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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.

The effects of water depth, operational and environmental conditions on bacterial communities were analyzed in microalgal-bacterial outdoor photobioreactors treating urban wastewaters from March to August 2014. Three raceway photobioreactors inoculated with Scenedesmus sp. and with different water depths (20, 12, and 5 cm) were used at different dilution rates (0.15, 0.3, 0.4, and 0.5 d−1). A thin-layer reactor with 2 cm water depth and operated at 0.3 d−1 was used as a control. The results showed that biomass productivity increased as water depth decreased. The highest biomass productivity was 0.196 gL−1d−1, 0.245 gL−1d−1, and 0.457 gL−1d−1 for 20, 12, and 5 cm depth raceway photobioreactors, respectively. These values were lower than the maximum productivity registered in the control reactor (1.59 gL−1d−1). Bacterial communities, analyzed by high-throughput 16S rRNA sequencing, were not affected by water depth. A decrease in community evenness was related to a decrease in nutrient removal. Hetetrotrophs and phototrophs, mainly from the family Rhodobacteraceae, dominated bacterial diversity. The community changed due to increasing temperatures, irradiance, and organic carbon, ammonia, and phosphate contents in the photobioreactor-influent as well as, microalgae inhibition and higher organic carbon in the effluent. The photobioreactors shared a core-biome that contained five clusters of co-occurring microorganisms. The bacteria from the different clusters were taxonomically and ecologically different but functionally redundant. Overall, the drivers of the community changes could be related to abiotic variables and complex biological interactions, likely mediated by microalgae excretion of organic substances and the microorganisms’ competence for substrates.

Isochrysis galbana is a widely-used strain in aquaculture in spite of its low productivity. To maximize the productivity of processes based on this microalgae strain, we have developed a model considering the influence of irradiance, temperature, pH and dissolved oxygen concentration on the photosynthesis and respiration rate. Results demonstrate that this strain tolerates temperatures up to 35 °C but it is highly sensitive to irradiances higher than 500 μE m −2 s −1 and dissolved oxygen concentrations higher than 11 mg l −1 . We have validated the developed model using data from an industrial-scale outdoor tubular photobioreactor demonstrating that inadequate temperature and dissolved oxygen concentrations reduce productivity to half that which is maximal, according to light availability under real outdoor conditions. The developed model is a useful tool for managing working processes, but especially in the development of new processes based on this strain and to take decisions regarding optimal control strategies.

In this paper, we have studied the outdoor production of the freshwater microalgae Scenedesmus sp. in two open reactors (32 m2): thin-layer (1.2 m3) and raceway (4.4 m3), using centrate from anaerobic digestion as the sole nutrient source. The aim was to recover valuable nutrients (nitrogen and phosphorus) from effluents in order to maximize biomass productivity. Experiments were performed in semicontinuous mode, modifying the centrate percentage within the culture medium. The optimal centrate percentage was 30% — above this value the culture's performance reduced, probably due to ammonium excess (above 122 mg l− 1). Using the raceway reactor, biomass productivity was 24 g m− 2 day− 1, whereas using the thin-layer reactor, it increased up to 42 g m− 2 day− 1. Nitrogen and phosphorus removal was demonstrated to be proportional to biomass productivity, with maximal values up to 38 mgN l− 1 day− 1 and 3.9 mgP l− 1 day− 1 being removed, respectively. Ammonium stripping was only relevant in the raceway reactor, due to its lower biomass productivity, with more than 40% of the inlet nitrogen being lost to the air. The thin-layer reactor also proved to be more photosynthetically efficient, with maximal values of 4.7% being measured. An economic analysis demonstrated that the thin-layer reactor allowed a reduction in the biomass production cost, in addition to utilizing waste as the nutrient source, with a minimum production cost of 0.9 €/kgbiomass being estimated. In conclusion, it is possible to use centrate from anaerobic digestion as the sole nutrient source for the large-scale production of Scenedesmus sp. biomass thus reducing the biomass production cost by avoiding the use of expensive and non-sustainable fertilizers; while also obtaining returns from the treatment of this type of residue. Such a combination helps to increase the possibility of producing commodities, or biofuels, from microalgae by coupling their production to treatment processes.

In this paper freshwater microalgae production is studied using centrate from anaerobic digestion as the only nutrient source. The objective is not only to maximize biomass productivity but also to eliminate contaminants in order to release water which is clean enough for reuse in compliance with regulations. Experiments were performed indoors using two microalgae strains, Muriellopsis sp. and Pseudokirchneriella subcapitata, to establish the optimal centrate percentages within the culture medium. Results demonstrated that with 40–50% of centrate in the culture medium, productivity reached values of up to 1.13 and 1.02 g·l− 1·day− 1 for Muriellopsis sp. and P. subcapitata, respectively. In addition, the removal of nitrogen and phosphorous contained in the culture medium exceeded 90%, the COD at the outlet being lower than 100 mg·l− 1. Above 50% of centrate, toxicity exists and culture performance decreases. No nitrification was observed as the ammonium was completely consumed at centrate percentages lower than 20%. Muriellopsis sp. demonstrated itself to be the more robust strain tolerating higher ammonium concentrations as well as achieving a higher quantum yield, of up to 0.50 g·E− 1, and a higher nitrogen removal rate, of up to 47.5 mgN·l− 1·day− 1 and 3.8 mgP·l− 1·day− 1. Consequently, it is possible to use centrate from anaerobic digestion as the sole nutrient source for the production of freshwater microalgae, thus reducing the biomass production cost by avoiding the use of expensive and non-sustainable fertilizers; not to mention the returns obtained from treating this type of residue. This combination can help to increase the possibilities of producing biofuels from microalgae by means of coupling their production to other treatment processes.

Microalgae have been proposed as an option for wastewater treatment since the 1960s, but still, this technology has not been expanded to an industrial scale. In this paper, the major factors limiting the performance of these systems are analysed. The composition of the wastewater is highly relevant, and especially the presence of pollutants such as heavy metals and emerging compounds. Biological and engineering aspects are also critical and have to be improved to at least approximate the performance of conventional systems, not just in terms of capacity and efficiency but also in terms of robustness. Finally, the harvesting of the biomass and its processing into valuable products pose a challenge; yet at the same time, an opportunity exists to increase economic profitability. Land requirement is a major bottleneck that can be ameliorated by improving the system’s photosynthetic efficiency. Land requirement has a significant impact on the economic balance, but the profits from the biomass produced can enhance these systems’ reliability, especially in small cities.

In this work, we studied the potential use of secondary-treated wastewater as nutrient source in the production of freshwater microalgae strains. Experiments were performed indoors in a semicontinuous mode, at 0.3 day(-1), simulating outdoor conditions. We demonstrated that all the tested strains can be produced by using only secondary-treated wastewater as the nutrient source. The utilization of secondary-treated wastewater imposes nutrient-limiting conditions, with maximal biomass productivity dropping to 0.5 g l(-1) day(-1) and modifies the biochemical composition of the biomass by increasing the amount of lipids and carbohydrates while reducing the biomass protein content. We measured fatty acid content and productivity of up to 25 %d.wt. and 110 mg l(-1) day(-1), respectively. We demonstrated that all the tested strains were capable of completely removing the nitrogen and phosphorus contained in the secondary-treated wastewater, and while the use of this effluent reduced the cells' photosynthetic efficiency, the nitrogen and phosphorus coefficient yield increased. Muriellopsis sp. and S. subpicatus were selected as the most promising strains for outdoor production using secondary-treated wastewater as the culture medium; this was not only because of their high productivity but also their photosynthetic efficiency, of up to 2.5 %, along with nutrient coefficient yields of up to 96 gbiomass gN (-1) and 166 gbiomass gP (-1). Coupling microalgae production processes to tertiary treatment in wastewater treatment plants make it possible to recover nutrients contained in the water and to produce valuable biomass, especially where nutrient removal is required prior to wastewater discharge.

Primary urban wastewater was processed using the microalga Scenedesmus sp. in an outdoor pilot-scale raceway reactor connected to an ultrafiltration membrane. The goal was to separate cellular retention time from hydraulic retention time. This strategy led to a 129.3% increase in the daily volume of wastewater treated per square meter, and to a 48.7% increase in biomass productivity to a final value of 22.2 ± 1.9 g·m−2·day−1. Nutrient removal was highly influenced by permeate rate, allowing to remove up to 0.65 mg·m−2·day−1 of phosphates. Over 99% of ammonia was removed when the ultrafiltration membrane was used, although this was partially due to nitrate production by nitrifying bacteria: higher permeate rates led to higher relative abundance of the nitrifying bacterial. The amplification and sequencing of the microalgae-bacteria samples led to the detection of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, such as Bradyrhizobiaceae, Nitrospiraceae, Nitrosomonadaceae, and Chromatiaceae. The most abundant families detected in the microalgae-bacteria biomass were Rhodobacteraceae and Comamonadaceae.