Metal supplementation to UASB bioreactors: from cell-metal interactions to full-scale application

Illuminating bioprocess responses to metal-based nanoparticles addition along hydrogen and methane production pathways: A review

Recent research has discussed the positive impacts of metal-based nanoparticles (NPs) on bioprocesses producing either hydrogen (H2) or methane (CH4). The enhancement has been explained by mechanisms such as direct interspecies electron transfer (DIET), metal corrosion, and dissimilatory reduction. Such interactions could induce further benefits, such as controlling oxidation-reduction potential (ORP), mitigating toxicants, promoting enzymatic activity, and altering the microbiome, which have not yet been comprehensively discussed. Factors like metal type, oxidation state, and size of NPs are crucial for their reactivity and corresponding responses. This review discusses how different redox potentials of metals can regulate metabolic pathways and how NPs and their reactive ions can eliminate toxicants (e.g., sulfate) and enhance the activity of intra- and extracellular enzymes. The enrichment of responsive microorganisms in correlation with NPs is further discussed. A better understanding of the multifaceted role of metal-based NPs can guide potential new incorporation strategies to improve bioprocesses.

Irons differently modulate bacterial guilds for leading to varied efficiencies in simultaneous nitrification and denitrification (SND) within four aerobic bioreactors

As a novel biological wastewater nitrogen removal technology, simultaneous nitrification and denitrification (SND) has gained increasing attention. Iron, serving as a viable material, has been shown to influence nitrogen removal. However, the precise impact of iron on the SND process and microbiome remains unclear. In this study, bioreactors amended with iron of varying valences were evaluated for total nitrogen (TN) removal efficiencies under aerobic conditions. The acclimated control reactor without iron addition (NCR) exhibited high ammonia nitrogen (AN) removal efficiency (98.9%), but relatively low TN removal (78.6%) due to limited denitrification. The reactor containing zero-valent iron (Fe0R) demonstrated the highest SND rate of 92.3% with enhanced aerobic denitrification, albeit with lower AN removal (84.1%). Significantly lower SND efficiencies were observed in reactors with ferrous (Fe2R, 66.3%) and ferric (Fe3R, 58.2%) iron. Distinct bacterial communities involved in nitrogen metabolisms were detected in these bioreactors. The presence of complete ammonium oxidation (comammox) genus Nitrospira and anammox bacteria Candidatus Brocadia characterized efficient AN removal in NCR. The relatively low abundance of aerobic denitrifiers in NCR hindered denitrification. Fe0R exhibited highly abundant but low-efficiency methanotrophic ammonium oxidizers, Methylomonas and Methyloparacoccus, along with diverse aerobic denitrifiers, resulting in lower AN removal but an efficient SND process. Conversely, the presence of Fe2+/Fe3+ constrained the denitrifying community, contributing to lower TN removal efficiency via inefficient denitrification. Therefore, different valent irons modulated the strength of nitrification and denitrification through the assembly of key microbial communities, providing insight for microbiome modulation in nitrogen-rich wastewater treatment.

Abiotic and biotic roles of metals in the anaerobic digestion of sewage sludge: A review

Anaerobic digestion (AD) is a promising technique for sludge treatment and resource recovery. Metals are very important components of sludge and can have substantial effects on its complex nature and microbial activity. However, systematic reviews have not addressed how metals in sludge affect AD and how they can be regulated to improve AD. This paper comprehensively reviews the effects of metals on the AD of sludge from both abiotic and biotic perspectives. First, we introduce the contents and basic characteristics (e.g., chemical forms) of intrinsic metals in sewage sludge. Then, we summarise the main mechanism by which metals influence sludge properties and the methods for removing metals and thus improving AD. Next, we analyze the effects of both intrinsic and exogenous metals on the enzymes and microbial communities involved in anaerobic bioconversion, focusing on the types, critical concentrations and valence states of the metals. Finally, we propose ideas for future research on the roles of metals in the AD of sludge. In summary, this review systematically clarifies the roles of metals in the AD of sludge and provides a reference for improving AD by regulating these metals.

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.

The Potential of Bioaugmentation-Assisted Phytoremediation Derived Maize Biomass for the Production of Biomethane via Anaerobic Digestion

Anthropogenic behaviors are causing the severe build-up of heavy metal (HM) pollutants in the environment, particularly in soils. Amongst a diversity of remediation technologies, phytoremediation is an environmentally friendly technology that, when coupling tolerant plants to selected rhizospheric microorganisms, can greatly stimulate HM decontamination of soils. Maize (Zea mays) is a plant with the reported capacity for HM exclusion from contaminated soil but also has energetic importance. In this study, Zea mays was coupled with Rhizophagus irregularis, an arbuscular mycorrhizal fungus (AMF), and Cupriavidus sp. strain 1C2, a plant growth-promoting rhizobacteria (PGPR), as a remediation approach to remove Cd and Zn from an industrial contaminated soil (1.2 mg Cd kg-1 and 599 mg Zn kg-1) and generate plant biomass, by contrast to the conservative development of the plant in an agricultural (with no metal pollution) soil. Biomass production and metal accumulation by Z. mays were monitored, and an increase in plant yield of ca. 9% was observed after development in the contaminated soil compared to the soil without metal contamination, while the plants removed ca. 0.77% and 0.13% of the Cd and Zn initially present in the soil. The resulting biomass (roots, stems, and cobs) was used for biogas generation in several biomethane (BMP) assays to evaluate the potential end purpose of the phytoremediation-resulting biomass. It was perceptible that the HMs existent in the industrial soil did not hinder the anaerobic biodegradation of the biomass, being registered biomethane production yields of ca. 183 and 178 mL of CH4 g-1 VS of the complete plant grown in non-contaminated and contaminated soils, respectively. The generation of biomethane from HM-polluted soils' phytoremediation-derived maize biomass represents thus a promising possibility to be a counterpart to biogas production in an increasingly challenging status of renewable energy necessities.

Effect of metals on mesophilic anaerobic digestion of strawberry extrudate in batch mode

According to recent studies, the anaerobic digestion of strawberry extrudate is a promising option with potential in the berry industry biorefinery. However, the lack and/or unbalance of concentrations of metals in some agro-industrial residues could hamper methane production during the anaerobic digestion of these kinds of wastes. In this study, a fractional factorial design was applied to screen the supplementation requirements regarding six metals (Co, Ni, Fe, Cu, Mn, and Zn) for methane production from strawberry extrudate (SE). The logistic model was used to fit the experimental data of methane production-time. It allowed identifying two different stages in the anaerobic process and obtaining the kinetic parameters for each step. Maximum methane production obtained in the first (B_max) kinetic stage, the methane production in the second stage (P), and the maximum methane production rates (R_max) concluded a statistically significant effect for Ni and Zn. The second set of experiments was carried out with Ni and Zn through a central composite design to study the concentration effect in the anaerobic digestion process of the strawberry extrudate. The parameters P and R_max demonstrated a positive interaction between Ni and Zn. Although, B_max did not prove a statistically significant effect between Ni and Zn.

A critical review of exogenous additives for improving the anammox process

Anammox achieves chemoautotrophic nitrogen removal under anaerobic and anoxic conditions and is a low-carbon wastewater biological nitrogen removal process with broad application potential. However, the physiological limitations of AnAOB often cause problems in engineering applications, such as a long start-up time, unstable operation, easily inhibited reactions, and difficulty in long-term strain preservation. Exogenous additives have been considered an alternative strategy to address these issues by retaining microbes, shortening the doubling time of AnAOB and improving functional enzyme activity. This paper reviews the role of carriers, biochar, intermediates, metal ions, reaction substrates, redox buffers, cryoprotectants and organics in optimizing anammox. The pathways and mechanisms of exogenous additives, which are explored to solve problems, are systematically summarized and analyzed in this article according to operational performance, functional enzyme activity, and microbial abundance to provide helpful information for the engineering application of anammox.

Research on the electrochemistry synergied cellulase enzymes strengthens the anaerobic fermentation of cow dung

Lignocellulose degradation (LCD) was the key factor limiting the development of anaerobic fermentation (AF) by the cow dung (CD). In the research, the effects of cellulase pretreatment (CP) and microvoltage (MV) alone and combination on the gas production and LCD during the AF were investigated. The results showed that both CP and MV had an significant effect on the AF, though the synergistic reaction was more pronounced. The total biogas yield (TBY) could reach 11521 mL, and the highest methane production rate was 73% in the synergistic reactions, which was increased by 18.7% and 10.0% compared to CP and MV alone respectively. Meanwhile, the degradation rates of cellulose and lignin could be increased by 33.44% and 22.23%, respectively. The results of SEM, FT-IR and excitation emission matrix demonstrated that CP and MV played an important role in improving the fermentation efficiency. The microbial biomass change results indicated that the synergistic effect of CP and 0.8 V MV on the LCD was achieved by promoting the growth of lignocellulose-degrading bacteria. Moreover, the electricity could not only accelerate the movement of microorganisms and enzymes, but also promoted and enhanced the activity of enzymes, which provided an important reference for further development of the AF technology and the biogas industry.

Long-Term Limitation Effects of Se(VI), Zn(II), and Ni(II) on Start-Up of the Anammox Process Using Gel Carrier

Anaerobic ammonia oxidation (anammox) bacteria are inhibited by heavy metals at high concentrations but require trace amounts of some heavy-metal elements for growth and activity maintenance. The present study evaluates the long-term limitation effects of Se(VI), Zn(II), and Ni(II) on the start-up period of an anammox reactor. To strictly limit the levels of heavy metals in the reactor, all tests used ultrapure water as the influent synthetic wastewater and all reactors were installed in a clean booth. The anammox biomass was maintained through the gel entrapment technique. In the absence of Se(VI) and Ni(II), the anammox reactor start-up was 18.9 kg-N (m³-carrier d)⁻¹ (nitrogen conversion rate (NCR) per gel carriers), indicating that Se(VI) and Ni(II) are not required or need not be continuously added to maintain the anammox process. Under Zn(II) limitation, the anammox process failed to start-up and the NCR tended to decrease rapidly. After readdition of 0.005 mg L⁻¹ of Zn(II), the NCR did not decline further and instead partially recovered at a very slow rate. The NCR was completely recovered after adding 0.020 mg L⁻¹ of Zn(II). These results reveal that Zn(II) limitation seriously affects the start-up of the anammox process while Se(VI) and Ni(II) are not required or need not be continuously added to the anammox process.

Strategy of optimizing anaerobic digestion of cassava distiller wastewater using a novel automatic biological incubation system

Due to the drawbacks of using fossil fuels and the need to mitigate global warming caused by increasing greenhouse gas emissions, agricultural biomass for bioenergy production is gaining great interest around the world. This work presented a study at a biochemical plant in Lianyungang, Jiangsu Province, China to maximize methane production from cassava distiller wastewater. The plant's annual production of cassava distiller wastewater is more than 3 million tons and currently was treated using a series of 5000 m³ Internal Circulation (IC) reactors. Modification was applied at No.19 IC reactor by connecting it to two 1 m³ automatic biological incubators called Information Bio-Booster (IBB). The effluent of the IC reactor was fed into the IBBs and iron, cobalt and nickel were added directly in the IBBs. The function of the IBBs was to regulate the microbial community. Afterwards, the microorganisms in the IBBs were pumped back into the IC reactor to participate in the methane production reaction. Daily net increase of methane content and COD removal reached 8.02% and 33% respectively in No.19 IC reactor comparing to the unadjusted reactors. Preliminary lab experiments found that improvements of biogas production, enhanced COD removal and VS removal was closely related to the enhancement of anaerobic microbial communities' diversity and the promotion of enzyme activity through the addition of the metal salts. Daily economic value could be estimated to be $218 which indicated the application potential of using the proposed system to enhance anaerobic digestion at industrial plants for bioenergy production.

Overview on agricultural potentials of biogas slurry (BGS): applications, challenges, and solutions

The residual slurry obtained from the anaerobic digestion (AD) of biogas feed substrates such as livestock dung is known as BGS. BGS is a rich source of nutrients and bioactive compounds having an important role in establishing diverse microbial communities, accelerating nutrient use efficiency, and promoting overall soil and plant health management. However, challenges such as lower C/N transformation rates, ammonia volatilization, high pH, and bulkiness limit their extensive applications. Here we review the strategies of BGS valorization through microbial and organomineral amendments. Such cohesive approaches can serve dual purposes viz. green organic inputs for sustainable agriculture practices and value addition of biomass waste. The literature survey has been conducted to identify the knowledge gaps and critically analyze the latest technological interventions to upgrade the BGS for potential applications in agriculture fields. The major points are as follows: (1) Bio/nanotechnology-inspired approaches could serve as a constructive platform for integrating BGS with other organic materials to exploit microbial diversity dynamics through multi-substrate interactions. (2) Advancements in next-generation sequencing (NGS) pave an ideal pathway to study the complex microflora and translate the potential information into bioprospecting of BGS to ameliorate existing bio-fertilizer formulations. (3) Nanoparticles (NPs) have the potential to establish a link between syntrophic bacteria and methanogens through direct interspecies electron transfer and thereby contribute towards improved efficiency of AD. (4) Developments in techniques of nutrient recovery from the BGS facilities' negative GHGs emissions and energy-efficient models for nitrogen removal. (5) Possibilities of formulating low-cost substrates for mass-multiplication of beneficial microbes, bioprospecting of such microbes to produce bioactive compounds of anti-phytopathogenic activities, and developing BGS-inspired biofertilizer formulations integrating NPs, microbial inoculants, and deoiled seed cakes have been examined.

The impact of calcium supplementation on methane fermentation and ammonia inhibition of fish processing wastewater

The effect of trace metal supplementation on the methane fermentation of fish processing wastewater (FPW) was studied in both batch and continuous experiments using a self-agitated anaerobic baffled reactor (SA-ABR). In the batch experiments, a single supplementation of Ca²⁺, Co²⁺ and Fe²⁺ was show to have a significant positive impact on the performance of methane fermentation. The continuous experiment results showed that supplementation with 1.5 g-Ca²⁺/L-substrate remarkably enhanced the performance of methane fermentation of the SA-ABR in treating FPW with the optimal organic loading rate achieved at 7.62 g-COD/L/d. During the steady states (stages 2 to 5), the average removal efficiencies of COD, protein, carbohydrate and lipid were 89, 85, 80 and 91%, respectively. The biogas conversion rates were in the range of 0.39 to 0.45 L-biogas/g-COD with a high methane content of 74%. Besides, Ca²⁺ supplementation also improved the resistance of the methane fermentation system to ammonia inhibition.

Evaluation of the influence of trace metals on methane production from domestic sewage, using the Plackett-Burman experimental design

Methanogenesis involves several enzymes with trace metal components that catalyze major metabolic pathways and, therefore, requires a sufficient supply of micronutrients such as iron, nickel or cobalt. The statistically-based Plackett-Burman experimental design was adopted in this study to identify which trace metal have a statistically significant effect on the maximum methane production from domestic sewage. The addition of Barium (Ba), Cobalt (Co), Copper (Cu), Iron (Fe), Manganese (Mn), Nickel (Ni) and Selenium (Se) was tested in batch reactors using domestic sewage as the substrate and sewage sludge as the inoculum. The results showed that the addition of Fe was statistically significant, positively affecting the maximum CH₄ production (p-value 0.05). The results are expressed in L of CH₄ per g of Chemical Oxygen Demand added, increasing it from 0.10 to 0.13 LCH₄ gCOD_add^-1. In L of CH₄ per g of Volatile Solids, increased it from 0.031 to 0.040 LCH₄ gVS^-1. The addition of Se was statistically significant, but with a negative effect on the maximum methane production (p-value 0.002), which decreased from 0.13 to 0.10 LCH₄.gCOD_add^-1. Moreover, six different Fe concentrations (0, 40, 80, 120, 160 and 200 mgL^-1) were tested, showing that the addition of 120 mgL^-1 had the greatest effect for statistically improving the maximum methane production, with 33% improvement (0.12 ± 0.003 to 0.16 ± 0.012 LCH₄ gCOD_add^-1) compared to no addition of Fe and the specific CH₄ production to 0.040 ± 0.001 LCH₄ gVS^-1.

Enhanced Anaerobic Biogas Production From Wheat Straw by Herbal-Extraction Process Residues Supplementation

Trace metals are essential constituents of cofactors and enzymes and that their addition to anaerobic digesters increases methane production. Many trace elements are contained in herbal-extraction process residues (HPR). The present study concerns the effect of six kinds of HPR [Danshen root (Dr), Astragalus membranaceus (Am), Isatis root (Ir), Angelica sinensis (As), and Pseudo-ginseng (Pg)] that were used as additives, respectively, in the anaerobic digestion of wheat straw on biogas and methane production. The ratios of HPR residues/wheat straw [based on total solids (TS), of wheat straw] were 3, 5, and 10%, respectively. The digesters were at 37 ± 1°C of water bath during 30 days of anaerobic digestion. The results showed that HPR had significant effects on the anaerobic co-digestion. The highest biogas productivity was achieved when treated with 10% Pseudo-ginseng residues (PGR), which yielded 337 ml/g TS of biogas and 178 ml/g TS of methane. Cumulative production of biogas and methane increased by 28 and 37% compared to the production achieved in the control. These results suggest that PGR is an effective HPR to enhance the production of methane.

Denitrification using permeable reactive barriers with organic substrate or zero-valent iron fillers: controlling mechanisms, challenges, and future perspectives

Nitrate as a diffusive agricultural contaminant has been causing substantial groundwater quality deterioration worldwide. In situ groundwater remediation techniques using permeable reactive barriers (PRBs) have attracted increasing interest. Particularly, PRBs based on biological denitrification, using the organic substrate as a biostimulator, and chemical nitrate reduction, using zero-valent iron (ZVI) as a reductant, are two major PRB approaches for groundwater denitrification. This review paper analyzed the published studies over the past 10 years (2010-2020) using laboratory, modeling, and field-scale approaches to explore the performance and mechanisms of these two types of PRBs. Important factors affecting the denitrification efficiencies as well as the influential mechanisms were discussed. Several research gaps have been identified and further research needs are discussed in the end.

A general framework to model the fate of trace elements in anaerobic digestion environments

Due to the multiplicity of biogeochemical processes taking place in anaerobic digestion (AD) systems and limitations of the available analytical techniques, assessing the bioavailability of trace elements (TEs) is challenging. Determination of TE speciation can be facilitated by developing a mathematical model able to consider the physicochemical processes affecting TEs dynamics. A modeling framework based on anaerobic digestion model no 1 (ADM1) has been proposed to predict the biogeochemical fate TEs in AD environments. In particular, the model considers the TE adsorption–desorption reactions with biomass, inerts and mineral precipitates, as well as TE precipitation/dissolution, complexation reactions and biodegradation processes. The developed model was integrated numerically, and numerical simulations have been run to investigate the model behavior. The simulation scenarios predicted the effect of (i) organic matter concentration, (ii) initial TEs concentrations, (iii) initial Ca–Mg concentrations, (iv) initial EDTA concentration, and (v) change in TE binding site density, on cumulative methane production and TE speciation. Finally, experimental data from a real case continuous AD system have been compared to the model predictions. The results prove that this modelling framework can be applied to various AD operations and may also serve as a basis to develop a model-predictive TE dosing strategy.

Enhanced anaerobic digestion with the addition of chelator-nickel complexes to improve nickel bioavailability

Nickel (Ni) is one of the most essential trace elements in the anaerobic digestion system. In this study, green chelating agent Ethylenediamine-N, N'-disuccinic acid (EDDS), common chelating agents with low biodegradability nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) were respectively used as ligands of Ni (II) to inspect the feasibility of enhancing methane production and reducing Ni dosage. In practice, continuous stirred-tank reactors (37 °C, 120 rpm) were operated with a mixture of pig manure and food waste as the substrate, and were supplied with extra Ni in the form of Ni (II) (0, 2.5, and 5.0 mg/L) or chelator‑nickel (EDDS-Ni, NTA-Ni and EDTA-Ni) complexes (2.5 mg/L). The results showed that compared with that of adding Ni (2.5 mg/L) individually, the methane production increased of 23.34%, 31.26% and 16.07% with the addition of EDDS-Ni, NTA-Ni and EDTA-Ni complexes (2.5 mg/L), respectively. Accompanying with that, the EDDS-Ni and NTA-Ni supplementations both significantly increased the F430 concentration of 28% and 36% on the day of peak methane production (day five). The BCR sequential extraction analysis indicated that the sum of Ni in water soluble and exchangeable fractions after digestion were increased of 43.28%, 39.41%, and 24.29%, respectively. Further, the acid-volatile sulfide (AVS) and the simultaneously extracted nickels (SEMNi) content in sediments confirmed that the chelator‑nickel improved Ni bioavailability due to dissolution of nickel ions from their sulfides. This study demonstrated that the addition of chelator-Ni complexes was a practicable method to enhance methane production and reduced Ni dosage.

Novel biorefining method for succinic acid processed from sugarcane bagasse

Sugarcane bagasse (SCB) was pretreated with hot water (HLW), ethanol (ETH), and sodium hydroxide (SH). The obtained residuals were hydrolyzed and applied as carbon sources for succinic acid (SA) fermentation, the residue digestibility and SA conversion rate of alkali-pretreated residual were superior to others. Considering the characteristics of alkali pretreatment, enzymatic hydrolysis and succinic acid fermentation, a novel in-situ semi-simultaneous saccharification and co-fermentation (SSSCF) procedure for SA production from SCB was developed. The yield, productivity, and conversion rates of SA from SCB raw material (DRM) processed by SSSCF were 41 g/L, 300 mg/L/h, and 320 mg/g dry, respectively. For every kilogram of SA production, the developed coupling method reduced the SH and water usages, energy consumption, and effluent emission by 0.14 kg, 233.5 L 14,000 kJ and 7 L, respectively, and enhanced the SA productivity by 1.7 times compared with the non-coupling procedure.

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.

Operational Parameters of Biogas Plants: A Review and Evaluation Study

The biogas production technology has improved over the last years for the aim of reducing the costs of the process, increasing the biogas yields, and minimizing the greenhouse gas emissions. To obtain a stable and efficient biogas production, there are several design considerations and operational parameters to be taken into account. Besides, adapting the process to unanticipated conditions can be achieved by adequate monitoring of various operational parameters. This paper reviews the research that has been conducted over the last years. This review paper summarizes the developments in biogas design and operation, while highlighting the main factors that affect the efficiency of the anaerobic digestion process. The study’s outcomes revealed that the optimum operational values of the main parameters may vary from one biogas plant to another. Additionally, the negative conditions that should be avoided while operating a biogas plant were identified.

Biogas Production from Food Residues—The Role of Trace Metals and Co-Digestion with Primary Sludge

The majority of municipal Wastewater Treatment Plants (WWTPs) in Sweden produce biogas from sewage sludge. In order to increase the methane production, co-digestion of internal sludge with Organic Fraction of Municipal Solid Waste (OFMSW) might be feasible in the future. The objective of this study was therefore to find a beneficial solution for the utilization of OFMSW at the WWTP in Varberg, Sweden. The effects of co-digesting primary sludge (PS) and OFMSW collected in the municipality, in different mixing ratios, were investigated by semi-continuous anaerobic digestion assays. Furthermore, the effects of the addition of a commercial trace elements mixture solution (CTES), available on the market in Sweden, were also examined. Co-digestion of OFMSW and PS resulted in specific methane yields of 404, 392, and 375 NmL CH4/g volatile solids (VS), obtained during semi-continuous operations of 301, 357 and 385 days, for the reactors fed with OMFSW:PS ratio of 4:1, 3:1, and 1:1, and at maximum organic loading rates (OLRs) achieved of 4.0, 4.0 and 5.0 gVS/L/d, respectively. Furthermore, mono-digestion of OFMSW failed already at OLR of 1.0 gVS/L/d, however, an OLR of 4.0 gVS/L/d could be achieved with addition of 14 µL/g VS Commercial Trace Element Solutions (CTES) leading to 363 mL CH4/g VS methane production. These experiments were running during 411 days. Hence, higher process efficiency was obtained when using co-digestion of OFMSW and PS compared to that of OFMSW in mono-digestion. Co-digestion is a more feasible option where a balanced Carbon/Nitrogen (C/N) ratio and nutrient supply can be maintained.

Effects of copper salts on performance, antibiotic resistance genes, and microbial community during thermophilic anaerobic digestion of swine manure

This study investigated methane production and ARGs reduction during thermophilic AD of swine manure with the addition of different Cu salts (cupric sulfate, cupric glycinate, and the 1:1 mixture of these two salts). Results showed methane production was increased by 28.78% through adding mixed Cu salts. The mixed Cu group effectively reduced total ARGs abundance by 26.94%, suggesting mixed Cu salts did not promote the potential ARGs risk. The positive effects of mixed Cu salts on AD performance and ARGs removal might be ascribed to the low bioavailability. Microbial community analysis indicated the highest abundances of Clostridia_MBA03 and Methanobacterium in the mixed Cu group might cause the increased methane production. Spearman's rank correlation analysis elucidated the succession in microbial community induced by environmental factors was the main driver for shaping ARGs profiles. Thus, mixed Cu salts could be an alternative to replace the inorganic Cu salt in animal feed additives.

Effects of temperature, hydrogen/carbon monoxide ratio and trace element addition on methane production performance from syngas biomethanation

Synthesis gas (Syngas) biomethanation is an environmentally friendly technology for fuel calorific value improvement. However, the slow mass transfer and poor product quality limit its development. In this study, the effects of temperature, hydrogen/carbon monoxide (H₂/CO) ratio and trace element addition on simulated syngas biomethanation were investigated in three batches of experiments. Results showed that (1) the temperature influenced little on the quality of produced biogas; (2) the methane content in the biogas production were 66.37 ± 4.04%, 70.61 ± 6.06% and 73.35 ± 2.39% respectively with the H₂/CO ratio of 3:1, 4:1 and 5:1; (3) after the addition of Fe, Co and Ni elements, the biogas quality was significantly improved (methane content was 79.76 ± 7.35%), but the microbial community structure did not change. This experiment provided a guidance for improving the biogas production performance of syngas biomethanation.

Impact of Fe and Ni Addition on the VFAs' Generation and Process Stability of Anaerobic Fermentation Containing Cd

The effects of Cd, Cd + Fe, and Cd + Ni on the thermophilic anaerobic fermentation of corn stover and cow manure were studied in pilot experiments by investigating the biogas properties, process stability, substrate biodegradation, and microbial properties. The results showed that the addition of Fe and Ni into the Cd-containing fermentation system induced higher cumulative biogas yields and NH₄⁺–N concentrations compared with the only Cd-added group. Ni together with Cd improved and brought forward the peak daily biogas yields, and increased the CH₄ contents to 80.76%. Taking the whole fermentation process into consideration, the promoting impact of the Cd + Ni group was mainly attributed to better process stability, a higher average NH₄⁺–N concentration, and increased utilization of acetate. Adding Fe into the Cd-containing fermentation system increased the absolute abundance of Methanobrevibacter on the 13th day, and Methanobrevibacter and Methanobacterium were found to be positively correlated with the daily biogas yield. This research was expected to provide a basis for the reuse of biological wastes contaminated by heavy metals and a reference for further studies on the influence of compound heavy metals on anaerobic fermentation.

Methanogenesis stimulation and inhibition for the production of different target electrobiofuels in microbial electrolysis cells through an on-demand control strategy using the coenzyme M and 2-bromoethanesulfonate

Electron allocation through the suppression or the stimulation of methanogenesis is critical for microbial electrolysis cells (MECs) to produce the desired target product (e.g., CH₄ or H₂). In this study, selective methanogenesis control using the coenzyme M (CoM) and 2-bromoethanesulfonate (2-BES) was investigated in a two-chambered MEC to evaluate the effect of CoM and 2-BES on the production of different electrobiofuels, net energy conversion efficiency and microbial community structure. Because the CoM is a crucial methyl-group carrier in the final process of methanogenesis, it was postulated that CoM would stimulate methanogenic activity at the anode, while a structural analog of the CoM (i.e., 2-BES) was expected to improve cathodic H₂ yield using electrons conserved because of methanogen inhibition (electron equivalence: 8 mol e^- = 1 mol CH₄ = 4 mol H₂). CoM injection in MECs significantly enhanced their CH₄ production rate, purity, and yield by 4.5-fold, 14.5%, and 76.1%, respectively, compared to the control. Moreover, microbial community analysis indicated that Methanosaeta, the major acetoclastic methanogen, continued to dominate the microbial community but steadily decreased in relative abundance after the CoM injection. On the other hand, drastic increases in hydrogenotrophic methanogens, such as Methanoculleus and Methanolinea, were observed along with potential syntrophic acetate-oxidizing bacteria. In contrast, CH₄ production in the 2-BES injected trials was significantly inhibited by 79.5%, resulting in a corresponding increase of H₂ production by 145.5% compared to the control. Unlike the CoM, the microbial community did not noticeably change when 2-BES was injected, although the population size gradually decreased over time. Also, a single injection of CoM and 2-BES, even at low concentrations (500 μM), enabled the desired allocation of electrons as characterized by a high sensitivity, fast response, and negligible interference. In terms of energy conversion efficiency, methanogenesis stimulation approach resulted in higher net energy production than inhibition approach, whereas the remained electrons were not fully converted to hydrogen in case of the inhibition trial, thus producing less energy.

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.

ADM1-based mechanistic model for the role of trace elements in anaerobic digestion processes

An original mechanistic model able to describe the fate of trace elements (TE) in anaerobic digestion systems has been synthetized from mass balance equations. The model takes into account the main biochemical and physico-chemical processes affecting TE bioavailability and it is aimed at evaluating the effect that the combination of such processes exerts on the system performance. Five main modules have been introduced: biochemistry, physico-chemistry, sorption, complexation and precipitation. The model is based on mass conservation principles and is formulated as a set of ordinary differential equations for the soluble and particulate components constituting the system. Model applications of two illustrative cases are provided. The first case is based on experimental results and examines the effect of TE depletion in an AD process of food waste (FW). The second case shows the effects of different metal supplements on methane production and biogas composition. The simulation results confirm that the model can fairly be used to predict the effect of TE dynamics and bioavailability, by considering biological, chemical and physicochemical processes in AD environments.

Improved anaerobic co-digestion of food waste and domestic wastewater by copper supplementation - Microbial community change and enhanced effluent quality

Anaerobic co-digesters are biorefineries for energy recovery from food waste and domestic wastewater via methane production. Nonetheless, the performance of this technology was not always satisfied due to the long chain fatty acids (LCFAs) generation from food waste. Micronutrient supplementation is an effective strategy that could be applied during the anaerobic (co-)digestion to further enhance the digestion efficiency while treating food waste. In this study, supplementing copper (as CuSO₄ and CuCl₂) at 10, 30, and 50 mg/L Cu²⁺ was selected to further enhance the methane production of anaerobic co-digester while treating food waste and domestic wastewater. Overall, with the supplementation of copper, the chemical oxygen demand (COD) removal efficiency was over 90%, while higher methane yields (0.260-0.325 L CH₄/g COD _removed) were obtained compared to the control without supplementation (0.175 L CH₄/g COD _removed). For the cumulative methane yield, the highest increment of 94.1% was obtained when 10 mg/L of Cu²⁺ were added. The results showed copper as a cofactor of many microbial enzymes and coenzymes involved in the methane production further improved both methane production and COD removal efficiency. Meanwhile, the microbial community analysis verified the copper supplementation significantly changed the bacterial communities but with the limited effect on the diversity of archaea. Furthermore, since the anaerobic co-digester was not that much efficient on the nutrients removal, the effluent from the upflow anaerobic sludge blanket (UASB) reactor was further treated by the anaerobic/anoxic/oxic (A²O) rector and the resulting effluent reached the satisfying quality in terms of COD, total nitrogen (TN), and NH₃-N removal, meeting the regional effluent discharge limits.

Mechanism of process imbalance of long-term anaerobic digestion of food waste and role of trace elements in maintaining anaerobic process stability

This study investigated mechanism of process imbalance of long-term anaerobic digestion (AD) of food waste (FW) and role of trace elements (TEs) in maintaining process stability. AD of FW was strongly inhibited by volatile fatty acids (VFA, mainly propionate). The deficiency of essential TEs in FW was the fundamental reason. TEs contents in digester gradually decreased with regular substrate feeding and digestate discharge, which greatly limited growth and metabolism of hydrogenotrophic methanogens and Methanosarcina. Finally, Methanosaeta replaced Methanosarcina and became dominant methanogen and hydrogenotrophic methanogens almost disappeared accompanied by declining methanogenic community diversity, which greatly suppressed ecological functions of methanogens and led to propionate inhibition. TEs supplementation eliminated all factors causing process imbalance and significantly enhanced process stability by maintaining strong ecological functions of methanogens via stimulating dominant growth of Methanosarcina (relative abundance between 67.2% and 87.5%), sustaining stable relative abundances of hydrogenotrophic methanogens (about 10%) and enhancing methanogenic community diversity.

Recovery of unstable digestion of vegetable waste by adding trace elements using the bicarbonate alkalinity to total alkalinity ratio as an early warning indicator

Vegetable waste (VW) is highly perishable and susceptible to acidification during anaerobic digestion, which inhibits biogas production. Effective process monitoring, diagnosis and control are necessary to maintain stable anaerobic digestion at a high organic loading rate (OLR). Continuous mesophilic digestion was conducted at OLRs of 0.5, 1.0, 1.5, 2.0, 3.0, 3.5 and 4.0 g volatile solids (VS)/(L d) with effluent recirculation (ER) in a reactor with total volume of 70 L. The effectiveness of three early warning indicators was validated. The ability of trace elements (TEs) (Fe, Co and Ni) to recover unstable VW digestion systems was evaluated. The results showed that the ratio of bicarbonate alkalinity (BA) to total alkalinity (TA) was a more effective warning indicator than the ratios of methane (CH₄) to carbon dioxide and volatile fatty acids (VFAs) to TA. When the ratio of BA/TA was lower than 0.9, the digestion system tended to be unstable. ER maintained a stable OLR of 1.5 g VS/(L d). The addition of TEs achieved a maximum stable OLR of 3.5 g VS/(L d) with an average volumetric biogas production rate of 1.91 L/(L d). Severe VFAs accumulation and unrecoverable instability occurred at an OLR of 4.0 g VS/(L d). The supplementation of ammonium bicarbonate was not useful for the recovery of the unstable system when the OLR was greater than 3.5 g VS/(L d) for the digestion of VW. The specific methane production was approximately 340 L/kg VS during the stable period with a digestion efficiency of 85%.

Adsorption mechanism of ZnO and CuO nanoparticles on two typical sludge EPS: Effect of nanoparticle diameter and fractional EPS polarity on binding

Worldwide application of nanotechnology has led to an increasingly release of nanoparticles in wastewater treatment systems, and thus into sewage sludge, which potentially impairs the disposal of sewage sludge. Here, the binding process, adsorption characteristics, and the contribution of fractional polarity of extracellular polymeric substances (EPS) of anaerobic granular sludge (AGS) and activated sludge (AS) to the nano-ZnO and nano-CuO adsorption were investigated. Briefly, CuO-NPs can be more efficiently adsorbed by the EPS-AGS than that of ZnO-NPs (1.31 ± 0.08 g/g VS vs 0.53 ± 0.04 g/g VS), and a smaller diameter of nanoparticles benefited the adsorption processes. Hydrophobic EPS (HPO-A and HPO-N) within these two sludge were more effective in removing nano-CuO and ZnO than were the hydrophilic fraction. For example, HPO-A and HPO-N obtained from AGS showed a relatively higher adsorption abilities (in g/g VS) of 2.09 ± 0.12 and 2.27 ± 0.14, respectively, for nano-CuO, much higher than HPI (0.76 ± 0.04 g/g VS). Structural variations of the EPS before and after nanoparticles sorption were evaluated via the analysis of infrared spectroscopy, which showed that the functional structures of hydroxyl, amino, carboxyl, amide groups and C-O-C groups played a major role in nanoparticles binding/removal. Sorption process of nano-CuO and nano-ZnO on unfractionated EPS well fitted by Langmuir isotherm, as well as a pseudo second-order kinetic model. However, adsorption process of HPO-A can be better simulated by Freundlich equation.

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.

ADM1 based mathematical model of trace element precipitation/dissolution in anaerobic digestion processes

Due to the complex biogeochemistry of trace elements (TEs, e.g. Fe, Ni and Co) in anaerobic digestion processes, their role and fate is poorly understood. Challenging, time consuming and low detection limits of analytical procedures necessitate recruitment of mathematical models. A dynamic mathematical model based on anaerobic digestion model no.1 (ADM1) has been proposed to simulate the effect of TEs. New chemical equilibrium association/dissociation and precipitation/dissolution reactions have been implemented to determine TE bioavailability and their effect on anaerobic digestion. The model considers interactions with inorganic carbonate (HCO₃^- and CO₃^2-), phosphate (PO₄^3-, HPO₄^2-, H₂PO₄^-) and sulfide (HS^- and S^2-). The effect of deficiency, activation, inhibition and toxicity of TEs on the biochemical processes has been modelled based on a dose-response type inhibition function. The new model can predict: the dynamics of TEs (among carbonate, sulfide and phosphate); the starvation of TEs; and the effect of initial sulfur-phosphorus ratio in an in-silico batch anaerobic system.

Real-time recovery strategies for volatile fatty acid-inhibited anaerobic digestion of food waste for methane production

This study investigated effects of real-time recovery strategies on VFA (volatile fatty acid)-inhibited anaerobic system of FW (food waste) and identified key driver of process recovery. The long-term anaerobic system of FW encountered serious VFA (mainly propionate) inhibition. The pH adjustment (PA) strategy could not reverse process imbalance but only delayed the process failure. The short-term effect of reinoculation (RI) strategy was greatly effective, but its long-term effect was non-sustainable. Trace elements were key drivers of process recovery owing to their indispensable roles in activating methanogenesis and therefore stimulating propionate conversion. From the viewpoint of economic feasibility, the single strategy of trace elements supplementation (TE) and the combinational strategy of PA + TE were respectively recommended in the initial and medium VFA-inhibition stages. The three-in-one strategy of PA + TE + RI was always effective but was costly. This study provided practical guidance on real-time recovery of VFA-inhibited anaerobic system of FW.

Impact of trace element supplementation on mesophilic anaerobic digestion of food waste using Fe-rich inoculum

Trace elements (TEs) play an indispensable role in enhancing the stability of anaerobic digestion (AD) of food waste (FW). Significant research on AD of FW with TE supplementation has been conducted with low Fe content inoculum. However, the use of Fe-rich inoculum is inevitable due to chemical phosphorous removal from wastewater in North America. We conducted comprehensive mesophilic batch tests to investigate the effect of TEs (Fe, Ni, Co, Se, and Mo) on FW digestion inoculated with Fe-rich sludge (≥ 1000 mg Fe L^-1). This paper presents the impact of supplementing various concentrations of TEs on specific methanogenic activity (SMA), maximum specific methane production rate (SMPR_max), and apparent hydrolysis rate constant (K_h). The addition of TEs adversely impacted methanogenic activity by 20 to 58% in the SMA tests. The effects of individual and mixed supplementation of TEs on the SMPR_max and K_h during FW digestion were negligible; exceptions include Fe, Mo, and Co. Final soluble TE concentrations were 10-29% of the initial soluble TEs. The high Fe concentration in the inoculum reduces the bioavailable fraction of added TEs via coprecipitation. Contrary with many literature reports indicating the need to supplement TE to improve FW digestion efficiency, with Fe-rich sludges, FW digestion does not require TE supplementation.

Role and application of iron in water treatment for nitrogen removal: A review

It is crucial to have a review on the role of iron in water treatment for the guidance towards the selection of appropriate processes, content of iron, and application conditions, as there are few reviews available at present and the systematic information is lacking for both researchers and engineers. The objectives of this review are to summarize the state of arts with respect to iron applied in nitrogen removal, discuss chemical and biological or bio-chemical combined nitrogen removal pathways and processes coupled with iron, and to reveal reaction mechanisms as well as providing references or even solutions to pertinent the practical engineering application of nitrate removal coupling with iron. The following information have been summarized and discussed in details: (1) iron based materials with varieties of preparations and forms, (2) major coupling ways of nitrogen removal methods or processes with iron application, (3) chemical reaction equations about a variety of chemical and biological or bio-chemical combined processes and the main mechanisms. In addition, challenges and/or drawbacks during the nitrogen removal processes will also be discussed in this paper, which is aimed to seek better practical engineering applications of nitrate removal coupling with iron.

Screening of trace metal supplementation for black water anaerobic digestion

Community on-site separation of wastewater is a treatment approach that leads to more efficient processes. Black water has high organic matter content and can be a suitable feedstock for anaerobic treatment systems. Biological methane production (BMP) tests were conducted using Plackett-Burman design to screen the effects of adding Fe, Ni, Cu, Co, Mn, Ba and Se, with simulated black water (SBW) as the substrate. In the inoculum, most metals were found mainly in the organic matter/sulfide and residual fractions except for Mn, which was present at 12.3% in the bioavailable fractions (exchangeable and carbonates), and Ba, which was evenly distributed among all the fractions. Ba had a significant negative effect on methane production and Mn addition enhanced the toxic effect. A specific methanogenic activity (SMA) between 18% and 27% lower than the control, was predicted at a total Ba concentration of approximately 1000-1200 mg L^-1. Similar SMA was predicted at Ba concentration between 400 and 600 mg L^-1 when 0.55 mg L^-1 of Mn is added. Se and Cu additions demonstrate the potential to improve the methane production from SBW. The SMA was predicted to reach 12 mLCH₄ gCOD^-1 d^-1 when Cu and Se are supplied at total concentrations of 3.0 mg L^-1 and 0.98 mg L^-1, respectively.

Bioavailability and dosing strategies of mineral in anaerobic mono-digestion of maize straw

The influence of the bonding form distribution of Fe, Ni, Co and Mn and their potential bioavailability during the anaerobic degradation of maize straw was investigated. Two reactors were operated over 117 days at 37°C and different dosage strategies of mineral were studied in reactor (R2). Control reactor (R1) was metal-limited over time. mineral supplementation (1 g L^-1) once a week reported the highest methane yield (257 mL g^-1 VS) with 30% of increment. Ni and Co predominated in their oxidizable bonding forms and Fe mainly existed as residual and oxidizable fractions. The potential bioavailability (Mn ˃˃ Co ≈ Ni ˃ Fe) of R2 was higher comparing to R1. Metal deprivation in R1 led to depletion of both sequential extraction fractions and total metal concentrations until the end of the process. This study confirmed that the dosage strategy of mineral has a stimulatory effect on methane production from crop maize waste.

Efficient Anaerobic Digestion of Microalgae Biomass: Proteins as a Key Macromolecule

Biogas generation is the least complex technology to transform microalgae biomass into bioenergy. Since hydrolysis has been pointed out as the rate limiting stage of anaerobic digestion, the main challenge for an efficient biogas production is the optimization of cell wall disruption/hydrolysis. Among all tested pretreatments, enzymatic treatments were demonstrated not only very effective in disruption/hydrolysis but they also revealed the impact of microalgae macromolecular composition in the anaerobic process. Although carbohydrates have been traditionally recognized as the polymers responsible for the low microalgae digestibility, protease addition resulted in the highest organic matter solubilization and the highest methane production. However, protein solubilization during the pretreatment can result in anaerobic digestion inhibition due to the release of large amounts of ammonium nitrogen. The possible solutions to overcome these negative effects include the reduction of protein biomass levels by culturing the microalgae in low nitrogen media and the use of ammonia tolerant anaerobic inocula. Overall, this review is intended to evidence the relevance of microalgae proteins in different stages of anaerobic digestion, namely hydrolysis and methanogenesis.

Enhanced performance and kinetics of marine anammox bacteria (MAB) treating nitrogen-rich saline wastewater with Mn(II) and Ni(II) addition

A sequencing batch reactor (SBR) was used to study nitrogen removal performance of marine anammox bacteria (MAB) with Mn(II) and Ni(II) addition. The reactor was operated at 25 ± 0.5 °C with influent pH of 7.5 ± 0.1. Optimal ammonium removal efficiencies (AREs) were 93.95% and 93.18% with 0.05 mM Mn(II) and 0.025 mM Ni(II), respectively. Both Mn(II) and Ni(II) played key roles in treating nitrogen-rich saline wastewater. However, the effect resulting from Ni(II) was far stronger than Mn(II). With optimal Ni(II) addition (0.025 mM), maximal nitrogen removal rate (NRR) and specific anammox activity (SAA) increased by 14.64% and 57.88%, respectively. Modified Boltzmann model was appropriate to describe nitrogen removal at low Mn(II) and Ni(II) concentrations while remodified Logistic model could be used at high Mn(II) and Ni(II) concentrations. Mn(II) and Ni(II) dosage should be controlled within 0.075 mM to achieve good nitrogen removal in nitrogen-rich saline wastewater treatment.

Instability mechanisms and early warning indicators for mesophilic anaerobic digestion of vegetable waste

In order to elucidate the instability mechanism, screen early warning indicators, and propose control measures, the mesophilic digestion of vegetable waste (VW) was carried out at organic loading rates (OLR) of 0.5, 1.0, and 1.5g volatile solid (VS)/(Ld). The process parameters, including biogas components, volatile fatty acids (VFA), ammonia, pH, total alkalinity (TA), bicarbonate alkalinity (BA), and intermediate alkalinity (IA), were monitored every day. Digestion was inhibited at OLR of 1.5gVS/(Ld). The primary causes of instability are a high sugar and negligible ammonia content, in addition to the feed without effluent recirculation, which led to BA loss. The ratios of CH₄/CO₂, VFA/BA, propionate, n-butyrate and iso-valerate were selected as early warning indicators. In order to maintain the digestion of VW at a high OLR, control measures including effluent recirculation and trace element addition are recommended.

Investigation of the impact of trace elements on anaerobic volatile fatty acid degradation using a fractional factorial experimental design

The requirement of trace elements (TE) in anaerobic digestion process is widely documented. However, little is understood regarding the specific requirement of elements and their critical concentrations under different operating conditions such as substrate characterisation and temperature. In this study, a flask batch trial using fractional factorial design is conducted to investigate volatile fatty acids (VFA) anaerobic degradation rate under the influence of the individual and combined effect of six TEs (Co, Ni, Mo, Se, Fe and W). The experiment inoculated with food waste digestate, spiked with sodium acetate and sodium propionate both to 10 g/l. This is followed by the addition of a selection of the six elements in accordance with a 2^6-2 fractional factorial principle. The experiment is conducted in duplicate and the degradation of VFA is regularly monitored. Factorial effect analysis on the experimental results reveals that within these experimental conditions, Se has a key role in promoting the degradation rates of both acetic and propionic acids; Mo and Co are found to have a modest effect on increasing propionic acid degradation rate. It is also revealed that Ni shows some inhibitory effects on VFA degradation, possibly due to its toxicity. Additionally, regression coefficients for the main and second order effects are calculated to establish regression models for VFA degradation.

Influence of zinc on biogas production and antibiotic resistance gene profiles during anaerobic digestion of swine manure

This study determined the accumulated biogas, methane content, and absolute abundances (AAs) of 14 common antibiotic resistance genes (ARGs) and two integrons during the anaerobic digestion of swine manure for 52days with different amounts of added zinc. The accumulated biogas increased by 51.2% and 56.0% with 125mgL^-1 (L) and 1250mgL^-1 (H) zinc, respectively, compared with the control with no added zinc (CK), but there was no significant difference between L and H. Compared with CK, excluding tetW and tetC, all the other ARGs detected in this study increased in the L and H reactors. However, the low concentration of zinc (L reactor) caused greater increases in the AAs of ARGs in the AD products. Redundancy analysis showed that NO₃-N and bio-zinc significantly explained the changes in genes, where they accounted for 60.9% and 20.3% of the total variation in the environmental factors, respectively.

Effect of magnetite powder on anaerobic co-digestion of pig manure and wheat straw

This study investigated the effects of different amounts of magnetite powder (i.e., 0g, 1.5g, 3g, 4.5g, 6g) on the anaerobic co-digestion of pig manure (PM) and wheat straw (WS). The variations in pH, alkalinity, cellulase activity (CEA), dehydrogenase activity (DHA) and methane production, were analyzed by phases. Correlation of the activities of the two enzymes with methane production was also analyzed, and the Gompertz model was used to evaluate the efficiency of anaerobic digestion (AD) with the addition of magnetite powder. The results showed that magnetite powder had significant effects on the anaerobic co-digestion of PM and WS. The maximum total methane production with the addition of 3g of magnetite powder was 195mL/g total solids (TS), an increase of 72.1%. The CEA and DHA increased with magnetite powder in the ranges of 1.5-4.5g, 1.5-6g, respectively, while the methane production showed a better correlation with DHA than with CEA. Using the Gompertz model, the efficiency of AD was optimal when adding 3g magnetite powder, with higher methane production potential (206mL/g TS), shorter lag-phase time (14.9d) and shorter AD period (44d).

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.

Metal fractionation in sludge from sewage UASB treatment

This study evaluates the trace metal composition and fractionation in sludge samples from anaerobic sewage treatment plants from six cities in Brazil. Ten metals were evaluated: Ni, Mn, Se, Co, Fe, Zn, K, Cu, Pb and Cr. Specific methanogenic activity of the sludge was also evaluated using acetic acid as the substrate. Among the essential trace metals for anaerobic digestion, Se, Zn, Ni and Fe were found at a high percentage in the organic matter/sulfide fraction in all sludge samples analyzed. These metals are less available for microorganisms than other metals, i.e., Co and K, which were present in significant amounts in the exchangeable and carbonate fractions. Cu is not typically reported as an essential metal but as a possible inhibitor. One of the samples showed a total Cu concentration close to the maximal amount allowed for reuse as fertilizer. Among the non-essential trace metals, Pb was present in all sludge samples at similar low concentrations and was primarily present in the residual fraction, demonstrating very low availability. Cr was found at low concentrations in all sludge samples, except for the sludge from STP5; interestingly, this sludge presented the lowest specific methanogenic activity, indicating possible Cr toxicity.

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%.

Start-up of an anaerobic fluidized bed reactor treating synthetic carbohydrate rich wastewater

The present work studied the start-up process of a mesophilic (37 ± 2 °C) anaerobic fluidized bed reactor (AFBR) operated at a hydraulic retention time (HRT) of 20 days using synthetic carbohydrate rich wastewater. Anox Kaldness-K1 carriers were used as biofilm carrier material. The reactor performance and biofilm formation were evaluated during the process. The start-up process at lower liquid recirculation flow rate enhanced the biofilm formation and reactor performance. The organic substrate composition had a major impact on early colonization of methanogenic archaea onto the surface of the Kaldness carriers during the start-up process. Specific organic substrates favouring the growth of methanogenic archaea, such as acetate, are preferred in order to facilitate the subsequent biofilm formation and AFBR start-up. The supply of 'bio-available' nutrients and trace elements, in particular iron, had an important role on optimal methanogenic activity and speeding-up of the biofilm development on the Kaldness carriers. This paper provides possible strategies to optimize the various operational parameters that influence the initial biofilm formation and development in an AFBR and similar high rate anaerobic reactors, hence can be used to reduce the long time required for process start-up.

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.