How to Maintain Industrial Reverse Osmosis Systems

14 Oct.,2022

 

Reverse Osmosis Membrane Elements

RO system membranes can collect fouling, which is rejected solute buildup that accumulates gradually. Flushing RO membranes can reduce buildup, but RO systems still require regular cleaning to operate properly and prevent membrane damage.

Cleaning RO membranes saves money on replacements because it prevents system damage and extends membrane lifespans. If the system operates in unclean conditions over an extended period, performance recovery can be difficult because unclean conditions can cause irreparable damage to membrane performance, resulting in expensive replacements. Following cleaning schedule guidelines can ensure an optimum cleaning response, and a clean reverse osmosis tank will maintain healthy water quality.

When Should I Clean a Reverse Osmosis System?

You should clean your reverse osmosis system when the normalized flow decreases from the preliminary acceptance test conditions by 10%, or when the overall pressure drop exceeds the preliminary acceptance test conditions by 15%. Increased salt passage can also indicate that a system needs to be cleaned. It’s best to clean systems during scheduled maintenance protocols or right before any medium or long-term shutdowns. You should typically clean your system every three to six months, or as soon as you notice any potential system issues.

Reverse Osmosis Cleaning Procedure

The RO membrane cleaning procedure is typically an easy task, but it can cause system issues if completed incorrectly. Problems can arise from applying the wrong cleaning solutions to improperly using the clean in place (CIP) system. Even if you perform periodic cleaning at the correct time, temperature and flow rate, using inappropriate chemistry can compromise the system.

With various RO membrane cleaning options, you should analyze your system operations and water to determine your system’s best cleaning option. Monitor the system’s performance and main plant parameters to detect any signs of membrane fouling. Increased salt passage, increased pressure loss and water flow loss are all indications to look for.

Before your system needs a chemical cleaning, determine the membrane’s most likely fouling. For example, you may notice increased pressure drops during the first system phase. This could indicate organic, inorganic or bacterial fouling, which typically benefits from a low pH soak and an alkaline cleaning. Elevated pressure drops during the final system phase can indicate mineral deposition.

To clean a reverse osmosis membrane, it’s important to recognize which minerals are most likely deposited. System or antiscalant projections, as well as new mineral analysis on the concentrate and feed, can be helpful. Carbonate scales may require low pH cleaners, and sulfate scales may require alkaline chelant cleaners. The following membrane foulants can compromise an RO system:

  • Suspended solids: An increased pressure drop and lower permeate flow can indicate that suspended solids are responsible for an unclean system.
  • Hydrogen sulfide: Hydrogen sulfide in the presence of metals will form sulfur and metal sulfides. These will decrease permeate flow and increase pressure drop.
  • Metals (iron, aluminum and manganese): Metal fouling can decrease permeate flow and increase pressure drop. When metals oxidize RO membranes, the permeate flow will be higher, and the salt rejection will be lower.
  • Microbial and biofilm foulants: Microorganisms, such as bacteria and organic compounds, will cause lower permeate productions and the need for higher pressure.
  • Silica: When colloidal silica causes membrane fouling, permeate flow will decrease and pressure drop will increase.
  • Hydrocarbons: Hydrocarbons will not enter the water unless human activities or ship spills leak them into it. This type of contaminant prevention is important because even a small concentration of hydrocarbons can have a significant membrane fouling effect.
  • Polymers: Polymers create a fouling film when they reach RO membranes, which prevents water flux. This increases pressure drop and decreases permeate flow.
  • Calcium sulfate and calcium carbonate (CaCO3): Low salt rejection and low permeate production can indicate the presence of calcium carbonate and calcium sulfate.

Cleaning the RO membrane is the only way to recover system capacity after contamination. Before choosing a cleaning method, check the membrane specifications to ensure you choose the correct RO membrane cleaning chemicals and sequence. Depending on the type of foulant, you will need to implement one of the following cleaning treatments:

  • Low pH chemicals: Low pH chemicals will clean inorganic colloidal fouling, sulfate scale and carbonate.
  • High pH chemicals: High pH chemicals will clean biofouling and organics.
  • Bacteria removal at high pH or with specific biocides: Specific biocides or high pH chemicals can remove bacteria.
  • Low pH chemical and high pH chemical combination: When mineral scale and organics are present, it’s best to start with a low pH cleaning chemical to remove the mineral scale and then follow with a high pH cleaning chemical to remove any organics.

To clean an RO membrane, you can follow these steps:

  • Open the concentrate valve and use permeate water to flush the system.
  • Attach the recirculation pump to the membrane feed inlet.
  • Attach the concentrate outlets and permeate to the chemical tank.
  • While controlling the pH, recirculate the cleaning chemical for one hour.
  • Use permeate water to rinse the membrane and remove any chemical traces.
  • Repeat the cycle for each cleaning chemical needed.

The most common RO membrane cleaning method for industrial and commercial systems is on-site cleaning, also referred to as Clean-in-Place (CIP). You can perform this type of cleaning while membranes remain inside pressure vessels. This method is often cheaper and faster than off-site cleaning, which involves removing membranes and shipping them to a separate facility.

While off-site cleaning can be expensive, it is typically a more efficient cleaning method. Off-site cleaning is more thorough and can help ensure you have a system that’s clean and running efficiently.