Bio-Conversion of Putrescent Waste

Black Solider fly: A Beneficial Arthropod

After seven years of research, ESR developed and patented a unique bioconversion process that effects a 95% reduction in the weight and volume of food waste within a matter of just a few hours. This process requires no energy, no electricity, no chemicals, not even water. It is totally self-contained and does not emit a drop of effluent, and aside from a small amount of carbon dioxide, it does not produce methane or any other greenhouse gases.

The larva that we have chosen for this waste disposal process is the larva of the black soldier fly (BSF, Hermetia illucens). It is a tropical fly indigenous to the whole of the Americas, from the southern tip of Argentina to Boston and Seattle. During World War II, the black soldier fly spread into Europe, India, Asia and even Australia.

Indigenous to the Whole of the Americas

Surprisingly this bioconversion process does not demand the introduction of anything foreign or exotic. It is powered by a creature indigenous to the whole of the Americas, and even though this creature may have lived alongside humans for thousands of years, it is not associated in any way with the transmission of disease. In view of the wide variability of putrescent waste presented to it, this benign creature possesses one of the most robust digestive systems within nature. It has the ability to thrive in the presence of salts, alcohols, ammonia and a variety of food toxins. In addition to food waste, it can also process swine, human and poultry waste. Upon reaching maturity, this creature is rigidly regimented by evolution to migrate out of the unit and into a collection bucket without any human or mechanical intervention. This self-harvesting grub represents a bundle of nutrients that rivals in commercial value the finest fish meal. In our effort to dispose of food waste, why waste this valuable resource? Why not boldly insist upon the reintegration into the feed chain of most of the nutrients and energy it contains?

Not Just Any Fly

Many of us immediately panic when we see the word "fly" just as we often panic when we see the word "bacteria." Yes, there are noxious, filth-carrying flies that transmit deadly, disease-bearing bacteria. But not all bacteria and not all flies are harmful to humans. Without bacteria and flies, life as we know it on earth could not exist. Both play an essential role in the recycling of nutrients within the food chain.

Hermetia illucens puparium
black soldier fly adult
mixed larvae - prepupae and final instar

Just as benign bacteria compete with harmful bacteria and block their proliferation, so too, the soldier fly aggressively competes with filth-bearing flies and very effectively blocks their proliferation. Just as certain Calliphorides are used to clean out necrotic human tissue, SF larvae can be used to dispose of the large quantities of putrescent waste generated through human activity.

Unlike many other flies, SF adults do not go into houses, they do not have functional mouth parts, they do not eat waste, they do not regurgitate on human food, and therefore, they are not associated in any way with the transmission of disease. SF adults do not bite, bother or pester humans in any way. Even though SF larvae have been known to survive inside the human gut if swallowed whole, this only happens under utterly extreme and bizarre conditions and poses no real danger to humans. True enteric myiasis does not exist in man through the agency of SF larvae or any other fly larvae, whereas pseudomyiasis can occur, even through the agency of ordinary houseflies, M. domestica.

Black Soldier Fly Life Cycle

Soldier fly adults congregate in small numbers near a secluded bush or tree in order to find and select a mate. After mating, the female searches for a suitable place to lay her eggs. She produces about 900 eggs in her short life of 5 to 8 days. Housefly adults, by contrast, live up to 30 days, and during this long period, they must eat, and in so doing, they are actively engaged in the spread of disease.

Hermetia illucens: in sand BSF prepupae Black Soldier Fly - shedded chitin skins remaining after molting

Half of the population of adult black soldier flies (the males) never goes near waste, since males do not lay eggs. Actually the females prefer not to lay their eggs upon the waste, but either above or to the side of the waste. In this way, the eggs have a far better chance of surviving. The eggs are relatively slow in hatching (102 to 105 hours). The newly hatched larvae then crawl or fall onto the waste and begin to eat it with amazing speed.

Under ideal conditions, it takes about two weeks for the larvae to reach maturity. If the temperature is not right, or if there is not enough food, this period of 2 weeks may extend to six months. This ability of BSF larvae to extend its life cycle under conditions of stress is a very important reason why it was selected for this waste disposal process.

SF larvae pass through 5 stages or instars. Upon reaching maturity, prepupal larvae are about 25mm long, 6mm in diameter, and they weigh about 0.2 grams. These larvae are extremely tough and robust. They can survive under conditions of extreme oxygen deprivation. It takes, for example, approximately 2 hours for SF larvae to die when submerged in rubbing alcohol. They can be subjected to several 1000 g’s of centrifugation without harming them in any way.

Texas Experiment

Research Background

In an experiment conducted in Texas over a period of one year, ESR LLC determined that SF larvae can digest over 15 kilograms per day of restaurant food waste per square meter of feeding surface area, or roughly 3 lbs per square foot per day. A 95% reduction in the weight and volume of this waste was also noted. This means that for every 100 lbs of restaurant food waste deposited into a unit, only 5 lbs of a black, friable residue remain!

Active BSF larvae devouring food scraps turning an active pile to reveal more larvae havesting bucket - fresh grubs

Over 100,000 active larvae can be found in a typical waste disposal unit, and in contrast to red worms, these larvae have the ability to eat and digest just about any type of putrescent waste, including meat and dairy products. On the surface of the disposal unit, we typically see a 2- to 4-inch layer of actively feeding larvae in several stages of growth.

larger vat of food wastes being recycled by black soldier fly larvae collection container - auto-harvesting biogrubs Hermetia larvae consuming a watermelon

The moment waste is deposited into the unit, the larvae begin to secrete powerful digestive enzymes into the waste long before it begins to rot and smell. Since thermophilic and anaerobic bacteria play no part in this process, these tiny creatures are able to conserve and recycle most of the nutrients and energy within the waste.

Rates of Bioconversion

What percentage of fresh food waste bio-converts into fresh prepupae? Over a period of one year, ESR LLC noted that roughly 20% by weight of the fresh food waste converted into fresh larvae. This food waste had an average dry matter content of 37%, and the prepupae had an average dry matter content of 44%. On a dry matter basis, the bioconversion of food waste situates at almost 24%.

The following flow diagram is based upon an input of 100 kg of food waste per day. Less than three 6-foot bioconversion units can handle this input.
Bioconversion Chart Detailing Rates
An Analysis of Dried Soldier Fly Prepupae

42.1% crude protein
34.8% ether extract (lipids)
7.0% crude fiber
7.9% moisture
1.4% nitrogen free extract (NFE)
14.6% ash
5.0% calcium
1.5% phosphorus

Competes with the Finest Fish Meal

Studies were conducted at the Coastal Plain Experiment Station in Tifton, Georgia, to examine the suitability of SF prepupae as a feed source for channel catfish and tilapia. The tests concluded that soldier fly larvae should be considered a promising source of animal protein in fish production. Taste tests were also conducted, and the results of these tests indicated that fish fed SF larvae are acceptable to the consumer.

About half of SF fresh weight translates into a dry meal or pellet, and two nutrition studies done under the supervision of Dr. Craig Sheppard suggest that this dry matter has roughly the same value as Menhaden fish meal valued at over $500 US dollars per ton. Live SF prepupae have been successfully fed to bull frogs, tropical fish, reptiles, snakes and many other creatures that have a strong preference for living food. Here the value of fresh SF larvae ranges from $4 to $20 /lbs.

If a unit is installed at a residence where the weekly or bi-monthly collection of larvae might be somewhat expensive, the larvae can be placed outdoors in a shallow plastic pan where birds will readily feast upon them. Chickens are especially fond of live SF larvae.

What Happens in Winter

BSF larvae have an amazing ability to dispose of putrescent waste. But as the temperature drops below 21 degrees, their ability to digest waste progressively grinds to a halt, and if they should freeze, they die. This tropical fly larva needs to be sustained at temperatures above 30 degrees if it is to continue to digest putrescent waste at the standard rate of roughly 15 kgs/m2 of unit surface per day.

To bring bioconversion units indoors during winter would be costly, and to equip them with heating coils is not necessary. The strategy proposed here involves nothing more than placing a styrofoam sheet on top of the larval residue to retain the heat generated by larval movement. If this heat is not allowed to escape, the temperature on the surface of the residue easily exceeds 35C.

Black Soldier Fly Colony in Winter - Temperature Chart

The following graphs plot daily temperature readings both outside the unit and underneath the sheet of styrofoam. Note that outside temperatures may fluctuate dramatically, but the temperature underneath the styrofoam sheet remains relatively constant. The difference in temperature between inside and outside the unit can exceed at times 82F or 45 C.

During summer, the conversion rate of fresh food waste into fresh larvae runs as high as 20%, but during winter, this conversion drops to less than 5%, in spite of the fact that the larvae digest roughly the same daily quantity of food waste per unit surface area. Under ideal summer conditions, it takes about two weeks for newly hatched larvae to reach their mature prepupal form, but during the cold of fall and winter, this two-week period may extend to six months. If SF larvae are able to generate their own heat throughout winter and if they are able to extend their life cycle until more favorable conditions return in spring, then the management of SF larvae becomes far easier than anyone had previously imagined.

If disposal units are well insulated, then SF technology could be introduced to some of the coldest regions of our planet. If so, the supply of eggs to such extreme areas will become an important technical issue, and all aspects of larval maturation must be researched in a definitive and conclusive manner.

During the hot summer months, overcrowding can easily occur, and this overcrowding gives rise to relatively high temperatures within the unit. In order to cool down, some actively feeding larvae are forced to exit the unit. This migration continues until the density of larvae and temperature within the unit drop to an acceptable level. But during the winter months, larvae can thrive in very large numbers without overheating, and as the mass of larvae increases in winter, so too, the amount of waste consumed within a given unit. Paradoxically it would appear that this bioconversion unit functions far better in winter than in summer.


In our search for an ideal bioconversion unit for putrescent waste, we do not have to go far to find what we are looking for. Nature freely gives us a voracious grub that is by far the pre-eminent recycler of the fresh putrescent waste generated by human activity. All that this fascinating creature demands of us is an appropriate apparatus and environment to do its job.

With soldier fly technology, gone are the garbage bins filled within food waste rotting and stinking within a residence. Out at the curbside, no longer will garbage bins serve as magnets for flies, rodents and stray dogs. Gone are the garbage trucks reeking of rotting food, and gone, too, are the methane and odiferous gases generated at landfills.

With soldier fly technology, all of this nonsense comes to an end. If, in our effort to dispose of putrescent waste, we do not take our cue from the best that nature has to offer us, when will we ever learn?

Evolution of the the Bioconversion Unit

The BioPod: Materials Make the Difference

ESR LLC has begun manufacturing the black soldier fly bioconversion unit in polyethylene. Due to cost and weight issues, we are not producing pre-cast concrete units and this time. These units resemble garbage bins, but these bins (US patent 6,780,637) are somewhat special in that they possess evacuation ramps that permit the larvae to self-harvest into a bucket. Ramps begin at the bottom of the unit and spiral up to the top.

BioPod Prototype - Concrete Design BioPod Prototype - Urine Diverting Toilet Design ProtaPod Prototype - larger 3 foot unit for commercial applications

The spiral ramps need not be wider than about one inch. Consequently they occupy little space and incur little loss in the holding capacity of the unit. In the plastic version of the unit, the ramps are created by means of a fold in the wall of the container. In this way, there is no underside of the ramp within the container where migrating larvae might uselessly congregate.

BioPod Prototype - one of the first designs in plastic BP5 BP6

The round shape of the unit greatly assists the mature larvae in exiting the unit. As they randomly orient toward the periphery of the waste, they encounter the rounded wall of the container, at which they may turn either right or left. If they turn right, they eventually come to the base of the right ramp, and if they turn left, they eventually come to the base of the left ramp. Since the total distance that the larvae must travel in exiting a unit is very small, the efficiency of larval crawl-off is fully optimized.

Unit Capacity

This 2-foot residential unit has an average feeding surface area of 0.34 m2. At a disposal efficiency of 15 kgs/m2/day, it can handle over 5 kgs of food waste per day. It can hold or contain over 144 liters of larval residue, and with a reduction in weight and volume of 95%, it must be emptied after receiving a total of 2.89 m3 of food waste. This unit serving a family of four people would have to be cleaned out approximately once every 8 years. With this larval bioconversion process, the costly transport of food waste to landfill is completely eliminated.

Old School: Precast Concrete

The most basic way to manufacture soldier fly bioconversion units is by means of pre-cast concrete. But a pre-cast unit molded as a single part will be difficult to handle and transport. However, if molded in three vertical sections of 120 degrees, these sections are easy to handle, and they can be stacked against one another to reduce transport volume.

Another advantage of molding the unit in three sections: no metal reinforcement of the concrete is required. Since the three sections are held together by three nylon straps in much the same way that an oak barrel is held together by bands of steel, stress on the unit is relieved at the points of intersection of the three sections.

All that is needed for the fabrication of the unit is a dollar or two of cement, and recycled materials such as stone, brick or broken glass can serve as aggregate or filler. To reduce the weight of pre-cast concrete, a lightweight aggregate such as perlite and vermiculite can be used. The construction of bioconversion units could take on many of aesthetic qualities of Hypertufa: lightweight, artificial stone containers.

No Bottom

Note that the concrete unit has no bottom. The unit can be situated above a bed of sand that would serve as a partial filter, and any nutrients that escape this filter could be absorbed by the roots of plants situated around the perimeter of the unit. In this way any free liquids liberated by the larvae in the digestion of the waste do not necessitate the introduction of bulking materials. This greatly simplifies the operation of the unit.

If left out in the open, the unit must have a lid to prevent rainwater from coming in. A lid could consist of nothing but a sheet of plastic or plywood. The fasteners that hold down the metal strips at the top of the unit create sufficient space in between the unit and the lid to allow soldier fly access into the unit.

Ideal for Developing Countries

Such a unit is ideal for use in developing countries where the cost of materials is high relative to the cost of labor. Since cement is abundant and readily available throughout most developing countries, since very little skill is needed to fill a mold, small workshops could be easily set up to serve a specific area or province, thereby eliminating the costly transport of units over long distances. Our goal is to sell a unit capable of disposing of all the putrescent waste from a single household for less than $10 US dollars. Larger units could be easily constructed in the same simple manner as indicated above by changing the angle from 120 to 60 degrees, and by increasing the number of ramps from two to four. In this case, a unit would consist of six vertical sections. Since one half of a unit would identical to its other half, the entire unit could be fabricated, once again, out of three molds.

Urine-Diverting Toilet

Another big advantage of using precast concrete: the unit could easily support the weight of a pre-cast concrete lid that could incorporate all of the essential features of a urine-diverting toilet. Into the one bin goes all putrescent waste.