Investigation of anaerobic digestion of Chlorella sp. and Micractinium sp. grown in high-nitrogen wastewater and their co-digestion with waste activated sludge

This study investigated two wildtype green algae, Micractinium sp. and Chlorella sp., for their growth in high nitrogen wastewater (mixture of sludge centrate and primary effluent wastewater) and subsequent anaerobic digestion under mesophilic conditions. Extraction and analysis of extracellular polymeric substances (EPS) in both algal species during cultivation showed that Micractinium generated larger quantity of EPS-proteins than Chlorella. Anaerobic digestion of harvested algae showed the opposite trend that Chlorella allowed a higher CH₄ yield on the volatile solids fed the digester (VS_fed) of 230 dm³ kg⁻¹ than Micractinium (209 dm³ kg⁻¹). These results suggested that different growth patterns of two types of algae, with different quantity of EPS expressed, affected anaerobic digestibility and biogas yield. Co-digestion of algae with waste activated sludge (WAS) improved the volatile solids reduction, hydrolysis efficiency as well as the biogas yields of algae.     

Isochrysis sp. IOAC724S,a newly isolated,lipid-enriched,marine microalga for lipid production,and optimized cultivation conditions

An oleaginous, unicellular, marine microalga termed IOAC724S was isolated from the South China Sea. Morphology and genetic analyses indicated it belongs to the genus Isochrysis. Gas chromatography (GC) results showed that more than 10 types of fatty acids existed in Isochrysis sp. IOAC724S and that 90% of them were suitable for lipid production. The culture conditions suitable for cell growth were progressively optimized through photosynthetic and respiratory analyses. The optimal culture conditions were: photon flux 200–500 μmol m⁻² s⁻¹, temperature 35 °C during daytime and 24 °C at night, pH value between 7 and 8, NaNO₃ 160 g m⁻³ and NaH₂PO₄·2H₂O 80 g m⁻³ for starting culture. When microalgal cultures were exposed to these optimal conditions, the specific growth rate reached to 0.26 d⁻¹ on average and 1.0 d⁻¹ in MAX. Lipid production was optimized through nutrient starvation processes, including nitrate or phosphate deprivation and simultaneous nitrate and phosphate deprivation. The highest lipid mass fraction of dry cell weight (about 55.6%) was obtained after the stationary phase algal culture was transferred into phosphate-free medium for 3 days. GC data demonstrated that the enhancement of lipid accumulation in algal cells maintained under nutrient starvation came mainly from an increase of C16:0 and C18:1 fatty acids; however, the lipids with a chain length appropriate for fuel use (C14 to C18) were unchanged at 90% mass fraction of the dry cell weight. Based on these good characteristics, Isochrysis sp. IOAC724S appeared to be a strong candidate for lipid production.     

Energy efficient growth control of microalgae using photobiological methods

Microalgae have garnered interest for the production of valuable molecules ranging from therapeutic proteins to biofuels. However, microalgae also are associated with the considerable problem of phytoplankton bloom. In this study, we demonstrated algal growth using Isochrysis galbana as a model can be controlled photobiologically. Long dark period (24-h light: 24-h dark) unlike common photoperiod resulted in biomass loss and slower growth rates, but were unlikely to cause fatal damage. Algal cell growth can be significantly recovered with the onset of light. Also, it was confirmed that blue light-emitting diode (LED) illumination was able to effectively support cell growth of I. galbana as the sole light source. The blue LED intensity with 200% (580 l×, 18.52 μmol m⁻² s⁻¹) based on 8000 l× (98.4 μmol m⁻² s⁻¹) fluorescent lamp provided the best support for growth of I. galbana. We verified excessive light intensities lead to inhibition of algal growth, whereas low light intensities also did not promote algal growth. Further, I. galbana cell growth can be controlled using blue LED with extremely high LED intensities. These results may provide means to control algal population for either goal of growth or inhibition through proper use of such illumination.     

A non-noble V2O5 nanorods as an alternative cathode catalyst for microbial fuel cell applications

This study assessed the feasibility of vanadium pentoxide (V₂O₅) as a novel cathode catalyst material in air-cathode single chamber microbial fuel cells (SCMFCs). The V₂O₅ nanorod catalyst was synthesized using a hydrothermal method. MFCs with different cathode catalyst loadings were studied. Cyclic voltammetry (CV) was used to examine the electrochemical behavior of the catalysts in the MFCs. The V₂O₅ cathode catalyst constructed with a double loading MFC exhibited the highest maximum power density of 1073 ± 18 mW m⁻² (OCP; 691±4 mV) compared with 447 ± 12 mW m⁻² (OCP; 594 ± 5 mV) and 936 ± 15 mW m⁻² (OCP; 647±5 mV) for the single loading MFC and triple loading MFC, respectively. The power density of MFC with double loaded V₂O₅ is comparable to the traditional Pt/C cathode (2067 ± 25 mW m⁻², OCP; 821 ± 4 mV), which covers up to 55% of the performance of Pt/C. This finding highlights the potential of the V₂O₅ cathode as an inexpensive catalyst material for MFCs that may have commercial applications.     

Pre-fermentation of waste as a strategy to enhance the performance of single chambered microbial fuel cell (MFC)

Functional role of pre-fermentation of food waste (PFW) was studied to enhance the performance of single chambered microbial fuel cell (MFC) (mediatorless; non-catalyzed graphite electrodes; open-air cathode). Significant improvement in power output was noticed after pre-treatment (391 mV; 530 mA/m²) compared to unfermented waste (275 mV; 361 mA/m²). MFC performance was found to depend on applied organic load and nature of substrate in terms of power generation and substrate degradation. The pre-fermentation of waste facilitated lowering of activation losses and in turn increased the bio-electrochemical activity of biocatalyst, leading to an effective MFC performance. Fuel cell behavior with respect to polarization, anode potential and bio-electrochemical behavior also supported the performance of MFC with PFW. PFW operation showed higher catalytic current in voltammograms with fine catalytic peaks supporting the positive role of pre-fermentation in discharging electrons effectively. VFA and pH profiles also correlated well with power generation and substrate degradation pattern.     

Coupling algal biomass production and anaerobic digestion: Production assessment of some native temperate and tropical microalgae

Coupling algal biomass production and anaerobic digestion is one of the most promising bioprocesses for economically viable algal production. This study assesses the production rates of some native microalgae growing in media supplemented with algal digestate, urban wastewater or digested sludge. Native microalgal populations isolated from temperate freshwaters (Scenedesmus spp.) and marine ecosystems (Nannochloris spp.) had the highest potential production rates (about 100 mg DW L⁻¹ d⁻¹) with algal digestate at about 20% loading ratio. However, no growth was measured for Nannochloris spp., when the ammonium concentration exceeded 100 mg L⁻¹ although Scenedesmus spp. appeared to be tolerant to higher NH₄⁺ concentrations. Very low production rates, or no growth, were measured when microalgae isolated from high salinity waters (Dunaliella salina, Lyngbya aestuarii) were used, suggesting that populations well adapted to extreme environmental conditions are not suitable candidates for growing on wastewater or anaerobic digestate.     

The effect of Fe on chemical stability and oxygen evolution performance of high surface area SrTix-1FexO3-δ mixed ionic-electronic conductors in alkaline media

Development of environmentally friendly, high performing oxygen evolution reaction (OER) catalysts is an important research challenge. In this work, iron doped strontium titanates with a general formula SrTi_1-xFe_xO_3-δ (x = 0.35, 0.50, 0.70, 0.90, and 1.00) denoted as STFx, were synthesized via a solid state reaction technique and characterized in terms of oxygen evolution reaction electrocatalysis in an alkaline electrolyte (0.1 M KOH). The produced powders were characterized by a high specific surface area (>20 m² g⁻¹), beneficial for OER. The evaluation of specific activity indicated the following trend of increasing performance: STF35 < STF50 < STF70 < SFO < STF90. The lowest overpotential at 10 mAcm⁻² _GEO of 410 mV (350 mV at 25 μA cm⁻²_OX) was achieved by STF90 with the corresponding Tafel slope of 60 mV dec⁻¹. The two materials with the highest Fe content (i.e. STF90 and SFO) showed, however, poor chemical stability in alkaline solution demonstrated by the dissolution of Sr. Based on the good electrochemical performance (~460 mV at 10 mA cm⁻²_GEO, ~405 mV at 25 μA cm⁻²_OX) and chemical stability for at least 30 days (no Sr dissolution) of STF50, it can be considered an interesting, working at room temperature OER catalyst based on non-toxic and abundant elements.     

Scaling up self-stratifying supercapacitive microbial fuel cell

Self-stratifying microbial fuel cells with three different electrodes sizes and volumes were operated in supercapacitive mode. As the electrodes size increased, the equivalent series resistance decreased, and the overall power was enhanced (small: ESR = 7.2 Ω and P_max = 13 mW; large: ESR = 4.2 Ω and P_max = 22 mW). Power density referred to cathode geometric surface area and displacement volume of the electrolyte in the reactors. With regards to the electrode wet surface area, the large size electrodes (L-MFC) displayed the lowest power density (460 μW cm⁻²) whilst the small and medium size electrodes (S-MFC, M-MFC) showed higher densities (668 μW cm⁻² and 633 μW cm⁻², respectively). With regard to the volumetric power densities the S-MFC, the M-MFC and the L-MFC had similar values (264 μW mL⁻¹, 265 μW mL⁻¹ and 249 μW cm⁻¹, respectively). Power density normalised in terms of carbon weight utilised for fabricating MFC cathodes-electrodes showed high output for smaller electrode size MFC (5811 μW g⁻¹-C- and 3270 μW g⁻¹-C- for the S-MFC and L-MFC, respectively) due to the fact that electrodes were optimised for MFC operations and not supercapacitive discharges. Apparent capacitance was high at lower current pulses suggesting high faradaic contribution. The electrostatic contribution detected at high current pulses was quite low. The results obtained give rise to important possibilities of performance improvements by optimising the device design and the electrode fabrication.     

Photoautotrophic microalgae Scenedesmus obliquus attached on a cathode as oxygen producers for microbial fuel cell (MFC) operation

Photoautotrophic algae Scenedismus obliquus could attach on the surface of a cathode electrode and produced oxygen for electricity generation in microbial fuel cell (MFC). Oxygen concentration by algae aeration in the cathode chamber increased from 0 to 15.7 mg/l within 12-h, and a voltage generation of 0.47 ± 0.03 V was obtained with 1000 Ω external resistance. In polarization test, MFC with algal aeration exhibited the maximum power density of 153 mW/m², which was 32% higher than the value (116 mW/m²) with mechanical aeration at oxygen concentration of 5.9 mg/l. The internal resistance of MFC with algal aeration decreased in ohmic resistance (5.9–5.2 Ω) and charge transfer resistance (9.6–7.2 Ω) over 72-h operation. Cyclic voltammetry of cathode during algal aeration revealed higher reduction current of −9.3 mA compared to mechanical aeration (−4.7 mA).     

Boron doped cryptomelane as a highly efficient electrocatalyst for the oxygen evolution reaction

In this study, cryptomelane-type (1D) MnO₂ was doped with boron powder by ball-milling in an inert organic solvent under various experimental conditions. The structural, thermal, morphological, and surface features of samples prepared by the ball-milling method were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, and surface measurements. The electrocatalytic oxygen evolution reaction (OER) performances of the samples were tested and compared with the bare cryptomelane to reveal the effect of boron doping into manganese oxide. It was found that boron particles transformed to trigonal BO₃ units in the cryptomelane structure via mechanical activation, and accordingly, the oxidation state of manganese in this structure relatively changed. The 0.25% B-doped cryptomelane sample prepared at 12 h grinding time exhibited the overpotential of 425 mV at a current density of 1 mAcm⁻² with a Tafel slope of ∼95 mV dec⁻¹. It showed a remarkable catalytic performance among the other electrocatalysts under neutral pH compared to bare cryptomelane. When the elemental boron doping exceeded 1%, the electrochemical performance dramatically decreased depending on the blocking of the Mn³⁺ active sites.     

Sustained hydrogen photoproduction by phosphorus-deprived Chlamydomonas reinhardtii cultures

This study demonstrates that, besides sulfur deprivation, sustained H₂ photoproduction in Chlamydomonas reinhardtii cultures can be generated by incubating algae under phosphorus-deprived (−P) conditions. However, phosphorus deficiency in algal cells could not be obtained by resuspension of algae in −P medium, evidently due to a significant reserve of phosphorus in cells. In this study, phosphorus deficiency was accomplished by inoculating the washed algae into the −P medium at low initial cell densities (below 2 mg Chl l⁻¹). After the initial growth period, where cells utilize intracellular phosphorus, algae established anaerobic environment followed by the period of H₂ photoproduction. The maximum H₂ output (∼70 ml l⁻¹) was obtained in cultures with the initial Chl content ∼1 mg l⁻¹. Cultures with Chl above 2 mg l⁻¹ did not produce H₂ gas. The physiological response of algal cultures to phosphorus deprivation demonstrated significant similarities with the response of algae to sulfur depletion.     

Preparation of Ni–P–La alloy as a novel electrocatalysts for hydrogen evolution reaction

Ternary Ni–P–La alloy was synthesized by the co-electrodeposition method on the copper substrate. The energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used for characterization of the synthesized alloy. The electrochemical performance of the novel alloy was investigated based on electrochemical data obtained from steady-state polarization, Tafel curves, linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) in alkaline solution and at ambient temperature. The results showed that the microstructural properties play a vital purpose in determining the electrocatalytic activity of the novel alloys. Also, the HER on investigated alloys was performed via the Volmer-Heyrovsky mechanism and Volmer step as RDS in this work. Ni–P–La catalyst was specified by ƞ₂₅₀ = −139.0 mV, b = −93.0 mV dec⁻¹, and j_o = −181.0 μA cm⁻². The results revealed that the Ni–P–La catalysts have a high potential for HER electrocatalysts in 1M NaOH solution.     

Influence of culture age,ammonium and organic carbon in hydrogen production and nutrient removal by Anabaena sp. in nitrogen-limited cultures

Hydrogen can be generated from cyanobacteria cultivation with light and organic carbon as an energy source. The aim of this study was to investigate the influence of ammonium and glucose concentration, and the culture age on the production of hydrogen and other products, and phosphate and organic carbon removal by Anabaena sp. (UTEX 1448) in batch anaerobic photobioreactors. Our results demonstrated an effect of culture age and ammonium concentration in hydrogen production, an average increase of 4.1 times (90.4 μmol H₂ mg Chl a⁻¹ h⁻¹ and 13.2 mmol mg Chl a⁻¹) in the conditions with younger biomass and without ammonium. Culture age also had an effect in phosphate removal, with 92% of removal efficiency, and ethanol production (an average increase of 2.9 times–97 mg L⁻¹), however the optimum conditions were obtained with older biomass. This study demonstrates efficient hydrogen production by this strain of Anabaena sp. fewer researched.     

One-step gas phase sulfided NiFe coating as self-supporting electrode for high efficiency oxygen evolution reaction

The development of cost-effective oxygen evolution reaction (OER) electrocatalytic electrodes is one of the essential means of applying green hydrogen energy. Due to the complex steps in the anodic OER, the high overpotential hinders the kinetics of water splitting. In this paper, the sulfided NiFe coating was innovatively designed as a self-supporting OER electrode by high-velocity oxygen fuel (HVOF) spraying coupled with one-step gas phase sulfuration. The thickness of the NiFe coating is approximately 20 μm. After the sulfuration treatment, the surface of the NiFe coating is remolded into a uniform rock sugar-like structure, and simultaneously forms new NiS and Ni₃S₄ phases. The sulfided NiFe coating electrode shows relatively low overpotentials of 220 mV and 253 mV at the current density of 10 mA cm⁻² and 100 mA cm⁻², respectively, and the Tafel slope is as low as 28.6 mV dec⁻¹. The excellent electrocatalytic activity is mainly attributed to the synergistic effect of sulfides, the adsorption of OH⁻ by the Ni³⁺ in alkaline electrolyte, and the acceleration of O₂ separation by the S²⁻ through promoting the cleavage of O–O bonds. In addition, the sulfided NiFe coating electrode also has a small charge transfer resistance, and the potential stability is as high as 98.1% in the 70 h stability test. Therefore, the development of sulfided transition metal coating electrodes can provide a new idea for the large-scale industrialization of water splitting.     

Formulation of a low-cost medium for mass production of Spirulina

A new medium was formulated for mass production of Spirulina sp. by incorporating selected nutrients of the standard Zarrouk's medium (SM) and other cost-effective alternative chemicals. This newly formulated medium (RM₆) contains single super phosphate (1.25 g l⁻¹), sodium nitrate (2.50 g l⁻¹), muriate of potash (0.98 g l⁻¹), sodium chloride (0.5 g l⁻¹), magnesium sulphate (0.15 g l⁻¹), calcium chloride (0.04 g l⁻¹), and sodium bicarbonate (commercial grade) 8 g l⁻¹. The alga was grown in an illuminated (50 μmol photons m⁻² s⁻¹ white light) growth room at 30±1 °C. Maximum growth rate in terms of dry biomass, chlorophyll and proteins in SM was recorded between 6 and 9 days of growth and values were 0.114, 0.003 and 0.068 as compared to 0.112, 0.003 and 0.069 mg ml⁻¹ d⁻¹ in RM₆,. No significant differences were observed in the protein profiles of Spirulina sp. grown in both the media. From the scale up point of view, the revised medium was found to be highly economical, since it is five times cheaper than Zarrouk's medium.     

Purification and characterization of exo-β-agarase from an endophytic marine bacterium and its catalytic potential in bioconversion of red algal cell wall polysaccharides into galactans

An extracellular exo-β-agarase was characterized from an endophytic bacterial strain Pseudomonas sp. isolated from the red alga Gracilaria dura. The enzyme was purified to homogeneity with a recovery of 28.2% and a purity fold of 8.33. The purified enzyme was composed of single polypeptide with a molecular mass of about 66 kDa. The enzyme exhibited a maximum activity of 81.74 U mL⁻¹ and a specific activity of 615.5 U mg⁻¹. The optimal pH and temperature for its maximum activity were 9.0 and 35 °C respectively. The enzyme stabilized its activity in alkaline pH 7–11 and high salt concentration up to 4 mol dm⁻³. The enzymatic hydrolyzed product of agar was characterized as neoagarobiose while the bacterium when incubated with G. dura biomass yielded galactose 20% on dry wt basis. The agarolytic ability of the former was further confirmed by release of protoplasts from G. dura tissue through digestion of cell wall polysaccharides. The bacterium investigated in this study could possibly be used for bioconversion of marine red algal polysaccharides into energy feedstock and the purified enzyme for preparation of compounds having pharmaceutical importance.     

A methodology to assess open pond,phototrophic, algae production potential: A Hawaii case study

Geographic information system (GIS) analysis was used to identify lands suitable for open pond production of phototrophic microalgae in the state of Hawaii where rainfall is less than 1.0 m y⁻¹, solar insolation is at least 4.65 kWh m⁻² d⁻¹, slope is ≤5%, zoning is non-residential, and contiguous area is at least 0.2 km² (Base Case). Eight sensitivity analyses were performed that varied these criteria and considered an added criterion stipulating a maximum distance from power plants that could serve as CO₂ sources. Results were overlaid with GIS layers for agricultural lands of importance to the State of Hawaii and land serviced by freshwater irrigation infrastructure. Base Case conditions were identified on 476 km², 2.9% of State land area. 60% of Base Case lands are important agricultural lands and of these, half are serviced by irrigation infrastructure. Assumed algae oil productivity of 1.87 dm³ m⁻² y⁻¹ would yield 0.9 hm³ y⁻¹, equivalent to 30% of the combined total consumption of distillate and jet fuel in the State in 2011.     

Energy potential of native shrub species in northern Spain

This paper we present an energy review of the waste generated by shrub species in soils of low fertility for use as fuel in a power plant. The residues analysed belong to the species: Rhamus alaternus, Ulex europaeus, Prunus spinosa, Smilax aspera, Erica sp., Rubus ulmifolius, and Pteridium aquilinum. Gross calorific value (GCV), net calorific value (NCV), density, elementary chemical analysis, moisture content, percentage of ash, productivity, energy density and FVI (fuel value index) have been measured. These parameters have been determined for three levels of moisture (maximum, medium and minimum). At medium moisture level, the residues of U. europaeus are those that reach the greatest FVI, 20,000. In the other extreme is the P. aquilinum with an FVI of 403. The average productivity of waste, in t ha⁻¹, of each species has been determined in order to know how much energy is stored per hectare. U. europaeus and P. spinosa are the species which accumulate more energy per hectare, with similar values of around 81 MJ ha⁻¹ yr⁻¹ and installed power of 2.59 W ha⁻¹. The energy recovery of the waste in a thermal power plant would generate an annual revenue of 14.6 M€, taking into account that 40% of the forest land covered by shrub in Cantabria is used for this purpose.     

Effects of shock loading versus stepwise acclimation on microbial consortia during the anaerobic digestion of glycerol

Sludge from a brewery was used to produce methane by feeding glycerol (propane-1,2,3-triol) at an organic loading rate (OLR) of Chemical Oxygen Demand (COD) at 2.5 g L⁻¹ d⁻¹. Results from two different substrate-feeding approaches were compared: one was the shock loading of glycerol (Run A) and other was a gradual increase in the glycerol amount in a mixture with other carbon sources including glucose, sodium acetate, and lactate (Run B). Methane production rate was similar for both experiments (approximately 21 mmol L⁻¹ d⁻¹). Dominant bacteria in Run A were closely related to Mesotoga sp., Alkalibacter sp., and Garciella sp., while the dominant bacteria in Run B were closely related to Trichococcus sp. Dominant archaea were similar for both experiments and were closely related to Methanosaeta sp. and Methanobacterium sp. From these results, it was confirmed that the microbial consortium, especially the bacterial consortium, was strongly dependent on the feeding approach of the glycerol in the anaerobic digestion system.     

Pd/Hemin-rGO as a bifunctional electrocatalyst for enhanced ethanol oxidation reaction in alkaline media and hydrogen evolution reaction in acidic media

H₂ generation needs a cost-effective, robust, stable, long-durable, and super-active electrocatalyst. This study reveals a rapid and facile method for fabricating Pd NPs on Hemin-rGO as novel support. The obtained electrocatalyst was characterized by UV–Vis, XPS, FESEM, EDS, HRTEM, and AFM. The electrochemical measurements reveal the superb effect of Hemin-rGO for enhancing the catalytic activity of Pd as bifunctional electrocatalysts for hybrid water electrolysis (hydrogen evolution reaction (HER) and ethanol electrooxidation reaction (EOR)). Pd/Hemin-rGO displays a low peak potential (−210 V) with remarkable current density (1.95 A mg⁻¹ Pd) in 0.1 M EtOH and 0.1 M NaOH. The ratio of j_f/j_b of Pd/Hemin-rGO compared with Pd electrocatalyst reveals this novel support's ani-poisoning effect. Besides, it shows the Tafel slope of 26 mV dec⁻¹ and overpotentials of 47 and 131 mV were obtained at 10 and 100 mA cm⁻² in acidic media toward HER. Exploring and designing new electrocatalysts may be enhanced by this research, which can use Hemin as a novel support for noble metals such as Pt, Pd, Rh, Au, and Ru for diverse energy-related applications.