Water to transport and operate the system is included in flush water, yes that is what I am referring . The point being that is not a zero in its volume, indeed many millions of gallons for even a modest size town. Water use for human purposes poses environmental impacts. If you live in the eastern US, water use issues are not a big debate. In contrast, for sourthern California and heck the Carib. Islands, it is a sustainability issue. I suppose we might debate waste stream use versus ag, industry, etc but again the point is it is not devoid of environmental impact. Regarding balance between intake and discharge to what we float in, I find your logic unconvincing. It is an issue not so much of total mass of water in and out but diversion and distribution. In California, diversion of western apline freshwater head stream water through pipes to the toilets of tens of millions of citizens and on out to the coastal estuaries, bypassing the natural catchments, has not been without substantial impacts to many river systems and a variety of species. We could debate the magnitude of such impacts as compared to agriculture in say the Central Valley but the point again is it is not a zero environmental impact.
Glad you agree that halomethanes and the like are a consequence of chlorination albeit under certain conditions the extent can vary. They are toxic, mutagenic and while the dose makes the poison, being privy to the decisions involved in setting allowable limits in discharge permits it is evident that such levels still produce biological response in receiving waters. It is a question of cost benefit, setting limits sustained by best available technology etc. The ecological and human health risk are not zero, but rather acceptable given the scenario under evaluation.. So once more it is not a zero effect and therefore obviates the assumption of zero environmental impact. And we have not even touched on the discharges of pharmaceuticals in the human municipal wastewater stream, the inability for existing tratment to achieve removalof such compounds and theior entry to receiving waters. Again not without environmental consequence and so not zero environmental impact.
While conceding design parameters for waterwater plants vary, one must recognize that throughput in excess of designed capacity either means reduced tratent efficiency or results in the need for expanded infrastructure. Poor nutirent removal, or he construction of more treatment capacity would not be devoid of environmental impact. Again the point is it is not zero. And we still have the issue of CSO's in municipalities that result in a lack of waste water treatment under high precipitation events.
Finally running sewage system requires energy, energy to lift stations, energy to run the batch systems, energy to monitor functions, energy to maintain the system. Good or bad, better or worse these are not zero impacts as well.
I am not trying to make an argument that boat-related human waste composting is "greener" than municipal waterwater treatment post pumpout. I was only observing that the "flush and forget" excuse that there is zero impact associated with ones contribution to wastewater treatment is spurious.
Regarding your questions posed I agree they are pertinent to ascertaining which method is right for one's situation. Ugly, beautiful, smelly or sweet, hard or easy are largely subjective though. Hence my own experiment into how well composting works for me. I got good ideas from this string and quite a few chuckles regarding folks misconceptions of different technologies and their own tastes. But it allhelps me frame my problem formulation and the tests I need to conduct.
I don't have ready access to pump out so it is not as easy for me to go that route vs composting. Your mileage may vary.
Resale of a used boat is so pitifully deflated at this point that flush head, porta potty, or composting would not likely avail one of a significant recoup of investment at this point, and really anyone expecting such return on a boat is probably way too hopeful. Maybe woquestion isbest framed which would have a bigger market pool. Incidentally I have a Thetford recirculating head in my boat at this time. Ha, anyone remember these little beauties? Recirculating chemical laden fecal/urine waste as flush water is a concept I tell ya that the significant other finds tantamount to supergluing live rats to the seat so it is all gravy from tis point forward :-)
A ship in harbor is safe, but that is not what ships are built for.