Final answer:
A combined sewer carries both sewage and storm water and can overflow during heavy rain, leading to pollution. Treatment of wastewater involves primary, secondary, and tertiary phases to ensure environmental safety.
Step-by-step explanation:
A combined sewer is a waste water collection system that carries both storm water and sewage. This type of sewer system is prone to overflows during heavy rain, which can lead to the discharge of untreated sewage into adjacent water bodies. Modern sewage treatment processes aim to prevent this type of pollution by treating wastewater in three phases: primary (solid removal), secondary (bacterial decomposition), and tertiary (extra filtration). The proper separation and treatment of sewage is critical for protecting water quality and public health.