Question of the Day : Normalizing RO Data

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Taxonomy

• Industrial Wastewater Treatment
• Industrial Water Treatment
• Reverse Osmosis
• Industrial Water Treatment

6 Answers

1. It is the only way to truly understand RO performance and predict membrane health. Membranes begin fouling as soon as they are put into service. Recording the initial performance data (a.k.a Normal) becomes the baseline of efficient operation. Understanding the deviation from normal provides the clear and predictive picture an operator needs to make informed decisions. (cleaning, replacement, repairs, etc...) 

2. HI,When the initial and normalising  load the RO we need to maintain total pressure drop not exceeds per PV 60 PSI.otherwise hydrolysis and damage of membrane will occur. First data of commissioning is very important to prepare history.

3. NPF cuts through the details and presents a clear, concise message for the customer to make educated decisions.  Predict vs. react. 

4. Normalizing is REQUIRED to properly analyze.  When you startup with new membranes, note the Primary Pressure, Pressure Drop (Perm to Conc) and Permeate Conductivity.  Track them and look to clean at 10% increase in any of the three.  Look to replace as the cleanings get less effective and time between cleanings decreases.

   Also, look for changes that could indicate other problems.  For example, a decrease in Primary Pressure and Pressure Drop with an increase in Permeate Conductivity could indicate membranes damaged by chlorine oxidation,

5. Normalizing performance relative to the manufacturer's specification of test data for the membrane model number installed (can be still useful in a mixed system if set up correctly), tells the operator several things:  the corrected net driving pressure at specified flow rate (hence also the normalized permeate flow rate at standardized pressure, as corrected for osmotic pressure, temperature, and average membrane applied pressure. 

   The normalization program then also provides the normalized salt rejection (and/or salt passage), which again is referenced to standard conditions of temperature, and corrected for recovery at the average membrane. 

   The differential pressure along the length of a vessel containing a number of membranes (often 6 elements), is also calculated from entered data, and a standardized differential pressure is calculated and this can be one of the key indicators of serious fouling within the spacer between leaflets of membrane material.

   Some normalization programs may have chemistry entries for sparingly soluble species such as Calcium (with carbonate or sulfate), and also for Silica.  Unless these calculations can also provide some indication of concentration gradient at and near the membrane surface, they can miss scaling indications.

   Most cases, a net loss of 15% of permeate flow is a strong indicator of scaling/fouling, and the fore-warned operator will begin a CIP (clean-in-place) sequence, alternatively selected membranes can be removed from the system and cleaned even individually, although this becomes very labor intensive.  If differential pressure coefficient and/or salt passage reveal statistically significant increases ~20%, cleaning is also recommended.  Caveat (warning): cleaning of an entire vessel or parallel set of vessels simultaneously may result in only shifting the fouling  or scale down the chain of elements, with a very ineffective result, where performance appears worse after cleaning.   In my opinion, this is usually the result of attempting to clean with too low a linear flow velocity along the length of the elements.

   In some cases, cleaning may be done with flow velocity counter to the normal feed direction to attempt to flush out particulates that are partially clogging the spaces between membrane sheets (envelopes).  Another technique is the very careful application of reverse pressure application from the permeate side of the membrane, although if not done with extreme control, can result in ripping the glued seams of the envelopes opening, and destroying the effective salt rejection of the system.  It is an effective way to induce sales of new membranes, however, as unethical as it is.

   1 Comment

   1. Great discussion based on many years of RO operation worldwide. Was recently introduced to Flow-Reversal RO that resolves many of these operational challenges by preventing scaling. Very intriguing, and existing systems can be retrofitted to achieve this operational goal. 

6. It allows you to determine when the membranes have lost 10% of their capacity.  Otherwise, pressure and temperature changes mask the severity of fouling.

   if fouling becomes severe, recovery of capacity with cleaning becomes impossible