# The Perspective of Large-Scale Production of Algae Biodiesel
## 3 Production of Algal Biodiesel
### The Production Process of Biodiesel from Algae  
The cycle of growing algae is very short (one to ten days), which allows for several harvests. This means that the yield of algal biomass is much higher compared to conventional crops, which further allows the production of a hundred times more oil than crops such as oilseed rape and soybeans [8]. After the growing cycle following harvesting: Applied harvesting technology depends on the type of algae grown and includes two major processes, namely collecting and algal biomass concentrating. Separation of water content is related to this biomass collection for further process of biodiesel production. The process involves two specific steps:1. Separation and thickening of microalgae from bulk suspension by micro strainers, by electrophoresis process, and by sedimentation, flotation, and flocculation;
2. Dewatering the microalgae slurry, applying filtration, or centrifugation [34; 50]. The first step taken in algae collection is to carry out a screening process or filtration. Micro strainers and vibrating screens are standard screening devices used. The high harvesting rates and removal efficiencies of 95% could be achieved by using vibrating screen filtration [51]. For concentration improvement, various centrifugation systems are in use, like the solid ejecting disc, solid bowl decanter, and hydro-cyclone. Centrifugation represents the fastest method with biomass recovery up to 95% under optimal conditions, so it is preferable over other methods [52]. For large algae systems (closed photobioreactor), it takes 73 kg of water to process 1 kg of algae biomass, which requires high-energy consumption. Applying flocculation reduces energy consumption. Flocculation involves several complex processes: first, the unstable impurities of colloid when neutralization, and the formation of aggregates; second, the adsorption of organic matter by the aggregate through a bridging process; finally, particle aggregates are removed by sedimentation, filtration, and sweeping [53]. Several flocculation techniques can be used like chemical flocculation, auto flocculation, inorganic flocculation, electrolytic process, etc. [54]. For large-scale plants, harvesting by a chemical coagulation-flocculation method is too costly. This method has another disadvantage that influences its quality, namely the difficulty in removing the separated algae from excess chemicals. Disruption of CO\({}_{2}\) into the algal system is a solution to this problem, which causes autoflocculation, where the algae flocculate by themselves [51]. In the flotation process, the air bubbles attach to the mass of algae and lift them to the surface of the fluid where they are easier to collect. After concentrating the algae suspension on 5% to 15% of dry matter, further slurry dehydration is needed to go to the following step of the production of biodiesel [8; 34]. A significant energy amount is required for drying of algal biomass, amounting to 11.22 MJ/kg [55]. Therefore, drying is a significant economic concern because the energy required to dry algae can account for the largest share of processing costs. Removing the moisture content from the algal biomass is necessary because moisture interferes with further processing steps, such as lipid extraction and transesterification. Many different dehydration techniques can be utilized for drying algae suspensions such as sun drying, freeze-drying, flashing drying, rotary drying, spray drying, toroidal drying, and fluid bed drying [34; 56; 57; 58]. All particular technologies of thickening and dewatering algal slurry have different characters. The microalgae strain, size, morphology, and composition of the medium used for cultivation, influence the effectiveness of these technologies. It is essential to analyze all available technologies before selecting which one to apply [59]. The dehydration of biomass is followed by lipid extraction from algae biomass. The process is much more demanding than oil extraction from terrestrial cultures.