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On the FO side, average apparent rejections of four ion species and DOC were obtained ranging from 79.5% (ammonium) to 100% (Table 2). Although the rejection of ammonium was lower than that of others, moderate rejection of ammonium in a FO system has been reported, and the rejection in this study was within the reported values in the laboratory- and pilot-scale experiments (Zhang et al. 2014; Xue et al. 2015; Wang et al. 2016; Li et al. 2018). The rejection of anions, such as NO3 and PO43− and DOC (>99.4%), was higher than that of ammonium. This result was possibly due to electrostatic repulsion between membrane and solute molecules (Xue et al. 2015).

Table 2

Rejection of cations, anions, and DOC in the FO and FO–MD hybrid systems (%)

FO sideaFO–MDa
Na+ b 99.8 ± 0.2 
NH4+ 79.5 ± 3.9 93.7 ± 2.2 
Mg2+ b 99.2 ± 0.7 
Ca2+ b 99.4 ± 0.6 
K+ b 100.0 
F 100.0 97.9 ± 2.3 
Cl b 97.9 ± 2.9 
Br b 98.8 ± 1.7 
NO3 100.0 100.0 
PO42− 100.0 100.0 
SO42− b 99.5 ± 0.8 
DOC c 99.1 ± 0.9 
FO sideaFO–MDa
Na+ b 99.8 ± 0.2 
NH4+ 79.5 ± 3.9 93.7 ± 2.2 
Mg2+ b 99.2 ± 0.7 
Ca2+ b 99.4 ± 0.6 
K+ b 100.0 
F 100.0 97.9 ± 2.3 
Cl b 97.9 ± 2.9 
Br b 98.8 ± 1.7 
NO3 100.0 100.0 
PO42− 100.0 100.0 
SO42− b 99.5 ± 0.8 
DOC c 99.1 ± 0.9 

aData show average and standard deviation for all the experiments (n = 3).

bRejection could not be calculated because of abundance in draw solution (seawater).

cDOC in draw solution could not be measured because of high salinity.

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