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.
