Short answers, pulled from the story.
A sanitary sewer is an underground pipe or tunnel system that carries sewage from houses and commercial buildings to a treatment plant while deliberately excluding stormwater. A combined sewer carries both sewage and surface runoff in the same pipe, which can cause untreated overflow during heavy rain when the volume exceeds a treatment plant's capacity.
Many cities built combined sewers before sewage treatment plants existed, using runoff to flush waste off streets and underground. Once those systems were in place, replacing them proved prohibitively expensive, so many older cities retained their combined infrastructure. Communities that urbanized in the mid-20th century or later generally built separate systems from the start.
A force main, also called a rising main, is a pumped sewer used when gravity alone cannot carry sewage to a treatment plant. It is typically constructed of welded steel or high-density polyethylene to resist internal pressure. A lift station collects accumulated sewage and pumps it to a higher elevation before it continues through the network.
Two main trenchless methods are used. Pipe relining coats the inside of a damaged pipe with epoxy resin, effectively creating a pipe within a pipe. Pipe bursting draws a new PVC or ABS plastic pipe through the old one behind an expander head that breaks apart the old pipe as the new pipe follows.
A simplified sanitary sewer uses small-diameter pipes, typically around 100 millimetres, often laid at gradients of roughly 1 in 200. The construction cost can be about half that of a conventional sewer, though maintenance requirements are generally higher. Simplified sewers are most common in Brazil and other developing countries.
A vacuum sewer uses differential atmospheric pressure to move wastewater through underground pipelines toward a central vacuum station. Pipelines range in diameter from 125 millimetres to 280 millimetres and are used mainly in low-lying communities where gravity drainage is impractical.