Reverse osmosis removes contaminants from unfiltered water, or feed water, when pressure forces it through a semipermeable membrane. Water flows from the more concentrated side (more contaminants) of the RO membrane to the less concentrated side (fewer contaminants) to provide clean drinking water. The fresh water produced is called permeate. The concentrated water left over is called the waste or brine.
A semipermeable membrane has small pores that block contaminants but allow water molecules to flow through. In osmosis, water becomes more concentrated as it passes through the membrane to obtain equilibrium on both sides. Reverse osmosis, however, blocks contaminants from entering the less concentrated side of the membrane. For example, when pressure is applied to a volume of saltwater during reverse osmosis, the salt is left behind and only clean water flows through
In wastewater treatment, ultrafiltration (UF) devices are used to recycle and reuse water that contains virtually no physical solids. Ultrafiltration (UF) is a membrane filtration process similar to Reverse Osmosis, using hydrostatic pressure to force water through a semi-permeable membrane.
Application of UF:
- UF can be used for removal of particulates and macromolecules from raw water, to produce potable water.
- It is used to either replace existing secondary (coagulation, flocculation, sedimentation) and tertiary filtration (sand filtration and chlorination) systems employed in water-treatment plants or as standalone systems in isolated regions with growing populations.
- When treating water with high suspended solids, UF is often integrated into the process, using primary (screening, flotation and filtration) and some secondary treatments as pre-treatment stages.
- Recycling benefits
- Ultrafiltration processes are preferred over traditional treatment methods for the following reasons:
- No chemicals required (aside from cleaning)
- Constant product quality regardless of feed quality
- Compact plant size
- Capable of exceeding regulatory standards of water quality, achieving 90-100% pathogen removal.
The membrane separation process known as nanofiltration is essentially a liquid phase one, because it separates a range of inorganic and organic substances from solution in a liquid – mainly, but by no means entirely, water. This is done by diffusion through a membrane, under pressure differentials that are considerable less than those for reverse osmosis, but still significantly greater than those for ultrafiltration. It was the development of a thin film composite membrane that gave the real impetus to Nano filtration as a recognised process, and its remarkable growth since then is largely because of its unique ability to separate and fractionate ionic and relatively low molecular weight organic species.
The membranes are key to the performance of Nano filtration systems. They are produced in plate and frame form, spiral wound, tubular, capillary and hollow fibre formats, from a range of materials, including cellulose derivatives and synthetic polymers, from inorganic materials, ceramics especially, and from organic/inorganic hybrids.
Application of NF:
- Food and Dairy Sector
- Chemical processing
- Pulp and paper industry
Although the chief application continues to be in the treatment of fresh, process and waste waters. In the treatment of water, NF finds use in the polishing at the end of conventional processes. It cannot be used for water desalination, but it is an effective means of water softening, as the main hardness chemicals are divalent.