Among the proposed photobioreactors, tubular photobioreactor is one of the most suitable types for outdoor mass cultures. Most outdoor tubular photobioreactors are usually constructed with either glass or plastic tube and their cultures are re-circulated either with pump or preferably with airlift system. They can be in form of horizontal / serpentine, vertical near horizontal, conical, inclined photobioreactor. Aeration and mixing of the cultures in tubular photobioreactors are usually done by air-pump or airlift systems.
Tubular photobioreactor are very suitable for outdoor mass cultures of algae since they have large illumination surface area. On the other hand, one of the major limitations of tubular photobioreactor is poor mass transfer. It should be noted that mass transfer (oxygen build-up) becomes a problem when tubular photobioreactors are scaled up. For instance, some studies have shown that very high dissolved oxygen (DO) levels are easily reached in tubular photobioreactors (Torzillo et al., 1986; Richmond et al., 1993; Molina et al., 2001).
Tubular photobioreactors consist of straight, coiled or looped transparent tubing arranged in various ways for maximizing sunlight capture. Properly designed tubular photobioreactors completely isolate the culture from potentially contaminating external environments, hence, allowing extended duration monoalgal culture.
Also, photoinhibition is very common in outdoor tubular photobioreactors .When a tubular photobioreactor is scaled up by increasing the diameter of tubes, the illumination surface to volume ratio would decrease. On the other hand, the length of the tube can be kept as short as possible while a tubular photobioreactor is scaled up by increasing the diameter of the tubes. In this case, the cells at the lower part of the tube will not receive enough light for cell growth (due to light shading effect) unless there is a good mixing system.
Also, it is difficult to control culture temperatures in most tubular photobioreactors. Although they can be equipped with thermostat to maintain the desired culture temperature, this could be very expensive and difficult to implement. It should also be noted that adherence of the cells of the walls of the tubes is common in tubular photobioreactors. Furthermore, long tubular photobioreactors are characterized by gradients of oxygen and CO2 transfer along the tubes . The increase in pH of the cultures would also lead to frequent re-carbonation of the cultures, which would consequently increase the cost of algal production.
Prospects
Large illumination surface area, suitable for outdoor cultures, fairly good biomass productivities, relatively cheap.
Limitations
Gradients of pH, dissolved oxygen and CO2 along the tubes, fouling, some degree of wall growth, requires large land space.
