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A secondary effluent from a wastewater treatment plant was used as the feed water in the membrane fouling experiment in which the FO membrane was fouled for 9 d and the feed water and draw solution were changed once a day during the operation. The flux decreased in FO and RO, but the membrane fouling observed in RO decreased the flux at double the rate of the corresponding rate in FO Figure 5. The degree of membrane fouling increased and the flux diminished over time for 9 d. Table 2 shows the performance of membrane operated in FO and RO processes with respect to water quality improvement. The removal efficiencies for TN, NH4-N and TP were greater than 90% in FO but less than 90% in RO (TN: 63.3%, NH4-N: 76.9% and TP: 88.8%). The reverse solute flux from the draw solution in FO may increase the removal efficiencies because the solute moves to the reverse of the permeate flux and blocks some foulants in the feed water.
Figure 5

Fouling experiment in the FO and RO processes with the secondary effluent in the wastewater treatment plant. Experimental conditions for fouling experiments: 3 M NaCl draw solution for FO or 16 bars (232 psi) for RO; feed solution containing secondary effluent; cross-flow velocity 8.5 cm/s; pH of 7.0; and temperature of 20 ± 1 °C; (n = 4,000).

Figure 5

Fouling experiment in the FO and RO processes with the secondary effluent in the wastewater treatment plant. Experimental conditions for fouling experiments: 3 M NaCl draw solution for FO or 16 bars (232 psi) for RO; feed solution containing secondary effluent; cross-flow velocity 8.5 cm/s; pH of 7.0; and temperature of 20 ± 1 °C; (n = 4,000).

Table 2

Influent and effluent concentrations and removal efficiencies in FO and RO mode

  FO mode
RO mode
ParameterInf. conc. (mg/L)Eff. conc. (mg/L)Removal efficiency (%)Eff. conc. (mg/L)Removal efficiency (%)
TN 16.9 1.3 92.3 6.2 63.3 
NH4-N 15.7 0.8 94.7 3.6 76.9 
TP 5.3 0.5 91.1 0.6 88.8 
  FO mode
RO mode
ParameterInf. conc. (mg/L)Eff. conc. (mg/L)Removal efficiency (%)Eff. conc. (mg/L)Removal efficiency (%)
TN 16.9 1.3 92.3 6.2 63.3 
NH4-N 15.7 0.8 94.7 3.6 76.9 
TP 5.3 0.5 91.1 0.6 88.8 
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