Black Soldier Fly Biowaste Processing
Urban solid waste management is considered one of the most immediate and serious environmental problems confronting urban governments in low- and middle-income countries. The severity of this challenge will increase in the future given the trends of rapid urbanization and growth in urban population. Recycling organic waste material (biowaste) is still fairly limited, especially in low- and middle-income settings, although this is by far the largest fraction of all generated municipal waste.
Scientists are searching for alternative solutions for bio-degradation of municipal waste and Insects larvae can provide a best possible alternative solution. In this regard Black Soldier Fly (BSF), Hermetia illucens, has obtained much attention in the past decade. Its popularity links to the promising opportunities of using the harvested BSF larvae as a source of protein for animal feed, thus, providing a valuable alternative to conventional feed.
Enterprises and small entrepreneurs are already investing significant amounts of money into this technology and are interested in keeping a competitive edge on the practical aspects of operating such a facility in a cost effective way.
In this article we will review an BSF model of waste treatment. Up scaling or transferring this information to a larger facility might require some adaptation or adjustment of equipment.
The economic viability of a BSF processing facility
generally it depends on the local conditions, but the key factors includes:
- Scale and respective capital and operating costs of the facility.
- Environmental conditions.
- Potential revenue from waste processing (tipping fees).
- Sales revenue from larvae derived products (e.g. whole larvae, protein meal, larval oil, etc.)
- Sales of the waste residue as soil amendment or its use in a biogas plant.
Waste Treatment by BSF
The natural life-cycle of BSF is the fundament requirements for an efficient and reliable waste treatment facility using BSF larvae. However, to treat biowaste on a regular basis, the operator has to take control over the entire life cycle and, thus, create and operate an engineered biosystem. To provide an environment that best mimics the natural habitat of the BSF, while at the same time ensuring a continuous waste treatment, the following points should be taken into consideration when selecting an appropriate site for a BSF processing facility:
- Availability of sufficient fresh waste at low cost.
- Routes for delivery of garbage and pickup of residue.
- Water and electricity supply and wastewater management options should be available.
- Adequate environmental buffers that separate the facility from the surroundings should be maintained (e.g. open areas, trees, fences, etc.)
- Facility should be downwind from the residential areas.
- Closed and ventilated room for the rearing, but sunlight for the love cages.
- Office and lab space.
Engineering the BSF waste Treatment Facility
In an engineered BSF processing facility, we can differentiate distinct processing units as shown below figure.
BSF rearing unit
This ensures that a reliable and consistent amount of small larvae (called 5-DOL) is always available to inoculate the daily amount of biowaste that is received for processing at the treatment facility. A certain number of larvae hatchlings are, however, kept in the rearing unit to ensure a stable breeding population.
Waste receiving and pre-processing unit
It is critical that the waste received at the facility is suitable for feeding to the larvae. A first step involves a control of the waste to ensure it contains no hazardous materials and no inorganic substances. Further steps then involve a reduction of the waste particle size, a dewatering of the waste if it has too high moisture and/or a blending of different organic waste types to create a suitable balanced diet and moisture (70-80%) for the larvae.
BSF waste treatment unit
This is where the 5-DOL from the rearing unit is fed with biowaste in containers called “larveros”. Here, the young larvae feed on the biowaste, grow into large larvae and, thus, process and reduce the waste.
Product harvesting unit
Shortly before turning into prepupae, the larvae are harvested from the larveros. The waste residue itself is also a product of value.
Both products, larvae and residue, can be further processed if required by the local market demand. We call this “product refining”. Typically, a first step will be to kill the larvae. Other steps of larvae refinement can be to freeze or dry the larvae, or to separate larvae oil from larvae protein. A typical step for residue refinement is composting or feeding the residue into a biogas digester for fuel production.
Waste receiving and pre-processing unit
Larvae are generally very tolerant when it comes to feeding substrates. Yet, it is important that the biowaste received at the facility is suitable as larva feed. With water content between 60% to 90% and a specific particle size, most organic materials will be treated in one way or the other.
The larvae strongly depend on symbiotic microorganisms which degrade cell structures and make nutrients available for the larvae to take up. With suboptimal feed, however, development time will be extended and the final larval weight will be lower. It is important to keep this in mind when looking at the BSF facility from an economic perspective.
A first step upon arrival of the waste involves a waste quality control to ensure that it contains no hazardous materials and no inorganic substances. With the waste quality ensured, the next required step then involves a reduction of the waste particle size. This can be achieved by using a shredder or hammer. Whatever type of technology is used, the equipment should shred the waste to particles of smaller than 1-2 cm in diameter.
BSF treatment unit
A specific amount of 5-DOL are transferred daily from the BSF rearing unit to the BSF treatment units containing the waste (we call these “larveros”). The number of 5-DOL added will depend on the amount of biowaste that is contained in a specified volume and surface area.
As a rule of thumb we can work with the following numbers: 10,000 5-DOL in a larvero (40x60x17cm) feeding on 15kg of wet waste (75% water) for 12 days.
While the 5-DOL feed and grow, more waste is added to the same larvero on day five and again on day eight, until the larvae have developed large enough to be harvested after 12 days of feeding, i.e. on the 13th day.
After 12 days of waste treatment by BSF larvae, each larvero is harvested. At this stage, the larvae have reached their maximum weight, but have not yet transformed into prepupae. Their nutritional value is, therefore, at its maximum. Harvesting is the process in which the larvae are separated from the residue. This can be done by using a manual or automated shaking sieve by which the larvae are easily separated from the residue. With a higher shaking frequency, the mesh size of the sieve can be bigger. A sieve mesh size of around 3 mm for manual sieving and 5 mm for automated sieving is considered suitable
Post-treatment of the larvae and residue
After harvesting, larvae may be sold alive to customers (e.g. reptile farms or bird markets). Another approach is to use them in the production of feed pellets. Freshly harvested larvae can be mixed with other ingredients (e.g. soy meal, sorghum, corn, etc.) to make a blend that meets the nutritive requirements of the targeted animal (broiler chickens, layer hens, different fish species, etc.)
Post-processing of the crumbly residue is required to produce stable, mature compost. Different measures can be envisaged to do this. Composting the residue for a period of two months is the simplest approach. This will result in a stable mature material that can be marketed in the same way as compost. Another option is to feed the residue into a vermicomposting facility to grow (and market) worms, as well as to obtain a stable and mature vermicompost. Finally, the third option proposed here, which is suitable when the residue is high in moisture and slurry-like, is to feed it into an anaerobic digester (biogas reactor).