A new trunk sanitary sewer is to be designed for a city of 50 km2 . The fully developed city will include 62% residential, 28% commercial, and 10% industrial development. The residential area will consist of 45% large lots (6 persons/ha), 50% small single-family lots (75 persons/ha), and 5% multistory apartments (2500 persons/ha). The design average wastewater flow rates are: 300 L/d/person for large and single-family lots, 200 L/d/person for apartments, 25,000 L/d/ha for commercial areas, and 40,000 L/d/ha for industrial areas. Infiltration and inflow is 900 L/d/ha for the entire area. It is anticipated that when the new trunk sewer is first installed the average flow will be 50% of the flow when the city is fully developed. Complete the following design steps: Step 1: Estimate the design flow rate (Q) for the trunk sewer of the full developed city.
Step 2: Determine the size (diameter) of the sanitary sewer that is designed to run three-fourths (75%) full (h/D=0.75). It is assumed that the concrete sewer is laid on a slope of 0.9%. Manning's n of the sewer pipe is 0.015.
Step 3: Calculate the flow velocity in the sewer and determine whether the velocity meets the self-cleansing velocity of 0.6 m/s (2 ft/s).
Step 4: Determine the potential for H2S generation in the sanitary sewer. It is assumed that the wastewater temperature is 25 degrees Celsius and the 5-day BOD of the wastewater at 20 degrees Celsius is 300 mg/L.