International Journal of Environmental Science and Technology
scaling and fouling in RO membranes. Hence, pressures of all stages did not increase rapidly (ASTM D3739-19 2019).
Free mineral acidity and p H at cation e x changer
Free mineral acidity (FMA) can be measured by the ani- ons of strong acids, namely sulfuric, nitric, and hydrochlo- ric acid, which is free to react at the outlet of the cation exchanger (Ramzan et al. 2012). In Fig. 6, a stable FMA value around 52 was observed due to the injection of sulfuric acid during acid treatment in the feed line. All the anions of strong acid were filtered at the primary anion exchanger, which was shown the value of FMA reduced to 0 at the primary anion exchanger outlet. This shows that the ion exchangers are relatively stable over time and there was no leakage at the primary anion exchanger. The pH at the cation exchanger was significantly decreased from about 7.7 to 2.9 due to acid injection, which is shown in Fig. 7. These pH values are still within the operational range of the RO plant, which is 2–11 for continuous operation, as given in Table 1.
Fig. 4 Effect of optimized pressure curve of all stages against time for RO plant after acid treatment. The pressure increases at each stage, but it is not a huge change from the pressure which is observed with- out the acid treatment
reduced three times. Pressure increment in the second stage, third stage, and total reject was 8.3%, 9.8% and 11.1% respectively. Cleaning frequency and operational cost were reduced while maintaining the quality of permeate (Redondo and Lomax 2001).
E ff ect of permeate water fl ow rate with time
Figure 8 shows clearly that product flow decreased with the passage of time due to fouling and scaling on RO mem- branes and resulted in a decrease of permeate flux as well as recovery rate. It clearly indicates that flux declined on the surface of the RO membrane was due to crystal forma- tion and these crystals caused fouling and scaling by which hindrance to feed water, and it caused decease in permeate water. Flux decline mechanism involved in the crystalline layer formation on the porous surface of membranes (Brusi- lovsky et al. 1992).
E ff ect on L S I factor after acid treatment
Figure 5 shows the controlled LSI value by H 2 SO 4 addition in the feed line to reduce pH from 7.6 to 7.2. Lowering the pH resulted in a lower value of LSI (1.6) in total reject. This controlled LSI factor resulted in minimizing the formation of
Fig. 5 LSI factor at RO membrane after acid addition with time . The scaling and fouling in RO membrane is reduced due to the acid treat- ment
Fig. 6 Free mineral acidity (FMA) versus time. Both cation exchanger outlet and primary anion exchanger outlet values remain extremely stable over a long period of time
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