Influence of biogas flow rate on biomass composition during the optimization of biogas upgrading in microalgal-bacterial processes

Innovative design and operational strategies to improve CO2 mass transfer during photosynthetic biogas upgrading

Several innovative strategies of design and operation, such as biogas recirculation, centrate pH manipulation and liquid nanoparticle addition, were tested to assess their potential to improve CO2 mass transfer during photosynthetic purification of biogas in a microalgae-bacteria pond connected to a biogas scrubbing column. Biogas recirculation in the column was not effective since the biogas and cultivation broth had reached chemical equilibrium under the operational conditions and configuration without biogas recirculation. Feeding the centrate at pH 10 (with and without ammonium desorption) directly to the absorption column substantially improved CO2 removal efficiency (from 58 to 91 %) achieving a biomethane complying with European standards. The supplementation of liquid nanoparticles considerably increased biomass concentration in the pond (from 1.2 to 3.5 g/L), revealing an enhanced photosynthetic activity. However, this promising approach requires additional research to elucidate the best conditions to boost CO2 absorption and guarantee a biomethane fulfilling most international standards.

Stress of cupric ion and oxytetracycline in Chlorella vulgaris cultured in swine wastewater

Chlorella culturing has the advantages in treatment of wastewater including swine wastewater from anaerobic digesters due to the product of biolipids and the uptake of carbon dioxide. However, there often exist high concentrations of antibiotics and heavy metals in swine wastewater which could be toxic to chlorella and harmful to the biological systems. This study examined the stress of cupric ion and oxytetracycline (OTC) at various concentrations on the nutrient removal and biomass growth in Chlorella vulgaris culturing in swine wastewater from anaerobic digesters, and its biochemical responses were also studied. Results showed that dynamic hormesis of either OTC concentration or cupric ion one on Chlorella vulgaris were confirmed separately, and the presence of OTC not only did not limit biomass growth and lipids content of Chlorella vulgaris but also could mitigate the toxicity of cupric ion on Chlorella vulgaris in combined stress of Cu2+ and OTC. Extracellular polymeric substances (EPS) of Chlorella vulgaris were used to explain the mechanisms of stress for the first time. The content of proteins and carbohydrates in EPS increased, and the fluorescence spectrum intensity of tightly-bound EPS (TB-EPS) of Chlorella vulgaris decreased with increasing concentration of stress because Cu2+ and OTC may be chelated with proteins of TB-EPS to form non-fluorescent characteristic chelates. The low concentration of Cu2+ (≤1.0 mg/L) could enhance the protein content and promote the activity of superoxide dismutase (SOD) while these parameters were decreased drastically under 2.0 mg/L of Cu2+. The activity of adenosine triphosphatase (ATPase) and glutathione (GSH) enhanced with the increase of OTC concentration under combined stress. This study helps to comprehend the impact mechanisms of stress on Chlorella vulgaris and provides a novel strategy to improve the stability of microalgae systems for wastewater treatment.

Effect of different mixed light-emitting diode light wavelengths on CO2 absorption from biogas and nutrient removal from biogas slurry by microalgae and fungi induced using strigolactone and endophytic bacteria

In this study, biogas and biogas slurry were simultaneously treated using two symbiotic systems: Chlorella vulgaris-Ganoderma lucidum-S395-2 (endophytic bacteria) and Scenedesmus obliquus-G. lucidum-S395-2. The influence of different mixed illumination (red and blue) intensity ratios on the algal symbionts' extracellular carbonic anhydrase activities was investigated, as well as the rates of microalgal growth and photosynthesis. The treatment performance was simultaneously assessed in terms of the efficiency of organic matter or nutrient removal and the level of CO2 absorption. The results indicated that red-blue light combinations with an intensity ratio of 5:5 were optimal. When comparing the performance of the two symbiotic systems, the C. vulgaris-G. lucidum-S395-2 symbiont co-culture system achieved significantly improved photosynthetic rates, biomass growth, and treatment effects. Under the optimal treatment conditions, the organic matter and nutrient removal rates were 81.06% ± 7.06% for chemical oxygen demand, 82.32% ± 7.18% for total nitrogen, and 82.98% ± 7.26% for total phosphorus. In addition, the rate of CO2 removal from biogas was 63.38% ± 5.35%. PRACTITIONER POINTS: The red and blue light intensity ratio of 5:5 showed the best removal performance. C. vulgaris-G. lucidum-S395-2 system obtained the best photosynthetic performance. The carbonic anhydrase activity had positive effects on CO2 removal performance.

Integrated role of algae in the closed-loop circular economy of anaerobic digestion

Following the surging demand for sustainable biofuels, biogas production via anaerobic digestion (AD) presented itself as a solution for energy security, waste management, and greenhouse gas mitigation. Algal-based biorefinery platform serves an important role in the AD-based closed-loop circular economy. Other than using whole biomass of micro- and macroalgae as feedstock for biogas production, the integration of AD with other bio- or thermochemical conversion techniques can achieve complete valorization of biomass residue after processing or valuable compounds extraction. On the other hand, anaerobic digestate, the byproduct of AD processes can be used for microalgal cultivation for lipid and pigments accumulation, closing the loop of resource flow. Furthermore, algae and its consortium with bacteria or fungi can be employed for combined biogas upgrading and wastewater treatment. Innovative strategies have been developed to enhance biogas upgrading and pollutant removal performance as well as minimize O₂ and N₂ content in the upgraded biomethane.

Influence of operational conditions on the performance of biogas bioconversion into ectoines in pilot bubble column bioreactors

The application of biogas as a low-priced substrate for the production of ectoines constitutes an opportunity to decrease their production costs and to enhance the viability of anaerobic digestion. The influence of operational conditions on CH₄-biogas biodegradation and on ectoines production yields was assessed in continuous pilot bubble column bioreactors. The rise in biomass concentration from 1 to 3 g L^-1 resulted in a decrease in the specific ectoine content from 42 ± 8 to 30 ± 4 mg_ectoine g_VSS^-1. The concentration of Cu²⁺ and Mg²⁺ did not impact process performance, while the use of ammonium as N source resulted in low CH₄ biodegradation and ectoine yields (13 ± 7 mg_ectoine g_VSS^-1). The increase in CH₄ content from 4.5 to 9 %v·v^-1 enhanced CH₄ removal efficiency. Process operation at NaCl concentrations of 3 %w·w^-1 instead of 6 %w·w^-1 decreased the ectoine yield to 17 mg_ectoine g_VSS^-1. Finally, Methylomicrobiumburyatense was identified as the dominant species.

Influence of the hydraulic retention time on the removal of emerging contaminants in an anoxic-aerobic algal-bacterial photobioreactor coupled with anaerobic digestion

This work evaluated, for the first time, the performance of an integral microalgae-based domestic wastewater treatment system composed of an anoxic reactor and an aerobic photobioreactor, coupled with an anaerobic digester for converting the produced algal-bacterial biomass into biogas, with regards to the removal of 16 contaminants of emerging concern (CECs): penicillin G, tetracycline, enrofloxacin, ciprofloxacin, sulfamethoxazole, tylosin, trimethoprim, dexamethasone, ibuprofen, naproxen, acetaminophen, diclofenac, progesterone, carbamazepine, triclosan and propylparaben. The influence of the hydraulic retention time (HRT) in the anoxic-aerobic bioreactors (4 and 2.5 days) and in the anaerobic digester (30 and 10 days) on the fate of these CECs was investigated. The most biodegradable contaminants (removal efficiency >80% regardless of HRT) were tetracycline, ciprofloxacin, sulfamethoxazole, tylosin, trimethoprim, dexamethasone, ibuprofen, naproxen, acetaminophen and propylparaben (degraded predominantly in the anoxic-aerobic bioreactors), and tetracycline, sulfamethoxazole, tylosin, trimethoprim and naproxen (degraded predominantly in the anaerobic reactor). The anoxic-aerobic bioreactors provided removal of at least 48% for all CECs tested. The most recalcitrant contaminants in the anaerobic reactor, which were not removed at any of the HRT tested, were enrofloxacin, ciprofloxacin, progesterone and propylparaben.

Simultaneous biogas upgrading and biogas slurry treatment by different microalgae-based technologies under various strigolactone analog (GR24) concentrations

In this research, the effects of exogenous strigolactone analog (GR24) on the growth rate, daily productivity and photosynthesis of symbiotes of Chlorella vulgaris (C. vulgaris)-Ganoderma lucidum (G. lucidum)-endophytic bacteria, C. vulgaris-G. lucidum-activated sludge and C. vulgaris-G. lucidum-multi-walled carbon nanotube, and the simultaneous treatment of biogas slurry and biogas were examined. The C. vulgaris-G. lucidum-endophytic bacteria symbiote achieved the best treatment effectiveness for biogas slurry and biogas, with removal efficiencies of chemical oxygen demand, total nitrogen, total phosphorus and CO₂ of 81.4 ± 7.6%, 79.6 ± 7.6%, 82.5 ± 8.2%, and 67.3 ± 6.3% under the optimal GR24 concentration of 10^-9 M. Moreover, the treatment effects were positively correlated with growth performance and photosynthesis efficiency of the symbiote. These findings advance the development and application of symbiotic systems in the field of wastewater treatment and biogas upgrading.

Microalgae Xanthophylls: From Biosynthesis Pathway and Production Techniques to Encapsulation Development

In the last 20 years, xanthophylls from microalgae have gained increased scientific and industrial interests. This review highlights the essential issues that concern this class of high value compounds. Firstly, their chemical diversity as the producer microorganisms was detailed. Then, the use of conventional and innovative extraction techniques was discussed. Upgraded knowledge on the biosynthetic pathway of the main xanthophylls produced by photosynthetic microorganisms was reviewed in depth, providing new insightful ideas, clarifying the function of these active biomolecules. In addition, the recent advances in encapsulation techniques of astaxanthin and fucoxanthin, such as spray and freeze drying, gelation, emulsification and coacervation were updated. Providing information about these topics and their applications and advances could be a help to students and young researchers who are interested in chemical and metabolic engineering, chemistry and natural products communities to approach the complex thematic of xanthophylls.

Contribution of vitamin B12 to biogas upgrading and nutrient removal by different microalgae-based technology

The algae-based technology has a positive effect on the treatment of biogas slurry and the purification of biogas, while vitamin B12 (VB12) is one of the important regulatory substances in the algae-based cultivation system. In this study, different concentrations of VB12 were used in three microalgal treatment technologies to assess their effect on simultaneous removal of nutrients from biogas slurry and removal of CO₂ from raw biogas. Results showed that Chlorella vulgaris exhibited higher growth rate, mean daily productivity, chlorophyll a content, carbonic anhydrase activity and better photosynthetic properties when co-cultivated with Ganoderma lucidum, rather than when co-cultivated with activated sludge or under mono-cultivation. Maximum mean chemical oxygen demand, total nitrogen, total phosphorus and CO₂ removal efficiencies were found to be 84.29 ± 8.28%, 83.27 ± 8.14%, 85.27 ± 8.46% and 65.71 ± 6.35%, respectively when microalgae were co-cultivated with Ganoderma lucidum under 100 ng L^-1 of VB12. This study shows the potential of microalgae and fungi co-cultivation supplemented with VB12 for the simultaneous upgradation of biogas production as well as for the purification of biogas slurry.

Integration of algae-based sewage treatment with anaerobic digestion of the bacterial-algal biomass and biogas upgrading

The domestic sewage treatment performance of an integrated anoxic-aerobic photobioreactor with biomass settling and recycling, coupled with anaerobic digestion of the produced bacterial-algal biomass and biogas upgrading in the photobioreactor was investigated. Hydraulic retention time in the photobioreactor initially was 4 days (stage I and II) and then reduced to 2.5 days (stage III). The integrated system supported high total organic carbon removals of 98.9 ± 1.1% regardless of the operational stage. A high total nitrogen removal of 90.8 ± 8.0% was recorded in the integrated system during the three operational stages, while total phosphorus removals accounted for 68.4 ± 20.1%, 68.3 ± 20.8% and 53.4 ± 25.0% in stages I, II and III, respectively. Biogas upgrading in the absorption column exhibited maximum removals of CO₂ and H₂S of 74.7 ± 3.0% and 99.0 ± 2.8%, respectively. Biomass settling and recycling resulted in overall improvement of biomass settleability.

Pollution prevention and waste phycoremediation by algal-based wastewater treatment technologies: The applications of high-rate algal ponds (HRAPs) and algal turf scrubber (ATS)

Following the escalating human population growth and rapid urbanization, the tremendous amount of urban and industrial waste released leads to a series of critical issues such as health issues, climate change, water crisis, and pollution problems. With the advantages of a favorable carbon life cycle, high photosynthetic efficiencies, and being adaptive to harsh environments, algae have attracted attention as an excellent agent for pollution prevention and waste phycoremediation. Following the concept of circular economy and biorefinery for sustainable production and waste minimization, this review discusses the role of four different algal-based wastewater treatment technologies, including high-rate algal ponds (HRAPs), HRAP-absorption column (HRAP-AC), hybrid algal biofilm-enhanced raceway pond (HABERP) and algal turf scrubber (ATS) in waste management and resource recovery. In addition to the nutrient removal mechanisms and operation parameters, recent advances and developments have been discussed for each technology, including (1) Innovative operation strategies and treatment of emerging contaminants (ECs) employing HRAPs, (2) Biogas upgrading utilizing HRAP-AC system and approaches of O₂ minimization in biomethane, (3) Operation of different HABERP systems, (4) Life-cycle and cost analysis of HRAPs-based wastewater treatment system, and (5) Value-upgrading for harvested algal biomass and life-cycle cost analysis of ATS system.

Treatment of Synthetic Ammonium Sulfate Wastewater by Mixed Culture of Chlorella pyrenoidosa and Enriched Nitrobacteria

Ammonium sulfate wastewater can cause eutrophication and black odor of water body. Although ammonia nitrogen can be used as nutrient of microalgae, high ammonia nitrogen levels could inhibit the growth of microalgae. Nitrobacteria can transform ammonia nitrogen into nitrate nitrogen. In this study, mono Chlorella pyrenoidosa culture (mono-C.py), synchronous mixed culture (mixed-a), and asynchronous mixed culture (mixed-b) systems were examined for their ability to treat ammonium sulfate wastewater. Nitrogen removal rate of mixed-b at the end of culture (52.96%) was higher than that of the mono-C.py (46.37%) and the mixed-a (39.11%). Higher total suspended solid concentration (2.40 g/L), crude protein yield (0.76 g/L), and heating value yield (35.73 kJ/L) were obtained in mixed-b, meanwhile with excellent settlement performance (91.43 ± 0.51%). Mechanism analysis of settlement showed that the relative abundance of floc-forming-related bacteria Sphingopyxis and Acidovorax were increased generally, while nitrification/denitrifying members were decreased in mixed-b along with the culture proceeding.

Influence of Leachate and Nitrifying Bacteria on Photosynthetic Biogas Upgrading in a Two-Stage System

Photosynthetic biogas upgrading using two-stage systems allows the absorption of carbon dioxide (CO2) in an absorption unit and its subsequent assimilation by microalgae. The production of microalgae requires large amounts of nutrients, thus making scale-up difficult and reducing economic feasibility. The photosynthetic process produces oxygen (O2) (1 mol per mol of CO2 consumed), which can be desorbed into purified biogas. Two-stage systems reduce its impact but do not eliminate it. In this study, we test the use of landfill leachate as a nutrient source and propose a viable and economical strategy for reducing the O2 concentration. First, the liquid/gas (L/G) ratio and flow mode of the absorber were optimized for 20% and 40% CO2 with COMBO medium, then landfill leachate was used as a nutrient source. Finally, the system was inoculated with nitrifying bacteria. Leachate was found to be suitable as a nutrient source and to result in a significant improvement in CO2 absorption, with outlet concentrations of 0.01% and 0.6% for 20% and 40% CO2, respectively, being obtained. The use of nitrifying bacteria allowed a reduction in dissolved oxygen (DO) concentration, although it also resulted in a lower pH, thus making CO2 uptake slightly more difficult.

Induction of vitamin B12 to purify biogas slurry and upgrade biogas using co-culture of microalgae and fungi

Different gradient concentrations of vitamin B12 (0, 10, 100, 1,000 ng L^-1 ) were used in the symbiosis system (Chlorella vulgaris-Ganoderma lucidum or Chlorella vulgaris-Pleurotus ostreatus) to assess their effect on simultaneous purification of biogas and removal of nutrients in biogas slurry using co-culture of microalgae and fungi. When B12 was added to the symbiosis system, biomass growth, intracellular carbonic anhydrase activity (CA), chlorophyll a content (CHL-a), photosynthetic characteristics of the two cultivation system, and removal efficiency of nutrients in biogas slurry and CO₂ in biogas were significantly higher than those in the control group. The optimal concentration of B12 was determined to be 100 ng L^-1 considering the removal efficiency of nutrients and CO₂ . Maximum mean chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), and CO₂ removal efficiencies were 75.98 ± 6.26%, 78.46 ± 6.21%, 80.21 ± 6.83% and 61.08 ± 5.21% in Chlorella vulgaris-Ganoderma lucidum, respectively. This study showed the potential of microalgae and fungi symbiosis system with B12 addition for nutrient removal and biogas upgrading. PRACTITIONER POINTS: Vitamin B12 had positive effects on algal-fungal pellets growth. The optimal vitamin B12 concentration was 100 ng L^-1 . The highest CO₂ remove rate was 61.08% by G. lucidum/C. vulgaris pellets. Vitamin B12 significantly improved photosynthetic performance of pellets.

Photoautotrophs-Bacteria Co-Cultures: Advances, Challenges and Applications

Photosynthetic microorganisms are among the fundamental living organisms exploited for millennia in many industrial applications, including the food chain, thanks to their adaptable behavior and intrinsic proprieties. The great multipotency of these photoautotroph microorganisms has been described through their attitude to become biofarm for the production of value-added compounds to develop functional foods and personalized drugs. Furthermore, such biological systems demonstrated their potential for green energy production (e.g., biofuel and green nanomaterials). In particular, the exploitation of photoautotrophs represents a concrete biorefinery system toward sustainability, currently a highly sought-after concept at the industrial level and for the environmental protection. However, technical and economic issues have been highlighted in the literature, and in particular, challenges and limitations have been identified. In this context, a new perspective has been recently considered to offer solutions and advances for the biomanufacturing of photosynthetic materials: the co-culture of photoautotrophs and bacteria. The rational of this review is to describe the recently released information regarding this microbial consortium, analyzing the critical issues, the strengths and the next challenges to be faced for the intentions attainment.

Chlorella vulgaris growth on anaerobically digested sugarcane vinasse: influence of turbidity

This paper shows the influence of turbidity (in Nephelometric Turbidity Units - NTU), chemical oxygen demand (COD) and aeration (CO2 supply) on the productivity and growth rate and lipid content of microalgae (a mixed culture predominantly composed of Chlorella vulgaris), using anaerobically digested vinasse as a culture medium. The microalgae can be cultivated in anaerobically digested vinasse, at turbidity and chemical oxygen demand of 690 NTU and 2.5 gCOD L -1, respectively, according to the modified Gompertz model, and removal of turbidity by filtration did not influence the microalgae productivity (≈ 77 mg L1 d1). Furthermore, aeration increased the productivity up to 139 mg L1 d1, with a biomass dry weight of 2.7 g L-1. Finally, a maximum lipid content of 265 mg L -1 was obtained, while a nitrogen removal of 98% was recorded for all conditions. Thus, the combination of anaerobic digestion followed by the use of the digestate for the cultivation of microalgae may be an efficient way to treat large quantities of this residue, in turn yielding large amounts of microalgae biomass, which can be transformed into fertilizer and biofuel.

Design, Commissioning, and Performance Assessment of a Lab-Scale Bubble Column Reactor for Photosynthetic Biogas Upgrading with Spirulina platensis

The two-step bubble column-photobioreactor photosynthetic biogas upgrading system can enable simultaneous production of biomethane and value-added products from microalgae. However, due to the influence of a large number of variables, including downstream processes and the presence of microalgae, no unanimity has been reached regarding the performance of bubble column reactors in photosynthetic biogas upgrading. To investigate this further, the present work documents in detail, the design and commissioning of a lab-scale bubble column reactor capable of treating up to 16.3 L/h of biogas while being scalable. The performance of the bubble column was assessed at a pH of 9.35 with different algal densities of Spirulina platensis at 20 °C in the presence of light (3–5 klux or 40.5–67.5 μmol m^–2 s^–1). A liquid/gas flow (L/G) ratio of 0.5 allowed consistent CO₂ removal of over 98% irrespective of the algal density or its photosynthetic activity. For lower concentrations of algae, the volumetric O₂ concentration in the upgraded biomethane varied between 0.05 and 0.52%, thus providing grid quality biomethane. However, for higher algal concentrations, increased oxygen content in the upgraded biomethane due to both enhanced O₂ stripping and the photosynthetic activity of the microalgae as well as clogging and foaming posed severe operational challenges.

Xanthophylls from the Sea: Algae as Source of Bioactive Carotenoids

Algae are considered pigment-producing organisms. The function of these compounds in algae is to carry out photosynthesis. They have a great variety of pigments, which can be classified into three large groups: chlorophylls, carotenoids, and phycobilins. Within the carotenoids are xanthophylls. Xanthophylls (fucoxanthin, astaxanthin, lutein, zeaxanthin, and β-cryptoxanthin) are a type of carotenoids with anti-tumor and anti-inflammatory activities, due to their chemical structure rich in double bonds that provides them with antioxidant properties. In this context, xanthophylls can protect other molecules from oxidative stress by turning off singlet oxygen damage through various mechanisms. Based on clinical studies, this review shows the available information concerning the bioactivity and biological effects of the main xanthophylls present in algae. In addition, the algae with the highest production rate of the different compounds of interest were studied. It was observed that fucoxanthin is obtained mainly from the brown seaweeds Laminaria japonica, Undaria pinnatifida, Hizikia fusiformis, Sargassum spp., and Fucus spp. The main sources of astaxanthin are the microalgae Haematococcus pluvialis, Chlorella zofingiensis, and Chlorococcum sp. Lutein and zeaxanthin are mainly found in algal species such as Scenedesmus spp., Chlorella spp., Rhodophyta spp., or Spirulina spp. However, the extraction and purification processes of xanthophylls from algae need to be standardized to facilitate their commercialization. Finally, we assessed factors that determine the bioavailability and bioaccesibility of these molecules. We also suggested techniques that increase xanthophyll’s bioavailability.

Innovative operational strategies in photosynthetic biogas upgrading in an outdoors pilot scale algal-bacterial photobioreactor

Three innovative operational strategies were successfully evaluated to improve the quality of biomethane in an outdoors pilot scale photobioreactor interconnected to an external absorption unit: i) the use of a greenhouse during winter conditions, ii) a direct CO₂ stripping in the photobioreactor via air stripping during winter conditions and iii) the use of digestate as make-up water during summer conditions. CO₂ concentrations in the biomethane ranged from 0.4% to 6.1% using the greenhouse, from 0.3% to 2.6% when air was injected in the photobioreactor and from 0.4% to 0.9% using digestate as make up water. H₂S was completely removed under all strategies tested. On the other hand, CH₄ concentrations in biomethane ranged from 89.5% to 98.2%, from 93.0% to 98.2% and from 96.3% to 97.9%, when implementing strategies i), ii) and iii), respectively. The greenhouse was capable of maintaining microalgae productivities of 7.5 g m^-2 d^-1 during continental weather conditions, while mechanical CO₂ stripping increased the pH in order to support an effective CO₂ and H₂S removal. Finally, the high evaporation rates during summer conditions allowed maintaining high inorganic carbon concentrations in the cultivation broth using centrate, which provided a cost-effective biogas upgrading.

Strategies for decreasing the O2 content in the upgraded biogas purified via microalgae-based technology

Microalgae-bacteria consortium based technology using a High Rate Algal Pond (HRAP) interconnected to an Absorption Bubble Column (ABC) has emerged as an environmentally friendly promising option to upgrade biogas. However, the oxygenic photosynthesis of microalgae induces oxygen contamination in upgraded biogas, which could limit its further applications. Several strategies were proposed to favor the oxygen desorption and oxygen uptake in parts and accessories of the upgrading system. The effect of the volumetric ratio liquid recirculation rate/biogas rate (L/G = 5.0, 1.0 y 0.5) was evaluated in conjunction with the application of a novel accessory called Open Trickling Column (OTC). The O₂ content in upgraded biogas was around 2.1%v, attaining CO₂ removal efficiencies around 90%, at L/G ratio of 1.0 during diurnal and nocturnal periods. The inclusion of an OTC at the previous L/G, enhanced 54% the removal of O₂ by stripping and uptake compared with the basal condition. Mass balances of H₂S and methane showed that L/G > 1.0 favored the complete oxidation of H₂S but promoted the loss of methane in dissolved form. Additionally the effect of increasing linear velocity of liquid broth in the lab-scale HRAP (from 15 cm s^-1 to 20 cm s^-1) showed to improve the O₂ stripping with a consequential increase of biomass concentration under steady-state (from 0.7 to 1.4 g L^-1) besides achieving O₂ content in the upgraded biogas around 1.5%v.

Anaerobic Co-Digestion Effluent as Substrate for Chlorella vulgaris and Scenedesmus obliquus Cultivation

Anaerobic digestate supernatant can be used as a nutrient source for microalgae cultivation, thus integrating phytoremediation processes with high value products storage in microalgae biomass. Microalgae are able to use nitrogen and phosphorous from digestate, but high nutrient concentration can cause growth inhibition. In this study, two microalgae strains (C. vulgaris and S. obliquus) were cultivated on the anaerobic co-digestion supernatant (obtained from the organic fraction of municipal solid waste (OFMSW) and waste activated sludge (WAS)) in a preliminary Petri plate screening at different dilutions (1:10 and 1:5) using a synthetic medium (ISO) and tap water (TW). Direct Nile red screening was applied on colonies to preliminarily identify hydrophobic compound storage and then a batch test was performed (without air insufflation). Results show that C. vulgaris was able to grow on digestate supernatant 1:5 diluted, while Nile red screening allowed the preliminary detection of hydrophobic compound storage in colonies. The analysis carried out at the end of the test on ammonia, phosphate, nitrate and sulphate showed a removal percentage of 47.5 ± 0.8%, 65.0 ± 6.0%, 95.0 ± 3.0% and 99.5 ± 0.1%, respectively.

Marine Microalgae for Potential Lutein Production

Lutein is particularly known to help maintain normal visual function by absorbing and attenuating the blue light that strikes the retina in our eyes. The effect of overexposure to blue light on our eyes due to the excessive use of electronic devices is becoming an issue of modern society due to insufficient dietary lutein consumption through our normal diet. There has, therefore, been an increasing demand for lutein-containing dietary supplements and also in the food industry for lutein supplementation in bakery products, infant formulas, dairy products, carbonated drinks, energy drinks, and juice concentrates. Although synthetic carotenoid dominates the market, there is a need for environmentally sustainable carotenoids including lutein production pathways to match increasing consumer demand for natural alternatives. Currently, marigold flowers are the predominant natural source of lutein. Microalgae can be a competitive sustainable alternative, which have higher growth rates and do not require arable land and/or a growth season. Currently, there is no commercial production of lutein from microalgae, even though astaxanthin and β-carotene are commercially produced from specific microalgal strains. This review discusses the potential microalgae strains for commercial lutein production, appropriate cultivation strategies, and the challenges associated with realising a commercial market share.

The effect of CO2 addition and hydraulic retention time on pathogens removal in HRAPs

The influence of CO₂ addition and hydraulic retention time (5 and 7 days) on removal of Pseudomonas aeruginosa, Clostridium perfringens, Staphylococcus sp., Enterococcus sp., and Escherichia coli was evaluated in a system with three parallel 21 L high rate algal ponds. Both the addition of CO₂ and an increase in HRT had no significant influence on bacterial removal, but bacterial removal was higher than found in previous studies. The removal was 3.4-3.8, 2.5-3.7, 2.6-3.1, 2.2-2.6 and 1.3-1.7 units log for P. aeruginosa, E. coli, Enterococcus sp., C. perfringens, and for Staphylococcus sp., respectively. Although CO₂ addition did not increase disinfection, it did significantly increase biomass productivity (by ≈60%) and settleability (by ≈350%). Additionally, even at the lower 5-day hydraulic retention time, CO₂ addition improves removal of chemical oxygen demand (COD), total organic carbon (TOC), total organic nitrogen and phosphorus by 97, 91, 12 and 50%, respectively.

Removal of pollutants from biogas slurry and CO2 capture in biogas by microalgae-based technology: a systematic review

Recent research interest has focused on microalgae cultivation for biogas slurry purification and biogas upgrading due to the requirement of high efficiency for nutrient uptake and CO₂ capture, with economic feasibility and environmental benefits. Numerous studies have suggested that biogas slurry purification and biogas upgrading can occur simultaneously via microalgae-based technology. However, there is no comprehensive review on this technology with respect to the nutrient removal from biogas slurry and biogas upgrading. This article summarizes microalgal cultivation with biogas slurry and biogas from anaerobic digestion. The parameters, techniques, and modes of microalgae cultivation have been discussed in detail to achieve high efficiency in biogas slurry purification and biogas upgrading. In addition, the evaluation of energy efficiency and safety has also been explored. Compared with mono-cultivation of microalgae and co-cultivation of microalgae and bacteria, microalgae-fungi symbiosis has demonstrated greater development prospect and higher energy efficiency and the energy consumption for pollutants and CO₂ removal were 14.2-39.0% · USD^-1 and 19.9-23.3% · USD^-1, respectively. Further, a sustainable recycling scheme is proposed for the purification of biogas slurry from anaerobic digestion process and biogas upgrading via microalgae-based technology.

Performance evaluation of a control strategy for photosynthetic biogas upgrading in a semi-industrial scale photobioreactor

The validation of a control strategy for biogas upgrading via light-driven CO₂ consumption by microalgae and H₂S oxidation by oxidizing bacteria using the oxygen photosynthetically generated was performed in a semi-industrial scale (9.6 m³) photobioreactor. The control system was able to support CO₂ concentrations lower than 2% with O₂ contents ≤ 1% regardless of the pH in the cultivation broth (ranging from 9.05 to 9.50). Moreover, the control system was efficient to cope with variations in biogas flowrate from 143 to 420 L h^-1, resulting in a biomethane composition of CO₂ < 2.4%, CH₄ > 95.5%, O₂ < 1% and no H₂S. Despite the poor robustness of this technology against failures in biogas and liquid supply (CH₄ concentration of 67.5 and 70.9% after 2 h of biogas or liquid stoppage, respectively), the control system was capable of restoring biomethane quality in less than 2 h when biogas or liquid supply was resumed.

Recyclable magnetite-enhanced electromethanogenesis for biomethane production from wastewater

Improving the yield and methane content of biogas is of great concern for wastewater treatment by anaerobic digestion. Herein we developed a nanomagnetite-enhanced electromethanogenesis (EM_nano) process for the first time, the sustainable utilization of which improved the biomethane production rate from dairy wastewater. The maximum CH₄ production rate in the EM_nano process is 2.3 ± 0.3-fold higher than it is in the conventional methanogenesis (CM) process, and it is accompanied by an almost delay-free start-up. The technical-economic evaluation revealed that an 82.1 ± 5.0% greater net benefit was obtained in the third generation of the EM_nano process compared with the CM process. The improved methanogenesis was attributed to the formation of dense planktonic cell co-aggregates that are triggered by nanomagnetite, which facilitated the interspecies electron exchange during acetoclastic methanogenesis. Simultaneously, a cathode biofilm with high viability and catalytic activity was also formed in the EM_nano process that decreased the biofilm resistance and facilitated the electron transfer during electromethanogenesis. This study is a worthwhile attempt to combine recyclable conductive materials with an electromethanogenesis process for wastewater treatment, and it effectively achieves energy recovery with high stability and cost-effectiveness.

Case study on the effect continuous CO2 enrichment, via biogas scrubbing, has on biomass production and wastewater treatment in a high rate algal pond

Microalgae grown in high rate algal ponds (HRAP) treating wastewater are considered a promising feed for biofuel production. Biomass productivity is often considered to be limited by carbon availability, with the addition of CO₂ being the proposed solution. Biogas from anaerobic wastewater treatment potentially provides a cheap, co-located CO₂ source. Two identical 223 m² HRAPs were constructed at Melbourne Water's Western Treatment Plant, where biogas from an anaerobic lagoon is used to generate electricity. One HRAP was fed secondary treated wastewater that had been enriched with CO₂ recovered from the biogas using industry standard biogas scrubbers, the Enriched HRAP, while the other HRAP was fed the same wastewater expect it had by passed the biogas scrubbers, the Control HRAP. The biomass production and wastewater treatment performance of the two HRAPs was compared over 12 months. The inlet to the Enriched HRAP had significantly higher free CO₂ and inorganic carbon, 175.00 ± 49.30 mg L^-1 and 110.00 ± 10.2 mg L^-1, than the inlet to the Control HRAP, 9.30 ± 7.08 mg L^-1 and 89.62 ± 5.12 mg L^-1. There were no significant differences in biomass production between the HRAPs as measured by dry matter, particulate organic carbon or nitrogen. Chlorophyll a was statistically higher in the Enriched HRAP, however, this measurement is potentially unreliable. Regarding wastewater treatment, only total nitrogen and ammonium removal differed significantly between the HRAPs, with the Control HRAP, 59.13 ± 21.13% and 76.46 ± 32.33%, slightly outperforming the Enriched HRAP, 53.52 ± 17.41% and 68.76 ± 31.17%. Overall, neither biomass production nor wastewater treatment was meaningfully improved by CO₂ enrichment, however, wastewater treatment was still effective in both HRAPs.

Integration of anaerobic digestion and microalgal cultivation for digestate bioremediation and biogas upgrading

Biogas is the gaseous byproduct obtained during anaerobic digestion which is rich in methane, along with a significant amount of other gases like CO₂. The removal of CO₂ is essential to upgrade the biogas to biomethane (>95% methane content). High CO₂ tolerant microalgae can be employed as a biological CO₂ scrubbing agent for biogas upgrading. Many microalgal strains tolerant to the levels of CO₂ and CH₄ seen in biogas have been reported. A CO₂ removal efficiency of 50-99% can be attained based on the microalgae used and the cultivation conditions applied. Nutrient-rich liquid digestate obtained from anaerobic digestion can also be used as the cultivation medium for microalgae, performing biogas upgrading and digestate bioremediation simultaneously. Mixotrophic cultivation enables microalgae to utilize the organic carbon present in the liquid digestate along with nitrogen and phosphorus. Microalgae appears to be a potential biological CO₂ scrubbing agent for efficient biogas upgrading.

Influence of three microalgal-based cultivation technologies on different domestic wastewater and biogas purification in photobioreactor

To investigate the effects of different microalgae and culture methods on the purification of domestic wastewater and biogas, Chlorella vulgaris and Scenedesmus obliquus were selected. Three different culture methods (monoculture, microalgal-fungi cocultivation, and microalgal-activated sludge cocultivation) were used to remove nutrients from four different domestic wastewaters and remove CO₂ from raw biogas in a photobioreactor. The results show that the effluent from the centrate of pretreated urban wastewater (WW4) achieved the highest nutrient and CO₂ removal efficiency. Cocultivation of C. vulgaris and activated sludge achieved the highest COD, TP, and CO₂ removal efficiencies of 79.27%, 81.25%, and 60.39% with WW4, respectively. Cocultivation of C. vulgaris and fungi achieved the highest TN removal efficiency of 78.46% with WW4. The contents of C, N, and P in the microalgal-activated sludge symbiont after treatment exceeded 50%, 8%, and 0.8%, respectively. Highly economically efficient energy consumption was achieved with WW4 for both C. vulgaris and S. obliquus. Microalgal-activated sludge cocultivation was identified as the optimal option for the simultaneous purification of wastewater and biogas based on its high pollution and CO₂ removal efficiency. This provides a reference for the microalgal-based purification of actual domestic wastewater and raw biogas. PRACTITIONER POINTS: Nutrient and CO₂ were efficiently removed by C. vulgaris with activated sludge. CO₂ was removed up to 60.4% and was economically viable. Cocultivation of C. vulgaris and fungi could achieve the highest TN removal with WW4.

Assessing the potential of purple phototrophic bacteria for the simultaneous treatment of piggery wastewater and upgrading of biogas

The potential of purple phototrophic bacteria (PPB) for the simultaneous treatment of piggery wastewater (PWW) and biogas upgrading was evaluated batchwise in gas-tight photobioreactors. PWW dilution was identified as a key parameter determining the efficiency of wastewater treatment and biomethane quality in PPB photobioreactors. Four times diluted PWW supported the most efficient total organic carbon (TOC) and total nitrogen removals (78% and 13%, respectively), with CH₄ concentrations of 90.8%. The influence of phosphorous concentration (supplementation of 50 mg L^-1 of P-PO₄^3-) on PPB-based PWW treatment coupled to biogas upgrading was investigated. TOC removals of ≈60% and CH₄ concentrations of ≈90.0% were obtained regardless of phosphorus supplementation. Finally, the use of PPB and algal-bacterial consortia supported CH₄ concentrations in the upgraded biogas of 93.3% and 73.6%, respectively, which confirmed the potential PPB for biogas upgrading coupled to PWW treatment.

Influence of liquid-to-biogas ratio and alkalinity on the biogas upgrading performance in a demo scale algal-bacterial photobioreactor

The influence of the liquid-to-biogas ratio (L/G) and alkalinity on methane quality was evaluated in a 11.7 m³ outdoors horizontal semi-closed tubular photobioreactor interconnected to a 45-L absorption column (AC). CO₂ concentrations in the upgraded methane ranged from <0.1 to 9.6% at L/G of 2.0 and 0.5, respectively, with maximum CH₄ concentrations of 89.7% at a L/G of 1.0. Moreover, an enhanced CO₂ removal (mediating a decrease in CO₂ concentration from 9.6 to 1.2%) and therefore higher CH₄ contents (increasing from 88.0 to 93.2%) were observed when increasing the alkalinity of the AC cultivation broth from 42 ± 1 mg L^-1 to 996 ± 42 mg L^-1. H₂S was completely removed regardless of the L/G or the alkalinity in AC. The continuous operation of the photobioreactor with optimized operating parameters resulted in contents of CO₂ (<0.1%-1.4%), H₂S (<0.7 mg m^-3) and CH₄ (94.1%-98.8%) complying with international regulations for methane injection into natural gas grids.

Technology validation of photosynthetic biogas upgrading in a semi-industrial scale algal-bacterial photobioreactor

The performance of photosynthetic biogas upgrading coupled to wastewater treatment was evaluated in an outdoors high rate algal pond (HRAP) interconnected to an absorption column at semi-industrial scale. The influence of biogas flowrate (274, 370 and 459 L h^-1), liquid to biogas ratio (L/G = 1.2, 2.1 and 3.5), type of wastewater (domestic versus centrate) and hydraulic retention time in the HRAP (HRT) on the quality of the biomethane produced was assessed. The highest CO₂ and H₂S removal efficiencies (REs) were recorded at the largest L/G due to the higher biogas-liquid mass transfer at increasing liquid flowrates. No significant influence of the biogas flowrate on process performance was observed, while the type of wastewater was identified as a key operational parameter. CO₂ and H₂S-REs of 99% and 100% at a L/G_max = 3.5 were recorded using centrate. The maximum CH₄ content in the biomethane (90%) was limited by N₂ and O₂ desorption.

A systematic comparison of the potential of microalgae-bacteria and purple phototrophic bacteria consortia for the treatment of piggery wastewater

This study evaluated the performance of two open-photobioreactors operated with microalgae-bacteria (PBR-AB) and purple photosynthetic bacteria (PBR-PPB) consortia during the treatment of diluted (5%) piggery wastewater (PWW) at multiple hydraulic retention times (HRT). At a HRT of 10.6 days, PBR-AB provided the highest removal efficiencies of nitrogen, phosphorus and zinc (87 ± 2, 91 ± 3 and 98 ± 1%), while the highest organic carbon removals were achieved in PBR-PPB (87 ± 4%). The decrease in HRT from 10.6, to 7.6 and 4.1 day caused a gradual deterioration in organic carbon and nitrogen removal, but did not influence the removal of phosphorus and Zn in both photobioreactors. The decrease in HRT caused a severe wash-out of microalgae in PBR-AB and played a key role in the structure of bacterial population in both photobioreactors. In addition, batch biodegradation tests at multiple PWW dilutions (5, 10 and 15%) confirmed the slightly better performance of algal-bacterial systems regardless of PWW dilution.

Long-term photosynthetic CO2 removal from biogas and flue-gas: Exploring the potential of closed photobioreactors for high-value biomass production

The long-term performance of a tubular photobioreactor interconnected to a gas absorption column for the abatement of CO₂ from biogas and flue-gas was investigated. Additionally, a novel nitrogen feast-famine regime was implemented during the flue-gas feeding stage in order to promote the continuous storage of highly-energetic compounds. Results showed effective CO₂ (~98%) and H₂S (~99%) removals from synthetic biogas, supported by the high photosynthetic activity of microalgae which resulted in an alkaline pH (~10). In addition, CO₂ removals of 99 and 91% were observed during the flue-gas operation depending on the nutrients source: mineral salt medium and digestate, respectively. A biomass productivity of ~8 g m^-2 d^-1 was obtained during both stages, with a complete nitrogen and carbon recovery from the cultivation broth. Moreover, the strategy of feeding nutrients during the dark period promoted the continuous accumulation of carbohydrates, their concentration increasing from 22% under normal nutrition up to 37% during the feast-famine cycle. This represents a productivity of ~3 g-_{carbohydrates} m^-2 d^-1, which can be further valorized to contribute to the economic sustainability of the photosynthetic CO₂ removal process.

Seasonal variation of biogas upgrading coupled with digestate treatment in an outdoors pilot scale algal-bacterial photobioreactor

The yearly variations of the quality of the upgraded biogas and the efficiency of digestate treatment were evaluated in an outdoors pilot scale high rate algal pond (HRAP) interconnected to an external absorption column (AC) via a conical settler. CO₂ concentrations in the upgraded biogas ranged from 0.7% in August to 11.9% in December, while a complete H₂S removal was achieved regardless of the operational month. CH₄ concentrations ranged from 85.2% in December to 97.9% in June, with a limited O₂ and N₂ stripping in the upgraded biogas mediated by the low recycling liquid/biogas ratio in the AC. Biomass productivity ranged from 0.0 g m^-2 d^-1 in winter to 22.5 g m^-2 d^-1 in summer. Finally, microalgae diversity was severely reduced throughout the year likely due to the increasing salinity in the cultivation broth of the HRAP induced by process operation in the absence of effluent.

Co-cultivation of fungal-microalgal strains in biogas slurry and biogas purification under different initial CO2 concentrations

The effects of five different microalgae-fungi on nutrient removal and CO₂ removal were investigated under three different CO₂ contents (35%, 45% and 55%). The results showed that the highest nutrient and CO₂ removal efficiency were found at 55% CO₂ by cocultivation of different microalgae and fungi. The effect of different initial CO₂ concentration on the removal of CO₂ from microalgae was significant, and the order of CO₂ removal efficiency was 55% (v/v) >45% (v/v) >35% (v/v). The best nutrient removal and biogas purification could be achieved by co-cultivation of C. vulgaris and G. lucidum with 55% initial CO₂ content. The maximum mean COD, TN, TP and CO₂ removal efficiency can reach 68.29%, 61.75%, 64.21% and 64.68%, respectively under this condition. All highest COD, TN, TP and CO₂ removal efficiency were more than 85%. The analysis of energy consumption economic efficiency revealed that this strategy resulted in the highest economic efficiency. The results of this work can promote simultaneously biological purification of wastewater and biogas using microalgal-fungal symbiosis.

Influence of the seasonal variation of environmental conditions on biogas upgrading in an outdoors pilot scale high rate algal pond

The influence of the daily and seasonal variations of environmental conditions on the quality of the upgraded biogas was evaluated in an outdoors pilot scale high rate algal pond (HRAP) interconnected to an external absorption column (AC) via a conical settler. The high alkalinity in the cultivation broth resulted in a constant biomethane composition during the day regardless of the monitored month, while the high algal-bacterial activity during spring and summer boosted a superior biomethane quality. CO₂ concentrations in the upgraded biogas ranged from 0.1% in May to 11.6% in December, while a complete H₂S removal was always achieved regardless of the month. A limited N₂ and O₂ stripping from the scrubbing cultivation broth was recorded in the upgraded biogas at a recycling liquid/biogas ratio in the AC of 1. Finally, CH₄ concentration in the upgraded biogas ranged from 85.6% in December to 99.6% in August.

Biocapture of CO₂ by Different Microalgal-Based Technologies for Biogas Upgrading and Simultaneous Biogas Slurry Purification under Various Light Intensities and Photoperiods

Co-cultivation of microalgae and microbes for pollutant removal from sewage is considered as an effective wastewater treatment method. The aim of this study is to screen the optimal photoperiod, light intensity and microalgae co-cultivation method for simultaneously removing nutrients in biogas slurry and capturing CO₂ in biogas. The microalgae–fungi pellets are deemed to be a viable option because of their high specific growth rate and nutrient and CO₂ removal efficiency under the photoperiod of 14 h light:10 h dark. The order of both the biogas slurry purification and biogas upgrading is ranked the same, that is Chlorella vulgaris–Ganoderma lucidum > Chlorella vulgaris–activated sludge > Chlorella vulgaris under different light intensities. For all cultivation methods, the moderate light intensity of 450 μmol m⁻² s⁻¹ is regarded as the best choice. This research revealed that the control of photoperiod and light intensity can promote the biological treatment process of biogas slurry purification and biogas upgrading using microalgal-based technology.

Evaluation of the dynamics of microalgae population structure and process performance during piggery wastewater treatment in algal-bacterial photobioreactors

The dynamics of microalgae population during piggery wastewater (PWW) treatment in four open photobioreactors operated at 27days of hydraulic retention time, and inoculated with Chlorella sp. (R1), Acutodesmus obliquus (R2), Oscillatoria sp. (R3) and in the absence of inoculum (R4), were evaluated for 6months. In addition, the algal-bacterial biomass concentration, removal of organic matter, nutrients and heavy metals were also assessed. The results revealed a high diversity and rapid variations in the structure of microalgae populations, Chlorella sp. being dominant in R4 throughout most of the operational period. Steady state average biomass concentration ranged from 2445-2610mg/L in R1-R3 to 3265mg/L in R4. No significant differences were recorded in the removal efficiencies (REs) of total organic carbon (86-87%), inorganic carbon (62-71%), total nitrogen (82-85%) and total phosphorous (90-92%). Finally, Zn-REs accounted for 26% in R3, 37% in R2, and 49% in R1 and R4.