Measurement of Settleable Solids
THEORY: Sewage contains 99.9% water and 0.1% solids. The solids are either
in dissolved form or in suspension. When sewage is passed through a filter the filtrate is not clear and looks turbid (turbidity is the obstruction in the passage of light). The turbidity is because of the filterable solids. This fraction consists of colloidal solids as well as dissolved solids. The colloidal fraction consists of the particulate matter of diameter ranging from 1 milimicron to 1 micron. The colloidal fraction consists of finely divided particles of gels, emulsions, grease, oil causing foams. The colloidal particles are also the fine clay particles with same charge repulsing each other. Because of their size and charge they can not settle by gravitational force. They have to be removed either by biological oxidation or chemical coagulation.
But a major portion of the suspended solids consists of larger, heavier particles that can settle in calm and quiescent conditions. They are known as settleable solids. In sewage treatment, after screening the first treatment unit is a sedimentation chamber known as the grit chamber. It is intended that particles of size 0.15mm to 0.2 mm with a specific gravity of 2.4 to 2.65 are settled in the grit chamber. These particles are generally inorganic particles and they are not degradable. The deposited particles, known as grit can be easily disposed of without any treatment. Even they can be used for filling purposes or low grade concrete.
The significance of this test is in finding the settelable portion of the suspended solids to design the grit chambers and to estimate the amount of deposited material daily in the grit chambers that is to be removed and disposed or used.
Apparatus
Imhoff Cone: It is a long glass cone specially designed with a mark on the top indicating its capacity as one litre; mounted on a stand.
Procedure
Pour the well mixed sample in the Imhoff cone upto 1 litre mark. Allow the sample to settle for 45 minutes. Gently stir sides of the cone with a knife so that the material sticking to the sides may also get settled. Allow it to settle for further 15 minutes. Thus the total settling time is 1 hour. The bottom of the cone is graduated in milliliters. Read from the graduated scale in terms of milliliters per litre.
CALCULATION AND RESULTS
Direct reading on the graduated bottom of the cone gives the amount of settleable solids in
milliliters per litre of sewage.
The settleable solid in the given sample of sewage are ……………milliliters /L
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