Microalgae-Based Biorefineries: Challenges and Future Trends to Produce Carbohydrate Enriched Biomass, High-Added Value Products and Bioactive Compounds.

Microalgae have demonstrated a large potential in biotechnology as a source of various macromolecules (proteins, carbohydrates, and lipids) and high-added value products (pigments, poly-unsaturated fatty acids, peptides, exo-polysaccharides, etc.). The production of biomass at a large scale becomes more economically feasible when it is part of a biorefinery designed within the circular economy concept. Thus, the aim of this critical review is to highlight and discuss challenges and future trends related to the multi-product microalgae-based biorefineries, including both phototrophic and mixotrophic cultures treating wastewater and the recovery of biomass as a source of valuable macromolecules and high-added and low-value products (biofertilizers and biostimulants). The therapeutic properties of some microalgae-bioactive compounds are also discussed. Novel trends such as the screening of species for antimicrobial compounds, the production of bioplastics using wastewater, the circular economy strategy, and the need for more Life Cycle Assessment studies (LCA) are suggested as some of the future research lines.

Pontederia sagittata and Cyperus papyrus contribution to carbon storage in floating treatment wetlands established in subtropical urban ponds.

Carbon sequestration is considered an ecosystem service of regulation provided by diverse ecosystems, including wetlands. It has been widely evaluated in the soil of natural wetlands while in constructed wetlands, there is scanty information. In Floating Treatment Wetlands (FTW) there is none. Previously, our research group reported the efficient performance of FTW in an urban polluted pond for two years. As a follow up, the aim of this work was to investigate the contribution of Cyperus papyrus and Pontederia sagittata to carbon storage (CS) in four FTW established in eutrophic urban ponds in a subtropical region. Plant growth, productivity, and CS were assessed in the aboveground biomass of C. papyrus and P. sagittata and the belowground biomass (root mix from C. papyrus and P. sagittata), throughout 26 months in 2 FTW with an area of 17.5 m2 (FTW1) and 33 m2 (FTW2) and throughout 19 months in 2 FTW with an area of 25 m2 (FTW3) and 33 m2 (FTW4), respectively. The macrophyte growth depended on various factors, such as the season, the plant species, and the location of the FTW. High relative growth rate values were found for both species (0.125 and 0.142 d-1 for P. sagittata and C. papyrus, respectively), especially during summer and early autumn. The highest values of productivity were 337 ± 125 gdw m-2d-1 for the aboveground biomass of C. papyrus in FTW2, 311 ± 96.90 gdwm-2d-1 for the aboveground of P. sagittata in FTW1, and 270 ± 107 gdw m-2d-1 for the belowground biomass in FTW2. The mean values of CS for P. sagittata found in FTW1 were 1.90 ± 0.94 kg m-2, while for C. papyrus in FTW2 they were 4.09 ± 0.73 kg m-2. The contribution of the belowground biomass to CS was also significant in FTW2 (4.58 ± 0.59 kg m-2).

Continuous dye adsorption and desorption on an invasive macrophyte (Salvinia minima).

The continuous adsorption-desorption of methylene blue (MB) on an invasive macrophyte, Salvinia minima, was investigated in fixed-bed columns. The effects of bed depth (h) (9.30, 18.70, and 28 cm), inlet dye concentration (C0) (51 ± 1.20, 154 ± 2.00, and 250 ± 1.50 mg L-1), and flow rate (Q) (7 and 14 mL min-1) on dye removal and breakthrough curves were assessed. Thomas, modified dose-response (MDR) and bed depth service time (BDST) models were fitted to the experimental data. Desorption and regeneration studies were also performed. The breakthrough time was affected by h, C0, and Q. The dynamic bed capacity at the breakthrough point (qb) increased with increasing h but decreased with increasing C0 and Q. Dynamic bed capacities (qe) from 318 to 322 mg g-1 were achieved at h = 28 cm, C0 = 154 ± 2.0, or 250 ± 1.50 mg L-1, independently of the Q value. High MB removals were also observed (75-78%). FTIR analysis revealed that hydroxyl and carboxyl groups could be involved in dye adsorption. MDR and BDST models were both successfully used to predict the breakthrough curves of MB adsorption onto S. minima. A high regeneration efficiency (> 87%) was obtained after three adsorption-desorption cycles. These results confirm that the use of S. minima biomass could be a very efficient and eco-friendly alternative for MB adsorption in continuous mode.

Carbon speciation and flocculation in Neochloris oleoabundans cultures using anaerobically digested stillage.

The effects of bicarbonate loading rate (BLR) and pH on growth kinetics, inorganic carbon speciation, carbon fixation and lipid content in Neochloris oleoabundans cultures using anaerobically digested stillage (ADS) (2% v/v) were investigated. Four different cultures were established: culture A with BLR = 1 g l-1 day-1 and no pH adjustment, culture B with BLR = 0.5 g l-1 day-1 and no pH adjustment, culture C with BLR = 1 g l-1 day-1 and pH adjustment at 7.0, and culture D with BLR = 0.5 g l-1 day-1 and pH adjustment at 7.0. The experiments were carried out in flat plate reactors (4 l) at controlled conditions (light intensity of 134 µmol photon m-1 s-1 and photoperiod 16 light/8 darkness; temperature of 32 ± 1 °C). The effects of pH (7, 10.41, 10.65, and 12), time (15, 30, 60, and 90 min), and concentration of a cationic polyelectrolyte (CP) (10 and 20 mg l-1) on the flocculation efficiency (FE) of N. oleoabundans were also investigated. The results showed that bicarbonate was the predominant carbon species in the media and the main carbon source for microalgae growth in all cultures. The highest productivity (87.70 ± 9.70 mg l-1 day-1) and CO2(aq) fixation rate (0.15 g CO2(aq) l-1 day-1) were found in culture B. The lipid content in N. oleoabundans was affected negatively by the pH adjustment at 7.0 during its growth; higher values were found in cultures with no pH adjustment (37.10% and 38.85% dw for culture A and B, respectively) as compared to those obtained in cultures with pH adjustment (27.35% and 22.20% dw for culture C and D, respectively) (p < 0.05). Regarding flocculation, the addition of 20 mg CP l-1 was required to obtain a FE > 95% in cultures A and B, although a significant FE (40-59%) occurred without CP addition at a high pH (≥ 10.41) in all cultures.

Phytofiltration of anaerobically digested sugarcane ethanol stillage using a macrophyte with high potential for biofuel production.

Anaerobically digested stillage (ADS) requires treatment before being discharged into water bodies or soils to avoid adverse effects. Phytofiltration systems are eco-friendly technologies for wastewater treatment, and they simultaneously serve as a source of biomass for biofuel production. The aim of the present study was to investigate the phytofiltration of ADS using Azolla sp. The effects of the ADS strength (dilutions 1: 50 and 1 : 25 v/v) and initial biomass density (IBD) [15.44 (IBD1) and 23.16 (IBD2) g dry weight (dw) m-2] on plant growth and pollutant removal were assessed. Productivities obtained at ADS 1: 50 (2.93 and 3.04 g m-2 d-1 for IBD1 and IBD2, respectively) were not significantly different from those of a synthetic medium (2.56 and 3.15 g m-2 for IBD1 and IBD2, respectively). Higher organic matter removal was found using ADS at 1: 25 than that obtained using ADS 1: 50 (52.16-53.34 vs 32.29-38.16%), while no IBD effect was observed. The nutrient concentrations in ADS were reduced significantly, especially the concentrations of NH4-N (75.11-82.54%), PO4-P (88.72-92.90%) and SO4-S (55.95-66.61%). The conversion of nutrients from ADS into Azolla biomass may result in an effective way to produce an attractive feedstock for biofuel production.

Year-round phytofiltration lagoon assessment using Pistia stratiotes within a pilot-plant scale biorefinery.

Phytofiltration lagoons are phytoremediation technologies suitable for tropical and sub-tropical regions requiring cost-effective and echo-friendly technologies. A biorefinery of fourth generation has been implemented at pilot plant level in Xalapa, Mexico, and the phytofiltration lagoon, being the first module for provision of treated water and plant biomass for biofuel production plays a key role. The aim of this work was to evaluate the performance of such phytofiltration lagoon with a working volume of 13,000 L for the removal of nutrients from an urban river polluted with domestic wastewater and the biomass productivity of the macrophyte Pistia stratiotes, during five different experimental periods, comprising 42 days each one. The maximum absolute growth rates (AGR, gdwday-1) registered for P. stratiotes during the Aug-Oct '15 and the March-Apr '16 and Apr-May '16 period were in the range of 13.51±2.66 to 16.54±2.02gdwday-1. The average biomass productivity was 5.808gdwm-2day-1. Productivities were similar during the periods of Aug-Oct '15, Mar-Apr '16 and Apr-May '16 and significantly higher (p<0.05) than those registered in Oct-Nov '15 and Jan-Feb '16. Removal percentages of COD and nutrients varied according to the season. COD was in the range of 47.82±39.3% to 88.00±15.0%. Ammonium N was in the range of 76.78±21% to 98.79±0.9%. Nitrates were removed in the range of 16.92±64%. to 97.14±4.5%. Finally, phosphates were removed very effectively, from 73.72±18.5% to 92.89±4.3%. A hydraulic retention time of 7 days was enough for the effective treatment of the water from the polluted river. It was concluded that the phytofiltration lagoon with P. stratiotes is very feasible within the biorefinery for providing biomass year-round and for treating the polluted water very effectively.

Long-term assessment at field scale of Floating Treatment Wetlands for improvement of water quality and provision of ecosystem services in a eutrophic urban pond.

Pollution of urban water bodies requires stringent control measures and the development of low-cost and highly efficient alternative technologies. In contrast to Constructed Wetlands, Floating Treatment Wetlands (FTWs) have the advantage of not requiring large surface of land since they operate in situ. However, there is limited information about their long-term evaluation while operating at field scale. The aim of this work was to assess the performance of FTWs using a combination of Pontederia sagittata and Cyperus papyrus for the improvement of the water quality and provision of ecosystem services of a eutrophic urban pond. The FTWs were built with low-cost material easy to acquire and to ensemble. Two FTWs (17.5m2 and 33m2) located in Pond 1 within a complex of 4 urban artificial ponds were evaluated for two years. They promoted an increase in the dissolved oxygen (D.O.) within a range of 15 to 67%, a removal of fecal coliforms in the range of 9 to 86% and a nitrate removal in the range of 9 to 76%. The plant productivity reached a maximum of 363gdmm-2d-1 in the FTW1 and 536gdmm-2d-1 in the FTW2 during the period March-June 2016. The TKN and the TP content in the plant were in the range of 18.3 to 28.1 and of 0.05 to 0.196gkg-1 dry matter, respectively. In conclusion, the tested FTWs have proved to be a very beneficial low-cost technology for the improvement of water quality and provision of ecosystem services.

Heavy metal removal in phytofiltration and phycoremediation: the need to differentiate between bioadsorption and bioaccumulation.

Phytoremediation and phycoremediation are cost-effective and environmentally sound technologies for the treatment of polluted streams and wastewaters contaminated with metals. Currently, the most commonly used parameter to assess the metal uptake of biomass is (q) expressed as mg metal g dry weight(-1). By contrast, the bioconcentration factor (BCF) is one of the most widely used factors to evaluate the metal uptake capacity of macrophytes. However, both parameters the metal uptake (q) and the BCF cannot be applied to differentiate between the ability of live plants or photosynthetic microorganisms to adsorb the metal onto their surface through passive mechanisms or to accumulate the contaminant at intracellular level through metabolically active mechanisms. This mini review has the objective of discussing the need to differentiate between bioadsorption and bioaccumulation of metals in live plants and photosynthetic microorganisms used in phytofiltration and phycoremediation processes, respectively. The use of two specific factors, the bioadsorption factor (BAF) and the intracellular accumulation factor (IAF) that have been previously reported in order to make a clear differentiation between these two metal removal mechanisms in Salvinia minima and Leptolyngbya crossbyana is highlighted. It is suggested that the BAF and the IAF can be used in phytofiltration wetlands and phycoremediation lagoons, where there is the need of specific information indicating the fate of the metal in order to gain information about possible removal mechanisms. These factors could also provide a tool to decide whether it is possible to harvest the biomass and to recover a fair amount of metal adsorbed onto the surface by means of desorbent agents. A critical assessment of the use of EDTA as desorbent agent is also included.

Constructed wetland mesocosms for the treatment of diluted sugarcane molasses stillage from ethanol production using Pontederia sagittata.

Sugarcane molasses stillage contains a very high concentration of organic matter and toxic/recalcitrant compounds. Its improper disposal has become a global problem and there is very scanty information about its treatment using phytotechnologies. This work aimed at evaluating the performance of subsurface flow constructed wetlands (SSF CWs) mesocosms planted with Pontederia sagittata and operating at two hydraulic retention times (HRTs), compared to an unplanted SSF CWs, for the treatment of diluted stillage subjected to no pre-treatment apart from an adjustment to pH 6.0. CWs were fed with very high surface COD loading rates (i.e. 47.26 and 94.83gCOD/m(2)d). The planted CWs were able to remove COD in the range of 80.24-80.62%, BOD(5) in the range of 82.20-87.31%, TKN in the range of 73.42-76.07%, nitrates from 56-58.74% and sulfates from 68.58-69.45%, depending on the HRT. Phosphate and potassium were not removed. It was concluded that this type of CWs is a feasible option for the treatment of diluted stillage.

Surface adsorption, intracellular accumulation and compartmentalization of Pb(II) in batch-operated lagoons with Salvinia minima as affected by environmental conditions, EDTA and nutrients.

The effects of environmental factors and nutrients on the various possible removal mechanisms (surface adsorption, intracellular accumulation and precipitation to sediments) and partitioning of lead among various compartments (plant biomass, water column and sediments) in Salvinia minima batch-operated lagoons, were evaluated. Surface adsorption was found to be the predominant mechanism for Pb(II) removal under all environmental conditions tested in the absence of nutrients (an average of 54.3%) and in a nutrient medium (modified Hutner 1/10 medium) free of EDTA and phosphates (54.41%) at "high" initial Pb(II) concentrations (in the range of 10.3+/-0.13 to 15.2+/-0.05 mg/L). Under these conditions, the bioconcentration factors (BCFs) were 2,431+/-276 and 2,065+/-35, respectively. Lead removal was very rapid during the first 4 h and reached 70% in the absence of nutrients at the "medium" light intensity and temperature (LIT) tested, 88% in nutrient medium free of EDTA and supplemented with synthetic wastewater (at the "lowest" LIT tested), and 85% in medium free of EDTA and phosphates. It was concluded that the mechanisms of lead removal by S. minima, and the compartmentalization of this metal in the microcosm of batch-operated lagoons, are primarily a function of the presence of certain nutrients and chelants, with secondary dependence on environmental conditions. In addition, the results indicate that the percentage of lead removed is only a gross parameter and that the complementary use of BCF and compartmentalization analysis is required to gain a full insight into the metal removal process.