# Effects of Caprolactam Wastewater on Algal Growth and Nutrients Removal by _Arthrospira platensis_
## 3 Results and Discussion
### Removal of N and P from WCP by A. platensis Cultivated in a Circular Photobioreactor  
To test the N and P removal efficiencies under the condition of fed-batch addition of mixed WCP, _A. platensis_ was cultured in a 25 L circular photobioreactor (Figure 8). Mixed WCP was added daily at a concentration of 1.25%, and the culture was continued for 10 d with total WCP addition of 3.125 L (12.5%, \(v/v\)). During cultivation, the daily evaporation of water was approximately 0.7 L; 0.3125 L (1.25%) of WCP addition compensated for some of the water evaporation, and deionized water was added to compensate for the remaining water evaporation so that the culture volume was maintained at 25 L. Changes in the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**WCP Supply Modes** & **Culture Phase** & \
\begin{tabular}{c} **Protein Production** \\ **(mg/L)** \\ \end{tabular} &
\begin{tabular}{c} **T-PC Production** \\ **(mg/L)** \\ \end{tabular} \\ \hline \multirow{4}{*}{Control} & 1–4 day & 1188.47 \(\pm\) 49.79 & 265.45 \(\pm\) 12.44 \\  & 5–8 day & 1470.28 \(\pm\) 77.17 & 376.37 \(\pm\) 37.86 \\  & 9–12 day & 1075.88 \(\pm\) 63.45 & 307.84 \(\pm\) 47.98 \\  & 13–16 day & 1229.85 \(\pm\) 42.60 & 326.72 \(\pm\) 32.81 \\  & 1–16 day & 4964.47 \(\pm\) 87.95 & 1269.79 \(\pm\) 55.49 \\ \hline \multirow{4}{*}{Fed-batch mode 1} & 1–4 day & 1153.67 \(\pm\) 7.48 & 246.50 \(\pm\) 11.46 \\  & 5–8 day & 1120.48 \(\pm\) 43.11 & 229.77 \(\pm\) 13.01 \\  & 9–12 day & 603.22 \(\pm\) 19.45 & 114.10 \(\pm\) 16.65 \\  & 13–16 day & 696.02 \(\pm\) 18.04 & 155.12 \(\pm\) 33.57 \\  & 1–16 day & 3573.38 \(\pm\) 37.40 & 745.49 \(\pm\) 58.20 \\ \hline \multirow{4}{*}{Fed-batch mode 2} & 1–4 day & 1057.46 \(\pm\) 30.57 & 228.50 \(\pm\) 6.47 \\  & 5–8 day & 100.11 \(\pm\) 86.82 & 203.33 \(\pm\) 18.53 \\  & 9–12 day & 524.67 \(\pm\) 69.16 & 100.50 \(\pm\) 16.83 \\  & 13–16 day & 585.11 \(\pm\) 44.49 & 118.99 \(\pm\) 4.57 \\  & 1–16 day & 3173.35 \(\pm\) 104.64 & 651.32 \(\pm\) 22.97 \\ \hline \multirow{4}{*}{Batch mode} & 1–4 day & 694.47 \(\pm\) 39.42 & 111.74 \(\pm\) 6.64 \\  & 5–8 day & 571.96 \(\pm\) 57.55 & 113.34 \(\pm\) 8.40 \\ \cline{1-1}  & 9–12 day & 301.61 \(\pm\) 20.49 & 60.81 \(\pm\) 8.96 \\ \cline{1-1}  & 13–16 day & 675.87 \(\pm\) 65.29 & 166.52 \(\pm\) 9.72 \\ \cline{1-1}  & 1–16 day & 2234.92 \(\pm\) 110.66 & 452.40 \(\pm\) 23.73 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Effects of the caprolactam wastewater supply mode on protein and total PC production (mean \(\pm\) standard deviation) of _Arthrospira platensis._NO3\({}^{-}\)-N and inorganic P concentrations in the algal suspension are shown in Figure 8. After adding wastewater every day, the NO3\({}^{-}\)-N and P concentrations reached 14.26 mg/L and 0.76 mg/L, respectively. After 24 h of cultivation, NO3\({}^{-}\)-N and P were not detected in the algal suspension, thereby indicating that all the N and P from WCP addition were assimilated by _A. platensis_. The biomass DW was 0.15 g/L at the beginning of the culture, and the biomass increased linearly during cultivation. After 5 d of culture, the biomass reached 0.7 g/L. To avoid a high biomass density, which might slow algal growth, the biomass was harvested to reduce the DW to 0.31 g/L. After harvest, the biomass increased linearly again and reached 0.92 g/L on day 10. The overall biomass productivity reached 17.48 g/(m\({}^{2}\)-d) with an average growth rate of 0.22 d\({}^{-1}\). The maximum protein content of _Arthospira_ on days 5 and 10 of culture was determined to be 67.56% and 69.93%, respectively. The high protein content resulted in high protein production of 12.03 g/(m\({}^{2}\)-d). The average biomass areal productivities were 17.25, 15.60, 14.75, 17.44, and 17.44 g/(m\({}^{2}\)-d) during days 0-4, 0-5, 0-6, 0-8, and 0-10, respectively. Although there were some fluctuations, the biomass productivities did not exhibit a trend of decline during cultivation and an accumulation effect of growth inhibition of WCP on _A.