Semi-continuous cultivation mode works!

At the beginning of November 2022, the last pilot trial of the year for ALGECO ran at NIVA’s pilot facility. Although the temperature outside and inside the greenhouse was not optimal for algae growth, the team wanted to test the infrastructure for the semi-continuous cultivation system, so new trials could be started in spring 2023.

The aim of the new trials will be to investigate how algae productivity responds to a semi-continuous cultivation mode. Testing if productivity rates could be sustained by an exchange of wastewater alone is a critical first step in identifying growth limiting resources and ultimately developing a continuous treatment system.

A raceway at Solbergstrand, NIVA's pilot facility, containing algae growing in wastewater and illuminated with LED lights while is dark outside.

Different methods for growing algae

There are three general modes of growing algae: batch, semi-continuous, and continuous cultivation:

Batch cultivation mode consists of a predetermined medium concentration in which the cells grow, and no further inputs are added. The culture experiences a lag phase, exponential growth, stationary phase, and decline (Algae cultivation method A). The best time for harvesting to maximize productivity is at the end of the exponential growth phase. To produce more biomass, a new batch is started. This is the easiest way to grow algae but also the least productive. The ALGECO team ran the optimization trials in 2022 as batch cultures to easily test that the algae could grow in wastewater, before moving on to a more productive and complex mode of cultivation.

A semi-continuous mode of cultivation implies that the culture is harvested at periodic time intervals while new medium is added. To maximise biomass produced, the harvesting is done at the highest biomass density possible before the culture enters the stationary phase. The biomass density is maintained continuously. The culture experiences the exponential phase repeatedly and never enters the stationary phase (Algae cultivation method B).

A continuous cultivation mode is the most efficient way to produce biomass. This cultivation method can be both a “chemostat” (when nutrient concentrations are held constant) or a “turbidostat” (when the density is held constant). In this method, a continuous supply of medium is added to the culture (Algae cultivation method C).

Algae cultivation methods: A. Batch culture, B. Semi-continuous culture, and C. Continuous culture. 1- lag phase, 2- exponential growth phase, 3- stationary phase, and 4- decline. Source: Modified from algaeresearchsupply.com.

The trial

During the ALGECO experimental trial, the raceways were filled with 1000 m3 of wastewater and inoculated with a co-culture of the algae Stigeoclonium sp. and Oedogonium sp. During the first phase, before the semi-continuous exchange was initiated, the co-culture grew exponentially and the culture density increased by more than 10 fold (0.02 g/l to 0. 24 g/l).

Then, the water exchange began where 25% of the cultivation medium was exchanged with new wastewater daily for eight days. The algae biomass decreased continuously, indicating that the exchange rate was unsuitable for sustaining the biomass density.

Conclusions: Although the trial did not give the desired results regarding algae growth and optimizations are needed, the team demonstrated that the infrastructure for the semi-continuous system works and is ready for new trials in 2023.

ALGECO researchers also proved that the co-culture could grow in VEAS industrial-relevant temperatures, which are lower than the ones experienced during past experiments.