reverse osmosis

Water Purification – Reverse Osmosis

by George Danellis

There are several methods available today to purify water. Filtering uses various types of membranes to strain unwanted elements. Distillation is a method to separate components with different boiling temperatures, and is commonly associated with the oil and alcohol industries. Reverse osmosis is a purification method used for the removal of salt and other substances from sea and brackish water. It is used for both household and commercial applications and has experienced significant levels of adoption since it was first developed in the 1970’s. RO, as reverse osmosis is commonly known, has its strengths and weaknesses as a water purification method. Here is a summary of these pros and cons.

Reverse Osmosis works by using pressure to force water in the opposite direction of osmosis; that is, from the stronger concentration to the weaker. Often UV lamps or ozone are used in combination to kill microbes that escape filtering by the RO membrane.

  • It is commonly used on a small scale for people living in rural or coastal areas without clean water supply, and as a solution for travelers on longer boating and camping trips.
  • The marine aquarium industry uses RO to remove many substances in common tap water that are detrimental to sensitive marine environments.
  • Car washes often employ RO water as its low mineral content reduces spotting and thus reduces the use of blowers or other drying systems.
  • The maple syrup industry has found RO the most economical solution to remove water from sap, and is likewise used for other food-liquid concentration purposes.
  • Larger applications of RO include wastewater purification, the production of bottled and supplied drinking water, and mobile military water purification. The minerals in wastewater at power plants are removed by the Reverse Osmosis process, improving efficiency.  With roughly 21,000 seawater desalination plants worldwide RO is now a common solution to desalinate seawater or brackish water to produce drinking water in water constrained situations.
  • The US military has invested heavily in RO systems for the portable purification of water, including on the battlefield and in training. In addition to purifying seawater, their Mobile Water Purification Units can also, impressively, purify water that has chemical, biological and radiologic contaminants. One of these units can produce between 12,000 and 60,000 gallons of water in a 24-hour period. For this to work there must be a 4-hour maintenance period to check and clean various parts of the system.

 

Although there are many useful applications for RO; as a water purification method Reverse Osmosis also has its weaknesses.

  • The size of the holes in the membrane block salt, but many contaminants are smaller and thus escape filtration by RO. Secondary filtration or light sterilization must also be used in these instances.
  • Household units use a great deal of water due to the lower water pressures available.
  • There is the challenge of waste: most of the salt from RO desalination is pumped back into the sea as brine, which is twice as salty as normal seawater and full of toxic chemicals.
  • RO removes many healthful, naturally occurring minerals from the water, leaving it highly acidic and corrosive and must be neutralized with “liming” agents to make to it safe for pipes and for people. Some countries restrict the flow of RO water into the system as it corrodes copper pipes. Similarly it can corrode steel storage tanks, and lead to contamination of water. There is a growing scientific consensus about the acidity of RO drinking water, that when it is consumed regularly causes loss of electrolytes and trace minerals as well as increasing acidity since Carbon Dioxide in the air reacts with RO water to become acidic.
  • In most applications, RO is not particularly water efficient with much wastage often occurring.
  • Reverse Osmosis operations require significant maintenance, which can drive up the cost.
  • Mineral scaling of the membranes is common, especially with Calcium and Magnesium, rendering the membrane ineffective.
  • Energy requirements are large – pumps must work at more than 800 psi to push the saltwater through the membrane. This can be compared with distillation which requires a heat source, but is much less energy intensive than RO and it could be argued without many of its other negatives.

 

Reverse Osmosis is a water purification solution that can provide clean water and contaminant removal in a variety of situations. But its efficiency of production and health impacts are ammunition to its detractors. It has also been noted that one of the challenges with a changing climate (access to fresh water) is being addressed with an approach that most often creates enormous GHG emissions (Reverse Osmosis). This certainly bears some thought when looking at methods for purifying water in our modern world.