What is reverse osmosis?

Reverse osmosis is a hot topic in the water treatment industry. With the lowest energy requirements, some of the highest recovery rates, and one of the best rejection rates on the market, it's no wonder people are interested in learning more about it. What is the definition of reverse osmosis, though? How does it work? Let's take a look into the heart of a Pure Aqua RO system and break it down for you to understand all about RO. 

As anyone could guess, it is the process of osmosis backwards. Osmosis is the passage of water through a reverse osmosis membrane (like our skin, or the inside of a plant cell) to equalize the concentration of particles dissolved in the water. As the diagram above shows, the semi-permeable membrane allows water to pass through, but molecules larger than water (things like minerals, salts, and bacteria) cannot. Water flows back and forth until the concentration is equal on both sides of the membrane, and an equilibrium is formed.

Let's apply this knowledge to water purification. We want to drink water from a lake or stream, but it contains too high concentration of contaminants like salt, minerals, and bacteria, that make it undrinkable. By applying pressure to water as it passes through a membrane, the water can be forced to move away from the membrane rather than attempting to form an equilibrium like normal. This against-flow motion is where the "reverse" in "reverse osmosis" comes from. A pump works well for this process. Water is forced through the membrane, which like a super-fine particle filter, blocks an extreme majority of contaminants from coming through.

As a water purification process, it has a number of advantages as well as disadvantages. In water treatment, TFC membranes can usually remove between 96 and 99% of most contaminants, including salts and minerals, dyes, particles, bacteria, and hazardous metals. Because of the way reverse osmosis works, however, you can never truly remove all of a contaminant. You may purify down to a fraction of a fraction of a percent, but the contaminant can never be truly eliminated with reverse osmosis. Water purification systems also requires a superior quality, high pressure pump, because the rejection rate is primarily dependent on the pressure applied to the membrane. That being said, smaller units have smaller ratios of permeate (clean, purified product) to waste water. This makes media filtration or other conventional filtration more effective in smaller scales (like residential settings).

Now that we've answered the biggest question about membranes systems by Pure Aqua, let's take a closer look at how we build our advanced water treatment systems and how reverse osmosis works.

The reverse osmosis process works by utilizing high pressure pumps to elevate the pressure on the concentrated side of the RO system and push the water through membrane(s). It is a highly effective water treatment process that needs a higher degree of pressure depending on the total amount of contaminants existing in the feed water.

The purified water that is removed of its dissolved solids is now referred to as permeate or product water. The concentrated stream of contaminants left behind in the brine is referred to as reject water and is ultimately discharged. 

The reverse osmosis membranes sole responsibility is to ensure efficient filtration of all unwanted salts and minerals from the feed water as it passes through the system. Membranes from top-branded manufacturers (Dupont Filmtec, Hydranautics) are capable of eliminating up to 99% of dissolved solids from the water source. The rejected stream is not always thrown away, it can be recycled through the reverse osmosis system for water conservation.

The reverse osmosis process is more similar to a cross-flow filtration method than a conventional one. Cross-flow filtration is more effective since it consists of two outlets for the solution to go in order to handle higher concentrations of pollutants. The flow of the permeate and concentrated water in opposite directions allows the membrane surface to remain clean and greatly minimizes buildup. This filtration method ensures a longer-life span of the system and components and reduced the need for membrane cleaning.

Now that we know how reverse osmosis works, let's take that and apply it to a real, working TWRO or BWRO system. If it only needed the membranes and a pump, it certainly wouldn't be so big, right?

Here is the complete reverse osmosis process, including the pretreatment and post-treatment that is generally required for highly contaminated water:

A) Pre-Chlorination Dosing System

If the feed water contains traces of heavy metals or contaminated, it is highly recommended to dose some chlorine to change the dissolved heavy metals to physical form, the media filter will be able to filter most of it.

B) Raw Water Storage Tank

Although some RO Systems can draw water right out of a well or pipe feed, most reverse osmosis systems start with a large tank that stores the contaminated water. Not having enough feed water can damage a pump, so having a large storage tank for your intake water is an easy way to make sure your pump lasts for as long as possible.

C) Feed Water Pump

A commercial or industrial pump provides the initial pressure for the Treatment System. This motor usually provides enough water pressure to get through any pretreatment as well as the RO membranes, but if it doesn't a booster pump may be necessary farther down the line.

D) Multi-Layer or Media Filter

As much as we hate to admit it, there are some things that membranes can't purify. Nitrates, a common contaminant found in fertilizers and animal waste, are a good example of particles that dissolve too well in water for reverse osmosis to catch them. Things like foul odor and taste usually aren't prevented by reverse osmosis, either. A Multi-Layer filter can be filled with media that specifically targets the things your RO system can't catch. If you need to eliminate these contaminants, a multi-layer filter is a must. An example of MMF or multi media filters is our water media filters MF-1000 Series.

E) Activated Carbon Filter

Activated carbon filters are a good solution to reduce organic, bad taste, smell and chlorine from the water.

F) Automatic Water Softener

Automatic water softeners are designed to remove water hardness, calcium and magnesium ions, for small RO systems, we usually recommend water softeners instead of the antiscalant chemical dosing. 

G) Antiscalant Chemical Dosing System

For larger RO systems, we use antiscalant dosing systems to dose our PA0100 antiscalant RO chemical, which helps in preventing membrane fouling. For more information, please visit our CDS-Series chemical dosing pumps.

H) Reverse Osmosis System

We finally have our reverse osmosis system. If a booster pump is necessary, it will typically be just before this step. The reverse osmosis system can produce up to one million gallons of product water a day from a steady intake, as well as a sizable amount of waste. Usually the wastewater can be dumped down the drain, but check with your local water authorities in case it needs to be handled with care.

I) Product Water Storage Tank

The permeate from the reverse osmosis system will usually go to a large tank, where it is held for use. If it didn't, the system would need to be running in order to have access to fresh water, which can be inconvenient. Sometimes, a RO system pumps water directly into a well or aquifer for recharging instead of being used in many of the normal industries or applications it is used in.

J) Post-Chlorination Dosing System

If the permeate water is intended to be stored for more than one day, it is highly recommended to dose some chlorine to maintain clean and non contaminated water.

K) Product Water Pump (Re-pressurization)

This pump repressurizes the permeate water to the point of use ends. This is selected based on the overall travelling distance and required head. This pump must be selected in stainless steel to prevent any contamination to the permeate water.

L) Product Water UV Sterilizer

The UV sterilizer is placed after the storage tank, and as a final disinfection device. Most of the time, we either use post chlorination as a disinfectant agent, or ultraviolet sterilization.

Reverse Osmosis is an ideal water treatment solution in most types of water. Generally speaking, all major water sources from a treatment standpoint can be broken down into three major categories: tap water, also known as municipal sources, groundwater, which includes brackish water, and saltwater. The biggest distinction between these three types is the Total Dissolved Solids (TDS) content of each type. As a rule of thumb, the American Health Association requires that drinking water is under 1,000 PPM TDS.

Tap water typically comes through a pre-existing infrastructure like city pipes or a damming system. Reverse osmosis is often used in a tap water environment to reduce hardness, or the debris deposited in water from traveling in metal pipes. Total dissolved solids is often a target of water purification in tap water systems. RO water is ideal in applications like power plants, pharmaceuticals, laboratories, and hospitals, where an extreme purity of water is crucial to the industry. Tap water typically has a TDS of under 1,000 PPM.

Underground reservoirs of water are often brackish or highly brackish, meaning they contain large volumes of salt, but not enough to be considered salt water. Groundwater reverse osmosis is very common, and one of the best uses of a reverse osmosis system to date. Groundwater is most often purified for the agriculture industry, the mining industry, and for residential use. Groundwater is also a prized target of the bottling industry, because the unique mineral combinations often have an appealing taste. Brackish water usually has a TDS of 5,000 PPM or less, but can come in concentrations of up to 12,000 PPM.

Reverse osmosis salt water (sometimes referred to simply as desalination) is the turning of salt water into drinking water. Ocean water has up to 45,000 PPM TDS. Typically, for environmental reasons, a bore hole is dug in the ocean for this kind of reverse osmosis water, but an open intake is more cost effective. The biggest uses of desalination come in providing water in areas that lack a regular supply of fresh water.

A detailed chemical analysis of the RO feed water is an absolute necessity for identifying potential foulants. This should include a measurement of the hardness (calcium and magnesium), barium, strontium, alkalinity, pH, and chlorine. The data from the chemical analysis can be used by the RO equipment designers to determine the optimum membrane array that will both minimize the tendency of scale and deposit formation and maximize the recovery and flux rate.

For example, the Langelier Stability Index (LSI), a measure of the calcium carbonate scaling tendency of the water, is computed from the water analysis to determine the maximum permissible concentration of dissolved minerals in the reject stream before scale deposition becomes a problem. Because of the number of variables that must be considered, these calculations are difficult to do with pencil and paper. Fortunately, the membranes manufacturers have developed computer programs that make these computations fast and easy to perform where the user can project the performance of membranes at actual feed conditions.

Although a water analysis is helpful in predicting the tendency of dissolved minerals to cause problems in the RO System, it does not always forecast the fouling tendency of colloids and other finely dispersed suspended solids. The Silt Density Index (SDI) is a useful tool for quantifying the fouling tendency of the feed water. This test is conducted by filtering a sample through a 0.45 micron (µm) filter and measuring the time required to collect a unit volume of filtrate. An index number is calculated from this data. Traditionally, a SDI value of less than 3.0 is desirable for reverse osmosis feed waters. The SDI measurement has certain limitations in that it does not model the cross flow design of an RO membrane.

Antiscalants have been shown to be effective in extending the intervals between chemical cleanings of the RO membranes. These products are generally formulated to include inorganic phosphates, organophosphonates, and dispersants. Use antiscalant products that have been approved by the membrane manufacturer, and follow all directions in applying and controlling the product dosage. Some antiscalant contain negatively charged polymers and dispersants that can react with cationic polymers that might be dosed up stream prior to the media filters. The antiscalant must be compatible with these polymers; otherwise, the reaction product will foul the membranes.