Fouling causes the membranes to work less effectively, which is one of the main issues in operating Commercial RO Plants. The most frequent type of membrane fouling, i.e., biofouling, is also the most difficult to manage. Common operational critical performance indicators that reveal the onset of fouling in large-scale systems, include an increase in the normalised feed channel pressure drop (NPD) over the feed spacer channel, a decrease in the normalised specific water permeability, and changes in salt retention.
A large rise in NPD and a moderate drop in NSP usually result from rapid biofouling, which typically appears in the modules of the first stage, but gradual biofouling may appear across the entire installation. Biofouling very seldom appears in other areas of the installation, like the tail elements.
Let’s get in-depth knowledge of the step-wise procedure of alkali and acid cleaning in Commercial RO Plants.
Chemical cleaning in Commercial RO Plants
Chemical cleaning in place (CIP) is used to restore the original RO performance in terms of NPD, and normalised salt rejection capacity, in order to solve the fouling issues. Commonly used acid-based CIPs frequently fall short of completely restoring RO performance, and eliminating all accumulated deposits from the membrane elements. The Commercial RO plant will continue to run with fouling issues, if the membrane performance cannot be recovered up to a certain level, for example, permanently increased NPD by > 15% after CIP.
Therefore, when a chemical cleaning is completed correctly and on time, it is reversible; in the opposite situation, it is not.
What are the various factors which indicate that chemical cleaning is required in Commercial RO Plants?
There are some factors that indicate that a chemical cleaning is required:
· When there is a 10 to 15% reduction in permeate flow.
· Pressure difference is determined by subtracting the pressure in the feed stream, from the pressure in the reject stream when it rises from 10% to 15%.
· When the rejection of salts falls by 1% to 2%.
· When sulphates, silicates, and carbonates can all contribute to soiling.
· When there are metal oxides of aluminium, iron, manganese, etc.
· When there are organic substances.
· When there is the growth of biological matter (growth of bacteria, mould, fungi, etc.).
Firstly, the fouling is brought on by the salts precipitating, and the fouling gets worse as their concentrations go closer, to the solubility limit in the rejection.
Also, the accumulation of organic matter causes fouling which gets worse with the higher concentration. And, when microorganisms impact the membrane and come into contact with biodegradable organic substances, fouling by biological matter happens.
What are the various factors to consider before cleaning Commercial RO Plant membrane?
Prior to cleaning of RO Membranes, keep the following in mind:
· Examine the dosage, pH, temperature, standing, and agitation times suggested by the cleaner's manufacturer.
· Verify the cleaner's compatibility with the components used in making membranes.
· Be cautious to stay within the prescribed pH range when making the cleaning solution using the cleaner. The membranes are not harmed by a range between 4 and 10.
· Remember that if cleaning is continued for an extended period of time, pH levels between 2 and 4 or between 10 and 12 can harm the membrane. It is true if the degree of fouling will necessitate the use of more aggressive solutions.
· Avoid combining caustic and acidic liquids. When introducing the following cleaning solution, make sure to thoroughly rinse the previous one.
· It is advised to wash each level separately in multi-stage systems.
· Clean in a space that has enough airflow.
· The cleaning system's components must all be resistant to chemical deterioration.
· The tank needs to be big enough to hold the cleaning agent. A capacity of 5 gallons per 4x40 "membrane and 20 gallons per 8x40 "membrane is advised. The tank where the solution is prepared needs to be simple to get to.
· To reduce foaming, the pipe configuration should provide recirculation, and recirculation should discharge at the bottom of the tank.
· The pump needs to be strong enough to deliver the necessary flow and pressure. The maximum pressure that is advised is 60 psi.
· Solids or contaminants cannot leave the membrane's surface at an excessive pressure. With an 8 x 40" membrane, the flow shouldn't be more than 40 gpm, and for a 4 x 40" membrane, it shouldn't be more than 10 gpm.
· To capture the sediments produced during cleaning, a 5 to 10 micron cartridge filter is advised. It needs to be inserted into the tank's recirculation pipe. At this stage, it's critical to keep an eye on the differential pressure, and swap out the cartridge when it reaches 15 psi.
· In order to assess pH and pressure at the pump discharge and in the recirculation return line, sampling points and pressure gauges are necessary.
· It is normal for the pH to vary since it indicates that pollutants and cleaning solution are interacting.
· Adjust the pH of the solution with the cleaning chemical to its initial value, if you notice a shift of more than 5 pH units.
Procedure of alkali and acid cleaning in Commercial RO Plant
The efficiency of the cleaning agents, the skill with which they are applied, as well as the construction and functionality of the cleaning equipment, are all necessary for good membrane cleaning in Commercial RO Plants. Cleaning the RO membrane with both high and low pH cleaning agents, will produce the greatest results.
· Cleaning high pH Reverse osmosis (RO) water: Very high pH cleaning is used to get rid of the organic as well as the biological foulants. Both of these contain the carboxylic functional groups. Eventually, the weak acids will develop an anionic charge at high pH levels, allowing them to scatter.
The calcium that holds biofilms and foulants to the membrane surface and to one another, is chelated by cleaning chemicals with high pH levels. Common products like NaOH are only somewhat effective, since they cannot eliminate chemicals.
· Cleaning low pH Reverse Osmosis water: In some cases, cleaning low pH RO water is better to cleaning high pH RO water. Organic foulants, however, may lose their anionic charge and end up compacted within the membrane, if a low pH cleaning is performed first.
· Examine the cleaning tank: You must examine the cleaning tank, the hoses, and the cartridge filters. If necessary, flush the hoses and clean the tank. Add fresh filter cartridges
· Fill the tank with RO permeate water: The cleaning tank should be filled with DI or RO permeate water. Activate the tank recirculation pump or agitator.
· Add your cleaning product: Based on the total CIP volume and the required product strength, add your chosen RO cleaning product gradually to the cleaning tank, and give it time to properly mix.
· Verify the solution's temperature: Adjust the heating control to deliver the ideal temperature, if the solution temperature is below the suggested level.
· Verify the solution's pH: The pH should be 12 or as the membrane manufacturer specifies. If pH is too low, raise it with sodium hydroxide (NaOH) or another chemical, as advised by the maker of the membrane. Adjust pH with hydrochloric acid (H2O:HCl), if it is too high.
· Circulate the solution: For 30 minutes, circulate the solution through each stage in the feed flow direction. Circulate at the flow rate suggested by the maker of the membrane or system. Pressure should never exceed 60 psig [4.2kg/cm2], but it should be low enough to ensure that, very less permeate is formed during cleaning.
· Avoid re-deposition of removed materials: To avoid the re-deposition of removed materials, the first return flow (up to 15% of the cleaning tank volume) should be routed to the drain, in cases of severe fouling. In a multistage system, each stage needs to be cleaned independently for best performance.
· Make a new solution: Dump the tank and make a new cleaning solution before continuing, if the first stage cleaning solution turns turbid or discoloured. A new solution should be made if the solution's pH or temperature deviates from the suggested range. In any case, a fresh cleaning solution needs to be made for each step.
· Rinse with RO permeate: Before putting the system back into service, rinse with RO permeate.
· Direct the product water to drain: When putting the device back in use, direct the product water to the drain until all cleaning solution residue, has been removed from the system.
How can we assist you?
Netsol Water professionals may look over your membranes, identify any issues they find, and then offer advice on what CIP procedure will keep such issues from happening again in the future.
From that point on, a new cleaning schedule may be put in place, and your Commercial RO Plant should continue to function flawlessly. Call us at +91 9650608473 or email at enquiry@netsolwater.com for further information.
