Recovery of microalgae by coagulationflocculation-sedimentation and characterization of the produced pastes

In recent years there has been a microalgae culture boom, with a wide range of applications, including their use as feed for crustaceans and fishes, production of pigments, special lipids, biodiesel, wastewater treatment, biomass to be converted in methane or hydrogen, biomass for energy production by pyrolysis, and many others. In this chapter, a brief introduction to the different microalgae cell recovery processes, as well as a more specific description of the triad coagulation-flocculationsedimentation, will be presented and discussed. Finally, the importance of characterizing the produced microalgal pastes through rheology studies will be discussed also. Recovery of biomass can become a bottleneck of the entire process, due to the small size of algal cells, besides the fact that microalga culture broths are, in general, very diluted and for many processes this concentration must be increased by 200 fold. This increment in microalgae concentration can contribute with 20 to 30% of the total cost of producing biomass. Among the most suitable methods for microalgae harvesting, are centrifugation, filtration, flotation, and coagulation-flocculation. These methods present advantages and disadvantages, but it is a priority to analyze the energetic cost involved in each of these methods. Coagulation-flocculation (CF) of microalgae can be achieved in several ways and a wide range of approaches have been explored in recent years. These approaches range from traditional CF to novel ideas based on the biology of microalgae, the use of walldeficient cells and the use of emerging technologies. After CF it is very common to add a sedimentation step. The simplest equipment to carry out the sedimentation process is the sedimenter or sedimentation device. It is generally a round or square tank designed in such way that the microalgae suspension goes through the device at a speed lower than the particle`s sedimentation rate, so the residence time allows sedimentation of most of the particles. Sludge is accumulated in the base of the sedimenter. Once the microalgae recovery process has been applied, the resulting product can be a concentrated suspension of cells or rather a humid cell paste. Using some specific procedures, a more concentrated cell dry paste can be obtained. It is necessary to characterize the concentrated pastes through rheological measurements. This step can be carried out by determining viscosity vs. shear rate profiles, as well as the storage and loss modulus and complex viscosities, which, in turn, define the paste's viscoelastic behavior.