I might be very ignorant here, but I would think that anything that goes down the drain is very biodegradable (it's food). Sure, it's better for the environment if we didn't contaminate the water with food, but I don't think it can compare to the time it takes for plastic or paper to break down.

And the soap/detergent and food remnants being processed at the sewage plant, or the septic pumping truck that cleans out your tank and transports the waste.