Osmosis: Safe Drinking Water Research Paper

Water is one of the many things that all human being need in order to survive. Considering the vast amount of water that is on the planet, it is reasonable to believe that there is more than enough water for everyone to have their fair share and more. Yet this sadly is not the case, as the majority of the water on Earth is unsuitable to drink due to the fact that it is tainted with a numerous amount of contaminants, such as bacteria and salt. The amount of safe drinking water is decreasing more and more each year and will soon become one of the more scarce resources on the planet.

Over the years, many different people all over the world have tried to create and refine a water purification process that is both effective and efficient. The majority of the processes developed are centered around filtration, separation, oxidation and the use of chemicals. Although many of these methods are viable options for purifying water, they do have their own drawbacks. Whether it be too costly or ineffective in the removal of contaminants, there is always a disadvantage to one method or another.

Yet the best of the water purification methods includes the use of reverse osmosis. Reverse osmosis is the process of forcing water or any other solvent against the natural flow, from an area of high solute concentration to an area of low solute concentration, separated by a semipermeable membrane. With a membrane only permeable to water molecules, only pure water will pass through, leaving behind all of the contaminants that were initially sustained within the water, with near perfect efficiency.

The discovery of reverse osmosis was not something spontaneous or out of the spur-of-the-moment. Its inception lies deeply in the saying “Necessity is the mother of invention”, which holds true for so many inventions and innovations of the past, present and future. Osmosis is a naturally occurring process in which water diffuses a semipermeable membrane from an area of low concentration to an area of high concentration. It was first discovered in 1748 by the French physicist, Jean-Antoine Nollet and later thoroughly studied in 1877 by German plant physiologist, Wilhelm Pfeffer.

Ten years prior, Moritz Traube prepared the first recorded synthetic membrane that was made from a precipitated film of copper ferrocyanide. It was not until almost two hundred years later that the process of osmosis was revisited and extensively reviewed by researchers from the University of California at Los Angeles. In 1959, Sidney Loeb and Srinivasa Sourirajan developed a membrane made from cellulose acetate, which removed at least ninety percent of salts from the water, but not at the expense of speed and efficiency.

Their design was later refined by John Cadotte in a way that improved both flow rates and salt rejection. The membrane Cadotte developed, known as thin film composite membranes, also had a high refusal for salts as well as high refusal for organic contaminants. With the constant development and improvement of reverse osmosis membranes, they were able to take a step towards the commercial market first in 1965 with a water filtration pilot program utilizing the method in Coalinga, California under the leadership of Joseph W. McCutchan and pioneer Sidney Loeb.

In 1977, Cape Coral, Florida started to use reverse osmosis on a citywide scale, being the first of many that would adopt the process. As time went on, reverse osmosis became one of the most prominent methods by which water is purified all over the world. Despite the enormity of its impact and its widespread usage, the process of reverse osmosis is quite simple. The naturally occurring phenomenon of osmosis is due to the difference in water potential between two regions of different concentration separated by a semipermeable membrane.

The water from the area of lower concentration flows through the membrane to the area of higher concentration. This action is driven by the osmotic potential of the two regions in respect to one another. The hypertonic solution has a higher potential because of the higher amount of dissolved solutes in the solution, therefore the water molecules flow towards the hypertonic solution to reduce the potential. The process of reverse osmosis uses the semipermeable membrane to remove dissolved solutes from the water to result in pure, distilled water.

This is done by applying a force greater than that of the osmotic pressure to the solution of higher concentration, thus forcing the water against its natural flow, leaving the dissolved solutes and other contaminants on one side of the membrane and pure water on the other. Loeb and Sourirajan synthesized the first reverse osmosis membranes through the acetylation of cellulose. The reaction would be cellulose with acetic anhydride in the presence of a catalyst, such as sulfuric acid. This would result in the formation of acetylated cellulose, the main component of the membrane, and acetic acid, a byproduct.

The cellulose based membranes were easy to make and are more resistant to chlorine than other reverse osmosis membranes. But they are sensitive to changes in pH and temperature, and they also tend to break down in the presence of water over time. Then came along the thin film composite membranes, first synthesized by John Cadotte. This membrane was composed of a polyamide coating made through the reaction between m-phenylenediamine and trimesoyl chloride. This new membrane allowed for higher salt rejection and water flow, while allowing for a reduced pressure to be applied to counteract the osmotic pressure.

But when the polyamide comes into contact with chlorine, it disrupts the hydrogen bonding breaking down the polymer and greatly reducing salt rejection. Due to the highly selective permeability of the reverse osmosis membrane, it has a long list of contaminants that it can remove from water. This list includes but is not limited to bacteria, cysts, salts, synthetic dyes, carbon, etc. The permeability of reverse osmosis filters puts it head and shoulders above all other water filtration techniques known to date.

But some of the major issues that arises with the use of reverse osmosis filtration are membrane fouling and deterioration. Fouling is when particles build up either on or within the membrane and cause a decrease in the flow of the water. There are multiple types of membrane fouling, such as biofouling, scaling, organic and colloidal fouling. Biofouling occurs when the feed water is contaminated with microorganisms which create a biofilm over the membrane, increasing the resistance of water flow through the membrane.

Scaling is when salts within the water precipitate and deposit onto the membrane decreasing flow. Organic fouling is when carbon-containing substances, such as hydrocarbons, cover the membrane surface and/or plug up the pores within the membrane. Colloidal fouling is similar to scaling, but instead of salts amassing on the membrane, it is other substances such as silica and clay. There are many ways in which fouling is attempted to be prevented, such as pretreatments with anti-scaling agents, disinfectants and basic sediment filtration.

Although these pretreatments reduce fouling, they do not ultimate eliminate the problem. Thus requiring for solutions to be make to clean the reverse osmosis filter Although all reverse osmosis systems are not entirely the same, they all do share similar components in their composition. They have a cold water supply line where the to be treated water enters the system and prepares to be treated. Next they reach a pre-filter or a series of pre-filters, which remove substances that would otherwise damage the reverse osmosis filter if allowed to come in contact with it.

This would include sediment, carbon and biofilters. After the water is treated, it would then go through the reverse osmosis filter at a high enough pressure to overcome the osmotic pressure of the solution. Then the water is put through a post-filter and/or a mineralizer to return desireable minerals into the water that were removed during the reverse osmosis process along with the undesirable contaminants. Finally, the purified water is either sent straight to a faucet or put into a storage tank for later usage.

The drain water (the wastewater that contains all of the removed contaminants previously sustained within the water) is then drained out the system into sewage or a septic tank. Reverse osmosis water filtration has many advantages to its use compared to other methods of water purification, such as multieffect distillation (MED) and multi-stage flash (MSF). The process of reverse osmosis does not require an evaporation step, so therefore there is no energy wasted in heating the water. Since this makes them energy efficient, it also makes them cost efficient as well.

Thus a power plant powering a reverse osmosis purification plant would produce less emissions than a standard power plant power another water treatment method. Heated brine discharge comes from the desalination of water through processes such as MED and MSF, and when this discharge is returned to the environment, it negatively affects the oxygen content in water, thus throwing off the balance of the water ecosystem. Reverse osmosis does not heat the water, so the brine discharge is not heated when it is returned to the environment.

These facts make reverse osmosis the more environmentally friendly option out of all of the commonly used methods to purify water today. Reverse osmosis systems are usually small in scale, allowing the ability to fit a fully functional system in a small area or a high occupancy system in a larger area. Due to its modular design, it allows a reverse osmosis system to be easily maintained, expanded and operated allowing small-scaled household models with the power and efficiency close to the that of factory models.

Compared to other filtration techniques, reverse osmosis membranes have the strongest durability and require less frequent changings in the filter. Although there have been numerous breakthroughs and innovations in the field of water filtration through reverse osmosis, there can always be more thorough testing and analysis done to further back up all of the points brought up so far proving the use and favorability of reverse osmosis filtration systems as opposed to other systems. Many other, less efficient methods and processes are still being used over reverse osmosis, even though it is starkly superior to them.

Therefore it is imperative that intensive testing is to be conducted on the effectiveness and efficiency of reverse osmosis filtration to signify it as the obvious and most trustworthy option for water filtration. There are numerous aspects and characteristics of water that have to be accounted for in such testing to deem it suitable for our many needs such as drinking, irrigation and cleaning. These tests would include but are not limited to total hardness, total chlorine content, pH, nitrates and nitrites, total alkalinity, bacteria content, lead content, total amount of pesticides, etc.

I plan on using these tests to compare the content and overall drinking quality of untreated tap water and tap water that is treated with a reverse osmosis filtration system. Due to the tap water having been already treated for certain substances such as sand, dirt and carbon, there is no need for pretreatment or post-treatment filters to be added to the reverse osmosis system being used. There is also no need for a water storage tank as there will not be a large volume of water being filtered throughout the process of this analysis.