Shane, based on your home port listing, my guess is that you'll be doing the bulk of your boating on the upper Chesapeake, Susquehanna, Del./Ches. Canal and possibly Delaware Bay, without spending much if any, time in the ocean. As you will be essentially in fresh and brackish water, I can't recommend an Electro-scan or a Hold N' Treat system with an Electro-scan, as there won't be enough salt in the water to support the treatment system.
That leaves a Hold N' Treat system using a Purasan (talk to Billy III or Mixman, who post here regularly), or a conventional holding tank. On a 32 foot boat, you don't have a whole lot of available space to mount anything along that line. My suggestion would be, if you're going to add a holding tank, to get the biggest one you can fit comfortably on the boat.
A general rule of thumb for holding tank capacity is 4 gallons per person per day. This is figuring a little "fat" and you may not actually need that much tank capacity. It basically comes down to how the boat will be used, the availability of nearby pumpout stations, how many people will be on board at any given time, and how long you expect to be out at a time.
In order to keep odors to a minimum, it isn't recommended to go much over two weeks between pumpouts, and less if possible, especially in the summertime.
If the existing head is of the pump type, either manual or electric, and is in good condition, it may be able to be retained. Distance between the head and the holding tank (length of plumbing run) should be kept to 10 feet or less. The pumpout is via a deck fitting, similar to a fuel fill fitting. The tank must have, at a minimum, an "in", an "out" and a vent line. The vent is run through the hull to a through-hull fitting, usually high on the side of the hull, up near the toe rail. The bigger the diameter of the vent line, the better. The tank must vent well in both directions - air that is contained in the tank must be able to get out, as the tank fills. Air must also be able to come into the tank when it is being pumped out, or it won't pump out.
Where to install the tank? Common spots are under a bunk, settee, or under the vee berth. Keep in mind that you must be able to run your plumbing lines from the head to the tank, and from the tank to the deck pumpout. You must also be able to run the vent line to a through-hull fitting, enabling it to vent outside the boat. If the tank will be mounted under the vee berth area, you should consider running two vent lines, if possible, exiting the boat on opposite sides of the bow. This will give you good cross-ventilation of the tank, helping to keep odors to a minimum.
Normal hose size is 1 1/2" I.D. for the line between head and tank, and from tank to deck pumpout fitting. For the tank vent lines, I recommend no smaller than 3/4" I.D. and larger if possible. The sewage entry to the tank (from the toilet) is on top or high on one of the sides of the tank. The pumpout fitting is on bottom of the tank or low on one of the sides of the tank.
Some people prefer to have the tank pumped out from the top, rather than the bottom. This requires that a "dip tube" be run down from the top, to just short of the bottom of the tank, enabling the pumpout to draw from the top of the tank. The advantage to doing it this way is that you will not have sewage laying in the pumpout hose all the time, eventually permeating the hose and stinking up the boat. If you choose to do that, you'll need at least 4" of clearance above the top of the tank, for the necessary fittings - and the dip tube fitting will have to be specified when you order the tank.
For the tank, I strongly urge that you stay away from a metal tank, and go strictly with high density polyethylene. The salts and acids in urine will eat holes in any metal tank over time. Poly tanks, if they're kept out of the sun, and are not damaged by the occupants of the boat, will last virtually forever.
That should be enough to ruminate on for the moment. More questions will come up over time, and we can deal with them as they come up, rather than me writing a complete novella now.
