Functionalities – Advantages and Disadvantages – Application Scenarios
Algae can be cultivated in open or closed photobioreactors (PBR). The main difference between these two approaches is whether the algae solution is separated from the atmosphere (closed systems) or not (open systems).
Closed PBRs are available in different varieties. They differentiate in the type of container in which the algae should proliferate. The most common types are cultivation in plastic bags, flat plate reactors and tubular reactors.
If planning to cultivate algae in a closed PBR, the use of plastic bags is an easy and affordable method.
Algae cultivation in plastic bags is convenient, a respective algae solution is filled in a transparent plastic bag. The algae should then proliferate under light irradiation. Usually, these bags are simple, straight tubes. The bags are installed in a specific bracket in which they are fixed vertically facing downwards. A hole is cut into the bottom of the bag to install a hose. The hose provides the algae solution with CO2 and nutrients. An opening needs to be created at the top end of the bag to allow excess CO2 to escape into the atmosphere. Plastic bags for algae cultivation are usually manufactured from polyethylene (PE) or polyvinylchloride (PVC).
Advantages of plastic bags in algae cultivation
Algae cultivation in plastic bags offers several advantages over other closed methods. Among these advantages, some of the most important are:
- Low Investment/Set up Costs
Low initial capital requirement is the main advantage of plastic bags.
- Convenient Technology
Algae cultivation in plastic bags is exceptionally easy to handle. It works with simple technologies and procedures.
- Calculability of Production Volume
The amount of algae that can be produced with plastic bags can be planned quite distinctively. Due to the necessary replacement of the bags once a year, a new and clean system is implemented every season.
Disadvantages of plastic bags in algae cultivation
Plastic bags have several problematic characteristics that have an unfavourable effect on algae cultivation. The most significant disadvantages of this cultivation method are:
- Suboptimal Impermeability
Typically, the holes on the bottom of the bag, where the hoses are attached, are usually sealed with either a tape or professional seals. Nevertheless, the bags leak frequently at these spots.
- Easier Contamination
Openings are jabbed into the top end of the bags to allow the excess CO2 to escape. The holes in such half-closed systems not only allow the CO2 to escape, but undesired organisms are also able to enter the bag.
- Fast Biofilm Development
The main issue with plastic bags is due to the surface roughness of the material, biofilm forms comparatively quickly on the inside of the bags.
- Poor Cleaning Capability
The issue of biofilm formation is exacerbated because the plastic bags can be very difficult to clean.
- Short Lifespan
The rapid biofilm formation and the difficult cleaning process result in the plastic bags need to be replaced at least once a year.
- Extremely high Operating Costs
The main cost item are the labour costs. The mounting and demounting of the respective installation requires manual labour and is time consuming.
- Below average Productivity
Algae cultivation in plastic bags is not very productive compared to other closed methods.
Sensible implementation scenarios for plastic bags in algae cultivation
When considering short time periods of production such as a year, algae cultivation in plastic bags made of PE or PVC is certainly the most cost efficient solution for algae production in a closed system. Plastic bags are suitable for quick trials in algae cultivation. However when aiming to reach and implement a productive, reliable and long lasting algae production, there is actually no possibility to do this economically with plastic bags. Unless the labour costs for cleaning and replacing the bags as well as the disposal costs for the bio contaminated polymer play a subordinate role in the calculation.
Algae cultivation in a plastic bag compared to a tubular glass PBR
An alternative to the use of plastic bags is algae cultivation in tubular glass PBRs. Tubular glass PBRs are particularly suitable for achieving successful algae cultivation on an industrial scale. In comparison to plastic bags, tubular glass PBRs are characterised by:
- Long Service Life
Plastic bags are usually replaced annually, once they are not productive anymore. Tubular glass can be used for up to 50 years at a consistent level of productivity.
- Lower Operating Costs
The plastic bags have significantly higher operating costs than tubular glass PBRs: The annual replacement of the plastic bags results in high costs due to the work being extensive and can only be done manually.
- Lower Total Cost of Ownership
If one plans to industrially produce algae over a longer period of time (several decades), then algae cultivation in plastic bags shows a significantly poorer Total Cost of Ownership in comparison to glass PBRs. This is mainly linked to a high amount of labour costs for the annual replacement of the bags and the disposal costs of the bio contaminated plastic waste.
- Reduced Biofilm Development
The polymer of plastic bags has a significantly higher surface roughness than glass. Therefore a large amount of algae quickly locate themselves there and results in the limitation of reactor productivity.
- Higher Productivity
The productivity of plastic bags is considerably poorer than that of tubular glass PBRs due to numerous reasons. Studies show that plastic bags produce between 30 and 60% less algae per year and area compared to tubular glass PBRs.
This article is an extract from our e-book “Algae cultivation in plastic bags in comparison to tubular glass PBRs“. If you would like to inform yourself in detail about the advantages and disadvantages of plastic bags in algae cultivation, you can download the e-book for free.