Membrane and Desalination Technologies - TCE Moodle Website

Potable Water Biotechnology, Membrane Filtration and Biofiltration 487
membrane processes that is most often used in drinking water treatment applications that
require the removal of dissolved contaminants, as in the case of softening or desalination.
Nanofiltration membrane modules are commonly fabricated in a spiral configuration for
drinking water treatment. An important consideration of spiral elements is the design of the
feed spacer, which promotes turbulence to reduce fouling.
Nanofiltration membranes are capable of removing bacteria and viruses as well as organicrelated
color without generating undesirable chlorinated hydrocarbons and THMs. Nanofiltration
is used to remove pesticides and other organic contaminants from surface and ground
waters to help insure the safety of public drinking water supplies. It is an attractive alternative
to lime softening or zeolite softening technologies for groundwater treatment. For many
groundwater applications, the pretreatment required for nanofiltration consists in adding acid
or antiscalant and then passing the feed through a cartridge filter. However, for surface
waters, more extensive pretreatment such as conventional treatment, microfiltration, ultrafiltration,
slow sand filtration, and/or GAC adsorption is necessary.
3.5. Reverse Osmosis
Reverse osmosis is the pressure-driven membrane separation process that employs the
principles of reverse osmosis to remove dissolved contaminants from water. Process mechanism
of this membrane is to employ the reverse of the natural osmosis process – i.e., the
passage of a solvent (e.g., water) through a semipermeable membrane from a solution of
higher concentration to a solution of lower concentration against the concentration gradient,
achieved by applying pressure greater than the osmotic pressure to the more concentrated
solution (2). An approximate osmotic pressure of fresh or brackish water is approximately
1 psi for every 100 mg/L difference in TDS concentration on opposite sides of the membrane.
Reverse osmosis can remove contaminants from water using a semipermeable membrane
that permits only water, and not dissolved ions (such as sodium and chloride), to pass through
its pores. Contaminated water is subject to a high pressure that forces pure water through the
membrane, leaving contaminants behind in a brine solution. Reverse osmosis is effective in
rejecting organic solutes with molecular weights, such as fulvic acids, lignins, humic acids,
and detergents. Low molecular weight, nonpolar, water soluble solutes (for example, methanol,
ethanol, and ethylene glycol) are poorly rejected. In addition, undissociated organic acids
and amines are also poorly rejected, while their salts are readily rejected.
Generally, reverse osmosis units for potable water treatment plants include raw water
pumps, pretreatment, membranes, disinfection, storage, and distribution elements. These
units are able to process virtually any desired quantity or quality of water by configuring
units sequentially to reprocess waste brine from the earlier stages of the process.
Reverse osmosis membranes have the ability to screen microorganisms and particulate
matter in the feed water. In addition, it can remove nearly all contaminant ions and dissolved
nonions. It is relatively insensitive to flow and TDS level and simple to operate due to its
automation, which makes it suitable for small systems with a high degree of seasonal fluctuation
in water demand. However, membrane units require high capital and operating costs.