Maybe it wasn't waste, and just all that old sludge getting flushed out.
If it dumped in your bilge, something is wrong. It should have first flowed out the vent, then when that plugged up, overflowed the deck fitting. It's possible that the water pressure blew the vent line off it's fitting, or that you have another line disconnected somewhere. Most of these boats are plumbed with an overboard discharge mascerator. You're just going to have to trace out the lines, and find the break.
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