For years, before anyone raised the red flag, guitar players (and anyone else that used any sort of battery) simply tossed them in the trash when they went dead (and back in the 1950s and ’60s, that was quite often, as the older type zinc-carbon and even the earliest alkaline batteries really did not hold a charge very well). We now know that batteries must be dealt with as toxic materials, and most cities have drop-off points where such materials may be taken before ultimately being destroyed or recycled in an ecologically sound manner. We strongly urge all battery users to properly dispose of dead batteries. A better alternative is to use rechargeable nickel metal-hydride (NiMH) batteries, which may be recharged thousands of times, or to simply buy a 9-volt adapter. There are special 9-volt adapters that will power up to 20 stompboxes, but use only one outlet in your power strip!
Bonus tip: Old 9-volt batteries make great sanding blocks for tight spaces, such as guitar fingerboards.