Black water holding tank

[gatehopper]

A few summers ago I plumbed all my grey water through the shower sump thus eliminating the need for the 70 gallon grey water holding tank. I would now like to plumb the black water into that tank and eliminate that 40 gallon holding tank completely. Does anyone see any reasons I should not attemp that project. The boat is my 1986. 3297 mariner
Thank you

[g36]

Never had a boat with gray water holding tank only black and fresh water tanks. I have a friend that has a mariner of that age he has 2 fresh water tanks and a black water tank no gray water one. If you sure what you have the tank materials are probably the same so that wouldn't matter but running lines for the pump out might be a challenge if it's hard to access.

[gatehopper]

I believe the second 70 gallon tank in most carver mariners are grey water tanks as a great number of them are in fresh water lakes where it is illegal to dump grey water. Mine has a pump out outlet at the bow aft of the black water outlet. I think it should be easy to change it the black water holding as long as the tank is an appropriate vinyl.

[mjk1040]

We had the same boat, how are you going to pump the black water up to the gray water tank? The black water tank is below the head and is plumbed into the top of the tank, you will have to plumb the inlet into the gray water tank at the top too. Don't think the head pump is going to be able to pump the black water up too the gray water tank! I feel your pain on the holding tank size, but do really want twice as much black water onboard? You could have run the 2 sink drains out the side of the boat. Did you plumb your AC drain into the sump too? Black water discharge is illegal unless you are multiple miles out to sea. Gray water discharge was illegal in some land locked lakes and reservoirs. Good Luck! Mike

[Viper]

Does the grey water tank outlet hose (pump out hose) come from the top of the tank or off the side close to the bottom?

[mjk1040]

Viper, the gray water tank set up in our Mariner only had one hose to it and it was at the bottom. The sump exit had a wye valve on it so you could pump the gray water into the tank or overboard from the sump. Mike

[Viper]

The only thing that raises a red flag in that type of setup is that I'm not crazy about inlets or outlets on the bottom of tanks used for this purpose or for fuel tanks. A fresh water leak there..no big deal, but a black water leak there even just a drip is not good. Also consider that no matter what you do or how you plumb the rest of the system, there will always be waste in that line. If you're going this route, use the best line out there or it will quickly permeate and it'll be all you smell in the boat.

[gatehopper]

Thank you for that. I always use the appropriate hose for the job. Your point about the waste in the line is well taken since the water tank is 5 feet further aft of the black water tank which is only about 18 inches from the toilet, all downhill. I don't think I could keep 6 or 7 feet of waste line running downhill in that location. I will take a serious look at the set up this weekend. I need to get rid of my existing black water tank anyway. Anyone have suggestions for where to buy the best fitted tanks for the 32 mariners.