Reverse Osmosis Purification Technology

Technology and Process Description

RO is a very reliable and time-tested treatment methodology to desalinate the water by reducing the total dissolved solids and other undesirable chemicals to drinking water standards. This technology is today acclaimed globally as the most advanced level of liquid filtration and is used for wider applications. Conventional filtration technology involves pumping the entire liquid stream through the filter medium. This is known as "dead-end" filtration. In the RO process, raw water (high salt concentration) - after necessary pretreatment - is passed under sufficient external pressure through a semi-permeable membrane. This filtration is termed as crossflow or tangential flow filtration where water flows over and parallel to the membrane surface. The RO membrane has a pore diameter of 0.0001 micron (ie.,0.00000004 inches) which is smaller than bacteria (0.5 mm to 1 mm) and virus (0.02 mm to 0.04 mm) thereby ensuring effective filtration (99%). Such fine pores ensure that up to 95% of the ionic impurities do not escape through the membrane. The turbulent flow of water over the membrane surface minimizes the accumulation of particulate matter on the membrane and facilitates continuous operation of the system. This membrane is made of thin, multi-layered high quality polyamide sheets. The design of the RO plant is primarily based on two factors . TDS content in the feed water and the recovery/reject ratio. In the separation process, purified water permeates the membrane and is collected separately, while the rejected water containing concentrated dissolved and suspended solids is discharged to the drain. The passage of water also depends on factors like molecular size and diffusability co-efficients.

The feed water flows continuously across the membrane and the rejected solids get into the back flow and is sent to the reject port. This process continues without clogging the membrane.

RO process is very effective in the removal of inorganic chemical content like chlorides, nitrates , fluorides, lead, copper, mercury, sulfates, calcium, and magnesium in addition to the removal of physical and microbial impurities.

Maintenance Operations

The filtration units (pre-treatment) will have to be periodically backwashed to ensure the removal of accumulated physical contaminants. The level of anti-scalant chemical will have to be monitored and periodically refilled. The UV lamp has to be checked for any drop in intensity.

No amount of pretreatment will completely eliminate long-term fouling of the membranes and hence periodical cleaning of the membrane is vital and absolutely necessary. Generally, the periodicity of membrane cleaning is dictated by a reduction in the permeate (product water) flow or permeate (product water) quality. As a result of concentration polarization with RO membranes, a drop in product water quality may signal the onset of fouling before a drop in permeate flow. Large RO systems are equipped with automatic units to backflush the membrane at a preset frequency. Apart from this routine backflushing, chemical cleaning may be required once in every six months depending upon the fouling potential of the feedstream.

Proper design, engineering perfection and sound maintenance will result in reduced running cost of the plant.