Methods: Open versus closed systems

Methods: Open versus closed systems

Phototrophic algae can generally be cultivated in two different ways, in open or in closed systems. The main difference between these two approaches is whether the algae solution is separated from the atmosphere or not. This has a crucial influence on which environmental impacts can affect the algae solution, and how well the algae cultivation can be controlled.

Open systems

Algae can be cultivated in natural open ponds. However, in industrial production, artificial man-made ponds are usually found. These are typically flat and no deeper than 30 cm. In the water of these ponds, the algae are able to conduct photosynthesis and form biomass with the help of sunlight radiation. Open ponds are typically built in circular or raceway configurations. The open raceway pond is particularly common. The water is kept in motion, for example by paddle wheels, to intermix the algae.

Closed systems

With algae cultivation in closed systems, the cultivation solution in comparison to open ponds, is found in closed containers. The algae are therefore isolated from the immediate atmosphere. There are different forms of closed systems, they are mainly dominated by tubular and flat panel reactors. Other options are bags, coils or domes. The algae in closed systems, depending on their construction are set into motion by induced CO2 or pumps.


The advantages and disadvantages of the two systems

Open Systems

Open ponds are seemingly inexpensive and easy to build. However, danger of contamination, high water evaporation and low volumetric productivities are the main challenges, which lead to lower quality biomass output, large water uptake and costly downstream processes. Due to the constant water loss, the ponds often over salt over time. Some difficulties can be overcome by rooftops – however, at a higher cost.

Closed systems
Closed systems initially require a higher investment and more knowledge about algae cultivation and production conditions. A closed system however, rewards one with significantly higher and especially reliable productivity and quality. This is because the impactful factors on cultivation in a closed system can be controlled accurately, one avoids bacterial contamination as well as dirt ingress, and the design permits a more effective use of light compared to open ponds. Additionally, closed systems are able to produce food grade biomass.


Where should closed systems be applied

Closed cultivation methods come especially into consideration when high quality biomass is the main cultivation goal. Or when the algae need an accurately controlled environment for their development. This is the case when the intended use of the algae is for human nutrition, as an integral part of cosmetics or in pharmaceutical products. Ideally, these products are produced with a cultivation method that is food grade.

Comparison of open ponds and closed systems

Open pond versus closed per systems
For further understanding: The table presents how well or poorly the alternatives perform regarding important evaluation criteria; from “very well“ (++) to “very poorly“ (--).
A closed PBR with comparable photoactive volume needs significantly less space than an equivalent open pond.

The advantages of closed systems at a glance

The most important advantages of closed systems are:
  • Significantly higher productivity
  • Lower risk of contamination
  • Low water consumption
  • Low space usage
  • Excellent purity of the biomass (food grade)
  • High reproducibility of results
  • Constant and predictable production volume
  • Cost efficient harvest due to higher algae concentration
  • The possibility of an easy change of the cultivated algae
  • Low weather dependency
  • The possibility of a 24h production with artificial lighting
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