Specific Gravity of Municipal Sewage Explained

Specific gravity is a measure of the density of a substance relative to the density of a reference substance, which is typically water for liquids. It is a dimensionless quantity defined as the ratio of the density of the substance to the density of the reference substance (usually water at 4°C).

The formula for specific gravity ($\text{SG}$) is:

$\text{SG} = \frac{\text{Density of substance}}{\text{Density of reference substance (water)}}$

Pure water at 4°C has a density of approximately 1000 kg/m³ or 1 g/cm³. Therefore, the specific gravity of pure water is 1.

Municipal sewage is essentially wastewater from homes, businesses, and potentially industries, containing a mixture of water, dissolved organic and inorganic solids, and suspended solids. While the bulk of municipal sewage is water, the presence of these dissolved and suspended solids increases its overall density compared to pure water.

Since the density of municipal sewage is slightly higher than the density of pure water due to the added solids, its specific gravity will be slightly greater than 1.

Let's look at the options provided:

• Slightly greater than 1: This is consistent with the presence of dissolved and suspended solids in water, which increases the density.
• Slightly lesser than 1: This would imply the sewage is less dense than water, which is not the case as it contains heavier components.
• Much lesser than 1: This would imply a significant density difference, far less dense than water, which is incorrect.
• Equal to 1: This would mean municipal sewage has the same density as pure water, which is not true because of the added solids.

Therefore, the specific gravity of municipal sewage is slightly greater than 1 due to the presence of dissolved and suspended impurities that increase its density compared to pure water.