F. G. Aden, J. M. Fernandez, E. Molina-Grima

Department of Chemical Engineering, University of Almeria, Almeria, Spain

13.2 INTRODUCTION

Microalgae have been proposed as the potential source for a wide range of products, rang­ing from fine chemicals and pharmaceuticals to nutraceuticals and additives, foods, and feeds and as a biofuel source as well as playing a role in wastewater treatment (Borowitzka, 1999; Richmond, 2000). However, of all these products and roles, only a few are performed on an industrial scale. Microalgae are produced as a source of certain carotenoids, such as p-carotene and astaxanthin; microalgae biomass is also produced as food in nutraceutial applications and as feed for aquaculture. The amount of microalgae produced worldwide for these markets is around 5 kt/year. The price of microalgae biomass ranges from €10-300/kg, and the size of these markets is from 10-50 kt/year (Pulz and Gross, 2004). The development of new applications for microalgae biomass can increase the present pro­duction capacity. Thus, large-scale markets such as energy or commodities have the potential to absorb enormous amounts of microalgae biomass—up to 104 kt/year—but the price of biomass in these markets is far lower, from €0.01-0.50/kg. For this reason, microalgae bio­mass production costs must likewise be reduced to comply with these markets (Chisti, 2007).

Even though biomass production is normally performed under continuous operation in order to maximize the system yield, some products can be produced by varying operation modes from discontinuous to continuous-discontinuous combinations, as is the case with astaxanthin. Whatever the final use of the microalgae biomass and whichever production mode is used, the steps required to produce it are the same. The culture medium has to be prepared and introduced into the photobioreactors, where the biomass is produced, then it has to be harvested and stabilized. Alternatively, it can be processed to create products according to adequate downstream schemes (see Figure 14.1). Each one of these steps requires

FIGURE 14.1 General scheme of microalgae biomass production systems. Major inputs are nutrients, water, and CO2 in addition to energy. Processes can be built to produce stabilized biomass or final products according to an adequate downstream process.

materials and energy input. In addition, waste released in each step has to be treated. Differ­ent possibilities exist for each of the necessary steps, the overall yield and cost of the finished product being a function of the final scheme used. For example, a culture medium might be prepared using fine chemicals, fertilizers, or wastes—the resultant costs using wastes being less but the final biomass quality produced significantly diminished.

In this chapter, the cost of producing microalgae biomass is reviewed for various applica­tions using various schemes. Analysis is performed based on (1) the product obtained, (2) the overall scheme of the process, and (3) the production capacity. In each case, the major factors determining total production costs are identified and strategies are discussed to reduce those costs.