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Feed Water Quality for Reverse Osmosis Systems:

Problems & Solutions

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Do RO Elements Remove Volatile Chemicals?

Do Brackish Membranes Work at 4000 TDS?

What is the Maximum Concentration of Chlorine for a Thin Film Membrane?


Is there a general rule of thumb regarding the Maximum TDS Concentration Treatable using RO?  I realize that there are many variables, however, I am just looking for a general number above which another treatment technology should be considered.

The maximum is
50,000 ppm.  For TDS values higher than this, the treatment process normally used is evaporation.

Do your Brackish membrane elements work in TDS of 4000?

Yes, Brackish Water Elements will typically work in a TDS of up to 10,000.  There are many variables which will affect membrane performance, but typically 10,000 TDS is fine for a Brackish element.

At what concentration of chlorine will the Thin Film Membrane Fail?

Maximum chlorine concentration should not exceed 0.1 ppm for a maximum of 1000 hours.

What is the conversion from conductivity to resistivity and vice versa?

Resistivity is inverse of conductivity. In other words resistivity = 1/conductivity. Conductivity of 1 mocromhos-cm [or micro Siemens]= 1,000,000 ohms of resistivity or 1 mega ohms. Conductivity of 10 micro ohms-cm= 100,000 ohms or 0.1 mega ohms. Etc.

What are the maximum levels of hydrogen sulfide, iron, and manganese that can be present in water in order to use a Pyrolox Filter?

Hydrogen Sulfide = up to 10 ppm

Iron = up to ppm

Manganese = up to 5 ppm

What analysis must be done in order to determine an acceptable chlorine level? What is the maximum level?

Maximum acceptable level for chlorine for thin film membranes is 0.1 ppm. However, the total cumulative chlorine tolerance of the membrane is 1000 ppm-hours [ppm x hours of exposure].

What values need to be considered when designing an RO membrane?

You need to consider the TDS for design purpose. The salinity [conductivity based] is a quick but indirect and approximate method of estimating the TDS. Conductivity works on the conductance of ions from the dissolved solids. The conductivity of an ion varies with its charge etc. So the same amount of TDS can give different conductivities depending on the types of dissolved solids. The conversion of conductivity to TDS is done using a make-up "standard" TDS solution that may be widely different in the water you are testing.

What is the chlorine value of tap water in the USA? How can you reduce/eliminate the chlorine?

In the USA and Canada, the amount of chlorine in water is 0.2 to 1 ppm. Chlorine can be removed by using an activated carbon system or cartridges.

What is the optimal pH of feed water to run RO systems?

There is no real optimal pH.  The best pH from rejection point of view for thin film composite membranes is 8 or 8.5.  In terms of pH being an optimal it is a question of doing an LSI check to make sure there is no problem with LSI being positive.  Don't worry about the pH as you can soften or inject antiscalant.  As long as you soften or inject antiscalant, it doe not matter if the pH is 6.5 or 8.5.

What effect does feed water pH have on R.O. throughput and membrane life. Our well water pH is 8.0. Will we need to adjust it?

There is no effect of pH on membrane throughput. A lower pH will help in keeping hardness from precipitating on the membrane but this is normally accomplished by antiscalant injection or water softening. The membrane life for a common RO membranes is unaffected by pH as long as pH is in 6-9 range. pH on the extreme points of membrane tolerance (2 and 12) affect other components of a membrane module and will lead to a shorter module life.

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Applied Membranes, Inc. 2007