When you own an RV one of the things you have to deal with is dumping the waste tanks at the end of your stay in a campground. This is generally not a big deal, you pull up to the dump station, usually located along the driveway on your way out, hook up your waste hose and dump your tanks by opening the 3" slide valve.
But what do you do if you go camping somewhere other than a campground? Well, south of the Mason-Dixon line you will find numerous facilities where you can dump for free or for a small fee, but such services seem to be a whole lot rarer up North. In my case we often take the RV up to visit my sister and her family in Maine and park in their driveway. There's really no conveinient place to dump when we return.
So I decided to see if I could cobble something up to make this task easier.
The solution to dumping your tanks without having to carry a 100' 3" diameter hose is to use a macerating pump so that you can pump your tanks out through a garden house and into a city sewer or private septic system. There are commercial pumps that you can buy for this but I had this old Insinkerator Dispos-all kicking around . . .
Step one was repairing the garbage disposer. This turned out to be pretty easy. Leakage around the seal had caused the top bushing to seize and spin in it's clip causing the rotor to bind against the stator. I freed everything up, re-secured the bushing, and lubed it with axle grease to help keep it free. My guess is that I would face the same problem if I re-installed the dispos-all in the sink, but it should last for many years with the light "duty" it will get as a macerator pump.
Once I got it working I found, to my delight, that the top of the disposer – once all of the gaskets and flanges were removed – was exactly 3" in diameter!
I drilled and tapped (4) 5/16" bolts into the top and had a perfect seal!
Next I drilled a 7/8" hole in a piece of 1/8" brass and soldering in a brass garden hose fitting. This bolted in place of the original drain flange.
The completed black water macerator pump! The dishwasher connection was plugged with a #2 rubber stopper inserted from the inside.
I made a waste hose with two 3" female fittings and hooked everything up.
I pried up the edge of our septic tank lid and tucked the hose in. Next I opened the waste valve and turned on the power. It worked like a charm!
It drained the half full 100 gallon tank in about 10 minutes and handled all of the solids without the slightest complaint. I briefly held the hose as high in the air as I could and the pump had sufficient head to lift the water 7' while maintaining plenty of flow.
Once the tank emptied the hose could easily be reposition to let the last bits dribble out.
I'm quite pleased with this quick little hack!