Novel biorefinery supply chains for wastewater valorization and production of high market value bio products using microalgae

Microalgae upstream

Introduction

For stable microalgae cultivations, several environmental factors have to be taken into consideration and be carefully designed. The supply of sunlight, CO2, water and nutrients such as phosphorus or nitrogen are only a few of the various parameters that need to be satisfied for the microalgae to grow and provide the desired products. Culture conditions such as pH or temperature are also critical to be maintained in desired levels.

Illumination

Sunlight is captured by microalgae in their chloroplast and is utilized for the conversion of carbon dioxide into glucose through the Calvin cycle process. Microalgae display better photon conversion effectiveness and overall photosynthetic efficiency than land plants. Microalgae are able to direct most of their energy into cell division (6 to 12-hour cycle) allowing for rapid biomass production. Also, unlike land plants, algae are lacking supportive structures such as stems and roots that are energetically expensive to produce. The source of light can be natural or artificial (typically through the employment of LED lamps). When in natural habitats, the light conditions (daily and seasonal changes in solstice altitude, light scattering by the atmosphere etc.) of the cultivation are set by the geography of the area are subject to significant variations and are beyond any control. In cases like this, specific microalgae strains that are native and capable to adapt in these specific conditions may be preferable to others. On the other hand, it is possible with artificial lighting to have a greater design input but this flexibility comes with increased operating costs and energy consumption.

The use of LEDs at night increased microalgae production CCMAR

Sunlight is captured by microalgae in their chloroplast and is utilized for the conversion of carbon dioxide into glucose through the Calvin cycle process. Microalgae display better photon conversion effectiveness and overall photosynthetic efficiency than land plants. Microalgae are able to direct most of their energy into cell division (6 to 12-hour cycle) allowing for rapid biomass production. Also, unlike land plants, algae are lacking supportive structures such as stems and roots that are energetically expensive to produce. The source of light can be natural or artificial (typically through the employment of LED lamps). 

When in natural habitats, the light conditions (daily and seasonal changes in solstice altitude, light scattering by the atmosphere etc.) of the cultivation are set by the geography of the area are subject to significant variations and are beyond any control. In cases like this, specific microalgae strains that are native and capable to adapt in these specific conditions may be preferable to others. On the other hand, it is possible with artificial lighting to have a greater design input but this flexibility comes with increased operating costs and energy consumption.

Light intensity

The availability of light is crucial since the photosynthetic activity of the cells increases with light intensity. A lot of problems with microalgae cultivations can be traced to light intensity and availability. As the microalgae grow and multiply, the cell density increases resulting in shading of distant microalgae from the cells closer to the light source. Likewise, in cultivation systems that display high depths, the biomass in the bottom might not see as much light as the one in the surface. In cases like this, it might be necessary to increase the light intensity in order to penetrate through the culture. In a comprehensive review by Matlsev et al, the optimal values of the light intensity at which the maximum growth rate was observed for different taxonomic groups and species of algae were in the range of 26–400 μmol photons m−2 s−1.

Light duration

Also known as photoperiod, this is another critical parameter, particularly for autotrophic cultivations. A common strategy is to subject the cultivation to a long illumination period, even a constant 24-hour cycle. Various microalgae species however, have distinct preferences for light-dark patterns, and aligning the photoperiod with their natural rhythms could be crucial for maximizing biomass production. In this context, the use of appropriate light-dark (L:D) photoperiods has been reported to reduce the light energy demand with similar or even higher productivity. There are reports that shorter illumination periods (also known as flashing light effect) might lead to increased biomass production as well as reduced operation costs. The studies available in the literature provide unfortunately conflicting reports so optimization of the PBR structure in each application is necessary.

Illumination depth

Microalgae illumination at different wavelengths

Light wavelength

The wavelength of light also affects the metabolism and pigment composition of the cells. Microalgae use light of wavelengths from 400 to 700 nm for photosynthesis although this might differ depending on the species. For example, the green microalgae absorb light energy for photosynthesis through chlorophylls in the range of 450-475 nm as a major pigment absorbing light energy in the range of 450–475 nm and 630–675 nm and carotenoids as an accessory pigment absorbing light energy of 400–550 nm. As reported by Chen et al., although red light is optimal for algae growth, yellow light resulted in the highest production rate of chlorophylla, and blue light was optimal for the production of specific pigments (chlorophylla and phycocyanin) in Spirulina platensis.

Nutrients

Microalgae need a number of nutrients for their growth and survival. Like all land plants, they need a source of carbon and illumination for photosynthesis but other nutrients such as nitrogen are crucial for their development.

Carbon

Carbon is the main component of the microalgae, reaching up to 50% w/w.  Microalgae are able to utilize carbon from different sources for their growth. The use of carbon dioxide, either from the atmosphere or industrial exhaust gases, can contribute significantly to the reduction of the production costs and at the same time the reduction of the emission of this greenhouse gas. The optimal carbon dioxide feed-stock can vary significantly not only between different microalgae strains but also for the same microalga when grown in different conditions. To a certain degree, increasing the CO2 concentration can accelerate photosynthesis with concentrations up to 10% is typically applied.

Other sources of carbon tested for microalgae cultivation include glucose, acetate and propionate. In this scenario, also known as heterotrophic cultivation, the microalgae are displaying significantly higher growth rates and biomass productivity. This is accompanied however, with an additional, usually expensive, raw material (for the heterotrophic cultivation of the aforementioned Auxenochlorella protothecoides, Li et al estimated that 80 % of the cost of culture medium belongs to the supplied glucose. But it is a very interesting option for commercial applications targeting high value products such as pigments.

Nitrogen

The second most abundant element in the microalga biomass, representing up to 10% of the cell. Nitrogen is an essential building block of nucleic acids and proteins and therefore a vital macronutrient for the microalgae development and reproduction. Nitrogen can be assimilated in the forms of nitrate, nitrite, urea and ammonium with nitrate being more preferable as it is more stable. The abundance of nitrogen has a direct effect in the evolution of the cultivation. A depletion of nitrogen, also known as ‘nitrogen starvation’,  in the cultivation medium causes a decrease in growth with concomitant increases in the lipid productivities and fatty acid profile.

Phosphorus

The last most important compound for the microalgal growth, lipid production and other metabolic processes. Phosphorus is used as an energy currency in signaling and driving reactions and as a building block for nucleic acids and lipid membranes. A typical algal cell appears to be dominated by phosphate, phosphoester (monoester and diester) and polyphosphate. There are two main processes for phosphorus accumulation. One is the extracellular phosphorus adsorption during which the extracellular polymeric substances (mainly protein and carbohydrates) form complexes with phosphate which can be accounted for 16-46% of total P in microalgae. The other is the intracellular P uptake which is carried out by various phosphorus entrapping mechanisms, mainly the phosphate metabolism and the formation of polyphosphate.

Wastewater

Wastewater is an unfortunate byproduct of most urban and industrial activities. The purification of these water bodies is a necessary step before their release into the environment. This is typically achieved through costly and energy intensive chemical and biological processes. There is a drive however, in the tenets of circular economy, to valorize these wastewater streams or their treatment. One such way is through microalgae cultivation.

Nutrient and water supplies for microalgae cultivation are the major cost-contributory factors. The cultivation of microalgae at industrial scale would require a substantial amount of nutrients, mainly nitrogen and phosphorus. If chemical or organic fertilizers are used for the supply of these nutrients, it might lead to a 50% overall energy usage and GHG emissions according to some LCA analysis. A pathway to minimize cultivation costs would be to utilize wastewater as a culture medium for the microalgae. Wastewaters are typically rich in these two elements which must be removed prior their discharge to avoid eutrophication of nearby water bodies. Microalgae cultivation in wastewater thus provides a very interesting solution for these two problems. Moreover, it would help wastewater treatment plants to reduce their energy consumption by 60-80% which typically is spent in the removal of nitrate and phosphate. A final advantage would be the reduction of the amounts of water required for the cultivation, especially in areas which are already facing water sparsity.  Cultivation of microalgae in wastewaters such as brewery wastewaters, cheese-whey, piggery wastewater is well documented and research on this topic is still very intense highlighting the potential microalgae role in a circular bioeconomy.

Bibliography

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The project “Novel biorefinery supply chains for wastewater valorization and production of high market value bio products using microalgae (BlueBioChain)” Project ID 31, is part of the BlueBio ERA-NET Cofund under the European Union’s Horizon 2020 Research and Innovation programme (Grant agreement No. 817992) (Project ID 31 BlueBioChain).

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