# Effects of Caprolactam Wastewater on Algal Growth and Nutrients Removal by _Arthrospira platensis_
## 3 Results and Discussion
### Fed-Batch Supply of WCP Decreasing the Impact on A. platensis  
#### 3.3.1 Impact of Fed-Batch Application of WCP on _A. platensis_ Growth  
To evaluate the effect of WCP supply modes on _A. platensis_ growth, one batch supply mode and two fed-batch supply modes were tested, as described in Section 2. Over the course of 16 d of culture, all cultivations were harvested every 4 d. The changes in the biomass DW are shown in Figure 7A. After 16 d of culture, the accumulated biomass concentration for the control, fed-batch supply mode 1, fed-batch supply mode 2, and batch supply mode were 7.07, 5.51, 5.19, and 3.88 g/L (Figure 7B), respectively. The biomass \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Wastewater**} & \multirow{2}{*}{**Concentration (\%)**} & \multicolumn{4}{c}{**Organic Matters in Wastewater**} & \multirow{2}{*}{**Inhibition (\%)**} \\ \cline{3-4} \cline{6-6}  & & **CHN (mg/L)** & & **CHL (mg/L)** & & **CHO (mg/L)** \\ \hline \multirow{3}{*}{WCP} & 12.5 & 20.6 \(\pm\) 1.2 & 4.1 \(\pm\) 0.4 & 5.8 \(\pm\) 0.4 & 7.1 \(\pm\) 0.4 \\ \cline{2-6}  & 25 & 41.1 \(\pm\) 2.3 & 8.2 \(\pm\) 0.6 & 11.5 \(\pm\) 3.1 & 14.8 \(\pm\) 1.2 \\ \cline{2-6}  & 50 & 82.3 \(\pm\) 3.3 & 16.4 \(\pm\) 2.1 & 23.0 \(\pm\) 2.0 & 29.6 \(\pm\) 1.8 \\ \hline \multirow{3}{*}{Reduced WCP} & 12.5 & 19.9 \(\pm\) 2 & 3.6 \(\pm\) 0.3 & 5.8 \(\pm\) 0.1 & 6.8 \(\pm\) 1.4 \\ \cline{2-6}  & 25 & 39.8 \(\pm\) 3.8 & 7.2 \(\pm\) 0.7 & 11.5 \(\pm\) 1.3 & 15.1 \(\pm\) 1.4 \\ \cline{1-1} \cline{2-6}  & 50 & 79.5 \(\pm\) 4.9 & 14.5 \(\pm\) 1.2 & 23.0 \(\pm\) 2.3 & 31.3 \(\pm\) 2.9 \\ \hline \multirow{3}{*}{Heat-treated WCP} & 12.5 & 0 & 0 & 0 & 9.9 \(\pm\) 3.0 \\ \cline{1-1}  & 25 & 0 & 0 & 0 & 13.6 \(\pm\) 1.4 \\ \cline{1-1}  & 50 & 0 & 0 & 0 & 29.9 \(\pm\) 2.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of the organic matter contents and growth inhibition effects of wastewaters. Figure 6: Effects of different concentrations of heat-treated caprolactam wastewater (HWCP) on the growth of _Arthrosipra platensis_. (**A**) Growth curves of _Arthrosipra platensis_, (**B**) final biomass concentration, (**C**) growth inhibition rate (%). Error bars represent the standard deviation (\(n=3\)). Data with different letters indicate significant differences (\(p<0.05\)) between the control and treatments. concentration of fed-batch mode 2 was significantly higher than that of the batch supply mode, and the biomass concentration of fed-batch supply mode 1 was significantly higher than that of fed-batch supply mode 2 and the batch supply mode (Figure 7B). Figure 7C shows the inhibition rates in the culture periods of days 0-4, 0-8, 0-12, and 0-16. These results indicated that by supplying the same amount of WCP, the fed-batch mode was more beneficial to algal growth than the batch mode. Moreover, a lower dose of each WCP addition resulted in a lower growth inhibition rate. Figure 3 shows that the growth inhibition rates of 12.5%, 25.0%, and 50.0% WCP on _Arthrospira_ increased as the culture time increased during the 6 d culture period. The inhibition of _Arthrospira_ by WCP showed an accumulation effect over culture time. What would happen if the culture time further increased? Figure 7C shows that the inhibition rate of the batch mode reached a maximum of 48.10% on day 12, and then decreased to 45.16% on day 16. After 12 d of culture, although the inhibition rates of the two fed-batch modes did not decrease, the upward trend of the inhibition rate began to slow. The phenomenon of the decline of inhibition rate, or the slowing of the upward trend of the inhibition rate, might indicate that under the combined effects of physical, chemical, and biological factors in algal culture, the inhibitory components in WCP could not exist stably for a long time, i.e., they were consumed as the culture time increased.