Green energy from marine algae: biogas production and composition from the anaerobic digestion of Irish seaweed species

Microalgae-based wastewater treatment for developing economic and environmental sustainability: Current status and future prospects

Over the last several decades, concerns about climate change and pollution due to human activity has gained widespread attention. Microalgae have been proposed as a suitable biological platform to reduce carbon dioxide, a major greenhouse gas, while also creating commercial sources of high-value compounds such as medicines, cosmetics, food, feed, and biofuel. Industrialization of microalgae culture and valorization is still limited by significant challenges in scaling up the production processes due to economic constraints and productivity capacities. Therefore, a boost in resource usage efficiency is required. This enhancement not only lowers manufacturing costs but also enhancing the long-term viability of microalgae-based products. Using wastewater as a nutrient source is a great way to reduce manufacturing costs. Furthermore, water scarcity is one of the most important global challenges. In recent decades, industrialization, globalization, and population growth have all impacted freshwater resources. Moreover, high amounts of organic and inorganic toxins in the water due to the disposal of waste into rivers can have severe impacts on human and animal health. Microalgae cultures are a sustainable solution to tertiary and quaternary treatments since they have the ability to digest complex contaminants. This review presents biorefineries based on microalgae from all angles, including the potential for environmental pollution remediation as well as applications for bioenergy and value-added biomolecule production. An overview of current information about microalgae-based technology and a discussion of the associated hazards and opportunities for the bioeconomy are highlighted.

Potential applications of algae in biochemical and bioenergy sector

Algae have gained substantial importance as the most promising potential green fuel source across the globe and is on growing demand due to their antioxidant, anticancer, antiviral, antihypertensive, cholesterol reducing and thickening properties. Therefore, it has vast range of application in medicines, pharmaceutical, cosmetics, paper and nutraceutical industries. In this work, the remarkable ability of algae to convert CO₂ and other toxic compounds in atmosphere to potential biofuels, foods, feeds and high-value bioactive compounds is reviewed. Algae produce approximately 50% of the earth's oxygen using its photosynthetic activity, thus acting as a potent tool to mitigate the effects of air pollution. Further, the applicability of algae as a desirable energy source has also been discussed, as they have the potential to serve as an effective alternative to intermittent renewable energy; and also, to combustion-based fossil fuel energy, making them effective for advanced biofuel conversions. This work also evaluates the current applications of algae and the implications of it as a potential substrate for bioplastic, natural alternative to inks and for making paper besides high-value products. In addition, the scope for integrated biorefinery approach is also briefly explored in terms of economic aspects at the industrial scale, as such energy conversion mechanisms are directly linked with sustainability, thus providing a positive overall energy outlook.

Energy Harvesting/Storage and Environmental Remediation via Hot Drinks Wastes

Providing energy and materials are considered one most important issue in the world. Produce and storage energy and also, prepare chemical substances from disposable biomass materials have been widely developed in recent decades to decrease environmental pollutions and production costs. The waste of hot drinks including coffee wastes and tea wastes have considerable potentials to provide energy and different chemical substances. Also, hazardous materials (especially aqueous ions) can be absorbed via hot drinks wastes to protect the environment against perilous pollutants. The low-cost and benign hot drinks wastes including tea wastes and coffee grounds and also the pyrolyzed of them as the hot drinks waste biochar materials have been widely used to produce and store green energies and also, absorb hazardous materials. Production and storage energy and environmental remediation in these sustainable procedures not only reduce the cost of energy but also protect the environment.

Valorisation of macroalgae via the integration of hydrothermal carbonisation and anaerobic digestion

This study investigates the integration of hydrothermal carbonisation (HTC) with anaerobic digestion (AD) as a valorisation route for two macroalgae species; S. latissima (SL) and F. serratus (FS). HTC reactions were conducted at temperatures of 150 °C, 200 °C and 250 °C, with resulting hydrochars, process waters and hydrothermal slurries assessed for biomethane potential yields. Un-treated SL generated similar biomethane levels compared to all SL slurries. Whereas all FS slurries improved biomethane yields compared to un-treated FS. Hydrochars represent a greater energy carrier if used as a solid fuel, rather than a feedstock for anaerobic digestion. Integrating HTC and AD, through hydrochar combustion and process water digestion has a greater energetic output than anaerobic digestion of the un-treated macroalgae. Treatment at 150 °C, with separate utilisation of products, can improve the energetic output of S. latissima and F. serratus by 47% and 172% respectively, compared to digestion of the un-treated macroalgae.

The Effects of Halogenated Compounds on the Anaerobic Digestion of Macroalgae

The urgent need to replace fossil fuels has seen macroalgae advancing as a potential feedstock for anaerobic digestion. The natural methane productivity (dry weight per hectare) of seaweeds is greater than in many terrestrial plant systems. As part of their defence systems, seaweeds, unlike terrestrial plants, produce a range of halogenated secondary metabolites, especially chlorinated and brominated compounds. Some orders of brown seaweeds also accumulate iodine, up to 1.2% of their dry weight. Fluorine remains rather unusual within the chemical structure. Halogenated hydrocarbons have moderate to high toxicities. In addition, halogenated organic compounds constitute a large group of environmental chemicals due to their extensive use in industry and agriculture. In recent years, concerns over the environmental fate and release of these halogenated organic compounds have resulted in research into their biodegradation and the evidence emerging shows that many of these compounds are more easily degraded under strictly anaerobic conditions compared to aerobic biodegradation. Biosorption via seaweed has become an alternative to the existing technologies in removing these pollutants. Halogenated compounds are known inhibitors of methane production from ruminants and humanmade anaerobic digesters. The focus of this paper is reviewing the available information on the effects of halogenated organic compounds on anaerobic digestion.

A Brief Review of Anaerobic Digestion of Algae for Bioenergy

The potential of algal biomass as a source of liquid and gaseous biofuels has been the subject of considerable research over the past few decades, with researchers strongly agreeing that algae have the potential of becoming a viable aquatic energy crop with a higher energy potential compared to that from either terrestrial biomass or municipal solid waste. However, neither microalgae nor seaweed are currently cultivated solely for energy purposes due to the high costs of harvesting, concentrating and drying. Anaerobic digestion of algal biomass could theoretically reduce costs associated with drying wet biomass before processing, but practical yields of biogas from digestion of many algae are substantially below the theoretical maximum. New processing methods are needed to reduce costs and increase the net energy balance. This review examines the biochemical and structural properties of seaweeds and of microalgal biomass that has been produced as part of the treatment of wastewater, and discusses some of the significant hurdles and recent initiatives for producing biogas from their anaerobic digestion.

Methane Production from Alginate-Extracted and Non-Extracted Waste of Laminaria japonica: Anaerobic Mono- and Synergetic Co-Digestion Effects on Yield

This study investigated the potentiality of methane production from alginate-extracted (AEWLJ) and non-extracted (NAEWLJ) waste of Laminaria japonica through batch anaerobic fermentation in mono- and co-digestion with rice straw (RS) at different mixing ratios. Optimal C/N ratio was demonstrated, and system stability was monitored in terms of the total ammonia nitrogen, total volatile fatty acids, and pH throughout the digestion period. The results show that the combination of AEWLJ/RS at 67% mixing ratio generated the highest biogas yield of 247 NmL/gVS, which was 36% higher than the AEWLJ alone. The synergetic effect was clearly observed leading to an increase in the total methane yield up to 78% and 88%, respectively, for arrays of AEWLJ/RS and NAEWLJ/RS. The kinetic model showed a high coefficient of determination (R2 ≥ 0.9803) when the modified Gompertz model was applied to predict methane production. These outcomes support the possibility of an integrated biorefinery approach to attain value-added products in order to achieve circular economies.

Inclusion of Saccharina latissima in conventional anaerobic digestion systems

Loading macroalgae into existing anaerobic digestion (AD) plants allows us to overcome challenges such as low digestion efficiencies, trace elements limitation, excessive salinity levels and accumulation of volatile fatty acids (VFAs), observed while digesting algae as a single substrate. In this work, the co-digestion of the brown macroalgae Saccharina latissima with mixed municipal wastewater sludge (WWS) was investigated in mesophilic and thermophilic conditions. The hydraulic retention time (HRT) and the organic loading rate (OLR) were fixed at 19 days and 2.1 g l^-1 d^-1 of volatile solids (VS), respectively. Initially, WWS was digested alone. Subsequently, a percentage of the total OLR (20%, 50% and finally 80%) was replaced by S. latissima biomass. Optimal digestion conditions were observed at medium-low algae loading (≤50% of total OLR) with an average methane yield close to [Formula: see text] and [Formula: see text] in mesophilic and thermophilic conditions, respectively. The conductivity values increased with the algae loading without inhibiting the digestion process. The viscosities of the reactor sludges revealed decreasing values with reduced WWS loading at both temperatures, enhancing mixing properties.

A review on the biomass pretreatment and inhibitor removal methods as key-steps towards efficient macroalgae-based biohydrogen production

(Red, green and brown) macroalgal biomass is a propitious candidate towards covenant alternative energy resources to be converted into biofuels i.e. hydrogen. The application of macroalgae for hydrogen fermentation (promising route in advancing the biohydrogen generation process) could be accomplished by the transformation of carbohydrates, which is a topic receiving broad attention in recent years. This article overviews the variety of marine algal biomass available in the coastal system, followed by the analyses of their pretreatment methods, inhibitor formation and possible detoxification, which are key-aspects to achieve subsequent H₂ fermentation in a proper way.

The effects of co-substrate and thermal pretreatment on anaerobic digestion performance

The influence of anaerobic co-digestion of leachate and sludge with organic fraction of municipal solid waste (OFMSW) under mesophilic condition in three batch digesters of 5 L capacity has been studied. OFMSW was mixed with leachate and sludge at three different ratios. Experimental results illustrated that the digester with a ratio of 2000/2500 (leachate (mL) or sludge/OFMSW (mL)) had significantly higher performance. Furthermore, this study compared the performance of anaerobic digestion of different substrates with three different mixing ratios with and without thermal pretreatment at low temperature (65°C) in terms of biogas production, chemical oxygen demand (COD) elimination as well as hydraulic retention time. In addition, to predict the biogas yield and evaluate the kinetic parameters, the modified Gompertz model was applied. Based on the results, the maximum biogas yield from adding different leachate and sludge ratios to OFMSW was recorded to be 0.45 and 0.38 m³kg^-1 COD which was higher about 7% in comparison with co-digestion original OFMSW without thermal pretreatment. In addition, thermal pretreatment accelerated the hydrolysis step. Moreover, the total COD elimination was relatively stable in the range of 52-60% at all types of substrate mixtures. Also, the modified Gompertz model demonstrated a good fit to the experimental results.

ABBREVIATIONS

AD: anaerobic digester; BOD: biochemical oxygen demand; COD: chemical oxygen demand; FAAS: flame atomic absorption spectroscopy; HS: high solids; HRT: hydraulic retention time; LS: low solids; MS: medium solids; OFMSW: organic fraction of municipal solid waste; TCD: thermal conductivity detector; TS: total solid; TSS: total suspended solids.

Microbial communities mediating algal detritus turnover under anaerobic conditions

BACKGROUND

Algae encompass a wide array of photosynthetic organisms that are ubiquitously distributed in aquatic and terrestrial habitats. Algal species often bloom in aquatic ecosystems, providing a significant autochthonous carbon input to the deeper anoxic layers in stratified water bodies. In addition, various algal species have been touted as promising candidates for anaerobic biogas production from biomass. Surprisingly, in spite of its ecological and economic relevance, the microbial community involved in algal detritus turnover under anaerobic conditions remains largely unexplored.

RESULTS

Here, we characterized the microbial communities mediating the degradation of Chlorella vulgaris (Chlorophyta), Chara sp. strain IWP1 (Charophyceae), and kelp Ascophyllum nodosum (phylum Phaeophyceae), using sediments from an anaerobic spring (Zodlteone spring, OK; ZDT), sludge from a secondary digester in a local wastewater treatment plant (Stillwater, OK; WWT), and deeper anoxic layers from a seasonally stratified lake (Grand Lake O' the Cherokees, OK; GL) as inoculum sources. Within all enrichments, the majority of algal biomass was metabolized within 13-16 weeks, and the process was accompanied by an increase in cell numbers and a decrease in community diversity. Community surveys based on the V4 region of the 16S rRNA gene identified different lineages belonging to the phyla Bacteroidetes, Proteobacteria (alpha, delta, gamma, and epsilon classes), Spirochaetes, and Firmicutes that were selectively abundant under various substrate and inoculum conditions. Within all kelp enrichments, the microbial communities structures at the conclusion of the experiment were highly similar regardless of the enrichment source, and were dominated by the genus Clostridium, or family Veillonellaceae within the Firmicutes. In all other enrichments the final microbial community was dependent on the inoculum source, rather than the type of algae utilized as substrate. Lineages enriched included the uncultured groups VadinBC27 and WCHB1-69 within the Bacteroidetes, genus Spirochaeta and the uncultured group SHA-4 within Spirochaetes, Ruminococcaceae, Lachnospiraceae, Yongiibacter, Geosporobacter, and Acidaminobacter within the Firmicutes, and genera Kluyvera, Pantoea, Edwardsiella and Aeromonas, and Buttiauxella within the Gamma-Proteobaceteria order Enterobacteriales.

CONCLUSIONS

Our results represent the first systematic survey of microbial communities mediating turnover of algal biomass under anaerobic conditions, and highlights the diversity of lineages putatively involved in the degradation process.

Valorisation to biogas of macroalgal waste streams: a circular approach to bioproducts and bioenergy in Ireland

Seaweeds (macroalgae) have been recently attracting more and more interest as a third generation feedstock for bioenergy and biofuels. However, several barriers impede the deployment of competitive seaweed-based energy. The high cost associated to seaweed farming and harvesting, as well as their seasonal availability and biochemical composition currently make macroalgae exploitation too expensive for energy production only. Recent studies have indicated a possible solution to aforementioned challenges may lay in seaweed integrated biorefinery, in which a bioenergy and/or biofuel production step ends an extractions cascade of high-value bioproducts. This results in the double benefit of producing renewable energy while adopting a zero waste approach, as fostered by recent EU societal challenges within the context of the Circular Economy development. This study investigates the biogas potential of residues from six indigenous Irish seaweed species while discussing related issues experienced during fermentation. It was found that Laminaria and Fucus spp. are the most promising seaweed species for biogas production following biorefinery extractions producing 187–195 mL CH₄ gVS⁻¹ and about 100 mL CH₄ gVS⁻¹ , respectively, exhibiting overall actual yields close to raw un-extracted seaweed.

Biogas production generated through continuous digestion of natural and cultivated seaweeds with dairy slurry

The technical feasibility of long term anaerobic mono-digestion of two brown seaweeds, and co-digestion of both seaweeds with dairy slurry was investigated whilst increasing the organic loading rate (OLR). One seaweed was natural (L. digitata); the second seaweed (S. Latissima) was cultivated. Higher proportions of L. digitata in co-digestion (66.6%) allowed the digester to operate more efficiently (OLR of 5kgVSm(-3)d(-1) achieving a specific methane yield (SMY) of 232LCH4kg(-1)VS) as compared to lower proportions (33.3%). Co-digestion of 66.6% cultivated S. latissima, with dairy slurry allowed a higher SMY of 252LCH4kg(-1)VS but at a lower OLR of 4kgVSm(-3)d(-1). Optimum conditions for mono-digestion of both seaweeds were effected at 4kgVSm(-3)d(-1). Chloride concentrations increased to high levels in the digestion of both seaweeds but were not detrimental to operation.

Comparison of microbial communities during the anaerobic digestion of Gracilaria under mesophilic and thermophilic conditions

Mesophilic and thermophilic anaerobic digesters (MD and TD, respectively) utilizing Gracilaria and marine sediment as the substrate and inoculum, respectively, were compared by analyzing their performances and microbial community changes. During three successive transfers, the average cumulative methane yields in the MD and TD were 222.6 ± 17.3 mL CH4/g volatile solids (VS) and 246.1 ± 11 mL CH4/g VS, respectively. The higher hydrolysis rate and acidogenesis in the TD resulted in a several fold greater accumulation of volatile fatty acids (acetate, propionate, and butyrate) followed by a larger pH drop with a prolonged recovery than in the MD. However, the operational stability between both digesters remained comparable. Pyrosequencing analyses revealed that the MD had more complex microbial diversity indices and microbial community changes than the TD. Interestingly, Methanomassiliicoccales, the seventh methanogen order was the predominant archaeal order in the MD along with bacterial orders of Clostridiales, Bacteriodales, and Synergistales. Meanwhile, Coprothermobacter and Methanobacteriales dominated the bacterial and archaeal community in the TD, respectively. Although the methane yield is comparable, both MD and TD show a different profile of pH, VFA and the microbial communities.

A Review on the Valorization of Macroalgal Wastes for Biomethane Production

The increased use of terrestrial crops for biofuel production and the associated environmental, social and ethical issues have led to a search for alternative biomass materials. Terrestrial crops offer excellent biogas recovery, but compete directly with food production, requiring farmland, fresh water and fertilizers. Using marine macroalgae for the production of biogas circumvents these problems. Their potential lies in their chemical composition, their global abundance and knowledge of their growth requirements and occurrence patterns. Such a biomass industry should focus on the use of residual and waste biomass to avoid competition with the biomass requirements of the seaweed food industry, which has occurred in the case of terrestrial biomass. Overabundant seaweeds represent unutilized biomass in shallow water, beach and coastal areas. These eutrophication processes damage marine ecosystems and impair local tourism; this biomass could serve as biogas feedstock material. Residues from biomass processing in the seaweed industry are also of interest. This is a rapidly growing industry with algae now used in the comestible, pharmaceutical and cosmetic sectors. The simultaneous production of combustible biomethane and disposal of undesirable biomass in a synergistic waste management system is a concept with environmental and resource-conserving advantages.

Thermochemical hydrolysis of macroalgae Ulva for biorefinery: Taguchi robust design method

Understanding the impact of all process parameters on the efficiency of biomass hydrolysis and on the final yield of products is critical to biorefinery design. Using Taguchi orthogonal arrays experimental design and Partial Least Square Regression, we investigated the impact of change and the comparative significance of thermochemical process temperature, treatment time, %Acid and %Solid load on carbohydrates release from green macroalgae from Ulva genus, a promising biorefinery feedstock. The average density of hydrolysate was determined using a new microelectromechanical optical resonator mass sensor. In addition, using Flux Balance Analysis techniques, we compared the potential fermentation yields of these hydrolysate products using metabolic models of Escherichia coli, Saccharomyces cerevisiae wild type, Saccharomyces cerevisiae RN1016 with xylose isomerase and Clostridium acetobutylicum. We found that %Acid plays the most significant role and treatment time the least significant role in affecting the monosaccharaides released from Ulva biomass. We also found that within the tested range of parameters, hydrolysis with 121 °C, 30 min 2% Acid, 15% Solids could lead to the highest yields of conversion: 54.134–57.500 gr ethanol kg⁻¹Ulva dry weight by S. cerevisiae RN1016 with xylose isomerase. Our results support optimized marine algae utilization process design and will enable smart energy harvesting by thermochemical hydrolysis.

The effect of seasonal variation on biomethane production from seaweed and on application as a gaseous transport biofuel

Biomethane produced from seaweed may be used as a transport biofuel. Seasonal variation will have an effect on this industry. Laminaria digitata, a typical Irish brown seaweed species, shows significant seasonal variation both in proximate, ultimate and biochemical composition. The characteristics in August were optimal with the lowest level of ash (20% of volatile solids), a C:N ratio of 32 and the highest specific methane yield measured at 327LCH4kgVS(-1), which was 72% of theoretical yield. The highest yield per mass collected of 53m(3)CH4t(-1) was achieved in August, which is 4.5 times higher than the lowest value, obtained in December. A seaweed cultivation area of 11,800ha would be required to satisfy the 2020 target for advanced biofuels in Ireland, of 1.25% renewable energy supply in transport (RES-T) based on the optimal gross energy yield obtained in August (200GJha(-1)yr(-1)).

Methanosarcina Play an Important Role in Anaerobic Co-Digestion of the Seaweed Ulva lactuca: Taxonomy and Predicted Metabolism of Functional Microbial Communities

Macro-algae represent an ideal resource of third generation biofuels, but their use necessitates a refinement of commonly used anaerobic digestion processes. In a previous study, contrasting mixes of dairy slurry and the macro-alga Ulva lactuca were anaerobically digested in mesophilic continuously stirred tank reactors for 40 weeks. Higher proportions of U. lactuca in the feedstock led to inhibited digestion and rapid accumulation of volatile fatty acids, requiring a reduced organic loading rate. In this study, 16S pyrosequencing was employed to characterise the microbial communities of both the weakest (R1) and strongest (R6) performing reactors from the previous work as they developed over a 39 and 27-week period respectively. Comparing the reactor communities revealed clear differences in taxonomy, predicted metabolic orientation and mechanisms of inhibition, while constrained canonical analysis (CCA) showed ammonia and biogas yield to be the strongest factors differentiating the two reactor communities. Significant biomarker taxa and predicted metabolic activities were identified for viable and failing anaerobic digestion of U. lactuca. Acetoclastic methanogens were inhibited early in R1 operation, followed by a gradual decline of hydrogenotrophic methanogens. Near-total loss of methanogens led to an accumulation of acetic acid that reduced performance of R1, while a slow decline in biogas yield in R6 could be attributed to inhibition of acetogenic rather than methanogenic activity. The improved performance of R6 is likely to have been as a result of the large Methanosarcina population, which enabled rapid removal of acetic acid, providing favourable conditions for substrate degradation.

The effect of pressure and temperature pretreatment on the biogas output from algal biomass

This paper presents data on methane fermentation of algal biomass containing Chlorella sp. and Scenedesmus sp. The biomass was obtained from closed-culture photobioreactors. Before the process, the algae were subjected to low temperature and pressure pretreatment for 0.0, 0.5, 1.0 and 2.0 h. The prepared biomass was subjected to mesophilic methane fermentation. The amount and composition of the biogas formed in the process were determined. The amount of biogas produced was larger when the biomass was subjected to thermal preprocessing. The proportion of methane in the gas also increased. Extending the heating time beyond 1.0 h did not significantly improve the biogassing effects.

Scope of algae as third generation biofuels

An initiative has been taken to develop different solid, liquid, and gaseous biofuels as the alternative energy resources. The current research and technology based on the third generation biofuels derived from algal biomass have been considered as the best alternative bioresource that avoids the disadvantages of first and second generation biofuels. Algal biomass has been investigated for the implementation of economic conversion processes producing different biofuels such as biodiesel, bioethanol, biogas, biohydrogen, and other valuable co-products. In the present review, the recent findings and advance developments in algal biomass for improved biofuel production have been explored. This review discusses about the importance of the algal cell contents, various strategies for product formation through various conversion technologies, and its future scope as an energy security.

Investigation of the optimal percentage of green seaweed that may be co-digested with dairy slurry to produce gaseous biofuel

Ulva lactuca, a green seaweed, accumulates on beaches and shallow estuaries subject to eutrophication. As a residue, and a macro-algae, it is a source of sustainable third generation biofuel. Production of biomethane from mono-digestion of U. lactuca, however is problematic due to high levels of sulphur and low ratios of carbon to nitrogen. Fresh and dried U. lactuca were continuously co-digested with dairy slurry at ratios of 25%, 50% and 75% (by volatile solid content) in 6 number 5L reactors for 9months. The reactors digesting a mix with 75% U. lactuca struggled to reach stable conditions. Volatile fatty acid levels of 14,000mgl(-1) were experienced. The levels of ammonia increased with percentage U. lactuca in the mix. Optimum conditions were observed with a mix of 25% fresh U. lactuca and 75% slurry. A yield of 170LCH4kg(-1)VS was achieved at an organic loading rate of 2.5kgVSm(-3)d(-1).

Microwave-induced low temperature pyrolysis of macroalgae for unprecedented hydrogen-enriched syngas production

An efficient methodology based on low temperature microwave-induced pyrolysis has been developed for syngas production from macroalgae.

A horizon scan of global conservation issues for 2014

•

This is the fifth in our annual series of horizon scans published in TREE.

•

We identify 15 issues that we considered insufficiently known by the conservation community.

•

These cover a wide range of issues. Four relate to climate change, two to invasives and two to disease spread.

•

This exercise has been influential in the past.

This paper presents the output of our fifth annual horizon-scanning exercise, which aims to identify topics that increasingly may affect conservation of biological diversity, but have yet to be widely considered. A team of professional horizon scanners, researchers, practitioners, and a journalist identified 15 topics which were identified via an iterative, Delphi-like process. The 15 topics include a carbon market induced financial crash, rapid geographic expansion of macroalgal cultivation, genetic control of invasive species, probiotic therapy for amphibians, and an emerging snake fungal disease.