Sustainable Waste Management at Household Level with Black Soldier Fly Larvae (Hermetia illucens)

Abstract

Waste management service is inefficient in peri-urban and rural areas where biowaste is a major component of the household waste produced. Biowaste recycling using black soldier fly larvae (BSFL) at source can reduce the burden on the authorities and add economic value to a yet underutilized resource. This study evaluated the practicability of BSFL bin use at the household level to handle kitchen biowaste by placing three bins per house after 15 days interval of larval feeding. It was found that 50% of the households contacted cooperated well to continue the experiment. A set of instructions for handling BSFL bins based on reasons of agreement and disagreement was developed and shared. Key parameters to evaluate waste treatment performance and larval development were waste dry matter weight reduction (89.66%, SD 6.77%), volumetric reduction (81.3%, SD 4.8), final prepupal dry weight (69 mg/larvae, SD 7.1), biomass conversion rate (12.9%, SD 1.7), metabolism (77.3%, SD 6.0) and residue (10.4%, SD 6.8). On average, 87.7% (SD 9.1) of waste was actually digested, with 16.6% (SD 2.2) efficiently converted into biomass. Initial moisture content of waste was found to be more significant for achieving greater waste dry weight reduction as compared with the feeding rate. Source separation of biowaste and cooperation between households and authorities may lead to successful implementation of BSFL-assisted biowaste bins.

1. Introduction

Solid waste management is a challenge for most of the settlements in the world and is especially acute in low- and middle-income countries such as Pakistan [1,2]. Primary waste collection and secondary transport components cost more than 70% of the total costs related to waste management in the developing countries [3]. In low- and middle-income regions, around 50–80% of municipal solid waste by weight consists of organic biodegradable material [4]. In peri-urban and rural areas, it is on the upper limit. However, it highly depends upon the economic and social status of the area. Therefore, whatever waste amounts can be sorted and recycled at the source of generation, especially household-level, will significantly relieve the burden on waste collection and transport services [5,6], especially in peri-urban and rural areas where services tend to be costly and inefficient. These large amounts of organics lead to the rapid filling of dump sites, resulting in bad odor, leachate, and methane gas generation that threatens water, air, and land quality, thus causing human health problems [7,8]. Recyclables with economic value are separated either at source or scavenged in the street and dump sites by the informal sector [1]. This, however, often does not apply to the organic fraction. Biowaste treatment at the household level may help divert waste from dumping sites, thus prolonging the lifespan of these sites [7].

The most common home-based biowaste recycling options are direct animal feeding and home composting. Direct animal feeding with source separated biowaste is widely practiced in rural areas and serves as a simple biowaste value recovery option for animal farmers [7]. This, however, implies that the household should have animals to feed. Home composting is another common small-scale biowaste recycling option in which households perform more or less controlled aerated decomposition of food, yard, and agricultural waste in small bins or heaps. This results in a nutrient-rich stable soil-like material known as compost, which is used for soil amendment [7,9,10]. Both of these recycling options aim at maximizing the value for the environment while at the same time minimizing the costs of waste collection and centralized treatment. However, there are certain risks and challenges associated with them. For example, directly feeding rotten items, meat, and cellulose-containing waste materials is not suitable for most of the animals. Furthermore, the pollution of biowaste with heavy metals, pathogens, molds, and pesticides may either directly affect the possible use or persist in the recycled product, thus requiring very careful waste segregation or pretreatment [7]. Composting further requires space, high temperatures, and time (around three months) and may face challenges such as vermin and odors. Lack of policy and governmental support, limited incentive systems, and marketing opportunities further hinder the mainstreaming of composting as a biowaste treatment approach. Apart from direct feeding and composting, anaerobic digestion has been shown to be another potentially feasible option [7,9]. However, substrate contamination, low energy density, and barriers in biogas storage and use as well as the need for more skilled labor and know-how may hinder widespread application of this technology.

Black soldier fly larvae (BSFL) Hermetia illucens L. (Diptera: Stratiomyidae) for biowaste recycling is an emerging technology that can convert a variety of decomposing biowastes into stable, environmental-friendly residue (similar to compost) and larval biomass, which can be used as animal feed [11,12]. Various studies have been conducted on the use of BSFL to treat municipal organic wastes such as kitchen and food wastes from households, institutions, and restaurants [13,14,15,16], animal manures such as from poultry [17] and dairy cows [18], agricultural waste and plant residues [19], human feces [20], and digested and undigested sludge [21]. Larvae of the black soldier fly are considered non-pest and non-vector organisms that reduce the breeding of houseflies [11,21]. They are also well known for reducing pathogens in organic waste such as from fecal sludge [20,22,23]. So far, municipal organic waste has been successfully treated through BSFL technology at laboratory and pilot-field scales. The study conducted in Costa Rica provides a good example of treating household biowaste at the source of waste generation [13]. Waste was collected from residents on a university campus and brought to the nearby research study site. Both household and fruit and vegetable market waste has been treated on a large scale through BSFL technology in Surabaya and Sidoarjo, Indonesia [4,24]. Application of this novel and environment-friendly [25] waste treatment technology at the source of waste generation can result in up to 80% biowaste waste reduction on wet basis, leading to a reduction in the typical risks associated with unmanaged or dumped organic waste, groundwater pollution, and greenhouse gas emissions. The remaining 20% of residue from BSFL treatment is not waste but can be used for soil amendment as compost.

Household kitchen waste is highly nutritional and heterogeneous in nature [26] and is composed of a variety of items such as fruit and vegetable peels, bread, rice, pasta, spent grain tea and coffee residues, cooking oil, decomposing yogurt, butter, milk, any meal, cooking residues, uncooked and cooked meat, etc. Kitchen waste is a preferred substrate for BSFL as it is rich in fats, proteins, and carbohydrates, and thus provides a good source of nitrogen (protein) and easily available carbon [12,16,27]. Both are essential for efficient larval growth and development. The BSF life cycle (fly—eggs—larvae—pupae—fly) can be engineered to a duration of 45 days under controlled environmental conditions to maximize its benefits, whereby the waste treatment phase is of 12–14 days [24]. An engineered BSF facility ensures the consistent supply of young larvae called 5-DOL (day-old-larvae), which are used and fed with specific amounts of biowaste at regular frequency until the larvae are grown and can be harvested. BSF life cycle starts with eggs, laid by female adult flies after 2 days of mating and copulation and which normally hatch in 4 days. The hatched young larvae are fed on a moist chicken feed mixture for 5 days and then transferred to the waste treatment unit, where they are fed on biowaste for 12 days. These fattened larvae can be then harvested and refined in product harvest units according to market demands. Around 2–5% is sent back to the rearing unit, where the 17-DOLs reach their 6th instar called prepupae [24]. After 2 days’ transformation period, the prepupae turn into pupae. The eclosion period starts in dark cages until the emergence of adult flies and life starts again [14]. The satellite waste management approach that treats waste at the source can be applied at the larval biowaste feeding stage of an engineered BSF life cycle. The 5-DOLs reared at the centralized BSF facility can be utilized by households where they can treat kitchen biowaste. The fattened larvae and residue can either be used by households as animal feed and compost or vended back to the centralized facility in exchange of a new BSFL bin with young larvae. Many commercial BSFL bins, also known as bio-pods, are now being introduced in the market. One can also design his own household-level bin according to the amount of biowaste generated every day. However, to what extent is placing these bins at the household level acceptable and practical is still not a fully established fact.

In the late-19th century, the household-level BSFL compost system was first investigated in Australia [28]. According to a study, household biowaste was used to raise BSFL as a potential ingredient in poultry feed [29]. Moreover, around 2000 BSFL can consume 1 kg of household biowaste per day [2]. Therefore, treating household kitchen waste would be beneficial as it is one of the major contributors of municipal organic waste and a preferred substrate for BSFL development due to its mixed composition [30].

Keeping all the waste management-related problems in view and the ideal properties of one of the most promising insects [31], a BSFL Satellite Waste Management (St WM) approach was tested in this study to treat kitchen biowaste at source, i.e., household level. The idea was inspired from a two-tier BSFL waste management model according to which a centralized rearing facility could serve several small, decentralized waste treatment units [7]. In the present study, the term “satellite” is used for small, decentralized units, which could be “household” or “community” level. The present study is innovative and unique in the sense that it provides a practical option of substituting the concept of traditional waste bins with BSFL-assisted waste bins. A BSFL waste bin can be defined as “the bin in which source separated biowaste is fed to the larvae of black soldier fly for waste treatment and its bioconversion into useful end products”. The purpose of the study was to evaluate the performance of BSFL-assisted waste bins to treat and convert household kitchen biowaste into useful end products and to identify the prospects and constraints of this emerging technology.

2. Materials and Methods

2.1. First Phase: Selection of the Households

A sample of 20 households was selected on the basis of the “non-random convenient sampling method” [32]. This maximum number of households selected resulted from the availability of larvae and time constraints of the project team. All these 20 households were provided an awareness and information campaign explaining the significance of BSFL biowaste treatment, which included the following main points: (i) benefits of BSFL waste treatment technology and its end products (residue and larvae), (ii) importance of treating biowaste at source, (iii) encouraging households to separate kitchen organic waste, and (iv) social and environmental benefits. In the first phase of the experiment, after the awareness and information campaign, 17 out of 20 households agreed to place an experimental BSFL bin in their houses for 15 days. For these 17 households, an introductory session was conducted about BSFL biowaste treatment and bin handling. Thereafter, a bin was provided to each household containing 5-day-old larvae (5-DOLs). The number of 5-DOLs per bin was in accordance with the expected amount of waste to be generated daily from each house and in reference to the requirement of 650,000 larvae per ton of waste fed over a period of 12 days [24]. The 5-DOLs were obtained from Pakistan’s first BSFL Municipal Organic Waste Treatment site of the Urban Unit, where hatchlings were fed on a feed mixture consisting of 30% chicken feed and 60% water [13,24]. Households were asked to daily add only organic kitchen waste to the BSFL bins and mix the contents of the bin at least once a day. The households were visited by the research team thrice a week or more frequently when summoned by the households. On the basis of general observations and the households’ feedback, and problems faced during the first phase, a set of instructions related to BSFL bin handling were documented.

After the initial placing of the bins, within 10 days, 7 households declined to continue with the experiment. Reasons for agreement and disagreement by households to participate were documented. The remaining 10 households completed the 15-day experiment and also agreed to continue with a second phase of experimentation (monitored by the research team).

2.2. Second Phase: Bin Setup and Handling at Households

Keeping in view the ideal bin for BSFL biowaste treatment at the household level, a horizontal crate style plastic bin was designed with dimensions 11 × 32 × 60 cm. It was provided with a removable thin net cloth to protect the waste and larvae from predators (lizards, mouse, cats, dogs, etc.), houseflies, mosquitoes and unfavorable weather conditions such as direct sunlight. Furthermore, a wooden stick for mixing waste and larvae, a small towel cloth for cleaning the wet bin walls in order to prevent larval crawl-out and some wheat bran was also provided. Wheat bran had the following purpose: (i) to absorb moisture from waste by sprinkling it over the waste top layer; (ii) to prevent unexpected migration of larvae by spreading it to the inner sides and edges of the bin.

The 10 households that agreed to continue in the second phase of the study were now provided with the improved BSFL bins. Larvae were added to each bin according to the expected amount of waste generated daily from each house on the basis of the total number of house members and the waste amount that can be treated by larvae within 24 h (

Table 1. Expected and actual amount of kitchen food waste generated from 10 different houses for 15 days and the number of larvae provided to each house per BSFL bin for waste treatment.

H No.	Bin No.	Total No. of Inhabitants/H	Expected Amount of Waste/H	Expected Amount of Waste/H in 15 Days	No. of BSFL Added/Bin	Total Amount of Waste Added in 15 Days (Ww)	Feeding Rate
			(kg/day)	(kg)		(kg)	mg/BSFL. day
H1	KW 1	2	0.5	7.5	3900	7	120
	KW 11				3900	6	103
	KW 21				3900	7	120
H2	KW 2	4	1	15	7800	15	128
	KW 12				7800	16	137
	KW 22				7800	13	111
H3	KW 3	4	1	15	7800	14	120
	KW 13				7800	13	111
	KW 23				7800	13	111
H4	KW 4	5	1.25	18.75	9750	18	123
	KW 14				9750	12	82
	KW 24				9750	17	116
H5	KW 5	6	1.5	22.5	11,700	18	103
	KW 15				11,700	22	125
	KW 25				11,700	23	131
H6	KW 6	6	1.5	22.5	11,700	20	114
	KW 16				11,700	21	120
	KW 26				11,700	18	103
H7	KW 7	7	1.75	26.25	13,650	25	122
	KW 17				13,650	27	132
	KW 27				13,650	27	132
H8	KW 8	8	2	30	15,600	30	128
	KW 18				15,600	30	128
	KW 28				15,600	32	137
H9	KW 9	8	2	30	15,600	30	128
	KW 19				15,600	32	137
	KW 29				15,600	30	128
H10	KW10	9	2.25	33.75	17,550	34	129
	KW 20				17,550	36	137
	KW 30				17,550	30	114

).

Each bin stayed in a certain house for 15 days; thereafter, the bin was replaced by a new bin with fresh 5-DOLs. This procedure was continued for three rounds (3 × 15 days). Houses (H) were labeled H1 to H3, and the bins were labeled KW1 to KW30 (Table 1). Households were asked to feed BSFL bins with daily generated kitchen waste once every 24 h, preferably at the same time every day. A weighing scale and data collection sheets were also provided so that households could document the amount of waste added to the bins on a daily basis. Households were instructed to mix the waste in the bin at least once a day and to drain excess water if required using a cloth or sieve. On the 15th day of treatment, the BSFL bin was collected from each household and transported back to the centralized rearing facility.

2.3. Waste and Larval Sampling for Moisture Content and Dry Matter

Moisture content of fresh incoming waste (added to the bins) was determined on the 1st, 7th, and 13th day of experiment. The mean values of three days gave the initial moisture content of waste for each bin. The final waste moisture content was determined at the end of the experiment after 15 days by taking three representative samples from each bin. Larvae were hand-picked from the sample and separated from the residue using tweezers. Moisture content of the waste was measured as the mean of three samples taken, analyzed in the laboratory by placing 30 g of waste sample in an oven for 105 °C for 24 h. Waste densities were measured from the three samples to calculate the initial volume of waste added to the bin. At the end of every 15-day period, 3 representative samples of 15 BSFL were removed from each bin and then surface-dried using tissue paper and weighed to obtain their wet weight. The samples were then dried in oven at 105 °C for 24 h to obtain the dry weight of the larvae. Moisture content of the larvae was calculated by subtracting dry weight from wet weight.

2.4. Determination of BSFL Biowaste Treatment Performance and Conversion Efficiencies

The following parameters were assessed to evaluate BSFL waste treatment efficiency in the bins: (i) initial wet weight of waste measured daily and total waste added over 15 days, (ii) wet and dry weight of waste residue after 15 days of experiment; (iii) dry weight of waste in bin measured on the 1st, 7th, and 13th day of the experiment; (iv) larval wet weight at the start (5-DOL) and after 15 days of the experiment; (v) larval dry weight after 15 days of the experiment.

Based on the above measurements, the following seven performance and conversion parameters were calculated: waste dry matter reduction (Wred), calculated using Equation (1), which is the ratio of ingested feed (calculated as the difference between dry weight of total feed (Wdi) and dry weight of residue (R)) and the dry weight of the total feed (Wdi) [6,18]

Wred (% DM) = ((Wdi − R)/Wdi) × 100

(1)

Volumetric waste reduction (VR) was calculated using Equation (2) as percentage based on the difference between initial volume (Vi) and final volume (Vf) in m³.

VR (%wet weight) = ((Vi − Vf)/Vi) × 100

(2)

The bioconversion rate (BCR) was calculated using Equation (3), which is the ratio of prepupal weight gain biomass (Bpp) on dry basis (kg) to the total amount of biowaste initially added (kg) on dry basis (Wdi) [15,18,30]. Prepupal weight gain biomass (Bpp) was obtained by measuring prepupal final dry weight, whereby the initial dry weight of 5-DOLs was assumed to be zero. Neonatal larvae have a negligible amount of dry weight, given that most of their body weight consists of water.

BCR (% DM) = ((Bpp)/Wdi) × 100

(3)

Metabolism (M) was calculated using Equation (4). Metabolism is the amount of food consumed that is used as energy by the larvae to maintain body homoeostatic balance utilization in respiration and larval activity. It is the difference between ingested food (Wdi-R) and prepupal weight gain on dry weight basis [6,33]

M (% DM) = ((Wdi-R − Bpp)/(Wdi)) × 100

(4)

The ingested food conversion efficiency (ECI) was calculated using Equation (5) [6,33,34] as the ratio of prepupal weight gain (kg) to the amount of ingested food on dry basis (kg).

ECI (% DM) = (Bpp/(Wdi-R)) × 100

(5)

Approximate digestibility (AD) was calculated using Equation (6), which is the ratio of the difference between the weight of ingested food and residue (kg) to the ingested food on dry basis (kg) [35,36,37,38].

AD = (Wdi-R) − R)/Wdi-R) × 100

(6)

Digested food conversion efficiency (ECD) was calculated using Equation (7) as the ratio of ingested food conversion efficiency (ECI) and approximate digestibility (AD) [35,36,38].

ECD = ECI/AD

(7)

Note: In the present study, ECI and ECD can be defined as the ability of BSFL to convert kitchen food waste into their body mass, where ECI and ECD represent ingested food and digested food conversion efficiencies. The higher the food conversion efficiencies are, the higher will be the ability of larvae to convert waste into body mass.

The study did not include structured interviews. It sought the feedback from households through an open discussion, which was recorded and analyzed to obtain the main reasons stated by the households for their agreement or disagreement to participate further after the first phase of the experiment.

2.5. Statistical Analysis

The response surface test was performed using Minitab software (version 19) to determine the factors showing a significant impact on BSFL biowaste treatment performance. Two-way ANOVA (p ≤ 0.05) was applied to determine the statistically significant differences between various factors affecting treatment performance parameters and larval development. The Pareto chart of the standardized effects was plotted to visually identify the factors having a major effect on performance parameters. 3D surface plot analysis was performed to identify the relationship between different variables.

3. Results and Discussion

3.1. First-Phase Experiment

3.1.1. Bin Design

During the first phase of the experiment, observations on bin design and its effect on BSFL-assisted biowaste treatment at the household level were documented. These can be summarized as follows:

• The bin must provide enough surface area for all larvae so that they can move freely and breathe easily within the waste.
• The bin should be lightweight and easy to handle and transport.
• The bin should allow easy mixing for uniform distribution of larvae within waste to prevent anaerobic conditions and higher level of heat.
• The bin should allow excess water to drain, else the accumulated excess water may affect larval activity.

3.1.2. Operational Aspects

During the first phase of the experiment, it was observed that some larvae started crawling out of the bins. Household members found this situation very difficult to handle and accept. Larvae crawl-out was observed as a result of delayed feeding by households and early pupation. This happened in the situation of limited waste supply at the household level as the larvae crawled out to search for food. In some cases, households counteracted this by adding fresh food to the bin instead of waste, which clearly was not the purpose of this initiative of waste treatment.

In other cases, the type of waste fed to the bin was not suitable as BSFL feed. This was observed when a house member added larger amounts of watermelons (very high water content), slimy, creamy waste (such as sauces, ketchups, creams), and woody waste (such as sugarcane husk and tree branches). Watery, slimy, or creamy waste inhibits larvae movement and creates anaerobic conditions, leading to very limited growth or even death. Woody waste cannot (or only to a very limited degree) be processed by the larvae due to the high cellulose and lignin content of such waste.

During first-phase experimentation and visits by the research team, it was observed that households sometimes had overloaded the bin with excess waste. Their intention was to feed larvae with more food so that they could grow bigger and fatter. Overfeeding, however, led to too much waste in the bin, which could not be processed quickly enough by the number of larvae in the bin and thus led to thick waste layers in the bin, which created anaerobic conditions suffocating the larvae or badly affecting their activity.

Some households forgot or ignored the recommendation to mix the waste on a daily basis. This led to uneven distribution of waste, and many larvae were found deprived of food. It was furthermore observed that the larvae colonized themselves, especially at the corners of the bins, leaving most of the waste untouched, which resulted in increased water content.

Another problem encountered at the household level was the unexpected presence of lizards and rodents entering the bins. In a few cases, cats also entered the BSFL bins. Birds preyed on the open bins to feed on the larvae. Moreover, housefly larvae were also found in BSFL bins. Although BSFL are known to prevent housefly breeding [21], this was not observed in this study. This might have been a result of overloading the bins, where larvae are not able to process the waste fast enough, and fly oviposition and hatching became possible.

Some households placed their BSFL bins in a location that received direct sunlight or rain. Sunlight slowed the activity of the larvae as they prefer shaded conditions. Sunlight also led to drying of the waste surface and even dehydration and death of the larvae in the bin. Exposure to rain turned the waste watery and led to larval migration out of the bin.

3.1.3. Set of Instructions and Precautionary Measures

Based on the observations of the first-phase experiment, a set of instructions was prepared for households to avoid problems with bins, and further training was provided. The set of instructions can be found in Appendix A.

Open discussion with all households at the end of the first phase allowed documentation of the reasons for agreement and disagreement to improve and use this waste treatment method again. Ten out of 17 households agreed to continue for a second phase of placing new BSFL bins in their houses.

Table 2. Reasons for Agreement and Disagreement.

#	Reasons for Agreement	Reasons for Disagreement
1	Almost half of the households foresaw an economic potential in the useful end products of the BSFL biowaste treatment method, i.e., fattened adult larvae as animal feed and residue as soil amendment. Households showed interest in learning this new technology to develop personal BSFL farms for business purposes.	The majority (86%) of the households that did not agree to participate in the second phase mentioned that placing BSFL waste bins required too much additional work given their daily busy routine.
2	About 30% of the households were convinced that in order to solve the solid waste management problem in the country, where land is costly, open dumping is common, and proper landfills are not available, it is important to treat biowaste at the household level. They were happy to see their kitchen waste being converted into useful, environment-friendly products.	Almost 57% of the households mentioned that they were not comfortable mixing waste on a daily basis as they found this activity smelly, unhygienic, and time-intensive.
3	Love of pets also convinced 20% of the households to place BSFL bins in their houses.	As source separation is not a common practice in Pakistan, 43% of the households disagreeing to the second phase mentioned that they felt it was too difficult to collect and sort kitchen biowaste separately on a daily basis.
4	Households that had spacious houses with indoor lawns were more inclined to accept BSFL bin placement in their houses. This was the main reason for participation mentioned by 70% of the households as they also commented that they would like to use the end product “residue’’ for gardening.	Overwhelmingly, 71% of all households had small houses and considered it too difficult to find a suitable place to place BSFL bins.
5	Households were provided the option to either return BSFL bins back to the facility or sell it to the poultry farmers directly. The direct sales option attracted 30% of the households to have BSFL bins.	One-third (29%) of the households reported that they needed to drain extra water from the BSFL bin. All these households found it an unhygienic, difficult, and time-consuming task.
6	Half of the households also showed a general interest in supporting this research activity as it was the first of its kind in Pakistan focusing on BSFL waste treatment technology.	A general feeling of fear of insects compelled 14% of households to refuse placing BSFL bins.
7		Approximately one-third (29%) of the households considered their amount of kitchen waste to be very small and felt a bin was not required for them. This conclusion by such households implies a lack of awareness and realization that even small amounts by individuals build up to big, unmanageable heaps at dump sites when considering the whole city.
8		Among all, 43% of the households considered solid waste management as not an important problem, or a problem that is already being addressed sufficiently by local authorities.
		The majority (71%) of the households disagreeing to enter the second phase mentioned the aspect of larval crawl-out from the bin as one of the reasons for refusal.

below presents the reasons for agreement and disagreement found during the study.

3.2. Second-Phase Experiment

Ten households agreed to continue for a second-phase experiment, which included three consecutive batch experiments of 15 days for each participating household. The results of this experiment were used to assess BSFL waste treatment efficiency.

3.2.1. Waste Dry Weight and Volumetric Reduction

Household-level BSFL biowaste treatment resulted in a mean waste dry matter weight reduction of 89.6% (range 77.0–96.1%; SD 6.77). The average waste dry matter weight reduction achieved was higher than values reported in previous studies on BSFL treatment using restaurant kitchen food waste, municipal organic waste, canteen waste, vegetable canteen waste, poultry feed, slaughterhouse waste, and wheat mill by-products [13,14,15,16,26,30].

The type and amount of waste, its composition, and moisture content added to the bin varied each day. This relates to kitchen activity and the household’s daily routine. In the present study, the moisture content of waste ranged between 58.5% and 78.5% with a mean of 69.9% (SD 5.5). Unfortunately, at household-level experiments, it was not practically possible to ensure an appropriate adjustment of moisture by household members with varying waste composition each day. Sometimes larvae would get only eggshells, stems of leafy vegetables, hard seeds, or banana peels as part of the kitchen waste, which are all not easily digestible [7,26]. Other times, the kitchen waste was rich in carbohydrates, proteins, and fats such as the remains of bread, decomposable cooked food, meat, rotten fruits, and vegetables. However, in the present study, the composition of kitchen waste was not monitored in detail. According to a study, biowastes comprising less amount of easily available carbon and a high proportion of carbon with lignin material are less likely to reduce easily through BSFL treatment [15].

Therefore, the moisture as well as composition of waste seem most probably the main reason for the variation in waste dry weight reduction in the bins placed in the same house but at different times. Other reasons for the variation in waste dry weight reduction between BSFL bins placed in the same or different houses may include inconsistency in the frequency of waste mixing and a variation of time between waste feedings. Although participating households were informed about handling procedures, their full compliance to the procedures could not be ensured.

Both factors, initial waste moisture and the waste amount added, significantly affect (ANOVA, p < 0.05) waste dry weight reduction achieved through BSFL treatment. Figure 1a illustrates that the initial moisture content of waste has a more significant effect on the dry weight waste reduction than the amount of waste added as it extends past the reference line at 2.06.

Surface plot analysis (Figure 1b) shows that the higher values of waste dry weight reduction are where high values of waste moisture content are. The lowest values of waste dry weight reduction are at low waste moisture and the mid-range of larval feeding rate. In the literature, the moisture content of waste, which should range between 65% and 80%, is reported to be even more important than carbohydrate and protein content of the waste for larval feeding efficiency and development [7,39].

BSFL biowaste treatment at the household level resulted in a volumetric reduction of 81.3% (range 72.8–89.0%; SD 4.8) (

Table 3. Summary table with means of BSFL household biowaste treatment performance parameters.

Performance Parameters	Mean	SD
Waste dry weight reduction (%)	89.6	6.8
WRI (%)	5.9	0.44
Volume reduction (%)	81.3	4.8
Wet wt of 1 adult BSFL (mg)	198.5	40.8
Dry wt of 1 adult BSFL (mg)	69	7.2
Feeding rate (mg/BSFL.day)	121	12.6
BCR (%)	12.9	1.73
Reside (%)	10.4	6.8
Metabolism (%)	77.3	6.0
ECI (%)	14.4	1.4
AD (%)	87.7	9.1
ECD (%)	16.6	2.2

). Previous studies on BSFL biowaste treatment do not provide data on this parameter. Volumetric reduction is important when considering the vehicle volume capacity requirements when transporting the bin content to a central recycling facility or when disposing of the residue in a dump site. In such a case, dump sites would receive waste in a stable form as soil cover. Furthermore, a high-volume reduction is beneficial for a large family of 8- to 9-member households to treat large quantities of waste generated but nevertheless not needing much bin volume.

3.2.2. Bioconversion Rate, Metabolism, and Residue

The average dry weight of adult larvae achieved in the study was 69 mg/BSFL (range 47–84 mg/pp; SD 7.1) and the average larval wet weight was 200 mg/BSFL (Table 3). Larvae dry weight is comparable to previous studies using municipal organic waste and fecal sludge treatment as the substrate [13,26]. However, it was higher than the dry weight of larvae fed on chicken feed, canteen waste, slaughterhouse waste, vegetable canteen waste, wheat mill by-products, wheat middlings, and brewery waste [6,19,30]. The average larval wet weight of 200 mg achieved in this study is comparable to larvae reared on restaurant food waste and fruit vegetable waste [15].

BSFL treatment of household kitchen waste resulted in an average BCR of 12.9% (range 9.6–15.5%, SD 1.73) (Figure 2). These results are higher than those reported in studies conducted on mixed biowaste [13], fruit and vegetable waste [15,40], human feces, and animal manures such as poultry, dairy, and horse manure [15,18,40,41] and similar to the one achieved for restaurant kitchen food waste treatment [15]. Different waste compositions and feeding rates from different households may affect the BCR values of BSFL. This study was performed at ambient temperatures ranging between 23 and 38 °C in a semi-arid environment. High temperatures could affect the larval activity and bioconversion process [41,42].

A very small proportion (10.4%, SD 6.8) of the material remained in the bin as residue (Figure 2), which consists of a mixture of larval feces and undigested feed [6,24,33]. The assimilated kitchen waste metabolized by BSFL to energy averaged 77.3% (range 65.1–85.6%, SD 6.0). This situation is comparable to the study conducted on bioconversion of restaurant food waste and vegetable waste by BSFL [26,33]. During food shortage, larvae prioritize to excrete less and metabolize more food to energy [33].

The higher proportion of metabolized waste and lower proportion of residue is an achievement from a waste management perspective. However, to achieve higher bioconversion rates and prepupa biomass, both quality and quantity of biowaste are important [14,41].

3.2.3. Digestibility and Food Conversion Efficiencies of BSFL for Household Kitchen Waste

For more accurate quantification of actually digested food by BSFL, its “approximate digestibility”, commonly known as assimilation efficiency (AD), was calculated [36,37]. However, this measure has not been determined in many past studies on the analysis of food waste conversion efficiencies for BSFL [6,33,34]. Another study specially highlighted the importance of BSFL assimilation efficiency as a measure for more accurate quantification of actually digested food [6]. In the present study, on average, 87.7% (SD 9.1) of the kitchen biowaste was digestible by BSFL. The BSFL efficiency of converting ingested (ECI) kitchen waste into biomass averaged 14.4% (range 11.6–17.0%, SD 1.40). ECI gives a rough and overall measure of the insect’s ability to use ingested food for its growth [38]. The higher the ingested food conversion efficiency, the higher will be the ability of the insect to convert food into its biomass [6,33]. The ECI values obtained in this study were higher than those reported for animal manures, rice straw waste, and brewer waste [19,43,44]. The food conversion efficiencies varied among BSFL bins in this study.

The measured AD and ECI were then used to calculate the efficiency of BSFL to convert actually digested food into its biomass (ECD) as many essential nutrients may not get absorbed into larval bodies [36,37,38]. The results showed that 16.6% (SD 2.2) of the digested kitchen waste was actually converted into larval biomass.

Results of the study showed that metabolism of BSFL significantly affected ECD values (ANOVA, p < 0.05), whereas the effects of AD were not statistically significant (ANOVA, p > 0.05; Figure 3). ECD may vary within same species of insects due to several factors. Higher metabolism rate of the insect to utilize food for energy may lower its ECD values [36]. This trend can clearly be observed in Figure 4. Therefore, the factors influencing metabolism of BSFL may affect its ECD value. In this study, the highest ECD (22.0%) was achieved in BSFL bin KW26 with the lowest metabolism (65.1%) and the lowest ECD (12.2%) was recorded for bin KW17, showing the highest metabolism (85.6%; Figure 4). It is also possible that due to higher metabolic activity, BSFL increased their actual digestibility, which may result in reduced ECD [36]. This trend can be observed in Figure 4 and also in a past study [37]. Figure 4 shows that both AD and metabolism are inversely proportional to ECD. However, all points are not linear to the straight line as this variation is not always due to the maintenance of homeostatic balance within an insect’s body [36]. It might be due to other factors, which include availability of essential nutrients (especially protein) in sufficient concentrations considered critical for larval growth [36,38,44]. ECD does not directly depend on digestibility. A higher ECD value is an indication of better larval efficiency to convert actually digested waste into its body mass [6]. In practice, it is difficult to identify cause–effect relationship for conversion efficiencies. It is possible that an insect consumes more because of low digestibility, or digestibility could be low because of a high food consumption rate [36].

4. Conclusions

About 50% of the households contacted for the study accepted to take part in the experiment and agreed to place BSFL bins in their houses. However, there was also considerable resistance to placing the bin inside the house, especially in smaller dwellings and from people who considered it to be an additional, time-consuming work or risk of larvae spreading out of the bin. The participating households faced few operational problems, which were addressed through brief training sessions and visits by the research team. This study documented operating procedures, precautionary measures, and a set of instructions for handling BSFL biowaste bins that would be helpful for academia and municipalities to conduct similar experiments. Furthermore, the reasons for agreement and disagreement for placing BSFL bins at the household level provided insights for future research and better planning.

It was found that the larvae successfully reduced biowaste in the range of 77.0–96.1% by weight and 72.8–89.0% by volume. These figures are quite important as any decrease in weight or volume of the organic waste requiring further transport and disposal is crucial for local authorities. Though the BSFL bins cannot be placed in all households of a city, the study provided an option that could be more suitable to peri-urban and rural areas. Waste management in rural areas is quite neglected in the developing countries, and this option may help to handle waste at source. The results confirmed the findings of other studies that moisture content of the waste in the range of 65–80% produces better results and BSFL are more suited to treat such waste. Training programs can increase the understanding level of the BSFL biowaste treatment process and bin handling, which in turn improve larval development and conversion efficiencies. In this regard, larval feeding rate, waste composition, and moisture content were found to be important constraints. However, these parameters cannot be made constant or exactly predictable as they depend on household kitchen activity.

The following recommendations are being suggested for similar studies as well as any implementation of the idea:

• In addition to the technical commodities required, the waste producers must be provided with a set of instructions similar to the one proposed in this paper to manage the bins.
• As the quantity and composition of waste fed to bins may vary even within a single household, operational problems should be expected and the households must be informed about the most common of these in advance.
• If households feel that the number of larvae is not in accordance with waste fed daily by observing leftover/non-treated waste at the end of the day, they should seek additional larvae.
• As moisture content of waste was found to be a crucial parameter, the participating households should be trained to maintain the moisture content in a range to the best possible level.
• Local authorities should encourage this solution, which can certainly decrease waste handling costs.
• This model is more suitable to large houses, especially with gardens and dwellings, in peri-urban and rural areas.
• This idea should be encouraged in rural settings where the end product can be used as a soil nutrient while larvae can also be fed to chicken and other animals as a protein source.