Solutions

Desalination of Brackish Water and Seawater

A shortage of drinking water throughout the world is a major problem today. Only 0.03 % of the world’s water is suitable freshwater for human use. For many countries, a solution to this problem is desalination of brackish or seawater through reverse osmosis.

The technology to accomplish desalination has become very sophisticated but yet affordable. This is of great interest especially for island locations. ProMinent offers its customers this technology, including the necessary pre and post treatment, all from one source. Our drinking water specialists choose the best process combinations as well as the best components for your application.

The main purpose of desalination of brackish and seawater, is to reduce the salt content. Salt amounts larger than 1,500 – 2,000 mg/L makes water taste salty. This water is also unsuitable for daily personal hygiene or watering generators. The typical salt content of brackish water is 2,000 to 10,000 mg/L and for seawater over 35,000 mg/L. WHO guidelines recommend no more than 500 mg/L for drinking water, i.e. there must be a reduction in salt content of between 75 % and 99 %, depending on the raw water.

This can be easily achieved with reverse osmosis equipment using the appropriate membranes. The requirements for this equipment are:

  • Appropriate reduction in salt content
  • Prevention of chemical and biological deposits on the membrane
  • Maximum exploitation i.e. little wastewater
  • Minimized energy consumption, i.e. low operating costs
  • Reliable, continuous operating life and easy-to-use operation
  • Guarantee of potability and hygiene in the drinking water produced

Procedure

  • Selection and design of the correct membrane equipment based on prior water tests, supported by modern design software and years of design experience
  • Selection of a suitable pre-treatment process: Depending on the source of the raw water (e.g. spring or open sea), very different methods are necessary. Otherwise organic, inorganic and biological substances will leave deposits on the membranes and considerably reduce the performance of the equipment. Problem substances include: oil, human substances, Fe/Mn, CaCO3, SiO2, algae, bacteria etc. As a rule they are removed or stabilized through a combination of oxidation/disinfection, flocculation, sedimentation, filtration, absorption and stabilization before reaching the membrane.
  • Finding optimum performance, so that no deposits are left on the membrane and water consumption is kept at a minimum.
  • Selection of a booster pump with maximum efficiency. With seawater, additional selection of an effective energy recovery system is required. Today, very effective systems are employed using pressure transducers, which can lower energy consumption in reverse osmosis equipment during desalination of seawater to under 3 kWh/cubic feet (cubic meters) in the drinking water produced.
  • The selection of long-life components and intelligent control systems make monitoring the process easy for the operator.
  • Selection of suitable post treatment of permeate from the reverse osmosis equipment. After the reverse osmosis process, the pH value is usually below the WHO recommended pH range of 6.5 to 8.5. The pH value must be raised by metering the appropriate alkaline chemicals or through filtration using calcite/dolomite filters. This also protects the customer’s piping system from corrosion. The permeate is usually disinfected so that fully cleansed drinking water comes out of the tap. In order to achieve a residual effect, chlorine is metered into the application.

Fulfilling the requirements mentioned above by appropriate methods and components to achieve smooth operation of the desalination generator is the every day work of ProMinent engineers.