Reverse 
OSMOSIS

MEMBRANE FILTRATION

Membrane filtration is a surface filtration technique, in which solids (dissolved and suspended, depending on type of membrane process) are separated from liquid by a semipermeable membrane. The commonly used membrane processes for water filtration are:

  • Microfiltration, MF (pore size 0.1-10 um), separation based on particle size and shape
  • Ultrafiltration, UF (0.01-0.1 um), separation based on particle size and shape
  • Nanofiltration, NF (0.001-0.01 um), solutes separated based on molecular size, diffusion rate, and electrostatic repulsive interaction with membrane (Donnan exclusion)
  • Reverse osmosis, RO (less than 0.001um) solute mainly separated by diffusion mechanism, although some reports indicate presence of pores in RO membranes and solute transport to be by capillary flow mechanism

The driving force for water filtration in these processes is positive pressure or vacuum (for submerged UF and MF membranes only) applied across the membrane. For removal of suspended solids and colloidal material, MF and UF processes are used, whereas for further purification of water, if required, NF (removal of dissolved salts and small organic molecules) processes are used.

MEMBRANE FOULING

Membrane fouling is an inevitable part of the membrane process and occurs due to deposition and adsorption of rejected solutes on the membrane surface or on the pore walls. Fouling results in decline in membrane productivity with time and sometimes in unfavorable separation characteristics. The loss of productivity means increased capital cost for higher membrane area and associated equipment to maintain required water production. The fouling also means increased operating cost, as membrane have to be cleaned frequently to maintain water production, labor cost for cleaning, plant downtime , and increased membrane replacement cost.

MEMBRANE EQUIPMENT

Membrane filtration systems primarily include:

Feed pumps

Per-filters or strainers

Membrane modules or cassettes

Filtrate tanks

Clean-in-place (CIP) tanks and pumps

Backwash water tanks (UF and MF applications)

For NF and RO applications, mainly spiral wound membranes are used: however, hollow fiber and tubular membranes have also been used at some plants. The widely used membrane materials for NF and RO is a thin film of polyamide formed on a polysulfone porous substrate (UF membrane). Polyamide membrane are superior to first generation homogenous (as opposed to composite) cellulose acetate membranes in terms of flux and salt rejection but do not tolerate free chlorine. Cellulose acetate  membrane, on the other hand, do tolerate continuous upstream use of up to 1 mg/L free chlorine, which helps control biofouling, but have a very narrow pH range of 4.5-6.5 and are used now only for selected situations.