**6. Microalgal biomass production**

## **6.1 Open ponds**

Cultivation of algae in open pond is oldest and simple practice in which algae are cultivated in similar condition as external environment. This type of system was first introduced in 1950s [56, 57]. Open pond consists of close loop system for circulation which is around 0.3 m depth with a paddlewheel constructed as clay, or plastic-lined ponds. Paddlewheel is used for circulation of water and for proper aeration. Open pond system is still used for large scale production of algae in outdoor condition. With time various designs has emerged for open-pond systems, but three designs (race-way ponds, circular ponds, and unstirred ponds) succeeded for mass multiplication of algae (**Figure 4**).
