Thank you Peggy and Keith,

I have the toilet removed so I have a larger hole for access. I have
been trying to get the sludge to break up. I have pumped a lot of
water into and out of the tank but quite a bit of sludge still
remains. I'll keep trying.

A note to builders: Do not position your washdown raw water intake
seacock close to your holding tank discharge seacock.

I am trying to convince myself to cut off the waste pump holding tank
diptube inside the tank to shorten it. I would be able to barely reach
it inside the tank through the toilet hole. I think it is now about 1"
above the bottom of the tank. Should it be maybe 2"? The tank is
plywood/epoxy/glass and the diptube is glued in PVC.

I had been using some enzyme based additives to try and break up the
sludge, including the KO brand. Perhaps I should have been using it
from the very first when the tank started seeing use about a year ago
but it doesn't seem to do much now. The tank has a 1 1/2" vent.

One thing I did do last night was to plumb in a connection to the
waste pump suction line from the washdown system. Using a Y valve I
can send raw water directly through the pump to rinse it out, it seems
to work great.

gaffcat wrote:

A note to builders: Do not position your washdown raw water intake
seacock close to your holding tank discharge seacock.

Or at least put it forward of the tank discharge. Same goes for toilet
intake thru-hull, btw

I am trying to convince myself to cut off the waste pump holding tank
diptube inside the tank to shorten it.

You don't want to do that. It should be about 3/4" off the bottom, cut
at a about 25 degree angle. Any shorter leaves too much in the tank. Cut
flat instead of at an angle and any undissolved TP or sludge can clog it.

I had been using some enzyme based additives to try and break up the
sludge, including the KO brand.

If you've used K.O. (which, btw, is live bacteria, not just
enzymes...enzymes alone are practically useless) according to
directions--which include flushing out the tank regularly--it's unlikely
your clog is sludge. If you've only used it to try to break up the clog,
you may not have used enough or may not have given it enough time to
work. It'll take a lot more of it to dissolve a sludge clog than for
regular tank maintenance...and it takes time to work--at least 48 hours.

Perhaps I should have been using it
from the very first when the tank started seeing use about a year ago
but it doesn't seem to do much now. The tank has a 1 1/2" vent.

Off the wall thought: have you checked your tank vent for a blockage?
'Cuz if the vent is blocked, it'll create a vacuum that'll definitely
prevent the pumpout from pulling more than about a gallon out, and could
prevent the tank from draining...like when you hold your finger over the
end of a straw in a glass of water and take the straw out of the water,
the water stays in the straw till you remove your finger so that air can
get into it to replace the water draining out. However, if you're able
to pump out, a blocked vent is unlikely. Still worth checking,
though...the two most common places for a vent blockage to occur are the
vent thru-hull and the connection to the tank--both the fitting and that
end of the vent line.

One thing I did do last night was to plumb in a connection to the
waste pump suction line from the washdown system. Using a Y valve I
can send raw water directly through the pump to rinse it out, it seems
to work great.

If it can generate enough pressure, you may be able to break up whatever
is clogging the line.

--
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/books...ku=90&cat=1304