I worked on this project when I was working for Middough on a part-time basis after I started my own consulting business. My background in both fermentation process development and horticulture was ideal for this project that was based on growing photosynthetic bacteria in miniature greenhouses.

Proterro, Inc.

Proterro had developed a genetically engineered cyanobacteria that produced sucrose when grown in sunlight and fed a minimal salts medium. They had also developed a conceptual design for what they called a “solid-state photo-bioreactor”. This design immobilized the cyanobacteria on a support that was hung vertically inside the bioreactor and was fed by dripping the feed medium onto the top of the support. The liquid medium then flowed down the support, collecting the synthesized sucrose and was then removed from the bottom of the reactor. Carbon dioxide for the photosynthesis was supplied in air blown into the bioreactor, with the intention that this could eventually be designed to use carbon dioxide-rich exhaust gas from a combustion source such as a power plant.

Pod Farm

Proterro’s early designs called for pod farms comprised of acres of modular photo-bioreactors connected to supply lines for the nutrient medium and carbon dioxide source and a return line for the sucrose solution. The collected sucrose solution would then be processed using conventional sugar refining technology. Needless to say, the prospect of maintaining hundreds or thousands of aseptic pods in outdoor fields presented a challenge. Growing up in the country, I also know what country kids will do for entertainment with a BB gun, .22 or shotgun and too much free time. However, Kevlar® pods were probably not going to be cost effective.

Energy Balance

As a greenhouse owner, my first thought when I looked at the Proterro pod concepts was that temperature control would be a challenge. Typical low-cost transparent plastic materials have a very low R-value. This is helpful during sunny days to dissipate heat, but also a detriment at night in a cold climate. Greenhouses maintain temperature control through a combination of ventilation, evaporative cooling and supplemental heat. However, these would be expensive to provide to hundreds or thousands of pods distributed over acres of land. Therefore, the size, shape and covering materials and the gaseous and liquid feed and return streams would all have to be balanced to maintain the optimum temperature inside the pods.

Working with the Middough engineers, we developed a material and energy balance for a pod that considered the solar radiation heat gain and heat losses to the environment through the covering. We then added in the enthalpy of all fluid streams entering and leaving the pod as well as evaporation of the water in the feed medium into the gas stream. To maximize the sucrose concentration, and keep from flooding the pod, the liquid feed and return streams were pretty much fixed in flow rate. This left only the gas supply stream as a tunable variable to control the pod temperature.

Modular Inflatable Bioreactor

The process economics dictated that the bioreactor pods must be either reusable or, if single-use, inexpensive. Single-use bioreactors should also be recyclable to reduce disposal costs. In addition, they needed to be able to be sterilized, and, as they were likely to be made of inexpensive plastics, thermal sterilization was not an option. This left either some form of liquid or gaseous sterilization or gamma-ray irradiation. However, gamma-ray irradiation would require that the bioreactors be able to be collapsed into a relatively small volume to be cost effective.

With all of these constraints in mind, I developed a concept for an inflatable pod design that could be manufactured as a single continuous sandwich containing the cyanobacteria support material on the inside with an external layer of greenhouse-grade polyethylene with ultraviolet-light stabilizers. This continuous sandwich would also have the supply and return lines molded or welded into it during fabrication. In this design, the continuous sandwich would then be cut into segments and the ends welded shut and connection fittings installed for the supply and return lines. The entire bioreactor could then be rolled or folded into a compact package for sterilization by gamma-rays.

Epilogue

Following this initial project with Proterro, I ended my part-time employment with Middough to focus on my independent consulting work. Ultimately, I believe, the realities of trying to produce a commodity product like sucrose cost-effectively in fields full of aseptic photo-bioreactors became apparent. My understanding is that the company changed its focus to higher-value products that could be produced using their technology.