Introduction: Microalgae, cultivated using algal technologies, represent an alternative raw material for various sectors, including agriculture. The biggest challenge in microalgae production is harvesting due to the high costs and inefficiency of the processes. Purpose: The aim of this master’s thesis is to successfully harvest the microalgal biomass, thereby separating it from the growth medium on a laboratory scale. Harvesting of microalgal biomass is a key step when reusing microalgal biomass in agriculture. For separating the two phases, we used electrochemical processes, with the aim of avoiding or minimizing the risk of toxic substance formation during the harvesting process. We compared the harvesting efficiency of microalgal biomass using aluminium electrodes, which release metal ions, and graphite electrodes, which are inert. Methods: We tested harvesting efficiency on laboratory-grown consortium of microalgae and on an environmental sample of a microalgae-bacterial consortium from the high-rate algal pond at the Central Wastewater Treatment Plant Ajdovščina. Experiments were conducted in the laboratory under key conditions for the experiment: electrode material (aluminium, aluminium-graphite, graphite), electric voltage (5, 8, 10 V), and harvesting time (4 and 8 min). For each electrode material, three replicates were performed at each voltage and time. Harvesting efficiency was determined by measuring absorbance at 680 and 750 nm and by determining total suspended solids and total solids. Results: The highest harvesting efficiency of the laboratory sample (58.0 %) was achieved using the aluminium electrodes at 10 V and 4 min. With the aluminium-graphite and graphite electrodes, harvesting in most laboratory experiments was unsuccessful (efficiency below 10 %). The highest harvesting efficiency of the environmental sample (~98 %) was achieved using the aluminium electrodes at all three voltages and 8 min. Comparable harvesting efficiency for the environmental sample (96.8 %) was achieved using the aluminium-graphite electrodes at 8 V and 8 minutes. Using the graphite electrodes, the maximum harvesting efficiency for the environmental sample was 66.5 % at 10 V and 8 min. Discussion and conclusion: The composition of the sample has the greatest impact on harvesting efficiency, followed by the electrode material, electric voltage, and harvesting time. Using aluminium-graphite electrodes under certain conditions, we achieved comparable efficiency to aluminium electrodes while reducing the amount of aluminium ions in the final product by almost 90 % compared to aluminium electrodes. To improve harvesting efficiency further, electrochemical processes can be combined with bioflocculation or filtration.
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