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.
... their answers. Heck, we're not paid for this. Libraries and more technical on-line sites can be better.
Assuming we are talking about a modern advanced wastewater treatment plant (some aren't and some discharge without treatment, but that's a very different discussion), there are few statements above deserving either discussion or correction.
* "Transport, disposal, and treatment takes water, a lot of it." Basically none. It is also basically illegal to add significant amounts of freshwater to wastewater for dilution or during treatment; check your local city code. I've designed and operated wastewater plants and we never used measurable amounts of city water. Only for drinking.
* "To my mind using treated drinking water to dispose of waste is non-sustainable and not green." Pure fallacy. Perhaps you are thinking of the flush water. But we flush with the water we are floating in and return it days later, so there is no water usage. The water cycle has been sustained for billions of years.
* "... 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." Part truth. With good treatment very little chlorine is used, only a few parts per million. Yes, there are halocarbons formed-low parts per billion--and the difference in what is acceptable discharged into the ocean and into drinking water is huge. These products will degrade in time; they are not durable molecules, like PCBs or such.
* Nutrient removal is not so much related to time as food:mass ratio and other design parameters. Asking "how long does it take waste to degrade" is like asking "how long does a bag of dog food last"; it depends a lot on the dogs. Plants are designed to reduce the time by recycling the best bacteria. Unless you understand the specific design very well, you cannot correlate time to effectiveness. A POTW can do in hours what takes nature many months.
I have no problem with composting. Love it and there is no problem where people have the space and take sufficient care. But let's not exaggerate the negatives of POTW disposal. I could find fault in composting, but I won't try. That only leads to exaggerated finger pointing on both sides.
To determine if a "dry" toilet is better, the real questions are these:
* Is it easier to pump out or to compost? The answer will be different for each boat and sailor and will vary regionally. No single answer.
* Is it cheaper? Generally a dry system is cheaper, it seems, though not so different if we compare equal quality. Very minor, compared to the boat.
* Is there less maintenance? IMHO, no. I have seen both and they are simply different. Both are easy once you understand them.
* Is it more odor free? Both can be acceptable and both can be bad. IMHO, MSD systems can reach zero odor, like home. dry systems are not quite there. That won't be acceptable to all.
* Yuck factor. It's not just you, it's the wife and family and friends. I'm skipping the details, but I've been told "hell no" by most people.
* Resale value. I doubt it will help, unless you find just the right buyer. Generally it will hurt. That is what the yacht broker told me. The bigger the boat, the greater the harm.
I would seriously consider a dry head for a trailer boat or small boat near home. Other wise....