I dunno, you really should check with CBOW (Composting Board of Wackos) and verify its certifiably OK to put something so daring in to your Tumbler. You also should get a permit as to when you will be adding the rinds. Pictures and documentation would also be helpful.....NAH just kidding, go ahead...if you read about some of the things I compost, you will find in comparison you might be a Saint.

The Joy of Pickling has 3 watermelon rind recipes. One is really sweet made with dark brown sugar, cinnamon and cloves. The ingredient list for minty watermelon pickles sounds interesting but it starts by soaking the watermelon in pickling lime which I don't want to get into. The first one is gingery watermelon pickles and it sounds pretty good but still has plenty of sugar: lemons, cinnamon stick, cardamon seeds, whole cloves, allspice berries, fresh ginger, vinegar, sugar (after being brined).

I've made this. I'm not a big fan of cinnamon, but the texture was just right. Next batch I put in my fav spices - cloves and nutmeg. I liked it a lot! I also made watermelon jelly and salted watermelon jelly which everyone loves, including me, from the fruit part, finding that the pulp nearest the seeds was the tastiest. I have a fellow farmer friend who gives me his recently cracked watermelons and I make some jelly for him in return. Nancy

If it hadn't been used before then I wouldn't worry about bacterial contamination (or possible dilution from water seeping out of the first batch of rind, which is what you were getting at when you asked about acidity ). It should have been heated (per the recipe) before adding to the rest, not added straight from the fridge b/c it would cool off the hot brine, but it's already done, if you stirred it in well I'm sure it was fine and the recipe does call for refrigerating overnight anyway. I don't know how much spilled, if 1C was enough to replace, if you haven't boiled the rind yet today you might want to borrow 3C of sugar from a neighbor and mix it with 1C of water and 1C of vinegar (1/3 recipe) to make sure you have enough to can. Worst case, if you find you don't have enough syrup to cover, just process what you can and stick the rest in the fridge, it should last until you can get some more sugar and then just make refrigerator pickle version out of it.