Puretec Industrial Water offers pre-treatment solutions for nearly any pure water application.
RO membranes are designed to remove dissolved solids (ions) from water. However, they can quickly foul with suspended particles and/or scale with dissolved solids and they do not tolerate oxidizers such as chlorine and chloramine. Therefore, RO pretreatment is critical to extending the life of your RO membranes, maintaining water production and quality, and reducing overall operating cost.
During the RO design process, a water analysis is performed to determine what pretreatment is required. It is important to understand that feedwater composition changes over time and a robust pretreatment system will help you address future changes in water composition. Likewise, an RO pretreatment system should be periodically reviewed based on changes in feedwater composition and system operating parameters.
An effective pretreatment system addresses the following:
A system may require some or all of these to effectively condition feedwater for an RO system and optimize performance.
Particulate fouling on the RO membrane happens when suspended solids and colloids accumulate on the membrane surface and prevent water from passing through the membrane. RO membranes are intended to remove dissolved solids and although they can remove suspended solids, they rapidly deteriorate when doing so because they plug so quickly. This results in higher pressure drops across the membrane, more energy cost, and less water production. Eventually the RO membranes will have to be cleaned or replaced depending on the severity of fouling.
Examples of particulate matter include suspended solids, minerals, clay, colloidal silica, and hydrous metal oxides.
To address particulate fouling, the following methods can be used:
RO feedwater after particulate removal should have a turbidity reading less than 0.2 NTU and an SDI value less than 3 to optimize RO membrane life and performance.
Chlorine and chloramine are essential to water treatment for their ability to eliminate most pathogenic microorganisms. However, chlorine and chloramine are not compatible with RO membranes. If chlorine and chloramine are not removed in the feedwater to an RO system, then irreversible damage is likely to occur requiring expensive membrane replacement. Symptoms include a drop in permeate quality (lower salt rejection) and higher permeate flow because there are now holes in the RO membrane that allow feedwater to bypass the membrane and enter the permeate stream.
To address chlorine and chloramine removal, the following methods can be used:
Biofouling is a direct result from the RO membranes inability to handle chlorine and chloramine. Since chlorine and chloramine must be removed prior to an RO system there is no oxidizer to deactivate microorganisms and they are free to rapidly multiply across the RO system. Biofouling quickly plugs RO feed spacers and the membrane surface resulting in a higher pressure drop across the RO system, loss of permeate flow and salt rejection and higher energy cost.
To address bio-growth, the following methods can be used:
Scale inhibitors are used to delay the formation of scale throughout the RO system. The consequences of inadequate scale control include a higher pressure drop, lower permeate flow and quality, increased energy cost. Scaling occurs when dissolved minerals become concentrated in the RO system and exceed saturation limits and then fall out of solution forming a scale on the RO membrane surface. Common scales include calcium carbonate (CaCO3), calcium sulfate (CaSO4), barium sulfate (BaSO4), strontium sulfate (SrSO4), calcium fluoride (CaF2) and iron and silica scales.
To address scale formation, the following methods can be used:
The objective of RO pretreatment is to condition the feedwater, so it is compatible with the RO membranes. Doing this will make sure your RO system operates properly and provides the quality and quality of water that it was designed for. A pretreatment failure can result in expensive membrane replacement and unnecessary cleanings.
There is not a single pretreatment solution for a RO system and what worked in the past might not work today due to seasonal water changes. Keep in mind that RO membranes still require occasional cleaning regardless of how well the pretreatment system is working.
Puretec Industrial Water provides a complete line of water treatment services and parts across California, Arizona, Nevada, and New Mexico.
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