Short answers, pulled from the story.
Sewage treatment is a type of wastewater treatment that removes contaminants from sewage to produce an effluent suitable for discharge to the environment or for reuse, preventing water pollution from raw sewage. It handles wastewater from households and businesses and possibly pre-treated industrial wastewater.
Sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also adds a tertiary stage with polishing processes and nutrient removal. A fourth, or quaternary, stage can be added to remove organic micropollutants such as pharmaceuticals.
At the global level, an estimated 52% of sewage is treated. Rates are highly unequal: high-income countries treat approximately 74% of their sewage, while developing countries treat an average of just 4.2%.
Disinfection kills disease-causing pathogens before treated sewage is discharged or reused, using methods such as ozone, chlorine, ultraviolet light, or sodium hypochlorite. Chlorination remains the most common method in many countries due to its low cost, while ultraviolet light is becoming more common because of concerns about chlorine by-products.
Nitrogen is removed by nitrification, the oxidation of ammonia to nitrate, followed by denitrification, which reduces nitrate to nitrogen gas released to the atmosphere. Phosphorus can be removed biologically through enhanced biological phosphorus removal, where polyphosphate-accumulating organisms store up to 20% of their mass as phosphorus, or by chemical precipitation with iron, aluminum, or lime.
The fourth treatment stage removes micropollutants such as pharmaceuticals and pesticides that slip through primary, secondary, and tertiary treatment, mainly using activated carbon filters that adsorb the pollutants. It has been implemented in Germany, Switzerland, Sweden, and the Netherlands, and has been enshrined in law in Switzerland since 2016.
Population equivalent, or PE, is a parameter based on the per person organic matter load used to design sewage treatment plants. Commonly used definitions hold that 1 PE equates to 60 grams of BOD per person per day and also equals 200 liters of sewage per day.