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18
Holding Tanks – General
Options
Recirculating pump out – often used
for a conversion installation where
the toilet compartment is sufficiently
roomy. The holding tank is usually
integrated with the toilet head, but
with the Lavac used in a recirculat-
ing system the holding tank can be
remote.
Fresh Water Flush with Pump-Out
Holding Tank – probably the most
acceptable system to the user.
Allows for a larger tank installation,
as the tank can be positioned any-
where in relation to the toilet head.
Holding Tank Capacity
If sink and shower waste is also
being contained the holding tank
must be substantially larger. Used
with care, a shower will require
about three gallons each time it is
used. Obviously both sink and
shower waste quantities depend
entirely on how the facilities are
used, as well as the frequency of
use. The size of a holding tank is
entirely dependent on the berth/size
of the boat and anticipated duration
between pump-outs. A holding tank
can never be too large but it can
easily be inconveniently small.
Restriction on size depends on
available space for installation and
effect on the boats trim. The follow-
ing formula will provide an approxi-
mate guide to the minimum size of
tank which is acceptable:
z Berths x days between pump-out
x Y = gallons capacity of tank.
z For recirculating systems factor Y
=0.5
z For fresh water flush systems factor
Y = 2.5
These calculations assume low water
use toilets are being used.
Materials
If a holding tank system is being
considered, some careful thought
must be given to the design and
construction of the tank. Four
materials are commonly used for
holding tank construction,
polypropylene, glass fibre, stain-
less steel and galvanised steel.
Polypropylene – the smooth inter-
nal and external finish of this
material has the benefit of prevent-
ing matter from adhering to the
walls of the tank and at the same
time presenting a clean, trim
appearance.
Glass Fibre – provides consider-
able flexibility in design and is
easy to alter during or after instal-
lation, should the need arise.
Stainless Steel – is strong and
resistant to corrosion but is expen-
sive, in both material and con-
struction.
Galvanised Steel – provides the
cheapest solution but is not totally
corrosion resistant. Once corrosion
starts it spreads rapidly.
Flexible Tanks
Flexible tanks are very easy to
install in retrofit situations. Ideal on
craft where holding facility is sel-
dom used, but two points should
be considered carefully:
(i) If the boat is likely to encounter
much sea movement then flexible
tanks are prone to wear and chaffe
(ii)some materials, although fluid-
tight, will omit odour and a suitable
grade flexible tank must be used.