Having seen loo pipes almost completely blocked by calcification, we have
always been ultra careful in flushing well after use. Until we had a
holding tank, the annual pipe inspection was more for peace of mind than one
of necessity. Deposits were usually minimal and cleaning simply by vinegar
solution and banging or flexing the pipes.

With a holding tank it is rather different. When in Mediterranean bays, or
harbours with remote loos, we can be up to 4 days or so with nasties
remaining in the pipes below the tank. Calcification then does become a
real problem. This year we had to do a cleaning job halfway through the
season. More serious, the outlet seacock also was affected. It could only
be opened halfway. I managed by some careful scraping and use of vinegar to
improve matters but it still did not open completely. By the time we got
back to Barcelona the seacock was only opening about one third of normal.

What should we do? How can the seacock be cleaned? Would we need a lift
and hold to change it? And the pipes? What must the inside of the tank be
like? There is no inspection hatch.

Jennifer (ex-catering lecturer) and Margaret (ex-Reader at the London
School of Pharmacy) put their intellectual and practical heads together.
Verdict - try a decalcification treatment, such as a kettle cleaner.

We found the necessary product in Barcelona, produced by a German firm for
cleaning water tanks in hard water areas, coffee percolators, water heaters
"und alle anderen Probleme mit Kalk". With the seacock closed but the
valve to the holding tank open, we filled up the system with the solution
and left it for about 6 hours except for draining a little off every hour or
so to get fresh fluid in contact with the pipes and the seacock. The
results were truly amazing. The seacock opened fully for the first time
since the start of the season and the pipes were squeaky clean.

The agent we used is based on citric acid. Margaret says that, at the
recommended strength, this would be sufficiently strong to deal with the
calc but, over the period of the treatment, not strong enough to damage the
pipes or the stainless steel and phosphor bronze of the tank and seacock.
We did flush the system well afterwards with fresh water. The whole process
is made easy with a deck pump out facility which I would encourage anyone
installing a tank to fit. To have to pump 60 or more litres into the tank
from the WC bowl would be a pain in the butt, to borrow a not inappropriate
phrase from our American cousins!