I posted a while back:
I'm having a stinky kitchen drain problem. It's a double bowl kitchen
sink with a food disposer on one side. The black ABS drain pipes are
gunked up with rot/slime. I've already tried flushing with bleach. but
that doesn't do the job.
I found a very simple solution. I used a long flexible bottle brush
dipped in bleach to scrub down the sides of the drain without the
disposer, and then bent the brush into a U to scrub down the sides of
the drain with the disposer (to get under the disposal's rubber rim).
Worked like a charm. Of course, it took several dippings/brushings.