In the last decennia, membrane technology has been receiving more and more attention and is becoming a true implemented separation technology in many industrial applications. Membrane technology has already gained significant attention for the removal of a broad spectrum of impurities from water and waste water since the 1990’s. Recently, membrane technology is being increasingly applied for water treatment. The growing interest of membrane technology applications for water treatment can be explained by the increasing demand for high quality water, the introduction of more stringent regulations, and the increasing interest in reuse. The first advantage of membrane technology is that this process requires less or no chemical reagents compared to conventional systems. In addition, separation takes place mainly by size exclusion, which is less sensitive to changes in the feed water quality. The membrane process is also a quite compact process and quite easy to extend and automate compared to conventional systems.

 

Membrane Performance

The performance of a membrane is characterized by its permeability (filtrate flow rate through a specific membrane surface area at constant transmembrane pressure), and its retention behavior or molecular weight cut-off (MWCO : refers to the lowest molecular weight of the solute retained for 90% by the membrane). There are a number of membrane properties such as membrane hydrophilicity/hydrophobicity, surface charge, porosity, roughness, physico-chemical nature of the surface that effect membrane performance and thus flux and MWCO.

Membrane Process Classifications

Membrane processes in aqueous applications are generally classified according to the driving forces applied. One identifies pressure, electrical potential, concentration, and chemical potential as drivers.

  1. Pressure driven processes: Microfiltration (MF), Ultrafiltration (UF),Nanofiltration (NF), and Reverse Osmosis (RO)
  1. Electrical potential driven process: Electrodialysis (ED)
  2. Concentration driven processes: Forward Osmosis (FO) and Dialysis
  3. Chemical potential driven processes: Membrane Contactors (MC), and Pervaporation (PV)

 

Normally, membranes for water treatment are mainly pressure driven and are categorized into four narrower ranges based on their pore size, and thus consequently, according to the size of particles, molecules and/or ions they retain. All four categories are given here in ascending order of pore size: RO, NF, UF, and MF. The suitable pore size depends on the specific requirements of the applications, and the pressure required differs inversely with the size of the pores (basically classical orifice theory).

 

Table 2.  The differences among the pressure driven membrane filtration alternatives.

  RO Membranes NF

Membranes

UF Membranes MF Membranes
Pore size (nm) <0.5 0,5-2 2-100 100-10000
Pressure (bar) 15-100 3-20 2-7 0.1-2
Permeability(L/hm2bar) 0.05-1.5 1.5-30 10-1000 >1000
Separation mechanism Solution diffusion Sieving/charge effects/affinity Sieving Sieving
Separation ability (retain = + and pass through = )
Suspended solids + + + +
Bacteria + + + +/-
Viruses + + +
Macromolecules + + +/-
Small organics + +/-
Multivalent ions + +/-
Monovalent ions +
Water

 

Membrane Filtration Operational Modes

In the membrane process for water treatment, water is pumped towards and through the membrane surface resulting in a stream of treated/purified water, called the permeate, and the stream containing the concentrated retained components known as retentate. Generally, there are two types of membrane filtration operational modes, i.e. dead-end and cross-flow. A schematic representation of both separation process modes is shown in figure 1

Figure 1. Two alternative filtration modes for the membrane separation process: (a) dead end flow and (b) cross flow set-up.

Membrane Fouling

(The process resulting in decrease of membrane performance during the filtration due to deposition or adsorption of suspended or dissolved substances on membrane external surfaces, at its pore openings, or within its pores).

Membrane fouling is one of the most critical problems of the membrane technology, decreasing the viability of the process. Fouling has a significant impact on the cost-effectiveness of the filtration process: it is strongly associated with an increase in the operational costs of the system and diminishes the attractiveness of membrane technology. There are four common forms of membrane fouling based on types of foulants.

  1. Organic fouling
  2. Inorganic fouling/scaling
  3. Biofouling
  4. Colloidal fouling

 

Membrane Fouling Mechanisms

Figure 2. Common forms of membrane fouling: adsorption, cake layer formation, pore blocking, and depth fouling.

Factors effect membrane fouling

  • Feed quality
  • Operational parameters
  • Membrane material
  • Membrane properties

 

Remedies of membrane fouling

  • Feed pre-treatment
  • Membrane modification
  • Suitable operational parameters
  • Membrane cleaning

 

We are immensely unique in providing membrane technological solutions for water and wastewater treatment e.g. choice of membrane chemistry and pore size for specific water stream, membrane modification to avoid membrane fouling, optimization of operational parameters to make process cost effective and to improve quality of produced permeate water, providing far less expensive membrane compared to the market (as we are also membrane manufacture)