The main difference between sewer and stormwater drainage is the source and destination of the water. Sewer drains carry wastewater and sewage from your home or office, while stormwater drains carry rainwater from various sites. Sewer drains deliver the water to a treatment plant, while stormwater drains discharge the water directly to waterways and the sea without any treatment. Sewer and stormwater systems are separate from each other to prevent contamination and flooding.