTPs) requires enhancing anaerobic digestion (AD) of excess sludge for a higher energy conversion efficiency. Among others, iron has been identified to function on enhancing methane production in AD. As an industrial residual, waste iron scraps (WISs) have been reported as potentially enhancing CH₄ production in AD. With this study, the mechanisms of AD enhanced by WISs are analysed in a two-phase process: acidogenic phase (AP) and methanogenic phase (MP). Semi-continuous tests substantially excluded ORP reduction and hydrogen-evolution corrosion induced by WISs in enhancing CH₄ production, although WISs (10 g Fe/L) could indeed increase CH₄ production by 10.1% and 21.4% when added in AP and MP respectively. Detection on both FISH and enzymatic activities of involved microorganisms revealed that the stimulating effects of WISs on anaerobes (both catabolism and anabolism) could play an important (96.3%) role in enhancing CH₄ production, which would facilitate hydrolysis of refractory organics and improvement of electron transport rate (ETR).

Trace metals supplementation in anaerobic membrane bioreactors treating highly saline phenolic wastewater

Biomass requires trace metals (TM) for maintaining its growth and activity. This study aimed to determine the effect of TM supplementation and partitioning on the specific methanogenic activity (SMA), with a focus on cobalt and tungsten, during the start-up of two lab-scale Anaerobic Membrane Bioreactors (AnMBRs) treating saline phenolic wastewater. The TM partitioning revealed a strong accumulation of sodium in the biomass matrix and a wash-out of the majority of TM in the reactors, which led to an SMA decrease and a low COD removal of about 30%. The SMA exhibits a maximum at about 6g Na⁺ L^-1 and nearly complete inhibition at 34g Na⁺ L^-1. The dose of 0.5mgL^-1 of tungsten increases the SMA by 17%, but no improvement was observed with the addition of cobalt. The results suggested that TM were not bioavailable at high salinity. Accordingly, an increased COD removal was achieved by doubling the supply of TM.

Effect of Ethylenediamine-N,N'-disuccinic acid (EDDS) on the speciation and bioavailability of Fe2+ in the presence of sulfide in anaerobic digestion

The effects of a biodegradable chelating agent, Ethylenediamine-N,N'-disuccinic acid (EDDS), on the speciation and bioavailability of iron (Fe²⁺) in anaerobic digestion were examined. Fe²⁺ supplementation at 10mg/L increased methane yield, but the presence of 8mg/L sulfide led to the precipitation of Fe²⁺ as FeS which limited its bioavailability. The results confirmed that the EDDS could replace common chelating agents with low biodegradability (EDTA and NTA), and improve the bioavailability of Fe²⁺ by forming an Fe-EDDS complex, thereby protecting Fe²⁺ from sulfide precipitation. Experimental findings from sequential extraction using the Community Bureau of Reference (BCR) method, and quantification of free EDDS and Fe-EDDS complex using UHPLC, confirmed that 29.82% of Fe²⁺ was present in bioavailable forms, i.e. soluble and exchangeable, when EDDS was added at 1:1 molar ratio to Fe²⁺. As a result, the methane production rate increased by 11.17%, and the methane yield increased by 13.25%.

Effect of energy grass on methane production and heavy metal fractionation during anaerobic digestion of sewage sludge

Anaerobic digestion (AD) is one of the most widely used processes to stabilize waste sewage sludge and produce biogas as renewable energy. The relatively low organic matter content and high heavy metal concentrations in sewage sludge have severely restricted the application and development of AD technology in China. In this study, the effect of energy grass (Pennisetum alopecuroides) addition on methane production and heavy metal fractionation during the AD of sewage sludge was evaluated. Methane production was enhanced by 11.2% by the addition of P. alopecuroides. The addition of P. alopecuroides significantly reduced the percentages of the water-soluble and exchangeable fractions of the target heavy metals in the sewage sludge after AD, and the dominant species were concentrated in Fe-Mn oxide-bound and organic- and sulfide-bound fractions of the digested sludge. The addition of P. alopecuroides at a dosage of 0.3kg significantly (P<0.05) decreased the mobility factors (MFs) of the target heavy metals after AD. In particular, the MFs of Cr and Ni were 61% and 32% lower, respectively, relative to the control. The increase in the added dose did not necessarily lead to further decreases in the MFs of the heavy metals. These results demonstrate that an appropriate addition of energy grass could enhance AD, decrease the mobility of heavy metals and promote heavy metal stabilization in sewage sludge during AD, which is beneficial for the subsequent land application of sewage sludge.

Enhancing biogas production from vinasse in sugarcane biorefineries: Effects of urea and trace elements supplementation on process performance and stability

In this study, the effects of nitrogen, phosphate and trace elements supplementation were investigated in a semi-continuously operated upflow anaerobic sludge blanket system to enhance process stability and biogas production from sugarcane vinasse. Phosphate in form of KH2PO4 induced volatile fatty acids accumulation possibly due to potassium inhibition of the methanogenesis. Although nitrogen in form of urea increased the reactor's alkalinity, the process was overloaded with an organic loading rate of 6.1gCODL(-1)d(-1) and a hydraulic retention time of 3.6days. However, by supplementing urea and trace elements a stable operation even at an organic loading rate of 9.6gCODL(-1)d(-1) and a hydraulic retention time of 2.5days was possible, resulting in 79% higher methane production rate with a stable specific methane production of 239mLgCOD(-1).

Iron deficiency and bioavailability in anaerobic batch and submerged membrane bioreactors (SAMBR) during organic shock loads

This study examined the effects of Fe(2+) and its bioavailability for controlling VFAs during organic shock loads in batch reactors and a submerged anaerobic membrane bioreactor (SAMBR). When seed grown under Fe-sufficient conditions (7.95±0.05mgFe/g-TSS), an organic shock resulted in leaching of Fe from the residual to organically bound and soluble forms. Under Fe-deficient seed conditions (0.1±0.002mgFe/gTSS), Fe(2+) supplementation (3.34mgFe(2+)/g-TSS) with acetate resulted in a 2.1-3.9 fold increase in the rate of methane production, while with propionate it increased by 1.2-1.5 fold compared to non-Fe(2+) supplemented reactors. Precipitation of Fe(2+) as sulphides and organically bound Fe were bioavailable to methanogens for acetate assimilation. The results confirmed that the transitory/long term limitations of Fe play a significant role in controlling the degradation of VFAs during organic shock loads due to their varying physical/chemical states, and bioavailability.

Impacts of trace element supplementation on the performance of anaerobic digestion process: A critical review

This paper critically reviews the impacts of supplementing trace elements on the anaerobic digestion performance. The in-depth knowledge of trace elements as micronutrients and metalloenzyme components justifies trace element supplementation into the anaerobic digestion system. Most of the earlier studies reported that trace elements addition at (sub)optimum dosages had positive impacts mainly longer term on digester stability with greater organic matter degradation, low volatile fatty acids (VFA) concentration and higher biogas production. However, these positive impacts and element requirements are not fully understood, they are explained on a case to case basis because of the great variance of the anaerobic digestion operation. Iron (Fe), nickel (Ni) and cobalt (Co) are the most studied and desirable elements. The right combination of multi-elements supplementation can have greater positive impact. This measure is highly recommended, especially for the mono-digestion of micronutrient-deficient substrates. The future research should consider the aspect of trace element bioavailability.

Does the addition of proteases affect the biogas yield from organic material in anaerobic digestion?

The aim of this study was to investigate the biochemical disintegration effect of hydrolytic enzymes in lab scale experiments. Influences of enzyme addition on the biogas yield as well as effects on the process stability were examined. The addition of proteases occurred with low and high dosages in batch and semi-continuous biogas tests. The feed mixture consisted of maize silage, chicken dung and cow manure. Only very high concentrated enzymes caused an increase in biogas production in batch experiments. In semi-continuous biogas tests no positive long-term effects (100 days) were observed. Higher enzyme-dosage led to a reduced biogas-yield (13% and 36% lower than the reference). Phenylacetate and -propionate increased (up to 372 mgl(-1)) before the other volatile fatty acids did. Volatile organic acids rose up to 6.8 gl(-1). The anaerobic digestion process was inhibited.

Comparison of start-up strategies and process performance during semi-continuous anaerobic digestion of sugarcane filter cake co-digested with bagasse

The anaerobic digestion of sugarcane filter cake and the option of co-digestion with bagasse were investigated in a semi-continuous feeding regime to assess the main parameters used for large-scale process designing. Moreover, fresh cattle manure was considered as alternative inoculum for the start-up of biogas reactors in cases where digestate from a biogas plant would not be available in remote rural areas. Experiments were carried out in 6 lab-scale semi-continuous stirred-tank reactors at mesophilic conditions (38±1°C) while the main anaerobic digestion process parameters monitored. Fresh cattle manure demonstrated to be appropriate for the start-up process. However, an acclimation period was required due to the high initial volatile fatty acids concentration (8.5gL(-1)). Regardless the mono-digestion of filter cake presented 50% higher biogas yield (480mLgVS(-1)) than co-digestion with bagasse (320mLgVS(-1)) during steady state conditions. A large-scale co-digestion system would produce 58% more biogas (1008m(3)h(-1)) than mono-digestion of filter cake (634m(3)h(-1)) due to its higher biomass availability for biogas conversion. Considering that the biogas production rate was the technical parameter that displayed the most relevant differences between the analyzed substrate options (0.99-1.45m(3)biogasm(3)d(-1)). The decision of which substrate option should be implemented in practice would be mainly driven by the available construction techniques, since economically efficient tanks could compensate the lower biogas production rate of co-digestion option.

The synthetic effect on volatile fatty acid disinhibition and methane production enhancement by dosing FeCl3in a sludge thermophilic anaerobic digestion system

A dosage gradient of FeCl₃was adopted and 9.92 mg Fe per g DS was favorable for the disinhibition of VFAs in sludge thermophilic digestion system.

Biogas Production from Sugarcane Waste: Assessment on Kinetic Challenges for Process Designing

Biogas production from sugarcane waste has large potential for energy generation, however, to enable the optimization of the anaerobic digestion (AD) process each substrate characteristic should be carefully evaluated. In this study, the kinetic challenges for biogas production from different types of sugarcane waste were assessed. Samples of vinasse, filter cake, bagasse, and straw were analyzed in terms of total and volatile solids, chemical oxygen demand, macronutrients, trace elements, and nutritional value. Biochemical methane potential assays were performed to evaluate the energy potential of the substrates according to different types of sugarcane plants. Methane yields varied considerably (5-181 Nm³·tonFM(-1)), mainly due to the different substrate characteristics and sugar and/or ethanol production processes. Therefore, for the optimization of AD on a large-scale, continuous stirred-tank reactor with long hydraulic retention times (>35 days) should be used for biogas production from bagasse and straw, coupled with pre-treatment process to enhance the degradation of the fibrous carbohydrates. Biomass immobilization systems are recommended in case vinasse is used as substrate, due to its low solid content, while filter cake could complement the biogas production from vinasse during the sugarcane offseason, providing a higher utilization of the biogas system during the entire year.