I’m taking a little break from the lithium battery project today.  Usually I don’t stray from hookups long enough to worry about emptying my waste tanks–it helps that I can handle 75 gallons down the toilet, and another 150 gallons of grey water–but for the first time in 10 years, I needed to empty my tanks and didn’t want to move the motorhome.

Where I’m parked, I’m about 150 feet away from a septic access.  I could get close moving the RV, but the tank outlet would be a little bit below the septic access.  So moving the RV doesn’t buy much, and the nearest RV park or dump station is about 20 miles away.

I’ve seen others with 12V macerator pumps and coils of hose, but was never impressed by them–especially considering the cost.  But if I wanted to empty my tanks without a drive, I’d need to pump.  I knew a garbage disposal could pump, and could grind up digested food just as well as undigested food, so I thought I’d give it a try.  A little searching showed that I wasn’t the first with the idea though, which just accelerated the project.

Note that I wanted to end up with bayonet fittings on the disposal, but where I’m at there isn’t one for sale within 30 miles.  Everything except the discharge hose was purchased at Lowe’s.

Here’s what’s needed:

• Garbage disposal.  I figure the cheaper the better, as I think the warranty will be void pretty quickly.  No worries about noise either.  Here’s what I used: InSinkErator Badger 1/3hp Disposer, $74.93.  You can get it online, though I paid a few bucks more to get it locally at Lowe’s.
• A power cord.   Any grounded cord with big enough wire will work, but here’s an example: Prime 8-foot 16/3 Power Supply Cord, $6.47
• Sewer cleanout adapter.  This fits snugly over the disposal inlet, and lets me use a normal sewer hose connection.  This one includes a cap (which I’ll need to buy separately): PVC Female Cleanout Adapter with Plug, $8.72
• 1-1/2″ Plastic straight coupling, $2.99 This connects to the discharge tailpiece on the disposal, and gives us the 1-1/2″ MPT fitting to connect the discharge hose.
• 1-1/2″ Lay-Flat Discharge Hose, $23.95 for 50′ with fittings.  This is perhaps the part of the project that excited me the most.  When rolled up, a 50′ section fits into a 10″x10″x5″ box.

Setting it up is pretty simple.

1. Pull the sink mounting stuff off of the top of the disposal and push the cleanout adapter on.  While I haven’t done it yet, it’ll be held in place permanently with PVC cement.
2. Connect power cord to the leads on the bottom of the disposal with wire nuts
3. Install tailpiece, bracket, and gasket to disposal outlet.
4. Install coupling on the end of the tailpiece
5. Connect discharge and sewer hoses.
6. Start disposal, open grey water valve momentarily, and check for leaks.
7. Empty black tank and rinse with grey.

In my case, I was pumping through 150′ of hose, with a few feet of elevation gain.  By the time I had walked the length of hose and checked for leaks, the black tank–which had over a month’s worth of stuff in it–was empty.  Rinsing with grey water, the inside of the discharge hoses looked cleaner than a 3″ sewer hose ever does, and laid flat as soon as the water drained out.

I expect to make a few tweaks still to make things even easier.  First, I’ll set it up to directly hook up to a short flexible sewer hose, and mount the disposal in the wet bay.  I’ll also add a shutoff valve on the end of the discharge hose, so that when I’m pumping uphill it’s easy to disconnect without draining it at the RV end of things.  And I might also connect it to the tank level monitor I’ve been working on, just so that I can push a button to start pumping and walk away, knowing the disposal will shut off when the tank is empty.

Quick update:  It’s almost over, but I just saw that Amazon has a more powerful (1 hp instead of 1/3 hp) disposal on sale for the same price I paid.  It’s physically a little bigger, but has over 4,000 largely very positive reviews: Waste King L-8000 Legend Series 1.0-Horsepower Continuous-Feed Garbage Disposal, $87.97