How should a septic tank be sized if receiving all flows?

The ideal sizing for a septic tank that will receive all flows is calculated based on the total anticipated volume of wastewater that will enter the system. Option C suggests sizing the septic tank as 50% of the designed tank size, in addition to the original tank size. This approach effectively accounts for peak flow conditions or unexpected surges in wastewater, which can occur due to factors such as increased usage during gatherings or events.

By having a tank that is 50% larger than the designed capacity, you ensure there's sufficient space for additional wastewater, allowing for better settling of solids and reducing the likelihood of solids escaping into the leach field. It also helps maintain the efficiency of the tank since it mitigates the risk of overloading during high flow periods.

The other options may not adequately address the potential for excess flow. For example, sizing exactly to the design specifications or only slightly increasing the size may not provide enough buffer against fluctuating volumes, leading to potential system failures, environmental hazards, and increased maintenance needs. Hence, the approach of expanding the size by an additional 50% effectively safeguards against these issues.