Sunday, May 10, 2015

DIY Trommel Sifter for Compost

My composting method makes compost with very little effort, but it needs sifting to separate the undecomposed material from the compost.

The first sifting method relied on rubbing the compost through a wire mesh placed over a wheelbarrow - that was way too much work. 

Then I found this instructable on making a rotating sifter, called a "trommel". The one I built from this idea used a wooden frame that could rest on my garden cart.  It was OK, but having to lift the trommel and frame off the cart to move the compost to the garden and then set it up again to continue sifting was a nuisance.

First trommel with supporting frame

I usually rested it on sawhorses and rolled a garden cart under the trommel to catch the sifted material. The frame was falling apart, so we redesigned the support system to be simpler. It's now just rails that bolt to sawhorses. The wheels the trommel rotates on are screwed to the rails. Disassembled it's 2 pieces of 4x4 lumber, the trommel, and 2 usable sawhorses.


  • 2 or 3 bicycle wheel rims, without the spokes.  You can usually get these free or cheap from a bicycle repair shop, because if the hole that holds the spoke is damaged the rim can't be fixed.  They just have to be round, and the same internal diameter.
  • 36" Hardware cloth with 1/4 mesh, a piece long enough to wrap around the wheels with an overlap of 4 to 6 inches.  (about 9 feet?)
  • Flexible stainless steel wire to attach the mesh to the rims.
  • 4 wheels or casters with bases, non-rotating, narrow enough to fit into the groove of the rims.  DO NOT GET SWIVEL CASTERS!!!!  They wobble and it's hard to spin the trommel.
  • 2 pieces of 4x4 or 2x4 about 5 feet long for the side rails. I had some scrap fence posts, so I used them. 2x4s would be strong enough and easier to find.
  • Pair of sawhorses with wooden tops that you can drill holes through.Tops should be at least 24 inches wide.
  • 4 carriage bolts long enough to  go through the top of the sawhorse and the side rails, with nuts and 4 large washers.

Part 1: Making the Trommel

  1. Cut a piece of mesh 4 to 6 inches longer than the diameter of the rims.
  2. Roll the mesh and slide it inside the rims.
  3. Align the ends of the mesh, and place one rim about 3 inches from the end of the mesh.
  4. If you are using three rims, place the center rim half way between the ends.
  5. Sew the mesh to the rims with wire. Make a running stitch through the holes where the spokes used to be. 
  6. Stitch the overlap with more wire.
Wire stitching through the spoke holes attaches mesh to rims

Part 2: Making the Trommel Support

The trommel rotates on 4 wheels mounted to the supporting frame.  How you make the frame can vary - a look at the variations in the comments of the instructables post shows many versions and they all work.

The basics ... the frame has to hold the wheels about 20 inches apart for stability and ease of rolling, and it has to be square so the trommel rolls smoothly. The wheels must hold the trommel 1/2 inch or more above the rails.

My version takes advantage of a pair of small, folding sawhorses with 2x4 lumber tops.  The tops are 24 inches wide.  Placing the 4x4 rails on the horses created the right spacing for the wheels.

  1. Place the sawhorses with the tops  parallel and about 3 feet apart.
  2. Place the rails on the horses so the facing sides are parallel and about 18 inches apart.
  3. Adjust the position of the side rails to square them to the horses.
  4. Place the trommel on the rails and adjust whatever you need to adjust so one end of the trommel is over the inside edge of the top of the sawhorse (as shown below) and the other end is over the outside edge of the other horse (see the last picture) Yes, this sounds odd, but the offset makes separating the material easier.
  5. Double check that the horses and rails are square and clamp them - or have a friend hold them.
  6. Drill the bolt holes and bolt the rails to the horses.  Check the squareness again and tighten the bolts.
  7. Place the trommel in position again to add the wheels. Screw them to the side rails so the trommel is not touching the rails and rotates smoothly.
    TIP: Do one end, then the other instead of doing both wheels for one side. It lines up more easily.
Position of wheels and bolts

Position of the trommel and sawhorse top, with deflector

Using the compost sifter

Set up the trommel and frame so the input side is a comfortable distance from the heap you are sifting and slightly higher than the output side. Tighten the bolts after you place the trommel on the wheels to make sure it's going to roll easily.

Toss a few shovelfuls of compost from the heap into the trommel and rotate it.  The fine material drops through the mesh and coarse material falls out the end after several rotations.  Rubbing the compost to break up the chunks increases the yield of fine material.

I had elaborate ideas about building some sort of exit chute to get the big pieces into a collecting bin, but realized that a piece of plywood leaning on the legs of the sawhorse would divide the sifted from the unsifted material just as well with less hassle. When the pile of material reaches the level of the trommel, I pick it up and toss it into the newest compost bin.

The fine material could be sifted into something or onto a tarp instead of the ground as I am doing here.

Compost sifter in use.
I will probably shorten the side rails

A Motor-Driven Version 

This is far more sifting power than I need, but if you have a large garden, commercial greenhouse or love powered everything, there are several ways to make a motor do the work for you. 

The Popular Mechanics how-to article.

And the You Tube video that inspired the article.

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