I want to isolate DNA from the bacterias (both Gram positive and negative bacterias) in drinking water. Do you have any protocols which is good for this purpose? THank you very much!

Without enrichments? And to what purpose (that is, how much yield/purity do you need)? if you just want to make some PCRs on it, it might be sufficient to concentrate the water (e.g. using filters) then take the concentrate and make a general lysis (e.g. cooking, bead beating....). Otherwise you might need a cleanup with phenol. Alternatively there are some kits, but you need quite a lot of bacteria to get enough DNA (otherwise they just get lost in the columns).