Dear grajekk, I have done algae reactors in the past and I think that the reasons for using flexible plastic bags instead of "hard" tubes are the following:
1. Price: the tubes should endure being in the outdoors for at least 18 months. This can only be achieved by using polycarbonate hard tubes or flexible bags with anisotropic properties treated to stand UV radiation. The polycarbonate tube cost around $3.00 dlls/linear ft while the flexible bags cost around $0.2 per sqr foot. A "coca-cola" PET bottle material will not make it for 18 months at the outdoors without costing the same as polycarbonate.
2. Operational considerations: if you are going to install thousands of 250 feet-tubes, 25 cm in diameter in the middle of the desert, you better be sure that they are easy to install and maintain. If you use hard tubing, you would not be able to transport whole 250ft-long pipes, so you would need to install them in sections of possibly 62.5 ft (so you'll be able to transport them in the flat bed of an 18-wheeler). For every joint between these sections, you'll get a place where algae can build up and under the right conditions, provide a focus of contamination that would induce the "crashing" of the culture. Another factor you should consider is that 62.5 ft of polycarbonate tube would weight more of what the workers are able to carry without the aid of a crane or any other kind of machinery (which would increase the costs even further).
3. Temperature control: A polycarbonate tube is thicker than the flexible bags and this induces a less efficient heat exchange beetween the algae water inside them and external agents such as wind or water that could be used as cooling or heating agents. For example, the water that you mention in the containment area probably is a tube-immersion test to determine the cooling behavior of the tubes when immersed in water. The deserts where BEHL wants to deploy its devices are 110-125 F in July, and most algae does not tolerate temperatures over 100F, so probably they will use a "geothermal heat pump" to control the temperature for very hot or very cold episodes. This means that they will pump water (could be saltwater, brine, wastewater or freshwater) out of a well (usually at around 60F), they will pass a flow of such water over the algae tubes to get some heat out from the algae water. The external cooling water then would be injected into the ground (not far from where it was pumped out in the first place), so they would use little water. The same could be done in winter when the air reaches freezing temps, this time to provide heat to the algae water to prevent it from freezing.
The issue on "contaminated" water is futile, given that even if that water has some algae from a leak in the tubes, once it gets injected in the ground, it would not have any sunlight to thrive and "contaminate" underground water.
What I see is that BEHL is getting the "right formula" on the bags to provide structural endurance, operational robustness, resistance to UV and competitive prices.
Another thing I see is that there's the need to get more "education" on algae production techniques. Without trying to offend anyone, some of the people that posts messages here, requires to know more about the processes, the metabolic pathways of algae (what happens inside of it) and operational capabilities of aquaculture facilites (the closest thing to algae farming). The problem is that "un-educated" comments here just create uncertainty that makes the stock of algae corps to be like a "yo-yo". This affect us at the end of the day, given that other possible fellow investors that could be key players to increase the stock value, regard algae companies as "too unstable", "too risky", "they are not making it", and they do not seriously consider investing the millions of dlls required for the consolidation of this industry.
I apologize if some regard this as offensive, but it is the awful truth... algae companies will make it someday just like automobile companies did it in the 1900's or personal computers did it in the 1980's... The ones "on board" for long will have the largest rewards... We just need to do our homework and hope for the best. Cheers, Maine