Pieter Oostlander

In this thesis the cost price of microalgae production in aquaculture hatcheries and the effect of cost reduction strategies on the biomass quality is studied. The cost price of microalgae production in aquaculture hatcheries and cost reduction strategies is described with a techno-economic model in. Using Rhodomonas sp. as an example species for microalgae production in aquaculture, the effect of growth conditions on the biomass yield on light and the biomass quality is studied on both laboratory scale and pilot scale.

The cost price of microalgae in aquaculture

A techno-economic model to describe the cost price for microalgae cultivation in aquaculture hatcheries was developed (chapter 2). The model is based on inputs from hatcheries in the Dutch aquaculture industry using commercially available microalgae production systems. The techno-economic model is used to calculate the cost price of microalgae production in modular production systems (bubble-columns), and scalable production systems (tubular photobioreactors). On a production scale presently applied in Dutch aquaculture hatcheries (125 kg year-1) the commonly used modular reactor systems result in a biomass cost price of €587 kgDW-1 for production under artificial light and €573 kgDW-1 using sunlight. The scalable production systems show lower production costs with €290 kgDW-1 with artificial light and €329 kgDW-1 under sunlight conditions. Three cost reduction strategies with a high impact on the cost price were identified: 1) increasing biomass yield on light, 2) applying more artificial light and 3) reducing labor requirements. The cost price can be reduced to €96 kgDW-1 by implementation of cost reduction strategies at the same scale (20 m2) using scalable production under artificial light conditions. Production of biomass at a larger scale (1500 m2) using scalable production systems combined with cost reduction strategies can result in a cost price of €23,47 kgDW-1 at a production scale of 6892 kg year-1. Implementation of scalable production systems in aquaculture is important for efficient cost production of microalgae. Modular systems always result in a high costs.

Light and temperature on Rhodomonas sp.

In chapter 3 the optimization of Rhodomonas sp. at lab-scale, under continuous cultivation conditions is described. This is the first study describing continuous cultivation of Rhodomonas sp.. The effect of both light and temperature on the growth rate, biomass production rate, biomass yield on light and the fatty acid content and composition of Rhodomonas sp. was studied. A wide range of light intensities (60-600 µmol m-2 s-1) combined with a wide range of temperatures (15-30 °C) was studied. The results describe growth rates of > 1.0 d-1, with biomass production rates up to 1.5 g l-1 d-1. The highest biomass yield on light (0.87 g mol-1) is found at a temperature of 22-24 °C and light intensity of 110-220 µmol m-2 s-1. The total fatty acid content fluctuates between 8-10% of the dry-weight with an EPA+DHA concentration of 14-25% of the total fatty acids. The total fatty acid content and EPA and DHA content of the cells was only influenced by the cultivation temperature with higher EPA and DHA content at lower temperatures.

Fatty-acid content of Rhodomonas sp. under day:night cycles of light and temperature

In chapter 4 the effect of daily oscillations of temperature and light on the biomass yield on light and on the biomass fatty acid content and profile was studied. Synchronized cultures of Rhodomonas sp. were found under day:night oscillations. Under the optimized growth conditions for biomass yield on light as described in chapter 3 the oscillations of both light and temperature in a 16:8 day:night cycle did not result in an increase of the biomass yield on light. At higher light conditions (600 µmol m-2 s-1) a 22% increase of the biomass yield on light was found with a day:night cycle compared to continuous light conditions. In a day:night cycle with daily oscillations for light and temperature the fatty acid content and compositions of the cells varied greatly with the moment of the day. Highest total fatty acid concentrations (91 ± 4 mg gDW-1) were found in the first hours after sunrise whereas the highest EPA+DHA content (16 ± 1 mg gDW-1) is found at the end of the dark period with a lower temperature.

Outdoor cultivation of Rhodomonas sp.

In chapter 5 Rhodomonas sp. was produced using pilot-scale tubular photobioreactors to study the effect of production at large scale under sunlight conditions on the biomass yield on light. Successful cultivation of Rhodomonas sp. at pilot-scale using sunlight conditions is described, for the first time in literature. Using three tubular photobioreactors with a working volume of 200L each, Rhodomonas sp. was produced over a period of 6 months, from February till July representing all sunlight conditions available in the Dutch climate. An average biomass yield on light of 0.29 ± 0.16 g mol-1 was obtained, which is lower that the yields obtained in the laboratory experiments (up to 0.89 g mol-1). The results show the potential of Rhodomonas sp. as a production species for aquaculture industry. The biomass production rates obtained (< 0.25 g l-1 d-1) were lower than those obtained in the lab experiments (< 1.5 g l-1 d-1). Further optimization of Rhodomonas sp. production at pilot scale seems to be possible. For that, the effect of high light intensities on the growth of Rhodomonas sp. should be studied at lab scale. New lab experiments with high light intensities could reveal the potential for and higher biomass production rates at large scale under sunlight conditions. General discussion In chapter 6 the integration of cost reduction strategies as proposed in chapter 2 with experimental lab data from chapters 3, 4 and 5 is discussed. With lab data on the effect of cost reduction strategies on the biomass yield on light and the biomass fatty acid content and composition a more realistic view on cost reduction potential is described. The combination of data of the combined effect of light and temperature on the biomass yield on light (chapter 3) and the techno-economic model (chapter 2) shows that optimization of the growth parameters towards cost efficient production of a strain can result in large cost reductions. However, the most cost-efficient production is not obtained at growth conditions for maximal biomass yield on light nor at the conditions where maximal biomass productivity is maximal. The most cost efficient growth conditions for Rhodomonas sp. production using scalable production systems and artificial light at a scale of 100m2 is at a temperature of 23-25 °C and light levels between 400-500 µmol m-2 s-1. Chapter 3 is the first study in which Rhodomonas sp. production is described under these growth conditions. In addition, the increased biomass yield on light resulting from the implementation of a day:night cycle (chapter 4) does not result in a cost reduction if applied at scales typically applied at aquaculture hatcheries, or with the modular production systems. A cost reduction (up to 10%) can be achieved at a scale >250m2 when using scalable production systems. Considering all experimental data and inputs on the techno-economic model it is concluded that the production of microalgae for aquaculture hatcheries should be performed under controlled growth conditions using artificial light and scalable production systems. The implementation of a centralized microalgae production facility will result in a great cost price reduction. A cost reduction of 80% can be achieved if algae production of ten hatcheries is combined in a production facility utilizing scalable production systems, compared to individual hatcheries maintaining a modular microalgae production facility.

The combination of the techno-economic model with laboratory data is proven as a powerful method for optimization of cost efficient microalgae production in aquaculture. This method can be applied to any microalgae species of which the effect of growth conditions on the biomass yield on light are known and for any production scenario based on scalable or modular production systems.