Digby was thinking very hard :
That's a good thought but my macine will be about 20 feet from the
nearest point I can connect to the sewer - doesn't this imply that I
will constantly have grey water lying in this pipe work ?

It sounds like you could just simply allow gravity to work, as that is
more than enough slope for the distance. Just run the pipe at a steady
fall down to the drain.

How should I protect against flowback into the outhouse?

I don't know if this is permitted, but an overflow pipe could be
installed which is at a slightly lower level than the level at which it
would overflow back into your outhouse.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org