Your disposal grinds up lots of stuff. A small amount of what you grind up stays behind (chicken, eggs, meat, vegetables, grease). Because disposals are warm, wet, and dark, mold and bacteria begin to breed. Every time you use your sink and grind up more food, the problem compounds. Once the mold and bacteria is established, it blooms regularly. You, as a homeowner, smell that bloom- a huge colony of mold & bacteria.