I did replace it not too long ago, thankfully. You are right about bacteria count. As soon as I had more or less mopped up, I went to the bathroom to wash my feet and legs with soap and warm water. I had exactly the same thoughtI wonder what the bacteria count was like. I bet none of us ever put bleach in that bucket of warm water that hangs around for great periods of time. Just to add to the level of yuck.
I have a slack bucket next to my forge, which I tip out regulalry.I wonder what the respective levels of yuck are for grinder buckets and quench tanks. I shudder to think how much ferric chloride, baking soda, borax, and other crap have accumulated from having been added to it over time to the scale, etc., in my quench bucket. I tend to just top it off as needed rather than dump and re-fill, so it's probably getting pretty concentrated.
This spring I left the quench tank uncovered for a few days, so I'm sure there's a healthy dose of mosquito larvae (and remaining bits thereof) thanks to the biblical quantities of mosquitoes here this year. At least that's the best explanation I've come up with for the swarms in the shop.
I suspect in my shop at least, the grinder bucket is relatively inert compared to the quench which may well have new life forms developing.