Frequently Asked Questions about Reverse Osmosis Membranes and Ion Exchange Resins (71-80)

71. Oil or other hydrophobic substances are harmful to membranes, so why can silicone grease be used as a lubricant?

This is because the high viscosity of Dow Corning 111 prevents these substances from entering the membrane surface.

72. Can reverse osmosis remove microorganisms such as viruses and bacteria?

Reverse osmosis (RO) is very dense and has a very high removal rate for viruses, bacteriophages, and bacteria, at least above 3 log (removal rate >99.9%). However, it should be noted that in many cases, microbial regeneration may still occur on the membrane permeate side. This mainly depends on the installation, monitoring, and maintenance methods. In other words, the ability of a system to remove microorganisms depends primarily on the appropriateness of the system design, operation, and management, rather than the properties of the membrane element itself.

73. The Dow FILMTEC NF90-400, NF270-400, NF200-400, and BW30LE-440, XLE-440 have larger permeate center tube diameters. Are special internal connectors and adapters required?

Each of the above Dow components comes with one internal connector. The adapters are provided by the pressure housing manufacturer. Please inform the housing supplier in advance of the model number of the selected component so they can provide the corresponding endplate adapter.

74. How does temperature affect permeate flow rate?

Higher temperatures result in higher permeate flow rate, and vice versa. When operating at higher temperatures, the operating pressure should be lowered to maintain a constant permeate flow rate, and vice versa. Please refer to the relevant section for the Temperature Correction Factor (TCF) for permeate flow rate variations.

75. What is membrane compaction? Why does it occur?

Under the combined effects of high temperature and high pressure, membrane compaction can occur, resulting in a decrease in permeate flow or system output. Compaction is an irreversible degradation of membrane performance. In fact, composite membranes are more resistant to compaction than cellulose acetate membranes, but frequent water hammer can still cause membrane compaction and must be avoided.

76. Why can water still exhibit oxidizing properties after adding SMBS (sodium bisulfite) reducing agent?

It has been found that even after adding SMBS, water can still exhibit oxidizing properties, and metal ions such as Fe and Cu can enhance this tendency, most commonly seen in seawater desalination systems. The synergistic effect of NaCl, NaHCO3, and copper can promote this oxidation capacity. In severe cases, oxidation-reduction potential (ORP) should be used for monitoring.

77. What is the temperature range for storing Dow FILMTEC dry and warm membranes?

0℃～45℃

78. Can formaldehyde (also known as formalin) solution be used as a protective solution for membrane systems or elements?

We do not recommend its use for the following reasons:

 It is suspected of having carcinogenic properties;

 Under certain conditions (especially within the first 6 hours of commissioning a new membrane), it can cause a decrease in permeate flow. This must be considered if you choose to use it.

79. What are particulate and colloidal fouling? How is it measured?

Particulate and colloidal fouling in reverse osmosis or nanofiltration systems can severely impact membrane permeate flow and sometimes reduce desalination rates. An early symptom of colloidal fouling is an increase in system differential pressure. The sources of particles or colloids in the membrane feed water vary depending on the location and often include bacteria, sludge, colloidal silica, iron corrosion products, etc. Chemicals used in the pretreatment section, such as polyaluminum chloride and ferric chloride or cationic polyelectrolytes, can also cause fouling if they are not effectively removed in the clarifier or media filter. Furthermore, cationic polyelectrolytes can react with anionic antiscalants, and their precipitates can foul membrane elements. The tendency for fouling in the water or the adequacy of pretreatment is evaluated using SDI15; please refer to the relevant chapters for detailed information.

80. What scale inhibitors are permitted for use with Dow FILMTEC membrane elements?

A variety of commercially available scale inhibitors are available. Their scale inhibition performance and membrane compatibility have been proven through long-term use by scale inhibitor manufacturers and water treatment system suppliers. Whether a new scale inhibitor is suitable should be confirmed by the scale inhibitor supplier. Water treatment system engineering companies will use their extensive engineering experience to responsibly select the most suitable scale inhibitor for the user's system (water type).