International Journal of Environmental Science and Technology
E ff ect of feed TDS with permeate TDS against time
Feed water quality directly affected the product. Higher TDS in feed resulted in higher TDS in the product, as shown in Fig. 9. For this reason, a high concentration of salts in feed was rejected by membranes and collected in the concentrate stream. These salts increased load on the membrane surface, and hence membrane permeates water quality automatically increased correspondingly (Sridhar 2002).
The normali z ed concentration of salt in re j ect water against time
The concentration of sodium, potassium, chloride, and sul- fate ions was measured at the reject water tank, as shown in Fig. 10. It was found that breakthrough of several ions such as sodium, potassium, and sulfate can be observed, and the normalized concentrations were increased by 6.12%, 6.06%, and 2.52% from 900 to 2400 h for sulfate, potassium, and sodium ions, respectively. On the other hand, no chloride ion leakage was found. The salt leakage may lead to calcium carbonate scaling formation due to the following reactions.
Fig. 7 The graph of pH at the cation exchanger versus time. The pH outlet is extremely stable, while the inlet pH shows a slight increase over a long period of time
CaCl 2(aq) + Na 2 CO 3(aq)
→ Ca
2 CO 3(s) + NaCl (aq)
(1)
CaCl 2(aq) + K 2 CO 3(aq)
→ Ca
2 CO 3 (s) + KCl (aq)
(2)
CaCl 2 , Na 2 CO 3 , and K 2 CO 3 are soluble in water while Ca 2 CO 3 is not soluble, creating precipitate or scaling (Jimoh et al. 2018). Due to no leakage in chloride ion, those break- through values still met the operational condition in which the recommended LSI is below 1.8 by the industrial standard
Fig. 8 Effect of flow rate at the inlet and outlet of the RO membrane with time. The outlet flow decrease over time is significant. The outlet flow increase causes an increase in reject flow in RO membrane
E ff ect of re j ect water fl ow rate with time
It is obvious from Fig. 8 that with the passage of time, highly concentrated water was not allowed to pass through the membrane pores, and it was directed into the reject stream. Therefore, fouling and scaling occurred and ulti- mately reject flow increased (Radu et al. 2012). The RO membrane was unable to handle the increased flow of highly concentrated water, this resulted in the increase of the Rejected water, which also caused more scaling. This resulted in a vicious circle, which is due to constant increasing scaling, the RO membrane becomes less and less effective, which causes more scaling and so on.
Fig. 9 Effect of TDS at the inlet and outlet of the RO membrane with time. The product and Feed TDS are almost the same. This is due to the feed TDS having an impact on the product TDS
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