## 2 Materials and Methods
### Raceways Design, Commissioning and Construction  
Based on the literature review and Allmicroalgae industrial experience in the growth of microalgae, two raceway pond prototypes were designed, as shown in Figure 1: one standard and one with a modified configuration. The standard raceway included the most common dimensions, configuration and construction materials reported, while the modified version considered several aspects previously reviewed from the literature and indicated as having a positive influence in the production of microalgal biomass.  
The starting point was a typical raceway with two bends, as ponds with more than two bends have higher energy consumption [32]. To avoid the formation of dead zones as much as possible, three baffles were included in the circular endings of the ponds. Raceway ponds with around 28 m\({}^{2}\) were developed. They were 15 m long, 2 m wide and 1 m channel width. The depth was fixed at 0.375 m and other structural dimensions are listed in Table 1. The mixer consisted of a single eight-flat-bladed paddlewheel installed right after one of the semicircular ends at the beginning of one of the channels.  
Figure 1: 3D prototype models of the two built raceways. (**a**) Standard raceway model with three baffles, an eight-bladed paddlewheel and 87 cm width; (**b**) Modified raceway model, with three deflectors, an eight blades paddlewheel and 47 cm width.  
The modified raceway model included a loop-shaped end of the central divider opposite to the paddlewheel. The 'loop' was intended to prevent the development of dead zones in the vicinity of this end of the divider. Besides, the paddlewheel hypothesized for the modified model had a width of 0.475 m, as compared to the standard model paddlewheel which measured 0.870 m. A picture of the raceways can be seen in Figure 2. Figure 3 shows a detailed view of the modified raceway.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**Standard Raceway**} & \multicolumn{2}{c}{**Modified Raceway**} \\ \hline \multirow{5}{*}{Dimensions} & Total area & 28.88 m\({}^{2}\) & 27.62 m\({}^{2}\) \\  & R (Width: Length) & 1.7.5 & 1:7.5 \\  & Total Width & 2 m & 2 m \\  & Total Length & 15 m & 15 m \\  & Channel length & 1 m & 1 m \\  & Channel width & & \\  & surrounding & 0.950 m & 0.545 m \\  & paddlewheel width & 0.870 m & 0.475 m \\  & Depth & 0.375 m & 0.375 m \\  & Volume & [2.89–8.66] m\({}^{3}\) & [2.76–8.28] m\({}^{3}\) \\ \hline \multirow{5}{*}{Operational specifications} & Water depth & [0.1–0.3] m & [0.1–0.3] m \\  & Speed & [5–60] rpm [0.1–0.3] m/s & [5–60] rpm [0.1–0.3] m/s \\ \cline{1-1}  & Paddlewheel & 8 blades, no slope & 8 blades, no slope \\ \cline{1-1}  & Carbonation & Box after paddlewheel & Box after paddlewheel \\ \cline{1-1}  & Number of baffles & Six & Three \\ \hline \multirow{5}{*}{Raw materials} & \multirow{5}{*}{Base of the raceway} & Polyethylene film, stone & Polyethylene film, stone \\  & & powder, PVC waterproofing & powder, PVC waterproofing \\ \cline{1-1}  & & geomembrane. & geomembrane. \\ \cline{1-1}  & External walls & Treated wood & Treated wood \\ \cline{1-1}  & Structure & Stainless steel & Stainless steel \\ \hline \hline \end{tabular}
\end{table}
Table 1: General characteristics of the two built raceways: dimensions, operational specifications, and raw materials.  
Figure 2: Pilot-scale raceway ponds used. Modified raceway at right and standard raceway at left.  
The raw materials used for the construction of both raceways were the same. Polyethylene film and PVC waterproofing geomembrane were used in the base of the raceways to waterproof them. The structure was supported by stone powder and stainless steel and external walls were made of treated wood.