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There is increasing interest in new protein sources for the food and feed industry and for the agricultural sector, and microalgae are considered a good alternative, having a high protein content and a well-balanced amino acid profile. However, protein production from microalgae presents several unsolved issues, as the biomass composition changes markedly as a function of cultivation operating conditions. Continuous systems, however, may be properly set to boost the accumulation of protein in the biomass, ensuring stable production. Here, two microalgae and two cyanobacterial species were cultivated in continuous operating photobioreactors (PBR) under nonlimiting nutrient conditions, to study the effects of light intensity and residence time on both biomass and protein productivity at steady state. Although light strongly affected biomass growth inside the PBR, the overall protein pool did not vary in response to irradiance. On the other hand, shorter residence times resulted in protein accumulation of up to 68 % in cyanobacteria, in contrast with green algae, where a minor influence of residence time on biomass composition was observed. Energy balance showed that light conversion to protein decreased with light intensity. Protein content was also related to energy costs for cell maintenance. In conclusion, it is shown that residence time is the key variable to increase protein content and yield of protein production, but its effect depends on the specific species., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Modeling microalgal mixotrophy is challenging, as the regulation of algal metabolism is affected by many environmental factors. A reliable tool to simulate microalgal behavior in complex systems, such as wastewaters, may help in setting the proper values of operative variables, provided that model parameters have been properly evaluated. In this work, a new respirometric protocol is proposed to quickly obtain the half-saturation constant values for several nutrients. The protocol was first verified for autotrophic exploitation of ammonium and phosphorus (Monod kinetics), as well as of light intensity (Haldane model), further elaborated on specific light supply basis. It was then applied to measure the kinetic parameters of heterotrophic growth. The half-saturation constants for nitrogen and phosphorus resulted comparable with autotrophic ones. The dependence on acetate and dissolved oxygen concentration was assessed. Mixotrophy was modeled as the combination of autotrophic/heterotrophic reactions, implemented in AQUASIM, and validated on batch curves with/without bubbling, under nutrient limitation, and different light intensities. It was shown that the reliability of the proposed respirometric protocol is useful to measure kinetic parameters for nutrients, and therefore to perform bioprocess simulation.

Microalgal-bacteria consortia application on wastewater treatment has been widely studied, but a deeper comprehension of consortium interactions is still lacking. In particular, mixotrophic exploitation of organic compounds by microalgae affects gas (CO 2 and O 2 ) exchange between microalgae and bacteria, but it is not clear how environmental conditions may regulate algal metabolism. Using a respirometric-based protocol, we evaluated the combined effect of organic carbon and light intensity on oxygen production and consumption by C. protothecoides, and found that the chemical oxygen demand (COD) was not consumed when incident light increased. Batch experiments under different incident lights, with C. protothecoides alone and in consortium with activated sludge bacteria, confirmed the results obtained by respirometry. Continuous system experiments testing the combined effects of light intensity and residence time confirmed that, under limiting light, mixotrophy is preferred by C. protothecoides, and the nutrient (COD, N, P) removal capability of the consortium is enhanced.

Microalgae-bacteria consortia application to wastewater treatment is considered as a potential and cheap strategy towards a self-sustaining oxygen-carbon dioxide gas exchange. However, microalgae can also carry out mixotrophy, thus reducing the net oxygen production, due to consumption of organic substrates. In this work, respirometric tests were used to quantify the oxygen reduction in the presence of biodegradable COD (chemical oxygen demand), which resulted up to 70%, depending on the biodegradability of the carbon substrate. The implication of mixotrophic metabolism on nutrient removal in urban wastewater was also measured by co-cultivating C. protothecoides with bacteria from activated sludge. To better understand the contribution of different populations, ad hoc experiments under controlled conditions were designed to quantify the nutrient consumption of bacteria and microalgae. Microalgae and bacteria were cultivated together and separately, with and without external bubbling, so to better ascertain the specific role of gas production and nutrient removal. Results showed that microalgae can remove up to 100 and 85% of P and N respectively, but the contribution on COD consumption may affect the net O 2 supply to heterotrophic bacteria. However, a mutual COD consumption by microalgae and bacteria was proved by both experimental growth curves and mass balance application, based on stoichiometry experimentally adjusted.

Renewable fuels and energy are of major concern worldwide and new raw materials and processes for its generation are being investigated. Among these raw materials, algae are a promising source of lipids and energy. Thus, in this work four different algae have been used for lipid extraction and biogas generation. Lipids were obtained by supercritical CO2 extraction (SCCO2), while anaerobic digestion of the lipid-exhausted algae biomass was used for biogas production. The extracted oil composition was analyzed (saturated, monounsaturated and polyunsaturated fatty acids) and quantified. The highest lipid yields were obtained from Tetraselmis sp. (11%) and Scenedesmus almeriensis (10%), while the highest methane production from the lipid-exhausted algae biomass corresponded to Tetraselmis sp. (236mLCH4/gVSadded)., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

One of the main parameters influencing microalgae production is light, which provides energy to support metabolism but, if present in excess, can lead to oxidative stress and growth inhibition. In this work, the influence of illumination on Scenedesmus obliquus growth was assessed by cultivating cells at different light intensities in a flat plate photobioreactor. S. obliquus showed a maximum growth rate at 150 μmol photons m(-2) s(-1). Below this value, light was limiting for growth, while with more intense illumination photosaturation effects were observed, although cells still showed the ability to duplicate. Looking at the biochemical composition, light affected the pigment contents only while carbohydrate, lipid, and protein contents remained stable. By considering that in industrial photobioreactors microalgae cells are subjected to light-dark cycles due to mixing, algae were also grown under pulsed illumination (5, 10, and 15 Hz). Interestingly, the ability to exploit pulsed light with good efficiency required a pre-acclimation to the same conditions, suggesting the presence of a biological response to these conditions.

In this study, the feasibility of integrating microalgae cultivation in a biogas production process that treats the organic fraction of municipal solid waste (OFMSW) was investigated. In particular, the biomass growth performances in the liquid fraction of the digestate, characterized by high ammonia concentrations and turbidity, were assessed together with the nutrient removal efficiency. Preliminary laboratory-scale experiments were first carried out in photobioreactors operating in a continuous mode (Continuous-flow Stirred-Tank Reactor, CSTR), to gain preliminary data aimed at aiding the subsequent scaling up to a pilot scale facility. An outdoor experimental campaign, operated from July to October 2019, was then performed in a pilot scale raceway pond (4.5 m2), located in Arzignano (VI), Italy, to assess the performances under real environmental conditions. The results show that microalgae could grow well in this complex substrate, although dilution was necessary to enhance light penetration in the culture. In outdoor conditions, nitrification by autotrophic bacteria appeared to be significant, while the photosynthetic nitrogen removal was around 12% with respect to the inlet. On the other hand, phosphorus was almost completely removed from the medium under all the conditions tested, and a biomass production between 2–7 g m−2 d−1 was obtained.

In this study, the feasibility of integrating microalgae cultivation in a biogas production process that treats the organic fraction of municipal solid waste (OFMSW) was investigated. In particular, the biomass growth performances in the liquid fraction of the digestate, characterized by high ammonia concentrations and turbidity, were assessed together with the nutrient removal efficiency. Preliminary laboratory-scale experiments were first carried out in photobioreactors operating in a continuous mode (Continuous-flow Stirred-Tank Reactor, CSTR), to gain preliminary data aimed at aiding the subsequent scaling up to a pilot scale facility. An outdoor experimental campaign, operated from July to October 2019, was then performed in a pilot scale raceway pond (4.5 m²), located in Arzignano (VI), Italy, to assess the performances under real environmental conditions. The results show that microalgae could grow well in this complex substrate, although dilution was necessary to enhance light penetration in the culture. In outdoor conditions, nitrification by autotrophic bacteria appeared to be significant, while the photosynthetic nitrogen removal was around 12% with respect to the inlet. On the other hand, phosphorus was almost completely removed from the medium under all the conditions tested, and a biomass production between 2-7 g m-2 d-1 was obtained. [ABSTRACT FROM AUTHOR]

Microalgal biomass has been considered as a possible alternative source of carbohydrates and lipids in fermentative/reactional processes, called third generation of biofuels. Carbohydrates from microalgae are mostly composed by glucose and some pentose-derived polymers that must be hydrolyzed to be efficiently used. When enzymatic hydrolysis is applied a pretreatment is required. Sonication/ultrasonication is one of the most promising methods, and in this paper the influence of pretreatment time, sonication intensity and biomass concentration was validated, and the energy consumed in the process compared as well. Sonication intensity had the major role on the enzymatic accessibility. Pretreatment time can be used to decrease hydrolysis time. More than 90% of hydrolysis efficiency was reached when higher amplitude (sonication intensity) and pretreatment time were used. The applied energy influenced indirectly the hydrolysis process. The best saccharification/energy relation was reached when 50% of amplitude for 25 min was applied, obtaining 91% of hydrolysis yield and spending 2.4 MJ/kg of dry biomass. [ABSTRACT FROM AUTHOR]

The supply of nutrients is a great issue to a sustainable scale-up of microalgal biofuels production, as these photosynthetic microorganisms require large amounts of N, P and other micronutrients to grow, which turns into high fertilizers demand. Additionally, recovery and reuse of nutrients (particularly N & P) are a must to reduce the non-point pollution emanating from their release into water or air during the downstream processing steps to biofuels or bioproducts. In the recent years, strong research efforts have been paid for developing nutrient recovery and recycling techniques, in order to reduce the net amount of fertilizers required. One possibility is exploiting nutrients from waste streams, such as wastewaters, while others focus on the recovery of N and P from the non-fuel fraction of the produced microalgal biomass, which is then recycled to the cultivation system, in a closed-loop perspective. In both cases, the presence of possible contaminants as well as nutrients bioavailability can impact the biomass productivity compared to standard synthetic media. Although the nutrients recovery and reuse has been in the forefront for a few years, there are no review publications available yet. In this paper, state-of-the art studies on nutrients recovery and recycling methods in microalgae processing from the last decade are reviewed. The study focuses on the different N and P recovery methods and yields, as well as on their subsequent use in algal cultivation and impact on algae productivity. Possible bioproducts exploitation is considered, and perspectives of closed-loop material balances on a large-scale are eventually provided. [ABSTRACT FROM AUTHOR]

In this study, acidic and enzymatic hydrolysis were carried out with Chlorella vulgaris and Scenedesmus obliquus , and a systematic evaluation of the yeast inoculum optimization ( Saccharomyces cerevisiae and Pichia stipitis ) during the hydrolysates fermentation was made. Acidic hydrolysis with 3% of sulphuric acid at 121 °C and 30 min of reaction time (50–100 g/L of biomass) was found as best condition, and more than 90% of sugars recovery in the liquid phase was achieved even though S. obliquus be more sensible to thermal degradation than C. vulgaris . On the other hand, during the enzymatic hydrolysis, ultrasonication showed to be an effective method of biomass pretreatment to provide higher enzyme accessibility what together with a combination between amylases, cellulase/hemicellulase mix and pectinases achieved more than 90% of sugars recovery in 8 h of hydrolysis time. Thus, both treatments could recover efficiently the sugars present in microalgae. After inoculum optimization (concentration and consortium between S. cerevisiae and P. stipitis ) and saline influence on yeast performance were evaluated, the fermentation of microalgal hydrolysate exhibited very different profiles in comparison with the control conditions and ethanol biochemical yield changed between 45% and 61%, emphasizing the importance to optimize this last but not less important step of ethanol production. [ABSTRACT FROM AUTHOR]

The cost-effective production of liquid biofuels from microalgae is limited by several factors such as recovery of the lipid fractions as well as nutrients management. Flash hydrolysis, a rapid hydrothermal process, has been successfully applied to fractionate the microalgal biomass into solid biofuels intermediates while recovering a large amount of the nutrients in the aqueous phase (hydrolyzate) in a continuous flow reactor. The aim of the work is to enhance the quality of a high-ash containing marine algae Nannochloropsis gaditana as biofuel feedstock while recycling nutrients directly for algae cultivation. Characterization of products demonstrated an increase in extractable lipids from 33.5 to 65.5 wt % (dry basis) while retaining the same fatty acid methyl ester profile, in addition to diminution of more than 70 wt % of ash compared to raw microalgae. Moreover, the hydrolyzate was directly used to grow a microalga of the same genus. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1494-1502, 2017 [ABSTRACT FROM AUTHOR]

• Aspen Plus simulation of small-scale anaerobic digestion with microalgae cultivation. • Optimal residence time for microalgae production found for different environmental conditions. • Microalgal growth is limited by the turbidity of the digestate. • The integrated process is economically profitable (profitability index >1) • Selling microalgae as biofertilizers is less profitable. The deployment of small-scale anaerobic digestion (AD) plants for the treatment of the organic fraction of municipal solid waste (OFMSW) and associated production of renewable energy at a local level has recently attracted attention as opposed to full-scale centralized plants. In this work, we evaluate the technical and economic feasibility of an integrated process comprising small-scale AD of OFMSW followed by microalgae cultivation, to valorise the waste streams of the former (off-gases produced by combined heat and power generation and liquid digestate) for the production of valuable biomass to be destined to either the biofertilizers or the biostimulants markets, in agreement with a circular economy perspective. The material and energy balances were obtained by means of process simulation for a plant located in Northern Italy, taken as a reference. The analysis revealed that, although the high turbidity of the digestate strongly hinders biomass production, the process is profitable from a techno-economic perspective, with discounted payback time ranging from 3.3 to 7.1 years, and profitability indexes greater than 1. The low market price of biofertilizers makes it the less profitable choice, even compared to the AD process alone, while selling the biomass for biostimulants applications could strongly increase the process profitability. [ABSTRACT FROM AUTHOR]

Abstract The development of new technologies which increase the production of biofuel without directly compete with food production is required. Microalgal biomass has recently been in the highlight. The role of this biomass is here discussed within the concept of biorefinery and industrial sustainability of bioethanol production. The process of cultivation in order to accumulate around 50% of carbohydrates in the biomass (dry weight) and the importance of water and nutrient recycling are reviewed. Saccharification of biomass using enzymes or acids and alternative processes such as hydrothermal liquefaction and flash hydrolysis are addressed. Since the main monosaccharide in microalgal biomass is glucose, high rates of hydrolysis and fermentation were, generally, achieved (more than 80% of the efficiency as a sum of these two processes). Anaerobic digestion to treat vinasse and the recycling of CO2 from the ethanolic fermentation and biogas could increase the process sustainability. Alternative techniques for the concentration of bioethanol from fermentation broth and for the optimization of fuel transportation are mentioned. Finally, the advantage of using microalgae rather than other sources is estimated with reference to the production rate, even though the cultivation costs are still high.

The increasing global demand for energy and advances in new biofuel production routes have increased the research on the potential of microalgae and cyanobacteria as a third generation of biofuels. The majority of research has been focused on using this type of biomass for producing biodiesel and biogas; however, more recent developments have indicated the potential of microalgae and cyanobacteria for the production of bioethanol. There are three routes for producing bioethanol from such microorganisms: the traditional one involving hydrolysis and fermentation of biomass with bacteria or yeast, the dark fermentation route, and the use of engineered cyanobacteria or “photofermentation.” In recent years, the use of engineered cyanobacteria to directly produce ethanol has gained enormous attention, mainly after the successful use of these microorganisms in industrial plants. However, only little information is available on the efficiency and the real advantages and disadvantages of these processes, and particularly, a comparison between traditional processes and engineered cyanobacteria. This study compiles the main publications on the production of bioethanol from microalgae and cyanobacteria, and summarizes the main features, advantages, and key aspects for each type of process. The industrial implementation of these technologies depends on the capability of reducing production costs through more scientific research and technical development, to become competitive with the lower cost of fossil fuels also thanks to public subsidies. [ABSTRACT FROM AUTHOR]

In this work, Nannochloropsis salina was cultivated in a continuous-flow flat-plate photobioreactor, working at different residence times and irradiations to study the effect of the specific light supply rate on biomass productivity and photosynthetic efficiency. Changes in residence times lead to different steady-state cell concentrations and specific growth rates. We observed that cultures at steady concentration were exposed to different values of light intensity per cell. This specific light supply rate was shown to affect the photosynthetic status of the cells, monitored by fluorescence measurements. High specific light supply rate can lead to saturation and photoinhibition phenomena if the biomass concentration is not optimized for the selected operating conditions. Energy balances were applied to quantify the biomass growth yield and maintenance requirements in N. salina cells. [ABSTRACT FROM AUTHOR]

This paper reports new experimental data of microalgae growth and lipid production under autotrophic conditions for the species Nannochloropsis salina. The effect of relevant operating variables is addressed and discussed, and some suggestions to better understand the process behavior are given with respect to lipid content maximization, carbon dioxide and nitrogen supply, and illumination conditions. The data obtained are finalized to the design of an environmentally and economically sustainable photobioreactor in view of achieving industrial photosynthetic biomass and natural oil production from large scale microalgae cultivation. [ABSTRACT FROM AUTHOR]

A technical evaluation of a plant for biodiesel production from microalgae was investigated in which a novel configuration of a CPR (closed pond photobioreactor) is proposed. The entire process was simulated by Aspen Plus ® and optimized energetically in order to obtain the best profits in energy terms. The design and sizing of the process equipment was performed to obtain a realistic estimation of costs, considering both CAPEX (capital costs) and OPEX (operating costs). The economic analysis evaluated the profitability of the complete process at industrial scale referred to a CPR of 1 km 2 of surface area as the base for calculation. In order to ensure an acceptable economic profitability a sale price of oil of $ 18.35/gal was obtained, corresponding to $ 21.11/gal of biodiesel. These results were compared to those of other recent studies, confirming that at the current state of technology the production of biodiesel from microalgae is not competitive with respect to that of conventional diesel, and a breakthrough in technology is needed to bridge this gap maybe by means of other valuable co-products from the same process. A future outlook is eventually reported. [ABSTRACT FROM AUTHOR]

Supercritical CO2 extraction from microalgae is applied with the aim of obtaining an oil rich in α-linolenic (ALA) essential fatty acid and with a low ω6:ω3 ratio. The maximum extraction yield is obtained at 60 °C and 30 MPa with 0.4 kg/h of CO2 and 5% of co-solvent (ethanol). When the effect of pressure, temperature and density on the supercritical extraction yield and solubility are studied, the thermodynamic cross-over is found at a pressure close to 30 MPa, while the extraction cross-over occurs at around 25 MPa. The experimental solubility data are correlated by literature empirical models. Mathematical models developed by Sovová are applied to describe the experimental extraction curves. Soxhlet extraction of lipids is also carried out, obtaining a similar fatty acids profile but proving to be less selective than SCCO2 method. Among the three species of microalgae examined, results show that Scenedesmus obliquus oil is richer in ω-3 fatty acids and ALA than Chlorella protothecoides and Nannochloropsis salina lipids. The effect of the extraction parameters on ALA content and the fatty acid profile is also analysed, concluding that the ω-3 percentage is favoured by lower temperatures, lower pressures and shorter extraction times. [ABSTRACT FROM AUTHOR]

The industrial production of microalgae in continuous photobioreactors (PBRs) was investigated using an integrated experimental-modeling approach. A lab-scale PBR was assembled for microalgae cultivation, in the shape of thin-layer flat-vertical panels of 250 mL, in order to minimize the effect of cell self-shading and optimize the light use. Two microalgal species, one from marine and the other from freshwater environments, were cultivated in continuous mode in this PBR. They were grown under non-limiting CO2 supply and with excess nitrogen and micronutrients availability. Steady-state operating conditions were achieved and the effect of partial recycle on biomass concentration and productivity was investigated. Light energy was supplied at a constant rate, and the net flux of photons absorbed by the culture was measured and correlated to the average internal biomass concentration. The reactor performances were evaluated in terms of photosynthetic efficiency, and the possibility of increasing the productivity of the system was discussed. The light attenuation along the culture depth was modeled to fit the light absorption and backscattering coefficients. The growth model proposed by Pruvost et al. (2011b) was applied to reproduce the experimental behavior of the lab-scale PBR and to simulate the performances of a pilot unit both without and with recycle, i.e. as a function of the axial mixing, under both transient and steady-state conditions. The optimized model parameters were used to predict the microalgal growth in a pilot-scale closed raceway-pond PBR. [ABSTRACT FROM AUTHOR]

Abstract: This work presents the results of a thorough EROEI (Energy Return on Energy Investment) analysis for biocrude production from microalgae processes. We have investigated different alternatives to maximize the energy recoveries of each process considered, which has been modeled and simulated by Aspen Plus™. The estimates for feeds and recirculation of microalgae and nutrients, working conditions, systemization and equipment requirements were obtained using both literature and own experimental data. The Pinch Technology Analysis was applied to optimize the process operating conditions. It was found that the process which uses the combustion of biomass after biocrude extraction is the most favorable one in energy terms. In particular, 2 cases of this type were addressed and compared to a Base case where all energy requirements (heat and electrical) are provided from external sources: Case 1, electricity is supplied externally whereas the biomass and part of the biocrude produced are burned to meet the requirements of thermal energy, and Case 2, where the energy recovered from both biomass and part of biocrude are used to fulfill all heat and electricity duties. These cases were analyzed also when the nitrogen needed for microalgae growth is obtained from wastewaters. Favorable EROEI figures could be calculated. [Copyright &y& Elsevier]

Nannochloropsis is commonly recognized as a valuable biomass source for the production of both biofuels and high value nutraceuticals compounds. Phosphorus is one of the main nutrients affecting microalgal growth and composition. In this work, the effect of using higher P concentrations in cultivation of Nannochloropsis salina was studied showing an increase of the phosphate uptake rate related with the higher P availability in the medium. This enhanced uptake was preferentially stored in the cells rather than being used for growth, indicating the activation of a luxury uptake phenomenon. The intracellular P storage in N. salina was measured and the presence of polyphosphate granules was detected. The kinetic of accumulation and consumption of internal phosphorus was quantified in batch cultures, as a function of P concentration and light intensity. Finally, the effect of P, also in combination with different light supply rates was analyzed in continuous steady-state experiments, showing that operating in this mode strongly influences not only the biomass productivity, but also the nutrient uptake. [ABSTRACT FROM AUTHOR]

Nutrient recycling is of paramount importance for microalgal cultivation aimed at biofuels production. Though different techniques have been assessed at lab-scale level, less information is available on the actual nutrient recovery yields and on the energy balance of a large-scale closed-loop process. In this work two technologies, namely anaerobic digestion (AD) and flash hydrolysis (FH), were investigated by process simulation analysis and compared in terms of both nutrients recovery and energy profitability (EROEI index). FH performs better in terms of nutrients recycling, with up to 70% and 60% of phosphorus and nitrogen recovered in bio-available forms, while incomplete biodegradability limits the nutrients recovery extent in AD. On the other hand, AD requires much lower energy inputs, achieving EROEI values always favorable (2.3–5), while the high thermal requirements of FH, even with heat integration, highlight the need of improving the operating conditions to reduce the energetic burden of this technology (EROEI = 0.8–2.4). [ABSTRACT FROM AUTHOR]

Microalgal industrial production requires high surface area, resulting in production costs currently unacceptable. A possible optimization of land use for the cultivation of Scenedesmus obliquus is proposed, by conjoint production of biomass and electricity using photovoltaic cells (PV). For this purpose, biomass cultivation in a continuously operated 1 ha open pond placed inside a greenhouse was considered, at two different Italian latitudes, as case studies. The greenhouse roof surface was partially covered with commercial PV modules, resulting in a reduced average irradiation. The light profiles and the average temperatures inside the greenhouse were simulated for different seasons, and the corresponding microalgal productivities were calculated based on a validated growth model. The partial pond shading limited photoinhibition in summer at Southern location, resulting in higher productivities. On the other hand, the loss of sunlight, due to the partial roof covering, resulted in a lower productivity in the other cases and for the Northern location. The presence of PV, however, allowed a better exploitation of light to produce electricity, which supports the energy duties of the process, with an additional net electricity production. Finally, an economic analysis was carried out showing a reduction of biomass production costs when PV is present. [ABSTRACT FROM AUTHOR]

Outdoor industrial-scale microalgae cultivation is limited by several factors, among which a low efficiency of overall light energy conversion plays a key role, mainly due to photosaturation and photoinhibition under high irradiations. This work aims at improving the overall photoconversion efficiency in a microalgal production photobioreactor (PBRs), by exploiting an advanced photovoltaic (PV) technology. A semi-transparent dye sensitized solar cells module (DSC) is placed on the irradiated surface, thus absorbing part of the incident light to produce electricity, while transmitted photons are used by algal cells for photosynthesis. Experiments are carried out in a continuous laboratory scale flat-panel PBR, at different constant light intensities and under a day-night irradiation regime, to ascertain the performances of this combined PV-PBR system in terms of biomass productivity and overall photoconversion efficiency, compared to traditional transparent PBRs. The results obtained show that the configuration proposed, combining biomass production with innovative photovoltaics technology, could be a valuable way to improve light energy utilization and efficiency in microalgal production. [ABSTRACT FROM AUTHOR]

Nutrient recycling is essential to make microalgae cultivation sustainable at industrial scale. To this aim, lab-scale anaerobic digestion experiments of Chlorella vulgaris after lipid extraction were carried out to evaluate biogas yield and nutrients recovery in the liquid digestate. Then, this liquid fraction was used as source of nutrients for cultivation of a closely related C. vulgaris species, in order to assess the possibility of re-cultivating microalgae towards a closed-loop nutrient recycling process. The biological methane potential tests resulted in biogas production of about 347 NmL gVS −1 with 43% v/v methane content. In re-growth experiments, the liquid digestate showed an insufficient amount of soluble sulfate and phosphorus. However, by amending these two nutrients, the specific growth rate and final biomass concentration increased to about 2 d −1 and 2 g L −1 , respectively, which were comparable to those obtained in a defined control medium. The low content of soluble phosphorus in liquid digestate was mainly due to precipitation and removal with the solid phase. Several techniques were hence tested to enhance phosphorus solubilisation, and highest recoveries of up to 41% were obtained when using NaHCO 3 . Finally, C. vulgaris was grown in such treated digestate, obtaining a final biomass production comparable to that of the control, without the need of external phosphorus supply. [ABSTRACT FROM AUTHOR]

The production of biofuels from microalgae is associated with high demands of nutrients (nitrogen and phosphorus) required for growth. Recycling nutrients from the residual biomass is essential to obtain a sustainable production. In this work, the aqueous phase obtained from flash hydrolysis of Scenedesmus sp . was used as cultivation medium for a microalga of the same genus, to assess the feasibility of this technique for nutrient recycling purposes. Batch and continuous cultivations were carried out, to determine growth performances in this substrate compared to standard media, and verify if a stable biomass production could be obtained. In continuous experiments, the effect of hydrolysate inlet concentration and of residence time were assessed to optimize nutrient supply in relation to productivity. Results obtained show that nutrient recycling is feasible by treating biomass with flash hydrolysis, and Scenedesmus is capable of recycling large amounts of recovered nutrients. [ABSTRACT FROM AUTHOR]

The performance of two 75-L high rate algal ponds (HRAPs) treating slaughterhouse wastewater was evaluated for 115 days with the aim of growing microalgae to produce biofuels (biodiesel, methane). One HRAP was placed indoors under controlled conditions of temperature (25 ± 2 °C) and light supply, while the other was placed in a greenhouse under 20 ± 6 °C and 9-fold higher light supply. The hydraulic retention time (HRT) was decreased from 15 to 10 days. High removal efficiencies were achieved in HRAP placed indoors (92% and 71%) and placed in the greenhouse (86% and 91%) for total chemical oxygen demand and soluble phosphorous, respectively. The maximum biomass production obtained was 12.7 g volatile suspended solids (VSS)/m 2 day. High quality fatty acids (FFA) were extracted by supercritical carbon dioxide, obtaining 142 mg FFA/g biomass. The highest CH 4 productions were obtained from lipid exhausted biomass corresponding to greenhouse HRAP at a HRT of 10 days (195 mL CH 4 /g VSS added ). [ABSTRACT FROM AUTHOR]