ANS:D - all the above.

1. Distances 150 m to 300 m: Sewer ventilating columns are often placed along sewer lines at regular intervals, typically spaced at distances ranging from 150 meters to 300 meters. This spacing allows for effective ventilation of the sewer system by providing adequate opportunities for the release of built-up gases and the intake of fresh air. The specific spacing within this range depends on factors such as the length of the sewer section, the magnitude of gas accumulation, and the design considerations of the sewer network.
2. Upper ends of branch sewers: In addition to being placed along main sewer lines, ventilating columns may also be installed at the upper ends of branch sewers. Branch sewers are smaller pipelines that connect to the main sewer line and collect wastewater from smaller areas or neighborhoods. Installing ventilating columns at the upper ends of branch sewers helps to ensure that gases generated within these sections are effectively vented and do not accumulate within the sewer network.
3. Every change in the size of sewers: Ventilating columns may also be placed at every change in the size of sewers, such as at junctions where sewer lines merge or split, or at transitions between different pipe diameters. Changes in sewer size can create areas where gases may accumulate or where ventilation may be restricted. Placing ventilating columns at these points helps to mitigate gas buildup and maintain proper airflow throughout the sewer system.