This system is without the ability to keep a control on the factors involved in the cultivation process. For example, algal cells have a propensity to settle in the form of residue under the influence of gravity and therefore, availability of CO2 and light is sufficiently reduced. In addition to this, this system also lacks the capacity to supply an efficient amount of CO2 to algal biomass, thus decreasing the yield.

13.3.1.1 Circular Ponds

This system has the credit for being the first design to be commercially used for algae fostering. The major drawback of this system is that its scale is limited to an upper range of approximately 1,000 m2 . At this range, the stress makes the core pivot mixer unmanageable.

13.3.1.2 Open Raceway Ponds

In early 1950s, Oswald and his colleagues introduced this system, which is also known as high rate algae ponds (HRAP). This system is basically used for the treat­ment of wastewater by supporting the symbiotic relationship between aerobically active bacteria and algae. Circulation of broth and nutrients using a paddle wheel is performed in looped channels. The ponds are made up of concrete, PVC, or clay and are about 0.2-0.5 m deep, allowing deep penetration of sunlight. Carbon dioxide is directly taken from the surface air but aerators can easily be installed inside the pond to increase the level of CO2. Although this system is well developed, yet it still presents the problem of infection with unwanted algal species.