Originally Posted by ZULU40
these are not unrelated topics, they are the same topic
Yes (mineral deposits and other inputs to a boat's sewerage system are related). And no (they are not the same topic).
Consider the problem of limescale deposits in other parts
of the boat
, such as:
* limescale deposits in engine heat exchanger
* limescale deposits in shaft tubes, such as associated with shaft seals
and shaft bearings (e.g. cutless bearings);
* limescale deposits in exhaust
* limescale deposits around external (to the hull) heat exchangers;
* limescale deposits on the external surfaces of portlights
* limescale deposits on metal fixtures (e.g. rudder
What these have in common with mineral deposits in the sewerage system is one important factor:
* seawater is saturated or supersaturated with calcium ions and others calcium anions (including calcium carbonates, calcium sulfates, calcium magnesium carbonates) and with high concentrations of a few others (such as calcium phosphates).
That supersaturation with calcium explains why sea animals
(everything from protozoa to molluscs and crustaceans) can very easily make shells and internal objects from calcium carbonate and/or calcium magnesium carbonate (with other inorganic and organic components).
That supersaturation explains why the first precipitation from seawater is calcium carbonate and calcium magnesium carbonate, not sodium chloride. Try it for yourself: get a cup of seawater. Let it evaporate (raise the temperature to speed the process).
Even before the water level has dropped 50% you'll see whitish deposits on the wall of the cup. You'll see it on your stanchions, your portlights
Only when about 75% of the seawater has evaporated from your cup will you find NaCl crystals (sure, you'll find salt
crystals on your stanchions and portlights too, but long after they're coated with a haze from CaMgCO3).
Take seawater and warm it. Such as in an engine heat exchanger
. Or your shaft tube after you've anchored or docked, as the prop shaft cools down. Mineral deposits, those same CaMg-carbonate deposits start forming because the solubility of calcium carbonates falls with an increase in temperature.
For that matter, landlubbers who use freshwater with a moderate calcium level will have noted that the heating
elements of some kettles and water heaters get coated with limescale. Same reason. Sidenote: and if you happen to live with a low level of calcium dissolved in your drinking water
, you're at higher risk of heart disease and so on!
Add some urine to the seawater. Your kidneys are excreting some calcium each and every day. That extra calcium, plus other chemicals in urine, add to the supersaturated calcium carbonate load of the seawater.
Let the urine and seawater solution sit around for a while. Bacteria (in the seawater and in your urine. Back in the 1950s people used to think the urine of human males was sterile (in contrast to the urine of a female, with shorter urethra. But that myth was exploded once microbiologists realised that they were just trying to grow urinary bacteria in conditions in which those bacters were not happy) grow. And cause chemical changes that cement the calcium magnesium carbonate into a concrete, with inclusions of uric acid.
Add some human faeces. About 70% bacteria (by dry mass). More bacteria and more nutrients for other bacteria that add to the concrete deposits.
It's that concrete, that mix of calcium magnesium carbonates + uric acid + chelates with lots of other inorganic and organic chemicals, which is the mineral deposit. And that concrete can resist low pH, or is a lot more resistant than what you see when you drop HCl and especially a weaker organic acid such as acetic acid on pure calcium carbonate.
So yes, it's all related.
But no, the limescale deposits can occur without your urine and faeces.
The urine is big contributor to the problem. The faeces less so.
Originally Posted by ZULU40
I would contend that the subject causes had evolved, and that adequate flushing
is a reasonable fix.
is better than inadequate flushing, by definition. One question is whether any level of flushing with seawater is adequate in the long run to prevent mineral deposits.
Flushing with freshwater is not a bad idea. For boats that have regular docking
, flushing with freshwater so the sewerage system is left soaking in freshwater is a good thing. But while cruising or otherwise being away from cheap
and abundant freshwater, there's a problem.
If you talk to people who deal with urinals in public toilets, you'll find that flushing human male urine is not that simple even with freshwater unless the freshwater is cheap
Urinals are however a step forward. The lack of separation of urine from faeces in the design of water closets was probably an error. If the urine is kept separate, reclaiming phosphates is relatively easy.
So excluding urine from a boat
sewerage systems is not a bad idea. Hard to accomplish in some cultural and social settings.