Sewer smell (hydrogen sulphide)

A sewer smell with a characteristic rotten egg quality is caused by hydrogen sulphide, a gas produced when sewage stagnates and anaerobic bacteria break down organic matter. The odour typically indicates that wastewater is sitting in the pipe for longer than intended, often due to a low spot in the drain, a partial blockage, undersized pipework or a failed pumping station. In London, the gas is also encountered near combined sewers during long dry periods when low flows allow septic conditions to develop. Hydrogen sulphide is corrosive to concrete and metal and can damage drainage infrastructure over time. Building Regulations Part H1 and BS EN 752 set out drainage design principles intended to maintain self-cleansing velocities. Engineers usually address the issue by jetting the affected line, ventilating the system and investigating the underlying cause.