Clearly we are over-simplifying something here, or we would see calcium build-up on the shoreline.
The mineral buildup in your hoses is not a mix of uric acid and water...the minerals are salt and sea water calcium carbonate, and can occur in ALL raw water intake hoses, not just sanitation hoses...so check your engine intake and exhaust hoses and clean them out if necessary. The mineral content in warm sea water is much higher than it is in cold water, which means that buildup will occur much faster in warm waters than in cold.
Muriatic acid will dissolve it...and distilled white vinegar WILL prevent it when used correctly. Use ONLY distilled white vinegar, never cider or any other kind...white vinegar is made from distilled alcohol and is more acidic--and therefore more effective--vinegars made from fermented fruit juices. A cupful flushed through the hoses once a week before the boat will sit will prevent build up. Do NOT leave it sitting in the bowl...that accomplishes nothing except a ruined joker valve. Geoff says he uses vinegar "regularly," but doesn't say how often 'regularly' is, which type of vinegar, or how long it stays in the system before toilet use flushes it out. But used correctly, it DOES work!
Geoff, your friend is right that fresh water does eliiminate SEA water buildup in hoses...although some fresh water has a high mineral content that can result in "scale"...but much more slowly. However, I hope your friend has not connected a sea water toilet to his fresh water plumbing, 'cuz that's a BIG no-no! It cannot be done without risk of e-coli contamination of the fresh water, damage to the toilet pump or both...and EVERY toilet mfr. specifically warns against it in their installation instructions. There are only two safe ways to supply fresh water to a sea water toilet: a separate dedicated flush water tank that shares no plumbing with the fresh water system, or the head intake line connected to a sink drain. (I'm not sure exactly what sailingdog means by "the sink acts as a separator"....it's considerably more simple than that: sink drains are not connected to the potable water supply.)
While it's not really necessary to use fresh water to eliminate sea water mineral buildup, nor replace hoses to clean it out (beating 'em on the dock will get rid of 99% of it), does have the added benefit of eliminating sea water odor problems. But just flushing all the sea water out before the boat will sit solves that problem and the correct use of white vinegar will prevent the buildup.
Visit the Salton Sea and you will certainly see some salt on the shoreline, but even there, not carbonates to any great extent. The only carbonates are those gathered by the myriad brine shrimp.
As for the raw water intake, at some point in the engine there be enough heat to take the calcium concentration above the scale point (calcium carbonate and calcium silicate are less soluable when hot), but that is fairly hot and generally indicates a design flaw. I work around cooling water systems, and if we are calcifiying lines, it is generally because we have let something get too hot through a design error. Heat is not a factor on the head lines.
In the discharge lines we have added additional chemistry, including uric and phosphate ions and a pH change. These are far less soluable than carbonate ions, and though I have not checke the pH of black water, that is an additional factor as we have pointed out; it takes strong acid to disolve deposites, but only very slightly acidic conditions to prevent the start.
But why would the inside of a cold intake hose build-up? Seems to me there must be a biological growth component here, because there is no chemisrty in the line that does not exist in the ocean. This seems obvious - there is no anti-fouling on the inside of the hoses. The only thing keeping growth in check is that, with the seacock closed, there is no oxygen, sunlight, or food supluy. Why dosen't the glass in a salt water aquarium collect calcium deposits?
On a different note, has anyone ever tried teflon-lined chemical hose? Though expensive, the stuff is very durable, should resits this sort of fouling by its shear sliperiness, and is quite odor-proof. There are reasons it is used extensively in the chemical trade. Other weaknesses are stiffness and difficulty in fitting, but these are managable for seldom-moved hose.
