It really is possible to have a completely odour free system! ……………..honest !
You may have read or heard, that the key to odour control is the hose, that hose permeates with sewage and causes the system to smell which leads to the kind of advice about hose we mentioned above. That’s a half-truth. The key to odour control is in the installation of the entire system. What very few people in the marine industry have learned is the very nature of sewage itself and how it breaks down. Once you understand it and it’s so simple, you can do the same thing.
First of all, a marine holding tank must be exactly the opposite of a septic tank. Holding tanks stink when they become “septic tanks.” Why does a septic tank stink? Because it is in an airless environment. So what keeps a holding tank from becoming a ‘septic tank’? A sufficient supply of fresh air, in and out of the tank.
Sewage contains two type of bacteria: aerobic bacteria (which need oxygen), and anaerobic bacteria which thrive in an airless environment; in fact anaerobic bacteria can’t survive in an aerobic environment, why is that important? Because it’s the anaerobic bacteria in sewage which produce the foul-smelling gasses; the aerobic bacteria break sewage down, but do not generate odour. So as long as there is a sufficient supply of air to the tank, the aerobic bacteria thrive and overpower the anaerobic bacteria, and the system remains odor free.
The anaerobic bacteria in sewage produce a variety of sulfur monoxides and dioxides (which are the malodorous gasses), methane, which has no odour but is flammable, and carbon dioxide which also has no odor but creates the environment in which the aerobic bacteria cannot live, but the anaerobic bacteria thrive. Carbon dioxide does not rise or fall, it is ambient, like the atmosphere. Without a sufficient flow of fresh air through the tank to allow it to dissipate, it simply lies like a blanket on top of any pool of sewage (whether inside a hose or a holding tank), suffocating the aerobic bacteria and creating the perfect environment for the anaerobic bacteria to take over. The system becomes “septic,” and the result is a smelly boat.
To prevent this, let’s start with the head: If at all possible, the discharge hose, no matter whether it goes overboard, or to a holding tank, should be installed, if at all possible, with no sags or low places where sewage can stand. When a marine head is not flushed sufficiently to clear the hose of sewage, that sewage sits in low spots in the hose. Bits of it cling to the walls of the hose. With no fresh air present the anaerobic bacteria thrive and create their stinking gasses. If sewage stands in a low spot, which gets no air, the smell will eventually permeate the hose. This is what has given rise to the myth that the “wrong” hose causes odour. Therefore, as we have already said, flush your head thoroughly enough to clear the entire hose of sewage and rinse behind it. And when you leave your boat to go home, flush the head thoroughly one last time, this time with fresh water. Until holding tanks came along, the hose was the source – but not the real cause of most odour.