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Is it normal for some solids to remain in the drain at all times??

[DIY Guy]

Waste lines rely on gravity for carrying waste out to a sewer or septic. The thing is, the slope of the pipes isn't all that steep for most of the run. Therefore, solids get washed down the waste line, but eventually the liquid moves faster and leaves solids behind. They get washed out with water from showers, tubs, sinks, laundry and other flushes.

So in short, yes, it is normal.