Characterisation of floc size, effective density and sedimentation under various flocculation mechanisms

Floc structure plays an important role in the separation of coal wastewater. In this study, a camera-based method is used to evaluate quantitatively the structural characteristics of flocs generated by different coagulants and flocculants. The correlations between particle size, settlement velocity and effective density of coal tailings flocs are analysed. The results show that the statistical settling velocity increases linearly with floc size, while the effective density decreases with increase in floc size. Different flocculation mechanisms lead to diverse growth abilities of flocs. When the flocculant is used alone, the quality of the flocs generated by the flocculants, cationic polyacrylamide (CPAM) and non-ionic polyacrylamide (NPAM), is better than that generated by anionic polyacrylamide (APAM). However, the combination of trivalent cations and APAM yields a much better effect than that obtained using CPAM and NPAM. Flocs become larger and more compact when treated with a coagulant combined with a flocculant.

Towards a better understanding of microalgae natural flocculation mechanisms to enhance flotation harvesting efficiency

In microalgae harvesting, flocculation is usually a compulsory preliminary step to further separation by sedimentation or flotation. For some microalgae species, and under certain growth conditions, flocculation can occur naturally. Natural flocculation presents many advantages as it does not require the addition of any flocculants to the culture medium and shows high efficiency rate. But because natural flocculation is so specific to the species and conditions, and thanks to the knowledge accumulated over the last years on flocculation mechanisms, researchers have developed strategies to induce this natural harvesting. In this review, we first decipher at the molecular scale the underlying mechanisms of natural flocculation and illustrate them by selected studies from the literature. Then we describe the developed strategies to induce natural flocculation that include the use of biopolymers, chemically modified or not, or involve mixed species cultures. But all these strategies need the addition of external compounds or microorganism which can present some issues. Thus alternative directions to completely eliminate the need for an external molecule, through genetic engineering of microalgae strains, are presented and discussed in the third part of this review.