OK, so back when I was looking into a black water tank (a real black water tank for an RV) a few people hear said they were using rectangular polyethylene tanks for black water. So I bought one of those to save $$.
Now I'm trying to figure out the best way to connect a 3" pipe to this tank and I'm finding it's not so straight forward.
I see two (maybe three) options:
1. Spin weld fittings. There are a number of places that sell polyethylene fitting meant to be "spin welded". This requires a special arbor to hold the fitting and a high speed router. (More on that here: http://www.spinwelding.com/page9.htm
) These seem like the best solution. The fitting are cheap and the result is as strong as the original tank. However the tool is expensive and it requires a bit of technique to do it right and I don't have any practice pieces I can ruin to learn this method.
2. Slip bulkhead fittings (or bolt-on bulkhead fittings) These look pretty good but you have to have an access hole to get at the inside in order to put the "nut" on the fitting. This means I'd have an access hole to fill in afterward. Also the damn fitting are pricey. Almost as much as the tank itself.
3. I could possibly make my own metal, through-bolt bulkhead adapter using plate steel and pipe. This would be cheap and if I made the plate rectangle or square I could put it through it's own hole and I would have to make an extra hole. Drawbacks here include: relying on my weld to be water tight and crating a big square hole in the tank which I'm guessing would weaken it quite a bit.
Had I know about this before I may have paid the extra to get the tank from plastic-mart.com they put any fittings you want on the tank for free!
I can probably use the 1 1/4" outlet to drain the gray water, but I'd like to hear suggestions/experiences on the black water outlet.