Desalination Post-treatment: Boron Removal Process
Why should Boron be removed from drinking water?
The human body contains approximately 0.7 ppm of boron, an element that is not considered as a dietary requirement. Still, we absorb this element from food , because it is a dietary requirement for plants. Daily intake is approximately 2 mg. The amount of boron present in fruits and vegetables is below the toxicity boundary.
A possible correlation exists between the amount of boron in soils and drinking water, and the occurrence of arthritis among people.
Why should Boron be removed from irrigation water?
Boron can be toxic at very low concentration levels. Boron concentration lower than 1mg/L is essential for plant development, but higher levels can cause problems in sensitive plants. Most plants exhibit toxicity problems when the concentration of boron exceeds 2mg/L (see table below).
Source: Extracted from the Australian Water Quality Guidelines for Fresh & Marine Waters (ANZECC)
Boron occurrence in seawater varies from 4 to 5.5 mg/L, proportionally to seawater salinity. It mainly comes from waste water treatment plants discharge, begin used in soap and detergents, as well as agricultural fertilizers.
Boron is present in water as Boric Acid H3BO3 and borate H3BO2-. The dominant form of boron species depends on the pH of the water. The pKa of H3BO3/H3BO2- is 9.2, therefore the equilibrium H3BO3 <--> H3BO2-+ H+ is typically towards the left at standard seawater pH 8.
Reverse Osmosis membranes are very efficient at removing charged species like the borate ion rather than neutral molecules like boric acid.
Typical Boron removal rates at pH 8 are between 73 and 90% for standard High Rejection Seawater Reverse Osmosis membranes, depending on the water temperature. Special High Boron Removal membrane can achieve a 95% removal.
Most of the time, high salinity seawater have high Boron content and are located in very hot climate area like the Persian Golf, the Red, the Eastern Mediterranean sea or the Caribbean Sea. At 30oC, Boron removal drops at about 78%, leaving 1.15 mg/L in the Pass-1 permeate stream. Therefore, a specific Boron Removal Process is required to achieve the 0.5 mg/L required by the WHO.
How can Boron removed be from desalinated water?
Depending on the water salinity, boron concentration and temperature, two main processes are used to produce drinking water below 0.5 mg/L of Boron:
2nd-Pass RO with caustic soda addition to raise pH around 9.5. Some of the pass-1 permeate can be by-passed in order to keep some minerals in the water. The 2nd pass RO can be made of Seawater Low Energy membranes if temperature and salinity are high or Brackish Water High rejection membrane in case of milder conditions.
Selective Boron Ion Exchange Resin with or without by-pass, depending on the residual boron concentration needed. The selective resin must be on-site regenerated with caustic soda and hydrochloric acid. A double column system is often required to ensure a continuous production.
LENNTECH engineers design and size the most cost-efficient Boron Removal Process that matches your water requirement.