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Articles of Purifiers :  RO / UV / TDS / AF / UF / Minerals / Filters & Prefilters 


Water is life, something without that life ceases to exist. Pure and clean water nurture our body and soul. Water we drink may contain various physical, microbiological, toxic Chemicals & Dissolved Impurities extremely harmful to our body. With so many waterborne diseases like jaundice,Typhoid, Cholera, Gastroenteritis challenging the human race, one should be extra conscious.

Dr. RO provides the highest level of purity, safety and convenience in your home or office. Assuring you the purest water possible with Reverse Osmosis. Reverse Osmosis is the new, advanced leading-edge technology, which separates even the smallest polluting particles, molecules and ions, which conventional technologies can't remove.

Dr. RO is a unique water purifier, based on the ultimate Reverse Osmosis technology and employs five stages for water purification. Especially designed for indian conditions with areas of high level of Total Dissolved Solids [TDS] and Hardness. Dr. RO makes the water both chemically and microbiologically safe by reducing pesticides, hazardous metal contaminants (Lead, mercury, Arsenic etc.) and waterborne disease causing Microorganisms. 

Want to PURCHASE an Water Purifier with RO / UV / TDS / UF / AF. Just CLICK HERE to go there for Dr. RO & OSO-Libra

OSO-Libra RO Leaflets                    OSO-Libra RO Booklet

Our industrial grade RO Systems will filter your raw water (from supply or bore-well) and deliver pure RO water directly to your overhead water tanks or into the RO units. So your full house can get RO water in all your taps. Imagine the purest water in your Bathroom Shower, Health Faucet, Washbasin, Kitchen, lawn.

If you live in a place where "supply water" or "bore-well water" quality is not very good, our RO systems can have dramatic effect on your quality of living. 

Can you imagine what all changes of good water can bring in your life, if you used RO water everywhere ?

  • 99.99% of bacteria, virus, cysts etc. will be removed. So even bathroom tap water will not cause any health problems.

  • The quality of your Hair will improve drastically. There will be no abrasion of hair, skin that is caused by salty hard water.

  • Dandruff and several skin problems will reduce or just disappear

  • The quality of your Skin will get affected drastically. Dryness caused due to Dirty or Hard water will not happen.

  • Children will love taking a bath and brushing their teeth because the taste of water will be very good and not salty.

  • Your expensive bathroom fittings will stop rusting or getting white/black scaling.

  • Water pipes will not clog due to calcium scales.

  • Water will become super-soft.

  • Less soap, shampoo and detergents will be required

  • There will be no white water marks on your expensive crockery, glasses and utensils

  • Bath-tubs, shower panels, taps mirrors will not require regular scrubbing to clean the scaling

  • Plants and lawn grass will grow better and look better with pure RO water used for gardening

  • RO water produced by our Dr.RO & OSO-Libra RO Water Purifierification can be used to nice to drinking, fooding, cooking, wash your car very easily, as it will leave no white marks on the paint or windows. Just spray the water on your expensive car, bike, SUV with a hose pipe and leave it!!  You may not even need to wipe it dry with a cloth, as there will be no residue from the water !

Reverse Osmosis is a technology that is used to remove a large majority of contaminants from water by pushing the water under pressure through a semi-permeable membrane.

Understanding Reverse Osmosis

Reverse Osmosis, commonly referred to as RO, is a process where you demineralize or deionize water by pushing it under pressure through a semi-permeable Reverse Osmosis Membrane.


To understand the purpose and process of Reverse Osmosis you must first understand the naturally occurring process of Osmosis.

Osmosis is a naturally occurring phenomenon and one of the most important processes in nature. It is a process where a weaker saline solution will tend to migrate to a strong saline solution. Examples of osmosis are when plant roots absorb water from the soil and our kidneys absorb water from our blood.

Below is a diagram which shows how osmosis works. A solution that is less concentrated will have a natural tendency to migrate to a solution with a higher concentration. For example, if you had a container full of water with a low salt concentration and another container full of water with a high salt concentration and they were separated by a semi-permeable membrane, then the water with the lower salt concentration would begin to migrate towards the water container with the higher salt concentration.

semi-permeable membrane is a membrane that will allow some atoms or molecules to pass but not others. A simple example is a screen door. It allows air molecules to pass through but not pests or anything larger than the holes in the screen door. Another example is Gore-tex clothing fabric that contains an extremely thin plastic film into which billions of small pores have been cut. The pores are big enough to let water vapor through, but small enough to prevent liquid water from passing.

Reverse Osmosis is the process of Osmosis in reverse. Whereas Osmosis occurs naturally without energy required, to reverse the process of osmosis you need to apply energy to the more saline solution. A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not the majority of dissolved salts, organics, bacteria and pyrogens. However, you need to 'push' the water through the reverse osmosis membrane by applying pressure that is greater than the naturally occurring osmotic pressure in order to desalinate (demineralize or deionize) water in the process, allowing pure water through while holding back a majority of contaminants. Below is a diagram outlining the process of Reverse Osmosis. When pressure is applied to the concentrated solution, the water molecules are forced through the semi-permeable membrane and the contaminants are not allowed through. 

How does Reverse Osmosis work?

Reverse Osmosis works by using a high pressure pump to increase the pressure on the salt side of the RO and force the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream. The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure.

The desalinated water that is demineralized or deionized, is called permeate (or product) water. The water stream that carries the concentrated contaminants that did not pass through the RO membrane is called the reject (or concentrate) stream.

RO Membrane Diagram

As the feed water enters the RO membrane under pressure (enough pressure to overcome osmotic pressure) the water molecules pass through the semi-permeable membrane and the salts and other contaminants are not allowed to pass and are discharged through the reject stream (also known as the concentrate or brine stream), which goes to drain or can be fed back into the feed water supply in some circumstances to be recycled through the RO system to save water. The water that makes it through the RO membrane is called permeate or product water and usually has around 95% to 99% of the dissolved salts removed from it.

It is important to understand that an RO system employs cross filtration rather than standard filtration where the contaminants are collected within the filter media. With cross filtration, the solution passes through the filter, or crosses the filter, with two outlets: the filtered water goes one way and the contaminated water goes another way. To avoid build up of contaminants, cross flow filtration allows water to sweep away contaminant build up and also allow enough turbulence to keep the membrane surface clean.

What will Reverse Osmosis remove from water?

Reverse Osmosis is capable of removing up to 99%+ of the dissolved salts (ions), particles, colloids, organics, bacteria and pyrogens from the feed water (although an RO system should not be relied upon to remove 100% of bacteria and viruses). An RO membrane rejects contaminants based on their size and charge. Any contaminant that has a molecular weight greater than 200 is likely rejected by a properly running RO system (for comparison a water molecule has a MW of 18). Likewise, the greater the ionic charge of the contaminant, the more likely it will be unable to pass through the RO membrane. For example, a sodium ion has only one charge (monovalent) and is not rejected by the RO membrane as well as calcium for example, which has two charges. Likewise, this is why an RO system does not remove gases such as CO2 very well because they are not highly ionized (charged) while in solution and have a very low molecular weight. Because an RO system does not remove gases, the permeate water can have a slightly lower than normal pH level depending on CO2 levels in the feed water as the CO2 is converted to carbonic acid.

Reverse Osmosis is very effective in treating brackish, surface and ground water for both large and small flows applications. Some examples of industries that use RO water include pharmaceutical, boiler feed water, food and beverage, metal finishing and semiconductor manufacturing to name a few.

Reverse Osmosis Performance & Design Calculations

There are a handful of calculations that are used to judge the performance of an RO system and also for design considerations. An RO system has instrumentation that displays quality, flow, pressure and sometimes other data like temperature or hours of operation. In order to accurately measure the performance of an RO system you need the following operation parameters at a minimum:

  1. Feed pressure
  2. Permeate pressure
  3. Concentrate pressure
  4. Feed conductivity
  5. Permeate conductivity
  6. Feed flow
  7. Permeate flow
  8. Temperature

Salt Rejection %

This equation tells you how effective the RO membranes are removing contaminants. It does not tell you how each individual membrane is performing, but rather how the system overall on average is performing. A well-designed RO system with properly functioning RO membranes will reject 95% to 99% of most feed water contaminants (that are of a certain size and charge). You can determine effective the RO membranes are removing contaminants by using the following equation: 

Salt Rejection Calculation

The higher the salt rejection, the better the system is performing. A low salt rejection can mean that the membranes require cleaning or replacement.

Salt Passage %

This is simply the inverse of salt rejection described in the previous equation. This is the amount of salts expressed as a percentage that are passing through the RO system. The lower the salt passage, the better the system is performing. A high salt passage can mean that the membranes require cleaning or replacement. 

Salt Passage Calculation

Recovery %

Percent Recovery is the amount of water that is being 'recovered' as good permeate water. Another way to think of Percent Recovery is the amount of water that is not sent to drain as concentrate, but rather collected as permeate or product water. The higher the recovery % means that you are sending less water to drain as concentrate and saving more permeate water. However, if the recovery % is too high for the RO design then it can lead to larger problems due to scaling and fouling. The % Recovery for an RO system is established with the help of design software taking into consideration numerous factors such as feed water chemistry and RO pre-treatment before the RO system. Therefore, the proper % Recovery at which an RO should operate at depends on what it was designed for. By calculating the % Recovery you can quickly determine if the system is operating outside of the intended design. The calculation for % Recovery is below: 

Recovery Calculation

For example, if the recovery rate is 75% then this means that for every 100 gallons of feed water that enter the RO system, you are recovering 75 gallons as usable permeate water and 25 gallons are going to drain as concentrate. Industrial RO systems typically run anywhere from 50% to 85% recovery depending the feed water characteristics and other design considerations.

Concentration Factor

The concentration factor is related to the RO system recovery and is an important equation for RO system design. The more water you recover as permeate (the higher the % recovery), the more concentrated salts and contaminants you collect in the concentrate stream. This can lead to higher potential for scaling on the surface of the RO membrane when the concentration factor is too high for the system design and feed water composition. 

Concentration Factor Calculation

The concept is no different than that of a boiler or cooling tower. They both have purified water exiting the system (steam) and end up leaving a concentrated solution behind. As the degree of concentration increases, the solubility limits may be exceeded and precipitate on the surface of the equipment as scale. 

For example, if your feed flow is 100 gpm and your permeate flow is 75 gpm, then the recovery is (75/100) x 100 = 75%. To find the concentration factor, the formula would be 1 ÷ (1-75%) = 4. 

A concentration factor of 4 means that the water going to the concentrate stream will be 4 times more concentrated than the feed water is. If the feed water in this example was 500 ppm, then the concentrate stream would be 500 x 4 = 2,000 ppm.

Flux :

Flux (Gfd) Calculation

For example, you have the following:
The RO system is producing 75 gallons per minute (gpm) of permeate. You have 3 RO vessels and each vessel holds 6 RO membranes. Therefore you have a total of 3 x 6 = 18 membranes. The type of membrane you have in the RO system is a Dow Filmtec BW30-365. This type of RO membrane (or element) has 365 square feet of surface area. 

To find the flux (Gfd): 

Flux Calculation Example

The flux is 16 Gfd. 

This means that 16 gallons of water is passed through each square foot of each RO membrane per day. This number could be good or bad depending on the type of feed water chemistry and system design. Below is a general rule of thumb for flux ranges for different source waters and can be better determined with the help of RO design software. If you had used Dow Filmtec LE-440i RO membranes in the above example, then the flux would have been 14. So it is important to factor in what type of membrane is used and to try and keep the type of membrane consistent throughout the system.

Feed Water SourceGfd
Sewage Effluent5-10
Sea Water8-12
Brackish Surface Water10-14
Brackish Well Water14-18
RO Permeate Water20-30

Mass Balance

A Mass Balance equation is used to help determine if your flow and quality instrumentation is reading properly or requires calibration. If your instrumentation is not reading correctly, then the performance data trending that you are collecting is useless. 

You will need to collect the following data from an RO system to perform a Mass Balance calculation:

  1. Feed Flow (gpm)
  2. Permeate Flow (gpm)
  3. Concentrate Flow (gpm)
  4. Feed Conductivity (µS)
  5. Permeate Conductivity (µS)
  6. Concentrate Conductivity (µS)

The mass balance equation is: 

(Feed flow1 x Feed Conductivity) = (Permeate Flow x Permeate Conductivity) 
+ (Concentrate Flow*Concentrate Conductivity) 

1Feed Flow equals Permeate Flow + Concentrate Flow

For example, if you collected the following data from an RO system:

Permeate Flow5 gpm
Feed Conductivity500 µS
Permeate Conductivity10 µS
Concentrate Flow2 gpm
Concentrate Conductivity1200 µS

Then the Mass Balance Equation would be: 

(7 x 500) = (5 x 10) + (2*1200) 

3,500 ≠ 2,450 

Then find the difference 

(Difference / Sum) ∗ 100 

((3,500 - 2,450) / (3,500 + 2,450)) * 100 = 18%

A difference of +/- 5% is ok. A difference of +/- 5% to 10% is generally adequate. A difference of > +/- 10% is unacceptable and calibration of the RO instrumentation is required to ensure that you are collecting useful data. In the example above, the RO mass balance equation falls out of range and requires attention.

Understanding the difference between passes and stages in a Reverse Osmosis (RO) system

The term stage and pass are often mistaken for the same thing in an RO system and can be confusing termonology for an RO operator. It is important to understand the difffernce between a 1 and 2 stage RO and a 1 and 2 pass RO.

Difference between a 1 and 2 stage RO System

In a one stage RO system, the feed water enters the RO system as one stream and exits the RO as either concentrate or permeate water. 

In a two-stage system the concentrate (or reject) from the first stage then becomes the feed water to the second stage. The permeate water is collected from the first stage is combined with permeate water from the second stage. Additional stages increase the recovery from the Systems.

 1 and 2 stage RO diagram


In a Reverse Osmosis System an array describes the physical arrangement of the pressure vessels in a 2 stage system. Pressure vessels contain RO membranes (usually from 1 to 6 RO membranes are in a pressure vessel). Each stage can have a certain amount of pressure vessels with RO membranes. The reject of each stage then becomes the feed stream for the next successive stage. The 2 stage RO system displayed on the previous page is a 2:1 array which means that the concentrate (or reject) of the first 2 RO vessels is fed to the next 1 vessel.

RO System with Concentrate Recycle

With an RO system that can't be properly staged and the feed water chemistry allows for it, a concentrate recycle setup can be utilized where a portion of the concentrate stream is fed back to the feed water to the first stage to help increase the system recovery.

RO System with Concentrate Recycle Diagram

Single Pass RO vs Double Pass RO

Think of a pass as a stand alone RO system. With this in mind, the difference between a single pass RO system and a double pass RO system is that with a double pass RO, the permeate from the first pass becomes the feed water to the second pass (or second RO) which ends up producing a much higher quality permeate because it has essentially gone through two RO systems.

Besides producing a much higher quality permeate, a double pass system also allows the opportunity to remove carbon dioxide gas from the permeate by injecting caustic between the first and second pass. C02 is undesirable when you have mixed bed ion exchange resin beds after the RO. By adding caustic after the first pass, you increase the pH of the first pass permeate water and convert C02 to bicarbonate (HCO3-) and carbonate (CO3-2) for better rejection by the RO membranes in the second pass. This can't be done with a single pass RO because injecting caustic and forming carbonate (CO3-2) in the presence of cations such as calcium will cause scaling of the RO membranes. 

Single Pass RO and Double Pass RO diagram

RO Pretreatment

Proper pretreatment using both mechanical and chemical treatments is critical for an RO system to prevent fouling, scaling and costly premature RO membrane failure and frequent cleaning requirements. Below is a summary of common problems an RO system experiences due to lack of proper pretreatment.


Fouling occurs when contaminants accumulate on the membrane surface effectively plugging the membrane. There are many contaminants in municipal feed water that are naked to the human eye and harmless for human consumption, but large enough to quickly foul (or plug) an RO system. Fouling typically occurs in the front end of an RO system and results in a higher pressure drop across the RO system and a lower permeate flow. This translates into higher operating costs and eventually the need to clean or replace the RO membranes. Fouling will take place eventually to some extent given the extremely fine pore size of an RO membrane no matter how effective your pretreatment and cleaning schedule is. However, by having proper pretreatment in place, you will minimize the need to address fouling related problems on a regular basis. 

Fouling can be caused by the following:

  1. Particulate or colloidal mater (dirt, silt, clay, etc.)
  2. Organics (humic/fulvic acids, etc)
  3. Microorganisms (bacteria, etc). Bacteria present one of the most common fouling problems since RO membranes in use today cannot tolerate a disinfectant such as chlorine and thefore microorganisms are often able to thrive and multiply on the membrane surface. They may product biofilms that cover the membrane surface and result in heavy fouling.
  4. Breakthrough of filter media upstream of the RO unit. GAC carbon beds and softener beds may develop an under drain leak and if there is not adequate post filtration in place the media can foul the RO system.

By performing analytical tests, you can determine if the feed water to your RO has a high potential for fouling. To prevent fouling of an RO system, mechanical filtration methods are used. The most popular methods to prevent fouling are the use of multi-media filters (MMF) or microfiltration (MF). In some cases cartridge filtration will suffice.


As certain dissolved (inorganic) compounds become more concentrated (remember discussion on concentration factor) then scaling can occur if these compounds exceed their solubility limits and precipitate on the membrane surface as scale. The results of scaling are a higher pressure drop across the system, higher salt passage (less salt rejection), low permeate flow and lower permeate water quality. An example of a common scale that tends to form on an RO membrane is calcium carbonate (CaCO3).

Chemical Attack

Modern thin film composite membranes are not tolerant to chlorine or chloramines. Oxidizers such as chlorine will 'burn' holes in the membrane pores and can cause irreparable damage. The result of chemical attack on an RO membrane is a higher permeate flow and a higher salt passage (poorer quality permeate water). This is why microorganism growth on RO membranes tends to foul RO membranes so easily since there is no biocide to prevent its growth.

Mechanical Damage

Part of the pretreatment scheme should be pre and post RO system plumbing and controls. If 'hard starts' occur mechanical damage to the membranes can occur. Likewise, if there is too much backpressure on the RO system then mechanical damage to the RO membranes can also occur. These can be addressed by using variable frequency drive motors to start high pressure pumps for RO systems and by installing check valve(s) and/or pressure relief valves to prevent excessive back pressure on the RO unit that can cause permanent membrane damage.

Pretreatment Solutions

Below are some pretreatment solutions for RO systems that can help minimize fouling, scaling and chemical attack.

Multi Media Filtration (MMF)

A Multi-Media Filter is used to help prevent fouling of an RO system. A Multi-Media Filter typically contains three layers of media consisting of anthracite coal, sand and garnet, with a supporting layer of gravel at the bottom. These are the medias of choice because of the differences in size and density. The larger (but lighter) anthracite coal will be on top and the heavier (but smaller) garnet will remain on the bottom. The filter media arrangement allows the largest dirt particles to be removed near the top of the media bed with the smaller dirt particles being retained deeper and deeper in the media. This allows the entire bed to act as a filter allowing much longer filter run times between backwash and more efficient particulate removal.

A well-operated Multi-Media Filter can remove particulates down to 15-20 microns. A Multi-Media Filter that uses a coagulant addition (which induces tiny particles to join together to form particles large enough to be filtered) can remove particulates down to 5-10 microns. To put this in perspective, the width of a human hair is around 50 microns.

A multi media filter is suggested when the Silt Density Index (SDI) value is greater than 3 or when the turbidity is greater than 0.2 NTU. There is no exact rule, but the above guidelines should be followed to prevent premature fouling of RO membranes.

It is important to have a 5 micron cartridge filter placed directly after the MMF unit in the event that the under drains of the MMF fail. This will prevent the MMF media from damaging downstream pumps and fouling the RO system.

Microfiltration (MF)

Microfiltration (MF) is effective in removing colloidal and bacteria matter and has a pore size of only 0.1-10µm. Microfiltration is helpful in reducing the fouling potential for an RO unit. Membrane configuration can vary between manufacturers, but the "hollow fiber" type is the most commonly used. Typically, the water is pumped from the outside of the fibers, and the clean water is collected from the inside of the fibers. Microfiltration membranes used in potable water applications usually operate in "dead-end" flow. In dead-end flow, all of the water fed to the membrane is filtered through the membrane. A filter cake that must be periodically backwashed from the membrane surface forms. Recovery rates are normally greater than 90 percent on feed water sources which have fairly high quality and low turbidity feeds.

Antiscalants and Scale Inhibitors

Antiscalants and scale inhibitors, as their name suggests, are chemicals that can be added to feed water before an RO unit to help reduce the scaling potential of the feed water. Antiscalants and scale inhibitors increase the solubility limits of troublesome inorganic compounds. By increasing the solubility limits, you are able to concentrate the salts further than otherwise would be possible and therefore achieve a higher recovery rate and run at a higher concentration factor. Antiscalants and scale inhibitors work by interfering with scale formation and crystal growth. The choice of antiscalant or scale inhibitor to use and the correct dosage depends on the feed water chemistry and RO system design.

Softening by ion exchange

A water softener can be used to help prevent scaling in an RO system by exchanging scale forming ions with non scale forming ions. As with a MMF unit, it is important to have a 5 micron cartridge filter placed directly after the water softener in the event that the under drains of the softener fail.

Sodium Bisulfite (SBS) injection

By adding sodium bisulfite (SBS or SMBS), which is a reducer, to the water stream before an RO at the proper dose you can remove residual chlorine.

Granular Activated Carbon (GAC)

GAC is used for both removing organic constituents and residual disinfectants (such as chlorine and chloramines) from water. GAC media is made from coal, nutshells or wood. Activated carbon removes residual chlorine and chloramines by a chemical reaction that involves a transfer of electrons from the surface of the GAC to the residual chlorine or chloramines. The chlorine or chloramines ends up as a chloride ion that is no longer an oxidizer.

The disadvantage of using a GAC before the RO unit is that the GAC will remove chlorine quickly at the very top of the GAC bed. This will leave the remainder of the GAC bed without any biocide to kill microorganisms. A GAC bed will absorb organics throughout the bed, which is potential food for bacteria, so eventually a GAC bed can become a breeding ground for bacteria growth which can pass easily to the RO membranes. Likewise, a GAC bed can produce very small carbon fines under some circumstances that have the potential to foul an RO.

The RO membranes are the heart of the RO system and certain data points need to be collected to determine the health of the RO membranes. These data points include the system pressures, flows, quality and temperature. Water temperature is directly proportional to pressure. As the water temperature decreases it becomes more viscous and the RO permeate flow will drop as it requires more pressure to push the water through the membrane. Likewise, when the water temperature increases the RO permeate flow will increase. As a result, performance data for an RO system must be normalized so that flow variations are not interpreted as abnormal when no problem exists. The normalized flows, pressures and salt rejection should be calculated, graphed and compared to the baseline data (when the RO was commissioned or after the membranes were cleaned or replaced) to help troubleshoot any problems and also determine when to clean or inspect the membranes for damage. Data normalization helps display the true performance of the RO membranes. As a general rule of thumb, when the normalized change is +/- 15% from the baseline data then you need to take action. If you don't follow this rule then RO membrane cleanings may not be very effective at brining the membranes back to near new performance.

RO Membrane Cleaning

RO membranes will inevitably require periodic cleaning, anywhere from 1 to 4 times a year depending on the feed water quality. As a general rule, if the normalized pressure drop or the normalized salt passage has increased by 15%, then it is time to clean the RO membranes. If the normalized permeate flow has decreased by 15% then it is also time to clean the RO membranes. You can either clean the RO membranes in place or have them removed from the RO system and cleaned off site by a service company that specializes in this service. It has been proven that offsite membrane cleaning is more effective at providing a better cleaning than onsite cleaning skids.

RO membrane cleaning involves low and high pH cleaners to remove contaminants from the membrane. Scaling is addressed with low pH cleaners and organics, colloidal and biofouling are treated with a high pH cleaner. Cleaning RO membranes is not only about using the appropriate chemicals. There are many other factors involved such as flows, water temperature and quality, properly designed and sized cleaning skids and many other factors that an experienced service group must address in order to properly clean RO membranes.


Reverse Osmosis is an effective and proven technology to produce water that is suitable for many industrial applications that require demineralized or deionized water. Further post treatment after the RO system such as mixed bed deionization can increase the quality of the RO permeate and make it suitable for the most demanding applications. Proper pretreatment and monitoring of an RO system is crucial to preventing costly repairs and unscheduled maintenance. With the correct system design, maintenance program, and experienced service support, your RO system should provide many years of high purity water. 

[All Data & images Collected from website of puretec water and it is puretec property. this article is published here for only for knowledge purpose to peoples know about RO systems]

What is TDS ?

Total Dissolved Solids (TDS) are the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm). A lot of minerals are found in water, like Calcium, Magnesium, Chlorine, etc. The RO system removes most of these from your water to make it safe and healthy for consumption and other domestic usage.

How much TDS is OK for humans ?

Threshold of acceptable human drinking water is 500 mg/l

1500 to 5000 mg/L TDS = Brackish water

> 5000 mg/L TDS = Saline water

What does a Dr.RO & OSO-Libra RO system do ?

  • If Raw feed water is entering into the Dr. RO / OSO-Libra RO System is about 2,000 TDS, then the output water TDS will become in the range of 80 to 150 TDS.
  • That is about 98% removal of unwanted Salts !!!
  • 80 PPM TDS is much LOWER than the TDS of water from many popular brands of bottled drinking water.

TDS (Total Dissolved Solids) is a measure of the combined content of all inorganic and organic substances contained in water in suspended form.

Primary sources for TDS are:

  • Agricultural and Residential runoff,
  • Soil contamination
  • Water pollution
  • Discharge from industrial or sewage treatment plants
  • Common chemical constituents like calcium, phosphates, nitrates, sodium, potassium and chloride
  • Pesticides arising from surface runoff
  • Certain naturally occurring TDS arise from the weathering and dissolution of rocks and soils 

What Are Total Dissolved Solids?

  • "Dissolved solids" refer to any minerals, salts, metals, cations or anions dissolved in water. This includes anything present in water other than the pure water (H20) molecule and suspended solids. (Suspended solids are any particles/substances that are neither dissolved nor settled in the water, such as wood pulp.)
  • In general, the total dissolved solids concentration is the sum of the cations (positively charged) and anions (negatively charged) ions in the water.
  • Parts per Million (ppm) is the weight-to-weight ratio of any ion to water.
  • A TDS meter is based on the electrical conductivity (EC) of water. Pure H20 has virtually zero conductivity. Conductivity is usually about 100 times the total cations or anions expressed as equivalents. TDS is calculated by converting the EC by a factor of 0.5 to 1.0 times the EC, depending upon the levels. Typically, the higher the level of EC, the higher the conversion factor to determine the TDS. NOTE - While a TDS meter is based on conductivity, TDS and conductivity are not the same thing. For more information on this topic, please see our FAQ page.

Where Do Dissolved Solids Come From?

  • Some dissolved solids come from organic sources such as leaves, silt, plankton, and industrial waste and sewage. Other sources come from runoff from urban areas, road salts used on street during the winter, and fertilizers and pesticides used on lawns and farms.
  • Dissolved solids also come from inorganic materials such as rocks and air that may contain calcium bicarbonate, nitrogen, iron phosphorous, sulfur, and other minerals. Many of these materials form salts, which are compounds that contain both a metal and a nonmetal. Salts usually dissolve in water forming ions. Ions are particles that have a positive or negative charge.
  • Water may also pick up metals such as lead or copper as they travel through pipes used to distribute water to consumers.
  • Note that the efficacy of water purifications systems in removing total dissolved solids will be reduced over time, so it is highly recommended to monitor the quality of a filter or membrane and replace them when required.

Why Should You Measure the TDS Level in Your Water?

The EPA Secondary Regulations advise a maximum contamination level (MCL) of 500mg/liter (500 parts per million (ppm)) for TDS. Numerous water supplies exceed this level. When TDS levels exceed 1000mg/L it is generally considered unfit for human consumption. A high level of TDS is an indicator of potential concerns, and warrants further investigation. Most often, high levels of TDS are caused by the presence of potassium, chlorides and sodium. These ions have little or no short-term effects, but toxic ions (lead arsenic, cadmium, nitrate and others) may also be dissolved in the water.

Even the best water purification systems on the market require monitoring for TDS to ensure the filters and/or membranes are effectively removing unwanted particles and bacteria from your water. 

TDS Graph

*Chart values represent national U.S. averages.  Actual TDS levels for geographic regions within the U.S. and other countries may vary. Click here for the U.S. EPA's list of National Secondary Drinking Water Regulations. 

How Do You Reduce or Remove the TDS in Your Water?

Common water filter and water purification systems:

Carbon filtration
Charcoal, a form of carbon with a high surface area, adsorbs (or sticks to) many compounds, including some toxic compounds. Water is passed through activated charcoal to remove such contaminants. 

Activated Carbon (Granular and Solid Block) :

Granular activated carbon is a well-established technology for the reduction of a wide range of aesthetic contaminants, and is quite effective in the reduction of some health contaminants such as volatile organic compounds (benzene, trichloroethylene, and other "petroleum"-based contaminants.

Because of its molecular makeup, activated carbon can adsorb well, meaning that it can take in or collect many organic molecules on its surface. Granular activated carbon filters are typically inexpensive, and maintenance involves replacing six to twelve cartridges a year, depending on the quality of the raw water and the filter media.

Specially designed solid block and precoat activated carbon filters are also available, which are effective at reducing heavy metals such as lead and mercury. Solid block filters with a pore size smaller than 0.2 microns are often effective against biological contaminants as well.

Microfiltration :

Microfiltration uses a filter media with a pore size smaller than 0.2 microns to physically prevent biological contamination from passing through. Ceramic and solid block carbon are commonly used to provide microfiltration. Ceramic filters have and advantage in that they can often be cleaned and reused a number of times before they lose effectiveness.

Carbon block media usually has to be disposed of after each use. This media, however, provides additional treatment for a variety of other health and aesthetic contaminants (see activated carbon section). Microfiltration is effective for treating the full range of biological contaminants, including hard-shelled cysts like Cryptosporidium.

Reverse osmosis (R.O.)
Reverse osmosis works by forcing water under great pressure against a semi-permeable membrane that allows water molecules to pass through while excluding most contaminants. RO is the most thorough method of large-scale water purification available. 

What is Reverse Osmosis? 

Anyone who has been through a high school science class will likely be familiar with the term osmosis. The process was first described by a French Scientist in 1748, who noted that water spontaneously diffused through a pig bladder membrane into alcohol. Over 200 years later, a modification of this process known as reverse osmosis allows people throughout the world to affordably convert undesirable water into water that is virtually free of health or aesthetic contaminants. Reverse osmosis systems can be found providing treated water from the kitchen counter in a private residence to installations used in manned spacecraft. [Read above topics of RO for all details]

Reverse Osmosis is a technology that is found virtually anywhere pure water is needed; common uses include:

  • Drinking Water
  • Humidification
  • Ice-Making
  • Car Wash Water Reclamation
  • Rinse Waters
  • Biomedical Applications
  • Laboratory Applications
  • Photography
  • Pharmaceutical Production
  • Kidney Dialysis
  • Water used in chemcial processes
  • Cosmetics
  • Animal Feed
  • Hatcheries
  • Restaurants
  • Greenhouses
  • Metal Plating Applications
  • Wastewater Treatment
  • Boiler Water
  • Battery Water
  • Semiconductor production
  • Hemodialysis

Distillation involves boiling the water to produce water vapor. The water vapor then rises to a cooled surface where it can condense back into a liquid and be collected. Because the dissolved solids are not normally vaporized, they remain in the boiling solution.

What is...Distillation?

Distillation is one of mankind's earliest forms of water treatment, and it is still a popular treatment solution throughout the world today. In ancient times, the Greeks used this process on their ships to convert sea water into drinking water. In far-eastern cultures, water was distilled for use in "Ranbiki" tea ceremonies. 

Today, distilled water is still used to convert sea water to drinking water on ships and in arid parts of the world, and to treat water in other areas that is fouled by natural and unnatural contaminants. Distillation is perhaps the one water treatment technology that most completely reduces the widest range of drinking water contaminants.

Not only is distillation one of the most effective forms of treatment, but it is also one of the easiest to understand: untreated water is converted into water vapor, which is then condensed back into liquid form. Most of the contaminants are left behind in the boiling chamber, with the condensed water being virtually contaminant-free. Anyone who has accidentally let a pot of water boil completely out on the stove is familiar with this process, and familiar with the crust of contaminants typically left behind after the water is gone.

In nature, this basic process is responsible for the hydrologic cycle. The sun causes water to evaporate from surface sources such as lakes, oceans, and streams. The water vapor eventually comes in contact with cooler air, where it re-condenses to form dew or rain. This process can be imitated artificially, and more rapidly than in nature, using alternative sources of heating and cooling.

Distillation Process 1

Early distillation equipment was very simple in design: a pot of undrinkable water (or water unfit for a ceremonial, commercial, or medical purpose) would be heated over an open flame until it boiled, forming steam. The steam would then condense on a cool surface suspended above the pot. The condensed water droplets would then run off into a storage container for future use. Alternatively, sponges could be suspended above the pot to collect the treated water. While such systems were relatively inefficient, it tended to be quite adequate for the limited water treatment needs of the time. 

The efficiency of the distillation process began to see improvements as distillation was adapted to commercially refine many different liquids such as alcohol, perfume, petroleum, and various solvents. Finally, population demands have strained water resources in the 20th century to the point where efficiently treating otherwise undrinkable sources of water for human consumption is increasingly important.

             [ Sixteenth Century Clay Porcelain Distiller ]

Deionization (DI)
Water is passed between a positive electrode and a negative electrode. Ion selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity deionized water results. The water is usually passed through a reverse osmosis unit first to remove nonionic organic contaminants.

Deionization (DI) is a water filtration process whereby total dissolved solids (TDS) are removed from water through ion exchange. In simple terms, by controlling the electric charge of ions in the water, it is possible to remove the TDS. Much like a positively charged magnet will attract a negatively charged magnet (and vice-versa), DI resins attract non-water ions and replace them with water ions, leaving a more pure water form.

The process of deionization uses two resins that are opposite in charges – the cationic (negative) and the anionic (positive). The cationic resin is typically made from styrene containing negatively charged sulfonic acid groups, and will be pre-charged with hydrogen ions. This resin will attract the positively charged ions in the water (Ca++, Mg++, Na+, etc.) and releases an equivalent amount of hydrogen (H+) ions.

Like the cationic, the anionic resin is also made from styrene, but contains positively charged quaternary ammonium groups, and will be pre-charged with hydroxide ions. This resin will attract the negatively charged ions (HCO3-, Cl-, SO4--, etc.) and releases an equivalent amount of hydroxide (OH-). The hydrogen and hydroxide ions then combine to form water. (H+ + OH- = HOH or H2O.)

Deionization Process


The two resins can be ionized at a certain level, usually weak or strong. The cationic can be either a strong or weak acid. Likewise, the anionic resin can be either a strong or weak base. A weaker ionization will exchange only the weak ions, providing for a greater capacity (meaning longer filter cartridge life), while a stronger ionization will provide a higher degree of ion exchange, but at the cost of reduced capacity (shorter filter cartridge life).

As with many other types of filtration or purification processes, a single deionization cycle may not remove all the TDS. Some of the ions will not be attracted by the resins, so running the DI water through a second cycle will provide for additional purification. In other words, the more you run the deionized water through the more pure the yielding water will be. However, it is important to test the filtered water with a TDS meter after each cycle to determine the effectiveness of your DI system. Compared with other filtration and purification methods, DI has a relatively short filter cartridge life and once it begins to fail, the TDS level of the purified will “rise” exponentially.

[All data collected from websites of tds meter for knowledge purpose only] Water Treatment Fundamentals, Seventh Edition, Joseph F. Harrison 
Water Technology Magazine (
The article below is provided by the Water Quality Association.

Filter Performance

  • If you have a filter or RO system in your home, you need to check the water it produces periodically to make sure it's working properly. The performance of RO systems and filters are measured by the amount of TDS Reduced by the filters and membranes. The reduction of TDS indicates the reduction of microorganisms and harmful non-solids such as chlorine and fluorine. 
  • So be sure to check the quality of your water every month to make sure your filters or membranes are working well! 

    Carbon Filters
  • Granular activated carbon filtration is the most common technology used in home filter systems. Unfortunately, these home systems are often poorly maintained. In many cases, filters are not cleaned properly, or filter elements are not changed at appropriate intervals. Over time, effectiveness declines, and in some cases the contaminants in the overloaded filter actually begin to discharge back into the water. 

    RO Percentage Rejection
  • The effectiveness of an RO unit is characterized by the rejection rate or rejection percentage.
  • The rejection rate is the percent of a contaminant that does not move through, or is rejected by the membrane. Rejection rates need to be high enough to reduce the contaminant level in the untreated water to a safe level.
  • To determine the needed rejection rate, it is necessary to consider the initial concentration. For example, when the feed water contains 300 ppm total dissolved solids (TDS), the product water may have 15 to 30 ppm (95% and 90% rejection ratio respectively).
  • An RO system design is based on a certain range of feed water TDS, the percentage of rejection and percentage of recovery desired. For a given system, the higher the percentage of recovery or the lower the percentage of rejection, the poorer the quality of product water becomes. (US FDA)


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