The Importance of Water Analysis for a Reverse Osmosis System Design

WaterTest

At AXEON Water Technologies we manufacture a large line of pre-engineered reverse osmosis systems and a wide range of energy efficient membranes. On our technical service team, we have application engineers skilled at selecting the optimal reverse osmosis system and membrane that best fits the customer’s needs.

By providing AXEON with a complete and accurate water analysis, our application engineers can provide proper guidance on how to operate the system as well as provide recommendations for pre-treatment to achieve the optimal permeate quality while extending the life of the RO membranes.

Many customers are familiar with the term TDS (total dissolved solids). TDS is a very important parameter in the design of RO systems. TDS is often reported in ppm, such as 300 ppm for tap water or 2,000 ppm for well water. TDS is most commonly associated with sodium chloride, typical table salt; however, it’s actually composed of a variety of salts and minerals.

Hardness and alkalinity are other terms familiar to many folks especially in discussions of traditional water softening. The reporting of both the hardness and alkalinity of water provides information relative to the levels of calcium and magnesium, carbonate and bicarbonate, respectively in the water. Although this information is more descriptive than knowing just TDS alone, it still does not paint the complete picture.

A more complete water analysis would include the major cations and anions. The cations are the positively-charged ions such as sodium, calcium, magnesium and potassium. The anions on the other hand are the negatively-charged ions such as chloride, sulfate, nitrate and bicarbonate. A water analysis is considered complete when the equivalent charge of the cations balances with the equivalent charge of the anions.

Click Here to view an example of a water analysis. On the first page, the anions are listed along with the concentration found in the water, measured in units of milligrams per liter (which can also be considered as ppm). The cations are shown as well under the metals category. Towards the bottom of the page, there is hardness and alkalinity. Note that the unit of measurement is expressed as milligrams per liter as CaCO3. This unit of measurement is different than the milligrams per liter used above for the ions. We will not go into discussing the conversion between these measurements; however, it is important to include the exact units of measurement when forwarding the water analysis to our application engineers.

At the bottom of the first page of the water analysis, bicarbonate is given. The presence of either carbonate or bicarbonate in the water is a function of the pH of the water. It is best to measure both temperature and pH with a handheld instrument when collecting the water sample at the project location as they will both change by the time the laboratory analyzes the sample.

AXEON’s technical service team will take the water quality data provided and enter it into the feed water input window of AXEON’s reverse osmosis projection design software. Below is a screen shot of the dissolved ions most often considered relevant when designing RO systems.

WaterAnalysis
In the screen shot below, we see the results after entering the data from the example water analysis seen above. As previously mentioned, mg/L can be considered the equivalent of ppm. Notice that the bicarbonate was entered as ppm as CaCO3.

WaterAnalysis
This sample water analysis turned out to be complete as the charge of the positive ions closely balanced that of the negative ions. If they had not been balanced, then some guess work would be required to determine what is needed to complete the balance.

There are some common foulants that can have a negative effect on the performance of RO membranes. It is important to know which ions and minerals are in the feed water because as these impurities become more concentrated, several of them such as the ones listed in the water analysis example that reach a point where they become no longer dissolved in water but settle onto the membrane and cause scaling.

With knowledge of the impurities in the water, AXEON applications engineers are better able to select a system design and membrane with an appropriate flux rate and recovery to suit the feed water conditions. Based on the water chemistry, pre-treatment methods such as adjustment of pH or the addition of anti-scalant may also be recommended.

Click Here to download AXEON’s Water Analysis Form.

For more information, please contact us at 800-320-4074 or email at sales@axeonwater.com.