Effect of temperature on kinetics of biogas production from macroalgae

Optimizing a biorefinery design for the valorization of Rugulopteryx okamurae by extracting bioactive compounds and enhancing methane production

The Mediterranean Sea has suffered recently from the unprecedent invasion of the alien macroalga Rugulopteryx okamurae due to global warming and climate change putting at risk the natural local ecosystem. Since 2015 this alga has colonized a great area on the western coast of the Mediterranean Sea and it has been also spotted in other areas such as the Azores Islands or the south of France. The arrival of alga tides into the coasts also provokes collateral environmental problems that need to be addressed. Seaweed-based biorefineries are considered a promising alternative within a circular economy model. This study aims to assess the potential of R. okamurae as raw material for the extraction of reducing sugars (RS) and total polyphenolic compounds (TPC) with antioxidant capacities, the subsequent production of methane from the extracted residue, and the final use of the anaerobic digestate as fertilizer. However, the presence of bioactive compounds greatly varies due to seasonality, location or even natural degradation. In order to provide some insights about these issues, two different batches were assessed: i.e. natural and ashore R. okamurae. As brown algae are characterised by a cell wall composed of crystalline cellulose and lignin, the biomasses studied were mechanically pretreated (dried at 100 °C and milled during 60 s) before the single and sequential extraction processes. Results showed that the extraction of the targeted compounds increased by 30-80 % when the biomasses were extracted sequentially. Similarly, the biochemical methane potential of the extracted solid residues increased as the RS and TPC content was reduced (120-150 NLCH4 kg-1VS), with no significant impact regarding the biomass origin (natural or ashore) or the extraction process order. An increase in the kinetic constant k (first-order model) of 150% and 75% was observed when the fresh biomass was extracted with water and ethanol, respectively, compared to the value obtained for the unextracted biomass. Finally, the physicochemical characteristics of the different anaerobic digestates generated were assessed for their potential use as biofertilizer. In this study, most of the digestate' liquid fractions (7 out of 10) comply with European regulations for organic fertilizers and could be used directly.

Adaptation of a microbial consortium to pelagic Sargassum modifies its taxonomic and functional profile that improves biomethane potential

In recent years, pelagic Sargassum has invaded the Caribbean coasts, and anaerobic digestion has been proposed as a sustainable management option. However, the complex composition of these macroalgae acts as a barrier to microbial degradation, thereby limiting methane production. Microbial adaptation is a promising strategy to improve substrate utilization and stress tolerance. This study aimed to investigate the adaptation of a microbial consortium to enhance methane production from the pelagic Sargassum. Microbial adaptation was performed in a fed-batch mode for 100 days by progressive feeding of Sargassum. The evolution of the microbial community was analyzed by high-throughput sequencing of 16S rRNA amplicons. Additionally, 16S rRNA data were used to predict functional profiles using the iVikodak platform. The results showed that, after adaptation, the consortium was dominated by the bacterial phyla Bacteroidota, Firmicutes, and Atribacterota, as well as methanogens of the families Methanotrichaceae and Methanoregulaceae. The abundance of predicted genes related to different metabolic functions was affected during the adaptation stage when Sargassum concentration was increased. At the end of the adaptation stage, the abundance of the predicted genes increased again. The adapted microbial consortium demonstrated a 60% increase in both biomethane potential and biodegradability index. This work offers valuable insights into the development of treatment technologies and the effective management of pelagic Sargassum in coastal regions, emphasizing the importance of microbial adaptation in this context.

The Gompertz model and its applications in microbial growth and bioproduction kinetics: Past, present and future

The Gompertz model, initially proposed for human mortality rates, has found various applications in growth analysis across the biotechnological field. This paper presents a comprehensive review of the Gompertz model's applications in the biotechnological field, examining its past, present, and future. The past of the Gompertz model was examined by tracing its origins to 1825, and then it underwent various modifications throughout the 20th century to increase its applicability in biotechnological fields. The Zwietering-modified version has proven to be a versatile tool for calculating the lag-time and maximum growth rate/quantity in microbial growth. In addition, the present applications of the Gompertz model to microbial growth kinetics and bioproduction (e.g., hydrogen, methane, caproate, butanol, and hexanol production) kinetics have been comprehensively summarized and discussed. We highlighted the importance of standardized citations and guidance on model selection. The Zwietering-modified Gompertz model and the Lay-modified Gompertz model are recommended for describing microbial growth kinetics and bioproduction kinetics, recognized for their widespread use and provision of valuable kinetics information. Finally, in response to the current Gompertz models' focus on internal mortality, the modified Makeham-Gompertz models that consider both internal/external mortality were introduced and validated for microbial growth and bioproduction kinetics with good fitting performance. This paper provides a perspective of the Gompertz model and offers valuable insights that facilitate the diverse applications of this model in microbial growth and bioproduction kinetics.

Impact of natural degradation of the invasive alga Rugulopteryx okamurae on anaerobic digestion: Heavy metal pollution and kinetic performance

This study shows, for the first time, how the natural biodegradation of the Phaeophyceae Rugulopteryx okamurae (R.o.) affects its methane yield, by biochemical methane potential assays, and the methane production kinetics. Additionally, a mechanical (zeolite-assisted milling) and a thermal (120 °C, 45 min) pretreatments were assessed. The highest methane yield was obtained from the mechanically pretreated fresh ashore biomass (219 (15) NLCH4 kgVS-1), which presents the use of zeolite during milling as an economical alternative for heavy metal toxicity reduction. Moreover, no significant differences were observed between the other tests (with the exception of the lowest value obtained for the mechanically pretreated fresh R.o.). Low methane yields were linked to the heavy metal content. However, an increase of 28.5 % and 20.0 % in the k value was found for the untreated fresh R.o. biomass and fresh ashore biomass, respectively, when subjected to thermal pretreatment. Finally, an enhancement of 80.5 % in the maximum methane production rate was obtained for the fresh ashore biomass milled with zeolite compared to the untreated fresh ashore biomass.

Algae biogas production focusing on operating conditions and conversion mechanisms - A review

Global warming is the result of traditional fuel use and manufacturing, which release significant volumes of CO₂ and other greenhouse gases from factories. Moreover, rising energy consumption, anticipated limitations of fossil fuels in the near future, and increased interest in renewable energies among scientists, currently increase research in biofuels. In contrast to biomass from urban waste materials or the land, algae have the potential to be a commercially successful aquatic energy crop, offering a greater energy potential. Here we discuss the importance of Anaerobic Digestion (AD) for enhanced biogas yield, characterization, and comparisons between algae pretreatment methods namely, mechanical, thermal, microwave irradiation, and enzymatic and catalytic methods. The importance of anaerobic digestion enhances biogas yield, characterization, and comparisons between mechanical, thermal, microwave irradiation, and enzymatic and catalytic treatment. Additionally, operational aspects such as algal species, temperature, C/N ratio, retention period, and particle size impact biofuel yield. The highest algal biogas yield reported was 740 mL/gVS, subtracted from Taihu de-oiled algae applying thermos-chemical pretreatment under conditions of temperature, time, and catalyst concentration of 70 °C, 3 h, and 6%, respectively. Another high yield of algal-based biogas was obtained from Laminaria sp. with mechanical pretreatment under temperature, time, and VS concentration of 38 ± 1 °C, 15 min, and 2.5% respectively, with a maximum yield of 615 ± 7 mL/g VS. Although biofuels derived from algae species are only partially commercialized, the feedstock for biogas might soon be commercially grown. Algae and other plant species that could be cultivated on marginal lands as affordable energy crops with the potential to contribute to the production of biogas are promising and are already being worked on.

An integration of algae-mediated wastewater treatment and resource recovery through anaerobic digestion

Eutrophication is one of the major emerging challenges in aquatic environment. Industrial facilities, including food, textile, leather, and paper, generate a significant amount of wastewater during their manufacturing process. Discharge of nutrient-rich industrial effluent into aquatic systems causes eutrophication, eventually disturbs the aquatic system. On the other hand, algae provide a sustainable approach to treat wastewater, while the resultant biomass may be used to produce biofuel and other valuable products such as biofertilizers. This review aims to provide new insight into the application of algal bloom biomass for biogas and biofertilizer production. The literature review suggests that algae can treat all types of wastewater (high strength, low strength, and industrial). However, algal growth and remediation potential mainly depend on growth media composition and operation conditions such as light intensity, wavelength, light/dark cycle, temperature, pH, and mixing. Further, the open pond raceways are cost-effective compared to closed photobioreactors, thus commercially applied for biomass generation. Additionally, converting wastewater-grown algal biomass into methane-rich biogas through anaerobic digestion seems appealing. Environmental factors such as substrate, inoculum-to-substrate ratio, pH, temperature, organic loading rate, hydraulic retention time, and carbon/nitrogen ratio significantly impact the anaerobic digestion process and biogas production. Overall, further pilot-scale studies are required to warrant the real-world applicability of the closed-loop phycoremediation coupled biofuel production technology.

Seaweed for climate mitigation, wastewater treatment, bioenergy, bioplastic, biochar, food, pharmaceuticals, and cosmetics: a review

The development and recycling of biomass production can partly solve issues of energy, climate change, population growth, food and feed shortages, and environmental pollution. For instance, the use of seaweeds as feedstocks can reduce our reliance on fossil fuel resources, ensure the synthesis of cost-effective and eco-friendly products and biofuels, and develop sustainable biorefinery processes. Nonetheless, seaweeds use in several biorefineries is still in the infancy stage compared to terrestrial plants-based lignocellulosic biomass. Therefore, here we review seaweed biorefineries with focus on seaweed production, economical benefits, and seaweed use as feedstock for anaerobic digestion, biochar, bioplastics, crop health, food, livestock feed, pharmaceuticals and cosmetics. Globally, seaweeds could sequester between 61 and 268 megatonnes of carbon per year, with an average of 173 megatonnes. Nearly 90% of carbon is sequestered by exporting biomass to deep water, while the remaining 10% is buried in coastal sediments. 500 gigatonnes of seaweeds could replace nearly 40% of the current soy protein production. Seaweeds contain valuable bioactive molecules that could be applied as antimicrobial, antioxidant, antiviral, antifungal, anticancer, contraceptive, anti-inflammatory, anti-coagulants, and in other cosmetics and skincare products.

Methane Production of Sargassum spp. Biomass from the Mexican Caribbean: Solid-Liquid Separation and Component Distribution

In the last decade, Sargassum spp. seaweed species have caused massive flooding on the Caribbean Sea coasts. These seaweed species have a high content of recalcitrant compounds, such as insoluble fibers and polyphenols, which generate low methane yields in anaerobic digestion (AD). This study investigated the effect of solid-liquid separation of Sargassum biomass on biodegradability and methane yield. A biochemical methane potential (BMP) test was conducted with both fractions and raw biomass (RB). A mass balance was developed to assess the distribution of the components. The obtained liquid fraction (LF) showed high biodegradability and a high methane production rate, and it generated a methane yield of 159.7 ± 7.1 N L kg VS-1, a value that corresponds to approximately twice that achieved with RB and the solid fraction (SF). The component distribution analysis showed that about 90% of total solids (TS), volatile solids (VS), ash, carbon, and cellulose were retained in the SF. In conclusion, the LF had high biodegradability and methane yield. This suggests the potential for LFs of Sargassum biomass to be treated in large-scale high-load reactors; however, studies applied to SFs are needed because they retain a large amount of organic matter with low biodegradability.

A perspective on the combination of alkali pre-treatment with bioaugmentation to improve biogas production from lignocellulose biomass

Anaerobic digestion (AD) is a bioprocess technology that integrates into circular economy systems, which produce renewable energy and biofertilizer whilst reducing greenhouse gas emissions. However, improvements in biogas production efficiency are needed in dealing with lignocellulosic biomass. The state-of-the-art of AD technology is discussed, with emphasis on feedstock digestibility and operational difficulty. Solutions to these challenges including for pre-treatment and bioaugmentation are reviewed. This article proposes an innovative integrated system combining alkali pre-treatment, temperature-phased AD and bioaugmentation techniques. The integrated system as modelled has a targeted potential to achieve a biodegradability index of 90% while increasing methane production by 47% compared to conventional AD. The methane productivity may also be improved by a target reduction in retention time from 30 to 20 days. This, if realized has the potential to lower energy production cost and the levelized cost of abatement to facilitate an increased resource of sustainable commercially viable biomethane.

Machine Learning Algorithms for Temperature Management in the Anaerobic Digestion Process

Process optimization is no longer an option for processes, but an obligation to survive in the market in any industry. This argument also applies to anaerobic digestion in biogas plants. The contribution of biogas plants to renewable energy can be increased through more productive systems with less waste, which brings the common goal of minimizing costs and maximizing yields in processes. With the help of data science and predictive analytics, it is possible to take conventional process optimization and operational excellence methods, such as statistical process control and Six Sigma, to the next level. The more advanced the process optimization aspect, the more transparent and responsive the systems. In this study, seven different machine learning algorithms—linear regression, logistic regression, K-NN, decision trees, random forest, support vector machine (SVM) and XGBoost—were compared with laboratory results to define and predict the possible impacts of wide range temperature fluctuations on process stability. SVM provided the best accuracy with 0.93 according to the metric precision of the models calculated using the confusion matrix.

Effects of NaOH, thermal, and combined NaOH-thermal pretreatments on the biomethane yields from the anaerobic digestion of walnut shells

Anaerobic digestion (AD) of walnut shells (WS) results in only a limited biomethane yield because of their high fibre content, which ultimately represents an essentially nonbiodegradable lignocellulosic biomass. In the present study, thermal (i.e. 50-250 °C), alkaline (i.e. 1-5% w/w NaOH) and combined alkaline-thermal (i.e. 4% w/w NaOH + 150 °C thermal) pretreatment methods have been applied to increase the anaerobic biodegradation of WS. The highest biomethane yields of 159.9 ± 6.8 mL CH₄.g VS^-1 and 169.8 ± 6.8 mL CH₄.g VS^-1 were achieved after pretreatment at both 250 °C and with 4% NaOH. After combined NaOH-thermal pretreatments, the AD process showed the largest total VFA concentration (i.e. 1280.1 mg Hac L^-1) but a relatively high lag phase (i.e. 3.90 days) compared to thermal and NaOH pretreatments alone, from which the highest biomethane yield (i.e. 192.4 ± 8.2 mL CH₄.g VS^-1 ) was achieved at the end of the AD process. The highest biomethane yield from the combined NaOH-thermal pretreated WS was corroborated by the corresponding highest SCOD/TCOD ratio (i.e. 0.37 ± 0.02) and the highest lignocellulosic fibre removal (i.e. 41.1 ± 2.7% cellulose, 35.6 ± 1.8% hemicellulose, and 58.7 ± 3.2% lignin). The cumulative biomethane yields were further simulated via a modified Gompertz model. This study provides a promising strategy in the sense that the biomethane yield of WS containing large amounts of lignin can be significantly increased via thermal, NaOH, and combined NaOH-thermal pretreatment methods.

Optimization of Reactor Temperature for Continuous Anaerobic Digestion of Cow Manure: Bangladesh Perspective

Converting organic waste into energy through anaerobic digestion is gaining popularity day by day. The reactor temperature is considered as one of the most vital factors for the digestion process. An experiment was conducted in the Biogas Laboratory of Green Energy Knowledge Hub at Bangladesh Agricultural University (BAU) to examine the influence of temperature on anaerobic digestion of cow-dung. Laboratory-scale continuous stirred tank reactors with a working volume of 15 L were operated for a 30-day retention time. The reactors were set at 20 °C, 25 °C, 30 °C, 35 °C, 40 °C and 45 °C, respectively to determine the effect of temperature on anaerobic digestion performance. Different parameters like total solids, volatile solids, pH, volatile fatty acids, ammonia nitrogen, total nitrogen, biogas production rate and methane concentration were examined. Among all the reactors, the reactor at 40 °C temperature produced maximum biogas (312.43 L/kg VS) and methane yields (209.70 L/kg VS), followed by the reactors at 35 °C and 30 °C, respectively. Statistical analysis of the obtained experimental results using Minitab® showed that the optimum process performance in terms of methane yield and volatile solid degradation is achieved at a reactor temperature of 35.82 °C.

Anaerobic co-digestion of sewage sludge with cellulose, protein, and lipids: Role of rheology and digestibility

Rheology is known to have an impact on the performance of digesters, but the effect of additional substrates (co-digestion) is poorly understood. The main objective of this study was to investigate the effects of the addition of cellulose, protein and lipids to substrates on the rheological behaviour and biogas production of the mixture of primary sludge (PS) and waste-activated sludge (WAS) in a batch system. A mixture of PS and WAS to form the main substrate was anaerobically co-digested with different types of organic matter (cellulose, protein and lipids) as co-substrates at different co-substrate to main substrate ratios of 2-8 (wt%) under mesophilic conditions and below ammonia inhibition levels. Yield stress (τ_y) and the flow consistency index (k) of the combined feed in the case of cellulose and protein were significantly dependent on the amount of co-substrate added, while there was an insignificant impact on these properties when lipids were added. Cellulose significantly increased τ_y and k in the feed, which resulted in poor fluidity and the improper homogenisation of the digester content, and consequently decreased the biogas yield. In contrast, the biogas yield was improved through the addition of 2% to 6% protein despite an increase in τ_y and k of the feed, but the methane yield decreased at 7% and 8% levels of protein concentration. This observation indicates that the threshold for τ_y and k of the digester media depends on the organic nature and digestibility of the substrate. There was no significant impact on the flow properties of the initial mixture when lipids were added, and their addition increased the biogas yield. A first-order kinetic reaction model was used for predicting the yield of methane from these digesters. The rate constant values revealed an increasing trend, with the highest for protein then lipids then cellulose.

Kinetic modeling of anaerobic carboxylic acid production from swine wastewater

This work aimed to evaluate the potential of anaerobic carboxylic acids (CA) production from swine wastewater (SW), perform modeling studies of the acidogenic process and estimate the kinetic parameters. Tests were carried out in four batch reactors with 250 mL reaction volume, with brewery sludge as inoculum and using chloroform (0.05%, v/v) for methanogenesis inhibition. Hydrolysis was the main limiting step of CA production from SW, once that it took more than twenty days for the particulate COD consumption to stabilize and fourteen days to produce 60% of the acids formed. A yield of 0.33 mg mgCOD_A^-1, corresponding to 0.40 mgCOD mgCOD_A^-1, was obtained. Kinetic models describing logistic growth functions were best suited to simulate CA production.

Effects of pressure and composition on Raman spectra of CO-H2-CO2-CH4 mixtures

There is great interest in the development of renewable and environmentally friendly fuel sources such as biogas and syngas for generation of heat and electricity. Raman spectroscopy is a promising method for rapid quantitative analysis of the composition of such gases. To improve the accuracy of the method, information is needed on changes in the spectra of the main components as a function of pressure and gas composition. In this work the spectral characteristics of the most intense peaks of methane, carbon monoxide, carbon dioxide and hydrogen in the range of 200-3800 cm^-1 were investigated in the range of 1-25 bar for pure gases and binary mixtures. It has been revealed that, under these conditions, the change in the characteristics of pure rotational hydrogen lines is negligible, while the Q-branches of the main vibrational bands of methane, carbon monoxide and carbon dioxide shift to a few tenths of cm^-1. The values of errors that can occur in the case of ignoring such changes in the spectra are estimated.

Co-digestion of Laminaria digitata with cattle manure: A unimodel simulation study of both batch and continuous experiments

This work investigated the thermophilic (55 °C) co-digestion performance both in batch and continuous mode operation. The biochemical methane potentials of L. digitata and cattle manure were 308 ± 24 and 203 ± 33 mL CH4/g VS, respectively. The optimum co-digestion feedstock ratio was found to be 80% macroalgae: 20% manure on a volatile solids basis, which produced 290 ± 19 mL CH4/g VS under long-term and stable continuous operation at an organic loading rate of 2 g VS/L/d and hydraulic retention time of 15 days. Simulations of the batch and continuous experiments were, for the first time, carried out using an integrated anaerobic bioconversion model without structural modifications. Close fits between measured and simulated data provided mutual confirmation of experimental reliability and model robustness, and provided new perspectives for the use of the software tool.