After many years of trying to stop the bad odour from my aft holding tank, I discovered that Nautor connect the holding tank breather to the grey water tank breather and pass the combination through a hole on the hull at deck level.

The problem with this is that the holding tank odour, can seep through to the grey water tank and permeate the cabin. Or worse if the holding tank fills some of the overflow can end up in the grey water tank – YUK

I think this is a design fault, although the same system applies to my forward holding tank and grey water tank, and there is no Odour. Perhaps this is because the fwd holding tank is bigger and does not overflow?

I have made separate holes in the hull for each breather/overflow pipe in the aft.