Production of microalgal concentrates by flocculation and their assessment as aquaculture feeds

Abstract

A novel technique was developed for the flocculation of marine microalgae commonly used in aquaculture. The process entailed an adjustment of pH of culture to between 10 and 10.6 using NaOH, followed by addition of a non-ionic polymer Magnafloc LT-25 to a final concentration of 0.5 mg L−1. The ensuing flocculate was harvested, and neutralised giving a final concentration factor of between 200-and 800-fold. This process was successfully applied to harvest cells of Chaetoceros calcitrans, C. muelleri, Thalassiosira pseudonana, Attheya septentrionalis, Nitzschia closterium, Skeletonema sp., Tetraselmis suecica and Rhodomonas salina, with efficiencies ≥80%. The process was rapid, simple and inexpensive, and relatively cost neutral with increasing volume (cf. concentration by centrifugation). Harvested material was readily disaggregated to single cell suspensions by dilution in seawater and mild agitation. Microscopic examination of the cells showed them to be indistinguishable from corresponding non-flocculated cells. Chlorophyll analysis of concentrates prepared from cultures of ≤130 L showed minimal degradation after 2 weeks storage. Concentrates of T. pseudonana prepared using pH-induced flocculation gave better growth of juvenile Pacific oysters (Crassostrea gigas) than concentrates prepared by ferric flocculation, or centrifuged concentrates using a cream separator or laboratory centrifuge. In follow up experiments, concentrates prepared from 1000 L Chaetoceros muelleri cultures were effective as supplementary diets to improve the growth of juvenile C. gigas and the scallop Pecten fumatus reared under commercial conditions, though not as effective as the corresponding live algae. The experiments demonstrated a proof-ofconcept for a commercial application of concentrates prepared by flocculation, especially for use at a remote nursery without on-site mass-algal culture facilities.

Biofuels from Microalgae

“…5 Selection of cost-effective technologies for biomass harvesting and drying Given the relatively low biomass concentration obtainable in microalgal cultivation systems due to the limit of light penetration (typically in the range of 1-5 g/L) and the small size of microalgal cells (typically in the range of 2-20 lm in diameter), costs and energy consumption for biomass harvesting are a significant concern needs to be addressed properly. Different technologies, including chemical flocculation, 75 biological flocculation, 76 filtration, 77 centrifugation, 78 and ultrasonic aggregation 79 have been investigated for microalgal biomass harvesting. In general, chemical and biological flocculation require only low operating costs; however, they have the disadvantage of requiring long processing period and having the risk of bioreactive product decomposition.…”

Placing microalgae on the biofuels priority list: a review of the technological challenges

“…However, flocculation caused by alkaline adjustment has been used to effectively remove Dunalliella testolata (Horiuchi et al 2003) and Chaetoceros sp. from fluids (Csordas & Wang 2004;Knuckey et al 2006).…”

“…Figure 7 illustrates such a concept in which oil is produced together with other valuable extracts and where nutrients and process water are recycled. Products derived from microalgal biomass can include commodity materials destined for a range of chemical products such as pharmaceuticals and platform chemicals including other fuels (by conversion to ethanol and methane); highly unsaturated fatty acids such as docosahexaenoic acid ); proteins and carbohydrates, which can be used as gross nutrients (Knuckey et al 2006); specific compounds such as pigments (Lorenz & Cysewski 2000); or silica derived from diatom cell walls (Gordon et al 2009). Each of these components could have considerable value; many could be simultaneously harvested, with contributions between them adjusted by modulation of growth conditions.…”

Energy Balance of Biogas Production from Microalgae: Effect of Harvesting Method, Multiple Raceways, Scale of Plant and Combined Heat and Power Generation

“…A major drawback of using mineral salts is that higher flocculant doses are required, ranging from 120 to 1000 g·m −3 , compared to 1-10 g·m −3 for organic flocculants [17]. The harvesting recovery efficiency of flocculation-assisted sedimentation ranges from 50% to 90% [56,71,72] and the concentration factor from 35 to 800, although concentration factors of 800 are generally not achievable for microalgae [21,73].…”