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Philip,
Vetus and others make a flexible bladder (bag) for black water (holding tank). This could be installed in your system by installing another Y valve in conjunction with your current Y valve. The current Y valve lets you pump overboard or to your holding tank, second Y would let you determine which holding tank you're pumping too. Your bladder can be purchased most supply stores, i.e. West Marine, Boats US etc. in various sizes (14, 18, 26 gals.) depending on which compartment you are giving up for bladder placement.
believe the pump out process would be handled much the same way as is currently done, except when first tank is empty you would have to switch the "new" Y valve to evacuate your second tank Cost for a 18 gal bladder, Y valve and hose should be less than $250.
Good luck on your new project.Fred,
aboard Am Morgenlicht, M-382 #165, Norfolk VA
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