Open Ponds vs Closed Reactors: some science behind how to grow lots of algae

Last week, Bodega algae was featured in an article in biomass magazine: http://www.biomassmagazine.com/article.jsp?article_id=3618

The article discusses the ongoing debate about open ponds vs closed photobioractors for growing algae on a large scale. Obviously, as part of a company working on photobioreactor technology, I am in favor of closed systems for industrial applications. I will go over some of the science here and say up front that I do see the advantages of open ponds - mostly their lower capitol costs than photobioreactors, but I think that with a little more research and development we will be able to engineer reactors that will not be prohibitively expensive.




1)Controlling growing conditions
The first thing to consider when thinking about cultivating microbes on a large scale is what are you growing. There is a lot of working being done on strain selection buy a number of government labs, academic labs, and private companies. They are all trying to determine which algae will grow best, under what conditions, and which will produce the most lipids. This is because not all algae are the same physiological and biochemically - not to mention their diverse evolutionary origins (see http://algaeenergy.blogspot.com/2010/03/what-are-algae.html ). At "normal" culture conditions algae do not grow that densely. Since the we are algae talking about grow photosynthetically in water, an algal culture is mostly water. A typical, run of the mill culture will yield roughly .1 g biomass/L. Now there is a huge range of growth rates in algae. Some are like weeds and will grow very fast, some are specialist and will grow really slow. The fastest growing ones, under normal condition can produce .5g biomass/L. The predictions to make growing algae as a biofuel feedstock economical is that we would have to grow at least 10 g/L but maybe even more like 50 g/L. Some people have achieved these densities in very specialized systems with small volumes on the order of 200 mL and I have heard claims that individuals or companies have achieved these densities in larger volumes recently but this is no simple task. Therefore, to produce very high density cultures in large volumes as the industry needs to, we need to do something clever. I often describe it as "factory farming" the algae. We need to engineer the algae or we need to engineer the system to be optimal growing conditions. Closed bioreactors allow for careful control of growing conditions where open ponds are subject to the weather. In the event that some group wants to grow genetically modified algae (I'll speak to this matter in another post) they would need to do this in a closed system so that the gentically modified organism was not allowed under any circumstances to be released to the environment.

2. Contamination
One of the biggest problems for the long term in any cultivation of algae is contamination - but this is especially problematic when growing on a large scale. In the lab, scientist take careful measures to make sure they are always working with sterile equipment, transfer algal culture in laminar flow hoods that limit the number of air born particles that come in contact with the culture, and they transfer the culture often to make sure an individual strain stays healthy. In large scale cultivation, it is difficult to control all these factors. In a open pond, it can be almost impossible. A friend mine who works on open pond systems said their group is focused on isolating natural strains that are already known to do well under local condition against competitors. This is true, and a cleaver strategy. However, we still do not understand all the aspects of the delicate balance found in microbial communities. Thus, I think it will always be difficult to control and maintain algal growth in open ponds where the culturing system is in contact with the open air. Close bioreactors are subject to contamination issues as well, but careful design and management can greatly reduce to chance of contamination.

3. Footprint
On of the benefits of using algae is their cultivation doesn't necessary require a large land footprint or arable land. Many of the open pond designs are large, shallow ponds that have a large surface area. Close bioreactors have flexibility in their design to allow for various shapes, stacking, integration with other building structures, and in general can have a much smaller footprint for the same amount of volume cultivated. In both cases, the biggest obstacle in scaling up is light limitation. In dense algal cultures, the optical path, or the distance light can travel through a material, is 3 in. That is why some many of the photobioreactor designs are thin plates, tubes, or bags. This is also why open ponds must be shallow. So photobioreactors techologies, such as Bodega Algae's reactor, work to get around this limitation by delivering light into larger volumes. If this can be done efficiently and with inexpensive materials, its possible to start cultivating much larger volumes without light limitation. For high cultivation densities, the algae need to have just the right amount of light. Too much light causes photoinhibition, where the cellular activities are shut down, and too little light means that the system is not as productive as possible. Light can be much easier to control in a closed system which optimizes the amount of biomass per unit area.


As things stand now, photobioreactors are still much more expensive than open ponds systems. What I like about our work at Bodega Algae is we are working toward bringing the costs down by engineering smart reactors with limited moving parts and inexpensive materials. As the article in Biomass Magazine says, there may be room for both open ponds and closed systems in the ultimate algae cultivation solution.