Closed systems: Widespread methods

Closed systems: Widespread methods

The geographical location of the algae cultivation and the available space are crucial for the choice of the cultivation method. These two factors potentially eliminate an open PBR. Furthermore, the significantly higher productivity and product quality as well as the enhanced calculability of production are also compelling reasons for using a closed cultivation method. There are numerous possibilities to choose from. The most common method of closed algae cultivation is the production in tubular PBRs, flat panel reactors and plastic bags.

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.

Types of closed systems

Tubular PBR systems

Tubes made of glass or polymer are arranged vertically or horizontally. In most cases a pump enables the algae culture to pass through the tubular system, after which the culture is collected by a tank and recycled again into the tubular system. Tubular PBRs allow to plan production reliably, considering the processes, the amount and quality. Moreover, they are especially productive, due to them being able to use the available floor space and available light radiation perfectly. They are also relatively easy to clean.
 

Flat panel PBR systems

This system uses plates made of glass or polymer. The plates are arranged vertically or horizontally with a thin layer of algae culture in between. The systems provide good lighting conditions. The systems however, suffer from heating problems and the tendency to form excess biofilm, especially when polymer plates are used. These biofilms are difficult to clean, as they are mechanically hard to access.
 

Plastic bag PBR systems

Plastic bags made of PVC or PE are fixed on special holder systems to intercept the culture supernatants. The investment costs are low, but strong biofilm formation followed by a frequent replacement of bags makes the process labour intensive and causes a lot of waste.

Learn more about this topic: Algae cultivation in plastic bags in comparison to tubular glass PBR

Where should tubular PBRs be applied

Closed tubular photobioreactors exist in laboratory and production sizes, as they are the only systems, which are able to produce high quality biomass for a cost efficient production of high end algae products like DHA, astaxanthin or spirulina. Algae for dietary supplements are best grown in closed systems as they provide a high degree of purity and productivity, which results in high quality of the final product. Furthermore, glass as a container guarantees that one can produce at a food grade level.

Comparison of different methods of closed systems

Different Methods of closed systems
For further understanding: The table presents how well or poorly the alternatives perform regarding important evaluation criteria; from “very well“ (++) to “very poorly“ (--).
Tubular glass PBRs have the lowest total cost of ownership of all closed systems. This is mainly because one is able to constantly produce for up to 50 years with glass tubes. Whereas plastic systems, depending on their method and material, need to be replaced every 1-10 years.

Learn more about this topic: Comparison of commonly used technologies for the cultivation of algae

The advantages of tubular PBRs at a glance

Regardless of the container material, due to their geometry, tubular PBRs have the following advantages in comparison to other closed systems:
  • Higher productivity, due to optimal light usage in the tubes and simultaneous rare downtime.
  • Excellent calculability of production, due to their hardly every being unplanned production interruption.
These advantages are especially applicable to tubular systems made of glass.
Glass systems additionally convince by:
  • Less biofilm formation, based on the low surface roughness of glass.
  • Easier cleanability with chemical and mechanical methods.
  • A very long lifespan of up to 50 years.
The lowest total cost of ownership at long operating periods.
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