With the introduction of new laws
regarding marine sewage disposal, holding tanks are becoming a neccessity. However, they often smell . The good news is they don't have to
smell.
You've probably read or heard that the key to
odour control is the 'right' hose, that the 'wrong' hose permeates with sewage
and causes the system to stink. That's folklore - a little truth coupled with a
lot of misunderstanding.
The real key to odour control is in knowing how to
incorporate proven sewage management principles - the same ones used in
composting and sewage treatment - into the installation of the entire system.
What very few people in the marine industry have learned, and why there is so
much folklore about odour is the very nature of sewage itself and how it breaks
down, what creates odour and what prevents odour from forming. Once we
understand these principles and learn how to apply them to onboard systems, we
are able to install systems that are completely odour free.
There are two ways to deal with holding tank odour: 1. Try to reduce
it, mask it and contain it after it is formed, by using chemicals and filters -
which have never been successful - or 2. Prevent it from forming in the
first place.
Sewage contains both aerobic (needs oxygen) and anaerobic
(functions in an airless environment); neither can function in the other
environment.Why is that important? It is important because only the
anaerobic bacteria in sewage produce foul smelling gases. Aerobic bacteria break
sewage down, as does anaerobic bacteria, but aerobic bacteria do not generate
odour. So as long as there is a sufficient supply of air to the tank and the
aerobic bacteria treatment is added to aid that which normally occurs in sewage, the aerobic bacteria thrive and overpower the
anaerobic bacteria and the system remains odour free.
A bio-active (live
aerobic bacteria) holding tank treatment, works with the
aerobic bacteria in sewage, eliminating odour, completely emulsifying solids and
paper and preventing sludge from forming.
Enzymes do little if anything - a
brief respite from odour immediately after adding them, the odour begins
to build again. Chemical products mask odour with another odour and they kill
not only odour causing anaerobic bacteria, but also beneficial aerobic bacteria
as well. This is not good, because the aerobic bacteria are needed in the
system to break down and emulsify solids and paper.
Otherwise, they only
break up, and dissolve them into tiny particles that settle to the bottom of the
tank, along with chemical residue, to become sludge that turns to concrete.
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 sulphur
monoxides and dioxides (which are malodorous gases), methane - which has no
odour but is flammable - and carbon dioxide, which also has no odour but creates
the environment in which no aerobic bacteria cannot function but allows the
anaerobic bacteria to thrive.
Carbondioxide does not rise or fall; it is
ambient - like the atmosphere, but heavier than air. 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)
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 gases out the
vent line every time the head is flushed.
To prevent this, let's start with
the head. The discharge hose, no matter whether it goes overboard - to a type 1
or 2 MDS, or a holding tank - should be installed, if at 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 will sit in low spots in the hose or bits
of it will cling to the walls of the hose - getting no air,
allowing the anaerobic bacteria to thrive and produce their
stinking gases.
If sewage stands in a low spot which gets no air in a hose
which is susceptible to a high rate of water absorption, it will
permeate the hose. This is what has risen to the myth that the
wrong hose causes odour.
Therefore, it's important to 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
odours, but incomplete flushing was the real cause.
In the holding tank, the key to odour control is the vent
line; it must allow a free exchange of fresh air for the carbon
dioxide generated by sewage. Therefore, those bladder tanks
that have no vents are all but guaranteed to stink; there's no
source of air for 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. Some
take the view that tanks must inevitably stink, so the thing to
do is run that vent line as far from people area - cockpits,
sundecks etc: - as possible, or make the vent line as small as
possible, or install a vent filter in it. All of these 'solutions'
create the 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 breeze
outside, just opening a window will allow,the fresh air
outside to exchange with the 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 (fan) of
pulling the air up and out - and that won't work unless
another window is open to create airflow. The only
window to a holding tank is at the end of the 'hallway' - the
vent line. So if the vent line is too narrow and goes around corners,
takes long and curved paths, or rises more than 450 above
horizontal, no ambient air can find its way. to the tank to
dissipate and exchange itself with the gases in it.
The tank vent lines should be short, as straight and as
horizontal as possible, with no sags, no arches and no bends.
The minimum I.D of the hose is 16 mm. Ideally it should be no
longer than a metre. If it has to be longer or if it is running
uphill more than 45°, or if it must go around a corner etc,
increase the size of the vent line to 25 mm. or even larger.
Preferably, holding tanks are placed in the bow of sailboats -
under the V berth - this will never be underwater even when
the boat is at maximum heel; it's the perfect place to install
a vent line through hulls because the bow is usually into the
wind, forcing air into the vent, when the boat is underway
or at anchor. On sailboats it is advisable to
vent from the top of the tank rather than the side because
heeling can cause the contents of the tank to run into the
vent line.
Because a filter blocks the flow of air into the tank, install
a tank filter only as a last resort; the filter does trap the gases
that try to escape through the vent line, but it will not stop
gases from forming and therefore going back up the inlet
hose into the boat or up the outlet hose - and eventually
permeating the best hose.
Check the vent lines regularly for blockages; little insects
love to build nests in thern. And remember the vent line is
not an overflow. So never try to overfill the tank - and if you
do, back flush the vent line thoroughly with water to remove
any bits of sewage which can clog it.
It's possible for enough air to pass through to allow the
tank to be pumped and gases to escape, but that doesn't
mean the line is completely clear of any blockage. Remember also that the vent mus be clear to allow discharge to a shore facility.
Finally, the system, including the tank, should at least be
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.
This article sourced from the Autumn 2005 VMRAQ magazine.
It is becoming apparent that chemical treatment units offer a better method of handling sewage generated on small craft. Approved on board sewage treatment units for small craft are the Lectrasan, and the Sani-Loo systems. They should be seriously considered as the prefered option.
Did you know. Sewerage is the infrastructure that transports and processes the sewage.