INTRODUCTION

Recently, the use of microalgae for obtaining biofuel has been investigated 1-4. There are different steps involved in the process of biodiesel production from microalgae: cultivation, harvesting, lipids extraction, and transesterification 5. All steps are suitable in order to improve the process.

Effects of cultivation conditions on the growth, total lipid content and fatty acid profiles have been analysed for different varieties of Pavlova microalgae. These studies show a maximum content in lipids of 36% of Pavlova Lutheri 6 and 32.10% for Pavlova Viridis 7, 8.

Once the microalgae have been cultivated, they must be separated from the water in a time as short as possible. It is necessary to develop technologies that enable effective separation, in terms of percentage of recovered microalgae and recovery time. To choose a separation method is necessary to consider different parameters such as the type of microalgae (size, density), operating conditions (temperature, pH), and the value of the products. Methods for culture dewatering include filtration, sedimentation, flocculation, centrifugation, or electrolytic filtration 9.

The flocculation is produced by addition of a chemical substance, which helps the solute particles to adhere together. These substances are called flocculants, and there are different types: inorganic metal salts 10, organic polymer 11, microbes 12, 13. Most microalgae cells have a small size, between 5 and 50 μm. These cells form stable suspensions in medium. The stability depends on forces interacting between the microorganisms themselves and between them and the water 14. In conventional coagulation–flocculation–sedimentation, a coagulant is added to create an attraction among the suspended particles. The mixture is slowly stirred to induce particles to join together into flocs. Then, the agitation is stopped and the flocs settle down.

Electrocoagulation–electroflotation (ECF) consists of applying electrical current to treat and flocculate contaminants in absence of flocculant. Electrocoagulation is a technique where metal anodes are used as coagulants. Using the principles of electrochemistry, the cathode is oxidized (loses electrons), while the water is reduced (gains electrons). When the cathode electrode makes contact with the water, the metal is emitted into the apparatus, so particulates are neutralized by formation of hydroxide complexes with the purpose of forming agglomerates. These agglomerates begin to form at the bottom of the tank and can be separated through filtration 15-17.

A microalga is more valuable for biodiesel production as a function of its lipid content, therefore it is important to know its content and remove these lipids. Microalgae are unicellular organisms with extremely tough cell walls that can be difficult to disrupt. They can accumulate a great amount of lipids (20–50% dry weight) 18 in their cytoplasm. Some researches 19-21 have reported different methods to disruption the cells, such as mechanical pressing, microwave‐assisted extraction, enzymatic extraction, ultrasonic‐assisted extraction, and solvent extraction. However, the efficiency of lipids extraction is highly dependent on the organic solvent used. These techniques involve the use of different solvents (chloroform, methanol, hexane, …) or solvent mixtures. In general, solvent mixtures containing a polar and a nonpolar solvent allowing to extract a greater amount of lipids 22.

Pavlova Lutheri is widely used in aquaculture as live feed for marine invertebrates (molluscs, crustaceans, zooplankton) 23. It contains a high amount of polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) 24 and it could be a good example of high lipid producing marine microalgae. Some researchers have analysed the potential of Pavlova Lutheri for biodiesel production 25. For this reason, the objective of this work is the study of two separation techniques for Pavlova Lutheri microalgae and two extraction lipids techniques as previous steps for biodiesel production. The analysed harvesting techniques to recovery the microalgae were coagulation and electrocoagulation–electroflotation ECF, and the extraction lipids techniques were Soxhlet and ultrasound methods (Figure 1).