Components of an RO System, Part 1
By Jason Rice
Training a new service technician on how an RO works can sometimes be difficult. Someone new to reverse osmosis might feel overwhelmed with the different configurations, filter options, membranes and technical terminology. By breaking down the individual components first, the technician will get a better understanding of how each part works, ultimately giving them a better understanding of the system. The following information is intended to explain the different components used in an RO system. Starting with the different types of sediment filters and why some are used in RO systems (point of use), while others are used at the point of entry, replaceable components are critical to its efficiency. It is important to remember that you should always refer to the manufacturer’s specifications and recommendations.
Sediment filters are sized with a micron (µm) rating. A micron is a metric unit of length equal to one millionth of a meter. The diameter of a human hair is roughly 80 microns. If a filter is rated at a nominal five microns, then 80 percent or more of the pores on that filter are five microns, though the rest could be larger. If a filter is rated to be five microns absolute, then 99.9 percent of the pores are five microns.
Spun melt-blown filters
Melt-blown filters are the most economical and are widely used in the market. They are noticeable for their hard cotton-like appearance. Typically made of three to four different layers, they will usually have the largest micron rating on the outermost layers. Melt-blown sediment filters are typically but not always made of polypropylene and the most popular type of thermoplastic that can be melted and bonded, which is how the filter got its name.
Melt-blown filters typically look like a paint roller (Figure 1). They do not require rubber gaskets on the top and bottom like some other filters. They are also relatively inexpensive and have a long service life. For these reasons, melt-blown sediment filters are widely used as the prefilter on many RO systems. Melt-blown (also referred to as spun) sediment filters, like most sediment filters, will lower the water pressure of the source water. Once the filter becomes clogged, in most applications there will be a significant drop in pressure. Most ROs require a minimum of 30 to 40 psi for proper operation.
String-wound sediment filters
The main purpose of a string-wound sediment filter is to remove silt, oil, grease and sand. It is commonly used as a general filter to remove dirt, particulate rust and iron. This filter would typically be installed at the POE and not used as a RO prefilter, because contaminants such as silt, oil and grease should be treated at the point of entry not the point of use.
String-wound filters resemble a spool of yarn (Figure 2) and are very similar to melt-blown sediment filters. Both have a nominal filtration and are made of fibrillated polypropylene or natural cotton. Like spun filters, these do not use rubber gaskets. They also have the largest dirt holding capacity. Like melt-blown filters, it is not uncommon to experience a significant pressure loss when the filter becomes clogged.
Pleated sediment filters
Pleated filters are easily recognized by their appearance: they resemble a well-folded piece of paper (Figure 3). Pleated filters typically only have one layer of filtration but are available in dual-pleated for additional sediment filtration. They can also be made of several different types of materials that include cellulose, polypropylene and polyester, each with its own advantages and disadvantages. Rubber gaskets are required for this sediment filter to seal properly in its filter housing.
Polyester pleated filters are used for removing suspended solids from water and are the most durable of the pleated filters because of their material. They are constructed of pleated polyester fabric that is bonded to the end caps. One advantage of the polyester filter is its resistance to many chemicals and bacterial damage. Polypropylene pleated filters are the most economical and have a higher dirt-holding capacity than polyester filters. Cellulose pleated filters can also be used for removing suspended solids from water. These filters are economical with multiple uses and have a long service life.
The main purpose for pleated filters is to remove solid particulates like grit, sand, particulate rust and other larger sediment particles. But pleated filters have the least dirt-holding capacity when compared to spun and string-wound filters. Although they may be used for dirt filtration, they may not perform well. Therefore, the pleated filter is rarely used as pretreatment on an RO. In addition, they will also experience the same flow restrictions as the other types of filters when clogged.
Sediment filters/carbon filters
Sediment filters can also serve a dual function. These filters are referred to as dual-function or carbon block. Sizing a dual-function filter uses the same micron rating as other filters. Carbon filters in general are commonly used in reverse osmosis as pretreatment.
One pound of granular activated carbon contains about 125 acres of filtering surface area. When talking about carbon, most people refer to it as an absorbent material like a sponge. With a sponge, water is absorbed into the pores and saturates the entire sponge. Carbon is adsorbent and everything takes place on its surface. Contaminants in the water are trapped in the surface pores. In Figure 4 you can see these pores.
Once all the pores on the carbon surface are filled, the carbon can no longer collect contaminants. This is referred to as becoming exhausted. When this occurs, there are only two options: replace the carbon or clean the surface of the carbon to expose new pores. In POU treatment, the option is to simply replace the filter. With a whole-house POE GAC automatic backwashing filter system, there would be a backwashing process to clean the surface of the carbon bed. It should be noted that once carbon has become exhausted, there is a possibility of leaching. This is when the carbon can no longer hold contaminants and releases them back into the water. Carbon can also be a perfect breeding ground for bacteria; therefore, filter changes and cleaning are vital to its performance.
Carbon block filters have been used in the water filtration industry since the 1970s. They were found to be most effective both as a standalone treatment method and within filtration systems with multiple stages. Most carbon filters are made with granular activated carbon (GAC) or powdered block carbon material. A carbon block filter is made up of activated carbon granules and a binding agent, allowing the carbon to maintain its circular form (Figure 5). It is also important to remember that the flow of water through carbon block and sediment filters is from the outside in, known as radial flow (Figure 6). This allows the water to surround the filter and water pressure forces the water through the filter to the center
Sediment filters/carbon filters
Carbon block filters are very effective because they have a large surface area and are very dense. With this density a carbon block can filter down to about one micron. For this reason, the filter is often used as a prefilter, which typically refers to any filtration on the RO system that is installed in front of a membrane. The more surface area the carbon has and the longer water is in contact with the carbon, the more contaminants are removed. This is referred to as empty bed contact time (EBCT). EBCT is the measure of time during which water to be treated is in contact with treatment media (carbon). Carbon block filters are rarely just exposed carbon; they will typically have filter paper covering the carbon. (see Figure 7).
Activated carbon water filters are rated by the size of contaminants they remove. These range from around 50 microns down to 0.5 microns. The smaller the measurement, the more effective the filtration will be. There are also several different substrates that are used to make carbon filters, such as bituminous coal, wood and coconut shell. Of these, coconut shell is the most expensive and has also been found to be the most effective.
Granular activated carbon
GAC is made from raw organic materials, such as coconut shells or bituminous coal, which are high in carbon. Heat is used to activate the surface area of the carbon. This type of filter is available in many different shapes and sizes and is typically a cartridge housing filled with loose granular carbon (Figure 8). The carbon is not packed tightly into the housing, which allows it to expand inside the cartridge once it has been saturated with water. GAC filters can be used in both pre and postfiltration. Most often the filter is used as a post filter (polisher). As the water comes out of the storage tank it will pass through the post filter before exiting the spigot. GAC filters will require additional rinsing of carbon fines. RO manufacturers will have detailed rinsing procedures when this type of filter is used.
Water enters the cartridge at the bottom and saturates the carbon inside (Figure 9). As water fills up the cartridge, it forces the water through the carbon. Treated water then exits the filter at the top center of the cartridge (Figure 10). This flow pattern of water allows for more contact time, greater capacity of the media and provides a consistent water quality. It is referred to as axial or longitudinal flow.
Inline carbon filter
There are only a few differences between a GAC filter and the inline carbon filter, the obvious one being the housing. With GAC, the cartridge is installed into a sump housing. Inline filters are typically self-contained (Figure 11). The housing can be packed full of carbon, creating a wall that the water must pass through. Unlike a carbon block, GAC filters will contain a lot of air. The housing will have a push-to-connect type fitting on each end, or female threaded ports where fittings can be attached. Some inline filters have incorporated a removable cap, which allows the housing to be re-packed (with carbon or other media) once the carbon has been exhausted. Others will be completely sealed and the entire housing needs replacement once the carbon is exhausted. The filter housing will also indicate the flow of water. One end will be dedicated inlet and the opposite will be the outlet.
The design of an inline filter has significant value as it helps increase reduction/removal of low molecular weight contaminants EBCT, as well as utilizing the bed depth of the filter. The placement of an inline filter can vary greatly. For optimal performance, the filter should be installed close to, but after, the membrane. Because the product water out of the membrane is produced at a slow rate, it will take several minutes for the water to pass from one end to the other, giving the water extended exposure to the carbon and increasing EBCT. There is also roughly 12 inches of bed depth (depending on the size of the filter) for the water to pass through. If an inline filter is installed before the membrane or after the storage tank, the flowrate will be higher, shortening the contact time. But there should be ample bed depth to impact taste, odor and some contaminants.
Like the GAC filter, the inline filter will require additional rinsing when new. It is very important to remember when replacing RO components, especially filters and membranes, that any modification other than the manufacturer’s recommendation will void any WQA certifications or contaminant reduction claims.
While covering a lot of information, there is much more to each filter type than discussed in this article. I encourage new service technicians and veterans alike to learn as much about their product line as possible, along with other products they may come across. Always follow the manufacturers’ guidelines and recommendations.
About the author
Jason Rice has been with Hague Quality Water International for the 23 years, 13 of those in manufacturing and a decade as Technical Advisor for international and domestic product and service training for residential and light commercial applications.
About the company
Hague Quality Water International exports water treatment solutions to over 50 countries. The company values quality and American-made goods. The headquarters and factory where its softeners are made are right here in the US. Hague’s flagship product, the WaterMax®, is the world’s most comprehensive, most efficient home water treatment system on the market today. Company engineers have iterated on those ideas for over 50 years to make sure that products are the most efficient and innovative for all.