Originally Posted by Nos4r2
If it's got petrol in the diesel, see if there's a stationary engine or vintage tractor club near you. They love contaminated fuel for making TVO.
Whereas if it has particulates / bug / water / sludge / crap in there then you will want to find a fuel "polisher" google can probably help, or your local marina may have someone they can point you towards. I think if you search the forum you will find previous references to someone but probably not local to you. Whether that is economical or not will depend on how much fuel you have in the tank...
If you have bug then you need to make sure you keep the fuel dry going forward, and probably add biocide for a while as it will only take one organism to survive and a small amount of water and it will recur.