Below is from the boatbuilding.community - Marine Sanitation: Fact vs. Folklore
It really IS possible to have a completely odor-free system!--honest!!! You have read or heard, over and over again, that the key to odor control is the hose, that hose permeates with sewage and causes the system to stink. That’s folklore. The key to odor 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, what creates odor and what prevents odor from forming. Once we understood proven sewage management principles and how to apply them to onboard systems, we were able to install systems that are completely odor-free and correct the ones that weren’t. Once you understand it--and it’s so simple!--you can do the same thing.
There are two ways to deal with holding tank
odor: try to reduce it, mask it, and contain it after it’s formed, by using chemicals and filters—which has never proven very successful…or prevent odor from forming in the first place by applying the same principles that are used to balance and maintain sewage treatment ponds. In fact, sewage treatment ponds only stink when they’ve been unbalanced biologically by an overload of chemicals! Here’s how it works:
Sewage contains both aerobic (need oxygen to survive and thrive), and anaerobic bacteria (thrive in an airless environment); neither can function in the other’s environment
. Why is that important? Because only the anaerobic bacteria in sewage produce foul-smelling gasses! Aerobic bacteria break sewage down, as does anaerobic bacteria--but aerobic bacteria do not generate odor. So as long as there is a sufficient supply of air to the tank, and an aerobic bacteria treatment is added to aid that which naturally occurs in sewage, the aerobic bacteria thrive and overpower the anaerobic bacteria, and the system remains odor free.
A bio-active (Iive aerobic bacteria) holding tank treatment such as our own "K.O." works with the aerobic bacteria in sewage, eliminating odor, completely emulsifying solids & paper, and preventing sludge from forming. Enzymes do little if anything--a brief respite from odor immediately after adding them, then odor begins to build again. Chemical products only mask odor with another odor, and they kill not only odor-causing anaerobic bacteria, but beneficial aerobic bacteria as well--not good, because the aerobic bacteria are needed in the system to break down and emulsify solids and paper. Chemicals only break them UP and dissolve them into little tiny particles that settle to the bottom of the tank, along with chemical residue, to become sludge that turns to concrete. Plus, chemicals, unlike bio-active products, are also unwelcome in landside sewage treatment facilities, and are especially unappreciated by those living and working near them!
The bacteria in sewage produce a variety of sulfur monoxides and dioxides (which are the malodorous gasses), methane--which has no odor 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 hose or a holding tank) and builds, suffocating the aerobic bacteria and creating the perfect environment for the anaerobic bacteria to take over. The system literally "turns septic," and the result: a stinking boat…or at least foul gasses out the vent line every time the head
To prevent this, let’s start with the head: the discharge hose, no matter whether it goes overboard
, to a Type I or II MSD, 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 and rinse the hose behind the sewage, that sewage sits in low spots in the hose or bits of it cling to the walls of the hose—getting no air, allowing the anaerobic bacteria to thrive and produce their stinking gasses. If sewage stands in a low spot which gets no air in hose which is susceptible to a high rate of water absorption, it will permeate the hose. This is what has given rise to the myth that the "wrong" hose causes odor. Therefore, as I’ve 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 of most odor, but incomplete flushing
was the real cause.
In the holding tank, the key to odor control is the vent line; it must allow a free exchange of fresh air for the carbon dioxide generated by the sewage. Therefore, those bladder tanks
which have no vent are all but guaranteed to stink; there’s no source of air into them at all. Boat builders, boat owners and boat yard personnel who install holding tanks have always viewed the vent line only as a source of enough air to allow the tank to be pumped out without collapsing and as an exhaust
for methane (Many even believe methane--which in fact is odorless--to be the source of odor.) Some take the attitude that tanks are going to stink so the thing to do is run that vent line as far from people areas—cockpits, sun decks, etc.—as possible, or make the line as small as possible, or install a filter in it. All of the above actually create the very problem you want to solve.
Think of the holding tank as a stuffy room which needs to be aired. You know that even if there isn’t a hint of a breeze outside, just opening a window will allow the fresh air outside to exchange with stuffy air in the room. Open another window for cross-ventilation, and the air exchanges even faster. However, just opening a skylight accomplishes nothing unless there’s also a mechanical means (an "attic fan") of pulling the air up and out--and that won’t work unless another window is open to create airflow. But the only "window" into a holding tank is at the end of a "hallway"--the vent line. If that "hallway" is too narrow and goes around corners, takes a long and curved path, or rises more than 45 degrees above horizontal, no ambient air can find its way to the tank to dissipate and exchange itself with the gasses in it.
Vent the tank with as short, as straight, and as horizontal a line as is possible, with no sags, no arches, and no bends. The minimum I.D of the hose (which is the "standard" size in use today, but for no reason other than being "standard" in fresh water and fuel
tainks) is 5/8"; we recommend that it be at least 3/4". Ideally, it should be no more than 3’ long. If it has to be substantially longer, or if running the vent line uphill more than 45 degrees off horizontal can’t be avoided, or if it’s impossible to run a vent line that does not go around a corner, increase the size of the vent line to 1" or even larger. If, for instance on a sailboat, the line must go up to the deck
, install a second vent line in order to create cross ventilation, or install some mechanical means of forcing air through the tank. We prefer to put holding tanks in the bow of sailboats--under the v-berth--because the hull
just behind the point of the bow is the only place on the hull
except the transom that will never be under water even when the boat is at maximum heel; it’s the perfect place to install vent-line through-hulls, because the though-hull is always into the wind
, forcing air into the vent line, when the boat is underway or on an anchor
. The vent through-hull should not be the same type as a fuel
vent through-hull (a cap with a slit in it), but should be a should be a straight open type through-hull.
On sailboats especially it’s advisable to vent off the top of the tank and not the side, because heeling can cause the contents of a half-full or more tank to run into the vent line. Because a filter blocks the flow of air into the tank, install a vent line filter only as a last resort; the filter does trap the gasses which try to escape through the vent line, but a filter will not stop gasses from forming, and therefore from going back up the inlet hose into the boat or up the outlet hose—and eventually permeating even the best hose.
Check the vent line regularly for blockages; little insects love to build nests in them. And remember--the vent line is not an "overflow!" So try never to overfill the tank; bits of sewage can clog the vent line. Enough air can pass through it to allow the tank to be pumped and gasses to escape, but that doesn’t mean the line is completely clear of any blockage.
Finally, the system, including the tank, should be at least nominally rinsed, through the head or back down the deck
fill—with fresh or salt
water—after each pump-out, and occasionally with fresh water. (If your marina doesn’t have a dock
water hose for this purpose, please ask them to install one. It should be separate from the potable water hose, and the two hoses should never be interchanged.)
We promise: if you install and maintain your system according to what I’ve said here, you will have NO odor! In fact, you can be standing next to the vent line through-hull when the head is flushed --and you won’t even notice it!