Open Ponds vs. Closed Systems

Open ponds

Open ponds are typically built in circular or raceway configurations. The water is kept in motion, for example by paddle wheels. 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. Some difficulties can be overcome by rooftops -  however at higher costs.

Closed systems

Closed systems are dominated by tubular and flat-plate reactors. Other options are bags, coils or domes. Flat plate systems have received a lot of attention due to their large illuminated surface area, but the technology suffers from heating problems and a strong tendency of building up biofilm formations on the inner walls where at the same time cleaning is difficult. Tubular systems on the other hand reduce these drawbacks while maintaining the advantages of optimal light input and high productivity. Therefore closed tubular glass Photobioreactors (PBRs) with long lifetimes and easy cleanability are ideally suited for the highly reproducible cultivation of algae resulting in the highest possible growth rates and qualities. Contamination-free tubular glass PBRs are also very well suited for a continous production of a high quality inoculum used in open ponds.

Comparison

Closed, tubular glass PBRs, with long lifetimes and easy cleanability, are very well suited for the highly reproducible cultivation of algae resulting in highest possible growth rates and qualities. As such, tubular glass PBRs are also best suited for providing high quality inoculum for open pond systems.

Contamination
Closed systems have a very low risk of contamination compared to open ponds, where other microorganisms or insects can have easy access. Birds and sand storms are additional hazards.
No limitation regarding the algae species that can be grown, due to effective blocking against competing organisms

Productivity
Significantly higher productivity in terms of mass per volume and day
Very efficient use of land

Algae contrentration at harvest
Significantly higher concentration in terms of mass per liter
More efficient harvesting procedure

Water loss
No evaporation within closed system compared in open ponds, which can lose significant water amounts, resulting in salt precipitation hazard
Water loss is limited to external factors, such as cooling processes, use of green houses, tube diameter, target temperature etc.

Biomass quality
Biomass quality is highly reproducible due to excellent process control of tubular PBR systems
High value products or high quality inoculum can be produced with optimum reliability

Production flexibilty
Easy cleanability allows defined initial status any time, thus switching algae species is possible and secure

Use of GMO* for improved production process
GMO* is possible in closed reactor design whereas open ponds are very limited


*GMO=Genetically Modified Organism

See also