Paint thinner, or mineral spirits, is commonly used to clean oil-based paints and stains from brushes and tools. Most people dispose of the thinner after just one use, but that's wasteful and unnecessary.
Next time, soak the brushes clean, then allow the dirty solvent to sit overnight. The paint sludge and pigment solids will settle to the bottom of the jar, leaving a layer of clear thinner on top. Gently pour the clear thinner into a second clean jar and reseal it for future use.
Seal the jar of paint sludge and save it for the next paint cleanup. When it's full, bring it to a hazardous-waste-disposal site or similar municipal facility. Never pour solvents or paint sludge down a sink drain or into a street gutter.