I am struck by the repeated statements in this thread that dumping your waste down the drain is a zero environmental impact scenario. First, transport, disposal, and treatment takes water, a lot of it. To my miond using treated drinking water to dispose of waste is non-sustainable and not green. Second in situations, and they occur often, where wasterwater treatment effluent requires final chemical treatment to address high coliforme loads, this involves chlorine treatment usually with the attendant production of chlorinated hydrocarbon by products. Not necessarily benign environmental stuff. Third, effiiency of nutrient removal in any treatment plant is a product of volume and holding time, too much of the former can adversely affect the later. Remove your small portion of the stream and you might be striking a small blow for staving off millions in facility improvements (I say this with tongue incheek, but hey as long as we are arguing here). Further, in many municipalities there still exist CSO systems. When it rains, raw sewage still enters waterways so your $4it may not end up where you think it is going. Finally, just where do you think your solids waste ends up? Well, after aerobic treatment, composting, and drying it is likely put back on the land, on crop land actually, oh and in the home Depot soil ammendment you put in your garden. Conversely I imagine there are still facilities that incinerate it...green as well?
I'm going to experiment with the compost approach at home first. If it passes the "gross test" with the significant other (and I mean that can be one tough test) I will construct a unit for my weekend boat. If my experience there proves viable for my lifestyle I will explore a commercial unit for longer term use aboard.
A ship in harbor is safe, but that is not what ships are built for.