Reverse Osmosis is an effective and proven technology to produce ultra-pure water that is suitable for many industrial applications that require demineralised or deionised water. Water treatment by reverse osmosis technology’s main aim is to eject the maximum percentage of ions from raw water, giving as much treated water flow as possible.

Untreated process water can lead to scaling and fouling, disrupting industrial processes and adding to the cost of operations. Proper industrial process water treatment can be used to reduce the presence of the minerals responsible for scaling and fouling, as well as unwanted: oils, grease, colours, odours, bacteria, turbidity, acidity and organic compounds.

Used in the production and treatment of potable, ultrapure and process water for municipal, agricultural and industrial applications. There are many applications for water purification, including: recycling wastewater, reducing chemical consumption, and reducing boiler blowdown. Significant amounts of water can be recycled and used again in the manufacturing process after undergoing a water purification process.

Reverse Osmosis (RO) Water Treatment Plants

Water has become a significant budget and environmental concern of industrial companies, and it is a commodity that is becoming scarcer and more expensive each year.

Noncompliance with regulatory agencies can result in financial penalties, downtime and adverse publicity. But significant amounts of water can be recycled and used again in the manufacturing process after undergoing a water purification process.

Reverse Osmosis is an effective and proven technology to produce water that is suitable for many industrial applications that require demineralized or deionized water. Water treatment by reverse osmosis technology main aim is to reject the maximum percentage of ions from raw water, giving as much treated water flow as possible.

What is a RO Plant?

In an RO system, pressure (usually from a pump) is used to overcome natural osmotic pressure, forcing feed water with its load of dissolved salts and other impurities through a highly sophisticated semipermeable membrane that removes a high percentage of the impurities. The product of this process is highly purified water. Commercial reverse osmosis is cost effective, and capable of removing particulates down to .0001 microns.

Reverse osmosis is used to produce highly purified water for drinking water systems, industrial boilers, food and beverage processing, cosmetics, pharmaceutical production, seawater desalination, and many other applications.

Reverse Osmosis (RO) Water Treatment Plants

How does Reverse Osmosis (RO) work?

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.

Reverse Osmosis system 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 of dissolved salts behind in the reject stream. An RO system uses cross-filtration, where the solution crosses the filter with two outlets: the filtered water goes one way and the contaminated water goes another way.

Design of RO Systems

RO Plants work on the Crossflow Filtration method, which takes the feed water and uses a percentage of it as a wash or reject stream, removing the solids during the filtration process. Designing an RO system requires a thorough understanding of a plant’s water supply and the technology’s capabilities.
Reverse osmosis membrane manufacturers offer software to assist in the design of RO systems. Using this tool, the design engineer enters the raw feed water data, calculates the scaling potential, evaluates various design configurations and projects the final permeated water quality. A full mineral and chemical water analysis is required in preparing an RO design.

What are the performance and design considerations for reverse osmosis plants?

Through technological advancements, reverse osmosis systems have become capable of economically filtering boiler feed water, cooling tower makeup water and process water at virtually any required flow.

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 several factors must be taken into consideration during the design process. These factors are as follows:

  • Feed water source
  • Feed pressure
  • Flow rate
  • Required product quality
  • Temperature
  • Microbiological contaminants
  • Silt density index
  • Pre-treatment

What contaminants do reverse osmosis plants remove?

Reverse osmosis is capable of removing dissolved salts (ions), particles, protozoa, organics, viruses, bacteria, pyrogens and colloids from the water.

Reverse Osmosis Systems will remove common chemical contaminants, including sodium, chloride, copper, chromium, and lead; may reduce arsenic, fluoride, radium, sulphate, calcium, magnesium, potassium, nitrate, and phosphorous.  A RO membrane rejects contaminants based on their size and charge.

Reverse Osmosis Plants
  • 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. The nature of the fouling may produce a different damage depending on its composition.
  • Scaling means the deposition of particles on a membrane, causing it to plug. As certain dissolved compounds become more concentrated, scaling can occur if these compounds exceed their solubility limits and precipitate on the membrane surface as scale. Scaling calculations are usually only based on the silicate concentration in the feed water.
  • Biofouling reduces actual membrane performance through microbial generation in a biofilm which is formed on the membrane surface.
  • Chemical damage on an RO membrane means a higher permeate flow and poorer quality permeate water. Dosing of oxidant agents such as chlorine or hypochlorite can reduce the performance and ultimately result in the failure of the reverse osmosis membranes. Use of aggressive cleaners can also cause chemical damage.
  • Mechanical damage: If the system is pressurized too quickly it can result in mechanical damage to the RO membrane elements. One of the most clear signs of damages on an RO membrane is the lack of salt rejection capabilities and very often an increase in permeate flow rate.

What type of RO Pre-treatment is there?

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

  • Multi Media Filtration
    A Multi-Media Filter is used to help prevent fouling of an RO system. This type of sediment filtration is ideal for a pre-treatment process to any reverse osmosis system helping to ensure long life of the RO membrane elements. A well operated Multimedia Filter can remove particulates down to 20 microns. A Multimedia filter that uses a coagulant addition can remove particulates down to 10 microns.
  • Micro Filtration
    The filters used in microfiltration have a pore size of approximately 0.1 micron. Bacteria and suspended solids are the only elements that can be removed through microfiltration.
  • Antiscalants and scale inhibitors
    There are a large number of chemicals which can be used as antiscalants and dispersants to improve the operation of RO. Antiscalants are a family of chemicals designed to inhibit the formation and precipitation of crystallised mineral salts that form scale.
  • Softening by ion exchange
    A water softener is a filtration system that removes hardness-causing calcium and magnesium minerals from water through a process called ion exchange. Standard water softeners are cation exchange devices. Cation exchange involves the replacement of the hardness ions with non-hardness ions.
  • Granular Activated Carbon (GAC) Filtration
    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.

What maintenance is required on a RO plant?

When using a membrane-based water treatment system such as Reverse osmosis (RO), fouling of membranes can create quite an issue. Membrane fouling will at the very least decrease production and use more energy and can even lead to much more costly equipment replacements. Preventative checks and maintenance can prevent fouling.
  • A decline in your water quality is generally a sign of membrane fouling.
  • Strange and strong smells can be a sign of biological growth on your membranes.
  • If you see an actual build-up of solids on your membranes.
RO Membrane Cleaning
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. Membrane cleaning is performed on a regular basis before the impacts of fouling become significant.

Preventative maintenance approach for RO systems

A preventative approach is always best to help avoid a shut down or the need to replace membranes earlier than necessary.
  1. A membrane cleaning schedule is a great start. RO membrane element needs periodic cleaning. The accumulated sediments on the surface of RO membrane affect membrane permeable capacity.
  2. Routine inspection including valves inspection and maintenance on pre-filtration equipment.
  3. Routine inspection of the conductivity gauge, various pressure gauges and PLC system, maintaining normal operation of RO system.
  4. Periodic inspection of the raw water pump and high pressure pump, and replace lubricating oil in time according to the maintenance manual.

How to calculate the recovery rate of RO plants?

The RO recovery is specified by the feed water salinity. Increasing the recovery raises the brine concentration and the osmotic pressure, thus decreasing the permeate flux and increasing the total dissolved solid (TDS) in the product. We can increase the water recovery by increasing the number of banks in the RO system.

How to calculate RO recovery:

Permeate rate divided by feed rate, expressed as a percentage.

For example, 33% recovery means that of a given feed rate, 33% is produced as purified water (permeate).

The re-use of waste water from RO plants

To meet growing water requirements, it is important to think of how to reuse the waste water from reverse osmosis water treatment plants wherever possible. There are several factors affecting exactly how much water your reverse osmosis system wastes.
  1. The design can include more than one bank to recover clean water.
  2. Pressure design
  3. Type of membrane selection for example using low fouling membrane technology to enable greater water recovery.

There are plenty of places that wastewater or grey water can be reused, although sometimes tertiary treatment is needed to ensure that the wastewater is clean enough for those uses. These reuse applications could be for industrial or municipal wastewater. Reuse applications could include:

  • Agricultural irrigation
  • Structural/non-structural firefighting
  • Toilet water
  • Cleaning
  • Decorative fountains
  • Industrial process water
  • Industrial or commercial cooling or air conditioning
  • Boiler feed water

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