Holding tank venting

[nickbrown]

Hi, I am going to be increasing the size of the holding tank capacity on our boat. It is a Nordic Tug 32.

The existing tank is glassed into the keel (about 30 gallons). As there is no easy way of getting to this and adding more capacity we plan to add another tank above it of about 25-30 gallons that will sit about 12 inches above the top of the existing one and off to the side on a shelf.

This new tank will be connected via hose to the lower tank so the idea is once the existing tank fills up the new tank will be the overflow for the additional 30 gallons. The head is a Vacuflush and the pump unit will be roughly at the height of the top of the new tank.

So, 2 questions

1) Does anyone see any issues with this set up.

2) I'm trying to aerate the new set up as much as possible. I was thinking about installing 2 vents, hopefully cross vented but it may be one side vent and one roof vent and probably 1 inch vents to maximise the air flow. My concern is with having a lower tank in the keel that is plumbed into the top tank and only the top tank is vented. Will having large cross vents in the top tank be enough to aerate the lower tank?

Many thanks!