I'm assuming the barrels are clean? Since DEF is urea & water, any residue should wash out pretty easily. I'm not sure what other additives might be in the solution, but as long as the barrels are rinsed well, they should be OK.
Here's another Don on TEG from quite a few years ago.
Most of his photos are no longer working. He must have been using a hosting service and his account is no longer active. Don's last thread was how he had everything set up for a new gardening season and that was the last we heard from him.
He had a nice set-up with plastic half-barrels TEG LINK .