MechChem Africa

Jens Lipnizki from the Membranes Liquid Purification Technologies business unit of LANXESS talks about keeping reverse osmosis water purification systems healthy and how to overcome some of the problems that inevitably occur when using this technology.

Lewabrane production smallReverse osmosis is a standard water treatment technology that has steadily gained increasing acceptance over the years. Handy design programmes, helpful literature and improved knowledge have led to this technology becoming much more commonplace. For example, reverse osmosis is no longer limited to use in industrial water treatment facilities, but now also has domestic applications such as those used for treating mains water.

Whatever the application, however, the fundamental problems that can be observed in membrane processes have not changed, namely: falling rejection rates; and reduction in performance due to reduced flow or higher pressure.

When such problems occur, the first question to be clarified is whether there have been any changes in water quality or temperature. To make it easier to localise problems, it is important to document basic parameters such as temperature, flow, pressure, yield and conductivity. In addition, it is also helpful to measure the pressure loss, preferably between the concentrate levels.

This data should be normalised, that is, expressed in relation to a standard situation so that it is possible to assess whether the change in performance is due to the system or changed inflow parameters. Calculation tables for this purpose are provided by the membrane manufacturers free of charge.

Aside from this, it is vital to check that other facility components such as measuring equipment, antiscalant dispensing units and ion exchange systems are running smoothly.

If, after normalisation, the data deviates as tabulated below, then the reasons for the deviation should be investigated more closely.

• 20% higher salt passage – compared to 100% salt rejection.
• 10% reduction in flow.
• Greater than 20% pressure loss along a pressure pipe.

Frequently, small facilities only record a few measurement values, and these are generally not normalised. Where this is the case, the influence of temperature and variations in the salt load in the water should be taken into account. The rule of thumb here is that for each degree Celsius the temperature drops, flow is reduced by approximately 3.0%.

Investigating the problem

It goes without saying that the investigation process depends on the problem observed. If the salt passage has increased, that is, the rejection percentage has worsened, this can indicate chemical or mechanical damage to the membrane or element. Reduced flow, on the other hand, is generally due to organic, biological or inorganic fouling. The table below provides an overview of the tendencies and potential reasons for a drop in performance.

Download the PDF.

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