Trouble Shooting Guide

Trouble shooting - Membrane Cleaning

Once it has been determined that a cleaning is necessary, there are several decisions that need to be made in order to maximize effectiveness. Not all foulant materials can be removed in the same way or with the same chemicals, so the very first task is usually to determine what type of foulant is suspected. The normalized trends are the best place to start as they can tell operators where problems are occurring within the system. If further investigation is warranted at that point it is usually prudent to look at a recent laboratory analysis of the feed water and – if available – the RO concentrate. It is also good practice to review general plant maintenance logs, especially if the normalized or raw data trends show a sudden and unexpected loss of performance. These sudden step changes can often be linked to events outside of the RO array, especially as they pertain to equipment upsets or failures of the upstream pretreatment equipment.

If the exact nature of the problem cannot be determined (or adequately guessed at) from these indirect methods, the operations staff will be faced with a decision; they can either proceed with a “standard” cleaning in the hope that this will do the job, or they can choose to send one or more of their membranes out for destructive autopsy and analysis.

Off-site membrane cleaning

The off-site autopsy method involves removing at least one – and usually several – membrane and sending it to a laboratory where it can be physically taken apart and the foulant materials can be collected and tested. The upside of this method is that in most cases the exact materials present on the surface of the membrane and on all the other various parts of the RO element can be determined in their exact proportions, allowing for a prescribed cleaning solution and regimen to be created. The downside is that this method requires both time and money.

Which chemical to use

Once the operations staff is satisfied that they know what they are trying to remove by cleaning the membranes, the next decision is which chemicals to use and for how long. This can be a complicated step because different foulants require different methods of removal and there are a large variety of cleaners (both generic and specialty) at the disposal of membrane operators. Most membrane cleanings are composed of multiple steps and chemicals and the process can often be complicated.

Basic steps in a “generic” cleaning regimen

  1. Prepare RO array/train for cleaning
    1. Flush train and connect CIP system
    2. Check CIP equipment for readiness
    3. Drain RO train
  2. Prepare first solution (usually high pH first)
    1. Mix proper volume of chemical with proper volume of RO permeate to create solution
    2. Heat solution
    3. Circulate solution locally to mix
    4. Check pH and adjust solution if necessary
  3. Send solution to train and circulate
    1. Valve CIP system to send solution to RO train
    2. Circulate solution within RO train or specific stage
    3. Dump first 10% of returning solution if necessary
    4. Check flows, pressures, pH to maintain specified values
    5. Circulate for specified amount of time
    6. Record data
  4. Soak solution
    1. Stop pumps and allow solution to soak for specified length of time
  5. Circulate solution again
    1. Re-start pumps and continue to circulate solution for specified length of time
  6. Dump spent solution
    1. Send used solution to drain/holding tank/neutralization tank
    2. Flush CIP tanks to remove remaining solution
    3. Flush RO train/stage being cleaned until pH is neutral
    4. Drain RO train
  7. Repeat steps 2-6 with second solution (usually low pH)
  8. Prepare train/stage to go back into service
    1. Flush membranes with RO permeate and ensure system full of water
    2. If CIP is complete, put train back into service
    3. If CIP is incomplete move on to next stage/train

Heat and pH

There are a variety of details to take into account once the general cleaning regimen and chemicals have been sorted out. It is extremely important to take into account the time it takes to fill tanks, add chemicals, heat solutions, etc. Heat is especially important as heated cleaning solutions tend to dissolve and remove foulant materials much more efficiently. Standard practice is to clean at the maximum temperature allowed by the membrane manufacturer. The same is also true of pH; the most efficient cleaning generally takes place at the extremes – both low and high – of what the membrane manufacturer will allow.

Circulation, soaking and flow direction

In terms of best practices for the actual cleaning regimen, it is generally accepted that the best way to dissolve and then remove foulant is to alternate between circulation steps and soaking steps. Each circulation step carries away foulant while each soaking step dissolves another layer of foulant on the surface of the membranes. Newer systems are also being designed to clean in the reverse direction of flow (as opposed to normal operation). Reversing the direction of the cleaning flow can be very beneficial in cases of heavy feed channel fouling at the front end of the membrane system as you would typically see with exposure to sand/filter media/biological growth, etc. Cleaning in the reverse direction of flow requires a more complex CIP system as well as membrane brine seals which allow flow in both directions (the standard “U-Cup” seals which have been supplied with membranes for decades only allow for flow in a single direction).

Contact PWT for more information on membrane cleaning.