Coagulation-Flocculation

Use of chemicals to inducecoagulation-flocculationof algal cells is a routine upstream treat- ment in various algae-harvesting technologies such as sedimentation (Friedman et al., 1977;

Mohn, 1980), flotation (Moraine et al., 1980), filtration (Danquah et al., 2009) and centrifuga- tion (Golueke and Oswald, 1965; Moraine et al., 1980). Coagulation-flocculation causes algal cells to become aggregated into larger clumps, which are more easily filtered and/or settle more rapidly to facilitate harvesting. Chemicals that were used as algal coagulants can be broadly grouped into two categories: inorganic and long-chain organic coagulants.

Inorganic coagulantsinclude metal ions as Al^þ3and Fe^þ3, which form polyhydroxy com- plexes at appropriate pH. Hydrated lime is a common coagulant inducer used in water and wastewater treatment. Its use would raise the pH to the point at which a milk-like inor- ganic compound, magnesium hydroxide, is formed and acts as a coagulant (Folkman and Wachs, 1973; Friedman et al., 1977). Aluminum sulphate (commonly calledalum,with the chemical formula Al2(SO4)318 H2O) or other salts of aluminum, common coagulants used in water treatment, have also been used as coagulants in algae harvesting (Golueke and Oswald, 1965; McGarry, 1970; Moraine et al., 1980). Ferric sulfate was found to be inferior in comparison with alum with respect to the optimal dose, pH, and the quality of the harvested algal paste (Bare et al., 1975;Moraine et al., 1980).

Satisfactory treatment of algal pond effluent has been achieved by lime addition (Folkman and Wachs, 1973; Friedman et al., 1977). However, satisfactory lime treatment was limited to algal cultures containing magnesium above 10 mg/L, and the quality of the harvested prod- uct was significantly affected due to excessive calcium content of up to 25% by weight.

Common flocculation theory states that alkaline flocculants neutralize the repelling surface charge of algal cells, allowing them to coalesce into a floc. Based on such electrostatic 91

5.3 METHODS OF ALGAE HARVESTING

flocculation theory, the more cells to be flocculated, the more coagulant would be needed in a linear stoichiometric fashion, rendering flocculation overly expensive. Contrary to this theory of electrostatic flocculation, a study found that the amount of alkaline coagulant needed is a function of the logarithm of cell density, with dense cultures requiring an order of magnitude less base than dilute suspensions, with flocculation occurring at a lower pH (Schlesinger et al., 2012). Various other theories abound that flocculation can be due to multivalent cross-linking or coprecipitation with phosphate or with magnesium and calcium. However, the study revealed that monovalent bases that cannot cross-link or precipitate phosphate work with the same log-linear stoichiometry as the divalent bases, obviating those theories and leaving electrostatic flocculation as the only tenable theory of flocculation with the materials used.

Long-chain organic coagulantsor polyelectrolytes could exist as anionic, cationic, and non- ionic synthetic or natural polymeric substances (Stumm and Morgan, 1981). In examining various organic polymers as algal coagulants, it was reported that only the cationic polyelec- trolytes were found to be efficient coagulants (Tenney et al., 1969; Tilton et al., 1972;Moraine et al., 1980). Organic cationic polyelectrolytes at low dosages (1–10 mg/L) can induce efficient flocculation of freshwater microalgae (Bilanovic et al., 1988). Effective flocculation was attained at salinity levels lower than 5 g/L. However, the high salinity of the marine environ- ment was found to inhibit flocculation with polyelectrolytes. The reduced effectiveness of cationic polymers to induce microalgae flocculation in high-salinity medium is primarily attributed to the effect of medium ionic strength on the configuration and dimension of the polymer, as indicated by changes in the intrinsic viscosity. At high ionic strength, the polymer shrinks to its smallest dimensions and fails to bridge between algal cells.

Studies also revealed that while anionic polyelectrolytes enhanced lime flocculation, most polyelectrolytes can be used in conjunction with alum or ferric sulfate as coagulant aids to strengthen the flocs, thus enhancing algae harvesting (Friedman et al., 1977). When used as coagulant aids, the polyelectrolytes can be applied at reduced dosages than they would have been used alone. This helps save chemical costs.

Algal coagulation-flocculation mechanisms based on the use of polymeric coagulants were postulated (Tenney et al., 1969; Tilton et al., 1972). Adsorption and the bridging model were hypothesized, and parameters affecting the process were investigated. It was reported that higher molecular weight cationic polyelectrolytes are superior in flocculating algal particles than their lower molecular weight counterparts. Optimal dose decreased with increasing mo- lecular weight. However, very high molecular weight polymers may reverse the algal surface charge, thus stabilizing the suspension (Tilton et al., 1972). The study also pointed out that for a given level of algal flocculation, variations in algal concentrations would affect the polyelec- trolyte dosage needed, and the relationship between algal concentration and polyelectrolyte dosage can be established based on stoichiometry (Tenney et al., 1969).

A commercial product calledchitosan, commonly used for water purification, can also be used as a coagulant but is far more expensive. To create chitosan, the shells of crustaceans are ground into powder and processed to acquirechitin, a polysaccharide found in the shells, from which chitosan is derived via deacetylation. Flocculation of three freshwater algae, Spirulina, Oscillatoria, and Chlorella, and one brackish alga, Synechocystis, using chitosan was examined (Divakaran and Pillai, 2002). With suspension in the pH range of 4 to 9 and chlorophyll-aconcentrations in the range of 80 to 800 mg/m³, the chitosan-aided flocculation achieved a clarified water turbidity of 10 to 100 NTU units. The chitosan was found to be

92 5. ALGAL BIOMASS HARVESTING

effective in separating the algae by flocculation and settling. It was found that the flocculation efficiency is very sensitive to pH, with optimal pH 7.0 for maximum flocculation of fresh- water algal species. The optimal chitosan concentration for maximum flocculation depended on the concentration of algae. Flocculation and settling rates were faster when higher than optimal concentrations of chitosan were used. The settled algal cells were intact and live and could not be redispersed by mechanical agitation. The clarified water may be recycled for fresh cultivation of algae. Studies of harvesting microalgae with chitosan flocculation were also reported (Lavoie and de la Noue, 1983;Morales et al., 1985).

In addition to the type of coagulant, the composition of the algal medium can also influence the optimum flocculation dosage. For lime treatment whereby magnesium hydroxide pre- cipitate is functioning as a coagulant, as discussed earlier, it was found that the higher the dissolved organic substances in the algal suspension, the higher was the concentration of magnesium hydroxide required for good algal flocculation (Folkman and Wachs, 1973). In- hibition of flocculation caused by the presence of dissolved organic matter was also observed in other investigations (Hoyer and Bernhardt, 1980; Narkis and Rebhun, 1981). Conversely, it was found in another study that algal exocellular organic substances reduced the optimal coagulant dose during the early declining growth phase of algal culture but increased the dose during the late growth stages (Tenney et al., 1969). The authors attributed the increased optimal dose to the development of the organic substances into protective colloid.

There are many variables that could affect algal coagulation-flocculation in a collective and complicated manner, rendering predictions for operational conditions almost impos- sible. Other than algal type, the optimal coagulant dosages can be dictated by the concen- trations of phosphate, alkalinity, ammonia, dissolved organic matter, and temperature of the algal medium (Moraine et al., 1980). In practice, optimal coagulant dosages are determined using bench-scale jar tests to simulate the complex coagulation-flocculation process.

Harvesting by chemical flocculation is a method that is often too expensive for large operations. The main disadvantage of this separation method is that the additional chemicals are difficult to remove from the separated algae, probably making it inefficient and uneco- nomical for commercial use, though it may be practical for personal use. The cost to remove these chemicals may be too expensive to be commercially viable. One way to solve this prob- lem is to interrupt the carbon dioxide supply to the algal system, which would cause algae to flocculate on its own—namely,autoflocculation. In some cases this phenomenon is associated with elevated pH due to photosynthetic carbon dioxide consumption corresponding to pre- cipitation of inorganic precipitates (mainly calcium phosphate), which cause the flocculation (Sukenik and Shelef, 1984). In addition to this coprecipitative autoflocculation, the formation of algal aggregates can also be due to excreted organic macromolecules (Benemann et al., 1980), inhibited release of microalgae daughter cells (Malis-Arad et al., 1980), and aggregation between microalgae and bacteria (Kogura et al., 1981).

A fungi pelletization-assisted bioflocculation process for algae harvesting and wastewater treatment was developed (Zhou et al., 2012). MicroalgaChlorella vulgaris UMN235 and two locally isolated fungal species,Aspergillus sp.UMN F01 and UMN F02, were used to study the effect of various cultural conditions on pelletization for fungi–algae complex. The results showed that pH was the key factor affecting formation of fungi–algae pellets, and pH could be controlled by adjusting glucose concentration and the number of added fungal spores.

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The best pelletization occurred when adding 20 g/L glucose and approximately 1.210⁸/L spores in BG-11 medium, under which almost all of algal cells were captured onto the pellets with shorter retention time. The fungi–algae pellets can be easily harvested by simple filtration due to their large size (2–5 mm). The filtered fungi–algae pellets were reused as immobilized cells for wastewater treatment. It was claimed that the technology developed is highly promising compared with current algae harvesting and biological wastewater treat- ment technologies in the literature.