Water Pollution Control By Membrane Based Technologies | Environmental Engineering - Civil Engineering (CE) PDF Download

MEMBRANE

Membrane can be described as a thin layer of material that is capable of separating materials as a function of their physical and chemical properties when a driving force is applied across the membranes. Physically membrane could be solid or liquid.  In membrane separation processes, the influent to the membrane module is known as the feed stream (also known as the feed water), the liquid that passes through the semipermeable membrane is known as permeate (also known as the product stream or permeating stream) and the liquid containing the retained constituents is known as the concentrate also known as retained phase. 

MEMBRANE PROCESS CLASSIFICATION 

Membrane processes can be classified in a number of different ways :  

• The type of material from which the membrane is made 
• The nature of the driving force 
• The separation mechanism 
• The nominal size of the separation achieved 

Table 3.11.1. General characteristics of membrane processes 

Table 3.11.2. Advantages & disadvantages of membrane technologies 

MEMBRANE MATERIALS & CONFIGURATIONS 

• Membranes can be made from a number of different organic and inorganic materials. The membranes used for wastewater treatment are typically organic. The principle types of membranes used include polypropylene, cellulose acetate, aromatic polyamides, and thinfilm composite (TFC).  
• Membranes used for the treatment of water and wastewater typically consist of a thin skin having a thickness of about 0.20 to 0.25 µm supported by a more porous structure of about 100 µm in thickness.
• Term ‘module’ is used to describe a complete unit comprised of the membranes, the pressure support structure for the membranes, the feed inlet and outlet permeate and retentate ports, and an overall support structure.  
• The principle types of membrane modules used for wastewater treatment are
   1) tubular, 2) spiral wound, 3) hollw fibre,4) flat. 

Table 3.11.3. Comparison of different membrane configurations  

MEMBRANE FOULING 

Membranes can be seen as sieves retaining part of the feed. As a consequence, deposits of the retained material will accumulate at the feed side of the membrane. In time this might hamper the selectivity and productivity of the separation process. This process is called fouling. koros et al gave the definition of fouling as “The process resulting in loss of performance of a membrane due to deposition of suspended or dissolved substances on its external surfaces, at its pore openings, or within its pores”. Membrane fouling is an important consideration in the design and operation of membrane systems as it affects pretreatment needs, cleaning requirements, operating conditions, cost, and performance. 

Three approaches are used to control membrane fouling:  

1) Pretreatment of the feed water: pretreatment is used to reduce the TSS and bacterial content of the feed water 

2) Membrane backflushing: to eliminate the accumulated material from the membrane surface with water and/or air. 

3) Chemical cleaning of the membranes: Chemical treatment is used to remove constituents that are not removed during conventional backwashing. Chemical precipitates can be removed by altering the chemistry of the feed water and by chemical treatment.