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Trace metals (TM) are often added to anaerobic digestors (AD) to improve digestor performance and methane yield. Though numerous experimental studies are done to study TM effects, a model based on ADM1 will help plant operators in case of metal deprivation as there are analytical limitations in quantifying metal speciation which controls TM effects on AD. This study developed a complete dynamic model based on ADM1 with least and unavoidable model input information to study TM speciation and its influence on AD. In addition to the biochemical and gas stripping processes described in ADM1, the model introduced new physicochemical processes with effects of operational conditions like pH, temperature and ionic strength. The biogeochemical processes influencing TM speciation include microbial uptake, precipitation (sulphides, phosphates, carbonates), adsorption (onto biomass, soluble inerts and precipitates), inorganic complexation and organic complexation (with EDTA, VFAs, amino acids (AAs)). The model was tested under different scenarios: variable EDTA concentration and dosing form, variable substrate and variable ionic strength. Complexation of TMs with EDTA and AAs influences metal bioavailability and methane yield. Supplementing EDTA reduces TM dosing and dosing EDTA as metal-EDTA complex is more effective than supplementing metals and EDTA separately. TM-AA complexation depends on substrates as amino acid compounds vary with substrates. Ionic strength influences TM adsorption and precipitation processes and hence is an important operational parameter controlling the system like pH and temperature. The model has been compared to experimental data obtained from this study and has been successfully applied to predict cobalt deprivation in an anaerobic digestor.

Strawberry extrudate (SE) is an underused by-product from strawberry industry. Recovery of the phenolic compounds present in SE would represent a very interesting valorisation option. Two main challenges need to be solved, firstly, the solubilisation and recovery of the phenolic compounds contained in SE, and, after that, the stabilisation of the resulted de-phenolized SE. The present research evaluates the potential of a biorefinery process combining a hydrothermal pre-treatment, followed by a phenolic extraction process and, finally, the anaerobic digestion of the remaining SE for producing energy that will contribute to compensate the energy requirements of the whole system. Following the hydrothermal pre-treatment at 170 °C for 60 min, an extraction of 0.6 ± 0.1 g of gallic acid per kilogram of SE was achieved using an adsorbent resin, representing a recovery rate of 64 %. Long-term semi-continuous anaerobic digestion of de-phenolized SE was evaluated at different organic loading rates to evaluate the stability of the process. The anaerobic digestion of pre-treated SE achieved a stable methane production value of 243 ± 34 mL CH4·g volatile solids-1·d-1 at an organic loading rate (ORL) of 1.25 g volatile solids·L-1·d-1. During the operation at this ORL, the control parameters including pH, alkalinity, soluble chemical organic demand (sCOD), and volatile fatty acid (VFA) remained stable and consistently constant. Specifically, the VFA in the reactor during this stable period achieved a value of 102 ± 128 mg O2/L. Also, an economic balance showed that the minimal price of the generated phenolic extract for having benefited from the proposed biorefinery system was 0.812 €·(g of gallic acid equivalents)-1, a price within the range of phenolic compounds used in the food industry.

The objective of this study is to assess the inhibitory effects of an aqueous extract from olive oil mill waste (alperujo) on the growth of a lactic acid bacteria (LAB) cocktail consisting of various strains of Lactiplantibacillus pentosus and Lactiplantibacillus plantarum species. For this purpose, response surface methodology was employed using two independent variables (pH levels 3.5-5.55; hydroxytyrosol concentration ranging from 0.93-2990 ppm). The response variable was the average inhibition per treatment on the LAB cocktail (expressed as a percentage). The developed model identified significant terms, including the linear effect of hydroxytyrosol and pH, their interaction, and the quadratic effect of pH. Maximum inhibition of the LAB cocktail was observed at progressively higher concentrations of hydroxytyrosol and lower pH values. Therefore, complete inhibition of LAB in the synthetic culture medium could only be achieved for concentrations of 2984 ppm hydroxytyrosol at a pH of 3.95. These findings suggest that extracts derived from "alperujo" could be utilized as a natural preservative in acidified foods with a bitter flavor and antioxidant requirements.

Copolymers based on vegetable oils and sulfur have recently been developed using the inverse vulcanization technique for water remediation. This study aimed to create a copolymer with heteroatoms, such as nitrogen, to improve adsorption efficiency and examine the competition and synergy between heavy metals in the adsorption process. FTIR, 1H NMR, solid 13C NMR, zeta potential, nitrogen adsorption, Elemental analysis, TEM, and SEM were used to characterize the copolymers synthesized. Once metal adsorption was optimized using the synthesized copolymers, a comparative adsorption study was conducted for the removal of individual heavy metals, Cd2+, Pb2+, Zn2+, Mn2+, and Cu2+ (100 mg/L), using two copolymers, S/CO:80/20 and S/CO/TAA:80/14/6. The copolymers exhibited a higher affinity for soft metals, such as Cd2+, Pb2+, and Cu2+, when they are individually and in a mixture. After 1 h, the S/CO:80/20 copolymer removed 47%, 40%, and 13% of these metals. While with the S/CO/TAA:80/14/6 copolymer removal increased to 10%, 24%, and 53%, respectively respect to S/CO:80/20 copolymer. The introduction of heteroatoms in the copolymers improved their removal efficiency, particularly for heavy metals like Pb2+. Overall, this study demonstrates a promising approach for sustainable water remediation using copolymers.

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.

Volatile fatty acids (VFAs) revalorisation from waste products are key in achieving industrial sustainability and circular economic goals. Hence, the objective of this work was to correlate the adaptability of the microbial community in olive mill solid waste (OMSW) anaerobic fermentation processes, to the production of VFAs under different pH conditions, i.e. under acidic (pH 4 &5), neutral (pH 6 & 7) and alkaline conditions (pH 9 & 10). At neutral conditions, anaerobic digestion exhibited minimal accumulation of VFAs, as they were primarily biotransformed to methane, where no significant changes in the microbial community were observed. At acidic conditions, a diverse profile of VFAs were present in the reactors, although the VFA production was limited to around 20 % of fed OMSW. Despite the low accumulation, the VFA profile at pH 5 was more complex than those at alkaline conditions, accounting propionic acid as the main VFA compound produced at pH 5 (60 % of the total VFAs). Acidic conditions entailed a shift in the microbial composition compared to the initial inoculum, although the reactors maintained similar diversity indices. At alkaline conditions, around 50 % of the fed OMSW was accumulated as VFAs, mainly as acetic acid. Overall, a lower diversity and higher dominance corresponded to a less diverse VFAs profile, such as the preponderance of acetic acid correlated with a microbial diversity decrease and the increased dominance of Tissirella.

This research evaluates process instability together with microbial population dynamics of the startup of an anaerobic digestion of a mild pretreated solid olive oil waste. The pretreatment consisted of a mild thermal treatment called thermo-malaxation and a subsequent dephenolized process of the olive mill solid waste. The anaerobic digestion process of the mild pretreated and partially dephenolized biomass was studied for three Hydraulic Retention Times (HRTs), with 21 days each HRT, with an organic load rate of 1 g VS/L d, carried out at mesophilic temperature (35 ± 1 °C). The average value of methane yield decreased from 204 ± 9 mL CH4/g VS d on day 21, the last day of the first HRT, to 87 ± 24 mL CH4/g VS d on day 60, the last day of the third HRT. The alkalinity decreased drastically, indicating instability of the anaerobic digestion process. Although phenolic compounds were partially extracted in the pretreatment, the observed increase in phenolic compounds during reactor operation might be contributed to the methane production decay. Interestingly, volatile fatty acids decreased with time, indicating that not only the methanogenic stage but also the hydrolysis stage was affected. Indeed, the microbial analysis showed that the abundance of hydrolytic bacteria decreased over time. It is also worth noticing that hydrogenotrophic methanogens, while present during the first two HRTs, were not observed at the end of the last HRT. This observation, together with the increase in the relative abundance of acetoclastic methanogens, showed a shift in the methane production pathway from hydrogenotrophic methanogenesis to acetotrophic methanogenesis.

The accumulation of volatile fatty acids (VFAs) through the anaerobic fermentation of organic waste, such as strawberry extrudate, is proposed for this work. A hydrothermal treatment was carried out, and it was proposed to break the complex matrix of the strawberry extrudate to favour the hydrolysis stage of the anaerobic digestion process. The production of volatile fatty acids from treated and untreated strawberry was evaluated by adjusting the pH to 5 and 9. After the hydrothermal treatment of the strawberry extrudate, an increase in the solubilisation of organic matter, such as sugars and phenols, was observed. In the production of VFAs by means of anaerobic digestion of the pretreated strawberry extrudate, a significant increase in the accumulation of volatile fatty acids was demonstrated at a pH of 9 with respect to the untreated strawberry extrudate. In addition, the operational pH also had a strong effect on the individual VFA profile. A stream enriched in acetic acid was obtained at a pH of 9 (around 65% of the VFAs), whereas the operation at a pH of 5 resulted in a more complex composition with a high percentage of propionic acid (29% of the VFAs).

Harnessing microbial capabilities for metal recovery from secondary waste sources is an eco-friendly and sustainable approach for the management of metal-containing wastes. Soluble microbial products (SMP) and extracellular polymeric substances (EPS) are the two main groups of extracellular compounds produced by microorganisms in response to metal stress that are of great importance for remediation and recovery of metals. These include various high-, and low, molecular weight components, which serve various functional and structural roles. These compounds often contain functional groups with metal binding potential that can attenuate metal stress by sequestering metal ions, making them less bioavailable. Microorganisms can regulate the content and composition of EPS and SMP in response to metal stress in order to increase the compounds specificity and capacity for metal binding. Thus, EPS and SMP represent ideal candidates for developing technologies for selective metal recovery from complex wastes. To discover highly metal-sorptive compounds with specific metal binding affinity for metal recovery applications, it is necessary to investigate the metal binding affinity of these compounds, especially under metal stressed conditions. In this review we critically reviewed microbial EPS and SMP production as a response to metal stress with a particular emphasis on the metal binding properties of these compounds and their role in altering metal bioavailability. Furthermore, for the first time, we compiled the available data on potential application of these compounds for selective metal recovery from waste streams.

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 (Bmax) kinetic stage, the methane production in the second stage (P), and the maximum methane production rates (Rmax) 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 Rmax demonstrated a positive interaction between Ni and Zn. Although, Bmax did not prove a statistically significant effect between Ni and Zn.

In recent years, there has been an increasing concern related to the contamination of aqueous ecosystems by heavy metals, highlighting the need to improve the current techniques for remediation. This work intends to address the problem of removing heavy metals from waterbodies by combining two complementary methodologies: adsorption to a copolymer synthesized by inverse vulcanization of sulfur and vegetable oils and phytoremediation by the microalga Chlorella sorokiniana to enhance the metal adsorption. After studying the tolerance and growth of Chlorella sorokiniana in the presence of the copolymer, the adsorption of highly concentrated Cd2+ (50 mg L-1) by the copolymer and microalgae on their own and the combined immobilized system (AlgaPol) was compared. Additionally, adsorption studies have been performed on mixtures of the heavy metals Cd2+ and Cu2+ at a concentration of 8 mg L-1 each. AlgaPol biofilm is able to remove these metals from the growth medium by more than 90%. The excellent metal adsorption capacity of this biofilm can be kinetically described by a pseudo-second-order model.

Olive mill solid waste (OMSW) is an agro-industrial waste that has a high content of recalcitrant lignocellulose, which can adversely affect the environment. This study aimed to evaluate the phenol and lignin removal and the enzyme activity involved in the biological pre-treatment of OMSW supplemented with aquaculture sludge (AS) as an external nitrogen source by Anthracophyllum discolor. The highest lignin removal and enzymatic activity performance was obtained in the mixture of OMSW and AS prepared at a C/N ratio 45. In these conditions, the pre-treatment could remove 66% of lignin and 68% of phenols in the solid phase and 56% of phenols in the liquid phase and the maximum activity of laccase, manganese peroxidase and manganese independent peroxidase were of 10, 289 and 75 U L−1 in 25, 30, and 15 days of pre-treatment, respectively. These results propose that the addition of AS as a co-substrate for adjusting the C/N ratio allows a 41 and 141% increase in lignin removal and manganese peroxidase activity respectively, enabling the treatment of both OMSW and AS wastes and the possible recovery of an enzymatic extract of biotechnological interest.