Here is my code. I still could not figure out.
\documentclass{article}
\usepackage{tabularx,booktabs}
\usepackage{array}
\usepackage{makecell}
\begin{document}
\begin{table}[htbp]
\centering
\caption{Anaerobic treatment of antibiotic wastewater}
\label{anaerob}
\vspace{0.2cm}
\newcolumntype{P}[1]{>{\centering\arraybackslash}p{#1}}
\renewcommand{\arraystretch}{1.5}
\begin{tabular}{p{0.25cm}P{2.2cm}P{2.4cm}P{2.4cm}P{2.5cm}P{2.5cm}}
\hline
No&\makecell{Influent\\COD\\(mg L)}&\makecell{Influent\\antibiotics\\(mg L)}&\makecell{COD\\removal\\efficiency (\%)}&\makecell{Antibiotics\\removal\\efficiency (\%)}&Reference\\
\hline
1&9~736 to 19~862& Ampicillin 3; aureomycin 1&91&Ampicillin 67; aureomycin 51&(Zhou, 2006)\\
2&3~000 to 43~000&Penicillin&97&N/A&(Degirmentas, 2004)\\
3&127~000&Erythromycin 1 to 200&94&N/A&(Amin, 2006)\\
4&3~000&Sulfamerazine 10 to 90&69 to 89&76 to 100&(Sponza, 2007)\\
\hline
\end{tabular}
\end{table}
\end{document}