A boat toilet ( called 'heads') uses seawater to flush the waste overboard or into a holding tank. Over time there is a serious "furring up" of the waste pipes - not the inlet pipes - which implies that there are precipitation salts produced from the reaction of the seawater with, presumably, urine. It is definitely hard and crystalline in structure and can be 'chipped' away with a screwdriver. The colour is a grey-brown. In order to find a way of removing the build up of salats I need to know what the likely composition is! Is there anyone here who has the chemistry knowledge to explain what reaction is ocurring and thus the most suitable chemical to re-dissolve them. By the way - I would imagine that if the system was properly flushed each time, then only salt water would remain in the waste outlet pipe and furring up would not ocurr in the same way as it doesnt ocurr in the inlets. Unfortunately people are lazy when 'pumping out' and I suspect that what gets left is a dilute mixture of urine and seawater. I dont actually know if the pipe remains full or not; if not I guess it could simply be evaporation salts from the urine?