Bill Watson wrote:
Does anyone know what the material is that builds up in the sewage lines
(with salt water flushing) on a boat? It is a whitish-grey crust or scale
which seems to build up surprisingly fast.

It's sea water calcium carbonates, maybe with some waste particles that
have been trapped by it as it builds up.

Is there any easy way to remove, or prevent it?

A cupful of undiluted distilled white vinegar flushed through the system
once a week when it can stand for at least several hours, especially
last thing before the boat will sit, will prevent it...and will also
dissolve it, albeit slowly. No need to rinse OR scrub.

If it's a severe buildup that's reduced the diameter of the discharge
line enough to impede flushing, you can dissolve it a lot faster with a
12% solution of muriatic acid (aka "brick cleaner"), available from any
hardware store. Or you can flush a cupful of vinegar daily till it's
done the job. Then you can prevent it in the future with a weekly cupful
of vinegar.

Use ONLY distilled white vinegar...not cider nor wine vinegars. Reason:
white vinegar is distilled from alcohol...cider and wine vinegars are
distilled from fruit juices. They are less acidic--and therefore less
effective at dissolving minerals--and also can leave a sticky residue
that helps to trap salts and mineral particles.

Do NOT leave vinegar sitting in the bowl...for one thing, that doesn't
accomplish anything because the buildup is in the hoses...and for
another, although soft rubber--i.e. the joker valve in the toilet--isn't
harmed by vinegar just passing through it, when soft rubber is left to
sit and soak in vinegar, it swells up and distorts. So pour in a cupful
of it, then flush it ALL the way through the line(s).


--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://shop.sailboatowners.com/boat_odors/