RO Water Treatment Series

The RO process uses pressure to overcome natural osmotic pressure and force feedwater with its load of dissolved salts and impurities through a semipermeable membrane. Rejected salts and impurities concentrate above the membrane and are discharged to drain.

The water that drains from an RO system is chemical-free and can be recycled back to the supply line.

Prefilter

A prefilter removes grit and sediment before the pressure pump drives water through an RO membrane. This step prevents premature fouling of the semipermeable membrane and increases the life span of the membrane.

Cartridge filters, made from wound strands of polypropylene, are commonly used as pretreatment for RO systems. They are cheap and effective for household RO applications. However, they lack the resilience to withstand high-pressure conditions. String prefilters, on the other hand, are built with multiple wounded layers around a central core. The outer layers serve as a filtration surface while the inner layers capture small particles. They also withstand high-pressure conditions and can be easily replaced, making them an excellent choice for industrial RO system pretreatment.

Reverse Osmosis systems require a high level of pressure to overcome natural osmotic pressure. This forces the salt side of the membrane against the reject stream, allowing filtered water to pass through while leaving most of the dissolved contaminants behind in the reject stream.

Most RO systems are equipped with a remineralizing filter to add the proper amount of minerals back into the drinking water. The remineralized water is then dispensed into dedicated faucets for use. The discarded contaminant-laden reject stream is typically sent to a drain. It RO Water treatment series is important to keep in mind that a higher rate of wastewater recovery will decrease the effectiveness of the remineralizing filter and lower overall contaminant removal rates.

Membrane

In an RO system water flows through the RO membrane under pressure. The membrane is made up of a series of interconnected polyamide or other semi-permeable films with a pore size of approximately 100 nanometers. Hydrogen ions pass through the membrane in a process called concentration polarization, while larger organic molecules are rejected. The rejection of ions is based on the valence of the ion, so monovalent ions such as chloride are rejected less effectively than divalent ions like calcium sulfate.

The membrane can reject up to 99 percent of dissolved solids. This includes most organics, salts, pyrogens, and some bacteria. The rejection of these contaminants is dependent upon the chemistry of the feed water and the pretreatment system. It is also dependent on how tightly the membrane is packed, which is usually accomplished by a filtration system known as a mixed bed ion exchange resin tank.

It is important that the feed water chemistry and system design be optimized to prevent fouling and scaling of the RO membranes. Fouling and scaling will increase the pressure drop across the membrane, which increases power consumption and decreases the permeate flow. A good pretreatment plan will minimize these effects. Also, the use of variable frequency drive motors to start high pressure pumps and check valves to prevent ‘hard starts’ will reduce mechanical damage to the membranes.

Post Filter

An RO system removes the unwanted dissolved solids from water and sends them down the drain as wastewater. A remineralizing filter can then be used to add back the minerals that are lost to the water. The result is RO Water treatment series very pure drinking water with a natural mineral balance. This type of water is ideal for saltwater aquariums.

A pretreatment cartridge filtration system can be added to prevent the membrane from fouling by removing larger sediment particles. This step also reduces the risk of clogging and extends the life of the membrane. Antiscalants and scale inhibitors can be added to help with the scaling potential of the feed water. These chemicals work by interfering with the formation and growth of troublesome inorganic compounds.

As the filtered water exits the RO membrane it goes through a postfilter to polish the water for consumption. This removes any remaining dissolved contaminants and improves the taste of the water. A water softener can be added to the system to prevent the RO membrane from fouling by exchanging scale forming ions with non-scale forming ions.

The waste stream from the system contains rejected contaminants that need to be disposed of properly. The challenge is to make sure the wastewater does not backflow into clean drinking water or contaminate other water sources. This is especially important in industrial applications where the recycle rate of the system may be high.

Storage Tank

The RO Storage Tank is where water is collected as it gradually purifies through the membrane. It is important to note that the RO process takes a while. As the water is pushed across the semi-permeable membrane, it loses much of its dissolved salts. This is why the RO system has a storage tank to collect that water for you so you have immediate access to it.

RO tanks are made out of a steel bladder lined with an inert material such as butyl. These tanks are designed to sit beneath your sink and inline with your reverse osmosis system. Before you begin working with the storage tank, be sure to shut off your water supply. You will also want to disconnect any lines running from your refrigerator or ice maker to the RO system so they do not get flushed with water.

A storage tank will have a hydraulic shutoff valve that is set to the pressure of your city water. When the water pressure reaches a certain level, the valve will shut off the flow of water to your faucet.

It is important to have a well maintained storage tank as it can be the source of fouling. A poorly maintained storage tank can develop bacteria or other microorganisms that can produce biofilms on the surface of your membrane and cause heavy fouling. This can be a big problem for your membrane and your entire RO system.