Evaluation of biochemical algal floc properties using Reflectance Fourier-Transform Infrared Imaging

Coagulation and flocculation (C-F) followed by a separation process such as sedimentation or dissolved air flotation is a major barrier to algal or cyanobacterial cells in water treatment plants and a common harvesting technique for algae cultured for high value products. Tailoring algal physical floc properties (size, strength, and density) has been shown to be important to optimise separation, however, the chemical properties of the flocs that lead to the physical properties are not yet well understood. This study investigated the biochemical composition of algal (Chlorella vulgaris) and cyanobacterial (Microcystis aeruginosa) flocs produced under sweep flocculation using aluminium sulphate as a coagulant. These flocs had very different physical properties. A new approach was developed to undertake biochemical characterisation of associated flocs using reflectance Fourier transform infrared (FTIR) imaging. Analysing the biochemical composition of the algal and cyanobacterial flocs revealed that the distribution of proteins and carbohydrates in the flocs are most likely responsible for the unique physical floc characteristics observed. The large flocs of C. vulgaris were characterised by a homogenous distribution of proteins and polysaccharides, and a high glycoprotein and biopolymer content. In contrast, the smaller but stronger flocs of M. aeruginosa had more concentrated protein regions within the flocs that tended to associate at the edge of regions which were absent of biomolecules, potentially comprising coagulant. Overall, the technique shows great potential for analysing algal flocs to gain a better understanding of the underlying biochemical composition and distribution of these biomolecules that leads to the varying physical floc properties.