Mumbai University > First Year Engineering > Sem1 > Applied Chemistry 1

Marks: 5M/10M

Year: May 2015

REVERSE OSMOSIS

• Reverse osmosis is the reversal of the natural flow of osmosis. It occurs when the water is moved across the membrane against the concentration gradient, from higher concentration to lower concentration.

• A pressure can be applied to the concentrated solution which is greater than that of the osmotic pressure. The direction of water passage through the membrane is reversed and the process that is called reverse osmosis.

• Reverse osmosis (RO) is a pressure driven membrane separation process that separates dissolved and suspended substances from water. The membrane acts as a selective barriers, removing unwanted substances such as salt, producing water safe for drinking.

Method:

1. In this process, pressure (of the order 15 to 40 kg $cm^2$) is applied to impure water to force its pure water out through the semi-permeable membrane; leaving behind the dissolved solids. The membrane consists of very thin films of cellulose acetate, affixed to either side of a perforated tube.

2. The material of membrane is semi-permeable, allows water molecules to pass through while acting as a barrier to dissolved solids, i.e. chemical contaminates, metals and organic compounds.

3. The reverse osmosis process cannot go on indefinitely without removing the contaminants. Ultimately the membrane could become clogged by salt and other impurities, requiring increasingly greater pressure to force water through the membrane.

Advantages:

1. It has distinct advantage of removing ionic as well as non-ionic, colloidal and high molecular weight organic matter.

2. The maintenance cost is almost on the replacement of the semi-permeable membrane.

3. Due to low capital cost, simplicity, low operating cost and high reliability, the reverse osmosis is gaining ground at present for converting sea water into drinking water.

ULTRAFILTRATION

• It uses a membrane very similar in design to reverse osmosis systems except that the ultra-filter’s pores are slightly larger. The ultra-filter is used to remove pyrogens and other long chain organic molecules as RNase from the purified water.

• Ultra filters use polymer film with chemically formed microscopic pores that can be used in place of granular media to filter water effectively without coagulants.

• Thus it is a membrane separation technique in which very fine particles or other suspended matters, with a particle size in the range of 0.005 to 0.1 microns are separated from a liquid. Because of the larger pore size in the membrane, UF requires a much lower operating pressure: 10 to 100 psi.

• It is capable of removing salts, proteins and other impurities within its range.

• Since a high percentage of the water passes through the ultra-filters, it will eventually plug if not carefully maintained. In a properly designed system, the ultra-filter is regularly and tangentially washed free to contaminants. With this type of design, ultra-filtration is an outstanding technology for ensuring very consistent ultra-pure water quality.

Advantages:

1. Effectively removes most particles, pyrogens, microorganisms, and colloids above their rated size.

2. Produces highest quality water for least amount of energy.

3. Regenerable using a hot (50C) 5% caustic solution, solving the problem of fouling.

Disadvantages:

It will not remove dissolved inorganic material.