Chemical flocculation adds flocculating and/or coagulating agents to the culture medium to speed cell aggregation. This process is often used with microalgae as a pretreatment in combination with other processes such as dissolved air flotation (DAF). While this improves the speed at which the cells are collected, it has the added complication of dosing chemicals at a specific desired concentration to achieve this rate. Generally, coagulants are used to neutralize the charges of the particles in the solution, and flocculants are the chemicals used to aggregate the particles. These chemicals (flocculants and coagulants) complicate the overall process in that they add additional cost, often add metals or other compounds that need to be disposed of in the resulting biomass, and complicate any downstream processing of the materials into primary and co-products. Negative impacts have been countered in a number of ways, including the use of degradable biopolymers as flocculants (e. g., polyacrylamide and starches) and electroflocculation where no flocculant is directly added.

Inorganic flocculants and coagulants are typically iron or aluminum based and are used to neutralize the surface charge. This method requires a significant input of the inorganic flocculant which adds to the sludge; this adds to the OpEx both in the inputs and in processing (to remove the chemicals). Additionally, the process is sensitive to pH, usually working best at higher pH but varies with strain and culture condition. All algal strains do not respond the same to a particular chemical, so tailoring will be required to fit the organism being harvested (Chen et al. 2011). The chemical flocculants can also be a problem for downstream use of the biomass for feeds, feedstock for anaerobic digesters, and residual ions can be a problem for use of digestates for land application as a soil amendment (Christenson and Sims 2011).

An example is the use of alum (hydrated potassium aluminum sulfate) for flocculation of Scenedesmus and Chlorella cultures (Molina Grima et al. 2003). Knuckey (2006) used ferric-induced flocculation at 0.5 mg L 1 to concentrate algae that had been pH-adjusted to about pH 10 by 200-800-fold. The floccs needed to be neutralized after concentration. They successfully flocculated Chaetoceros calci — trans, C. muelleri, Thalassiosira pseudonana, Attheya septentrionalis, Nitzschia colesterium, Skeletonema sp., Tetraselmis suecica, and Rhodomonas salina all with >80 % efficiency. All were marine algae useful for aquaculture feeds (Knuckey et al. 2006). As an example of strain differences, it has been reported that B. braunii flocculated better at pH 11 (Lee et al. 1998).

Organic flocculants and coagulants can also be used; these are high molecular weight bridging polymers (e. g., chitosan and starch) that react with cells in the culture to make large aggregates and help speed up flocculation (Edzwald 1993). It is reported that these biodegradable polymers do not contaminate the biomass as much as inorganic coagulants and that cationic polymers are superior to anionic and neutral ones. Cationic polyelectrolytes (e. g., Dow C-31) induced algal cells to flocculate while anionic and nonionic polymers were shown to be ineffective (Tenney et al. 1969). Changing the pH was necessary to optimize the flocculation with these cationic polymers, with essentially no flocculation at high pH (above 8) and maximal flocculation between pH 2 and pH 4. The flocculation was attributed to a bridging by the polymers between algal cells to form a matrix. The chemicals necessary for pH adjustment (and neutralization) significantly add to the overall OpEx of flocculation processes. The flocc formation is inhibited by high salt concentrations, forms better at high biomass density, and is subject to shear dis­ruption, which slows the flow rates possible.

Growth stage has also been reported to have an impact on the flocculation procedure. For example, Botryococcus braunii cultures flocculated easiest after two weeks of culture regardless of harvesting method (Lee et al. 1998).