Effect of hydroxyapatite, octacalcium phosphate and calcium phosphate on the auto-flocculation of the microalgae in a high-rate algal pond

Abstract

Recovering microalgae is one of the main technological and economic concerns in a high-rate algal pond (HRAP) because of their small size and their low density. This paper emphasizes the characterization (identification and assessment of potential flocculation) of chemical compounds involved in microalgae auto-flocculation in a HRAP. First, thermodynamic simulations were performed, using two models (i.e. Visual Minteq and a simplified thermodynamic model) in order to determine the chemical compounds of interest. Experimental tests were then carried out with these compounds for assessing their flocculation ability. Both models revealed that precipitates of calcium phosphates and their substituted forms were the compounds involved in the auto-flocculation. Moreover, experimental tests showed that the stoichiometric neutralization of algal charges by calcium phosphates (i.e. hydroxyapatite (Ca5(PO4)3OH), octacalcium phosphate (Ca4H(PO4)3) and amorphous calcium phosphate (Ca3(PO4)2)), at a pH within the range 7-10 yields 70-82% recovered algal biomass. The optimum ratio required for algae auto-flocculation was 0.33 Ca5(PO4)3OH/g DM(algae) at pH 10, 0.11 Ca4H(PO4)3/g DM(algae) at pH 7 and 0.23 g Ca3(PO4)2/g DM(algae) at pH 9. Auto-flocculation appears as a simple, sustainable and promising method for efficient harvesting of microalgae in a HRAP.

Flocculation of Chlorella vulgaris with alum and pH adjustment

“…Flocculation is considered as a more economical alternative technique for microalgae biomass harvesting. It is mainly composed of three stages: neutralization of charged microalgae cells with the addition of flocculant materials, forming aggregates (flocs) of cells, and, finally, settlement of the flocs 41–48 …”

“…It is mainly composed of three stages: neutralization of charged microalgae cells with the addition of flocculant materials, forming aggregates (flocs) of cells, and, finally, settlement of the flocs. [41][42][43][44][45][46][47][48] One of the microalgae strains with advantageous properties is C. vulgaris. It is a green eukaryotic microalgae species from Protista, division of Chlorophyta, class of Trebouxiophyceae, order of Chlorellales, and family of Chlorellaceae .…”

Cultivo de >i<Chlorella sorokiniana>/i< em mistura de esgoto sanitário e suíno e separação da biomassa por sedimentação e flotação por ar dissolvido (FAD)

“…Outro ponto é que devido à alta alcalinidade do EFBR (0,41 g CaCO3•L -1 ), uma quantidade considerável de base é necessária para atingir os altos valores de pH. Esta premissa está de acordo com os estudos anteriores para recuperação de fosfato de cálcio por precipitação(BAYA et al, 2013;SONG;HAHN;HOFFMANN, 2002).…”