Sewage and waste waters contain plenty of solids in suspension or in colloidal solution or simply as floating objects.
These have to be removed before the biological treatment. To separate large floating objects like pieces of vegetables, rags, dead animals and other junk the sewage is passed through screens or nets in chambers which are called screening chambers.
Larger suspended or floating objects are held back in the screening chambers. After screening waste waters are run through a series of grit chambers. These chambers contain large stones or brick ballast. Passage through these chambers removes much of coarse particulate material. Special mechanical devices are also available to remove coarse particles from waste waters.
These are known as hydrocyclones or centrifugal separators. The sewage is subjected to rapid circular motion in hydrocyclones which generates strong centrifugal force. The coarse particles are pushed towards the periphery of the circular chambers. Clear water is drawn out from the centre of the tank.
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Lighter particles float over the surface. Heavier ones settle down. Later the liquid is drawn out from the middle layer which leaves the upper and lower layers back in the sedimentation tank. Oils, fats and greases are also retained in the upper layers as they are lighter. These tanks, therefore combine, the process of flotation as well as sedimentation.
In order to make the process of flotation more effective some emulsifying or foaming agent may be added to the tank and the contents of the tank agitated with the help of compressed air or mechanically. Particles get adsorbed on to the surface of the bubbles and come up on the surface. This process is known as Froth floatation. Certain chemicals and coagulants may also be used to raise the efficiency of the process of sedimentation.
A coagulant is a finely powdered material which is mixed with waste waters. Its particles add on to the lighter particles present in the sewage and make them heavier which facilitates their sedimentation. The process is commonly known as Coagulation. Salts of iron, aluminum, activated silica, polyacrylamides etc. are usually used as coagulants.
The practice of adding coagulants or flocculants improves the efficiency of processes in primary treatment. But it suffers from the basic disadvantage that such substances are added in waste waters which later interfere with the process of biodegradation in the secondary phase of the treatment. Various soaps, oils and other flocculants may clog the filter beds which are extensively used in the secondary treatment.