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Consider a Spherical Man
A Simple Model to Include Human Excretion
in Life Cycle Assessment of Food Products
Ivan Muñoz, Llorenç Milà i Canals, and Roland Clift
Keywords:
carbon cycle
feces
industrial ecology
nutrients cycle
urine
wastewater
Summary
Emissions derived from human digestion of food and subsequent excretion are very relevant from a life cycle perspective,
and yet they are often omitted from food life cycle assessment
(LCA) studies. This article offers a simple model to allocate
and include these emissions in LCAs of specific foodstuffs.
The model requires basic food composition values and calculates the mass and energy balance for carbon, water, nutrients
(mainly nitrogen [N] and phosphorus [P]), and other inorganic
substances through different excretion paths: breathing, feces,
and urine. In addition to direct excretion, the model also allocates some auxiliary materials and energy related to toilet use,
such as flushing and washing and drying hands. Wastewater
composition is also an output of the model, enabling water
treatment to be modeled in LCA studies. The sensitivity of
the model to food composition is illustrated with different
food products, and the relative importance of excretion in a
product’s life cycle is shown with an example of broccoli. The
results show that this model is sensitive to food composition
and thus useful for assessing the environmental consequences
of shifts in diet. From a life cycle perspective, the results show
that postconsumption nutrient emissions may dominate the
impacts on eutrophication potential, and they illustrate how
the carbon cycle is closed with the human emissions after food
preparation and consumption.
Address correspondence to:
Llorenç Milà i Canals
SEAC, Unilever Colworth
Colworth House, Sharnbrook
Bedfordshire MK44 1LQ, United Kingdom
[email protected]
c 2008 by Yale University
DOI: 10.1111/j.1530-9290.2008.00060.x
Volume 12, Number 4
www.blackwellpublishing.com/jie
Journal of Industrial Ecology
521
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Introduction
Food is a basic human need, recognized as
one of our most resource-demanding and polluting daily activities when the complete life cycle of food is considered. Food production causes
many environmental impacts through its supply chain, which includes agricultural production, storage, several transport steps, processing,
cooking and consumption, and waste disposal.
Several studies have identified food as one of
the main contributors to the environmental impact of private consumption at both the national and the international level (Nijdam et al.
2005; Tukker et al. 2006). It is not surprising,
then, that life cycle assessment (LCA) studies
are increasingly directed at food to find ways to
make its production and consumption patterns
sustainable.
LCA has been applied to many different food
products, including basic carbohydrate foods,
fruits and vegetables, dairy products, meat, fish,
and alcoholic and nonalcoholic drinks, among
others (Foster et al. 2006). Although some practitioners have conducted full LCAs for particular products or product groups (Andersson and
Ohlsson 1998; Jungbluth et al. 2000; Ziegler et al.
2003), many studies tend to focus on a particular stage of the product’s life cycle, such as
agriculture (Antón et al. 2004; Milà i Canals
et al. 2006), industrial processing (Sonesson,
Mattsson, et al. 2005), transport (Milà i Canals
et al. 2007; Sim et al. 2007), retailing (Carlson
and Sonesson 2000), industrial processing and
packaging (Hospido et al. 2006), home storage
and processing (Sonesson et al. 2003; Sonesson,
Anteson, et al. 2005; Sonesson, Mattsson, et al.
2005), and waste management (Sonesson et al.
2004; Lundie and Peters 2005).
Human digestion and excretion remains the
least studied life cycle stage of food products; so
far, only nutrients in food have been included. In
their case study on seafood, Ziegler and colleagues
(2003) included nutrients in the food and their
fate through sewage treatment. Sonesson and colleagues (2004) studied the importance of postconsumption waste treatment in the life cycle of
food products, proposing a systematic procedure
for modeling the nutrients balance. Nonetheless, besides nutrients, published studies have not
522
Journal of Industrial Ecology
covered human metabolism and excretion as a
whole.
The biochemical transformations undergone
by food in the human body give rise to different pollutants released to air and water, which
should be included within the system boundaries
of a complete food LCA, similar to the way food
waste is treated when it is landfilled or composted.
Therefore, why has human excretion been systematically omitted by LCA practitioners up to
date? We can envisage at least three reasons for
this:
1. It is not necessary in case studies comparing similar products, because the environmental burdens would also be similar.
2. LCA is a tool intended to support decision
making at many levels in the food chain;
it can guide decisions about producing or
consuming more or less organic food, fresh
or frozen products, and so forth, but human
metabolism is a constraint, something we
can hardly influence and therefore must
accept as a limitation. In particular, LCA
has been traditionally used mostly for sustainable production, and hence the focus
has been on cradle-to-gate studies.
3. There are no available models to calculate
the environmental burdens of this stage as
a function of the type of food; that is, there
is no allocation procedure analogous to
those developed for other multi-input processes, such as solid waste and wastewater
treatment (Doka and Hischier 2005). Even
though Sonesson and colleagues (2004)
suggest some hints for calculating postconsumption emissions from food, to our
knowledge, these have not been used in
any published food LCA studies.
As pointed out by Andersson (2000) and
Sonesson and colleagues (2004), the relevance
of including human excretion depends on the
goal of the study. Digestion and excretion are
clearly relevant when the aim is to close the balance of materials in the life cycle or to compare the environmental effects of different diets
(Jungbluth et al. 2000; Alfredsson 2002; Kytzia
et al. 2004) or ways to provide food (Sonesson,
Mattsson, et al. 2005) due to the dependence of
excretion emissions on food composition. Human
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excretion should also be included in attributional
food LCA studies, which aim to identify the life
cycle hot spots.1
In this work, we address this methodological gap by providing a simple model to calculate product-related life cycle inventories of human excretion. The title of this article refers to
the spherical cow metaphor, where a theoretical
physicist started a calculation on a dairy’s production with “Consider a spherical cow. . .”. This
metaphor is often used to refer to simplified scientific models of reality, which help understand
more complex problems. The article sets out the
model fundamentals, tests the model with different food types, and positions it in the context of
the whole life cycle of a particular product, broccoli. The final section discusses the results and
highlights the main conclusions of the article.
Model Description
The model has been designed as a MS Excel spreadsheet, a comprehensive description of
which is offered by Muñoz and colleagues (2007).
General Structure and System Boundaries
The model can be divided in two main parts:
The first addresses the global balance of materials
and energy in the human body as a consequence
of ingestion of food with a specific composition,
whereas the second part concerns the auxiliary
materials and energy associated with toilet use.
Figure 1 shows a flow diagram of the system modeled and its boundaries.
It is worth noting that the only emissions
to nature calculated by this model are to air,
mainly from respiration, whereas the wastewater
containing human excrement and toilet paper is
considered as an output to the technosphere. It
is assumed that toilets discharge wastewater to a
sewer connected to a wastewater treatment plant.
This means that, to determine the final emissions to the environment, an additional model
for wastewater treatment must be used. Several
models are available (Dalemo 1997; JimenezGonzalez et al. 2001; Doka 2003) to calculate
inventories of wastewater treatment for userdefined wastewaters. If a scenario with no sewage
treatment is considered, the emissions quantified
Figure 1 Modeled system. COD = chemical oxygen demand; BOD = biological oxygen demand.
Mu ñoz et al., Human Excretion in LCA of Food Products
523
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by the human excretion model must be taken as
the final mass of pollutants released to the aquatic
environment.
Food Composition
Any kind of food, including plain water, can
be assessed by the model, as long as its composition is known. The input parameters to be defined, as g/100 g on a fresh weight basis, are the
following:
• water content;
• protein content;
• fat content, including all lipids (saturated
and nonsaturated fatty acids, cholesterol,
etc.);
• carbohydrate content, including all sugars
and starch;
• Fiber content, including lignin, pectin, and
cellulose;
• alcohol;
• organic acids not covered by any of the
above categories, such as acetic acid or lactic acid;
• inorganic elements, such as phosphorus,
sodium, chloride, magnesium, potassium,
iron, and heavy metals.
Raw and cooked food composition can be
found in handbooks such as that published by
the UK Food Standards Agency (2002). It is important to consider the composition of the food as
it is ingested, because cooked or boiled food can
have a very different composition as compared to
raw food. As an example, broccoli loses 30% of
its protein and 39% of its carbohydrate content
when boiled (Food Standards Agency 2002).
Inorganic constituents must also be included,
especially if they represent a significant part
of the food. The occurrence of toxic organic
compounds, such as pesticide residues, is not
taken into account because the added complexity that would be introduced by their modeling
in the human body is out of the scope of this article. In fact, pesticides’ (and other substances’)
metabolites are not even considered in sophisticated pesticide fate models in LCA. Only heavy
metals are included in the food composition, but
it is important to bear in mind that the purpose
of this model is to obtain a life cycle inventory;
impacts on human toxicity of exposure to heavy
metals in food are not assessed.
Human Metabolism Modeling
One of the basic assumptions of the model is
that a “steady-state” person is considered. This
means that all material entering the body as food
is excreted, including proteins and fat. No accumulation of fat or synthesis of additional proteins
is considered; this is in accordance with the analysis of Sonesson and colleagues (2004). Food is
entirely converted to excretion products and expelled from the body in one of the following flows:
breath, urine, feces, and skin/sweat.
Figure 2 shows an overview of the transformations and fate of food entering the human body
according to this model. Food constituents are
divided into four categories: water, degradable
material, nondegradable material, and inorganics. The model considers as degradable organic
Figure 2 Overview of the fate of food constituents in the human body as considered in the model.
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Journal of Industrial Ecology
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Table 1 Elemental composition of organic constituents in food
Food
constituents
Elemental composition (kg/kg)
C
H
O
N
S
Protein
0.47
0.07
0.29
0.15
0.02
Fat
0.77
0.12
0.12
0.00
0.00
Carbohydrate
0.42
0.06
0.52
0.00
0.00
Alcohol
0.52
0.13
0.35
0.00
0.00
Organic acids
0.40
0.07
0.53
0.00
0.00
Fiber
0.44
0.06
0.49
0.00
0.00
Comments
Average C, H, N, O, and S content in each of
the 20 amino acids: alanine, arginine,
asparagine, aspartic acid, cysteine, glutamic
acid, glutamine, glycine, histidine, isoleucine,
leucine, lysine, methionine, phenylalanine,
proline, serine, threonine, tryptophan,
tyrosine, valine
Based only on triglycerides, which constitute
more than 90% of total fat intake in western
diets (Boron and Boulpaep 2003). As C, H,
and O content in two triglycerides used as
models: triglyceride of palmitic acid, oleic acid,
alpha-linoleic acid, and triglyceride of palmitic
acid, palmitic acid, palmitoleic acid.
Average obtained by the sum of C, H, and O
content of the following carbohydrates:
fructose, sucrose, maltose, lactose, and starch
Based on the empirical formula of ethanol,
C 2 H 5 OH
Based on the empirical formula of acetic acid,
CH 3 COOH. The weight fractions are equally
valid for lactic acid, C 3 H 6 O 3 , and for any
carbohydrate with formula (CH 2 O) n
Dietary fiber includes lignins, pectins, and
cellulose (Boron and Boulpaep 2003). The
composition of fiber is based on the empirical
formula of cellulose (C 6 H 10 O 5 ) n
Note: C = carbon; H = hydrogen; O = oxygen; N = nitrogen; S = sulfur.
materials all the organic constituents listed in
the section on food composition above, with the
exception of fiber (nondegradable organic matter in figure 2). For carbohydrates, particularly
starch, the availability for digestion seems to be
lower in processed and reheated food (Clifford
2007); this might introduce differences between,
for example, ready-meal and home-prepared versions of the same foodstuffs. This has not been
taken into account in the model, however: All
organic degradable materials are assumed to be
fully available for digestion.
Inorganics and water are not subject to any
chemical transformation; the latter is partitioned
between air and wastewater, whereas the former are assumed to report entirely to wastewater. The main transformation described by the
model is that undergone by degradable organic
material as a result of human digestion. To
model this transformation, a general biochemical reaction has been defined, which, in turn,
requires the chemical composition of the reactants to be defined. Table 1 summarizes the average elemental composition of the food constituents and how they have been estimated. The
data in table 1, with the exclusion of fiber, are
used along with food composition to estimate a
weighted empirical formula for digestible organic
matter.
Nondegradable material is basically excreted
via feces without taking part in any human
metabolic process. An allowance has been made
for fiber, however, as well as for degradable organic matter, to be converted to methane by the
colon bacterial flora. This is further described in
the next section.
Mu ñoz et al., Human Excretion in LCA of Food Products
525
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The overall biochemical reaction proposed for
degradable organic matter is shown in equation 1.
This equation implies that organic degradable
matter is converted, by cell respiration, to carbon dioxide and water, whereas some carbon is
lost in urea (CH 4 ON 2 ) and feces (C 2 H 4 O). To
simplify the calculations, it is assumed that all
nitrogen from protein degradation ends up in
urea, so that feces contain only carbon, hydrogen, and oxygen, in molar proportions similar to
those in activated sludge in wastewater treatment
plants. All sulphur ends up as sulphate, reported
as H 2 SO 4 . Some sulphur will actually be “excreted” via growth of hair and nails, but this has
been omitted from the model in the interest of
simplicity.
Ca Hb Oc Nd Se + A O2 → B CO2
+ C H2 O + D C H4 ON2
+ E H2 S O4 + F C2 H4 O
(1)
where
E =e
(2)
D = d /2
(3)
F = 0.055a
(4)
B=a−D−F
(5)
C = (b − 4D − 2E − 4F )/2
(6)
A = (C + D + 4E + 2F + 2B − c)/2 (7)
For equation 1 to be solved, the share of carbon incorporated in either carbon dioxide (B) or
feces (F) has to be defined. We have done this
by calculating a balance for degradable carbon in
the human body, with the following assumptions:
• Alveolar volume in respiratory system is
350 milliliters (mL),2 of which 5% is carbon dioxide (Boron and Boulpaep 2003).
Breathing rate is taken as the average
of 12/min (Boron and Boulpaep, 2003)
and 20/min (Marieb 1995). This leads
to an average output of 195 grams carbon/person/day as carbon dioxide exhaled.
• Urine production is 1.5 L/day (Boron and
Boulpaep 2003), with a dry weight of 5%
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Journal of Industrial Ecology
(Mara 2003) and a carbon content 14% in
dry weight (Feachem et al. 1983). The carbon loss in urine follows as 11 g/person/day.
• Feces production is around 0.15 kilogram
(kg)3 per day, with a dry matter content
of 25% and a carbon content of 50% in
the dry matter (Feachem et al. 1983). This
gives 19 g C, from which the contribution of
nondegradable fiber must be excluded. Average intake of fiber is 15 g/person/day, with
a carbon content of 44% (table 1). If we
assume that fiber is excreted in feces without any transformation, the contribution of
fiber to carbon in feces is 7 g. Therefore,
the output of degradable carbon via feces is
19 − 7 = 12 g C/person/day.
These calculations lead to a loss of degradable
carbon via feces of 5.5% (equation 4). It is worth
noting that a similar amount is lost via urine,
around 90% of the amount of carbon effectively
used by cell respiration and transformed to carbon
dioxide. This balance has omitted several carbon
flows that were estimated and found to be negligible: carbon dioxide and methane via intestinal
gas, and methane expelled via lungs; altogether,
these account for less than 0.1% of the carbon
output.
The fate of each of the final products obtained
in equation 1 is defined in the model as follows:
• Carbon dioxide is entirely emitted to atmosphere via the lungs.
• Urea, sulphate, and feces are expelled as
liquid and solid excreta: urea dissolved in
urine, and feces as solid, whereas sulphate
seems to be almost entirely excreted in
urine (Florin et al. 1991).
• Water will be emitted both as a liquid and
as a gas. To determine the share of each,
we have used the water balance suggested
by Boron and Boulpaep (2003), according
to which 64% of the water output corresponds to the liquid phase (60% by urine
and 4% by feces), whereas the remaining
36% corresponds to the air phase (22% by
skin/sweat and 14% by breathing). In addition to the water produced by cell respiration, the model must also determine the
fate of water originally present in food, usually a much larger quantity. The fate factors
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already above also apply to the water in
food.
Fiber is the only category of organic constituents in food not affected by the biochemical
transformation in equation 1. Fiber is assumed to
be emitted to wastewater via feces with no chemical transformation except methane production,
as described below.
Methane Emissions
Besides cell respiration, the only additional
chemical transformation considered by the model
is the formation of methane by colonic bacteria.
In carbon terms, the amounts may seem negligible (see above), but from a greenhouse gas perspective they may not be. For this reason, an attempt has been made to estimate the amount of
methane emitted by the human body due to the
activity of anaerobic bacteria in the intestine.
Human cells have no metabolic pathway capable of producing or metabolizing methane. Therefore, the model attributes all methane production
to the action of intestinal bacteria and assumes
that all methane is excreted in intestinal gas or
exhaled breath (Bond et al. 1971). Methane production varies widely among individual humans:
Some subjects, approximately one third of the
population, continually produce large quantities
of this gas, whereas others consistently excrete little or no methane at all. This appears to be related
to the presence or absence of methane-producing
flora: Familial (not necessarily genetic) factors
play an important role in determining whether
a subject produces methane (Levitt and Bond
1980).4
According to Bond and colleagues (1971), the
average methane excretion rate of methane producers is 0.33 mL/min and 0.45 mL/min via lungs
and intestine gas, respectively. If a pressure of
1 atmosphere (atm)5 and a body temperature of
310 kelvin (K)6 are considered, this suggests that
a methane producer emits 0.52 g C in CH 4 per
day, or 0.69 g CH 4 per day. If this is corrected to
take into account that only 33% of the population are considered to be significant methane producers, we obtain an average emission of 0.17 g CCH 4 per person per day, or 0.23 g CH 4 per person
per day. This implies that around 0.08% of the
total carbon emitted by the human population is
in the form of methane.
Degradable organic material contributes to
methane production, but so does dietary fiber.
Tomlin and colleagues (1991) found that a fiberrich diet implies an increase in intestinal gas production as compared to a fiber-free diet. Bond and
colleagues (1971), however, found that methane
production is insensitive to changes in nonabsorbable carbon intake, whereas the production
of other gases, in particular hydrogen, is clearly
enhanced by fiber intake. In view of this uncertainty, the model allocates methane emissions to
all carbohydrates present in the food ingested on
the basis of carbon content, regardless of whether
they are digestible.
Metabolic Energy Balance
With regard to the energy balance of the overall process, the chemical energy stored in all the
inputs and outputs to and from the human body
is calculated, on the basis of their heating values.
The model uses the upper heating value, because
most water is excreted as the liquid, whereas even
the vapor emissions (exhaling and perspiration)
actually pass through the skin and lung surfaces
as liquid. Upper heating values are calculated for
food, methane, urea, and feces derived from both
degradable and nondegradable organic material
(fiber). All the remaining materials (oxygen, water, carbon dioxide, sulphate, and phosphorus)
are assigned a null energy content. The heat
content calculation is based on elemental compositions according to the formula proposed by
Michel (1938):
Upper Heating Value (MJ/kg)
= −9.8324O + 124.265H
+ 34.016C + 19.079S + 6.276N (8)
where C, H, O, N, and S are the mass fractions of
each element. The figures obtained with equation
8 for the energy input in fiber-rich food will be
higher than those reported in food labels, which
normally exclude the calorific value of fiber because it is nonabsorbable and not actually digested.
The difference between the energy input and
output, each calculated with equation 8, is the
Mu ñoz et al., Human Excretion in LCA of Food Products
527
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fraction of energy effectively used by the cells
in their metabolic processes. The model assumes
this energy to be emitted eventually to the environment as heat.
Allocation of Technosphere Processes
Using the toilet to evacuate liquid and solid
excretion products implies, directly or indirectly,
the use of ancillary materials and energy. The
model allocates these processes to food intake
on the basis of mass of excretion products. The
following basic assumptions are made:
• Every time the toilet is used, it is flushed.
• After each toilet use, hands are washed with
soap and water at ambient temperature.
• At home toilets, hands are dried by means
of a towel, whereas at workplace toilets,
hands are dried by means of a hot air
blower.
• Towel production is excluded because of
the long service life of the towel, but washing and drying at home are included.
• Transport of ancillary materials (soap, detergent, toilet paper) is not included.
A set of parameters have been defined and
given default values intended to be representative
of UK conditions (table 2). The user can modify
the parameter values to make them representative of other regions or scenarios.
The different environmental burdens can be
calculated per person per day with the data in
table 2. Next, these figures are divided by the
average daily solid plus liquid excreta production
by an average person, which is taken as 1.65 kg,
made up of 1.5 kg (i.e., 1.5 L) urine and 0.15 kg
feces. The allocation to food intake on the basis
of food excreta is finally carried out by means of
equation 9:
Toilet related burden
kg food intake
=
Toilet related burden
kg solid and liquid excreta
×
528
kg solid and liquid excreta
kg food intake
Journal of Industrial Ecology
(9)
Model Output
The output of the human excretion model
consists of a disaggregated inventory table including inputs from nature (oxygen), inputs from the
technosphere (food itself and those related to toilet use), outputs to nature (emissions to air from
respiration and digestion), and outputs to the
technosphere (wastewater). The pollution load of
the resulting wastewater is expressed in the model
by the parameters total organic carbon (TOC),
biological oxygen demand (BOD), chemical oxygen demand (COD), N-total, P-total, and other
inorganic elements. A small amount of these pollutants arises from toilet use (hands washing with
soap, towel washing, etc.), but the highest share
is related to human excreta; they are calculated
by the model as follows:
• TOC is determined from the carbon content in the solid and liquid excretion products, namely fiber, and products of equation 1: feces and urea. COD and BOD are
estimated from TOC according to the following ratios (Doka 2007): TOC/BOD =
0.641, and TOC/COD = 0.479. It must
be noted that these three parameters are
related; just one of them—COD in this
work—must be used in the eutrophication
potential, because otherwise we would be
double counting carbon emissions.
• Nitrogen from human metabolism is considered in the model to be excreted only as
urea. Thus, from the amount of urea produced and its empirical formula, the nitrogen released into wastewater is calculated.
• Phosphorus and other inorganics are just
expressed as the initial amounts in food,
because they are not subject to any transformation in the model.
We can calculate the concentration of pollutants in wastewater by dividing the amounts
released by the total water discharged, including
water from toilet use and water in the food itself and resulting from digestion (see equation 1).
Impacts related to wastewater treatment and the
final amount of pollutants released to the environment must be subsequently modeled by the
LCA practitioner, using, for example, the models
suggested in the General Structure and System
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Table 2 Parameters and default values used for allocation of toilet use processes
Parameter
Toilet flush volume
(L)
Hand-washing water
use (L/wash)
Toilet uses
(times/day)
Toilet uses at home
(%)
Default
value
Comments
11
Measured volume of a standard toilet tank at the University of
Surrey
Assumption
1.5
5
Assumption; this includes both urination and defecation
57
Toilet paper use
(kg/day/person)
0.02
Hand-washing
(liquid) soap use
(g/wash)
Electric hot air
blower power
(kW)
Time needed to dry
hands (s)
Towel weight (kg)
Number of persons
per household
Frequency of towel
washing (days)
Power demand of
washing machine
(kWh/kg towel)
Detergent use by
washing machine
(g/kg towel)
Water use by
washing machine
(L/kg towel)
Power demand of
towel drier
(kWh/kg towel)
Hand-washing
wastewater
composition
(mg/L)
3.3
This parameter is used to estimate the share of hand drying by
means of a cotton towel. The remaining 33% is assumed to
be done at work with a hot air blower. The value is an
assumption based on the following figures: 5 working days
per week, 2 weekend days per week. In a working day, three
toilet trips are made at the workplace, and two at home. On
weekends, all toilet trips are made at home.
Calculated with the following data: tissue paper consumption
in Western Europe in 2004 was 4.1 million tonnes, of which
62% was toilet tissue, and 18% was consumed in the United
Kingdom and Ireland (European Tissue Symposium, 2005).
The population of the United Kingdom and Ireland in 2004
was 63,727,560 (Eurostat 2007).
Measured weight at University of Surrey toilet was 100 g liquid
soap dispensed per 60 pushings. The figure considers two
dispenser pushings per wash.
Average power of a hand dryer (Handryers.net 2005)
2
30
Average drying time of a hand dryer (Handryers.net 2005)
0.35
2.4
Assumed for a cotton towel
Average for the United Kingdom (Office for National
Statistics 2007)
Assumption
7
0.43
Washing of the cotton towel (Group for Efficient Appliances
1995)
45
135 g detergent for a typical 3-kg load. Process related to
washing of the cotton towel (Group for Efficient Appliances
1995)
Washing of the cotton towel (Group for Efficient Appliances
1995)
17.2
0.7
COD: 400
Drying of the cotton towel (Group for Efficient Appliances
1995). Average of three technologies: air vented tumble
driers, condenser tumble driers, and condenser washer driers
Representative averages from several studies (Eriksson et al.
2002)
Continued
Mu ñoz et al., Human Excretion in LCA of Food Products
529
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Table 2 Continued
Parameter
Laundry wastewater
composition
(mg/L)
Default
value
BOD: 190
N-total: 10
P-total: 1
COD: 1270
Comments
Representative averages from several studies (Eriksson et al.
2002)
BOD: 260
N-total: 10
P-total: 25
Note: COD = chemical oxygen demand; BOD = biological oxygen demand; N-total = nitrogen total; P-total =
phosphorus total. One kilowatt (kW) ≈ 56.91 British Thermal Units (BTU)/minute ≈ 1.341 horsepower
(HP).
Boundaries section of this article. Table 3 shows
an example of an inventory table obtained for
a particular product, namely boiled broccoli, by
applying the model to the composition for this
product given in table 4.
Results
Model Sensitivity to Different Food Types
The sensitivity of the model to food composition has been tested on eight food products
commonly present in daily western diets, with
typical compositions given in table 4: bread, broccoli, apple, chicken meat, beer, cheese, a chocolate snack, and coffee. Selected inventory results
are given in figure 3. Rather than comparing the
environmental performance of these food items,
the following discussion aims at exploring how
the model responds to extremely different data
inputs.
Figure 3 shows that some parameters are
highly variable from one food type to another,
whereas others remain quite similar. The amount
of solid and liquid excretion products emitted to
wastewater (figure 3a), for example, is rather similar for all these food products, in the range 0.5
to 0.65 kg per kg food, primarily because water
is one of the main components in food and 64%
of the water is assumed to report to urine and
feces regardless of the food type. Nevertheless,
foods with low water content, such as the choco530
Journal of Industrial Ecology
late snack or the parmesan cheese, lead to similar
values. This is explained by the fact that one of
the main outputs of the organic degradation reaction, as shown in equation 1, is also water. If the
mass of excretion products per kilogram ingested
is broadly comparable for all foods, then the environmental burdens from the technosphere processes described above (e.g., wastewater volume,
figure 3d) will also be similar for all food products,
because all these processes are allocated on the
basis of the amount of solid and liquid excretion
products.
Carbon dioxide emissions due to respiration
(figure 3b), conversely, are highly variable depending on the food type; between coffee and the
chocolate snack, for instance, there is a difference
of three orders of magnitude. This is clearly related to the degradable carbon content in food,
which is very high in dry foods, such as cheese
and chocolate, and very low in drinks, such as
coffee or beer. Methane emissions (figure 3c) follow a pattern similar to that for carbon dioxide
emissions, because this pollutant is allocated on
the basis of carbon content in food. In this case,
not only degradable carbon but also nondegradable carbon (present as fiber) contributes; however, we see in table 4 that in most of the food
products considered, the main source of carbon is
other than fiber.
The volume of wastewater discharged to the
sewer (figure 3d) shows a similar pattern for all
foodstuffs. Across all products, the quantity of
wastewater is 20 to 26 liters per kilogram of food,
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Table 3 Inventory table for excretion of boiled broccoli
Inputs and outputs
Inputs
From nature
Oxygen (g)
From the technosphere
Broccoli, boiled in
unsalted water (g)
Toilet paper (g)
Tap water (L)
Comments
71
985
Grams of food ingested
7.8
24.4
Allocated on the basis of solid plus liquid excreta mass
Toilet flushing plus hand washing, plus towel washing, allocated
on the basis of solid plus liquid excreta mass
Hand washing, allocated on the basis of solid plus liquid excreta
mass
Detergent for washing machine used for towel washing, which, in
turn, has been used to wash hands
Electricity for hot air blower, washing machine, and drier. All
these processes are related to hand drying
Soap (g)
6.5
Detergent (g)
0.36
Power (kWh)
0.023
Outputs
To nature
Air emissions:
Carbon dioxide (g)
80
Methane (g)
0.037
Water (g)
Oxygen needed for cell respiration of degradable constituents in
food (carbohydrates, fat, and protein)
337
Heat (MJ)
1.0
To the technosphere
Toilet paper (g)
7.8
Wastewater volume (L)
25
Wastewater emissions from food:
Urea (g)
9.8
N-urea (g)
4.6
TOC (g)
13
BOD (g)
21
COD (g)
28
Sulphate (g)
2.2
P-total (g)
0.57
Na (g)
0.13
K (g)
1.7
Ca (g)
0.4
Cl (g)
0.23
Produced by catabolism of degradable constituents in food
(carbohydrates, fat, and protein)
Produced by colonic bacteria. Degradation of all
carbon-containing compounds, including fiber
36% of all water ingested or produced by catabolism is evaporated
by the body through skin or breathing. Main source of water
here is the initial content in food, but there is also water
produced in cell respiration
Energy actually used by metabolic processes
Present in wastewater
Sum of solid plus liquid excreta plus tap water
All nitrogen in food is assumed to be included here
Urea expressed as nitrogen mass
Carbon content in urea and fiber
Related to carbon content from urea and fiber
Related to carbon content from urea and fiber
From protein metabolism
Inorganic constituents in food are 100% allocated to solid plus
liquid excreta.
Inorganic constituents in food are 100% allocated to solid plus
liquid excreta.
Inorganic constituents in food are 100% allocated to solid plus
liquid excreta.
Inorganic constituents in food are 100% allocated to solid plus
liquid excreta.
Inorganic constituents in food are 100% allocated to solid plus
liquid excreta
Continued
Mu ñoz et al., Human Excretion in LCA of Food Products
531
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Table 3 Continued
Inputs and outputs
Comments
Wastewater emissions from toilet use:
BOD (g)
0.59
COD (g)
1.3
N-total (g)
0.030
P-total (g)
0.0064
Related to gray wastewater: hand washing and towel washing
Related to gray wastewater: hand washing and towel washing
Related to gray wastewater: hand washing and towel washing
Related to gray wastewater: hand washing and towel washing
Note: N, P, Na, K, Ca, and Cl refer to the elemental symbols. COD = chemical oxygen demand; BOD = biological
oxygen demand; N-total = nitrogen total; P-total = phosphorus total.
Megajoule (MJ) = 106 joules (J, SI) ≈ 239 kilocalories (kcal) ≈ 948 British Thermal Units (BTU).
associated mainly with toilet use (flushing, washing hands, etc.).
The mass of pollutants discharged to the sewer
(figure 3e) shows a different picture. In this case,
big differences are seen between products, up to
two orders of magnitude in all three parameters.
There is a clear relationship between protein content and discharge of urea and sulphate, as proteins are the only food constituents containing
nitrogen and sulphur in their empirical formula.
Carbonaceous organic matter, measured as COD,
is related not only to proteins but also to fiber and
degradable organic matter in general. This figure
shows only the amount of pollutants derived from
solid and liquid excretion products, whereas the
contribution of toilet use (gray wastewater from
hands washing and towel washing) is excluded.
Nonetheless, table 3 shows that the contribution
of toilet use is one order of magnitude lower for
the particular case of broccoli.
Finally, as expected, the energy content of
food is also highly variable (shown in parentheses on top of the bars in figure 3f), as also is the
energy efficiency of the human body, depending
on the composition of food. The efficiencies go as
high as 95% for beer and as low as 63% for broccoli. The most energy-efficient foods appear to be
alcoholic drinks, due to the ethanol content, and
also fat- and carbohydrate-rich foods, whereas the
least efficient are those containing a high share
of proteins, such as chicken, and especially, fiber,
as in the case of broccoli. Proteins show a lower
efficiency due to an important energy loss in the
form of urea, whereas the human body is simply
Table 4 Composition of several food products, in grams per 100 g edible portion
Component
Broccoli,
boiled
Roasted
chicken
Lager
beer
Chocolate
snack
Parmesan
cheese
Apple,
nonpeeled
Coffee,
infusion,
average
White
bread,
sliced
Water (g)
Protein (g)
Fat (g)
Carbohydrate (g)
Fiber (g)
Alcohol (g)
P (g)
Na (g)
K (g)
Ca (g)
Cl (g)
91.1
3.1
0.8
1.1
2.3
0
0.057
0.013
0.17
0.04
0.023
65.3
27.3
7.5
0
0
0
0.2
0.1
0.3
0.017
0.088
93
0.3
Tr
Tr
Tr
4
0.019
0.007
0.039
0.005
0.02
2
7.5
26
63
N
0
0.2
0.12
0.33
0.2
0.21
27.6
36.2
29.7
0.9
0
0
0.68
0.756
0.152
1.025
1.26
84.5
0.4
0.1
11.8
1.8
0
0.011
0.003
0.12
0.004
Tr
98.3
0.2
Tr
0.3
0
0
0.007
Tr
0.092
0.003
0.003
38.6
7.9
1.6
46.1
1.9
0
0.095
0.461
0.137
0.177
0.829
Source: Food Standards Agency (2002).
Note: P, Na, K, Ca, and Cl refer to the elemental symbols. Tr = trace (considered as zero); N = no reliable information
(considered as zero).
532
Journal of Industrial Ecology
FORUM
Figure 3 Selected model results for several food products.
unable to digest the fiber in fiber-rich foods and
use its chemical energy.
Importance of Human-Excretion-Related
Impacts in the Overall Life Cycle
of Foodstuffs
Figure 4 shows the cradle-to-grave results for
consumption in the United Kingdom of 1 kg
broccoli grown in Spain. Details of the life cycle modeling for these crops are provided by Milà
i Canals and colleagues (2008). The excretion
and wastewater stage includes the emissions described in this article in addition to the treatment of the wastewater described in table 3; the
latter has been modeled as described by Muñoz
and colleagues (2007). Global warming potential
(GWP) and eutrophication potential (EP) have
been assessed according to the CML2001 method
(Guinée et al. 2002); in addition, the inventory
indicators water use (WU) and primary energy
use (PEU) are shown in figure 4.
EP (figure 4a) is dominated by the home
and excretion and wastewater stages, which contribute 32% and 45%, respectively, to the total
EP related to the broccoli life cycle. The former
contributes due to the loss of nitrogen and phosphorus from broccoli to the boiling water during
cooking. Nitrogen and phosphorus are not effectively removed in the sewage plant and so become
emissions to aquatic ecosystems; note that these
figures assume that only 11% of wastewater is
treated in plants equipped with nutrient-removal
processes, because this is the current situation in
the United Kingdom (Muñoz et al. 2007). Also,
there is the contribution from leachate emissions
from landfilling of food waste (uneaten broccoli;
see the top section in the Home bar in figure 4a).
Mu ñoz et al., Human Excretion in LCA of Food Products
533
FORUM
Figure 4 Contribution of different life cycle stages and items to eutrophication potential, global warming
potential, water use, and primary energy use for consumption in the United Kingdom of 1 kg Spain-grown
broccoli. WWT = wastewater.
Nonetheless, wastewater treatment of feces and
urine creates the biggest contribution to EP; this
is mostly due to nitrogen and phosphorus compounds.
In the case of GWP (figure 4b), the contribution from the excretion and wastewater phase is
not as significant as in the nutrient-related impact. Energy use at home (mainly for boiling
the broccoli) dominates GWP through carbon
dioxide (CO 2 ) emissions, whereas fertilizer related nitrous oxide (N 2 O) and CO 2 from fuel
use by farm machinery dominate the cropping
stage. The GWP reduction in cultivation, due to
the C embodied in broccoli through photosynthesis (seen as a negative bar in Figure 4b), is
almost entirely reemitted to the atmosphere during the excretion and wastewater stage. The remaining C is emitted in the landfilled food waste
and/or remains in the landfill or sewage sludge
from wastewater.
Concerning WU (figure 4c), the cropping
stage is clearly the most important one, as it is
responsible for 73% of the overall water consump-
534
Journal of Industrial Ecology
tion. Broccoli is an irrigated crop in Spain, using
something less than 200 L/kg. The contributions
of the home (15%) and excretion and wastewater
(9%) stages are not negligible, however. It must
be highlighted that the water use associated with
the home stage is mainly cooling water used in
electricity production, rather than water actually
used in the kitchen for cooking. Although the
contribution of these two stages may seem low,
it must be borne in mind that if UK-grown broccoli were considered, these relative contributions
would be much higher, because in the United
Kingdom broccoli is rain fed.
Finally, PEU (figure 4d) is clearly dominated
by the home stage, due to the electricity and gas
consumed for broccoli cooking, which account
for 65% of the PEU. Retail and distribution and
cropping are responsible for 22% and 11%, respectively. In this case study, it is concluded
that human excretion and further wastewater
treatment have a negligible contribution from a
PEU perspective. Nevertheless, it is interesting
to observe that the PEU of toilet use processes
FORUM
(production and delivery of toilet paper, soap,
tap water, etc.) is three times higher than that
related to treating the fecal wastewater in the
sewage plant.
Discussion and Conclusions
Human excretion has proven to be significant in the overall life cycle of food products.
Particularly for the nutrient-related EP, emissions from postconsumer wastewater treatment
are of paramount importance, together with other
home-related nutrient losses through boiling water and food waste. This is crucial, as it is often
concluded from partial LCA studies, cradle-togate or cradle-to-retail, that EP is dominated by
the cropping stage. Obviously, nutrient emissions
from agriculture merit attention due to their diffuse nature and the fact that it is possible to reduce
them. Nonetheless, nutrient emissions from domestic activities should not be overlooked. The
results obtained in this work suggest the importance of educating consumers on “healthy cooking” to avoid flushing so much of the food’s nutrient content down the drain (e.g., 30% of proteins
and 34% of phosphorus are lost to the water when
broccoli is boiled). The amount of nutrients lost
in the cooking stage will vary significantly with
food type, particularly with foodstuffs that are
eaten raw (e.g., lettuce, fruit); in this case, there
would be less nutrient loss in the kitchen but
more releases after treatment of fecal wastewater.
It should be noted that the contribution of excretion and wastewater to GWP would also be more
significant for foodstuffs that are eaten raw (i.e.,
when no energy is used for cooking). In addition, the contribution from the distribution stage
is relatively high, and the relative importance of
excretion is therefore reduced in the example presented here because broccoli is transported over
2,600 kilometers (km).7
The model presented here provides a tool to
enable human excretion to be included in food
LCAs. This tool might also be of interest for other
environmental analysis methods, such as material flow analysis and substance flow analysis, as
the model can be used to close the balances for
carbon, nitrogen, and phosphorus, among other
substances present in food.
Concerning the application of this model to
regions different from the United Kingdom and
other western countries, we can make a distinction between human body modeling, on the one
hand, and toilet use and wastewater treatment
plants, on the other. The default values used
here to model the latter are representative of
the United Kingdom, but they can be modified
at will by the user. The balances obtained from
human body modeling are based on figures from
western sources but should be generally applicable. For most of the variables included, data from
different regions have not been found, but the
model should give reasonable estimates of emissions from the macronutrients in food.
The case study presented here can be considered as an attributional LCA. Our model has
proven to be useful in highlighting the relative
importance of excretion in a food product’s life
cycle. Nevertheless, it has also been shown to be
very sensitive to food composition, which suggests that it may also be useful in consequential
LCA, dealing with such topics as the environmental assessment of dietary shifts. Until now,
only the comparative environmental impacts of
producing the ingredients for different diets have
been assessed; this study shows that differences
related to excretion emissions from different diet
compositions may also be important, particularly
when changes in the balance of macronutrients
(proteins, fats, carbohydrates, fiber) occur. Future
research should check the significance of the excretion stage for a range of food products. In addition, the model could be developed further to allow for differing N content in proteins from different sources (e.g., to distinguish between animalbased and plant-based proteins); however, initial exploration of variations in protein/N factors
suggests that the resultant changes in excretion
impacts are likely to be small.
Acknowledgements
This research was carried out under
project RES-224-25-0044 (http://www.bangor.
ac.uk/relu), funded as part of the UK Rural
Economy and Land Use (RELU) programme.
Dr. Milà i Canals acknowledges GIRO CT
(http://www.giroct.net) for its logistic support.
The authors kindly thank Prof. Mike Clifford for
Mu ñoz et al., Human Excretion in LCA of Food Products
535
FORUM
his useful comments on the model, Gabor Doka
for his support in wastewater treatment modeling,
and the three anonymous reviewers who have
provided constructive comments on this article.
Notes
1. Depending on the goal of the study, LCAs
are usually classified as follows (Weidema, 2003):
• Attributional: life cycle assessments of the accountancy type, typically applied for hot-spot
identification, for product declarations, and for
generic consumer information. This would correspond to the type of study carried out in our
article.
• Consequential: they study the environmental
consequences of possible (future) changes between alternative product systems, typically applied in product development and in public policy
making. This type of LCA would correspond to a
case study dealing with diet shifting.
2. One milliliter (mL) = 10−3 liters (L) ≈
0.034 fluid ounces.
3. One kilogram (kg, SI) ≈ 2.204 pounds (lb).
4. The presence of mercaptans in intestinal
gas is regarded as a local environmental quality
issue and not included in the model.
5. One atmosphere (atm) ≈ 760 torr ≈ 14.70
pounds/inch2 .
6. 310 ◦ K ≈ 36.85 ◦ C ≈ 98.33 ◦ F.
7. One kilometer (km, SI) ≈ 0.621 miles
(mi).
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About the Authors
Ivan Muñoz was a research fellow at the Centre for Environmental Strategy, University of
Surrey, Surrey, United Kingdom, at the time the
article was written. He is currently a researcher
at the Department of Hydrogeology and Analytical Chemistry, University of Almeria, Almeria,
Spain. Llorenç Milà i Canals was a research fellow at the Centre for Environmental Strategy,
University of Surrey, at the time the article was
written. He is currently life cycle analysis manager within Unilever’s Safety and Environmental
Assurance Centre (SEAC) group, Unilever Colworth, Colworth, United Kingdom. Roland Clift
is distinguished professor of environmental technology in the Centre for Environmental Strategy at the University of Surrey and presidentelect of the International Society for Industrial
Ecology.