Sewers freezing is almost impossible if there are regular flows, even when mounted external to a building. There are frequent discharges from toilets, sinks, showers and baths that are well above zero even during freezing conditions. Sewers are typically buried, almost always below frost level.
Sewers can be built at great depth (I inspected a sewer that was 20 metres below ground level) but typically there are pump stations at intervals that lift the flows closer to ground level where it flows under gravity until the next pump station or the treatment plant.
Minimum grades for sewers depends on the sewer diameter and flow rates/ velocity. Continuously flowing water will tend to keep ‘sediment’ mobile, even at a lower velocity. If the flow is intermittent, then you need a higher velocity to re-mobilise sediment, otherwise the sediment volume just keeps growing, once deposited.
