Cleaning Solution Formula Differs as RO Membrane Contaminants Vary

RO Membrane Cleaning

Cleaning Criteria

Microorganisms, colloidal solids and other impurities cannot be completely removed from water in the pretreatment process. As a result, RO membranes are fouled when operating for a period, leading to a decline in membrane element performance. Clean the membrane elements when the following cleaning criteria are met.

• Normalized permeate flow drops 10% since the last cleaning;
• Normalized salt passage increases 5%;
• Normalized pressure drop between the feed and concentrate goes up 15%;
• Before the RO system turns into in idle.

Membrane cleaning is also required in daily RO system maintenance.

Contaminant Determination

• Visual Inspection
  • Check if the membrane element outer wrap is intact;
  • Check if there is silt and other foreign matters attached to the membrane outer wrap at the feed end;
  • Check if there are large particles, active carbon granules and other objects on the membrane outer wrap at the feed end;
  • Check if the feed carrier or permeate carrier at the concentrate end is washed out or if sediment occurs.
• Weight Comparison
  • Take out the membrane element and put it vertically. When the water drains out, weigh the membrane element and compare its weight with that of unused membrane element to determine the contaminant type of the membrane element.

Cleaning Scheme

• Cleaning Flow
  Cleaning flow refers to the flow of the membrane cleaner entering into the pressure vessel in the cleaning process. The cleaning flow varies as the pressure vessel diameter varies.

  Cleaning Flow of Membrane

  Membrane Element Diameter (inch)	Flow of Every Pressure Vessel in 1st Stage GPM (L/min)
  2.5	5 (19)
  4	10 (38)
  8	40 (151)

• Membrane Cleaner Selection

  Membrane Cleaner Selection

  Contaminant	Common Features	Membrane Cleaner
  Inorganic salt deposits	Significant decrease in salt rejection, increase in the feed-to-concentrate pressure drop, and slight decrease in the system productivity.	Ammonia is adjusted to pH 4.0 by using 2% citric acid solution at 40 °C; pH 2–3 (0.5%) hydrochloric acid solution is also used for membrane cleaning.
  Oxide/hydrated oxides (iron, nickel, copper, etc.)	Significant fast decrease in salt rejection, significant fast increase in the feed-to-concentrate pressure drop, and significant fast decrease in the system productivity.	Ammonia is adjusted to pH 4.0 by using 2% citric acid at 40 °C; sometimes pH 2–3 (0.5%) hydrochloric acid solution is also used for membrane cleaning.
  Mixed colloids (iron, organic substances and silicates)	Slight decrease in slat rejection, gradual increase in the feed-to-concentrate pressure drop, and gradual decreases in system productivity.	Sulfuric acid is adjusted to pH 10.0 by using 2% sodium tripolyphosphate (STTP) solution at 40 °C; sometimes NaOH solution (pH ＜ 10 ) is also used for membrane cleaning.
  Calcium sulfate	Considerable decrease in slat rejection, slight or moderate increase in the feed-to-concentrate pressure drop, and slight decrease in system productivity.	0.8% EDTA (ethylene diamine tetraacetic acid) tetrasodium salt solution (0.25% sodim dodecyl benzene sulfonate solution may be used if necessary) is adjusted to pH 10.0 by using 2% sodium tripolyphosphate solution at 40 °C; sometimes NaOH solution (pH ＜ 10 ) is also used for membrane cleaning.
  Organic deposits	Possible decrease in salt rejection, gradual increase in the feed-to-concentrate pressure drop, and gradual decrease in system productivity.	0.8% EDTA (ethylene diamine tetraacetic acid) tetrasodium salt solution (0.25% sodim dodecyl benzene sulfonate solution may be used if necessary) is adjusted to pH 10.0 by using 2% sodium tripolyphosphate solution at 40 °C.
  Bacteriological contamination	Possible decrease in salt rejection, significant increase in the feed-to-concentrate pressure drop, and considerable decrease in system productivity.	0.8% EDTA (ethylene diamine tetraacetic acid) tetrasodium salt solution (0.25% sodim dodecyl benzene sulfonate solution may be used if necessary) is adjusted to pH 10.0 by using 2% sodium tripolyphosphate solution at 40 °C; 0.1% NaOH is adjusted to pH 11.5 by using 0.03% sodim dodecyl benzene sulfonate solution.

• Precautions
  • Membrane element cleaning must be operated by professionals;
  • When performing chemical cleaning, corresponding protective measures must be taken to prevent chemical agents from damaging operators, for example, wear protective glasses, gloves and work clothes. Especially when using some strong corrosive chemical agents, if contact the agents accidentally, please dispose in accordance with MSDS supplied by chemical suppliers.
  • During the cleaning process, any chemical agent that may do harm to the membrane elements are prohibited.

RO Membrane Maintenance

1. Storage before Installation
   When leaving the factory, membrane elements shall be enclosed in a sealed bag containing a storage solution. During the period of use and transportation, the elements shall be stored in a cool, dry place with an ambient temperature range of 20 °C to 35 °C. The elements shall be protected from direct sunlight and avoid contacting with oxidizing gases.
2. Storage after Installation
   1. Short-Term Storage
      1. It is suitable for periods where an RO system must remain out of operation for more than 5 days, but fewer than 30 days, with the RO elements in place. Store the elements as follows:
      2. Flush the RO system with feed water, while simultaneously venting any gas from the system;
      3. Run the system 1–2 hours every day. Close relevant valves to prevent gas from entering the system.
   2. Long-Term Storage
      It is suitable for where an RO system must remain out of operation for more than 30 days with the RO elements in place. Store the elements as follows:
      1. Flush the RO system with feed water, while simultaneously venting any gas from the system;
      2. Fill the RO system with biocide solution prepared from permeate and 1% sodium hydrogensulfite, and make sure that is fully filled;
      3. Repeat step ② with fresh solution every 30 days if the temperature is below 27 °C, or every 15 days if the temperature is above 27 °C;
      4. When the RO system is ready to be returned to service, flush the system for one hour with low pressure feed water. Before returning the RO system to service, check for any residual biocide in the permeate.
3. RO Membrane Sterilization
   1. Biocide Sterilization Speed (precondition: remove 99.9999% of spores at 20 °C)

      Biocide Sterilization Speed

      Biocide	Duration of Exposure
      2% formaldehyde solution	12 hours
      0.2% hydrogen peroxide solution	25 hours
      5% hydrogen peroxide solution	2–3 hours
      10% hydrogen peroxide solution	1–2 hours
      1% hydrogen peroxide solution and 400 mg/L peracetic acid solution	0.5–1 hours

   2. Biocide for Composite Membrane Element Sterilization
      • Formaldehyde
        • Dose: 0.1% – 1.0% (for new membranes, it must be used only after 24 hours of operation, otherwise it will lead to irreversible loss of water flux. It can be used as a biocide solution for long-term storage)
      • Isothiazolinone
        • Dose: 15–25 mg/L
      • Sodium hydrogensulfite
        • Dose: 500 mg/L (1.0% solution can be used for long-term storage)
        • Duration of exposure: 30–60 minutes
      • Hydrogen peroxide/peracetic acid
        • Dose: 0.2% (total content of both compounds), 1.0% solution can be used for long-term storage.
        • pH value: 3–4; temperature: 25 °C (maximum);
        • Recycle for 20–30 minutes, soak for 2–4 hours and then flush
        • It is the most effective and rapid oxidizing biocide for destructing biological mucosa, which may take a 4-hour exposure; the biocide solution is not suitable for long-term storage.