There is so much food that half of it is easily not finished and thrown. It is either thrown out as trash or digested.
Everything that goes into the canyon must come out of the canyon. It is all about preserving the natural state of the canyon. I respect that. Better explained, if you pee in a bush, the sun bakes the urine, thus causing a bad smell and destruction of animals and plants.
Liquid waste goes into the river. Everything from toothbrush water, water that is not drunk, and, well, ALL liquid waste. A porter potty is setup for solid waste. Normally, the facility is setup so that a view provides a moving experience.
My only complaint was having to climb into the river when it is 50 degrees. The best part of the trip was the hike up to a waterfall and standing under warm water that was as close to a shower as I was going to get. At the end of the trip, I greatly appreciate the luxury of running water.