REVERSE OSMOSIS

Principle: 

Osmosis is a natural process that when a pure water and saline water are separated by a membrane which can permit only water molecules to pass through. Pure water will flow into the saline water due to a tendency called osmosis. This natural tendency can be reversed by applying an opposite pressure to the saline water so that water molecules from saline water tend flow out as pure water. This is called reverse osmosis.

Process:

·         It consists of several horizontal compartments separated by semi-permeable membranes.

·          A pressure of 15-40 kg/cm² is applied to the saline water to force its pure water to pass through the semi-permeable membrane leaving behind the dissolved salts.

·         The pure water from different compartments are collected and the concentrated saline water left out is discarded.

·         The membrane is generally a thin film of cellulose acetate or polymethyl methacrylate.

Advantages:

1. This method is economical, compact and very simple.

2. Life time of the membrane is high about 2 years.

3. Easy to replace the membrane within two minutes.

4. Ionic, non ionic, colloidal and high molecular weight organic matters are easily removed.

Experimental determination of COD by iodometric titration

Principle:

 A known volume of water sample is taken and refluxed with known volume of K₂Cr₂O₇ in conc. H₂SO₄ in presence of Ag₂SO₄ and HgSO₄. The unused potassium dichromate is titrated against FAS, using ferroin as indicator. It is back titre value. Blank titration is carried out without water sample; this titre value is blank titre value. The difference between the two titre value curresponds to the amount of potassium dichromate required to oxidize the impurities present in water sample.

Procedure:

Back titration:

·         A known volume waste water sample is pipette out into conical flask.

·         Add 10 ml of K₂Cr₂O₇ solution using pipette. About 1 test tube full of conc. H₂SO₄ followed by 1g of Ag₂SO₄ and 1g of HgSO₄ is added to it.

·          The contents are refluxed for 30 minutes, on a water bath. The solution is cooled to room temperature, 3 drops of ferroin indicator is added to the flask.

·         It is titrated against standard FAS solution till the color changes from bluish green to redish brown. Volume of FAS consumed is noted ( ‘V₁’ ml)

Blank titration:

·         The above procedure is repeated by taking the same volume of distilled water in place of the waste water sample.

·         Volume of FAS consumed is noted (‘V2’ ml) and COD is calculated.

Calculations:

The volume of waste water sample = V ml

Normality of FAS used= N

Volume of FAS from back titration = V₁ ml

Volume of FAS from blank titration = V₂ ml

Amount of K₂Cr₂O₇ consumed interms of FAS = (V₂ - V₁) ml

Therefore normality of oxygen =    (V₂ - V₁) x N

                                                        V

                                               = N₁

 Hence, COD= N₁ x Equivalent weight of oxygen

= N₁ x 8 x 1000 mg/dm³ = …………….. mg/dm³

Chemical Oxygen Demand (COD)

Definition

COD is defined as the amount of oxygen used while oxidizing the total organic load of the sample with a strong chemical oxidant like K₂Cr₂O₇ in acid medium. It is represented in mg/dm³ or ppm.

Importance of COD:

1. It is satisfactory, quantitative method for measuring total organic load.

2. Rapidly measurable parameter and needs about 3 hours for completion.

3. In general COD>BOD since both biodegradable and non biodegradable organic load are completely oxidized.