Thank you, Tom...I try to be!
You said, Even though having LOTS of holding tank ventilation seems counterintuitive (more vents "seems" like that might just allow more smell to escape), Peggy is so correct. It reduces the odour, so dont' restrict the air flow in any way.
A short course in bio-physics 101 may be useful here:
When organic material breaks down ANAEROBICALLY (without oxygen), it generates hydrogen sulfide and sulfur dioxide which are not only stinky, but highly toxic, and methane which is odorless but flammable...septic tanks are one example.
But when organic material breaks down AEROBICALLy (with oxygen), it converts to CO2 which is odorless. This explains why compost piles must be tossed frequently to aerate--oxygenate--them (otherwise the material just rots and stinks) and why decorative water elements in office buildings and malls always include fountains and/or waterfalls, why mountain streams never stink but swamps do: running water doesn't stink, stagnant water does (and so do wet dirty bilges and inadequately ventilated waste tanks).
So the key to an odorless holding tank (odor out the vent) is plenty of fresh air. In most cases, it can be supplied via a vent line that's short, straight relatively horizontal and has a larger diameter than the standard 5/8" vent line that's also on fuel and water tanks. But tank location and other factors can make it impossible to have a short, straight, horizontal vent line...in which case, aeration may be the only solution. But effective aeration isn't a "one-size-fits all" solution..it has to be designed to push air throughout the tank.
So while larger vent lines and thru-hulls and/or a system that pushes the gasses in the tank out the vent may seem counter-intuitive when it comes to eliminating odor, it's actually just applying plain ol' bio-physics 101.
All this is explained in a lot more detail in my book, the title of which (my publisher's idea) is a bit misleading 'cuz although it does deal with every source of odor on a boat and how to cure, or better yet PREVENT 'em, it's actually a comprehensive "marine toilets and sanitation systems 101" manual that explains the laws, describes all the types of systems and how they work, and will help you learn how to operate and maintain your system to prevent 99% of problems instead of having to cure 'em. 'Cuz you get to do any preventive maintenance on your terms when it's convenient...the need to cure a problem never happens when it is!
And because no book can answer every possible question, I'll do my best to answer any that it doesn't.
--Peggie
