Control of residual aluminum from conventional treatment to improve reverse osmosis performance

Abstract

Soluble aluminum (Al 3+ ) may react with both ambient silica and antiscalant components to form colloidal foulants during reverse osmosis (RO) treatment. Whereas conventional treatment (coagulation/filtration/sedimentation/dual-media filtration) was being used prior to RO, aluminum sulfate (alum) and polyaluminum chloride (PACl) coagulants were evaluated at ambient pH (pH 7.8 to 7.9) and suppressed pH (pH 6.7) in an effort to lower the total aluminum to below 50 µg/L-a level previously observed to prevent RO membrane fouling. Additional tests were conducted with 5 mg/L citric acid added to the RO influent to chelate the soluble aluminum fraction. All tests were conducted with 1.5 to 2.5 mg/L chloramines present. Testing of a RO process fed with optimized alum-or PACl-coagulated water showed that PACl outperformed alum regardless of pH. Alum coagulation at ambient pH resulted in 184 to 273 µg/L total aluminum passing through the filtration process. Only by lowering the mean influent water pH to 6.7 was the mean soluble aluminum residual (45 µg/L) for alum coagulation reduced to below the 50 µg/L aluminum goal. Regardless of pH, for alum-coagulated waters, the higher aluminum carryover resulted in severe RO membrane fouling within 500 hours of operation. Only when a chelating agent (citric acid) was added to the RO feed was the loss in productivity and selectivity arrested. However, PACl consistently met the 50-µg/L goal for both total and soluble aluminum for all pH levels tested, which resulted in more stable membrane performance over time. Further research on the compatibility of PACl and polyamide membranes in the presence of chloramines is needed as data from this project suggest PACl coagulation may facilitate membrane oxidation.

Degradation of polyamide reverse osmosis membranes in the presence of chloramine

“…28% is observed and after 17 h of exposure and it has been suggested that aluminium can accumulate on the surface of membranes over time resulting in reduced flux [51]. A plot of the normalized absorbance ratio versus time for a sample of the membrane exposed to 25 ppm chloramine and 0.3 ppm Fe 2+ solution is shown in Figure 7(c).…”

“…The images of the membranes exposed to Cu 2+ (Figure 3(c)) and the combination of Al 3+ /Fe 2+ (Figure 3(d)) show evidence of ubiquitous particles on the surface that varied in size and shape. The presence of these particles could potentially block some of the membrane pores resulting in changes in permeate conductivity and flux [51].…”

Case Study of a Small Scale Reverse Osmosis System for Treatment of Mixed Brackish Water and STP Effluent

“…The active layer of the membrane was shown at 1656 cm -1 , indicating amide I group (C=O) [25], while the amide II group (N-H) that should be visible at 1540 cm -1 did not appear in the FTIR. Another band that could be characteristic of the polyamide group is C=C ring vibration at 1490 cm -1 [25].…”

Investigation of the Potential of Monomeric and Polymeric Coagulants in the Treatment of Raw Water used at a Coal-Fired Power Station

“…Poly-aluminium chloride is advantageous over monomeric flocculants as it i) is more thoroughly dispersed, ii) doesn't have an impact on the clarifier, iii) forms no insoluble residues and iv) achieves better turbidity removal than alum. However, it is less effective in improving the water colour than traditional flocculants [27,28].…”