Mastering semi-continuous algae cultivation

The ALGECO Team ran the last algae cultivation trial at NIVA’s pilot facility in Solbergstrand in spring 2024. During 2023, NIVA improved the semi-continuous cultivation mode after setting up the infrastructure for it at the end of 2022 (read more about this work here).

2023 Trials

During 2023, we ran the raceways in a semi-continous mode for 30-40 days, three times in total (Fig 1). This allowed us to improve and tune our approach and logistics. We ran the experiments with 1000 L of culture, and exchanged 40-50% of water approximately twice a week. The co-culture grew well in semi-continuous mode. The last round may indicate that a continuous system with biomass densities between 0.1-0.2 g/L is achievable.

Fig 1. Dry weights (g/L) of the biomass grown during the three runs in 2023. In all runs the biomass reached a maximum during the first week and the last run may indicate that a continuous system is achievable. Plot by Ethan Wood.

We treated 5400–6400 L of wastewater in total, producing an average of 0.03 g/L/day of dry algal biomass. So, the total biomass theoretically produced was 3800 g. The dried biomass was sent to our partners for further experimentation.

2024 Trial

The last ALGECO run lasted six weeks in the spring of 2024, using 10000 L of wastewater in total. We carried five water exchanges, with 40% of the total water volume in each raceway. We collected culture samples before and after each exchange, which Veas is currently analyzing. With these results, we will understand how efficiently the algae culture depleted the nutrients from the wastewater. We harvested 1 kg of dry algal biomass and sent it to our partners for further experiments.

Fig. 2. A. algae culture in the raceway; B. algae paste during harvesting; C. freeze dryer with bags with algal biomass; and D. freeze-dried algal biomass ready to be sent to the partners. Photos: (A. & B.) Ethan Wood; (C. & D.) Tommy Marelius.

Conclusions

Using a semi-continuous mode of cultivation, we produced 2.5 kg of dry algae biomass and cleaned about 20000 L of wastewater. A semi-continuous mode of cultivation allowed for a faster biomass turnout and less logistic pressure than a batch mode of cultivation.

ALGECO researchers have also shown that a continuous mode of cultivation might be possible.