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Primary urban wastewater was treated in outdoor raceways using microalgae-bacteria consortia dominated by Scenedesmus almeriensis. The current study aimed at assessing the effect of operational conditions, namely, culture depth and dilution rate, on the following: (i) biomass productivity, (ii) the nutrient removal capacity and (iii) the composition of the microalgae-bacteria consortium and the presence of unwanted microorganisms. Optimum dilution rates to process large quantities of wastewater during summer and achieve high biomass productivities were 0.3–0.5 day−1. Under the optimum operational conditions, nitrogen and phosphorus removal rates were higher than 90% while removal of chemical oxygen demand was 70%. Operating at different culture depths had a striking effect on the composition of the microalgae-bacteria consortium. The relative abundance of nitrifiers increased with culture depth and was minimised at 0.05 m: larger culture depths led to enhanced nitrifying activity and therefore to nitrate production and accumulation in the system. Results demonstrate the potential of microalgae-based wastewater treatment processes and the importance of selecting suitable operational conditions to maximise both, biomass production and nutrient removal by minimising the occurrence of nitrifying bacteria. [ABSTRACT FROM AUTHOR]

The depth of the culture and the dilution rate have a striking effect on the biomass productivity and the nutrient recovery capacity of microalgal cultures. The combination of culture depth and dilution rate that allows to maximise the performance of the system depends on environmental conditions. In the current study, a response surface methodology was used to explore the relationship between the two most relevant operational conditions and the biomass productivity achieved in 8.3 m2 pilot-scale raceways operated using urban wastewater. Four polynomial models were developed, one for each season of the year. The software predicted biomass productivities of 12.3, 25.6, 32.7, and 18.9 g·m−2·day−1 in winter, spring, summer, and autumn, respectively. The models were further validated at pilot-scale with R2 values ranging within 0.81 and 0.91, depending on the season. Lower culture depths had the advantage of minimising nitrification and stripping but allow to process a lower volume of wastewater per surface area. Biomass productivity was higher at culture depths of 0.05 m, when compared to 0.12 and 0.20 m, while the optimal dilution rate was season-dependent. Results reported herein are useful for optimising the biomass productivity of raceway reactors located outdoors throughout the year. [ABSTRACT FROM AUTHOR]

Enzymatic hydrolysates containing peptides with biological activity are being studied as potential novel ingredients in functional foods, cosmetics, and pharmaceuticals. Recently, the microalga Spirulina was suggested as a potential source of bioactive peptides. However, the industrial production of bioactive peptides derived from Spirulina has not yet been achieved mainly caused by high production costs and a limited number of scientific reports validating the observed bioactivities in vivo. In this review, current challenges and needs for reducing the costs of Spirulina production are described. This study emphasises the importance of designing the process based on the end application of the produced biomass, focusing on those requirements needed to improve protein content. Moreover, current methods used to extract proteins and produce bioactive peptides are also summarised and described. Large-scale production of Spirulina using raceway reactors is a reality. Different types of water and nutrients have been studied and their selection will depend largely on the end application of the produced biomass. The performance of large raceway reactors is unknown and more sophisticated systems such as thin-layer cascade reactors have not yet been assessed for the large scale production of Spirulina. Spirulina proteins are a promising source of bioactive peptides, although further in vivo studies are needed to validate the several bioactivities observed in vitro. • Spirulina can be produced using different types of water and nutrient sources. • Further studies using larger and more modern reactors are needed. • Spirulina is rich in proteins that can be used to produce bioactive peptides. • Antioxidant, antihypertensive and antidiabetic peptides were reported. • In vivo assessment of bioactivity is rare but necessary for further development. [ABSTRACT FROM AUTHOR]

There are places in our planet where environmental conditions are beyond normal limits for growth but still harbour life. Extremophile microalgae are the candidates for large‐scale production because (i) their extreme growth conditions help to avoid unwanted contaminations, facilitating their production and (ii) in order to survive under harsh environments, these microorganisms synthesise valuable uncommon biomolecules. Extremophile microalgae include Arthrospira platensis, used as a source of phycocyanin and produced under highly alkaline conditions, and Dunaliella salina, which is industrially produced as a source of β‐carotene and can naturally resist salinity conditions exceeding 300 g L−1. Microalgae can be produced on non‐arable lands, using seawater, and within a wide range of environmental conditions, allowing their production in almost any part of the planet. The current paper aimed at highlighting the biotechnological potential of extremophile microalgae as sources for carotenoids, valuable molecules used as pigments and health‐promoters. [ABSTRACT FROM AUTHOR]

The role of microalgae in the production of bioproducts and biofuels, along with their ability to provide a sustainable pathway for wastewater treatment, makes them promising alternatives to conventional processes. Nevertheless, large-scale downstream processing requires an understanding of biomass rheology that needs to be addressed further. This study aimed to characterize microalgal concentrates rheologically in different culture media. The presence of bacteria was quantified by photorespirometry and plate counting techniques. The culture medium was found to significantly influence viscosity, with primary wastewater exhibiting the highest viscosity and seawater plus pig slurry the lowest. The concentration of heterotrophic bacteria was directly related to the viscosity. Extracellular polysaccharides (EPS) in supernatant exhibited an inverse viscosity trend compared to biomass concentrates, with pig slurry cultures having higher concentrations. These findings emphasize the profound influence of culture medium and EPS on the rheology of microalgal biomass, underscoring the need for continued research aimed at facilitating and optimizing large-scale downstream processes within the framework of a circular economy and the attainment of the Sustainable Development Goals (6,8, and 12). [Display omitted] • Culture medium highly influences the rheology of microalgae concentrates. • Heterotrophic bacteria concentration was related to microalgae concentrate viscosity. • Dynamics of EPS production/binding were associated with microorganism activities. • Pig slurry cultures presented higher EPS production indicating its market potential. [ABSTRACT FROM AUTHOR]

The microalga Tetradesmus almeriensis was produced in a 63 m2 thin-layer cascade photobioreactor using either freshwater supplemented with fertilisers, primary urban wastewater, secondary urban wastewater, or diluted fresh pig slurry. The reactors were operated in semi-continuous mode at dilution rate of 0.3 day−1 for over a month. Overall, the biomass productivity was highly influenced by the type of water used, the highest being that produced using freshwater (30.3 g m−2·day−1) given its greater nutrient content. The composition of the produced biomass was also significantly influenced by the type of water used. The protein and lipid contents were higher when the biomass was produced using wastewater. These differences were not only attributed to the microalgal cells' response to the different nutrient availabilities but also to the microbial diversity of the biomasses themselves. Indeed, Illumina sequencing revealed that the inoculated strain was only the most abundant in the culture produced using freshwater (44.6%) and diluted pig slurry (52.9%). When produced using primary or secondary wastewater the genus Tetradesmus represented only 1.6 and 4.9% of the total eukaryotic populations, respectively. Several algal predators were identified in the cultures including parasites of the genus Amoeboaphelidium and rotifers from the genus Adineta. The results highlighted the importance of characterising microalgal cultures to identify potential pathogens and predators, and to implement contingency actions. • Microalgal biomass was produced using freshwater, wastewater and diluted pig slurry. • The highest biomass productivity was achieved with freshwater (30.3 g m−2·day−1). • Algal predators including parasites and rotifers were identified. • T. almeriensis was outcompeted by other microbials when produced using wastewater. • The microbial diversity in the biomass affected its biochemical composition. [ABSTRACT FROM AUTHOR]

The microalga Chlorella vulgaris UAL-1 was produced using urban wastewater that was either left untreated or sterilised. The biomass productivity achieved when using the untreated wastewater was comparable to that of the control, which was freshwater supplemented with synthetic chemicals (0.58 g·L−1·day−1). The microalga C. vulgaris UAL-1 was able to recover up to 93.8, 73.1, 80.5, and 85.2% of the N-NH4+, N-NO3−, P-PO43− and COD present in the unprocessed wastewater. The thermal treatment (120 ºC, 10 min) and covering the reactors led to lower biomass productivities and nutrient recoveries. The biomass was then disrupted by sonication (280 W, 24 kHz) and assessed for biostimulant effects. Overall, although the germination index of watercress seeds was improved with the microalgal extracts (p < 0.05), the observed increase was around 1.0%. All the microalgal extracts significantly promoted the development of roots in soybeans, especially those generated from the biomass produced using freshwater and unprocessed wastewater (p < 0.05). The percentage increase was close to 200%. Moreover, the results from the cucumber cotyledon expansion test revealed that the microalgal extracts led to a greater weight gain; no major differences were observed between the biomass produced using freshwater and unprocessed wastewater while the autoclaved media and the biomass produced in the dark led to a lower weight gain (p < 0.05). The microalga C. vulgaris UAL-1 is adaptable, robust, and can be produced using wastewater; the biomass shows potential for evaluation in field trials as a plant biostimulant. [ABSTRACT FROM AUTHOR]

Dissolved oxygen concentration and pH are controllable and cost-effective variables that determine the success of microalgae-related processes. The present study compares different control strategies for pH and dissolved oxygen in pilot-scale microalgae production systems. Two 80 m2 raceway reactors were used, one operated with freshwater plus fertilizer and the other with wastewater as the nutrient source. Both were in semi-continuous mode at a fixed dilution rate of 0.2 day−1. A comparison between the classical On-Off and more advanced pH control strategies, such as PI and Event-based control, was performed, focusing on biomass productivity and the influence of all the process parameters on microalgae growth; "No control" of pH was also assayed. The results show that Event-based control was the best algorithm when using freshwater plus fertilizer. In contrast, no significant differences were observed using the different control strategies when wastewater was the nutrient source. These experiments were performed through selective control strategy, prioritizing pH over dissolved oxygen; however, it was demonstrated that they did not allow to achieve satisfactory dissolved oxygen removal results, especially for the fertilizer system. After modifying the gas diffuser configuration and improving the mass transfer, independent on-off strategies have been developed, permitting effective control of both variables and increasing productivity by up to 20% in both systems. Concluding, a detailed analysis of the energy demand for each strategy implemented in terms of gas consumption and gas flow to biomass ratio is provided. • Optimal pH and dissolved oxygen control strategies for pilot-scale raceway reactors • Improved gas-liquid mass transfer in raceway reactors • Comparing control strategies for microalgal cultivations in wastewater and freshwater • Analysis of energy demand for pH and dissolved oxygen control in microalgal cultivations [ABSTRACT FROM AUTHOR]

• Large-scale production of microalgal biomass in fresh and wastewater. • Comparable productivity and photosynthetic efficiency in the two systems. • Nutrient reduction under the European limitation for water discharge. • Study of the impacting process and environmental parameters on productivity. • Modeling the biomass productivity as function of temperature and irradiance. The present work aims to assess the treatment of unprocessed urban wastewater using the microalga Scenedesmus almeriensis. Two 12 m3 raceway reactors, one supplemented by wastewater and the second by chemical fertilizer, operating outdoors in a semi-continuous mode, were used for eight months. Results suggested that S. almeriensis can be produced in wastewater without affecting the photosynthetic apparatus reaching a productivity of 13 g·m−2·day−1 on average in both the systems. Furthermore, the nutrient content in terms of nitrogen, phosphorous and chemical oxygen demand of the wastewater was reduced under the European limitations during most of the period, with an average removal rate of 2.2, 0.2 and 3.0 g·m−2·day−1 respectively. Therefore, raceways demonstrated a high potential for microalgal production and successful biotreatment, proving robust and reliable. Finally, the effect of environmental conditions on biomass productivity of the clean system was evaluated in a model with high accuracy (R2 = 0.9, p = 0.0002). [ABSTRACT FROM AUTHOR]

One of the main social and economic challenges of the 21st century will be to overcome the worlds' water deficit expected by the end of this decade. Microalgae based wastewater treatment has been suggested as a strategy to recover nutrients from wastewater while simultaneously producing clean water. Consortia of microalgae and bacteria are responsible for recovering nutrients from wastewater. A better understanding of how environmental and operational conditions affect the composition of the microalgae-bacteria consortia would allow to maximise nutrient recoveries and biomass productivities. Most of the studies reported to date showed promising results, although up-scaling of these processes to reactors larger than 100 m2 is needed to better predict their industrial relevance. The main advantage of microalgae based wastewater treatment is that valuable biomass with unlimited applications is produced as a co-product. The aim of the current paper was to review microalgae based wastewater treatment processes focusing on strategies that allow increasing both biomass productivities and nutrient recoveries. Moreover, the benefits of microalgae based agricultural products were also discussed. [Display omitted] • Microalgae based wastewater treatment is sustainable and economically viable. • Raceway reactors allow processing large volumes of wastewater. • Operational conditions can influence the microalgae-bacteria consortia. • Up-scaling of current processes to over 100 m2 reactors is required. • Microalgal biomass can be used to produce high-value agricultural products. [ABSTRACT FROM AUTHOR]

Microalgae and microalgae-derived ingredients are one of the top trends in the food industry. However, consumers' acceptance and purchase intention of a product will be largely affected by odour and flavour. Surprisingly, the scientific literature present a very limited number of studies on the volatile composition of microalgae and cyanobacteria. In order to fill the gap, the main objective of the present study was to elucidate the volatile composition of seven microalgal and cyanobacterial strains from marine and freshwaters, with interest for the food industry while establishing its potential impact in odour. Among the seven selected strains, Arthrospira platensis showed the highest abundance and chemical diversity of volatile organic compounds (VOCs). Aldehydes, ketones, and alcohols were the families with the highest diversity of individual compounds, except in Arthrospira platensis and Scenedesmus almeriensis that showed a profile dominated by branched hydrocarbons. Marine strains presented a higher abundance of sulfur compounds than freshwater strains, while the ketones individual profile seemed to be more related to the taxonomical domain. The results of this study indicate that the VOCs composition is mainly driven by the individual strain although some volatile profile characteristics could be influenced by both environmental and taxonomical factors. [ABSTRACT FROM AUTHOR]

• A comprehensive model for simulating microalgae-bacteria consortia in wastewater treatment systems. • Validation and calibration using outdoor data from an 80m2 raceway reactor over a period of 206 days. • High accuracy and predictability • Simulation of microalgae-bacteria consortia as function of the operational conditions Modelling microalgae-bacteria in wastewater treatment systems has gained significant attention in the last few years. In this study, we present an enhanced version of the ABACO model, named ABACO-2, which demonstrates improved accuracy through validation in outdoor pilot-scale systems. ABACO-2 enables the comprehensive characterization of microalgae-bacteria consortia dynamics, allowing to predict the biomass concentration (microalgae, heterotrophic bacteria, and nitrifying bacteria) and nutrient evolution. The updated version of the model incorporates new equations for nutrient coefficient yields, oxygen mass balance, and microorganism cellular decay, while significantly reducing the number of calibrated parameters, simplifying the parameter identification. Calibration and validation were performed using data from a 80 m2 raceway reactor operated in a semicontinuous mode over an extensive period (May to November, total of 206 days) at a fixed dilution rate of 0.2 day-1 (corresponding to 5 days of hydraulic retention time), where untreated urban wastewater was used as culture medium. ABACO-2 exhibited robustness, accurately forecasting biomass production, population dynamics, nutrient recovery, and prevailing culture conditions across a wide range of environmental and water composition conditions. Mathematical models are essential instruments for the industrial development and optimization of microalgae-related wastewater treatment processes, thereby contributing to the sustainability of the wastewater treatment industry. [Display omitted] [ABSTRACT FROM AUTHOR]