Unit Operations and Processes

UNIT OPERATIONS AND PROCESSES

Unit Operations: Treatment done by physical or mechanical methods are called as unit operations.

Eg: Screening, Filtration, Sedimentation etc.

Unit Processes: Treatment done by employing chemical or biological methods are called as Unit Processes.

Eg: Coagulation, Chlorination etc.

The important unit operations employed in water and waste water treatment are:

1. Gas transfer: Aeration

Water is exposed to oxygen, to remove dissolved harmful gases (CO2, H2S) and Iron-Manganese from water. Aerators may be Gravity aerator, spray aerators, diffusers etc.

 2. ION Transfer

(i) Chemical coagulation - Removal of colloidal particles - Coagulant is added to water to form clusters which settle rapidly.

(ii) Chemical precipitation - Chemicals are added to precipitate out the dissolved impurities. Eg: Precipitation of carbonate hardness by adding lime.

(iii) Ion exchange - Interchange of ions between water and solid ion exchange media. Eg: Water softening process.

(iv) Adsorption - Ions are removed from solution and concentrated at surfaces of adsorbent. Eg: Activated carbon adsorption.

3. Solute stabilization

Water is stabilised and objectionable solutes are converted into unobjectionable forms.

(i) Chlorination - H₂S is oxidised into sulfate

(ii) Liming-CO₂ is converted to soluble bicarbonate by passing through limestone.

4. Solids transfer.

Removal of solids from water.

(i) Straining - Removes floating and suspended impurities using Screens.

(ii) Sedimentation - Removal of suspended solids in settling tanks under gravity.

(iii) Floatation - Suspended solids gets attached to floatation agents that lift the particles to surface - removed by floatation devices.

(iv) Filtration - Suspended solids are trapped on the grains of filter media (sand, coal, granular materials)

5. Nutrient or Molecular Transfer

6. Interfacial contact

7. Miscellaneous operations:

(i) Disinfection - Removal of pathogens from water

(ii) Desalination - Convertion of salt water into fresh water

(iii) Fluoridation - Adding fluoride to water

8. Solids concentration and stabilization – Used in waste water (sludge) treatment to increasw the soilds concebtration of  sludge.

 (i) Thickening

(ii) Centrifuging

 (iii) Sludge digestion