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]{>{centeringarraybackslash}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}