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Producing high productivity microalgae with flexible composition

October 2, 2017 — by Mohammad-Matin Hanifzadeh

Editor’s note: Matin is a PhD graduate research assistant at the University of Toledo. He has worked for more than five years on research related to improving economic and environmental sustainability of algae production. During his research, he was able to reduce 50-80% of the cost of biodiesel production from microalgae through developing (1) a cultivation technology for continuous high rate production of lipid and starch by optimizing nutrient input, and (2) low cost harvesting by manipulating the particle size. His recent research was scaled up from 3 L indoor to 30 L and 2000 L systems. Out of these research studies, several publications were submitted and a patent was filed. He received several awards including the Young Researcher award from the Algal Biomass Organization. Most recently a proposal was submitted, based on his research projects, to DOE which won a $2.4 million federal grant.

Fig. 1. Outdoor sequential batch cultivation experiments with various initial-N concentrations (5-27 mg/L).

Ijoined Dr. Viamajala’s lab in 2013 working on improving economic and environmental sustainability of algal biodiesel production. My research (funded by DOE and NSF) has focused on addressing the current challenges of the upstream processing (cultivation and harvesting):

  1. eliminating cost and siting limitations posed by CO2 availability and delivery,
  2. maintaining sustained productivity of desirable species without contamination by other microorganisms,
  3. producing biomass with a desired composition (e.g. high lipid content for biofuel production), and
  4. cost- and energy-efficient recovery of biomass that also allows reuse of media.

Within our group, we have isolated a high productivity microalgae strain (Chlorella sorokiniana str. SLA-04) that thrives in extreme pH conditions (>10) and is thus resistant to detrimental contamination. In previous work in our group, a pH> 10 growth medium containing high media alkalinity (>100 mM) was developed. In this medium, a non-limiting concentration of bicarbonate for photosynthetic carbon fixation can be maintained while simultaneously allowing a high rate of CO2 mass transfer from the atmosphere.

Cultivation experiments with this medium showed long term (> 6 months) high biomass productivity under outdoor conditions without culture crash and CO2 supplementation. Building upon previous works, the additional goals of my research was to design cultivation strategies that allow flexibility in biomass composition (for fuel, feed, or nutritional products) without compromising productivity. Further, we enhanced the environmental/economic sustainability of microalgae production by (a) assessing the potential for cultivation of str. SLA-04 in low quality water, and (b) investigating low-cost harvesting using natural sedimentation.

These specific aspects in this project are described below:

Aspect #1: Modulation of biomass composition by manipulation of media components

Biomass composition is strongly influenced by media components. Specifically, our preliminary experiments have shown that changing the concentration of N, Mg or Ca in the media is effective in altering biomass composition.

N is a macronutrient and N content in biomass can determine the end-use of microalgae. For instance, high N-content (i.e. high protein) is desirable for microalgae use as a food/feed ingredient. However, for biofuel production low N in biomass is desirable since presence of N in fuel is detrimental to fuel quality. Conventional cultivation methods use a high concentration of N in the medium which leads to production of biomass with high N content.

However, our studies have shown that cultivation of str. SLA-04 in a low N media significantly lowers biomass N- and increases lipid- and carbohydrate-content with a relatively low impact on biomass productivity. In this regard, we propose cultivation with different N input (5 – 27 mg/L) as a strategy to yield biomass with flexible composition (suitable for fuel, food or nutraceutical use). Our experiments showed that cultivation in sequential batches (each batch of duration 2-3 days) allows application of this strategy without compromising the biomass productivity (Fig.1).

Later, we scaled up the cultivation experiments to 1000 L ponds where the total initial chlorophyll concentration was normalized for the cultures with various N-content. Our results showed higher biomass productivity for the cultures with low N- and high lipid- /carbohydrate-content relative to high N cultures. So, our studies suggest that the efficient light absorption by microalgae cells and therefore higher biomass, lipid and carbohydrate productivities can be achieved through optimizing N input during cultivation.

Fig. 2. Efficient light absorption into the cultures with low

While the effects of macronutrients have been reported in literature, the effects of micronutrients on microalgae have not been well investigated. Standard media recipes usually contain excess Mg and Ca. We have shown that microalgae cultivation under Mg or Ca limitation resulted in similar or higher biomass productivities and significantly higher lipid/starch concentrations relative to a micronutrient-excess medium. The results suggest that optimization of micronutrient concentrations in the medium can be employed as strategy to improve the overall biomass, lipid and starch productivity for biofuel production.

Aspect #2: Saline/wastewater use

Lowering fresh water requirements is critical to sustainable microalgae cultivation. Saltwater is a more sustainable water source than fresh water. In our studies, we observed that high salinity (30 g/L) didn’t impede growth of str. SLA-04; on the contrary, a modest improvement in productivity was observed. The partial improvement in biomass productivity can be attributed to (a) better light penetration into the culture as the result of lower cell pigmentation and (b) improved CO2 absorption (higher Henry’s by increase in salinity).

Fig. 3. Cultivation experiment in outdoor raceway ponds using diary waste as nutrient source.

In addition to salt water, we were able to obtain similar biomass and lipid productivity from str. SLA04 by using dairy waste as the nutrient source (Fig.3). Our results showed that cultivation in wastewater at pH (>9.5) allow sustainable high biomass productivity without any culture crash/contamination and any CO2 supplementation.

Aspect #3: Harvesting of SLA-04 using low-cost natural sedimentation

In addition to cultivation, developing cost-efficient harvesting methods would also significantly improve economic sustainability of microalgae upstream processes. In sedimentation experiments with SLA-04 performed on both small scale (1 L graduated cylinder) and large scale (500 L tank), we observed much higher settling rates than natural sedimentation rates reported for other species. We hypothesized that the improvement in natural sedimentation with SLA-04 was due to larger particles diameter as a result of partial flocculation of the cultures grown in high pH. Our experiments also showed that the relatively long sedimentation didn’t cause the biomass deterioration due to low chance of microbial contamination in the extreme pH medium.

Fig.4. Schematic of proposed cultivation/harvesting of alkaliphilic microalgae

In the future, we plan to scale up cultivation in sequential batches using waste/saline water and optimized nutrient input to commercial scales. In such systems, culture at the end of the light cycle would be transferred to a settling tank. The overnight sedimentation allows partial harvesting of microalgae without interference with the daily growth cycle. The settled biomass at the bottom of tank would be further harvested using conventional harvesting methods (centrifugation). The sedimentation effluent can be used as the starter culture for the next growth batch (Fig.4).

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