Importance of reduced sulfur for the equilibrium chemistry and kinetics of Fe(II), Co(II) and Ni(II) supplemented to semi-continuous stirred tank biogas reactors fed with stillage

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.

Dynamic modelling the effects of ionic strength and ion complexation on trace metal speciation during anaerobic digestion

Dosing trace metals into anaerobic digestors is proven to improve biogas production rate and yield by stimulating microorganisms involved in the metabolic pathways. Trace metal effects are governed by metal speciation and bioavailability. Though chemical equilibrium speciation models are well-established and widely used to understand metal speciation, the development of kinetic models considering biological and physicochemical processes has recently gained attention. This work proposes a dynamic model for metal speciation during anaerobic digestion which is based on a system of ordinary differential equations aimed to describe the kinetics of biological, precipitation/dissolution, gas transfer processes and, a system of algebraic equations to define fast ion complexation processes. The model also considers ion activity corrections to define effects of ionic strength. Results from this study shows the inaccuracy in predicting trace metal effects on anaerobic digestion by typical metal speciation models and the significance of considering non-ideal aqueous phase chemistry (ionic strength and ion pairing/complexation) to define speciation and metal labile fractions. Model results show a decrease in metal precipitation and increase in metal dissolved fraction and methane production yield with increase in ionic strength. Capability of the model to dynamically predict trace metal effects on anaerobic digestion under different conditions, like changing dosing conditions and initial iron to sulphide ratio, was also tested and verified. Dosing iron increases methane production and decreases hydrogen sulphide production. However, when iron to sulphide ratio is greater than 1, methane production decreases due to increase in dissolved iron which reaches inhibitory concentration levels.

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.

Nickel complexation as an innovative approach for nickel-cobalt selective recovery using sulfate-reducing bacteria

This study evaluated the differences in nickel (Ni) and cobalt (Co) solubility in the presence of sulfate reducing bacteria (SRB) to evaluate the feasibility of selective recovery of both metals from mine-impacted waters. A series of sulfate reducing activity tests with Ni, Co and both metals showed that up to 99 % Ni remained soluble despite the availability of sulfide for precipitation, while Co sulfide precipitation always occurred (over 84.5 %). The characterization of proteins in the liquid phase of the experiments revealed that some proteins were only produced in the experiments where Ni displayed higher solubility, suggesting their involvement in metal complexation. Some functions of these proteins included maintaining Ni homeostasis, acting as metalloenzymes and containing Ni-binding ligands. Desulfomicrobium baculatum, Stenotrophomonas maltophilia, and Desulfovibrio magneticus, were the main responsible species producing these proteins.

The effects of temperature shock on the treatment of high-concentration organic wastewater by an Fe0/GO-anaerobic system

Temperature changes are inevitable during the actual operation of anaerobic systems. Adding an Fe⁰/GO composite is shown to improve microbial activity and alleviate the adverse effects caused by temperature shock.

Behavior of trace elements and micronutrients in manure digestate during ozone treatment

Digestate treatment techniques have recently been proposed as a strategy to increase the ultimate biogas yield from dairy manure and to improve the digestate quality as an organic fertilizer. These studies however rarely take the trace elements (TE) and nutrient partitioning into account. This study focusses on ozone treatment (5-40 g O₃ kg^-1 Total Solids (TS)) as a digestate treatment technique to control the concentration of TE and nutrients in the liquid phase of the digestate. Controlling the TE and nutrient concentrations in the liquid and solid digestate can improve the agronomic value of dairy manure digestate. The ozone concentration of the gas stream entering reactor was 48.53 g O₃/Nm³ or 3.4% w/w O₃ in O₂-gas. The experiments were repeated using pure oxygen gas to investigate its influence. The results from ozonation and oxygenation of the dairy manure digestates revealed that O₃ treatment up to 40 g O₃ kg^-1 TS did not have a more pronounced effect on the biochemical parameters compared to supplementation of pure O₂. Ozonation of the digestate and the supernatant showed that the TE concentration in the liquid phase followed a parabolic profile. The observed initial increase in this parabolic profile was explained by the release of TE from the organic matter to the supernatant causing an increase in TE concentration, followed by a decrease due to precipitation of TE as hydroxides and sulfides, due to the increasing pH and sulphur concentrations.

A simultaneous assessment of organic matter and trace elements bio-accessibility in substrate and digestate from an anaerobic digestion plant

This study evaluates a simultaneous assessment of organic matter (OM) and trace elements (TE) bio-accessibility in substrate and digestate from a full-scale anaerobic digester by a sequential OM extraction method. Simultaneous release of TE was determined along with the extraction of different OM fractions and the effects of extracting reagents on characteristics of OM were evaluated by nuclear magnetic resonance (NMR) spectroscopy. The reagents used for sequential extraction of OM were not enough selective. However, proteins were particularly removed by 0.1 M NaOH, while 72% H₂SO₄ mainly extracted hemicellulose and cellulose. The OM fractionation allowed for simultaneous extraction of >60% of total As, Cd, Co, Fe, Mn, Ni and Zn, while the extraction was limited for Al, Cr, Cu, Mo, and Pb. In substrate, >50% of total As, Co, Mn and Ni and <40% of total Fe, Zn and Mo were identified in bio-accessible fractions. In digestate, all elements demonstrated poor bio-accessibility except for As.

Process analysis of anaerobic fermentation of Phragmites australis straw and cow dung exposing to elevated chromium (VI) concentrations

Anaerobic fermentation is considered as a cost-effective way of biomass waste disposal. Chromium (Cr) is one of the heavy metals that often been blamed for unsatisfactory operation or failure of anaerobic fermentation. The impact of Cr (added as K₂Cr₂O₇) on mesophilic anaerobic fermentation of Phragmites australis straw and cow dung was demonstrated by investigating the biogas properties, process stability, substrate degradation and enzyme activities during the fermentation process. The results showed that 30, 100 and 500 mg/L Cr⁶⁺ addition increased the cumulative biogas yields by up to 19.00%, 14.85% and 7.68% respectively, and brought forward the daily biogas yield peak. Meanwhile, the methane (CH₄) content in the 30 (52.47%) and 100 (40.57%) mg/L Cr⁶⁺-added groups were generally higher than the control group (37.70%). Higher pH values (close to pH 7) and lower oxidation-reduction potential (ORP) values in the Cr⁶⁺-added groups after the 15th day indicated the better process stability compared to the control group. Taking the whole fermentation process into account, the promoting effect of Cr⁶⁺ addition on biogas yields was mainly attributable to better process stability, the enhanced degradation of lignin and hemicellulose, the transformation of intermediates into VFA, the higher coenzyme F₄₂₀ activities and the efficient generation of CH₄. These results demonstrate that an appropriate addition of Cr⁶⁺ could enhance the anaerobic fermentation which support the regulations utilizing of the Cr⁶⁺ contaminated biowaste.

Effects of molybdenum, selenium and manganese supplementation on the performance of anaerobic digestion and the characteristics of bacterial community in acidogenic stage

The addition of trace elements to aid anaerobic digestion has already been widely studied. However, the effects of rare trace elements on anaerobic digestion remain unclear. In this study, the effects of Mo, Se and Mn on anaerobic digestion of rice straw were explored. The results showed the methane yield increased by 59.3%, 47.1% and 48.9% in the first 10 days following addition of Mo (0.01 mg/L), Se (0.1 mg/L) and Mn (1.0 mg/L), respectively. Toxic effects and the accumulation of volatile fatty acids (VFAs) were observed when the Se, Mo and Mn concentrations were greater than 100, 1000 and 1000 mg/L, respectively. The half-maximal inhibitory concentrations (IC₅₀) for Se, Mn and Mo were 79.9 mg/L, 773.9 mg/L and 792.3 mg/L, respectively. The addition of trace elements has changed the bacterial structure of the bacteria, which in turn has affected the digestion performance.

Risks of using EDTA as an agent for trace metals dosing in anaerobic digestion of olive mill solid waste

Low concentrations of trace elements in many organic wastes recommend their supplementation in order to avoid potential limitations. Different chelating agents have been used to ensure an adequate trace metal pool in the soluble fraction, by forming dissolved complexes. Ethylenediaminetetraacetic acid (EDTA) is probably the most common, although several negative effects could be associated with its usage. Biomethane potential tests were performed using Olive Mill Solid Waste as the substrate, supplementing different combinations of Fe, Co, Ni, Ba, always under the presence of EDTA. Results show that Ni and Co slightly recovered biodegradability. However, Ba supplementation resulted in worsening the methane yield coefficient in all cases. High concentration of EDTA led to decrease in the activity of anaerobic digestion. High availability of EDTA induces the capture of trace metals like Co or Ni, key trace metals for anaerobic biomass activity. While supplementing trace metals, the addition of Ba and/or EDTA must be carefully considered.

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.

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.

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.

Chemical speciation of sulfur and metals in biogas reactors - Implications for cobalt and nickel bio-uptake processes

This article deals with the interrelationship between overall chemical speciation of S, Fe, Co, and Ni in relation to metals bio-uptake processes in continuous stirred tank biogas reactors (CSTBR). To address this topic, laboratory CSTBRs digesting sulfur(S)-rich stillage, as well as full-scale CSTBRs treating sewage sludge and various combinations of organic wastes, termed co-digestion, were targeted. Sulfur speciation was evaluated using acid volatile sulfide extraction and X-ray absorption spectroscopy. Metal speciation was evaluated by chemical fractionation, kinetic and thermodynamic analyses. Relative Fe to S content is identified as a critical factor for chemical speciation and bio-uptake of metals. In reactors treating sewage sludge, quantity of Fe exceeds that of S, inducing Fe-dominated conditions, while sulfide dominates in laboratory and co-digestion reactors due to an excess of S over Fe. Under sulfide-dominated conditions, metals availability for microorganisms is restricted due to formation of metal-sulfide precipitates. However, aqueous concentrations of different Co and Ni species were shown to be sufficient to support metal acquisition by microorganisms under sulfidic conditions. Concentrations of free metal ions and labile metal complexes in aqueous phase, which directly participate in bio-uptake processes, are higher under Fe-dominated conditions. This in turn enhances metal adsorption on cell surfaces and bio-uptake rates.

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).

Micronutrient dynamics after thermal pretreatment of olive mill solid waste

This study investigated metal dynamics, and their bioavailability, before and after thermal pretreatment of olive mill solid waste (OMSW), using a sequential metal extraction scheme. The 11.5% increase of cobalt in the most available fraction after the pretreatment coupled to the increase of methane production rate have been a good indicator that the OMSW anaerobic digestion might be metal limited due to the lack of cobalt.

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.

Dosing time of ferric chloride to disinhibit the excessive volatile fatty acids in sludge thermophilic anaerobic digestion system

An investigation into the effect of ferric chloride (FeCl3) on the disinhibition of excessive volatile fatty acids (VFAs) in sludge thermophilic anaerobic digestion (AD) system was performed. The optimum dosing time of FeCl3 was tested with the time interval of 0 h, 36 h, 72 h, 108 h and 144 h. The maximum biogas production was obtained in the case of 72nd hour dosing group, and the biogas production potential was 293.13 ± 11.38 mL/gVS based on modified Gompertz predicted model with the maximum rate of 8.55 ± 0.38 mL/(gVS day), which was triple as that in the control group. More biodegradable organic matters were generated from sludge with FeCl3 additive and then consumed efficiently according to excitation-emission matrix (EEM) fluorescence spectra analysis in the dissolved organic matter (DOM). Acetic acid was the main inhibitor and synthetic effects occurred for the disinhibition of excessive VFAs with the additive of FeCl3, except to direct removal of acetic acid in the system.

Fate of Trace Metals in Anaerobic Digestion

A challenging, and largely uncharted, area of research in the field of anaerobic digestion science and technology is in understanding the roles of trace metals in enabling biogas production. This is a major knowledge gap and a multifaceted problem involving metal chemistry; physical interactions of metal and solids; microbiology; and technology optimization. Moreover, the fate of trace metals, and the chemical speciation and transport of trace metals in environments--often agricultural lands receiving discharge waters from anaerobic digestion processes--simultaneously represents challenges for environmental protection and opportunities to close process loops in anaerobic digestion.