# The Perspective of Large-Scale Production of Algae Biodiesel
## 3 Production of Algal Biodiesel
### Algae Farming Systems
There is also an alternative approach to growing algae in the dark, known as heterotrophic fermentation. Although photosynthesis is based on CO2 and light to meet the organisms' needs, another source of carbon has to be added to heterotrophic cultivation. Many organisms have developed mechanisms for the assimilation of carbon sources other than CO2. The ability to grow on sugar or acetate allows these organisms to survive even in dark conditions. In the mid-1990s, the heterotrophic algae production technology for Chlorella production was developed in Japan. Heterotrophic microalgae can take carbon and nitrogen from a variety of sources, but inexpensive materials such as inedible lignocellulosic feedstocks are preferred. The most cost-effective sources of nitrogen are ammonium or nitrates. The transition to heterotrophic production leads to a significant increase in volumetric production rates. While photobioreactor dries mass concentrations with peak values only slightly above 3 g/L, the heterotrophic processes easily reach ten to twenty times higher cell densities. [PERSON] et al. [36] think that by applying photobioreactor technology, over five years, it is possible to increase the average biomass productivity from 5 to 8 g/L/day. When considering the algal biodiesel production, an important aspect is a yield obtained from algal biomass (Table 2). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Microalgae Species**} & \multirow{2}{*}{**Lipid Productivity**} & **Lipid Content** & **Biomass** & \multirow{2}{*}{**Reference**} \\  & & **(g/L/day)** & & **(\% Dry Weight)** & \\ \hline _Chloorella vulgaris_ & 0.1837 & 32.5 & 0.2341–0.5322 & ([PERSON] et al., 2018) [37] \\ _Chloorella vulgaris_ & 0.0421 & 47.53 & - & ([PERSON] et al., 2018) [58] \\ _Chloorella vulgaris_ & 0.300 & 31 & 0.71 & ([PERSON] et al., 2012) [39] \\ _Desmallosmps. Sp. 881_ & 0.01983 & 48.41 & - & ([PERSON] et al., 2016) [40] \\ _Nammodloropsis acamica_ & 0.05691 & 46.14 & 0.91 & ([PERSON] et al., 2015) [41] \\ _Paracholorella kesseri_ & 0.590 & 66 & 0.82 & ([PERSON] et al., 2018) [42] \\ _Paracholorella kesseri_ & 0.500 & 25 & 0.64 & ([PERSON] et al., 2013) [43] \\ _Chloydomonas reinhardtii_ & 0.08092 & - & 3.33 & ([PERSON] et al., 2019) [44] \\ _Scendesmus quadricauda_ & 0.13955 & 38.61 & 0.3614 & ([PERSON] and [PERSON], 2018) [45] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The yield of biomass and oil from different species of algae. The presented results refer to laboratory research. In real conditions of algae cultivation, it is impossible to maintain ideal conditions like in the laboratory, so the realistically achievable results are much lower [46]. Biomass productivity can vary by more than one order of magnitude depending on location, farming system, and algal species. The efficiency of converting sugar into algae biomass is from 0.2 to 0.3 g of the lipid per gram of sugar to produce cells with high lipid content, usually with over 50% fat. The cultivation of heterotrophic microalgae typically takes place under strictly sterile conditions. Costs of contamination control or strict sterile farming can amount to 20-30% of production costs. Recently, there has been growing attention to a combination of heterotrophic and photosynthetic production (mixotrophic production) as a way to increase production in photobioreactors [45]. While most manufacturers have focused solely on open-pond systems or only photobioreactors, some manufacturers have sought to combine the benefits of separate systems. This kind of algae cultivation that combines two or more of these systems is called a hybrid system. These may be small or more significant photobioreactors used in combination with outdoor ponds. Recently, the possibility of connecting the process of wastewater treatment and algae cultivation into one integrated system is being considered. In such a system, algae would take nutrients and CO\({}_{2}\) from wastewater for their growth and at the same time decompose unwanted and toxic substances.