Provided you've done your your zoning and EPA homework on Septic Tanks and "Cesspools" (draining into a lagoon would qualify for a "Cesspool"), your plan is doable. It can be as simple as digging a hole, filling the bottom with stone, taking a 55 gallon barrel and cut one end out, cut a hole for a pipe, drop the barrel in the hole with the cut out end - down, attach a pipe for the sewage to run in, and bury the thing.
Here's the drawbacks. For water only, it will work forever. If you put any solids in it, it will eventually fill up. You can put fingers on it, just like a household septic tank system, and then you probably can put solids in it.
Remember, in a septic tank, the bottom of the tank (about 1/4 of it) is the "sludge" which is vital to keep the tank "fermenting" to disolve solids and then run out the fingers as liquid, which then get's absorbed in the ground. The more fingers you have, the more you can dump in it. The draw back is, if you're tank is too small (like a 55 gallon drum), about 17 gallons at the bottom will need to be used for sludge, other wise, those solids will just keep building up on the bottom because the gravel will allow that vital liquid to drain away. You could use the barrel uncut.
Here's the second thing to consider: If you use a metal barrel, it will eventually rust out. It won't take long. Once it rusts out, it will collapse and you'll have to redo it or make a new one and fill the hole.
If plan to make the holding tank out of concrete, or a fiberglass tank, or PVC tank of some sort, it might last longer than metal. Concrete brick it up or pour your own holding tank with concrete.
Any one of these methods will work. But there again, if you put solids in it, you'll need a way to open it to clean it out. Even household septic tanks get too full of solids and have to be pumped out. It may take 25 years, but eventually they all do!
You can do it, provided EPA and Zoning permit it. Actually, the cesspool idea might be easier, although if the wind blows the right direction, you might re-think your plans.
Do you know 'poop' burns? Ever thought of that. In the Army, when I was in Korea on those training expeditions, an area of our camp was sectioned off as a latrine. The latrine was a barrel cut in half with a make-shift toilet seat. Every morning someone was scheduled for "latrine duty." They'd remove the seat, pour a bit of diesel fuel in it and set the contents on fire. Once it burned up, they dumped the ashes in a little bitty hole in the ground. When we moved from that geographical area, that little hold was buried over. Maybe you can think of some how to use this process!
Either way you go... good luck! Hope it all works out well for you!
