Membrane scaling
precipitate and form a dense layer on the membrane surface in reverse osmosis (RO) applications.
[1] Figures 1 and 2 show scanning electron microscopy (SEM) images of the RO membrane surface without and with scaling, respectively.
Membrane scaling, like other types of membrane fouling, increases energy costs due to higher operating pressure, and reduces permeate water production.
Membrane scaling can occur when sparingly soluble salts in RO concentrate become supersaturated, meaning their concentrations exceed their equilibrium (solubility) levels.
The ratio of permeate water to feedwater is known as recovery which is directly related to membrane scaling.
Recovery needs to be as high as possible in RO installations to minimize specific energy consumption.
The most common salts that cause scaling in RO processes are: There are a number of indices available to determine the scaling tendency of sparingly soluble salts in a water solution.
These indices provide information if a given scale-forming specie is undersaturated, saturated, or supersaturated.
[6] Another method of preventing scaling is to operate RO at low recovery (ratio of permeate water to the feedwater).
Low recovery reduces the adverse effect of concentration polarization because there is less solute concentration on the membrane surface, reducing the potential for scale formation.
This approach, however, is not very appealing or economical because it results in high specific energy consumption.
Antiscalants addition to the RO feed is one of the most widely applied strategies in term of scale control.
[7] Antiscalants can be used to increase the recovery of RO process and are primarily contains organic compounds such as sulphonate, phosphonate, or carboxylic acid functional groups.
[8][9] The addition of antiscalants hinder the crystallization process, i.e., nucleation and/or growth phase of scaling compounds.
Antiscalant prevent scale formation by three mechanisms, namely threshold inhibition, crystal modification and dispersion.
