Pre-Treatment

Puretec Industrial Water offers pre-treatment solutions for nearly any pure water application.

Filtration

Overview of pretreatment methods for industrial reverse osmosis systems

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:

  1. Particulate removal
  2. Chlorine/chloramine removal
  3. Biofouling
  4. Scale inhibitors

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:

  • Multi-media filtration: A vessel contains different layers of media that can remove suspended solids down to 10 microns in size. MMF’s periodically need to be backwashed to remove accumulated solids on the media. Coagulants can also be injected to the MMF that help particles combine and be retained easier by the MMF media. It is important to have a pre-filter to the RO system to catch any loose media that occasionally escapes the MMF unit. Also, a coagulant may be added prior to a MMF to increase particulate removal capability. It is important not to overdose the coagulant as it can foul RO membranes downstream.
  • Ultrafiltration (UF) and Microfiltration (MF) membrane filtration can remove suspended solids as well as bacteria. The main difference between the two is the pore size in the membrane. Ultrafiltration, with its smaller pore size, can also remove viruses, silica, plastics, and endotoxins. A UF can consistently filter down to 0.1 micron and MF membranes range from 0.1 to 10 microns.
  • Cartridge Filtration: In smaller RO systems a replaceable cartridge filter is the economical choice to remove suspended solids before an RO.

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:

  1. Sodium Bisulfite (SBS) also referred to as a chlorine scavenger can be injected after the cartridge filter and prior to the RO feed pump to remove any oxidizers present such as chlorine and chloramine. It is important not to overfeed SBS since excess bisulfite can promote anaerobic bacteria growth and foul membranes. SBS should be mixed with DI water or RO permeate in the hold tank and replaced frequently to avoid going stale since SBS will react with oxygen in the atmosphere and deteriorate. A ORP sensor can be used to control the SBS dosing pump to address the dynamic changes in chlorine/chloramine levels in the feedwater but maintaining the ideal dose rate can be very challenging.
  2. Granular Activated Carbon (GAC) will also remove chlorine and/or chloramines from RO feedwater. The advantage of a GAC is that it can handle fluctuations in chlorine/chloramine concentrations in the feedwater without having to rely on a SBS dosing pump and/or ORP sensor, not to mention having to constantly add and mix SBS to a chemical feed tank. The disadvantages of GAC are the GAC can promote bio-growth since the top layer removes chlorine/chloramine and the remainder of the bed does not have an oxidizer to control growth and as a result bacterium can thrive and pass downstream to the RO system. Likewise, GAC must be used prior to the cartridge filters before the RO system whereas SBS can be applied after the cartridge filters. This helps prevent bio-growth inside the cartridge filter. Also, if poor quality GAC is used or the GAC bed is not backwashed properly at startup then GAC’s can bleed small carbon particles that make their way past cartridge filters and into the RO system and can foul membranes. From time to time the GAC bed will need to be replaced with fresh carbon.

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:

  1. Do not overfeed sodium bisulfite (SBS) into the RO system. Excess bisulfite can promote bio-growth since any surviving bacteria will use the sodium bisulfite as nutrition. A rotten egg smell when opening the RO vessel is a sign that anaerobic bacteria have been thriving on excess bisulfite.
  2. Perform an adequate post shutdown flush with RO permeate water.
  3. Biocides can be injected to help prevent bio-growth in non-potable RO systems.
  4. Clean and sanitize equipment and piping.
  5. Ultraviolet radiation can reduce bio-growth before an RO but can increase silica fouling since silica particles aggregate under UV radiation.
  6. Any air that is drawn into the RO system while not in use can promote bio-growth. The correct choice and maintenance of fittings, gaskets and valves can help prevent this.

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:

  1. Anti-scalant chemicals are used to prevent scale formation on an RO membrane by delaying the precipitation of dissolved salts so they can make it through the RO system before they form a scale. Anti-scalant is typically injected before the cartridge filter housing. There are different anti-scalants available depending on the feedwater scaling tendencies and the correct one can be determined by conducting a water analysis. The proper anti-scalant will allow the RO system to run at the highest possible recovery rate which will result in saving water by sending less concentrate waste to drain.
  2. A water softener can remove hardness causing calcium and magnesium minerals but is not effective with other scaling potential in the water such as silica scale formation. Water softeners can be either provided by an outside service provider who exchanges the softener or a permanently installed softener that self-regenerates onsite. The latter requires the frequent addition of salt and ability to discharge spent brine.
  3. Acid can be injected prior to an RO system, with or without anti-scalant, to control carbonate scale. However, acid reacts with bicarbonate and creates CO2 which puts an extra load on ion exchange resin after the RO.
  4. A post flush during shutdown to rinse out saturated minerals is important to prevent scale formation while the RO system is idle.

Summary:

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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