This paper outlines the development of a novel mode of growing microalgae using the enclosure method of microalgae immobilised within a semi-solid gel membrane disk which can be floated upon seawater.
Optimisation of nutrient delivery and ease of harvesting using immobilised microalgae cultivation methods are beneficial. Further research would need to develop flotation via foam based technologies to reduce density and increase buoyancy, as well as reduce incipient costs of the gelling agents. If a gelled immobilised sheet sank into the depths of the sea, not only would it be unrecoverable and therefore not able to be harvested, but it could cover the benthic ecosystem with unknown consequences. Control of flotation is ubiquitous to microalgae in their natural environment as it allows them to go nearer and remain in the presence of the source of sunlight. As light enters deeper into the water column, there is a gradual reduction in both the intensity and frequency of
photosynthetic active radiation which is able to penetrate such depths down to a maximum of 100 m in clear pelagic oceanic seawater known as the light extinction coefficient. Future research would also identify the nutrient profiling of harvested microalgae biomass processed simultaneously with dried pectin and alginate hydrogels. Compositions of hydrogel formulations are worth investigating further due to the relative cost fluctuations of the respective ingredients, some of which are low grade by-products of industrial food and feed processing. Though this mechanism of production has proven to be uneconomically viable in the present form for biofuels, development of foam technologies would simultaneously reduce inclusion of ingredients and improve flotation characteristics. An alternative strategy to achieve a profitable production system would be to increase the cost return per unit volume or mass on the microalgae product composition with high value biochemicals such as nutraceutical or feed products potentially in shallow ponds. Density control of floating disks is possible; though density derived flotation rather than displacement derived flotation is desired and not achieved during these trials. At current market prices, the intended gel substrate is too expensive as to be discussed further in the final chapter.
However, seaweed farming is growing in interest and this could potentially affect the supply and demand as well as influencing market prices.
Continued research for the next chapter of this thesis considers the beneficial attributes of alginate with regard to microalgae gels in an alternative function, using a lower inclusion concentration and for another application; to investigate harvesting of microalgae biomass from growth media.
4 Low-cost microalgae harvesting
(This chapter has contributed to the formation of a new International PCT application PCT/GB2015/050298 priority claimed 11thFebruary 2014).
4.1 Introduction
A technology to harvest microalgae simultaneously requires an appreciation of the aqueous liquid entity from which algal cells are separated. Microalgae live within liquids. Liquids flow; are amorphous, have a fixed volume and resist compression. The flow rate of a liquid depends on the magnitude of the intermolecular forces and the shapes of the molecules, the larger the molecule the slower they move (Toledo, 2008). Microalgae are themselves comparable particulate matter equally distributed within a liquid. Different species of microalgae vary in size from a few to a few hundred microns. Most of the commercially useful microalgae species including those used in marine algaculture and aquaculture are typically in the lower range of this size spectrum between 5 to 30 microns.
Molecular associations interact between water molecules and non-water particulate matter irrespective of size, to form coagulated mixtures of variable homogeneity and inter-particulate affinity. High turbidity values 150 Nephelometric Turbidity Units (NTU) and 80 NTU calcium alginate proved to be a very effective coagulant causing turbidity removals generally over 98% in drinking water (Devrimci et al. 2012). Pectin gelation requires a reduction in water activity, induced by co-solutes such as ethylene glycol or sucrose. Co-solutes play an active role in stabilisation of gels by formation of hydrogen bond links contributing to overall gel strength (Mitchell, 1976). A characteristic of sodium alginate solutions is the ability of gel formation in the presence of polyvalent cations, such as Ca2+ (Draget, Skjåk-Bræk & Smidsrod, 1997). Extraction and purification processes of alginates from seaweed are based on the conversion from the insoluble form in the algae to the soluble one, normally the sodium salt, followed by successive dissolutions and precipitations to eliminate impurities (Larsen, Salem, Sallam, Mishrikey and Beltagy, 2003; McHugh (2003).
Alginate extraction from seaweed is a multiple step process. The process can either be preceded by drying seaweed and re-hydration within solution or by the use of wet seaweed. In summary, 1) Size reduction of raw material by chopping 2) Dilute acid treatment whereby calcium alginate is converted into alginic acid which is more readily extracted with alkali than the original calcium alginate (Haug, 1964), 3) Formaldehyde treatment for additional phenolic compound removal, 4) Alkaline alginate extraction with 1.5% sodium carbonate to solubilise sodium alginate from insoluble calcium alginate, 5) Separation of insoluble seaweed residue by either flotation or filtration, 6) Precipitation of calcium alginate from calcium chloride, 7) Bleaching to reduce colour and odour with sodium hypochlorite, 8) Conversion of calcium alginate into alginic acid with acid filtration, 9) De-watering alginic acid with a screw press, 10) Conversion of alginic acid into sodium alginate with solid sodium carbonate for paste formation of pellet extrusion and drying on a fluid-bed dryer, 11) Milling sodium alginate into a powder form (McHugh, 2003).
In the early patents for the industrial production of alginate, the acid pre-extraction step was described as an essential step to make the alginate more readily soluble in the alkaline solution (Clark & Green, 1936; Bescond, 1948; Secconi, 1967).
None of the above authors’ gave any chemical explanation for the use of the acid treatment. The first scientific explanation was given by Haug (1964) who assumed that alginate occurs as an insoluble salt with calcium as the main cation, so the extraction of alginate could be regarded as a two-step process, a transformation of insoluble alginate into soluble alginate i.e. sodium alginate, followed by a diffusion of the soluble alginate into the solution. He proposed that this transformation could be conveniently carried out by converting the algal alginate into alginic acid, followed by neutralization of the alginic acid with an alkaline sodium salt (Hernández-Carmona, 1999). However, alginate extraction can occur by omission of the acid pre-treatment as with the patented boiling alkali method (Lukachyov & Pochkalov, 1965; Baranov et al., 1967). Acid pre-treatment for alginate extraction from Sargasssum spp. gave 13.8% alginic acid on extraction with sodium carbonate, while the untreated seaweed gave 13.7%
extraction of sodium alginate. These authors’ also proposed that alginic acid is mostly present as free acid rather than calcium salts as reported by other authors (Shah, Mody and Rao, 1967). Polymeric chain degradation is high in the presence of HCl solution during the acid pre-treatment of the alginate extraction process, the acidity must be constantly kept at pH 4. Additionally, acidic pre-treatment produces hydrolysis of some linkages with a decrease in the molecular weight and a lower rheological performance for these samples. (Gomez et al.
2009).
Alginates are a widely abundant food commodity and vital pharmaceutical component (Gombotz & Wee, 2012). Experiments to test the mode of action of low concentrations of alginates as initiators of mild gelation, in proximity to microalgae cells, and as an agent of applied microalgae harvesting in culture were conducted using a non-motile freshwater species Chlorella vulgaris, flagellated marine species Tetraselmis chui, and non-flagellated marine species Chlorella salina. It was hypothesised that at very low concentrations of gelation, microalgae will bind together in a gel like manner and effectively increase their relative size enabling an improved filtration procedure. This process allows “free water” to pass through the filter but does not allow water in the gel to pass through. Upon filtration, as the gelatinised microalgae cells become concentrated within the filter, pressure forces more water out of the gel, thus concentrating filtered microalgae cells and concomitantly separating microalgae from culture media. This succinct hypothesis was tested in the laboratory.
The Bligh & Dyer 1959 lipid extraction method uses the miscibile and immiscible gravimetric chemical analysis properties with the ratio of 0.8:1:2 of citrate buffer:chloroform:methanol. It has been the widely applied benchmark for the extraction of lipids from microalgae (Mercer & Armenta, 2011).