The present doctoral thesis aimed to achieve a robust cultivation of microalgae by recycling nitrogen from a liquid digestate. However, the use of non-sterile nutrient source increases the risks of contamination of undesired organisms such as bacteria, viruses, protozoa, and rotifers. Therefore, the thesis also aimed to develop a cheap and bio-compatible decontamination method, which can be applied to microalgae during their cultivation. To reach these objectives, three projects were carried out.
In the first project, microalgae (Chlorella vulgaris) were successfully cultivated at high-density with liquid digestate from anaerobic digestion as a nitrogen source. The cultivation was carried out at laboratory and pilot scales in glass-column and thin-layer photobioreactors, respectively. Biomass dry weight and productivity reached values up to 18.6 grams per liter and 0.93 grams per liter per day, respectively. To obtain these results, a cultivation method was developed to overcome inherent issues associated to the use of liquid digestate such as the acidification of the water, ammonium toxicity, turbidity, and nutrient imbalance. The nitrogen mass balance was carried out during the cultivation and it shows that only 40 to 60 % of the nitrogen supplied to the cultures were assimilated in the biomass. Surprisingly, accumulation of nitrogen in the supernatant accounted only for approximatively 3 %. Therefore, a large fraction of the nitrogen was lost to the atmosphere.
In the second project, a citrate-modified photo-Fenton (PF) process was used to treat microalgae cultures contaminated with the bacterium Escherichia coli. The aim was to set experimental conditions where physiological and morphological differences can be used to favor the inactivation of bacteria and to minimize the loss of microalgae. Results showed that the citrate-modified PF was more effective against bacteria than a solar light/hydrogen peroxide treatment, while being less effective against microalgae. If the treatment was applied at an early-stage contamination, bacteria were fully inactivated, while the microalgae loss of viable cells was about three orders of magnitude.
In the third project, the citrate-modified PF process was used to treat microalgae cultures contaminated by the rotifer Brachionus calyciflorus, a predator of microalgae, which can lead cultures to failure due to its high consumption rate of algal cells. Then, treated cultures were cultivated up to 14 days to assess the efficacy of the treatment. Results show that the citrate-modified PF and solar light/hydrogen peroxide treatments had equivalent efficacy to inactivate rotifers. However, the PF process remained less harmful for microalgae. When the citrate-modified PF treatment was applied to an early-stage contamination (5 rotifers per mL), the decontamination was effective and no regrowth of rotifers was observed. However, the cultivation of contaminated cultures, which were not treated, failed due to an increasing rotifer population (>1000 individuals per mL after 14 days of cultivation).
As a whole, the present doctoral thesis contributes to the development of more robust and sustainable cultures of microalgae. Methods were developed to cultivate microalgae at high-density with liquid digestate and to control biological contaminants. Finally, challenges and opportunities were discussed as a perspective for further works.