Reverse osmosis (RO)

Reverse osmosis (RO) is a membrane treatment process mainly used to separate dissolved solutes from water. It is a process exploited by many industries, such as the textile and apparel industry has always covered an important role in the world economy and sustainability. This process allows the deionization of water.

In fact, the dyeing and finishing process consumes a large amount of fresh water and discharges significant volumes of wastewater, and is now under increasing pressure because of its enormous environmental impact.
Reverse osmosis is most commonly known for its use in purifying wastewater to make it reusable, particularly in removing salt and other effluent materials from water molecules.

WHAT DOES WATER DEIONIZATION MEAN?
Water deionization is a chemical and physical process used to remove inorganic ions from water, thus making it virtually free of mineral salts and other ionic solutes. This process is used to produce purified water or demineralized water.

What is reverse osmosis and how does it work?

To better clarify the concept of osmosis, let’s imagine that we have a vessel divided into two compartments of equal volume (A and B) by a semipermeable membrane (i.e., permeable only to the solvent, in this case water, and not to the solute, in this case glucose).

In compartment A there is an aqueous solution in which one tablespoon of glucose has been dissolved.
In compartment B we have an aqueous solution of equal volume in which three tablespoons of glucose has been dissolved. This difference creates a concentration gradient for the glucose on either side of the membrane.

Since it cannot cross it, equilibrium is reached by the passage of water from compartment A (where glucose is more dilute) to compartment B (where it is more abundant).
Water can be said to flow by osmosis from the solution in which it is most concentrated (A) to that in which it is less concentrated (B).
As a result of this flow, the level of water in B increases and decreases in A, creating a certain difference in level between the two. This phenomenon ends when the two solutions reach the same concentration.

If a higher pressure than osmotic pressure is applied in B, this is called reverse osmosis. Due to this pressure (in the range of 40-70 bar for seawater), the solvent moves from the solution in which it is less concentrated (B) to the one in which it is more concentrated (A).

The reverse osmosis process

Reverse osmosis is a process of separating foreign matter from water using semipermeable membranes. Membranes allow water to pass through, but retain dissolved mineral elements, colloids and bacteria.

An osmotic membrane consists of a central core around which a semi-permeable fabric made of synthetic material is spirally wound. Membranes are generally classified according to size according to standards generally expressed in inches (e.g., a 4040 membrane corresponds to a module 40 inches long and 4.0 wide), but also according to production capacity, generally indicated in GPD (gallons per day).

The water to be treated is pushed into the membrane by a pump, which exerts a higher pressure than the osmotic pressure, so that two outflows are obtained: the part of the incoming water that passes through the membrane constitutes the permeate (low in salts) that goes to use, while the remaining part escapes with a high salt concentration, due to the accumulation of all the salts that have not passed through the membrane; this is the concentrate (rich in salts) that must be discarded.

Advantages and disadvantages of reverse osmosis

The advantage over other water treatment methods is that reverse osmosis, in addition to removing dissolved salts, also retains bacteria, germs, particles, and dissolved organics.

Reverse osmosis succeeds in completely removing pollutant particles as small as one ten-thousandth of a micron in size, as well as pesticides, pyrogens, viruses and bacteria, thus ensuring the absolute bacteriological purity of the water. It has been estimated that reverse osmosis can retain 90 to 99.9 percent of dissolved substances in water. This is why it has become the absolute most widely used filtration technology in industry, from the biomedical sector to the food sector, industrial effluent treatment and the generation of drinking water from seawater. The cons of this system are very few, but let’s look at them in detail.

Benefits of Reverse Osmosis

Reverse osmosis implies substantial benefits, including:

• Improved energy efficiency;
• Consistency of permeated water quality;
• Enhancement of overall system performance;
• Round-the-clock availability of purified water;
• Minimized maintenance;
• Bacteria, viruses and pyrogenic materials are repelled by the intact membrane;
• On average, the membrane lasts one to two years before needing replacement;
• Periodic complete sterilization of the RO system with formalin is practical.

Disadvantages of Reverse Osmosis

The disadvantages of Reverse Osmosis systems include the following:

• Cellulose acetate membranes have limited pH tolerance. They degrade at temperatures above 35 degrees;
• Water obtained from Reverse Osmosis can significantly reduce good minerals that for overall heart and muscle health, which is why those who use Reverse Osmosis as their primary source of drinking water may suffer from side effects such as fatigue, muscle cramps, general weakness, and, in severe cases, cardiovascular disorders;
• Like any equipment, the reverse osmosis machine needs regular maintenance every 6 to 12 months, and a routine cleaning procedure of the system should be carried out concurrently;
• To install a reverse osmosis purifier takes up a lot of space.

Curiosities about Reverse Osmosis: Frequently Asked Questions – FAQs

Is it healthy to drink reverse osmosis water?

Yes, reverse osmosis water is not bad for human beings.

In fact, water treated with reverse osmosis is recommended for food use, so much so that even in places where hygiene is paramount, such as hospitals and professional laboratories, it is commonly used.
Reverse osmosis is the most effective and safe technology for refining and purifying drinking water primarily for two reasons: it is a natural process within which no chemicals are used. In the course of this process, unwanted pollutants and those of organic nature are removed.

What industries can take advantage of reverse osmosis for wastewater recovery?

Reverse osmosis plants find application in a wide range of fields. For example:

• drinking water potabilization for civil centers
• boiler feeding and condensate treatment
• cooling and process water in refineries
• high-purity water for specialized industries, such as textiles
• irrigation in brackish coastal areas
• demineralized water for medical uses
• treatment of industrial effluents
• recycling of process water
• seawater potabilization
• medicine, cooling circuits, horticulture.

For these sectors, the most effective approach is usually a multistage treatment system that incorporates several purification technologies, chosen according to purification needs.