Baking soda and
Pour some of each down the drain, watch it foam and froth and eat the gross stuff living in your pipes. Follow with a kettle full of boiling water. I’m not a plumber, but I’d assume it’s safe for both PVC and metal pipes, new and old, since it’s way less caustic than something like Drano.
*ETA: OK, use just the hottest water your tap makes if you have PVC pipes, so they don’t melt, I guess.