(click photos to enlarge)
A full function BSF composter
The second generation of the bucket composter features everything needed for efficient BSF composting on a small scale. The surface area of this design is relatively small which means that you won’t be able to use this unit to process large quantities of waste. My main goal for this design is that of an introductory BSF unit which people can use to learn about black soldier fly larvae behavior and about how to keep a colony in balance. I process only a few ounces of waste per day in this unit and under optimal conditions it might handle one pound (.4kg) per day. A few advantages of a small unit is that 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. When you’re ready to try a larger unit you can apply the same construction methods on a larger container or use your knowledge of BSF to design your own.
The design is based on a common 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.
The biggest issue with version 2.0 was that the larvae shredded the filter material. I may experiment with ways to make that first filter work, but for v2.1 I’m testing a synthetic material. All the other changes in this version are for the purpose of tweaking performance.
Measurements
I used the American System of measurements for obvious reasons (I live in America) and any conversions to metric are approximate.
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. Also, BSF will often lay eggs in and around where the lid meets the bucket so I try not to disturb it too much. For those reasons 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 fasten all four hooks but the result is fairly secure. I assume it wouldn’t deter a big raccoon, but it might keep the family dog out.
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 113ºF (46ºC) 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 optimal internal temperature range) The lid vent doubles as a handle for lifting the lid and in v2.0 I expected it would also be used by BSF females as an entrance when looking for egg laying sites, but I never observed this happening. I increased the size of the vent fitting and pipe to 1½ inches. This may encourage the females to use this as an entrance, but the main purpose is to increase ventilation. In this version I drilled 8 holes, 5/8 inches (16mm) in diameter and equally spaced around the circumference. This is an increase from the previous 3/8 inch holes. 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. (This added ventilation is important in hot climates such as the southeast where I live, but smaller slits may be fine in cool climates such as Seattle and Vancouver.) Keep the slits close to the overhang to help avoid rain water intrusion into the unit.
I used a razor knife to cut the slits but it’s a dangerous operation. 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 recommend practicing on a spare bucket before drilling or cutting the one you intend to use.
Lid vent – I increased the size of the pvc pipe from ¾ inches (19mm) to 1½ inches (38mm). The portion of this upper vent located below the lid offers a handy place to affix a strip of corrugated cardboard which is an attractive egg laying substrate for the BSF. Drill a 2 inch (48mm) hole in the lid, again with a hole saw, to accept the short length of pipe that passes through the lid. A short length of 1½ inch pipe passes through the hole in the lid and into the tee fitting above and a 1½ inch connector fitting* below. This 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. Use pvc pipe cement to connect the fittings being careful to keep them under pressure for 15-30 seconds for a tight fit. The pipes that extend out horizontally from the tee are cut at a 45º angle to shield the inside from rain intrusion. It’s not necessary to glue the horizontal pipes in place.
*The length of pvc connectors vary by manufacturer. It’s better to use a shorter fitting because it will allow for better ventilation because the opening to the vent will be higher. You can also cut the connector to achieve this but please be careful if you try this.
The photos above are from version 2.1 and below from version 2.0.
Larva barrier
Without some method of containing the larvae they could easily escape via the vent holes whenever condensation is present on the inside of the bucket. Water droplets have surface tension which allows the larvae to stick to, and climb the vertical surface. For the past few years I have had success using Velcro as a barrier. The hook part of the “hook and loop” material is best because larvae sometimes become entangled in the hook side of Velcro. If a larva tries to cross a vertical strip of Velcro the raised surface of the hooks breaks the surface tension of the water and the larva falls back to the compost. The previous version used standard black Velcro and this version uses 1 inch wide (2.5cm) “Ultra Thin” “Sticky Back” Velcro. I’ve also had good results with transparent Velcro which is also pictured below. Both the transparent Velcro and the ultra thin are a different design and seem to have better adhesion than the black. I have seen a few dozen larvae defeat the Velcro, but this is a tiny percentage of the many thousands of larvae that I’ve seen contained by it. Even the commercial BioPod allows the occasional larva to escape. I should mention that it isn’t a big problem if a few larvae escape the unit, but overall we want to contain them so that we can have control over the process.
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 tape 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 the different rates of expansion of the bucket and tape will cause it to pull away from the bucket in places leaving spaces that the larvae might fit through. The gaps I leave are small enough that any medium or larger larvae should be prevented from escaping. The small larvae typically don’t try to migrate out of the unit unless it becomes too hot. Even if you’re careful about installing the tape you may see bumps appear. If that happens use a razor knife to cut the area that popped up and convert it to an expansion gap. If you have a problem with adhesion in a larger section then simply cut it away, clean it with alcohol, and fill the gap with a new piece of tape.
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 promoting the growth of undesirable anaerobic bacteria. Anaerobic bacteria thrive in material with little or no oxygen and produce foul, sewer-like 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. This 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 threaded adapter and 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.
Drainage tube – There are many possible variations, but the liquids are drained from this bucket via a ¾ inch (19mm) internal diameter vinyl tube fitted to the bucket with common plumbing fittings. I used about 18 inches (45cm) of tubing which allows the user to set the bucket on a raised platform for draining the liquids into a container for disposal. Alternatively you could use whatever length of tubing required to direct the liquid into the ground or a drain.
Update-July 14, 2010 – Initially I didn’t use clamps to secure the drain tubing to the barbed adapters. After a few weeks of use I find that the connection is becoming loose and I will be adding hose clamps to the fittings on both ends of the drain tube.
In this version the drain is placed as low as possible leaving a little space for tightening the “nut” that holds it. You can mark the center of the outlet hole by setting the fitting in place and holding the bucket up to the light to see where the hole would be centered. A 1 inch hole is drilled and a nylon ¾ inch barb-¾ inch male hose adapter is inserted into it. The tubing slips onto the barbed side of the adapter. The threads on the other end are the same size as those found on the male fittings of common garden hoses. I couldn’t find a nut to fit these threads so I made one out of a threaded plastic cap made for garden hoses. I did this by drilling a large hole in the flat side of the cap. It’s a little tricky to do but the result was a tight fitting nut that holds the adapter firmly in place for a watertight seal. An o-ring slipped onto the adapter threads, on the outside of the bucket, will allow a watertight seal.
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, as in the last, 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.
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. The previous bucket composter used an air conditioning filter material that is made from hog’s hair. This worked well initially, but it ultimately failed for the same reason most filters fail with BSF; the larvae expanded and shredded it. Whatever material is used, the larvae will tend to congregate in it. As they burrow through the filter they expand the fibers, and after a few weeks of this the filter often loses its effectiveness. Coir or the hog’s hair filter from v2.0 will work fine if a method is found for keeping them intact. I considered sandwiching the filter medium between two discs of galvanized wire hardware cloth. I wanted to avoid using metal for fear of it rusting, but I may experiment with this in the future. For this new version I’m testing a synthetic cut-to-fit air conditioner filter. I don’t have a large colony working in this unit yet, but preliminary results look promising. With this new material I did nothing except cut a single disc to fit the bottom of the bucket about 2 inches (50mm) off of the bottom. Hopefully it will remain intact without modifying it or adding other layers, but you have the option of experimenting with wire cloth (¼ inch or larger) or with “quilting” it with a yarn needle and string.
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. Too big is better than too small.
UPDATE 7/23/2010 – The filter system I used above gradually failed over a one month period. I believe the modifications needed to resolve the issue are simple. What caused the failure was the effect of the incessant churning action of the larvae. Imagine each individual BSF larvae as a tiny muscle and then multiply that by a colony of many thousand and the collective strength is impressive. I think this filter material will work fine, but it must be contained so that the larvae can’t expand it excessively as they did in the first test. The solution I’ll be testing next is a simple layering of the filter between two rigid meshes. Here I’m using the “egg crate” type panels that are normally used to cover florescent light fixtures in dropped ceilings. Wire mesh (¼ or ½ inch) would also work, but something with small holes like window screen would probably clog too easily. To connect the two rigid pieces I simply tied them together by threading nylon string through the filter with a large yarn needle. I didn’t compress the filter, I just wanted to prevent the larvae from expanding the filter beyond it’s original size.
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
Automatic harvesting of the mature larvae is achieved with; a funnel that rests against the bucket wall, which is inserted into a tube, which spirals up to ¾ inch pvc piping, which passes through a hole in the bucket and leads into a collection container. I made the funnel from a generic one gallon plastic water bottle because it is cheap, pliable enough to easily work with and it can be found anywhere. The handle section of the water jug fits snugly inside of the ¾ inch I.D. (internal diameter) vinyl tubing used here. Below the handle section 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 is 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. Beware; not all magnets are created equal. It tried using the same size and type magnets from a discount store and they didn’t have nearly the strength as those from the hardware store. Also, I discovered that most Home Depot employees don’t know that they sell magnets. In fact, most of them insisted that they didn’t until I found them myself. They’re in the fastener isle mixed in with the brads. 
*To maintain good harvesting it’s important to check the position of the funnel often. The larvae will constantly move the scraps around and if the opening to the funnel gets covered then the larvae will pass it by and continually circle within the bucket. A larva that enters the funnel and tube isn’t “aware” that it is finding a path out of the unit; it simply encounters the funnel entrance as is crawls. Sometimes small adjustments are necessary to keep the opening at the same elevation that migrating larvae are likely to use, which is normally the surface level of the compost.
The photos should make it fairly simple to see what you need to do for the harvest system. It isn’t really necessary to glue all of the pvc fittings since the parts hold together well due to friction. I do recommend gluing the 90º elbow fitting that is inside the bucket to the short length of ¾ inch pipe that passes through the bucket. I would not glue that same short piece of pipe to the outside elbow so that you will be able to remove the assembly for cleaning or storage, etc. For a harvest container you can use anything 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.
I don’t give exact measurements for the pvc pipe length because it’s not practical to limit the design to one specific harvest container. Almost any container will work, and if you’ll be keeping your unit sheltered from rain then you can even make a straight drop into the container instead of the right angle I used here. The only reason for the last 90º elbow in the run is that it creates an uphill elevation into the harvest container to channel rain water away from it. It isn’t necessary to glue the pipe together so you can cut the pieces a little long and make gradual adjustments until you get the proper fit.
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. 
People have requested a shopping list and I will do my best to post one in the next few days.
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:
These videos feature version 2.0
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
Stumble upon it!