A Comprehensive Guide to Reverse Osmosis Process

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What is Reverse Osmosis?

The process of reverse osmosis involves removing dissolved salts, minerals, and pollutants from tap water through a filtration process using a semi-permeable membrane under pressure. This results in cleaner, odour-free, and better-tasting water. Reverse osmosis is widely used in both industrial and residential settings.


Comparing Osmosis and Reverse Osmosis

Unlike osmosis, where water molecules naturally flow from a low-concentration area to a high-concentration area to achieve equilibrium, reverse osmosis is an energy-intensive process that involves applying pressure to force water through a semi-permeable membrane.

This membrane filtering selectively removes dissolved impurities such as salt, minerals, organic compounds, and contaminants while allowing water to pass through. 

Stages of Reverse Osmosis (RO) Filtration

The typical filtration system of this kind consists of four stages:

1. Mechanical Filtration

 In the initial stage of RO filtration, large particles such as sand, dirt, and rust are effectively removed. This crucial step prevents clogging and potential damage to the subsequent filters.

2. Activated Carbon Filtration

During the second stage of filtration, activated carbon is utilised to eliminate chlorine, organic compounds, and other pollutants from the water. These impurities are commonly responsible for negatively impacting the flavour and odour of the water.

3. Reverse Osmosis Membrane Filtration

During the third stage of the filtration process, the water undergoes passage through a semi-permeable RO membrane. This specialised membrane effectively separates and filters out heavy molecules present in the water, including sodium, dissolved minerals, high levels of lead, fluoride, and various other impurities.

Only water molecules are permitted to pass through the membrane, resulting in purified water.

4. Polishing Stage

The final phase of the RO filtration process, known as the Polishing Stage, enhances the taste and appearance of water by providing additional purification before it reaches your faucets.

Common Pollutants Removed by Reverse Osmosis

Reverse osmosis can remove up to 99.9% of the impurities, contaminants, and sediments having particle sizes up to 0.001 microns. It makes water pure and healthy for drinking. Some of the common pollutants removed by the RO system include:

  • Dissolved minerals like calcium, magnesium, sodium
  • Fluoride
  • Chlorine and chloramine
  • Heavy metals such as lead, mercury, arsenic
  • Pesticides and herbicides
  • Nitrates and sulphates
  • Radioactive elements such as radium and uranium
  • Detergents
  • Sediments
  • Volatile organic compounds

Why is Reverse Osmosis beneficial?

The RO Water Filtration System is beneficial for everyday drinking for various reasons:

  • Enhanced taste and smell
  • Provides safe and pure water
  • Easy installation
  • Health benefits
  • Cost saving
  • Reduced environmental impact
  • Simple maintenance

In today's context, there is a growing awareness among people about the potential dangers and problems associated with using contaminated water. As a result, there has been a shift towards adopting filtration systems that offer convenient and effortless access to pure and clean water.

Calculations for Reverse Osmosis Performance and Design

To determine the accurate performance of our RO system, you must consider to ensure accurate performance evaluation and design considerations for our RO system, it is essential to conduct the following calculations based on operational parameters: feed water flow rate, feed water pressure, feed water temperature, feed conductivity, recovery rate, permeate pressure, permeate flow, concentrate pressure, and permeate conductivity.

To make design considerations for our RO system, it is necessary to perform the following calculations:

Calculation of Salt Rejection Percentage

The effectiveness of RO membranes in removing contaminants can be measured by the percentage of salt rejection. A well-designed RO system has the capability to remove 95% to 99% of contaminants present in the feed water.

The salt rejection percentage can be calculated using the following equation:

Salt rejection % = (Conductivity of feed water - Conductivity of permeate) * 100.

Calculation of Salt Passage Percentage

The salt passage percentage indicates the amount of salts that are able to pass through the RO membrane. A lower percentage signifies better performance of the system, while a higher percentage indicates the need for cleaning or maintenance.

The salt passage percentage can be calculated using the following equation:

Salt passage % = (1 - salt rejection %)

Calculation of Recovery Percentage

The recovery rate of an RO system refers to the amount of water that is successfully recovered rather than being discharged as concentrate waste. A higher recovery rate indicates efficient water conservation. However, an excessively high recovery rate can lead to scaling and fouling issues.

The recovery percentage can be calculated using the following equation:

Recovery % = (Permeate flow rate in gallons per minute (gpm) / Feed flow rate in gpm) * 100. 

Calculation of Concentration Factor

The concentration factor is a parameter that is closely tied to the recovery percentage. As the recovery rate increases, the concentration of salts in the reject stream also increases, which raises the risk of scaling within the RO membrane.

The concentration factor can be calculated using the following equation:

Concentration factor = 1 / (1 - recovery %) 

Calculation of Flux Rate

Flux refers to the rate at which water flows through each square foot of the RO membrane. The flux value can range from 5 to 30 gallons per square foot per day (gfd), depending on the characteristics of the feed water source.

The calculation for flux is as follows:

Flux (gfd) = (Permeate flow rate in gallons per minute) * (1440 minutes per day) / (Number of RO elements in the system) * (Square footage of each RO element)

Mass balance

It can determine whether the quality and flow instruments provide accurate measurements or if they need calibration. 

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