Permeate conductivity

Figure 4.2 Permeation conductivity for some elastomers in the presence of air...

Figure 14.4 Bar graph showing permeate conductivities after profiling RO system.

Why is the Permeate Conductivity High after Cleaning the Membranes ... 

The raw water conductivity decreased 5 to 10 % during microfiltration. No rejection of salt is expected with MF, however, the ions may be associated with the particulates or retained organics. The RO permeate was very low in conductivity, as expected, and is not very concentration dependent. During the concentration experiment, the permeate conductivity increased from 3 to 10 pS. 

Figure A 1.14 Rejection of conductivity and permeate conductivity as a function of concentrate conductivity (25 10196).

The first-pass RO unit is a three-stage (2 1 1) array and the recovery is 75%. The permeate flows to the second-pass RO unit and the reject flows to drain. The second-pass RO unit is a two-stage (1 1) array operating at a recovery of 90%. The reject flows back to the first-pass RO pump inlet. The second-pass permeate conductivity is <0.2 pS/cm. The overall recovery of the double-pass RO system is 69%. Caustic soda solution is injected in the second-pass feed water fine (first-pass permeate) to raise the pH from 6.0 to 8.0. At alkaline pH, free CO2 in water is converted to sodium bicarbonate, which is easily rejected by the RO membrane. 

The single-pass RO unit is a three-stage (1 1 1) array designed to produce purified water at an overall recovery of 75%. The TFC RO membrane average rejection is 98%. The permeate conductivity should be less than 10.0 pS/cm. When the AP across the membrane exceeds the maximum allowable value, the RO unit is taken out of service to clean the membranes. The frequency of cleaning the RO unit is typically 3-6 months for softened RO feed water. 

Caustic soda (50% NaOH solution) may be injected into the RO feed water line by the chemical dosing pump (one pump is on standby) to maintain the pH of RO feed water between 7.5 and 8.5 if there is no hkehhood of carbonate scaling. The iiyection rate is controlled by the PLC based on the pH value measured downstream of the in-hne mixer. At alkaline pH conditions, dissolved CO2 gas is converted to bicarbonate ions, thereby enhancing membrane rejection (lower permeate conductivity). 

RO permeate flows to the RO product water storage tank at 53 m /h. If the tank is full or if the permeate conductivity exceeds the maximum permissible value (see Table 4.5) for an extended period of time (minutes), the RO system should be shut down. The feed water and permeate flow rate date is used by the PLC to calculate % recovery. Similarly, RO feed water and permeate conductivity is monitored continuously, and the data are used by the PLC to calculate salt rejection or simply %rejection. 

The RO unit design is based on meeting the specified product water quality and flux. RO unit service run is based on permeate conductivity (% rejection), productivity... 

Figure 13.9 reports the trend of the permeate conductivity with F concentrations. 

Figure 13.9 shows that for F concentrations ranging from 5 to 1000 mg L , there was no remarkable impact on the rejection and the permeate conductivity was constant and equal to 2piScm. However, when the feed F concentration increased to 5000 mg L , the permeate conductivity increased as well, reaching the value of 3 p,Scm . This result was attributed by authors to the partial wetting of larger pores present on the membrane surface. 

Figure 13.9. Variation of the permeate conductivity with fluoride concentrations. Vfeed=0.52ms 7feed = 50°C udist = 0.15mTdist =20°C (adaptedfromHou ci a/., 2010).

In the Figures 13.10 and 13.11, a comparison, in terms of permeate flux and permeate conductivity, between the gradual concentration of natural groundwater and pre-acidified groundwater is presented. When pre-acidified brackish groundwater was considered as feed, the tests were carried out at pH 4.0. 

Figure 13.11. Comparison of the permeate conductivity measured during the concentration of natural groimdwater and of pre-acidified groundwater. ufeed=0-52ms Tfeej = 50°C ujist = 0.15ms 2dist = 20°C (adapted from Hou ef a/., 2010).

Figure 13.11 shows that there is a linear increase of the conductivity with the concentration factor, reaching a maximum value of about 15 xS cm . After the acidification of the feed, the permeate conductivity stabilized at about 2.0 (xS cm , until concentration factors were lower than 5.0 when the concentration factor increased from 5.0 to 10.0 the permeate conductivity increased as well, from 3.0 to 8.0 (xS cm, respectively, because of deposit formation. 

The authors performed also long-term experimental tests (300 h, concentration factor of 4.0) with the pre-acidified brackish groundwater as feed solution. The permeate flux was constant at about 11.5 kg m h during all the experiment and the permeate conductivity stabilized at about 3.0 xS cm with a F content in the permeate below the detection limit. 

---