Make "Black Gold" With DIY Worm Compost Bins

Composting with worms (vermicomposting) is a great way to cycle your kitchen scraps and yard waste back into the 'circle of life'. When compared to your average compost pile, there is much less physical labor involved (frequent mixing not necessary), the process produces results sooner, and can even be done indoors or in confined spaces. When such bins are regularly maintained, following some basic rules, there will be very little-to-no odor. Similar "worm tower" systems exist, but they cost no less than $80-100 and don't hold much material.

With just a few plastic storage bins/totes, a bit of window screen material, some metal mesh (hardware cloth), some wood, a few screws and some rivets, you can make a space-efficient powerhouse of a composter that can handle all of your kitchen scraps for well under $50 (purchasing all materials new, and not including the cost of worms).

About vermicompost:

In home-vermicomposting systems, red wiggler worms (Eisenia fetida) are the most commonly used species in temperate climates as they will tolerate confinement and a relatively wide range of temperatures, but are still highly productive eating machines - they will process about half their body weight in kitchen scraps every day! One can find red wigglers for sale in many places, including online (check out: https://unclejimswormfarm.com/).

Vermicast/castings (worm poo) in the bins & leachate (liquid worm waste and condensed liquids) collected in the bottom of the base bin/tray can be used as a soil amendment and wide-spectrum natural fertilizer which is significantly more nutrient-dense than traditional aerobically-produced compost. If *fresh*, and from a reasonably well-maintained set of worm bins, vermicast and the accompanying leachate are also a potent source of beneficial microorganisms with the power to breathe life back into dead soils! That is why worm castings are sometimes called "black gold"! The big bags of castings available at the local garden supply store are a waste of money compared to what one can make at home - store-bought castings sitting around in non-ideal conditions for any amount of time practically guarantees that much of the nutrients have degraded, and that the majority of the accompanying microbial population have died off.

Most of the castings that I harvest from my bins are used to make "worm tea" - a biologically-rich and readily bioavailable organic fertilizer. I will cover the fabrication and usage of an actively-aerated tea-brewing system in a later Instructable...

Design basics:

In stacking tower-style systems, small plastic trays with perforated bottoms stack on top of a solid-bottomed base, with a vented lid on top of the stack of trays. Starting with one of the perforated trays nested above the base, worms are added to the tray along with food and bedding (diet-balancing high-Carbon material such as shredded paper), and more food scraps and bedding are piled in successive layers until the heap of worms/food/bedding is high enough to contact the underside of the next tray above. At this point, all fresh material is added to this upper tray. The worms will eventually finish processing the food in the lower tray(s), and thanks to the perforations in the tray bottoms, the worms are free to migrate upwards in search of fresh food, or downwards in search of suitable mating conditions. The stacking arrangement also allows the most fully-processed tray (the lowest) to be removed for vermicast harvesting, and returned empty to the top of the stack continuing the process indefinitely. Most worm composters have air vents in both the lid and in the base to keep the compost oxygenated, and often feature a means with which to empty any collected leachate from the base.

About this build:

I built this set of worm bins at TechShop Menlo Park in early 2013, documented it as well as I could (at the time, there were little-to-no Instructables detailing such a build and its use), and over three and a half years later they (and the worms) are still performing wonderfully! In that time, I have brewed hundreds of gallons of worm tea from harvested castings, and my gardens are doing quite well!

This design is based on the following plans for the "Oregon Soil Corporation Reactor Jr.".

Note that the following linked file is not consistently available online:

https://ucanr.edu/sites/EDC_Master_Gardeners/files/306167.pdf

The upper bins each hold a large number of worms, the resulting vermicompost, and food scraps in various states of decomposition/consumption. The base bin is constructed differently than the upper bins, as the purpose of the base bin is to collect leachate and facilitate air movement into the upper bins.

The design linked above is a good start, but I decided to make a few adjustments/improvements. After inspecting the shape of these storage bins, drilling all those holes high on the sides of the upper bins seemed unnecessary as the adjacent nesting bins would cover the holes, preventing air from passing through while still allowing foreign insects to enter the top bin with ease. Additionally, drilling even several times the recommended number of holes throughout each bottom of the upper bins may still enable the formation of potentially dangerous pockets of anaerobic bacteria. Drilling a sufficient quantity of holes in the bottoms, to ensure that the contents stay aerated on their own, would significantly weaken the bins, aside from being highly tedious work. Here, I've used sections of hardware cloth riveted over the floor of the upper bins, allowing much more oxygen to permeate the composting material, and allowing the worms a minimally-impeded passage between bins. Don't worry, the rough metal mesh doesn't cut up the worm's bodies - when they run into something hard/scratchy, they just wiggle backward and alter course!

Step 1: Construction Materials

Gather up the following:

10-gallon *opaque* storage bins (3-5x, with 3x matching lids) - many kinds of plastic nesting containers made with LDPE or HDPE should work (I used 4x Rubbermaid bins @ ~$7/each)
1.5"x1.5" redwood square rod (60" length) - rot-resistant spacers sandwiched between lids + frame for fiberglass screen affixed to the bottom of the lower lid (DO *NOT* USE TREATED WOOD) (~$5)
fiberglass screen (9"x15") - physical barrier in lid assembly which keeps foreign insects out of the top bin (free, scrap)
1/4" hardware cloth (2'x4', or enough to fit the bottom of each of the upper bins) - mesh must be coarse enough to allow worms to pass between bins, but fine enough to retain vermicast, i.e. fine fiberglass window screen material isn't strong enough and wouldn't allow transit of worms, and chicken wire would not prevent vermicast from dumping into the base bin... (a 2'x5' roll is ~$9)
~3/4" long wood screws (12x) - I used self-drilling pan-head-type screws for convenience (free, on-hand)
~2" long wood screws (8x) - must be longer than one width of the wood (free, on-hand)
1/8" *aluminum* rivets (~48x, more than is pictured here) - small grip range (1/16-1/8"), and *aluminum* backup plates (~$6 total)
bricks (2x), or other inert 2-3" spacers ($1, not shown here)
[250+ red wiggler worms (Eisenia fetida) - (~$15 shipped, not shown here)]

Note: the metal screen shown wrapped in blue tape was not used.

Step 2: Tools

These tools represent (in my view) a fairly ideal "fabrication situation"; however, there may be plenty of suitable alternatives at your disposal.

You will need:

heavy-duty shears, snips, or wire cutters - suitable for cutting the hardware cloth
utility knife, shears, or heavy-duty scissors - suitable for cutting fiberglass screen AND removing sections of the bin bottoms and lid
1/4" to 1/2" drill bit - for drilling aeration holes through the sides of the bottom bin
small drill bit - for drilling pilot holes into wood
countersink bit - for use with flathead wood screws
power drill (I use a battery-powered impact wrench for most everything)
tape measure - for sizing cuts of hardware cloth and wood
fine-point marker - for marking hole locations on the base bin & cuts to be made out of the hardware cloth
straight edge - could be a metal ruler (like I used) or a straightish piece of wood, used to roughly align aeration holes
rivet gun - suitable for ~1/8" rivets
saw (not pictured here) - for cross-cutting (chopping) the square wood rod
safety glasses - especially if using a power saw!
protective gloves (not pictured) - freshly-cut hardware cloth edges can be really sharp!

Optional - staple gun & medium-duty staples - to help with stretching the fiberglass screen over the frame

Step 3: Upper Bins: Remove the Bottoms

Using a utility knife, cut out the bottoms of three of the bins (if using four bins total), using care to guide the blade around the outer edge of the ridge closest to the bin's walls (see pictures).

It is important to cut outside the ridge so that liquids do not collect in the valley between the inner ridge and the side walls. Use care when cutting, as any stray cuts beyond this outline may weaken the bins.

Step 4: Upper Bins: Attach Hardware Cloth

Measure the inside dimensions of your container, from the flat edge of one side of the bottom to the other.

Using snips/shears, cut three (3x) 12"x18" pieces of the 1/4" hardware cloth.

Trim each corner and both ends so that the hardware cloth rests only on the flat parts of the bottom of the bin, and not on the sloped corners.

Flip each container upside down and hold the respective piece of hardware cloth roughly centered in place (adhesive tape helps). Make ~16 evenly-spaced marks around the bin's bottom, taking care to center each mark in the middle of the space between the wires of the hardware cloth. Make sure these marks are approximately halfway between the edge of the hole in the bottom of the bin and the edge of the fitted hardware cloth. I made some holes closer together than others, hoping to counteract any flexion of the mesh.

Use a 1/8" (or the width of your rivets) bit to drill the holes.

Align the hardware cloth with the holes from the inside of the bin. One at a time, insert the heads of the rivets through the bottom of the bin, through the spaces in the hardware cloth. Finally, hold a backup plate in place over the hardware cloth, and pop the rivet with your rivet gun.

Step 5: Base Bin: Drill Air Inlets

To further ensure that the worm population & all decomposing material gets enough oxygen throughout the stack, it's important to add some holes to the sides of base bin.

Placement of these holes requires some considerations - they must be high enough up the walls so that a fair amount of leachate can be collected without any movement of the bin risking unintended spills, but not so high as to be covered by the sides of the bin above. For 10-gallon Rubbermaid bins, I recommend 2-3" from the bottom. To guide vertical placement, use a straightedge on the outer walls of the bin (see picture #3).

The holes should be drilled fairly close together, as to maximize airflow, but not so close as to structurally weaken the walls. A good rule of thumb with most plastics is to keep a distance of at least 2x the hole diameter between the sides of each hole. LDPE/HDPE is quite forgiving, and since we'll be using sturdy spacers inside, the base bin will not have to support much weight, so don't worry too much about the spacing of these holes.

To facilitate the controlled removal of leachate (avoiding spills through the air inlets), I suggest leaving as much as 1/4 of the total perimeter (i.e. half the length of two adjacent walls) of the bin undrilled (see pictures #4 & #6).

With some reasonably-spaced marks made around most of the bin, it's time to "DRILL BABY DRILL" (see picture #5)!

Step 6: Lid: Preparation

Using a utility knife, cut out a rectangular hole in one lid, following the outer edge of the indented region (see picture #1).

Cut a piece of fiberglass screen to overlap the hole on all sides by an amount equal to or greater than the width of your redwood square rod (see picture #2).

For this size of Rubbermaid bin, the frame to hold the fiberglass screen will need to be about 9"x15" (see picture #4).

Using a saw, cut 2x 9" pieces and 2x 12" pieces of the 1.5"x1.5" redwood square rod.

Next, cut 2x 7" pieces and 2x 2" pieces for the lid spacers (not shown).

Drill pilot holes and countersink them, and assemble the frame using 4x long screws (see picture #5).

Step 7: Lid: Final Assembly

Lay each of the 2x 7" spacers on top of the narrower ends of the completed frame. Drill 2x pilot holes, passing fully through each spacer piece and penetrating into the shorter end pieces of the frame, making sure to avoid drilling into the frame's existing screws.

Note: I had more than 60" of redwood on hand, so my lid spacers are actually 9" long.

Make sure to keep track of the orientation of the spacers in relation to the frame - marking each spacer to match each side of the frame may be helpful.

If using flat-head [long] screws, countersink the pilot holes.

Place the screen on top of the frame and align the screen to best cover the frame. Tacking a few staples around the frame may help to keep the screen aligned & lightly stretched. The screen and frame should now look similar to picture #3.

Place the lid with the large hole roughly centered on top of the frame, thus sandwiching the screen as shown in picture #4. Important: make sure the lid is oriented with its regular "outer/top" side pointed up towards you (same orientation as when the lid is locked on top of a bin) as shown in picture #4.

Hold/clamp the stack down and fasten it together with 6 short self-drilling screws spaced evenly and aligned on the centerlines of the long (12") pieces of wood (see picture #4).

Place the two spacers on top of the lid, aligned with the left & right sides of the frame. Drive 4x long screws through this sandwich (2x in each spacer piece), ensuring that the screws are driven into the pre-drilled holes in both the spacers and the frame, so as to avoid cracking the wood (see picture #5).

Place one of the unused lids on top of the whole assembly and align the sides of this lid with that of the lower lid. Drive 6x short self-drilling screws through the top lid and into the two spacer pieces below.

Wedge the remaining 2" spacers in between the lids, and guide them to an area that needs support (the lids tend to warp) but don't allow them to push against the fiberglass screen.

Step 8: Final Preparation & Usage

The bins are built, so now comes time to put them to use!

Scrub and rinse each bin thoroughly with a bristle brush and water.

Place *one* of the mesh-bottomed upper bins into the solid-bottomed lower bin.

Spread some pre-moistened bedding into this bin, covering the entire piece of mesh, and keep piling more on top until this initial layer of bedding is ~1-3" deep.

If your worms have just been shipped to you, they'll need a drink of water first - add just enough water into the sack of worms to moisten most of the bedding material they were shipped with.

Next, gingerly spread your worms around the bin (see picture #5). Apply a little more pre-moistened bedding on top of them, then scatter a shallow layer of food on top of this layer of bedding. Finally, cover the food completely with another layer of moist bedding.

Now, secure the lid on top of these two bins, and place the unit in a somewhat warm location. It's best to put the bin in a bright/sunny location when first adding worms, as the light will discourage the most curious worms from wiggling out and away from the bins.

Red wiggler worms will tolerate temperatures from 40-90° Fahrenheit (inside the bins), but will do best with a relatively consistent temperature between 55-77° Fahrenheit. Freezing temperatures should definitely be avoided. Exposure to intense sunlight for long periods may cause large temperature swings and may overly stress the worms, resulting in reduced feeding & reproduction activity. Protect your bins from rain, which may penetrate the lids and oversaturate the contents.

Check the bins daily during your worms' first week in their new home. Always keep the pile inside the top bin moist, but not dripping wet, with a spray bottle, and slowly add alternating layers of food and bedding as the worms & microbes consume the food and bedding, and (hopefully) multiply like crazy!

When adding food to the top bin, try to evenly distribute the food over the surface of the previous layers, then cover the food entirely with a fluffy layer of moistened bedding.

About Bedding:

Bedding should ideally:

Be of neutral pH
Retain moisture, but drain well
Allow oxygen to permeate
Have a high carbon content
Have a low nitrogen content (e.g. dried grass cuttings instead of fresh)

I recommend the following options:

Shredded brown cardboard
Shredded paper (not bleached white office paper)
Shredded newspaper (not colored, NOT glossy magazines)
Aged compost
Aged horse or cow manure
Coco coir
Peat moss (may be somewhat acidic)
Dried straw, hay, dried leaves or other dried & seed-free yard waste
Wood chips

Bedding should be lightly moistened (not saturated) just before use, such that squeezing a handful of the new bedding only produces a few drops of water.

About worm food:

"The worms’ primary food source isn’t the food or bedding you’re putting in the bin – it’s the [microoganisms] that grow on that food[...]" From: http://www.oregonmetro.gov/sites/default/files/201...

Worms are happiest when given fruit and vegetable scraps, moistened cereals, unbuttered bread, and oil-free pasta; you may also add coffee grounds (mixed with other things or in moderation, due to acidity), coffee filters & tea bags (with staples removed), eggshells, and even dryer lint obtained from natural fibers (cotton, wool, linen, etc.). Eggshells are a great addition to your bins (calcium boost) provided they are finely crushed, or pulverized into a powder, as the worms won't be able to deal with coarse chunks.

Acidic foods, such as citrus fruits, should be used sparingly as the worms will not tolerate excessively acidic conditions. It is possible to neutralize citrus using gardening lime (calcium carbonate), but the pulp & skin should first be ground up fairly well.

Worms will consume & digest microorganisms easily due to their size, but larger pieces of solid food must be mechanically broken down by abrasive 'grit' in the gizzard, just like the digestive systems of birds. Adding a handful of grit every 1-2 months is recommended.

Recommended sources of grit are:

pulverized egg shells
very-fine to medium-grind coffee grounds (will tend to lower pH)
crushed oyster shells (will tend to raise pH)
vermiculite
garden soil
sand (may increase compaction)

Do *NOT* use any:

Dairy (milk, cheese, yogurt), meat, and oily foods (e.g. potato chips).

Amazingly, the gut of composting worms is able to digest *most* human pathogens, so you don't have to worry too much about Salmonella or E. coli running rampant in the bins.

However, some pathogens present in pet feces, various protozoans in particular, may *NOT* be digested by the worms, and can cause serious illness if ingested by humans (e.g. toxoplasmosis). Since we may want to use our worm castings in food gardens, DO NOT ALLOW PET WASTE ANYWHERE NEAR THE BINS!