The tap water supply? Im assuming you mean any supply of water that comes out of the tap, i.e. faucet, sink, hose that is supplied to us through either city or well water? First of all.. Those are municipal water supplies for cities, which is used for drinking, which is metered, and you are charged for use. If its supplied by a well... Well, now you have some freedom. But most well pumps, are not enough to supply you volume AND pressure needed to run a dredge... let alone an 8" dredge. You cant even use a garden hose to run a 2" dredge. I dont see how you're going to get a "tap" supply to power your idea.
If you live somewhere where you can actually tap into a water main without anyone saying "HEY YOU CANT DO THAT!!", id be VERY SURPRISED! But then again, you're from the Western Sahara? So Africa. So that 7-10bar pressure... Is also, more than likely, part of a bigger system with pumps somewhere, which will me monitored in case of leaks(pressure drops) or unchecked pipes. Again, if you can tap into a main, and people dont bat an eye... Then sure.. You might have a chance... But again.. those pipes are probably a LONG way from where you need to get the water to. Water running through pipes creates friction, which equates to loss of pressure over the amount of hose you put down. Quoting "several hundred feet" I went with a nice even number of 500ft of hose. Out of a 3" hose, running 200GPM, your friction loss will be 13.5 PSI of friction loss. at 250gpm at @3"in hose, your friciton loss will be 21psi. So keep even that small equation in mind. Now you're going AT depth, say 15ft DOWN, PLUS now pushing that water through that 8" hose....
I think you need to just bite the bullet and buy an 8" dredge...
But then again... I'm following along to see where this goes :O