ANS:D - all the above.

1. Must be of adequate size to avoid overflow: Sewers must be sized appropriately to accommodate the expected volume of wastewater flow without causing overflow. If sewers are undersized relative to the flow they need to convey, they can become overwhelmed during periods of heavy rainfall or high wastewater discharge, leading to backups, flooding, and pollution of the environment. Proper sizing of sewers involves considering factors such as peak flow rates, hydraulic capacity, and future growth projections to ensure adequate capacity and prevent overflow.
2. Must flow under gravity to full: Gravity flow is the preferred method for conveying wastewater in sewer systems. Gravity sewers rely on the natural slope of the terrain to transport wastewater from higher elevation areas to lower elevation treatment facilities or discharge points. Maintaining gravity flow ensures efficient and cost-effective operation of the sewer system, as it minimizes the need for mechanical pumping and reduces energy consumption. Additionally, sewers must be designed to flow to full capacity under gravity to prevent the accumulation of solids and sediment, which can lead to blockages and reduced flow capacity.
3. Must be laid at least 2 to 3 m deep to collect water from basements: Sewers should be installed at a sufficient depth below ground level to effectively collect wastewater, including drainage from basements and underground structures. By laying sewers at a depth of 2 to 3 meters or more, engineers can ensure that the sewer lines are below the level of basement floors and can efficiently collect and transport wastewater from these areas. Adequate depth also helps protect the sewer infrastructure from potential damage due to excavation activities, traffic loading, and frost penetration.

• All of the above.