click photos to enlarge
This is no longer the current version of the bucket composter. Please see v2.1 HERE.
A full function BSF composter
I referred to my previous bucket composter as a “no frills” approach. This new version features everything needed for efficient and convenient BSF composting. The design is based on a typical 5 gallon bucket with lid, and other easy to find hardware items. I limited myself to new materials that I could find in national chain stores, but you can modify the design to take advantage of what you already have or what you can salvage.
I think the size of this composter is ideal for someone just learning about BSF composting. It can easily be moved as you find the best location, and there is less temptation to overload it with food scraps, something that I believe is a very common mistake with larger units. If you do feel like you want a larger unit you can simply apply the concepts for this bucket to a larger storage container.
I haven’t fully tested this design yet and I will post updates as I discover and address any weaknesses as I identify them. Please check the comments below for new information.
The bucket
The bucket I used is made by Encore Plastics but any brand can work. I like this particular bucket because it’s lightweight which makes it easy to cut the various slits and holes. I chose white because it will stay cooler if any direct sunlight hits it, but they do make a nifty green one.
Lid
Snapping the lid in place isn’t convenient when you’re making regular additions of food scraps or if, like me, you enjoy checking on the progress of your colony several times a day. For that reason I don’t snap the lid down, I just let it rest on the bucket. If you have critters (wild or domestic) that might get into the bucket you can make a fastener with two small bungee cords. Simply lay one bungee across the middle of the other bungee to form an “X” shape, and then tie the bottom cord around the top cord with a square knot. Next tie another square knot with the top cord so that the two are joined in the middle. It takes few seconds to secure all four hooks but the result is fairly secure.
Ventilation
BSF larvae generate a lot of heat as they metabolize food scraps and other waste. Overheating leads to premature crawl-off of juvenile larvae as they try to escape dangerously high temperatures. BSF larvae perish when their body temperature reaches 115ºF (46C) and in most climates the composter should be kept away from any direct sunlight. (Those of you who live in cool climates may find that keeping the unit in partial sunshine could help maintain the proper internal temperature range) You can’t have too much ventilation so this new design includes a vent in the lid. The lid vent doubles as a handle for lifting the lid and I expect it will also be used by BSF females as an entrance when looking for egg laying sites. The first version of the bucket composter used several 5/8 inch (16mm) holes for ventilation drilled just under the reinforcing ring on the outside of the bucket. For this version I drilled 8 holes, 3/8 inches (10mm) in diameter and equally spaced around the circumference. Then 4 slits are formed using the 4 pairs of holes making sure that the area directly under the bucket handle is left intact so that it can support the weight of the unit when full. I think these slits will provide more surface area than a series of holes and also a more attractive entrance for the female BSF. Keep the slits close to the overhang to help avoid rain water intrusion into the unit.
I won’t be converting to metric measurements for the rest of this article because the size of the holes will be effected by the size of the fittings available where you live. Your goal should be to drill holes that allow a snug fit, even more so where the drain adapter passes through the bucket.
I’m not going to describe how I cut the slits because it’s risky if you don’t know what you’re doing. The bucket I use is made of fairly soft plastic but your bucket may be heavier and may require a different technique. If you’re not a confident and experienced do-it-yourself type then please get help with this step. You can opt to use only drilled holes as I did in the first bucket composter version. I find that a sharp spade or paddle bit is the best way to drill through plastics although a friend has suggested using a hole saw with a blank drill bit for more precision. I use the higher speed on my cordless drill and it’s best to use very light pressure. If you drill the hole too quickly it tends to tear the plastic instead of cutting a nice clean hole. I don’t think it’s possible to cut a clean hole in this type of material with a regular twist type bit. I recommend practicing on a spare bucket before drilling the one you intend to use.
Lid vent – Like the lower vents, this opening serves two purposes; the obvious one of ventilation plus it allows the female BSF easy access to the inside of the composter where we want them to lay their eggs. The portion of this upper vent located below the lid offers a handy place to affix corrugated cardboard which is an attractive egg laying substrate for the BSF. The top part of the pvc tee accommodates 3/4 inch pipe and the vertical section is for 1 inch pipe. I chose this format over the standard 3/4 inch-only tee to create a larger hole going through the lid allowing more air movement. The reducer bushing which is fitted beneath the lid is normally used for transitioning from 1 inch pvc pipe to 2 inch. I drilled a 1 3/8 inch hole in the lid, again using a spade or paddle bit. A short length of 1 inch pvc pipe passes through the hole in the lid and into the tee fitting above and reducer bushing below. The pipe should be short enough to allow the two fitting to touch each other so you get a snug fit on both sides of the lid. I put a bead of silicone around the circumference of the lid hole on both sides to make it relatively water tight. The silicone may be enough to hold the fittings in place but it might be best to use a little pvc cement on the pipe also. The 3/4 inch pvc pipes that extend out horizontally from the tee are cut at a 45º angle to shield the inside from rain intrusion.
Update: May 5, 2010 – I had thought the vent fitting and pipe for the lid vent would be used by the female BSF as an entry point. Over the past several days I’ve observed a few dozen BSF entering the bucket but none used the lid vent. I think the next one I make will use a larger pipe/fitting. More ventilation can’t hurt and I want to do everything I can to make easy access for the female BSF.
Larva barrier
Without some method of containing the larvae they could easily escape via the vent holes whenever condensation was present on the inside of the bucket. For the past few years I have had good success using Velcro as a barrier. I’ve primarily used the hook part of the “hook and loop” material but I think both would work. When the larvae and container are damp, the surface tension of the water allows the larvae to stick to the vertical surface of the container. If a larva tries to cross a vertical strip of Velcro the raised surface of the Velcro hooks (and probably of the loops too) break the surface tension of the water and the larvae falls back to the compost.
It’s important when applying the Velcro that the surface of the bucket is dry and free of oil or dirt. I avoid touching the sticky back of the Velcro by using the point of a razor knife to hold it while I peel off the backing instead of my fingers. Leave expansion gaps in the Velcro because otherwise it tends to pucker or pull away from the bucket in places leaving spaces that the larvae might fit through. The small gaps I leave are probably too small to allow a full size larva to pass through, but I put another piece of Velcro directly above expansion gaps to container smaller larvae. Since Velcro usually comes with both hook and loop you will probably have extra loop material which you can use for this purpose. I used a longer strip of loop above the gaps than necessary because I think the female BSF might use it as an egg laying substrate. Even when I’m careful it’s not unusual to see bumps form in the barrier. When that happens use a razor knife to cut the area that popped up and convert it to an expansion gap. I currently have one DIY composter that spent an entire year outdoors through a wide range of temperature and humidity. The Velcro is still firmly attached and working well.
Drainage
The most important function of a BSF composter is that it drain excess liquids quickly and thoroughly. Accumulated liquids will starve the compost of oxygen which promotes the growth of anaerobic bacteria. Anaerobic bacteria thrive in material with little or no oxygen and produce foul sewer type odors. A properly designed and managed BSF composter has a pleasant earthy aroma; if you smell bad odors it’s a sign that your colony is out of balance. I think the filter and drain of this design will function very well. The drainage system consists of a filter medium, a collection area and a drain tube. You may set up your system to drain into the ground or you may collect the liquids via a cap on the drain tube when convenient. Some people claim that the liquid resulting from BSF composting has value as a fertilizer but I have not tested that theory. I don’t save the liquid.
Filter – I’ve used coconut fiber (coir) in the past to filter liquids in my original BioPod. It works well unless the larvae expand it and mix it into the compost. You can use coir but you must find a method for keeping it in place. As an alternative to coir I used an air conditioning filter material that is made from hog’s hair. It’s biodegradable and is designed for use in wet applications. The brand I used is NaturalAire and I found it at a few different national chain stores. One advantage of this material over coir is that the fibers are glued together so it’s less likely the BSF will expand it. It has a mesh fabric on one side which will also help. For a template to shape the filter I used a bowl that was the same diameter as the inside of the bucket about two inches from the bottom, where the filter will be installed. It’s important the filter fit tightly against the wall of the bucket so that food scraps don’t fall through. The larvae will be able to crawl into and around the filter, but that shouldn’t be problem. Cut two discs and sandwich them together with the mesh on the outside of each. Then, using a large yarn needle, stitch the two discs together with some type of string, twine, fishing line, etc. Don’t pull the string too tight because you don’t want to compress the filter; you just want to prevent the larvae from expanding it.
Update: May 5, 2010 – Well, I only have 400-500 medium to large larvae in this unit now and they’re already distorting the filter. Never underestimate the power of the grub! 
I didn’t know how strong or weak the mesh was but the larvae have pulled the strings wide apart in some spots. In the future I will probably add several concentric circles of stitches to help hold it all together. Maybe a synthetic material will hold up better…
Update: June 2, 2010 – The original filter seems to be working but I’m testing a new filter material now which is synthetic. I like that the other stuff is bio-degradable but I don’t think it will function as well for as long as a synthetic. Of course you can also work with coconut husk (coir). You will always have the option of removing the larvae and compost from your unit in the future and installing a different material. Below are photos of the new filter I’m testing.
Liquid waste holding area – Accumulated liquids need an unobstructed area so that they can flow freely. This is achieved by supporting the filter a few inches above the bottom of the bucket. In this version I’m using plastic practice “golf balls” which are inexpensive and effective. You could use a wide variety of materials to accomplish this but I like the fact that the balls are hollow and perforated so most of the space is utilized for holding the liquid waste away from the filter and compost.
Drainage tube – The liquids are drained from the bucket via 1/2 inch diameter vinyl tubing fitted to the bucket with common plumbing fittings. To get a watertight seal it’s important that you drill a precise 3/4 inch hole through the bucket at the proper location assuming you use the standard adapter that I used here. The size of the hole was such that it took considerable force to thread the fitting through it. Before drilling I set the fitting in place and held the bucket up to the sunlight to see where the hole would be centered (see photo). The nylon barb adapter passes through the hole you drilled and into the female threads of the pvc fitting inside the bucket. I modified the pvc fitting with notches so that it could rest on the bottom of the bucket and still drain. Since I used a downward facing elbow fitting I will have the option of creating a light siphon effect if I choose to. You could also opt to use a straight fitting instead of an elbow. It can be difficult to thread plastic fittings tightly together so I applied pvc pipe thread sealant to the female threads of the elbow to help lubricate them. I also placed a bead of silicone sealant around both fittings where they contact the bucket. Silicone doesn’t adhere well to the bucket I used, but maybe it will at least act as a gasket. I used the same 1/2 inch thread – 1/2 barb adapter on the outlet of the drain tube so that I could fit a 1/2 inch pvc threaded cap to it. There are hundreds of variations for the drainage plumbing that would be equally as effective and if you have difficulty or suggestions please post them in the comment box at the bottom of the page.
Update-5/21: The next bucket composter I make will have the drain located as low as possible. That means I’ll need to use a straight fitting instead of the 90º elbow I used here.
Optional tap – I also considered ordering a tap like those used on buckets for brewing beer or wine. Those come with a handy nut and gasket and you could probably attach a tube to the outlet for easy dispensing. The only reason I didn’t get one was because it would have added $4-$5 dollars to the cost of the bucket. If you can’t get a water tight seal with the system I used then you might try getting a tap.
Harvest system
Unlike the first bucket composter this design will collect mature larvae as they migrate away from the food source in search of a pupation site. This is achieved with a funnel that rests against the bucket wall which is inserted into a tube that spirals up to a pvc pipe leading into a collection container. I made a funnel from a generic one gallon plastic water bottle. I chose this because it is cheap, pliable enough to easily work with and it can be found anywhere. You could also use a proper funnel if you can find one with a flat or straight side that will mesh with the bucket wall vertically. The handle section of the water jug fits snugly inside of the 3/4 inch internal diameter vinyl tubing used here. Below the handle I left portions of two adjacent sides of the bottle which are at right angles from each other. One side will be placed vertically against the bucket wall and the other will be partially buried beneath the compost. As the level of the compost rises the flexible tubing will be adjusted accordingly to keep the funnel properly placed at the surface. Since the funnel and tube will need to be adjusted over time I use a pair of magnets to secure the funnel to the bucket. Magnets can be found at hardware and craft stores.
Traction issues – One issue I had with several different types of tubing was that they were all very smooth on the inside and the larvae had difficulty getting traction. This isn’t a problem given sufficient humidity, but we can’t always count on that. The angle can be lessened by using a longer tube to make a more gentle spiral, but I would prefer a shorter tube if possible. I did manage to etch the inside of the tube for this unit by running some pvc cement through it. It was a messy, stinky process but it seems to have worked. I removed the glue by rinsing with water which seemed to transform the glue to something like a solid plastic. If I try that again I’ll try removing the glue by running a strip of fabric through the tube instead. Of course you would need to allow the fumes from the glue to evaporate for several hours or overnight. Mechanical scratches should work fine and I picture using a long drill bit or a rod of some type to reach into the tube and roughen the surface. Any suggestions for de-glossing or scratching the inside of the tubing would be appreciated.
Once you exit the bucket with the pvc fitting it’s a matter of simple plumbing to direct the larvae into a harvest container. I haven’t yet glued most of the pvc fittings and it doesn’t seem necessary since the parts hold together well due to friction. You can use any container that has a water tight lid and if your unit will be protected from rain you really don’t even need a lid. In a dry location you could opt to use only a vertical pipe that enters the harvest container through the top. The purpose of the horizontal pipe is to allow a side entry to minimize rain intrusion. The horizontal pipe is angled slightly upwards as it enters the container so that rainwater will run away from the container and not into it. An inch or two of dry sawdust will serve two purposes; it will quiet the larvae by giving them cover and it keeps the larvae dry so they can’t crawl up the sides of the container.
Conclusion
I’m sure I haven’t included every piece of information possible for constructing this composter, but most of it is common sense if you understand what the goals are. I hope that the photos will supply enough information that might be lacking in the text. This unit is cheap enough to build that you can afford to engage in a little trial and error until you get exactly what you want. There are numerous opportunities to hurt yourself constructing this so please be careful.
Questions, comments and updates
I want to be clear that I have not tested this composter yet. I believe it will work fairly well as described above but I expect to make changes as I work with it. If you have questions or suggestions please use the comment box located at the bottom of this page. I’m especially interested in variations on the funnel and tubing that directs the larvae into the collection container.
Follow my progress via youtube:
DIY black soldier fly bucket composter – part 1
DIY black soldier fly bucket composter – part 2
BSF bucket composter April 26 2010
BSF bucket after two weeks
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