A Class 3 operator is expected to evaluate design tradeoffs, not just identify components — starting with the reality that the single greatest long-term threat to a concrete or metal sewer pipe isn't the wastewater itself, but the gas above it.
Pipe Material Selection: Corrosion Resistance vs. Structural Loading
Every material decision in a gravity collection system balances two competing pressures: the corrosive environment created by sewer gas, and the structural demands of the trench.
Hydrogen sulfide corrosion. Wastewater under anaerobic (low-oxygen) conditions generates hydrogen sulfide (H2S) gas. In a partially full gravity pipe, H2S off-gasses from the flow and reaches the exposed, damp pipe crown above the water line. There, naturally occurring sulfur-oxidizing bacteria convert the H2S into sulfuric acid, which attacks the calcium-based compounds in ordinary concrete —