Freshwater microalgae harvested via flocculation induced by pH decrease

Abstract

BackgroundRecent studies have demonstrated that microalga has been widely regarded as one of the most promising raw materials of biofuels. However, lack of an economical, efficient and convenient method to harvest microalgae is a bottleneck to boost their full-scale application. Many methods of harvesting microalgae, including mechanical, electrical, biological and chemical based, have been studied to overcome this hurdle.ResultsA new flocculation method induced by decreasing pH value of growth medium was developed for harvesting freshwater microalgae. The flocculation efficiencies were as high as 90% for Chlorococcum nivale, Chlorococcum ellipsoideum and Scenedesmus sp. with high biomass concentrations (>1g/L). The optimum flocculation efficiency was achieved at pH 4.0. The flocculation mechanism could be that the carboxylate ions of organic matters adhering on microalgal cells accepted protons when pH decreases and the negative charges were neutralized, resulting in disruption of the dispersing stability of cells and subsequent flocculation of cells. A linear correlation between biomass concentration and acid dosage was observed. Furthermore, viability of flocculated cells was determined by Evans Blue assay and few cells were found to be damaged with pH decrease. After neutralizing pH and adding nutrients to the flocculated medium, microalgae were proved to maintain a similar growth yield in the flocculated medium comparing with that in the fresh medium. The recycling of medium could contribute to the economical production from algae to biodiesel.ConclusionsThe study provided an economical, efficient and convenient method to harvest fresh microalgae. Advantages include capability of treating high cell biomass concentrations (>1g/L), excellent flocculation efficiencies (≥ 90%), operational simplicity, low cost and recycling of medium. It has shown the potential to overcome the hurdle of harvesting microalgae to promote full-scale application to biofuels from microalgae.

Effect of eggshells for the harvesting of microalgae species

“…At neutral pH, flocculants possess a coil-like structure. [2] The surface of C. vulgaris cells is likely to be protonated at very low pH values and to become deprotonated with increasing the pH. Variations in the surface charge affect the interaction between particles, and in turn, the flocculation efficiency as well.…”

“…[15,21À24] Such metal ions in the growth medium are hydrolyzed to form positive precipitates which coagulate negatively charged microalgal cells by sweeping flocculation and charge neutralization. [2] In dense microalgal cultures, pH is often highly variable: it may increase to 10 due to intensive primary production or decreased to 6 during CO 2 addition or as a result of respiration. [10] The pH values affect the zeta potential of charged particles, and interfere with the flocculation capacity.…”

“…[1,2] Microalgae have been found to be an economical and potential raw material for biomass energy, [3] since they do not require large areas of land for cultivation, have a short growth period and a high growth rate and contain more high-lipid materials than the usual food crops. [2,4] However, as pointed out by Malm and Tanner [5], harvesting microalgae biomass is challenging mainly due to the small size of microalgae, their tendency to grow as single cells and the low cell density in the culture medium. [6,7] The solution volumes to be treated are large and the amount of processing and energy needed renders the separation of pure microalgae biomass demanding and expensive.…”

The Environmental Biorefinery: Using Microalgae to Remediate Wastewater, a Win-Win Paradigm

“…Moreover, this is a method that may be easily scaled up by reproducing and adapting processes that are currently used in the treatment of wastewater. Various techniques are available: bioflocculation (use of bacteria to enhance the flocculation of microalgae, [59]), autoflocculation (induced by a pH increase, [77] or a pH decrease, [78]) and electroflocculation (use of an electric field, [79]). Although only a few studies exist on the use of flocculation to harvest microalgal biomass grown on wastewater [80,81], the technique should efficiently work for harvesting microalgae since it is frequently used in both the wastewater and microalgae industries.…”

Assessment of the shelf life of Nannochloropsis oculata flocculates stored at different temperatures

“…This procedure consisted in extracting and homogenizing a sample of each of the flocculates stored at different temperatures; two cell counts were performed with a Neubauer hemocytometer. The samples were then stained with 0.5 mL of stock solution of Blue Evans Dye (1%) at 10 mL microalgae (Liu et al, 2013). This was left for 15-20 min at room temperature, after which two further cell counts were performed at room temperature (15 ± 2ºC) with a Neubauer hemocytometer, in order to observe the dead cells under a microscope.…”