We were confused too.
Instead, this is apparently a pressure line. We had high hopes when we found it on the main road — which is downhill — from the building lot. (Very important for all things potty.) But according to the local sewer district, this is unusable.
So we’re back to a septic system. Welcome back, stone & pipe.
A House In A Field 
At least your septic will be downhill. Ours goes uphill. Don’t ask.
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