Chlorella sp. culture in coagulation and dissolved air flotation separation process using a poly-ε-Lysine coagulant

Abstract

Harnessing the cultivation and utilization of different microalgae is an ongoing global project since microalgae grow via photosynthesis. They can build organic molecules from inorganic compounds and solar energy, which, compared to plants, results in nutritious biomass rich in protein with a low water footprint. Microalgae harvesting technologies have seen significant technological advancement in the latest years. This progress has improved the feasibility of the microalgal biorefinery since they lead to a great reduction in harvesting costs. Coagulation-based harvesting research is directed to biobased coagulants that are sustainable, harmless, efficient, and inexpensive. Polysaccharide bio-polymers herd the greatest part of research publications, while limited literature is available for protein-based coagulants (1). It is hypothesized that protein and polypeptide coagulants could achieve high separation efficiencies due to their foaming properties. A native Cuban Chlorella sp. strain (Universidad de Oriente, Cuba) was cultivated in BG-11 medium in 30L tubular photobioreactors. The growth was monitored on a dry weight basis, with an optical density of 750 nm (2). Coagulation and dissolved air flotation (C-DAF) jar tests were conducted on day 6 of growth in late-exponential phase biomass resuspended in fresh medium to 0.15 g/L dry weight at pH 8. The jar testing consisted of a homogenization (10 min, 200 rpm) with the addition of poly-ε-lysine (PL) (molecular weight: 3.5–4.5 kDa, Carbosynth, Ltd., Compton, UK) and floc growth phase (20 min, 20 rpm) followed by DAF (450 kPa, 31% recycle ratio) (Platypus DAF jar tester, Aquagenics Pty Ltd, Australia) in 1 L. The separation efficiency (η%) is the difference in absorbance of the subnatant at 750 nm. Zeta-potential and pH were measured in the Zetasizer Nano (Malvern Panalytical, UK) and the Knick PH-meter 764 Multi-Calimatic, respectively (3). Dissolved organic carbon (DOC) and total nitrogen (TN) were measured on the filtrate (0.45 μm PES membrane filter, VWR, Belgium) with the TOC-L analyser (Shimadzu, Germany). All solutions were freshly prepared for each experiment. At 80 mg PL per g of algal biomass, there is a sudden increase in separation efficiency (89.5%), with the zeta potential increasing from -42.3 to -18.3 mV with weak foam formation. When increasing the dosage to 300 mg PL per g of algal biomass, the separation efficiency did not rise; the zeta potential increased slightly, reaching values closed to zero but did not become positive. The foam layer remained extremely thin. The pH dropped with an increasing dose of PL but did not go lower than 7. Similar observations were made by Noh et al. for a small-scale coagulation and sedimentation concept (4). The PL is positively charged at pH 8 and of low molecular weight; thus, it successfully produces flocs of small size, around 80 μm. The weak foam formation could be explained due to the 100% ratio of hydrophilic residues to the total number of residues which does not support the interaction with the bubble-air interface even though it is known to be slightly negative (5). Concluding, PL is prominent coagulant (electrostatic interactions with the cells) but not as efficient for flotation processes (weak hydrophobic interactions). Follow-up C-DAF tests with medium rich in organic matter could result in superior separation efficiencies since there might be a synergistic effect between the polypeptide and the extracellular organic matter.