Our planned returned flight leaves Treviso at 10.00 a.m. Would it be difficult to get from St Mark's Sq area to the airport by 8.00 a.m. on a weekday.

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Check ATVO timetables they run buses from Piazzale Roma that co incide with the majority of flights from Treviso especially Ryan Air.
A Vaporetto line one from San Marco area to Piazzale Roma for the Treviso Bus, take approx 35 minutes line 1. and theyu run every Ten Minutes,
See http://www.atvo.it/index.php?lang=en&area=23&menuid=36&view_pc=! for links to the ATVO bus timetable - it looks like you would need to leave Piazza Roma at about 0710.
The daytime Vaporetto run frequently from about 5 am so you should be OK.
Phil-the-Bus
Thanks for your quick responses, much appreciated and helpful.