I'd been weighing the pros and cons of compost tea for a while now, particularly after realizing that the base of my store-bought compost tumbler was accumulating, essentially, compost tea from the rainwater filtering down through the contents. I'd piped a hose from the base to my garden irrigation system, but the tumbler was on its way out and I figured I could improve on the system.
What I don't like about the traditional compost tea process is how labor intensive the batch method is. A continuous method would really streamline the process.
Step 1: Gather Materials
At about the same time I started brainstorming the new compost tea bin, I was dismantling my biogas digester, which wasn't working as I had hoped. I figured, however, that I could reuse the plastic 30-gallon and 55-gallon drums from the collector part of the digester as the basis of the new compost tea bin. I'd just need to cut them lengthwise, as seen in the photo above to make, essentially, two troughs.
Of course, reusing old parts means working around some obstacles. In this case, I'd already cut the tops off both drums, so I'd have to butt the open ends of each drum half together to allow them to hold liquids and solids; were I working with unmodified drums, I wouldn't need to go through that step. I'd also drilled an overflow drain hole in the larger drum, which was in a less than ideal location for draining the compost tea from what would become the larger trough; again, were I working with an unmodified drum, I'd have drilled the drain hole in the larger trough's lowest point.
Step 2: Build the Outer Trough
The outer trough needs to be watertight, so when butting the open ends of the drum together, I simply overlapped them and started drilling a series of holes just large enough for nuts and bolts to pass through. Because each end was so wobbly, I started by drilling a hole in the center, bolting the two halves together, and working my way out toward each end. After I had all the bolt holes drilled and the bolts cinched up, I put a good bead of silicon down where the two halves of the drum overlapped.
Step 3: Build the Inner Trough
The inner trough, on the other hand, needs to act more or less like a sieve to allow water to percolate through the compost and drip through to the outer trough. I butted the open ends of the drum together simply by drilling two parallel series of smaller holes and running zip-ties through them. I then drilled a row of holes along the bottom of the trough for drainage; the holes you see in the photo work well, add more as you feel necessary.
What I don't want is for the compost in the inner trough to remain soaked, so I needed to lift the underside of the inner trough up off the bottom of the outer trough. To do that, I grabbed four sections of scrap PVC pipe and screwed them to the underside of the inner trough.
Step 4: Add a Drain and Build a Stand
As mentioned above, the drain hole was not in an ideal location partway up the side of the outer trough; a small amount of tea will remain below that level, and I'll have to take measures to keep mosquitos from laying eggs in that water. On the other hand, that small amount of water will keep any solid matter from the inner trough from passing on into my irrigation system; I'll just have to muck out the bottom of the outer trough from time to time. A bulkhead fitting in the drain hole will allow me to mount a spigot and a length of garden hose to connect to my irrigation system.
I also needed a stand to keep the bin elevated and to keep it from rolling. The four pieces of pallet wood I notched above proved insufficient once the bin started to fill up; I've since replaced them with something sturdier.
Step 5: Using the Finished Product
My garage has no gutters, so I situated the new bin to catch the rainwater coming off the garage roof. The spigot is on the back side, closest to the buried drain pipe that leads downhill to my gardens and irrigation system.
I bought an old blender from the thrift store for the sole purpose of grinding up kitchen scraps, which then go into the inner trough. I also empty my basement dehumidifier into the bin when it's full.
A couple months in and it seems to be working well. I've yet to plant my vegetable gardens (we don't plant until Memorial Day around here), but the weeds are really digging the tea. The older compost in the inner bin also breaks down quicker than it would in my old compost tumbler, though I'm sure that grinding the kitchen scraps before adding them to the compost has a lot to do with that.
If I were to do this again, I'd start with unmodified drums or even actual troughs rather than the cut-up drums I had on hand.