2.2.5.1 HARVESTING

As the biomass cannot be utilised efficiently at low concentrations in me­dia, the first step in the biomass processing stage is harvesting the algae for subsequent processing. The method of harvesting used depends very much upon the type of algae which is under cultivation. Microalgae re­quire more intensive harvesting methods in comparison to macroalgae, because of their cell size. Depending upon circumstances, often a series of harvesting methods is required to produce a final biomass below a desired moisture content. Common methods of harvesting of algae are: microfil­tration, flocculation, sedimentation, flotation and centrifugation [58].

One of the most effective methods of harvesting is filtration using mi­cro-filters. This method of filtration generally uses a rotary drum covered with a filter to capture the biomass as the influent passes through from the centre outwards [59]. Initial harvesting tests in the 1960s tested micro­filters but found that the majority of algal cells simply passed through most of the filter types [60]. It was later suggested that micro-filtration was suitable for strains of algae with a cell size greater than around 70 pm and was not suitable for those species with cell sizes lower than 30 pm

[61] . The size of the opening in the filter mesh dictates what percentage of biomass is captured likewise with the size of the biomass cells. The pore size also affects how much pressure is required to facilitate the flow of water through the filter which will in turn affect the energy consumption

[62] . The concentration of the algae in suspension also influences the ef­ficiency of removal as highly concentrated biomass will foul the filter very quickly causing reduced performance and a requirement for backwashing and thus further energy consumption. If filtration is to be used it is essen­tial that the method suits the species of algae which is being harvested, otherwise the filtration will be ineffective and provide low yields of bio­mass. If the cultivated algal species allows for filtration (e. g., Spirulina, Spirogyra, Coelastrum), the filtration method can prove very efficient and cost effective method of harvesting. Mohn [63] for example found that gravity filtration using a microstainer and vibrating screen both provided good initial harvesting of Coelastrum up to a total suspended solid of 6% with low energy consumption (0.4 kWh/m3). Mohn [63] also investigated pressure filtration of Coelastrum which provided even higher total solids of concentrate up to 27%, although requiring more than twice the energy. Clearly inexpensive and low energy harvesting of biomass is possible with filtration, providing the dominant algae being harvested is of a suitable cell size and optimal concentration level.

Sedimentation and flotation have also been proven as viable options for harvesting algal biomass with no requirement for specific cell size. Both sedimentation and flotation rely on biomass density to facilitate the process, both processes are aided by flocculation and flotation is aided additionally with bubbling. As a method of biomass removal, sedimentation was consid­ered a viable process in the 1960s due to its prominence in wastewater treat­ment and its low energy requirement [60]. Due to the low specific gravity of algae, the settlement process is, however, slow but, under certain conditions, the self-flocculation of some strains of algae is possible. Nutrient and carbon limitation and pH adjustment appear to be methods of auto-flocculation of algae which may provide a low-cost solution to the initial harvesting process [64]. Recent studies have focussed upon bio-flocculation which occurs as a result of using several bacteria or algal strains to flocculate with the desired algal biomass to allow settlement. Gutzeit et al. [65] found that gravity sedi­mentation was possible using bacterial-algae flocs developed in wastewater for the removal of nutrients, and reported that the flocs of Chlorella vulgaris were stable and settled quickly. Other approaches investigated the combined use of autoflocculating microalgae (A. falcatus, Scenedesmus obliquus and T. suecica) to allow for flocculation of non-flocculating oil-accumulating algae (Chlorella vulgaris and Neochloris oleoabundans) [66], which re­sulted in a faster sedimentation as well as a higher percentage of biomass harvested. This method of harvesting appears viable due to its low energetic inputs but also because it does not rely on chemicals, thus allowing the water to be discharged or recycled without further treatment. However, it should be noted that this method of flocculation may not be suitable for all types of algae, and thus further research is required in this area.

Conventional methods of flocculation using flocculants common to wastewater treatment such as alum, ferric chloride, ferric sulphide, chi — tosan among other commercial products are likely to provide a more consistent and effective solution to flocculation. Much research has been conducted upon the removal of algae using flocculants with vary­ing degrees of success (Table 4). For example, a complete removal of freshwater microalgae, Chlorella and Scenedesmus, using 10 mg/L of polyelectrolytes while 95% removal using 3 mg/L of polyelectro — lites has been reported [60]. A comparative study where alum and fer­ric chloride were use as flocculants for three species of algal biomass (Chlorella vulgaris, I. galbana and C. stigmatophora) indicated the low dosages of alum (25 mg/L) and ferric chloride (11 mg/L) were sufficient for optimal removal of Chlorella vulgaris, while higher dos­ages of alum and ferric chloride were required for the removal of ma­rine cultures I. galbana (225 mg/L alum; 120 mg/L ferric chloride) and C. stigmatophora (140 mg/L alum; 55 mg/L ferric chloride) [67]. Additionally it has been reported that the combined use of chitosan at low concentrations (2.5 mg/L) and ferric chloride provided much quicker flocculation of the algal cells, Chlorella vulgaris, I. galbana and C. stigmatophora, and reduced the requirement of ferric chloride [43]. The use of chitosan as a flocculant for the removal of freshwater algae (Spirulina, Oscillatoria and Chlorella) and brackish algae (Syn — echocystis) has been investigated [43], and chitosan has been found to be a very effective flocculant, at maximum concentrations of 15 mg/L removing about 90% of algal biomass at pH 7.0. The use of conven­tional and polymeric flocculants for the removal of algal biomass in piggery wastewater has been recently investigated [43]: ferric chloride and ferric sulphate were found to be effective flocculants at high doses (150-250 mg/L) providing removal rates greater than 90%; polymeric flocculants required less dosing (5-50 mg/L), although provided lower biomass recoveries; chitosan performed poorly at both low and high dosages for each of the algal species types with a maximum removal of 58% at a dose of 25 mg/L for a consortium of Chlorella.