Quantifying the components of the mass and energy
balance of a black soldier fly (BSF) food waste
composting system
Tiny but Hungry
Shwe Sin Win
Ph.D. Sustainability
Alicia Piscitelli
B.S Chemical
Engineering
Problem Statement
The Burden of Food Waste: Existing waste management systems for handling the large volume
of food waste generated in the United States have been facing numerous environmental, economic,
and political challenges. Landfill sites occupy valuable arable land, generate leachate and emit
greenhouse gases and odor [1]. Methane (a potent greenhouse gas) emissions are generated during
the decomposition of food waste under the aerobic conditions at the landfill. Greenhouse gasses
are also emitted during transportation to the sites. Building one new landfill to meet regulations is
politically challenging and operating it for a 21 year life span can cost as much as $12-37 billion
according to World Bank estimates [2]. In 2015, the U.S. government declared a goal of reducing
food waste by 50% by 2030 [3]. Massachusetts, Connecticut, Vermont, Seattle, San Francisco and
Portland have already banned landfilling food waste from large commercial food waste generators
[4].
Black solider fly (BSF) waste management systems could potentially reduce the amount of food
waste needing to be landfilled in areas of concentrated generation, such as urban areas and
institutions like universities and hospitals while also providing products with added value.
Currently, BSF waste management systems are employed large-scale in warmer climates [5] and
have been studied only in laboratories and pilot-scale facilities in colder climates [6]. Little
information is known about the economic and environmental viability of scaling up these systems
for colder climates, nor optimal parameters (i.e. heat input, reactor design, and emission output,
etc.) to use in locations with high food waste generation [6]. To scale and optimize the system,
more information needs to be known about mass and energy generated, consumed, and
transformed during degradation of food waste by BSF and emissions from BSFL composting
system [7].
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Introduction and Background
Black Solider Fly Composting: Black soldier fly (Hermetia illucens; Diptera: Stratiomyidae)
larvae (BSFL) have been used in backyard and animal farm composting systems for decades [8].
Recently, researchers have explored how BSFL can be used in other applications, particularly
urban or institutional food waste management [9]. BSFL by nature are a decomposer that can decay
a wide variety of organic wastes; however, to date, they are primarily used for treating only animal
waste. They can significantly reduce the mass of wastes such as animal manure, fecal sludge,
municipal waste, and food waste [10]. Sheppard et al. reported that larvae could reduce manure
dry matter by up to 56% and total nitrogen concentration by 62% [11]. They can convert a wide
variety of low valued organic waste into protein and lipid rich biomass and decompose large
amounts of wastes quickly with a small carbon footprint. BSFL composting could be used to
reduce wastes sent to landfills, incinerators, bio-digesters and composting facilities. Larvae can
then be used as fish/chicken feed or, for the production of secondary biofuel products (biodiesel
and biogas), and their digested residue is a valuable fertilizer.
Academic institutions represent a constrained system that generates a consistent amount of waste.
Bioconversion of organic food waste with BSFL on the campuses of institutions could be an
attractive solution to reduce emissions by diverting food waste from the landfill [12]. 1.2 million
BSFL can compost 105 kg/week (15 kg/day) in a square meter [13]. Currently, RIT diverts 2-3
tons per week of pre and post-consumer waste from its largest dining halls to an anaerobic digester,
but the waste must travel by truck to reach the digester or landfill site releasing carbon emissions
in the process [14]. Using BSFL composting would also reduce RIT’s waste disposal cost, and
biomass of BSFL could be transformed for bioenergy generation and/or profitable products. BSFL
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may be a viable option for helping RIT meet its commitment to become waste and carbon neutral
by 2030 providing a model for other institutions with similar goals [15].
BSFL in Cold Climates?: Although utilization of black soldier flies to eliminate organic food
waste is promising, there are some technical challenges of scaling up the production of BSFL from
organic waste to an industrial scale rearing facility that would be necessary for an institution or
city. Since BSF are native to warmer climates, a breeding colony of BSF in cold weather conditions
requires additional energy inputs [6]. In the long-term, we plan to develop and implement a pilot
scale passive-house style facility for maintaining a BSF colony at RIT, but the heat and gas
production of the colony must be known in order to properly design the heating and ventilation
system for lowest energy consumption and optimal waste conversion in this colder climate. The
amount of energy generated by the BSF while they decompose waste is unknown. Alvarez
explored mass and water balances in a medium scale reactor, but amount of mass lost through
respiration (i.e. CO2 and H2O) and the heat generated was not quantified [6]. During larval
processing of organic waste, an unquantified amount of byproduct greenhouse gases are also
released to the environment. Although these emissions are expected to be much lower than
standard composting, to date, there has been no effort to measure these gases from the air in
comparable insect-based biodegradation facilities [7]. In this project, we used bench experiments
to analyze and determine the energy requirements for maintaining the colony through the winter,
quantify the methane and CO2 released from composting with BSFL, and calculate the value of
the larvae and their wastes from an environmental perspective. Wastes and/or by-products
including emissions were also be quantified.
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Relationship to Sustainability
This project promotes sustainability through several environmental, economic, and social benefits.
Planet: BSFL composting provides environmental benefits by reducing emissions and waste
during the decomposition process, and because it can be conducted on-site, provides additional
emission reduction by avoiding transportation. These advantages are achieved in the short-term
and cumulative emission and waste reduction provide long-term benefits in relation to climate
change. Some research suggests that diverting 1000 kg of food waste from landfills saves 900 kg
of CO2eq [16]. Green and Popa [17] recorded 5–6 times higher ammonium (NH4+) concentrations
of organic leachate processed by BSF larvae than unprocessed leachate, suggesting further
advantages in reducing emissions.
Prosperity: In the long-term, BSFL could eliminate the cost of waste disposal for organic materials
while generating biodiesel and biogas and/or other profitable products (feed for fish/poultry),
which would potentially turn institutional costs into profit. RIT currently pays $85/ton to ship
food waste to a digester in addition to $60/ton tipping fees for landfilled waste. Because larvae can
be used to create energy products, the energy return on energy invested may be positive.
People: Social benefits in the short-term will be to provide senior design projects and
undergraduate research opportunities for students at RIT that focus on sustainability. Long-term
social advantages support institutions, like RIT, in achieving their commitments to carbon
neutrality in order to reduce greenhouse gas emissions and mitigate global climate change. Once
a robust system is established, BSFL could provide a solution to urban composting as it is compact
and accepts commonly unaccepted organic waste (e.g. meat, dairy and post-consumer waste).
Additionally, BSFL could also provide waste treatment and reduction of both human and market
wastes in developing countries, turning waste into animal feed to reduce hunger.
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Materials and Methods
Our long term goal is to maintain ideal conditions for BSFL with as little added energy and as
much food waste reduction and accumulate the highest biomass yield as possible. To address this
aim, we developed bench scale experiments to help us understand the parameters (heat, ventilation)
of maintaining BSF colonies in cold climates, quantify the type and quantity of gases released
from BSFL and the degradation of food waste in the system, understand the mass and energy
balances of the BSFL food waste system, and determine quantities and types of solid products, byproducts, and/or wastes generated by the system. This activity involved:
1) Cultivation and measuring consumption rates: At 6 days old, 1,000 larvae (15mg (w)/larvae)
were inoculated into 900 g (60 per larvae per day) of either post-consumer
food waste or dog food and measure for biomass accumulation. The BSFL
were kept at 25ºC and humidity (60-70%) in laboratory incubators for 2025 days as shown in Fig 1. Feed was replenished until the larvae reached
Fig. 1 Laboratory reared
BSFL
the prepupae stage when they also reached their highest biomass. Larvae
can decompose 100-150 mg of food waste per day and convert into high nutrient biomass (50.4%
protein, 38.5% lipid and 6.8% carbohydrate).
2) Batch heat experiments: Short batch experiments investigated energy produced as heat by the
BSFL alone, dog food alone, and the mixture of dog food with BSFL for a duration of 5 hours.
Specific heat capacities of BSFL, dog food and food waste were calculated with the empirical
formula from ASHEAE (2006) [18]. Samples of 25 larvae alone, 15g dog food alone, and 25
larvae with 15g dog food and a blank were placed in 58 mL test tubes, insulated with fiberglass
and foam insulation, and kept in an incubator at 25 ºC and 60% RH during the test as shown in Fig
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2 and 3. The temperature and humidity inside each test tube was logged every 10 seconds using
HOBO External Temp/RH Data Logger.
Figure 2 (Top). Assembly of the system with fiberglass and foam insulation.
The images depict (2-a) the test tube with the sensor, needle and thread sealing
tape, (2-b) the system wrapped in fiberglass insulation, (2-c) the system in both
the fiberglass and foam insulation.
Figure 3 (bottom). The system containing the variables for the batch
experiments. Depicted are (3-a) larvae in the system, (3-b) larvae and food
waste in the system, and (3-c) larvae and dog food in the system.
3) Batch respiration experiments: Using the same set up as above,
short batch experiments were conducted to measure the respiration
rate of BSFL alone, dog food alone, and 25 larvae while eating dog
food over 6 hours. The gas composition of air in a fixed volume
reactor (58 mL test tube) was analyzed hourly using a Shimadzu
2014 GC TCD gas chromatograph. No methane (CH4) was
detected over a period of 6 hours from all the test tubes.
Modeling Heat Transfer Data: To obtain the overall heat transfer coefficient of the insulation
used in the test bench, the change in temperature over a period of time was recorded through
experiments conducted with hot and cold air. The results from the experiments were interpreted
and the system was modeled for conduction using Fourier’s Law. Eq [2] – [6] allowed the team to
model the heat transfer of the insulation.
Figure 4. Modeling the overall heat transfer coefficient by looking at the flow of mass and energy into and out of the
system
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Plotting the left side of the equation as a function of time, the graph shows a positive linear trend.
From the slope of the trend line fit to the data, the overall heat transfer can be determined.
Graph 1 (left). The negative natural log of the temperature difference (y value) graphed as a function of time in
seconds. The value for the slope of the linear trend line was used in determining the overall heat transfer coefficient
for both the fiberglass and foam insulation combined.
Graph 2 (right). The negative natural log of the temperature difference (y value) graphed as a function of time in
seconds. The value for the slope of the linear trend line was used in determining the overall heat transfer coefficient
for the larvae and the dog food.
Key milestones and project tasks: The experiment conducted in phase I will provide valuable
information to design a continuous feed BSFL food waste reactor prototype and conduct
continuous system, and collect necessary information to scaled up for a pilot scale BSFL
continuous reactor housed in RIT. The key milestones of the project are outlined as the following:
Phase I:
i.
BSFL cultivation and measurement of consumption rates
o Purchase BSLF from Biogrubs Worm store, California.
ii.
Purchasing the materials and equipment from local and online suppliers
iii. Perform batch heat experiments
iv.
Perform batch respiration experiments
v.
Perform theoretical energy balance calculation Eq. [1] 1 using information from experiment
data
Phase II (upcoming April 2016)
vi.
Design continuous experiment using (nearly complete)
vii. Conduct continuous system and data analysis
1
𝑡𝑡=𝑡𝑡
�𝑚𝑚𝑎𝑎𝑎𝑎𝑎𝑎 𝐶𝐶𝑉𝑉,𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑚𝑚𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 𝐶𝐶𝑉𝑉,𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 + 𝑚𝑚𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝐶𝐶𝑉𝑉,𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 ��𝑇𝑇𝑓𝑓 − 𝑇𝑇0 � + 𝑈𝑈𝑈𝑈 ∫𝑡𝑡=𝑡𝑡 𝑓𝑓(𝑇𝑇 − 𝑇𝑇∞ )𝑑𝑑𝑑𝑑 = 𝑚𝑚̇𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑞𝑞̇ 𝑔𝑔𝑔𝑔𝑔𝑔 [1]
0
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Results, Evaluation and Demonstration
The results from the fitted linear trend line of the heat transfer data yielded the overall heat transfer
of the insulation as 5.13 mW/m2.K, the heat capacity of the larvae as 4.86 kJ/kg.K which was
slightly higher than the ASHEAE empirical calculated value of 3.4 kJ/kg.K based on the larvae
nutritional composition [19], and the heat capacity of the dog food as 7.03 kJ/kg.K. Since these
values are higher than the heat capacity of water, errors in experimentation and the assumptions
made during analysis may have occurred. The theoretical values from literature were used for the
cost analysis of the BSFL waste system. With the overall heat transfer of the insulation and heat
capacities of the larvae and food, the heat generation of larvae in the system can be calculated
through further analysis.
Determination of Carbon Dioxide (CO2) emitted by BSFL during respiration
The CO2 production rate of one larvae per hour can be derived from the accumulative CO2
production data. The average CO2 production rate of larvae alone, food waste alone, and larvae
and food waste from three batch experiments in their larvae and prepuae stages are 0.027mL of
CO2/ larvae (0.149 mL of CO2/g larvae), 0.019 mL/gram and 0.07 mL/gram. Total CO2 production
of larvae and food waste was lower than larvae alone and it can be due to CO2 was absorbed by
water in the food waste (70% moisture content). This allowed for an analysis of the amount of CO2
and/or CH4 generated from BSFL composting system. Using the data from the batch experiments,
we are able to calculate the continuous reactor size (~ 2L), air flow rate of a fan to maintain the
concentration of CO2, sensors to monitor the temperature and humidity in the reactor for longerterm experiments.
Economic benefits: As a pilot program, RIT can divert 3 tons per week and total 150 tons per year
of food waste by BSFL composting, and 80 million of larvae are required to eliminate 1 ton of
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food waste per day. The manufacturer suggested that 25 lbs. of food waste can be treated in one
ProtaPodTM (D” 44 x H” 34) [23] and estimated 20 ProtaPodsTM would be required to treat a daily
input of 940 lbs. (0.43 MT) of food waste. The estimated cost of building one BSFL rearing super
insulated “smart” shed (8’x 8’) is $4,000 based on the size of two ProtaPodsTM. The estimated
initial capital cost is $50,000 (excluding O&M and transportation costs) and the return from
displaced inorganic fertilizer and sale of BSFL as a live food for fish and chicken were not included
in this analysis. Therefore, RIT current waste disposal cost of $13,000 per year can be averted.
Environmental benefits: 26 kg CO2-eq per 1 ton of food waste and 80 million larva was produced
from composting 1 ton of food waste per day. Energy required to raise 1 ton of food waste to
reach 25◦C from -7◦C per hour is 417 MJ and emissions attributed to utilizing grid electricity is 22
kg CO2-eq per 1 ton of food waste. The proposed system is based on the ideal conditions and the
heating requirements and energy used by the system will be less in the passive house. Results are
reported based upon the functional unit of one metric ton (MT) of food waste processed.
Table 1. Economic and Environment benefits of BSFL food waste composting system.
Economic Metric
Environment Metric
Economic Metric
Environment Metric
Net Economic benefits
Net Environmental benefits
•
Reference Case
2.5 – 3 (4,400 - 6,600 lbs.)
85
13,000
720
57
BSFL Composting
$50,000
~4
417
26
22
54,387
9 (=57-(26+22)
Units
MT per week food waste diverted to anaerobic digester
$/MT (food waste hauling fees0
$ Disposal cost/ year (150MT * $85/MT)
kg CO2-eq /ton of waste emission from landfill [20]
kg CO2-eq/ ton of waste diverted to AD [21]
Units
$ Capital Cost (Shed + supplies ( one time-charge))
Years (Simple Pay Back Period)
MJ ( Heat Demand)
kg CO2-eq /ton food waste + 80 million larvae per day
kg CO2-eq/ton food waste (required to raise 25◦C [22]
$ NPV (Interest rate = 3%, IRR = 10%)
kg CO2-eq /ton of food waste
2016 NYSP2I Earth day Student Competition Exhibition Event: BSFL in different stages
and food waste will be added to the small reactor (larvae house) and will serve as a
prototype and heat, water and CO2 released by BSLF will be measured with the Temp/RH
data sensor with data logger and mini CO2 meter to demonstrate the real-time
biodegradation process.
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Conclusions
Food consumption and larval growth and development are known to be sensitive to the temperature
and humidity conditions, but the team has successfully cultured BSF at 25 ºC and RH 50-60%,
reaching the optimal weight (0.15-0.2g/larvae). During batch experiments, no other gas besides
CO2 were detected in the system over a period of 5 hours. The rate of CO2 generation depends on
the size and age of larvae and should be investigated more fully.
Quantifying greenhouse gas (GHG) emissions due to implementation of BSF composting is
important to achieve the goal of this project. A simple lifecycle analysis was performed on the
basis data from the bench experiment, resulting in net reduction of 9 kg CO2-eq/ ton of food waste
by avoidance of conventional anaerobic digestion treatment. GHG emissions estimates are subject
to uncertainty and variability and the impacts of other parameters should be explored for further
analysis.
If RIT converts 3 MT per week of food waste by the proposed BSFL composting, the five years
of NPV is $54,387 and discounted payback period is less than 4 years. The revenue from fertilizer,
and animal feed production could cover the operation and maintenance cost and maintenance
supplies. In phase II, this initial batch experiment data (heat & CO2 emission) by the system will
be utilized to design a lab-scale continuous feed BSFL food waste reactor.
Because so little is known about the potential of BSFL composting, applied scientific research is
needed to fill the knowledge gaps and there are extensive opportunities for undergraduate and
graduate research through innovative design across multiple disciplines and generations of
students at RIT, well beyond the scope of this initial project.
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