I did quite a bit of marketing work for a composting toilet mfr. a few years ago. One of the things that I found was that the by-product (regardless of whether the product really works or not) is not compost, but classified by the EPA as the same as sludge. Whether you agree with that or not, whoever ultimately is left holding the bag cannot legally throw it in the garbage and pat themselves on the back. You also cannot spread it on your garden, but most users do this.

As to the composting marine toilets, my understanding of the process is that the way big units work is they separate the urine from the shit and air dry the solid matter. If you don’t get enough air through it, you get pathogens and smell. So any boat unit is going to require lots of air through it – some commercial units use a fan and a heating coil. Regardless, the vent pipe, even on fully functioning units is always way above the nose line, because shit stinks and composting shit really stinks. The reason working units don’t smell at the seat level is because of the downdraft air circulating, pulling air over the seat downwards – the first test I have seen technicians use on units is to put a piece of toilet paper over the seat to see if it pulls down. Also, on the big units, someone has to rake the pile in order to prevent coning and also to stir things up.

Most commercial units get emptied once every few months by septic trucks that take the compost to the sewer plant where it is processed.