6.2 What are the process requirements?
To learn how to set up a complete BSF processing facility with biowaste, one can read Black Soldier Fly Biowaste Processing – A Step-by-Step Guide 2nd edition by Dortmans et al. Since no guidelines are available for faecal sludge processing yet, one would need to conduct an experimental phase to determine the bioconversion rates, feeding frequency, larva density, and faecal sludge mass reduction. Additionally, one must monitor the operation continuously.
The suggestions are based on experimental research studies and are applied to the treatment process and not for breeding.
Bioconversion Rate
The bioconversion rate is a metric that measures the amount of larval biomass generated in comparison to the feedstock given to the larvae. Studies conducted in Sweden and Kenya have shown that black soldier fly larvae can process faecal sludge with a bioconversion rate ranging from 10% to 20% of TS. This implies that if 100 grams of total solids (TS) of feedstock is fed to the larvae, their weight can increase from 10 to 20 grams of total solids (TS).
Feeding Rate
Feeding rate is a metric that measures the amount of feedstock provided per larva and day, expressed in milligrams per larva and day. An optimal feeding rate for faecal sludge from pit latrines in Uganda was identified at 50 mg/larvae/day (at 23 – 24°C, 60% moisture content, for 10-day-old larvae). Larvae can be fed once or periodically, for example, every fourth day, depending on the larval density and the feedstock quality.
Larval Density
Larval density is the number of larvae per m2 surface area of the container; one study in Uganda identified an optimal larvae density for the treatment of FS from pit latrines to be 13,300 larvae/m2 (1.33 larvae/cm2). The guideline for biowaste suggests a density of 33,000 larvae/m2 (3.3 larvae/cm2).
Temperature
BSF larvae grow best at temperatures between 24 and 30°C. Higher temperatures make them leave the food and look for a cooler place. Lower temperatures make them less active and slow down their metabolism.
Age of Larvae
To treat a defined amount of waste regularly, the rearing unit must provide a limited number of days. In Uganda, 10-day-old larvae were fed to treat FS. The guideline for biowaste suggests a supply of 5-day-old larvae for the treatment process.
Moisture Content:
The ideal moisture content for black soldier fly larvae (BSFL) feedstock is between 60% and 90%, with 70% being the optimum moisture content for the process. A study conducted in Uganda demonstrated that faecal sludge from pit latrines with moisture content as high as 90% can be effectively treated with black soldier fly larvae.
Particle Size
The larvae have no chewing mouthparts. Therefore, they access nutrients more quickly if the substrate is in small pieces or slurry form.
Light
BSFL’s avoid light and prefer a shaded environment. To ensure uniform treatment, it is essential to use a black cover to darken the container and prevent sunlight from interfering with BSFL feeding. The cover also helps prevent other flies from laying eggs in the container.
Depth of Organic Waste
BSF larvae feed on the top organic waste surface. In Uganda, the waste layer was 50 mm deep to treat FS.
Ventilation
Ventilation is crucial for providing oxygen to the larvae and removing air saturated with moisture. Increasing ventilation, particularly with fans, before harvesting can improve evaporation and produce a crumbly waste residue that is easy to sift from the larvae.
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Further Reading:
