A 'départ' is some kind of outgoing feed (looked at from the POV of "where it is coming from"), and the 'surpressé' refers to the fact that the water has had its pressure boosted by pumps (cf. the non-surpressé = incoming mains pressure) — this might, for example, be in order to maintain sufficient water pressure on the higher floors of a tall building.

In typical engineering jargon, 'boost feed' would be fine, though more properly, of course it ought literally to be 'boosted'.

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Note added at 15 hrs (2018-07-26 00:20:48 GMT)
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Here are some examples of translations relating to surpresseur / boost in connection with water kindly found for us on the 'Net by Linguee.

By no means all of them are entirely relevant, but there is sufficient consensus for even non-technical experts to surely get the gist?

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Note added at 12 days (2018-08-07 05:02:52 GMT)
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Sorry, omitted to post the link!

Here it is now — though thee are plenty of other examples to be found

https://www.linguee.fr/francais-anglais/traduction/surpresse...