A review on microalgae and cyanobacteria in biofuel production

Today, fossil fuel shortages and climate change impacts have led mankind to the search for an alternative energy. With many advantages, bioenergy is a promising source to replace conventional energy. However, biofuel productions in the first and second generation are likely to add more concerns to the problems in water scarcity and threats to food security. Meanwhile, third generation biofuels obtained from microalgae and cyanobacteria are able to overcome existing challenges thanks to their rapid growth rates, abilities to fix CO 2 , high yields in lipid extraction and capabilities to be grown in non-arable lands. Microalgae and cyanobacteria appear to be the only ones among renewable sources that are capable of producing a wide range of biofuels including biohydrogen, biomethane, bioethanol and biodiesel.In this study, we present an overview about microalgae and cyanobacteria use for the production of biofuels in fundamentals, including their biology, cultivation systems taking into account the hydrodynamic conditions, harvesting, and processing. The review also provides a general picture at the current status of this renewable energy industry

Microalgae for municipal wastewater nutrient remediation: mechanisms, reactors and outlook for tertiary treatment

This review explores the use of microalgae for nutrient removal in municipal wastewater treatment, considering recent improvements in the understanding of removal mechanisms and developments of both suspended and non-suspended systems. Nutrient removal is associated to both direct and indirect uptake, with the former associated to the biomass concentration and growth environment (reactor). Importantly, direct uptake is influenced by the Nitrogen:Phosphorus content in both the cells and the surrounding wastewater, with opposite trends observed for N and P. Comparison of suspended and non-suspended systems revealed that whilst all were capable of achieving high levels of nutrient removal, only non-suspended immobilized systems could do so with reduced hydraulic retention times of less than 1 day. As microalgae are photosynthetic organisms, the metabolic processes associated with nutrient assimilation are driven by light. Optimization of light delivery remains a key area of development with examples of improved mixing in suspended systems and the use of pulsating lights to enhance light utilization and reduce costs. Recent data provide increased confidence in the use of microalgae for nutrient removal in municipal wastewater treatment, enabling effluent discharges below 1 mg L−1 to be met whilst generating added value in terms of bioproducts for energy production or nutrient recovery. Ultimately, the review suggests that future research should focus on non-suspended systems and the determination of the added value potential. In so doing, it is predicted that microalgae systems will be significant in the delivery of the circular economy

Pilot Scale of Microalgal Production Using Photobioreactor

Microalgal gained much interest as a promising sustainable feedstock for the production of food, feed, bulk chemicals and biofuels. Pilot scale of microalgal is needed to bridge the gap between laboratory scale research and commercial application. Commercial applications of microalgal have been used for a wide array of functions including, pharmaceutical, health sector, nutraceutical, cosmetics and agriculture. Numerous photobioreactors (PBRs) of different volume and shapes have been designed. Cost of PBR has a major influence on production cost for large scale biomass. There are several ways to reduce production cost depends on the type of algal strain, type of PBRs, CO2 and the production technology of the biomass. Dilution rate is an important factor, which affects the biomass productivity, rate and ultimately what needs to be maximized

Establishment of an effective photobioreactor for growing microalgae: A review

The premise that microalgae could be used to produce landscapes of biofuel, nutrition, and bioremediation is gaining popularity. The four main factors influential to microalgae growth are light, CO2, nutrients, and process conditions-including temperature and pH. Compared to other open systems such as ponds, control and efficiency in flat plate and tubular type photobioreactors are much higher. A photobioreactor needs to be developed to enhance the mass transport, and light penetration, and to reduce contamination. Every kind of photobioreactor has its advantages and limitations in using the airlift, bubble column, and stirred tank. Thus, the use of hybrid bioreactors makes it possible to eliminate individual limitations. This review discusses and analyzes the features of photobioreactor systems, their drawbacks, and the progress achieved in the field of microalgae production. Int. J. Agril. Res. Innov. Tech. 14(2): 153-162, December 202

Advantages and challenges of microalgae as a source of oil for biodiesel

Microalgal oil is currently being considered as a promising alternative feedstock for biodiesel. The present demand for oil for biofuel production greatly exceeds the supply, hence alternative sources of biomass are required. Microalgae have several advantages over land-based crops in terms of oil production. Their simple unicellular structure and high photosynthetic efficiency allow for a potentially higher oil yield per area than that of the best oilseed crops. Algae can be grown on marginal land using brackish or salt water and hence do not compete for resources with conventional agriculture. They do not require herbicides or pesticides and their cultivation could be coupled with the uptake of CO2 from industrial waste streams, and the removal of excess nutrients from wastewater (Hodaifa et al., 2008; An et al., 2003). In addition to oil production, potentially valuable co-products such as pigments, antioxidants, nutraceuticals, fertilizer or feeds could be produced (Mata et al., 2010; Rodolfi et al., 2009)

Luminescent solar concentrators to increase microalgal biomass productivity

Light is the main limiting factor of any mass microalgal cultivation resulting in relatively low biomass productivity in raceway ponds. Microalgal cells in open ponds are normally photoinhibited on the surface and photolimited at the depth of the cultures where there is total darkness. Delivering light to the microalgal cells at the depth of cultures in large scale raceway ponds can increase biomass productivity. Luminescent solar concentrators (LSCs) can potentially be an economical light-diffusing system to be used in algal biotechnology. The main advantage of luminescent solar concentrators is that a solar tracking system is not needed. This results in less cost compared to other diffusing systems. Luminescent particles such as organic dyes or quantum dots (QDs) are the main constituents of LSCs. Luminescent particles absorb photons when light hits the surface of LSCs and the absorbed light is reflected internally and emitted from the edges at a longer wavelength. To the best of my knowledge, to date, there have been no attempts in using LSCs as a light guide for the growth of microalgae in any open system. Thus, the main aim of this study was to evaluate the effect of LSCs as a light guide to deliver light to the depth of microalgal cultures in raceway ponds to increase both biomass and high-value productivities. To assess the viability and efficacy of the LSCs system in an algal raceway pond, it is first necessary to select the most suitable microalgae species for this purpose. Three species, Arthrospira platensis (MUR 129), Scenedesmus sp. (MUR 268) and Chlorella sp. (MUR 269). were chosen for a laboratory experiment to investigate the effect of red and blue LSCs on the productivity of cultures. Arthrospira platensis showed up to 9% higher productivity when red LSCs were used compared to control and blue LSCs. The biomass productivity of Scenedesmus sp. cultures under red LSCs was also 30% and 4.5% higher compared to that in control and blue LSCs. The growth rate of Chlorella sp. cultures did not improve under red and blue LSCs. Furthermore, Scenedesmus sp. culture resulted in 30% higher cell density in cultures with red LSCs compared to that in control. Thus, Arthrospira platensis and Scenedesmus sp. were chosen as the most suitable species for further outdoor investigations using micro raceway ponds. In the next stage, Arthrospira platensis and Scenedesmus sp., were grown using red and blue LSCs and compared with control cultures with no LSCs using micro raceway ponds (0.1 m2) with the final culture volume of 21.5 L. The LSCs were installed on the edge of raceway ponds to have 200 mm of a panel inside the raceway pond and 100 mm of the panel out of the pond facing the sun to collect visible and diffuse light from sunlight, downgrade and, transfer it to the depth of A. platensis cultures. The bottom part of LSCs inside the A. platensis culture was also laser-cut to have enough surface area to increase the irradiance. Arthrospira platensis cultures when grown with red LSCs, reached a significantly higher biomass yield (1.77 ± 0.014 g L−1) compared to control (1.53 ± 0.002 g L−1) and blue LSCs (1.59 ± 0.056 g L−1). The biomass productivity of 57 ± 3.2 mg L−1 d−1 (12.2 g m−2 d−1) was obtained when Arthrospira cultures in raceway ponds were equipped with red LSCs. This was 24% and 26% higher than the biomass productivity of Arthrospira cultures when grown in raceway ponds with blue LSCs and control. There was no significant difference between the productivity of Arthrospira cultures with blue LSCs and control. Furthermore, the maximum phycocyanin productivity in Arthrospira cultures with red LSCs was 8.49 ± 0.9 mg L−1 d−1, which was 14% and 44% higher than that in cultures with blue LSCs and control cultures. In addition, the phycocyanin content of A. platensis was 136 mg L−1 (77 mg gbiomass−1) and 141 mg L−1 (89 mg gbiomass−1) under red and blue LSCs, respectively. The results of showed that red LSCs can significantly increase Arthrospira’s growth and productivity. Based on the outcome of this study, only red LSCs were applied to outdoor Scenedesmus sp. cultures in the next experiment. When grown with red LSCs, Scenedesmus sp. cultures reached a higher cell density compared to the control. Furthermore, the maximum specific growth rate (µ) of Scenedesmus sp. cultures with red LSCs was 16% higher than control with no LSCs. The biomass productivity of 43.6 ± 1.3 mg L-1 d-1 (9.4 g m-2 d-1) was obtained for Scenedesmus sp. cultures equipped with red LSCs which was 18.5% higher than that for Scenedesmus sp. cultures when grown in raceway ponds with no LSCs. Further, the protein content of Scenedesmus sp. under red LSCs was 436 mg gbiomass-1 (43.6%) which was 17.5% higher than that in control. The lipid content of Scenedesmus cultures under red LSCs (133 mg gbiomass-1) was also 10% higher compared to control with no LSCs. However, the carbohydrate content of Scenedesmus sp. cultures with red LSCs and control was not significantly different. The results of all indoor and outdoor experiments showed that using red LSCs on Arthrospira platensis and Scenedesmus sp. cultures was promising. More light availability to microalgal cells into the depth of the cultures is the most likely reason for having higher productivity in cultures with red LSCs. From the energy perspective, the results showed that the total amount of photosynthetic active radiation (PAR) available for A. platensis and Scenedesmus sp. cells at the depth of each pond emitting from four red LSCs is 34 µmol photons s−1. In other words, using red LSCs in each outdoor raceway pond bring about 34 µmol photons s−1 more light to the depth of A. platensis and Scenedesmus sp. cultures. This means injecting 34 µmol photons s−1 deep into the A. platensis and Scenedesmus sp. cultures where it would otherwise be in full darkness. This helps move the light from the photosaturated surface to the depth of the microalgal cultures. Moreover, based on the mixing rate, the thickness of the LSCs and surfaces of each red LSC, A. platensis and Scenedesmus sp. cells received brief bursts of light when they pass an edge and a surface of LSCs. For instance, considering PAR emitting from an edge of a red LSC (110 Wm−2/506 µmol photons m−2 s−1), A. platensis and Scenedesmus sp. cells received around 506 µmol photons m−2s−1 in 27 ms from each edge and 276 µmol photons m−2 s−1 in 218 ms when they pass each surface of a red LSC. In other words, it can be said that A. platensis and Scenedesmus sp. cells with red LSCs received brief bursts of light with different intensities for durations less than a second inside the cultures while there was total darkness for the cultures without LSCs. Finally, the costs of biomass and phycocyanin production using luminescent solar concentrators as a light delivering system on an industrial scale raceway pond cultivation of Arthrospira was assessed. The results showed that using red luminescent solar concentrators would result in a biomass and phycocyanin production costs of AU$3.16 and AU$ 125 per kg, respectively, which are 14% and 35% lower than the corresponding costs in a conventional raceway pond with no LSCs. The biomass and phycocyanin production costs of Arthrospira cultivation in conventional raceway ponds (with no LSCs) were AU$3.67 and AU$ 187 per kg, respectively. These results showed that using LSCs for growing Arthrospira can significantly lower the cost of biomass and phycocyanin production if the same size production facility is used. In conclusion, this study clearly showed that using LSCs in a raceway open ponds can be a promising method to increase the biomass productivity of a microalgal culture while reducing the production costs of biomass and the desired high-value product

Optimal periodic resource allocation in reactive dynamical systems: Application to microalgal production

In this article, we focus on a periodic resource allocation problem applied to a dynamical system which comes from a biological system. More precisely, we consider a system with $N$ resources and $N$ activities, each activity use the allocated resource to evolve up to a given time $T > 0$ where a control (represented by a given permutation) will be applied on the system to reallocate the resources. The goal is to find the optimal control strategies which optimize the cost or the benefit of the system. This problem can be illustrated by an industrial biological application, namely, the optimization of a mixing strategy to enhance the growth rate in a microalgal raceway system. A mixing device, such as a paddle wheel, is considered to control the rearrangement of the depth of the algae cultures, hence the light perceived at each lap. We prove that if the dynamics of the system is periodic, then the period corresponds to one reallocation whatever the order of the involved permutation matrix is. A nonlinear optimization problem for one reallocation process is then introduced. Since $N!$ permutations need to be tested in the general case, it can be numerically solved only for a limited number of $N$. To overcome this difficulty, we introduce a second optimization problem which provides a suboptimal solution of the initial problem, but whose solution can be determined explicitly. A sufficient condition to characterize cases where the two problems have the same solution is given. Some numerical experiments are performed to assess the benefit of optimal strategies in various settings.Comment: International Journal of Robust and Nonlinear Control, Wiley, 202