By this technique, about 60-80% of the suspended particles can be removed. However, addition of certain chemicals aids sedimentation, a process referred to as chemical precipitation or chemical-aided sedimentation. It is not always possible to remove the colloidal wastes in sewage by plain sedimentation. Primary, secondary, coagulation-cum-sedimentation tanks, grit chambers, septic and Imhoff tanks.įor the sake of illustration, some sedimentation tanks are depicted in Fig. Longitudinal, vertical, radial and spiral. This categorization is mainly based on the tendency of the particles to interact and form solids. There are four major types of settling-discrete settling, flocculent settling, hindered or zone settling and compression. These include the size, shape and specific gravity of particles, besides viscosity and flow velocity of sewage. The process of sedimentation is influenced by several factors. If the flow of the sewage is stopped and if it is stored in a tank referred to as sedimentation tank, the solid particles can settle down at the bottom. However, most of the solid particles of organic compounds remain in a suspended state in a flowing sewage. The solid particle of the sewage tend to settle down due to gravity. It is sometimes necessary to use chemical coagulants to facilitate or aid sedimentation, and this process is referred to as chemical precipitation or coagulation-aided sedimentation. In the normal process of sewage treatment, sedimentation is usually carried out twice-once before the secondary treatment, referred to as primary sedimentation, and then after the secondary treatment is complete, a process known as secondary sedimentation. Primary treatment basically involves the process of sedimentation or settling. Primary treatment is aimed at the removal of fine suspended organic solids that cannot be removed in the preliminary treatment. This material is pushed to the side compartment referred to as stilling compartment from where it can be removed manually or mechanically. The skimming tank is divided into three compartments that are interconnected.Īs the compressed air is pushed from the floor of the tank, the raising air bubbles coagulate and solidify the oily and greasy materials present in the sewage. They can be removed by using a skimming tank which is fitted with baffle walls that divide the tank (Fig. Several greasy and oily materials (fats, oils, waxes, soaps etc.) from the domestic or industrial outlets find their entry into the sewage. A diagrammatic representation of a typical grit chamber is depicted in Fig. Grit chambers may be kept either before or after the screens.

This technique is based on the process of sedimentation due to gravitational forces. The heavy inorganic materials (specific gravity 2.4-2.7) like sand, ash and others can be removed by using grit chambers.

The most unknown thermal ponds