An energy and resource efficient alkaline flocculation and sedimentation process for harvesting of Chromochloris zofingiensis biomass

Abstract

Harvesting microalgal cultures is often energetically intensive and costly. To improve efficiencies, a two-step harvesting method utilising alkaline flocculation and sedimentation to pre-concentrate cultures can be used prior to centrifugation. When applied to the microalga Chromochloris zofingiensis, high rates of sedimentation (> 90%) were found at low concentrations of base (< 10 mM), with the addition of magnesium to the media (via NaOH/ MgSO 4 or Ca(OH) 2 /Mg(OH) 2 ) to form Mg(OH) 2 . The process was scaled to 180 L, where sedimentation was as efficient as that achieved at bench scale. Characterisation of the harvested biomass showed comparable composition (following neutralisation of pH) to biomass recovered solely by centrifugation. The alternative two-step processes were assessed for environmental impacts and cost, which indicated that a two-step harvesting generally performs better than centrifugation alone, but that the locally available electricity source is a critical parameter for optimal solution.

Progress towards a targeted biorefinery of Chromochloris zofingiensis: a review

“…There is little information available about the proteins and amino acids of C. zofingiensis likely due to the focus on increasing TAGs and astaxanthin, which causes a reduction in protein content [1]. The highest protein content recorded for SAG 211-14 biomass is 48.4% where the nitrogen source was ammonium nitrate [114]. Protein content also varied under different light intensities, 13% at 300 µmol/m 2 /s and 20% at 50 µmol/m 2 /s which could imply that excess light induces protein degradation or that nitrogen uptake is better at lower light intensities [115].…”

“…The harvesting method used can affect the biochemical and elemental profiles of the algal biomass. For example, the protein content was 47.4%, 30.5%, and 48.4% DW while the carotenoid content was 0.79%, 0.37%, and 0.76% DW when centrifugation, sedimented non-neutralised centrifugation, and sedimented neutralised (to pH 7) centrifugation were compared on SAG 211–14 [ 114 ]. This could be because certain salts and hydroxides precipitated with the biomass when the non-neutralised technique was used causing an increase in ash content.…”

“…Research into harvesting methods of C. zofingiensis has demonstrated that the most effective mechanisms are flocculation and coagulation. The most significant research on flocculation was by Mayers et al [ 114 ] and Wyatt et al [ 165 ] where both an NaOH and Mg 2+ combination and FeCl 3 achieved high flocculation efficiencies of 89.4% and 90%. It is suggested that bio-flocculants should be studied to determine the influence on the quality of other products despite their efficiency being reduced compared to regular flocculants [ 166 ].…”

Harvesting Porphyridium purpureum using polyacrylamide polymers and alkaline bases and their impact on biomass quality

“…purpureum biomass is cultivated in seawater culture medium that contains a large amount of alkaline earth metal ions such as Mg 2+ and Ca 2+ . At high pH >9 and under atmospheric conditions, these ions can precipitate as magnesium hydroxide and calcium carbonate (Besson and Guiraud, 2013;Mayers et al, 2020;Vandamme et al, 2015). The large mass of precipitates, while settling down due to gravitational force, entangles with the algal cells by a sweeping effect.…”

Evaluación ambiental del efecto del uso de aditivos químicos en la estabilización de suelos viales

“…Se concluyó que el contenido de óxido de calcio (CaO) tuvo un efecto significativo en la precipitación de la materia orgánica contenida estas muestras, lo que condujo a una disminución de la DBO. Esto se atribuye principalmente a la floculación inducida por el aumento del pH debido al agotamiento del CO2, como se afirma por(Branyikova, et al, 2018) y(Mayers, Landelsb, Allenb, & Albers, 2020).…”