Electrocoagulation of unicellular algae

0574969 - ÚCHP 2024 CZ eng D - Dizertace
Lucáková, Simona
Electrocoagulation of unicellular algae.
Ústav chemických procesů AV ČR, v. v. i. Obhájeno: Praha. 29.08.2023. - Prague, 2023. 126 s.
Institucionální podpora: RVO:67985858
Klíčová slova: electrocoagulation * microalgae * cultivation * chlorella vulgaris
Obor OECD: Environmental biotechnology related ethics

In large-scale autotrophic production of microalgae, the separation of algal cells from the medium (so called harvesting) is one of the most challenging steps due to the low biomass concentration, which is usually in the range 0.2-2 gDW/L depending on the type of cultivation device. Nowadays, centrifugation is the most common harvesting method. Although, it is economically inefficient due to its high energy consumption caused by processing large volumes of culture medium. Due to the small diameter of microalgal cells (in the case of Chlorella cells about 2-10 µm), low cost separation methods such as sedimentation or filtration are not applicable. The use of a pre-concentration step is an option to reduce the volume of algal suspension that needs to be centrifuged and thus significantly reduce the separation costs. One possible method of pre-concentrating the suspension is coagulation, a process by which cells are induced to form well-sedimenting aggregates called flocs. Unfortunately, effective chemical coagulants are more or less toxic to humans. Therefore, in this thesis, a process of electrocoagulation with iron sacrificial anode was tested and found to be an efficient method for harvesting of food-grade biomass of Chlorella vulgaris, a common single-cell microalga. The influence of the following parameters on the separation efficiency was studied during laboratory-scale experiments: pH, temperature, electric charge, agitation intensity, initial biomass concentration and residual salts concentration in cultivation media. The optimization of these process parameters was carried out to achieve high biomass separation efficiency and low biomass contamination by the electrode material (iron). Subsequently, three types of continuous bench-scale harvesting devices (aerated reactor, fluidized bed reactor, channel flow reactor) were tested. The best results were achieved in the channel flow reactor, so in the next step, an integrated pilot-scale continuous electrocoagulation-sedimentation device with a working volume of 110 L was designed and tested for Chlorella harvesting. At laboratory scale, harvesting efficiencies of more than 95 % were achieved over a wide range of conditions. In channel flow reactors in both scales, the harvesting efficiencies were higher than 90 %. In all three measures, the iron content in the harvested biomass met the legislative requirements for food. Using electrocoagulation as a pre-concentration step prior to centrifugation, the total energy cost was reduced to 0.136 kWh/kgDW, which represents less than 14 % of the cost of centrifugation alone. In addition, it has been shown that the biomass after electrocoagulation is capable of further growth with unchanged biomass productivity. Thus, electrocoagulation proved to be a feasible and cost-effective method for harvesting of Chlorella vulgaris.
Trvalý link: https://hdl.handle.net/11104/0344856