What is the minimum gradient for a 9 inch foul pipe?

The minimum gradient for a 9-inch foul pipe, which is typically used for wastewater, is generally recommended to be at least 1% to 2% (1/8 inch to 1/4 inch per foot). This ensures proper flow and prevents blockages. However, local codes and specific conditions may dictate different requirements, so it's essential to consult relevant regulations or guidelines for your area.