Pre-concentration strategies for microalgae harvesting as biorefinery process chain

Abstract

Over the past few decades, microalgae become focus of the researches again as a possible raw material for the production of biodiesel due to the myth of the oil crisis and also Kyoto Protocol which entered into force in 2005. Although microalgae have proven to be very efficient at producing oil-rich lipids, the optimum conditions for algae cultivation and methods for harvesting and oil extraction have not been determined in details yet. In particular, the harvesting process is especially important to the effectiveness of the overall process because of the large volumes of algae-rich water that must be processed. The requirements for downstream also makes complicated the chosen harvesting system. Several solid-liquid separation technologies have shown some potential for achieving microalgae/water separation; however, application of these processes to biofuel production requires an evaluation of treatment effectiveness as a function of water quality, algae particle characteristics and process chemistry. Pre-concentration (bulk harvesting) aims to separate the micro algae from culture medium (bulk solution). Flocculation, flotation or gravity sedimentation are the frequently used methods. Filtration, centrifugation and ultrasonic aggregation are mostly used in thickening. Of these technologies, from chemical and/or biological harvesting, by means of flocculants e.g. auto flocculation, chemical coagulation, inorganic coagulants, organic flocculants, combined flocculation, electro-coagulation and ultrasonic aggregation. Flocculation is used in order to ease the following dewatering steps in this study. Flocculation is also a factor that affects the mechanical properties of the culture. Therefore, it is important to study the flow behaviour of flocculated suspensions by additives. The flocculation mechanism determines the properties of the flocs and therefore the rheological behaviour of the suspension. The rheological properties of algae slurries have a direct impact on the agitation and pumping power requirements as well as process design for producing algal biofuels. Flocculation efficiency is commonly evaluated by measuring the settling rate of flocs, the percentage of solids settled, the sediment volume/weight, the moisture content and strength of flocs, and the suspension viscosity and turbidity in water treatment processes. Particle size analysis is usually applied in drinking water treatment for monitoring and controlling filtration process performance. Analysis of sizes of flocs formed in the coagulation and flocculation processes are not routinely conducted. However, particle size distribution (PSD) analysis can produce direct information about the flocs in the fluid. Through measuring and analysing the amount and size of particles, we can evaluate the efficiency of the process; assess design of the treatment or harvesting systems. A primary factor controlling the performance of sedimentation is the particle size distribution (PSD) of the incoming sediment. Particle size distribution information is needed to model the sedimentation process. From biological harvesting methods, auto-flocculation is one of the promising methods for microalgae. The mechanism of auto-flocculation depends on the alkaline conditions. According to several studies two major reactions are effective: the precipitation of calcium carbonate (CaCO3) and the precipitation of magnesium hydroxide (Mg (OH) 2), depending on the primary particles and the ions contained in the solution. To sum up, this thesis aims; (i) to determine a simple, rapid and an efficient pre-concentration method -preferentially cost effective- for Phaeodactylum tricornutum and Nannochloropsis gaditana through testing, (1) natural sedimentation; (2) flocculation with commercial flocculants (aluminium sulphate, polyaluminium chloride) and chitosan; and (3) pH induced flocculation. The drawbacks of the methods, state of the concentrated algae according to studied species after every studied treatment method, sedimentation rates of pH induced flocculation method were also shown; (ii) to know more about pre-concentration processes of studied species, characteristic properties of downstream process after treating the pre-concentration methods under author chosen conditions (viscosity, particle size and Ca and Mg ion determination analysis) were investigated after applying different methods of harvesting; (iii) to try a mathematical model and a numerical simulation procedure for the flocculation process, using usual parameterization solutions to estimate the sedimentation rate and floc size distribution for pre-concentration process. The main objective is estimating sedimentation rate coupled with that of the particle size distribution (PSD), through a population balance equation (PBE). From the thesis we could conclude; the selection of the proper harvesting method depends mainly on the process product targets. To decide which method is better for harvesting, it should be checked not only how efficient the agent as a flocculant and how appropriate to provide a cost effective harvesting, but also how feasible to use the agent in the case of using concentrated biomass in the further processes without any problem and managing the residual water after harvesting. Depending on the chemical properties of the seawater and on the properties of the outer surface of the algae, pH induced flocculation seems promising for P. tricornutum and N. gaditana species. It is effective, economical and eco-friendly. Sedimentation rates of the author chosen method are also helpful to scale-up the process. The process is also very reproducible. It is observed that after a certain pH, increasing the pH didn’t change the flocculation efficiency, however changed the mass content of the concentrated sample (higher ash content). The comparative study of pre-concentrated sample properties shows that; (I) Pre-concentration processes make both pumping and mixing easier because of the Newtonian behaviour of the samples than Non-Newtonian ones. (II) Particle size analysis of the pre-concentrated samples support settling properties sufficiently. (III) Ca and Mg ion concentrations of the pre-concentrated samples substantiate the fact that the Mg ion is the protagonist in the alkalinity-induced flocculation mechanism. A population balance model (PBM) has been tried to build to predict the floc size distribution for P. tricornutum and N. gaditana. The model was ineffective to simulate alkalinity induced flocculation -a mimic of autoflocculation in lab scale. High shear rates were needed in the model to let the floc to appear. The mathematical model for flocculation is still a framework that should be developed. Additionally, the model needs to be further adjusted and integrated with the sedimentation model simultaneously with PBM.