Microalgal-biotechnology as a platform for an integral biogas upgrading and nutrient removal from anaerobic effluents

Getting Grip on Phosphorus: Potential of Microalgae as a Vehicle for Sustainable Usage of This Macronutrient

Phosphorus (P) is an important and irreplaceable macronutrient. It is central to energy and information storage and exchange in living cells. P is an element with a "broken geochemical cycle" since it lacks abundant volatile compounds capable of closing the P cycle. P fertilizers are critical for global food security, but the reserves of minable P are scarce and non-evenly distributed between countries of the world. Accordingly, the risks of global crisis due to limited access to P reserves are expected to be graver than those entailed by competition for fossil hydrocarbons. Paradoxically, despite the scarcity and value of P reserves, its usage is extremely inefficient: the current waste rate reaches 80% giving rise to a plethora of unwanted consequences such as eutrophication leading to harmful algal blooms. Microalgal biotechnology is a promising solution to tackle this challenge. The proposed review briefly presents the relevant aspects of microalgal P metabolism such as cell P reserve composition and turnover, and the regulation of P uptake kinetics for maximization of P uptake efficiency with a focus on novel knowledge. The multifaceted role of polyPhosphates, the largest cell depot for P, is discussed with emphasis on the P toxicity mediated by short-chain polyPhosphates. Opportunities and hurdles of P bioremoval via P uptake from waste streams with microalgal cultures, either suspended or immobilized, are discussed. Possible avenues of P-rich microalgal biomass such as biofertilizer production or extraction of valuable polyPhosphates and other bioproducts are considered. The review concludes with a comprehensive assessment of the current potential of microalgal biotechnology for ensuring the sustainable usage of phosphorus.

Mini-review: Nanoparticles for enhanced biogas upgrading

This mini-review is intended to explore the innovative applications of nanoparticles (NPs) in biogas upgrading, emphasizing their capacity to enhance biogas quality. Numerous studies underscore how NPs, when applied during anaerobic digestion, can boost not only the quantity but also the quality of the produced biogas, leading to reduce significantly the concentration of hydrogen sulphide or even to remove it completely. Moreover, NPs are proving to be excellent alternatives as adsorbent materials, achieving up to 400 mgH2S g-1 NPs. In addition, new studies are exploring the application of NPs to increase the efficiency of biological treatments thanks to their unique features. This review also emphasizes the potential benefits and addresses the challenges that need to be overcome for these technologies to reach their full potential, ultimately contributing to the development of a sustainable and environmentally friendly energy landscape.

Cultivation of microalgae-bacteria consortium by waste gas-waste water to achieve CO2 fixation, wastewater purification and bioproducts production

The cultivation of microalgae and microalgae-bacteria consortia provide a potential efficient strategy to fix CO2 from waste gas, treat wastewater and produce value-added products subsequently. This paper reviews recent developments in CO2 fixation and wastewater treatment by single microalgae, mixed microalgae and microalgae-bacteria consortia, as well as compares and summarizes the differences in utilizing different microorganisms from different aspects. Compared to monoculture of microalgae, a mixed microalgae and microalgae-bacteria consortium may mitigate environmental risk, obtain high biomass, and improve the efficiency of nutrient removal. The applied microalgae include Chlorella sp., Scenedesmus sp., Pediastrum sp., and Phormidium sp. among others, and most strains belong to Chlorophyta and Cyanophyta. The bacteria in microalgae-bacteria consortia are mainly from activated sludge and specific sewage sources. Bioengineer in CBB cycle in microalgae cells provide effective strategy to achieve improvement of CO2 fixation or a high yield of high-value products. The mechanisms of CO2 fixation and nutrient removal by different microbial systems are also explored and concluded, the importance of microalgae in the technology is proven. After cultivation, microalgae biomass can be harvested through physical, chemical, biological and magnetic separation methods and used to produce high-value by-products, such as biofuel, feed, food, biochar, fertilizer, and pharmaceutical bio-compounds. Although this technology has brought many benefits, some challenging obstacles and limitation remain for industrialization and commercializing.

Influence of pH on the Morphology and Cell Volume of Microscopic Algae, Widely Distributed in Terrestrial Ecosystems

Terrestrial algae are a group of photosynthetic organisms that can survive in extreme conditions. pH is one of the most important factors influencing the distribution of algae in both aquatic and terrestrial ecosystems. The impact of different pH levels on the cell volume and other morphological characteristics of authentic and reference strains of Chlorella vulgaris, Bracteacoccus minor, Pseudoccomyxa simplex, Chlorococcum infusionum, and Vischeria magna were studied. Chlorella vulgaris, Pseudoccomyxa simplex, and Vischeria magna were the most resistant species, retaining their morphology in the range of pH 4-11.5 and pH 3.5-11, respectively. The change in pH towards acidic and alkaline levels caused an increase in the volume of Pseudoccomixa simplex and Vischeria magna cells, according to a polynomial regression model. The volume of Chlorella vulgaris cells increased from a low to high pH according to a linear regression model. Changes in pH levels did not have a significant impact on the volume of Bracteacoccus minor and Chlorococcum infusionum cells. Low and high levels of pH caused an increase in oil-containing substances in Vischeria magna and Bracteacoccus minor cells. Our study revealed a high resistance of the studied species to extreme pH levels, which allows for us to recommend these strains for broader use in biotechnology and conservation studies of natural populations.

Strain-resolved metagenomics approaches applied to biogas upgrading

Genetic heterogeneity is a common trait in microbial populations, caused by de novo mutations and changes in variant frequencies over time. Microbes can thus differ genetically within the same species and acquire different phenotypes. For instance, performance and stability of anaerobic reactors are linked to the composition of the microbiome involved in the digestion process and to the environmental parameters imposing selective pressure on the metagenome, shaping its evolution. Changes at the strain level have the potential to determine variations in microbial functions, and their characterization could provide new insight into ecological and evolutionary processes driving anaerobic digestion. In this work, single nucleotide variant dynamics were studied in two time-course biogas upgrading experiments, testing alternative carbon sources and the response to exogenous hydrogen addition. A cumulative total of 76,229 and 64,289 high-confidence single nucleotide variants were discerned in the experiments related to carbon substrate availability and hydrogen addition, respectively. By combining complementary bioinformatic approaches, the study reconstructed the precise strain count-two for both hydrogenotrophic archaea-and tracked their abundance over time, while also characterizing tens of genes under strong selection. Results in the dominant archaea revealed the presence of nearly 100 variants within genes encoding enzymes involved in hydrogenotrophic methanogenesis. In the bacterial counterparts, 119 mutations were identified across 23 genes associated with the Wood-Ljungdahl pathway, suggesting a possible impact on the syntrophic acetate-oxidation process. Strain replacement events took place in both experiments, confirming the trends suggested by the variants trajectories and providing a comprehensive understanding of the biogas upgrading microbiome at the strain level. Overall, this resolution level allowed us to reveal fine-scale evolutionary mechanisms, functional dynamics, and strain-level metabolic variation that could contribute to the selection of key species actively involved in the carbon dioxide fixation process.

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.

From anaerobic digestion to single cell protein synthesis: A promising route beyond biogas utilization

The accumulation of a large amount of organic solid waste and the lack of sufficient protein supply worldwide are two major challenges caused by rapid population growth. Anaerobic digestion is the main force of organic waste treatment, and the high-value utilization of its products (biogas and digestate) has been widely concerned. These products can be used as nutrients and energy sources for microorganisms such as microalgae, yeast, methane-oxidizing bacteria(MOB), and hydrogen-oxidizing bacteria(HOB) to produce single cell protein(SCP), which contributes to the achievement of sustainable development goals. This new model of energy conversion can construct a bioeconomic cycle from waste to nutritional products, which treats waste without additional carbon emissions and can harvest high-value biomass. Techno-economic analysis shows that the SCP from biogas and digestate has higher profit than biogas electricity generation, and its production cost is lower than the SCP using special raw materials as the substrate. In this review, the case of SCP-rich microorganisms using anaerobic digestion products for growth was investigated. Some of the challenges faced by the process and the latest developments were analyzed, and their potential economic and environmental value was verified.

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.

Integration of Waste to Bioenergy Conversion Systems: A Critical Review

Sustainable biofuel production is the most effective way to mitigate greenhouse gas emissions associated with fossil fuels while preserving food security and land use. In addition to producing bioenergy, waste biorefineries can be incorporated into the waste management system to solve the future challenges of waste disposal. Biomass waste, on the other hand, is regarded as a low-quality biorefinery feedstock with a wide range of compositions and seasonal variability. In light of these factors, biomass waste presents limitations on the conversion technologies available for value addition, and therefore more research is needed to enhance the profitability of waste biorefineries. Perhaps, to keep waste biorefineries economically and environmentally sustainable, bioprocesses need to be integrated to process a wide range of biomass resources and yield a diverse range of bioenergy products. To achieve optimal integration, the classification of biomass wastes to match the available bioprocesses is vital, as it minimizes unnecessary processes that may increase the production costs of the biorefinery. Based on biomass classification, this study discusses the suitability of the commonly used waste-to-energy conversion methods and the creation of integrated biorefineries. In this study, the integration of waste biorefineries is discussed through the integration of feedstocks, processes, platforms, and the symbiosis of wastes and byproducts. This review seeks to conceptualize a framework for identifying and integrating waste-to-energy technologies for the varioussets of biomass wastes.

Microalgae Biomass as a New Potential Source of Sustainable Green Lubricants

Lubricants are materials able to reduce friction and/or wear of any type of moving surfaces facilitating smooth operations, maintaining reliable machine functions, and reducing risks of failures while contributing to energy savings. At present, most worldwide used lubricants are derived from crude oil. However, production, usage and disposal of these lubricants have significant impact on environment and health. Hence, there is a growing pressure to reduce demand of this sort of lubricants, which has fostered development and use of green lubricants, as vegetable oil-based lubricants (biolubricants). Despite the ecological benefits of producing/using biolubricants, availability of the required raw materials and agricultural land to create a reliable chain supply is still far from being established. Recently, biomass from some microalgae species has attracted attention due to their capacity to produce high-value lipids/oils for potential lubricants production. Thus, this multidisciplinary work reviews the main chemical-physical characteristics of lubricants and the main attempts and progress on microalgae biomass production for developing oils with pertinent lubricating properties. In addition, potential microalgae strains and chemical modifications to their oils to produce lubricants for different industrial applications are identified. Finally, a guide for microalgae oil selection based on its chemical composition for specific lubricant applications is provided.

Phycoremediation integrated approach for the removal of pharmaceuticals and personal care products from wastewater - A review

Pharmaceuticals and personal care products (PPCPs) are of emerging concerns because of their large usage, persistent nature which promised their continuous disposal into the environment, as these pollutants are stable enough to pass through wastewater treatment plants causing hazardous effects on all the organisms through bioaccumulation, biomagnification, and bioconcentration. The available technologies are not capable of eliminating all the PPCPs along with their degraded products but phycoremediation has the advantage over these technologies by biodegrading the pollutants without developing resistant genes. Even though phycoremediation has many advantages, industries have found difficulty in adapting this technology as a single-stage treatment process. To overcome these drawbacks recent research studies have focused on developing technology that integrated phycoremediation with the commonly employed treatment processes that are in operation for treating the PPCPs effectively. This review paper focuses on such research approaches that focused on integrating phycoremediation with other technologies such as activated sludge process (ASP), advanced oxidation process (AOP), Up-flow anaerobic sludge blanket reactor (UASBR), UV irradiation, and constructed wetland (CW) with the advantages and limitations of each integration processes. Furthermore, augmenting phycoremediation by co-metabolic mechanism with the addition of sodium chloride, sodium acetate, and glucose for the removal of PPCPs has been highlighted in this review paper.

Potential application and beneficial effects of a marine microalgal biomass produced in a high-rate algal pond (HRAP) in diets of European sea bass, Dicentrarchus labrax

Microalgae have been used as live food in aquatic species. In recent years, the interest in microalgae has considerably increased, thanks to the evolution of production techniques that have identified them as an ecologically attractive aquafeed ingredient. The present study provides the first data about the effects of dietary inclusion of a microalgae consortium grown in a high-rate algal pond system on zootechnical performance, morphometric indices, and dietary nutrient digestibility as well as morphology and functionality of the digestive system of European sea bass, Dicentrarchus labrax. A dietary treatment including a commercial mono-cultured microalgae (Nannochloropsis sp.) biomass was used for comparison. Six hundred and thirty-six European sea bass juveniles (18 ± 0.28 g) were randomly allotted into 12 experimental groups and fed 4 different diets for 10 weeks: a control diet based on fish meal, fish oil, and plant protein sources; a diet including 10% of Nannochloropsis spp. biomass (100 g/kg diet); and two diets including two levels (10% and 20%) of the microalgal consortium (100 and 200 g/kg diet). Even at the highest dietary inclusion level, the microalgal consortium (200 g/kg diet) did not affect feed palatability and fish growth performance. A significant decrease in the apparent digestibility of dry matter, protein, and energy was observed in diets including 10 and 20% of the microalgal consortium, but all fish exhibited a well-preserved intestinal histomorphology. Moreover, dietary inclusion with the microalgal consortium significantly increased the enzymatic activity of maltase, sucrase-isomaltase, and ɤ-glutamil transpeptidase in the distal intestine of the treated European sea bass. Algal consortium grown using fish farm effluents represents an attempt to enhance the utilization of natural biomasses in aquafeeds when used at 10 % as substitute of vegetable ingredients in diet for European sea bass.

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.

Resource recovery from sugarcane vinasse by anaerobic digestion - A review

The increase in biofuel production by 2030, driven by the targets set at the 21st United Nations Framework Convention on Climate Change (COP21), will promote an increase in ethanol production, and consequently more vinasse generation. Sugarcane vinasse, despite having a high polluting potential due to its high concentration of organic matter and nutrients, has the potential to produce value-added resources such as volatile fatty acids (VFA), biohydrogen (bioH₂) and biomethane (bioCH₄) from anaerobic digestion. The objective of this paper is to present a critical review on the vinasse treatment by anaerobic digestion focusing on the final products. Effects of operational parameters on production and recovery of these resources, such as pH, temperature, retention time and type of inoculum were addressed. Given the importance of treating sugarcane vinasse due to its complex composition and high volume generated in the ethanol production process, this is the first review that evaluates the production of VFAs, bioH₂ and bioCH₄ in the treatment of this organic residue. Also, the challenges of the simultaneous production of VFA, bioH₂ and bioCH₄ and resources recovery in the wastewater streams generated in flex-fuel plants, using sugarcane and corn as raw material in ethanol production, are presented. The installation of flex-fuel plants was briefly discussed, with the main impacts on the treatment process of these effluents either jointly or simultaneously, depending on the harvest season.

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.

An evolved native microalgal consortium-snow system for the bioremediation of biogas and centrate wastewater: Start-up, optimization and stabilization

It is necessary to develop sustainable technologies for centrate wastewater (CW) and biogas treatment from sludge anaerobic digestion (AD) systems in an environmentally friendly and economical manner. The microalgae-based bioremediation approach presents a competitive alternative due to its capacity for nutrient recovery and carbon sequestration. However, process instabilities and operating challenges limit its development and implementation largely due to the complexities in the CW and biogas. In this study, the evolved native microalgal consortium (ENMC) was firstly developed using the gradual stress increase method to enhance their adaptation in high ammonium condition. The supplementation of local snow (with Ca²⁺ and Mg²⁺) and biogas into CW significantly enhanced ENMC growth through batch tests. Subsequently, an integrated ENMC-snow (ENMCS) system was proposed consisting of a hydrolysis-acidification reactor (HAR), biogas upgrade reactor, and photobioreactor (PBR). The ENMCS system was systematically investigated under both batch and semi-continuous operations, by adjusting primary process parameters including the fill ratio, feeding time, hydraulic retention time (HRT), wastewater pretreatment, and PBR type. It was eventually optimized as a 24 h, 70% fermented CW diluted with 30% snow water, semi-continuous feeding system with a fill ratio of 50% and HRT of 6 d in an open-PBR. Long-term operation (310 days) showed superior biomass yield (0.3059 ± 0.0039 g/(L•d)) and nutrient removal efficiencies (95.6 ± 0.13% and 90.8 ± 0.44% for NH₄⁺-N and PO₄^3--P removal). Meanwhile, biogas was upgraded with an 82.2% CO₂ reduction. The economic and environmental analysis further demonstrated the ENMCS system as an effective alternative for the bioremediation of AD effluents while simultaneously producing value-added biomass, especially applicable to snowy regions.

Biogas from Anaerobic Digestion as an Energy Vector: Current Upgrading Development

The present work reviews the role of biogas as advanced biofuel in the renewable energy system, summarizing the main raw materials used for biogas production and the most common technologies for biogas upgrading and delving into emerging biological methanation processes. In addition, it provides a description of current European legislative framework and the potential biomethane business models as well as the main biogas production issues to be addressed to fully deploy these upgrading technologies. Biomethane could be competitive due to negative or zero waste feedstock prices, and competitive to fossil fuels in the transport sector and power generation if upgrading technologies become cheaper and environmentally sustainable.

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.

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.

Improving biogas upgrading and liquid chemicals production simultaneously by a membrane biofilm reactor

In this study, a novel membrane biofilm reactor (MBfR) was developed for simultaneously biogas upgrading and liquid chemicals production. With external hydrogen supplied from inside of the gas permeable hollow fiber of the MBfR, CO₂ in biogas could be captured via a biological process as liquid chemicals and simultaneously producing high-purity methane. Continuous operation of MBfR further confirmed that higher solubilized hydrogen was favorably affecting acetate and ethanol titer and rate, and methane purity. Moreover, by retaining biomass on the outer surface of hollow fiber, the highest biogas purity (96.7%) and acetate and ethanol production rates (37.8 and 13.5 mmol L^-1d^-1) were achieved at a hydraulic retention time of 2.0 d. Meanwhile, the CO₂ and hydrogen conversion efficiency reached to the maximum of 93.8% and 98.1%, respectively. The findings obtained can pave a new way for efficient liquid chemical production and biogas upgrading with both economic and environmental benefits.

Autotrophic sulfide removal by mixed culture purple phototrophic bacteria

Current H₂S treatment methods for sour gases require considerable amounts of chemicals and energy, or in case of biological treatment, unwanted diluents such as oxygen or nitrogen may be introduced. In order to reduce those requirements, the viability of an anaerobic biological H₂S removal process using purple phototrophic bacteria (PPB) was investigated in this study. PPB can use sunlight, and centrate as nutrient source, thus potentially reducing energy and chemical requirements. An added benefit is the production of biomass with potential uses, such as single cell protein. An inoculum of PPB enriched from domestic wastewater was grown photoautotrophically with sulfide as the electron donor and inorganic carbon in a mixed culture. Additionally, synthetic medium and centrate as well as high (56 ± 11 Wm^-2) and low (27 ± 3 Wm^-2) IR irradiation were trialled. Finally, a process model was developed to study biomass specific removal rates and yield. The results showed that a mixed culture of PPB removed sulfide completely in synthetic media (121 ± 9 mg-S.L^-1) at a maximum rate of 1.79 ± 0.16 mg-S(Lh)^-1 (low irradiance) and 2.9 mg-S(Lh)^-1 (high irradiance). The pH increased in both experiments from about 8.5 to 9. Sulfide removal rates using centrate and low irradiance were similar. However Fe and Mn were found to be limiting growth and sulfide removal. In all experiments, Chromatiaceae (purple sulfur bacteria) were most abundant at the end of the experiment, while at the start purple non-sulfur bacteria were most abundant (from the inoculum). Process modelling and experimental work identified the sulfide oxidation to be a multi-step process with accumulation of intermediates. Specific rates were directly dependent on light input, doubling at high irradiance. Sulfide oxidation was estimated at 0.100 ± 0.014 h^-1 (0.085 ± 0.012 g-S(g-VS.h)^-1) at low irradiance, and the biomass yield at 0.86 ± 0.05 mg-COD.mg-COD^-1. This process model enables the virtual evaluation of autotrophic sulfide removal by PPB in a continuous scaled-up process. Overall, the photoautotrophic removal of sulfide seems to be a viable option, especially because of the possibility of using sunlight as an energy source and centrate as a nutrient source.

The Effect of Chemical Sulfide Oxidation on the Oxygenic Activity of an Alkaliphilic Microalgae Consortium Deployed for Biogas Upgrading

The oxygenic photosynthetic activity (OPA) of an alkaliphilic microalgae consortium was evaluated at different concentrations of dissolved sulfide under room temperature and well-defined conditions of irradiance and pH in a tubular closed photobioreactor. The kinetic assays showed that it was optimal at a sulfide concentration of 3.2 mg/L under an external photosynthetically active radiation of 50 and 120 μE/m2 s together with a pH of 8.5 and 9.2. In contrast, the oxygenic photosynthetic activity was insignificant at 15 μE/m2 s with a pH of 7.3, both in the absence and presence of sulfide. Consecutive pulse additions of dissolved sulfide evidenced that the accumulation rate of dissolved oxygen was decreased by the spontaneous chemical oxidation of sulfide with dissolved oxygen in alkaline culture media, mainly at high sulfide levels. At 3.2 mg/L of sulfide, the oxygenic photosynthetic activity was improved by around 60% compared to the treatment without sulfide at external irradiances of 120 μE/m2 s, 30 °C, and pH of 8.5 and 9.2. Additionally, an even higher OPA enhancement (around 85%) was observed in the same previous conditions but using 16 mg/L of sulfide. Thiosulfate was the major end-product of sulfide by oxic chemical reaction, both in biotic and abiotic assays with yields of 0.80 and 0.68, respectively.

Biomitigation of CO2 from flue gas by Scenedesmus obtusiusculus AT-UAM using a hybrid photobioreactor coupled to a biomass recovery stage by electro-coagulation-flotation

The microalga Scenedesmus obtusiusculus AT-UAM efficiently captured CO₂ from two flue gas streams in a hybrid photobioreactor located in a greenhouse. Uptake rates of CO₂, NO, and SO₂ from a formulated gas stream were 160.7 mg L^-1 day^-1, 0.73 mg L^-1 day^-1, and 1.56 mg L^-1 day^-1, respectively, with removal efficiencies of 100% for all gases. Exhaust gases of a motor generator were also removed with uptake rates of 111.4 mg L^-1 day^-1, 0.42 mg L^-1 day^-1, and 0.98 mg L^-1 day^-1, obtaining removal efficiencies of 77%, 71%, and 53% for CO₂, NO_x, and SO₂, respectively. On average, 61% of the CO₂ from both flue gas streams was assimilated as microalgal biomass. The maximum CO₂ uptake rate of 182 mg L^-1 day^-1 was achieved for formulated flue gas flow rate above 100 mL min^-1. The biomass recovery of 88% was achieved using a 20-L electro-coagulation-flotation chamber coupled to a settler with a low specific power consumption of 0.27 kWh kg^-1. The photobioreactor was operated for almost 7 months without contamination of invasive species or a decrease in the activity. It is a very encouraging result for long-term operation in flue gas treatment.

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.

Development and cost-benefit analysis of a novel process for biofuel production from microalgae using pre-treated high-strength fresh cheese whey wastewater

In this study, a novel two-step integrated process is proposed to facilitate the microalgae biofuel production as well as fresh cheese whey wastewater (FCWW) treatment simultaneously. The pre- and post-treatment of high-strength FCWW were performed by means of coagulation and algal cultivation, respectively. The pre-treatment of FCWW for maximum removal of chemical oxygen demand (COD), turbidity (TUR) and total solids (TS) as responses was obtained by statistical optimization of coagulation parameters. The maximum removal of COD, TUR and TS at the optimum level of variables was obtained as 68.09%, 47.80% and 73.63%, respectively. The pre-treated FCWW was further treated by Chlorella pyrenoidosa and observed a significant reduction in the above-mentioned responses (87-94%). The maximum algal biomass yield and lipid productivity were observed as 2.44 g L^-1 and 77.41 mg L^-1 day^-1, respectively. Based on promising results of FCWW treatment and its use as a third-generation biodiesel feedstock, a cost-benefit analysis of the developed process was assessed for microalgal oil production. The total profit earned by the integrated process model was $9.59 million year^-1. Accordingly, the estimated production cost of algal oil (TAG) from the developed system was estimated to be $79.03 per barrel.

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.

A perspective on novel cascading algal biomethane biorefinery systems

Synergistic opportunities to combine biomethane production via anaerobic digestion whilst cultivating microalgae have been previously suggested in literature. While biomethane is a promising and flexible renewable energy vector, microalgae are increasingly gaining importance as an alternate source of food and/or feed, chemicals and energy for advanced biofuels. However, simultaneously achieving, grid quality biomethane, effective microalgal digestate treatment, high microalgae growth rate, and the most sustainable use of the algal biomass is a major challenge. In this regard, the present paper proposes multiple configurations of an innovative Cascading Algal Biomethane-Biorefinery System (CABBS) using a novel two-step bubble column-photobioreactor photosynthetic biogas upgrading technology. To overcome the limitations in choice of microalgae for optimal system operation, a microalgae composition based biorefinery decision tree has also been conceptualised to maximise profitability. Techno-economic, environmental and practical aspects have been discussed to provide a comprehensive perspective of the proposed systems.

Simultaneous biological nitrification and desulfurization treatment of ammonium and sulfide-rich wastewater: Effectiveness of a sequential batch operation

Sulfide inhibition to nitrifying bacteria has prevented the integration of digestate nitrification and biogas desulfurization to simplify anaerobic digestion systems. In this study, liquid digestate with NaHS solution was treated using nitrifying sludge in a sequential-batch reactor with a long fill period, with an ammonium loading rate of 293 mg-N L^-1 d^-1 and a stepwise increase in the sulfide loading rate from 0 to 32, 64, 128, and 256 mg-S L^-1 d^-1. Batch bioassays and microbial community analysis were also conducted with reactor sludge under each sulfide loading rate to quantify the microbial acclimatization to sulfide. In the reactor, sulfide was completely removed. Complete nitrification was maintained up to a sulfide load of 128 mg-S L^-1 d^-1, which is higher than that in previous reports and sufficient for biogas treatment. In the batch bioassays, the sulfide tolerance of NH₄⁺ oxidizing activity (the 50% inhibitory sulfide concentration) increased fourfold over time with the compositional shift of nitrifying bacteria to Nitrosomonas nitrosa and Nitrobacter spp. However, the sulfur removal rate of the sludge slightly decreased, although the abundance of the sulfur-oxidizing bacteria Hyphomicrobium increased by 30%. Therefore, nitrifying sludge was probably acclimatized to sulfide not by the increasing sulfide removal rate but rather by the increasing nitrifying bacteria, which have high sulfide tolerance. Successful simultaneous nitrification and desulfurization were achieved using a sequential-batch reactor with a long fill period, which was effective in facilitating the present acclimatization.

Pretreatment and co-digestion of microalgae, sludge and fat oil and grease (FOG) from microalgae-based wastewater treatment plants

The aim of this study was to assess the co-digestion of residual biomass flows generated in microalgae-based wastewater treatment plants: microalgae, primary sludge and fat, oil and grease (FOG), with and without microalgae thermal pretreatment. The results evidenced the high methane yield of FOG (563 mL CH₄/g VS) as compared to microalgae (140 mL CH₄/gVS) and sludge (299 mL CH₄/g VS). The methane yield of microalgae and sludge co-digestion (50-50% VS) was increased by 25 and 42% by adding 10 and 20% VS of FOG, respectively. Moreover, co-digestion trials improved the anaerobic digestion first-order kinetics by up to 67%. Regarding the thermal pretreatment, it increased the methane yield of microalgae by 60%, and 15% upon co-digestion with sludge and FOG. Therefore, co-digestion of microalgae, primary sludge and FOG appears as a promising strategy to enhance the biogas production, hence bioenergy recovery from wastewater, even without pretreatment.

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.

Saline wastewater treatment with purple phototrophic bacteria

Biological removal of organics, nitrogen and from saline wastewaters is adversely impacted by high salinity, which can be a major concern for treatment of industrial or domestic saline wastewater. In anaerobic treatment systems, sulfidogensis, especially when treating sulfate-rich saline wastewaters (e.g. seawater has 930 mgSO₄-S L^-1, or 2800 mg L^-1 as SO₄^2-) can cause additional biological, operational, and safety issues, due to H₂S toxicity. Here, the use of anaerobic purple phototrophic bacteria (PPB) is tested as mediator to treat high salinity domestic wastewater (NaCl), and marine wastewater (Red Sea Salt - high sulfate, potassium, etc.) in a continuous anaerobic infra-red photo bioreactor, operated over 372d. Saline adapted PPB simultaneously removed COD, nitrogen and phosphorus with biomass yields of 0.8 gCOD gCOD^-1. Batch activity tests found a broad optimum peak for saline adapted PPB between 30 and 70 mS cm^-1, and 50% reduced activity at 140 mS cm^-1 (3.5x seawater). For marine wastewater, high sulfate influent concentrations (770 mgSO₄-S L^-1) did not result in substantial H₂S production (<1.6 mgS L^-1) over 80 d. When irradiation was removed, sulfide rapidly rose to >90 mgS L^-1 and the process failed. The results indicate rapid adaptation to high-salt conditions (both NaCl and marine), and the capacity for PPB to form a combined wastewater treatment/resource recovery process, particularly for salty industrial wastewater.

Cultivation of Chlorella pyrenoidosa in anaerobic wastewater: The coupled effects of ammonium, temperature and pH conditions on lipids compositions

Anaerobic wastewater potentially was an ideal medium for cultivating microalgae. The coupled effect of ammonium, temperature and pH on lipids accumulation was a core issue during algal culture using anaerobic wastewater. Therefore, their combined effects on Chlorella pyrenoidosa culture and lipids accumulation in anaerobic effluent were investigated. Free ammonia induced from the rising pH and temperature inhibited algal growth, but significantly promoted lipid accumulation. The highest lipids content reached 30.2% when pH rose to 8.3-8.5 (25 °C, ammonium 280 mg/L), which was 1.6-fold higher than that under neutral condition. Moreover, the percentage of unsaturated fatty acids (un-SFAs) increased to 74.8-77.9% at pH 8.3-8.5, whereas it was only 56.1-58.9% under neutral condition. The C18:2 and C18:3 dominated the un-SFAs increase at high pH, typically the percentage of C18:3 increased by 74.5-153.1%. This study provides a potential way for lipid accumulation in algal culture using anaerobic wastewater.

Photorespiration in an outdoor alkaline open-photobioreactor used for biogas upgrading

The rates of oxygenic and carbon fixation photosynthetic processes of a microalgae consortium were simultaneously evaluated under steady-state performance in an bench scale alkaline open-system exposed to outdoor conditions in Mexico City. A synthetic methane-free gaseous stream (SMGS) similar to biogas was used as inorganic carbon source and model of biogas upgrading. The microalgae CO₂ fixation rates were calculated through a novel methodology based on an inorganic carbon mass balance under continuous scrubbing of a SMGS similar to biogas, where the influence of pH and temperature time-depended oscillations were successfully incorporated into the mass balances. The oxygenic activity and carbon fixation occurred at different non-stoichiometric rates during the diurnal phase, in average carbon fixation predominated over oxygen production (photosynthesis quotient PQ≈ 0.5 mol O₂ mol^-1 CO₂) indicating photorespiration occurrence mainly under dissolved oxygen concentrations higher than 10 mg L^-1. The oxygen and inorganic carbon mass balances demonstrated that photorespiration and endogenous respiration were responsible for losing up to 66% and 7% respectively of the biomass grew at diurnal periods under optimal conditions. In favoring photorespiration conditions, the microalgae biomass productivity (CO₂ effectively captured) can be severely decreased. A kinetic mathematical model as a function of temperature and irradiance of the oxygenic photosynthetic activity indicated the optimal operation zone for this outdoor alkaline open-photobioreactor, where irradiance was found being the most influential parameter.

The Potential of Biogas; the Solution to Energy Storage

Energy storage will be essential for balancing the renewable energy systems of tomorrow, especially if excess electricity from wind and solar power requires immediate utilization. The use of biogas as a carbon source can generate carbon dioxide-neutral carbon-based energy carriers, such as methane or methanol. The utilization of biogas today is limited to the generation of heat/power or biomethane (first-generation upgrading); both processes disregard the potential of the coproduced carbon dioxide during the fermentation process. By using renewable energy, biogas upgrading systems can convert carbon dioxide into hydrocarbon-based high-energy-density fuels, which can replace fossil-based fuels for applications in which they are hard to decarbonize. The possibilities for the future utilization of biogas are discussed, and the terminology for "second-generation upgrading" is introduced to help research and development within this field. It is believed that second-generation upgrading of biogas will have a huge potential for dynamic energy storage.

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.

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.