Outdoor pilot production of Nannochloropsis gaditana : Influence of culture parameters and lipid production rates in flat-panel photobioreactors

Preventing biofouling in microalgal photobioreactors

Photobioreactors (PBRs) are used to grow the light-requiring microalgae in diverse commercial processes. Often, they are operated as continuous culture over months period. However, with time, biofouling layer develops on the inner surfaces of their walls. The fouling layer formation deteriorates the PBR performance as foulants reduce light penetration in it. Light is essential for photosynthetic cultures, and a deterioration in lighting adversely impacts algae growth and biomass productivity. Fouling requires a frequent shutdown to clean the PBR and add to the environmental impact of the operation by generating many wastewaters contaminated with the cleaning chemicals. Antibiofouling coatings could be used to modify the surfaces of existing and future PBRs. Therefore, transparent and non-toxic fouling-release coatings, produced using hydrogel technology, could transform the existing PBRs into efficient and enduring microalgae culture systems, requiring only the application of the coating to the inner walls, without additional investments in new PBRs.

Perspectives on cultivation and harvesting technologies of microalgae, towards environmental sustainability and life cycle analysis

The development of algae is seen as a potential and ecologically sound approach to address the increasing demands in multiple sectors. However, successful implementation of processes is highly dependent on effective growing and harvesting methods. The present study provides a complete examination of contemporary techniques employed in the production and harvesting of algae, with a particular emphasis on their sustainability. The review begins by examining several culture strategies, encompassing open ponds, closed photobioreactors, and raceway ponds. The analysis of each method is conducted in a systematic manner, with a particular focus on highlighting their advantages, limitations, and potential for expansion. This approach ensures that the conversation is in line with the objectives of sustainability. Moreover, this study explores essential elements of algae harvesting, including the processes of cell separation, dewatering, and biomass extraction. Traditional methods such as centrifugation, filtration, and sedimentation are examined in conjunction with novel, environmentally concerned strategies including flocculation, electro-coagulation, and membrane filtration. It evaluates the impacts on the environment that are caused by the cultivation process, including the usage of water and land, the use of energy, the production of carbon dioxide, and the runoff of nutrients. Furthermore, this study presents a thorough examination of the current body of research pertaining to Life Cycle Analysis (LCA) studies, presenting a perspective that emphasizes sustainability in the context of algae harvesting systems. In conclusion, the analysis ends up with an examination ahead at potential areas for future study in the cultivation and harvesting of algae. This review is an essential guide for scientists, policymakers, and industry experts associated with the advancement and implementation of algae-based technologies.

Transparent antibiofouling coating to improve the efficiency of Nannochloropsis gaditana and Chlorella sorokiniana culture photobioreactors at the pilot-plant scale

The implementation of industrial-scale facilities for microalgae cultivation is limited due to the high operation costs. One of the main problems in obtaining an efficient and long-lasting microalgae culture system is biofouling. The particular issue when developing antibiofouling surfaces for microalgae cultures is that the material must be transparent. The main purpose of this work was to evaluate the antibiofouling efficiency of a non-toxic polydimethylsiloxane-based coating prepared with polyethylene glycol-based copolymer on different photobioreactors at the pilot-plant scale. The antifouling properties result from the development of a fouling-release coating utilizing hydrogel technology. Nannochloropsis gaditana and Chlorella sorokiniana were cultured outdoors for 3 months over the summer, when biofouling formation is at its highest due to environmental conditions, to test the coating's antibiofouling efficiency. Although biofouling was not completely prevented in either photobioreactor, the coating significantly reduced cell adhesion compared to the polydimethylsiloxane control (70% less adhesion). Therefore, this coating was shown to be a good alternative for constructing efficient closed-photobioreactors at the pilot-plant scale, at least for cultures lasting 3 months.

Competitive algae biodiesel depends on advances in mass algae cultivation

The aim of this review was to study why, despite large investments in research and development, algae biodiesel is still not price competitive with fossil fuels. Microalgal production was confirmed to be a critical cost item (84 up to 93 %) for biodiesel regardless of the production technology. Techno-economic assessment revealed the main cost drivers during mass cultivation. It is argued that a breakthrough in the cultivation efficiency of microalgae is identified as a necessary condition for achieving price-competitive microalgal biodiesel. The key bottlenecks were identified as follows: (1) light and O₂ concentration management; (2) overnight respiratory loss of oil. It is concluded that most of the research on microalgae biodiesel yields economically over-optimistic presumptions because it has been based on laboratory scale experiments with a low level of interdisciplinary overlap.

In situ monitoring of chlorophyll a fluorescence in Nannochloropsis oceanica cultures to assess photochemical changes and the onset of lipid accumulation during nitrogen deprivation

In situ chlorophyll a fluorescence measurements were applied to monitor changes in the photochemical variables of Nannochloropsis oceanica cultures under nitrogen-deplete and nitrogen-replete (control) conditions. In addition, growth, lipid, fatty acid, and pigment contents were also followed. In the control culture, growth was promoted along with pigment content, electron transport rate (ETR), and polyunsaturated fatty acids, while total lipid content and fatty acid saturation level diminished. Under nitrogen-deplete conditions, the culture showed a higher de-epoxidation state of the xanthophyll cycle pigments. Fast transients revealed a poor processing efficiency for electron transfer beyond Q_A , which was in line with the low ETR due to nitrogen depletion. Lipid content and the de-epoxidation state were the first biochemical variables triggered by the change in nutrient status, which coincided with a 20% drop in the in situ effective quantum yield of PSII (ΔF'/F_m '), and a raise in the V_j measurements. A good correlation was found between the changes in ΔF'/F_m ' and lipid content (r = -0.96, p < 0.01). The results confirm the reliability and applicability of in situ fluorescence measurements to monitor lipid induction in N. oceanica.

Effects of different nitrogen sources and light paths of flat plate photobioreactors on the growth and lipid accumulation of Chlorella sp. GN1 outdoors

To assess the potential of Chlorella sp. GN1 for producing biodiesel raw materials in flat plate photobioreactors (FPPs) outdoors, we optimized the nitrogen sources and concentrations for the growth of the algae. The effects of different light paths of FPPs on the growth, lipid accumulation, and fatty acids of Chlorella sp. GN1 were also studied. As the light path of the FPPs was reduced, the alga could accumulate lipids rapidly, achieving high lipid content and lipid productivity outdoors. The highest lipid content obtained was 53.5%, when the light path was 5 cm. In addition, the lipid productivity was 66.7 mg L^-1 day^-1. The main fatty acids were C16/C18, accounting more than 90% of the total fatty acids. Results showed that Chlorella sp. GN1 had the ability to accumulate large quantities of lipids in FPPs outdoors and was a promising microalgal species for biofuel production.

Factors Affecting Microalgae Production for Biofuels and the Potentials of Chemometric Methods in Assessing and Optimizing Productivity

Microalgae are swift replicating photosynthetic microorganisms with several applications for food, chemicals, medicine and fuel. Microalgae have been identified to be suitable for biofuels production, due to their high lipid contents. Microalgae-based biofuels have the potential to meet the increasing energy demands and reduce greenhouse gas (GHG) emissions. However, the present state of technology does not economically support sustainable large-scale production. The biofuel production process comprises the upstream and downstream processing phases, with several uncertainties involved. This review examines the various production and processing stages, and considers the use of chemometric methods in identifying and understanding relationships from measured study parameters via statistical methods, across microalgae production stages. This approach enables collection of relevant information for system performance assessment. The principal benefit of such analysis is the identification of the key contributing factors, useful for decision makers to improve system design, operation and process economics. Chemometrics proffers options for time saving in data analysis, as well as efficient process optimization, which could be relevant for the continuous growth of the microalgae industry.