Looks like we (W.Aus) have it easier than other jurisdictions. No raw sewerage zones in typical places (marinas, most inland waterways etc), and not within 500m of land or 100m from other boats or person in the water.

From my reading, human faecal matter breaks down very quickly in salt water.  Whether this is because e.coli and its mates doesn't like a saline environment or due to bacteriophageous elements in the water - or to both - I'm not sure.  But when human faecal matter is mixed with fresh water, as with the usual Western toilet, the bacteria have a field day.

Now obviously, in the enclosed waters of a marina, the harmful bacteria may well be introduced in quantities too great for the natural system to cope with, as happens around fish farms, but I cannot see any harm in discharging faecal matter in a tidal mooring or anchorage, where there aren't many other occupied boats, apart from the aesthetics of it all.

As for urine, surely no-one would think it is other than harmless.

I think that those jurisdictions that control boats' overboard discharges except (and accept for that matter!) a 'bucket and chuck it'.  Don't ask me why.  In NZ we have a rather more practical approach to the use of a marine toilet - don't empty it within 500 metres of the shoreline or aquaculture.  So the environmental aspect of using a marine toilet is muddied, so to speak, with poor biology.  On the other hand, I'm perfectly willing to be set right on this one, by someone who knows more about it.  But I have heard - and of course it may be apocryphal: maybe QLD Gary could find out more - that a yachtsman in QLD successfully defended himself against a charge of illegal discharge by convincing the judge with the science.

Grey water is, IMHO, even less of an issue.  Don't forget we are talking anyway about tiny quantities from most boats, compared with houses.  If you use plant-based soaps, I can't see that you are doing any harm at all.  In some countries you are actively encouraged to re-cycle your grey water into your garden to grow veg!

The third aspect is the one of plumbing and holes in the hull.  Getting rid of both of these makes sense and so some sort of on-board 'sewage system' can only be a good thing.  I have to say that for myself, who feels the pollution is over-rated and the attitude towards pollution from yachts verges on the hysterical, the draconian US rules have had the advantage of encouraging a lot of clever people to update the 'bucket and chuck it' to something acceptable to most parties and appropriate to most boats.  May their improvements continue and their profits sky-rocket!

These, of course, are my personal prejudices backed up by carefully-edited data and I stand to be corrected.

Yes, I think that to try and put the argument that composting toilets on boats are a means towards saving the environment is pushing things a bit too far but there cannot help be some environmental benefits if we are not dumping our toilet waste directly into our pristine coastal waters. We recently undertook a survey of 1000 odd boat owners (moored boats) in the area where I am Harbourmaster and we found that only slightly over 50 percent of the boats had holding tanks so here in my part of the world there are a lot of boats dumping toilet waste into the shallow waters of our coastal anchorages. At least with a composting toilet we are holding that waste either for dumping on shore, or at sea away from the coast, by which time a lot of the waste is well on the way to becoming humus. Also with the Airhead we get at least a month of holding capacity whereas most holding tank systems only provide several days. Also just about every holding tank I have come across has some odour issues. I don't think that dumping urine over the side has any negative effects on the coastal environment.

The real reason for installing the Airhead toilet in Footprints was to find a practical marine toilet which suited our cruising needs. Our chemical toilet was good but not really suited to 3 people been on board for any length of time. Because of my experiences with normal marine heads, and holding tanks, I just did not want one of those in Footprints. Not the least reason was not wanting hoses, through hulls that can leak, and a toilet which will eventually start leaking. We have had composting toilet systems  in each of the two houses we have owned over the last 18 years so a composting toilet seemed the best way to go for Footprints. We are very happy with the Airhead, the Naturehead I am sure is a good product as well, and the C Head looks to be a good lower cost option but without the holding capacity of the other two. I am sure that with a bit of creative engineering anyone could cobble together some sort of composting toilet for a boat. As well as being well put together and hopefully looking a bit attractive there will need to be a way of separating out the uring from the solids, and a ventilation fan fitted to the toilet to take any odours to the outside is also a must. I would like to think that the peat moss we use is harvested in an environmentally friendly manner but I am not sure, however we do not use very much. As has been discussed there are other alternatives to peat moss, although peat moss has the benefits that it is light weight, . It is very good at absorbing moisture, and it is good at covering the solid dumpings and masking odours.

We have used various composting toilets in land based environments since the 1970's. Our more recent homemade unit is based on Joe Jenkins Humanure, http://humanurehandbook.com/, process and works quite well. While it is a a full cycle process it is not compatible with a boat unless you are close to a land base where composting the end result is possible. The process emphasizes the use of partially composted sawdust from industrial sources. 

Annie Hill wrote:Sphagnum moss and peat are essentially the same thing.  They both come from bogs (wetlands) that have taken thousands of years to evolve.  The peat industry claims that it can restore the bogs after they've 'harvested' them, but like so many of these claims, this is greenwash.  The original bogs are a complex eco-system that can't be engineered anew.

Thank you Annie, I was about to point that out but you beat me to it.

I've been using a composting head since 1997, at first using a self made system that was just a ss bucket (with seat) and you just added saw dust after the offering was made (you just need to cover it, no more). Liquids went straight over the side or into a bottle and then over the side as circumstances permitted. The system worked very well with little or no smell, even in the Caribbean (I cruised the Caribbean for nearly three years). Incidentally, any saw dust seems to work as long as it is not from treated wood. The treatments that stops the wood from rotting also stops it from composting.

Now however I'm giving myself a treat and have installed a Natures Head in LC. The reason for choiceing the Natures Head over the Air Head was that the seat of the Natures Head is built in. I've had to repair far to many head seats so when I saw that built in, was how it was done on the Natures Head, that was the one for me.