13.2.1 Production of High-Value Carotenoids

Carotenoids such as astaxanthin and beta-carotene are examples of high-value products obtained from microalgae. Astaxanthin is a carotenoid that is naturally synthesized in some plants and bacteria but especially in the microalga Haematococcus pluvialis. It is widely used in aquaculture for salmon and trout farming as well as in dietary supplements (Guerin et al., 2003; Higuera-Ciapara et al., 2006).

Astaxanthin can be produced synthetically at a cost of $1,000/kg, its market size being more than $200 million per year, with the market price above $2,000/kg (Olaizola, 2003). However, because synthetic astaxanthin is derived from petrochemicals, its use is only permitted in aquaculture. It is not allowed for human consumption nor in animal feed other than in aquaculture applications, so for these other uses natural astaxanthin production is required (Li et al., 2011).

Astaxanthin is produced from Haematococcus pluvialis using a two-step strategy. First, green vegetative cells are produced under optimal growth conditions; they are then put un­der stress to trigger the accumulation of astaxanthin (Guerin et al., 2003; Olaizola, 2003). Because Haematococcus pluvialis is easily contaminated with other fast-growing strains such as Scenedesmus or Chlorella, the production is ideally performed in discontinuous mode. To enhance the process yield, repeated batches or semicontinuous cultures can be used to produce green vegetative cells, but this has always to be carried out using closed photobioreactors to avoid contamination problems. The second step is performed for a short time, 5-10 days, under nutrient deprivation conditions and high irradiance, so this step is usually carried out in cheaper open photobioreactors. Although a one-step production tech­nique has been reported at the pilot scale, no commercial production using this methodology exists (Del Riio et al., 2008; Garcia-Malea et al., 2009).

One of most extensive cost analyses carried out on astaxanthin production from Haematococcus pluvialis has recently been published (Li et al., 2011). Cost analysis data were obtained from the operation of a pilot-scale facility consisting of an 8,000 L airlift tubular photobioreactor and a 100 m2 raceway photobioreactor located in Shenzhen, China. Biomass is harvested by sedimentation and centrifugation, and then it is stabilized in a dryer, then additionally disrupted by pulverization (see Figure 14.3). The production capacity of the pilot

FIGURE 14.3 Block diagram of the process for the production of astaxanthin from Haematococcus. (Adapted from Li et al., 2011.)

plant is estimated at 140kg/year of dry Haematococcus pluvialis biomass with 2.5% astaxanthin content: meaning an astaxanthin production capacity of 3.5 kg/year. From these data, the authors extrapolate the production cost of a projected facility, located at a different location under better environmental conditions, producing 900 kg/year of astaxanthin (36 t/year of biomass); that is, a 260 times greater production capacity than demonstrated. Scale-up is performed by multiplying the number of units equal to that used on the pilot scale; thus a total of 30 airlift tubular photobioreactors and 200 raceway photobioreactors were con­sidered. From this analysis the total fixed capital required to build up the facility is close to $1.5 million, the direct production cost including manpower being close to $0.5 million/year. Thus, the expected biomass production cost is $14/kg and $718/kg for biomass and astaxanthin, respectively. These costs are much lower than usually reported for this process (which range from $2,000-3,000/kg); the authors attributing this fact to the low cost of the photobioreactors used and of manpower in China. Thus, if the same production facility were located in the United States, labor would cost approximately $600/kg of astaxanthin, com­pared to only $120/kg of astaxanthin in China.

Data from Li et al. (Li et al., 2011) demonstrated that to produce high-value biomass, the complexity of the process is greater and the use of reactors with adequate control systems is mandatory. In this case, depreciation represents more than 23% of the total production cost, although major costs relate to utilities (33%) and labor (30%) (see Figure 14.4). The utility cost is mainly a result of the facility’s high power consumption, whereby temperature is controlled by cooling the culture volume in tubular photobioreactors; the cost of water is not relevant. Raw material cost is principally due to fertilizer use, representing up to 54% of raw material cost, in addition to pure CO2, which represents up to 39%. The depreciation cost is mainly a function of tubular photobioreactor and raceway pond costs, representing 25% and 16% of total fixed capital, respectively. Machinery cost related to harvesting is low because Haematococcus pluvialis is easily separated from the supernatant by sedimentation.

From these data, it is clearly shown that a reduction in power consumption is a major factor in reducing the production cost of this facility. Consequently, one third of power consump­tion comes from the cooling of the tubular photobioreactors, one third is related to raceway power consumption, and the rest is consumed in gas supply and harvesting (including drying). Any reduction in cooling requirements or improvements in raceway reactor fluid dynamics can help significantly improve the economic viability of the system.

raw materials, the fertilizer cost cannot be reduced, so the only possibility is to reduce their consumption. Finally, with regard to the depreciation cost, it is possible to reduce the cost of tubular photobioreactors by increasing their size instead of installing multiply units. The pre­cise total production cost obtained using these improvements can be evaluated only if their viability is previously demonstrated not to influence the overall process yield, but it can reach up to 30% of total production.