Introduction to the Continuous Flow Through (CFT) worm bins

For a home scale use, the most popular systems are homemade Rubbermaid worm bins or stacking trays commercial products such as the Worm Cafe, the Worm Factory etc... Those stacking trays systems are what call flow through or vertical flow systems, this means we add the food scraps at the top, then, the content migrate to the bottom and when mature, you collect the compost from the bottommost tray. Both systems work well but they have their limits and become inefficient if you have a lot of food waste to compost. In fact, a single worm bin of that style (about 55cm x 37cm) is usually not enough to compost all the food waste generated by a small family, especially when you follow safe feeding techniques such as the pocket feeding method and use a good quantity of bedding material at each feeding to reduce chance of failure. My family of 3, generates on average 8 to 10 Litres of food waste per week and I need at least 4 worm commercial bins like the Worm Cafe or Can-O-Worms. At one time, I had 8 of them so I could compost more waste but it was tedious to maintain.

As I got more involved in worm farming, I was browsing a lot of forums and especially Facebook groups on the topic. That's where I discovered another type of worm bins called CFT (Continuous Flow Through) systems.

The idea is to have a bin that is not only large enough to be able to compost a larger volume of waste, but it would also be optimized to compost the waste more efficiently and to reduce the effort from the harvest of the vermicompost. Like the flow through bins, the CFTs are also fed from the top but being a single compartment, you don't have any heavy trays to move around. The vermicompost is harvested from the false bottom. This process allows you to not interrupt the worms in their continuous work.

Here are some specifications of a CFT worm bin

The height

To be considered a real CFT, the bin needs to ideally at 60 cm (24 inches) in height from bottom to the top. 50 cm (20 inches) is the strict minimum.

The reasons behind this are:

• compost worms prefer to live near the surface close to the food layer. The minimum height is given to the CFT so that it keeps most of the worms from the bottom, thus, allowing harvesting an almost worm-less vermicompost. That means you don't have to sift to separate the worms, that saves you time but also contribute to a better quality vermicompost because the sifting process destroys some microorganisms such as fungi.
• the height is also to give enough time for the food waste to properly compost and for the vermicompost to mature. It also gives time for microorganisms with slow cycle such as fungi to develop and for cocoons to hatch and baby worms to migrate to the surface.
• at this height, the larger mass will compact the vermicompost at the bottom, preventing the vermicompost from self-harvesting and forcing the worms to migrate upward.

The false bottom

A CFT does not have a closed bottom like regular worm bins. The solid bottom is replaced with bars or a grid (for large commercial scales systems). The CFT is elevated off the ground by 30 cm (12 inches) or more to facilitate the harvest from the bottom. With bars, this process is done by using a rake and scratching the compost through the bars. With the grid system, a blade will be pulled along the bottom by a motor, slicing the vermicompost few centimetres above the grid, the motor also vibrates the bin and the vermicompost falls through the grid.

One other benefit of the false bottom is that it provides an additional air input at the bottom. During the waste composting process, gases are formed and water evaporates, this leaves a void which will be replaced by air sucked in from the sides and from the bottom when possible. In a regular plastic worm bin, this natural aeration/ventilation process is not very efficient, this is why the vermicompost from these systems is often wet or muddy. With a CFT, this extra input of air (oxygen) in large quantity, especially those made of wood where air can get in through the cracks and gaps on the sides, will boost the number of all the beneficial aerobic organisms including the worms. More organisms means a more performant composting process. A CFT can, hence, host population of double the size of that from a plastic worm bin of same dimensions.

The setup of a CFT

We usually start a CFT made of bars with a layer of about 5 cm (2 inches) of corrugated cardboard sheets folding them upwards against the sides going up about 20 cm (8 inches). Then you need to add fresh and moist (not wet) bedding material such as corrugated cardboard or newspaper (shredded or hand-torn) for a thickness of 30 cm (half the height of the bin). This serves two purposes:

1. It will hold the content of the bin while it is breaking down and start compacting. After a while, that layer of cardboard will also break down by the organisms of the worm bin and the vermicompost should be able to hold in place on its own.
2. This thick bedding material will keep the worms off the bottoms at the beginning.

Here is a video of the setup of Gargantua, my homemade CFT:

The surface area

In order to efficiently compost food waste, we need more worms, however, increasing the volume by increasing the depth of the bin is not the solution because compost worms prefer to live near the surface. So we need to increase the surface area. Personally, I think a CFT should have a surface area of at least 60x60 cm (24x24 inches), Gargantua (my homemade CFT) has a surface area of 100x60 cm (40x24 inches).

Easy maintenance

With a single compartment, you don't have heavy trays to lift and move around. This makes it much easier to maintain, feed the worms and harvest the vermicompost.

Due to its larger volume, the mass of content is also increased. That larger mass provides thermal stability of the vermicompost. It's like leaving a glass of cold water and a jar of cold water at ambient temperature, the glass of cold water will warm up faster than the jar. What that means for the worm farmer is that he or she won't have to worry too much about a heatwave during the summer, the worms will have a lot of cool places to retreat to and in a mild winter, the heat is preserved long enough during the nights to keep the worms active.

With a regular plastic worm bin, it is generally advised to not water it at all because it keeps moisture too well. However, with a CFT, we need to water it once a week in order to keep the bottom moist as the constant contact with air will dry it out. Start with one litre and adjust accordingly if you see the water seep through the bottom.

Feeding the worms in a CFT

Overfeeding is one of the major issues with new worm farmers. With a CFT, you can feed the worms once a week by adding food and bedding material all over the surface area to about 5 cm (2 inches) of thickness. If something goes wrong in the food layer, the worms will bury themselves down, there is plenty of neutral space for them. That does not mean you can be careless though...

Easy harvest

The harvest is easy, but the wait is long.

You need to wait until the CFT is full up to the top before harvesting it for the first time. This can take anywhere from 9 to 24 months! It took mine about 16 months, if memory serves. All depends on various criteria including the initial worm population size and all sort of conditions. But once that level is reached, you can harvest regularly. If you are feeding the worms pre-hot-composted horse manure, you can harvest 5 cm (2 inches) weekly, if you are feeding food waste, you will be harvesting monthly as food waste compacts much more than manure as it breaks down. With Gargantua (my DIY CFT), 5cm of harvest is 100x60x5 cm = 30 litres = 8 gallons of vermicompost.

Here is a video of me harvesting Gargantua for the first time:

With a 60x60 cm (24x24 inches) CFT, a harvest of 5 cm is about 18 litres or 4.7 gallons for vermicompost. You just need to be patient.

For those who are handy, here are some construction plans:

If you are not a keen DIYer, you can buy a CFT. One of the size of Gargantua can be rather expensive to buy, commercial ones are in 4 digits prices... But here are some possible products:

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Related content

• Use plenty of bedding material in your worm farm
• Four common worm species used in worm farming
• The better the ventilation, the happier the compost worms
• Blending or freezing food waste for compost worms
• Composting with worms in an apartment
• Do worms really not eat meat. Understanding relative quantities
• How to feed your compost worms
• How to set up a worm bin
• What does a worm cocoon look like
• 10 myths in worm farming

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