Question

Explain the phenomenon of concentration polarization in ED systems (i.e. depletion of ions….etc). What should be done to minimize fouling? and define current density, and CD/N ratio. How is CD/N ratio related to concentration polarization in ED systems?

Answer 1

Concentration polarization controls the performance of practical ED system.

Lets consider a salt solution in which the ions are transported across the membrane. The resistance to ion transport in ED system can be represented by a thin film adjacent to the membrane surface. The concentration of some of the ions in the solution immediately adjacent to the membrane surface becomes significantly depleted compared to the bulk solution concentration as ions selectively permeate the membrane. As the voltage across the stack is increased to raise the flux of ions through the membrane, the solution next to the membrane surface becomes increasingly depleted of the permeating ions.

Depletion of ions at the membrane surface means that an increasing fraction of the voltage drop is dissipated in transporting ions across the thin film rather than through the membrane. Therefore the energy consumption per unit of ions transported increases significantly.

A point can be reached at which the ion concentration at the membrane surface is zero. This represents the maximum transport rate of ions through the thin film. The current through the membrane at this point is called the limiting current density, i.e. current per unit area of membrane (mA/cm2).

Once the limiting current density is reached, any further increase in voltage difference across the membrane will not increase ion transport or current through the membrane.

To minimize fouling various physical, chemical and physicochemical methods are available.

The selection of the chemical to be applied for membrane cleaning depends on the membrane type, intensity of the pollution and pollutants type. Chemicals that usually recommended by membrane manufacturer are; weak and strong acids, alkali (NaOH), detergents, complex agents (EDTA), disinfectants (H2 O2, NaCl) and enzymes. Also operation conditions can be considered to reduce fouling.

Ultrasound is an effective technique for cleaning foulants.

Colloids that commonly found in natural water are clay, silica, iron, and aluminum hydroxide, organic deposits, and carbon dust. Pre-filtration, membrane cleaning with chemicals, cross- flow velocity should be provided for control of colloidal pollution.

Many foulants move to ion exchange membranes under an electric field by their surface charge and electrical conductivity. After that the foulants accumulate on membranes and the electrical resistance increases. These deposits can be removed with an alternating electrical current.