Domestic Sewage

Abstract Fresh sewage typically has a light brown color. As it ages, oxygen is depleted, and anaerobic microorganisms begin to use other electron acceptors, including sulfates, nitrates, iron, and manganese. As sulfates are reduced, hydrogen sulfide is produced, which can combine with metals in the sewage. The color gradually changes from light brown to gray and eventually to black as a result of the metal sulfides. The odor of fresh sewage is usually not objectionable, but as it ages, anaerobic microorganisms produce odors that are much more offensive than the odor of fresh sewage. Total solids are made up of soluble solids and suspended solids and are determined by evaporating a known volume of sewage at a temperature of 103–105°C and weighing the residue. The temperature of sewage is generally warmer than that of the water supply because of heat added by homes, businesses, and industrial activities. Sewage temperatures vary with the seasons and geography. The organic strength of sewage is measured by a test called biochemical oxygen demand (BOD). The test does not directly measure the concentration of the organics, but rather measures the amount of oxygen that is used as the organic compounds are degraded by microorganisms.