Validation of a cationic polyacrylamide flocculant for the harvesting fresh and seawater microalgal biomass

Highlights

• Fast and efficient microalgae harvesting was achieved with polymer flocculation.

• The technique was demonstrated using a freshwater and a marine microalgae species.

• The optimal dose was smaller than value of flocculants in the existing literature.

• Microalgae recovery performance was independent of the culture solution pH.

Abstract

A simple, efficient, and fast settling flocculation technique to harvest microalgal biomass was demonstrated using a proprietary cationic polyacrylamide flocculant for a freshwater (Chlorella vulgaris) and a marine (Phaeodactylum tricornutum) microalgal culture at their mid-stationary growth phase. The optimal flocculant doses were 18.9 and 13.7 mg/g of dry algal biomass for C. vulgaris and P. tricornutum, respectively (equivalent to 7 g per m³ of algal culture for both species). The obtained optimal dose was well corroborated with changes in cell surface charge, and culture solution optical density and turbidity. At the optimal dose, charge neutralization of 64 and 86% was observed for C. vulgaris and P. tricornutum algal cells, respectively. Algae recovery was independent of the culture solution pH in the range of pH 6 to 9. Algal biomass recovery was achieved of 100 and 90% for C vulgaris and P. tricornutum respectively, and over 98% medium recovery was achievable by simple decanting.

Introduction

Microalgal biomass is a renewable feedstock for the production of biochemicals for food additives and the biotechnology industry, animal feed, and biofuel (Vo et al., 2018, Vadivel et al., 2019, Jacob-Lopes et al., 2019, Poddar et al., 2018, Khalid et al., 2019, Ma et al., 2018). Microalgae production includes two major steps, namely culturing and harvesting. As progress has been achieved to optimize growth conditions and nutrient requirements for effective microalgae culturing, the second step has emerged as a major bottleneck for cost-effective microalgal biomass production. Large-scale harvesting of microalgal biomass is challenging due to low cell concentrations (less than 1 g/L in a mature culture), small cell sizes (3–$30\mathrm{\mu }\mathrm{m}$), stability of cell suspension, and complex culturing solution matrix (Vandamme et al., 2010, Zheng et al., 2012, Li et al., 2017a, Augustine et al., 2019, Muylaert et al., 2017). An estimation of 30% of the total production cost is attributed to microalgal harvesting (Şirin et al., 2012), which is arguably the most energy-intensive step in the production of microalgal-based materials.

Several microalgae harvesting techniques including membrane filtration, centrifugation and flocculation have been explored and reported in the literature (Vandamme et al., 2010, Şirin et al., 2012, Rashid et al., 2013, Bilad et al., 2012). Amongst them, flocculation is the most promising option for low cost microalgae harvesting, although the biomass recovery efficiency is often low (Vandamme et al., 2010, Şirin et al., 2012, Rashid et al., 2013, Pandey et al., 2019, Ummalyma et al., 2017). Common flocculants for microalgae harvesting can be, divided into three groups: (i) inorganics such as ferric chloride and aluminum sulfate; (ii) synthetic polymers such as polyacrylamide and polyethyleneimine; and (iii) bio-agent flocculants such as fungi, protein, and chitosan (Vandamme et al., 2010, Rashid et al., 2013, Horiuchi et al., 2003, Li et al., 2017b). Inorganic flocculants are required at high doses (up to g/L) and increase the impurity of microalgal biomass, limiting its application and necessitating downstream processing (Şirin et al., 2012). Recently, the second and third groups of flocculants have been extensively studied. Performances of these flocculants are often dependent on pH, long settling time and growth medium matrix (freshwater vs. seawater). Vandamme et al. (2010) reported that cationic starch could effectively recover freshwater Parachlorella and Scenedesmus but not marine microalgae such as Phaedactylum and Nannochloropsis. Likewise, chitosan can be effective for harvesting marine microalgae at high dose (e.g. 40 mg/L or more) (Cheng et al., 2011). The culture media matrix (i.e. ionic strength, cell structure of fresh and marine microalgae) influences the efficiency of previous flocculants (Pandey et al., 2019, Roselet et al., 2015). Moreover, previous studies reported that a relatively higher dose of inorganic and polymer flocculant is needed for marine microalgae (Bilanovic et al., 1988, Fabrizi et al., 2010, Uduman et al., 2010). Given the performance of current available flocculants and the wide range of applications of marine microalgal-based products, it is essential to identify a versatile flocculant that can be used for both freshwater and marine microalgae, over a wide pH range and at low doses.

This study aims to validate the efficiency of a cationic polymer on the recovery of the freshwater (Chlorella vulgaris) and marine (Phaeodactylum tricornutum) microalgae. The polymer has been widely used in the water industry but has not been applied to harvest microalgae. A dose–response experiment was performed to determine the optimal polymer dose. Optical density removal, turbidity, zeta potential and biomass recovery were examined to evaluate flocculation efficiency and mechanisms.