This study sets out to harvest microalgae through auto-flocculation by manipulating the interactive effect of calcium, phosphate, and polysaccharides (PSAs). The harvesting efficiency (H.E) of 91 ± ۲٫۷% was achieved for Ettlia sp. through auto-flocculation. The H.E was attributed to the chelation of calcium and PSAs present in microalgae medium. In the absence of PSAs, the H.E reduced to 64 ± ۰۵% only. The addition of phosphate (34 ± ۰٫۱۳ mg L
۱ ) increased the H.E to 73 ± ۱٫۵%. Zeta-potential measurements showed that the harvesting was induced by charge neutralization and inter-particle bridging. The PSAsbased auto-flocculation was tested for Chlorella sp. too but it turned out the H.E of 51 ± ۱٫۳% only. The flocculation did not take place when the co-harvesting of Ettlia sp. and Chlorella sp. was carried out. It is concluded that each microalgae specie shows different auto-flocculation mechanism due to variation in their PSAs characteristics. It triggers up the need for setting up a distinct protocol for different microalgae species to assess their auto-flocculation potential.

Introduction

Harvesting or dewatering is a critical step of microalgae bioprocessing (Gejji and Fernando, 2018; Pradhan et al., 2019). Harvesting accounts for 20e30% of the total cost of microalgae bioprocessing (Sahoo et al., 2017a; Yoo et al., 2013). So far, various methods for microalgae harvesting including centrifugation, dissolved air flotation, and filtration have been developed; however, none of them is cost-effective yet (Sahoo et al., 2017b). To address this challenge, auto-flocculation has been recognized as an alternative method for microalgae harvest (Raheem et al., 2018). Autoflocculation is a cheap and efficient method (Lai et al., 2016; Mathimani et al., 2018; Wang et al., 2018). It demands less resource input than any other method of microalgae harvest (Ding et al., 2017; Jimenez et al., 2017 ). Auto-flocculation is based on the principle of the well-known chemical flocculation process; however, it does not involve the addition of expensive chemicals. Autoflocculation is mainly driven by the interaction of polysaccharides (PSAs), calcium, and phosphate (Tran et al., 2017; Ummalyma et al., 2017; Vandamme et al., 2012). PSAs are the form of polymeric carbohydrates, which are produced by microalgae under specific cultivation conditions. PSAs are composed of glucose, arabinose, galactose, xylose, mannose, and fructose. Each sugar group has a different role in cell metabolism. For example, arabinose controls cell aggregation properties, and galactose is responsible for providing energy. It is widely known that microalgae produce PSAs under light and nutrients stress condition. The yield and the characteristics of PSAs depend on the microalgal species (Lee et al., 2016; Vandamme et al., 2012).