Question

What are the factors that affect the different fouling behaviours of thin film composite (TFC) and cellulose triacetate (CTA) forward osmosis (FO) membranes when treating wastewater.

Answer 1

Forward osmosis process shows great potential in the sustainable water purification and waste water treatment. Since it exhibit low energy consumption and membrane fouling propensity.

Membrane fouling is an another issue that brings in additional hydraulic resistance and reduce the effective osmotic pressure and water permeability in FO process.

Membrane fouling in FO process is impacted by various factors, including operatinf conditions, water condition and membrane structure.

Factors affecting the fouling behaviour:

1) Organic concentration:

The special membrane fouling increases with organic concentration when water flux is decreases and hence the membrane fouling resistance increases.

2) Influence of positive ions and its fouling rate:

If the wastewater contains Na⁺ and Ca²⁺ inpurities,then with increasing concentration of Ca²⁺ , the water flux decreases and Ca²⁺ can bind with the natural organic matter, leading to massive fouling.

3) Fouling behaviour under varying wastewater temperature:

As the temperature of feed is increased, the net bulk osmotic pressure is increased, which causes increase in net driving force. The absolute viscosity of feed is decreases with increasing temperature, which enhances the diffusivity of water flux through the membrane active layer and porous layer, therby inducing a positive influence on pure water flux.

4) Membrane orientation in flux decline:
Larger the pore size, greater membrane surface porosity and rougher structure enabled serious pore blocking and formation of cake layer.

5) Effect of pH in wastewater on membrane fouling:

The membrane water flux increased with increasing pH. This behaviour is attributed to zeta potential profiles of the active layer. As water flux is increase the viscosity will decrease causing the protonation and deprotonation process inside the membrane. The protonation/deprotonation is caused by the membrane zeta potential and hence it influences the membrane fouling in different pH.