an evaluation of cassava as a biofuel crop focusing on crop
TRANSCRIPT
An evaluation of cassava as a biofuel crop focusing on crop yield, ethanol
conversion rate, and water, soil and atmospheric contamination
by
Sarah Thompson
An Undergraduate Thesis
Submitted in Partial Fulfillment for the Requirement of
Bachelor of Arts
In
Environmental Science: Policy Analysis
Carthage College
Kenosha, WI
May, 2012
An evaluation of cassava as a biofuel crop focusing on crop yield, ethanol
conversion rate, and water, soil and atmospheric contamination
Sarah Thompson
May 12, 2012
____________________
Abstract:
An evaluation of biofuels is imperative to relieve human dependence on fossil fuels as
well as to reduce negative impacts on the environment that result from the burning
conventional fuels. Due to the exponentially growing global population, demands for fuel are
constantly on the rise. Biofuels are seemingly the perfect alternative to conventional fuels;
however it is important that their use is not creating more substantial issues than the ones they
are attempting to solve. Cassava is an energy crop that is becoming increasingly popular,
especially in China who purchased 98% of Nigeria’s cassava starch production in 2010 to
convert to ethanol. Cassava contains potentially toxic levels of cyanogenic glycosides which,
when disturbed, produce the toxic chemical hydrogen cyanide (HCN). Due to this toxic
release, I hypothesize that cassava production on a large scale for the purpose of
manufacturing biofuels will have a more so negative effect on both human and environmental
health with regard water, soil, and the atmosphere. This assessment was conducted by
compiling previous data from online sources and scholarly articles. The results support the
hypothesis that cassava production and processing for biofuel use is negatively impacting
environmental health however, further research is required to accurately address cassava as a
biofuel as production increases.
____________________
Introduction
The global human population has recently exceeded 7 billion. Of those alive today and
the 385 thousand more that will be here tomorrow (Bureau, 2010) there are countless opinions
about what should be done regarding everything from local politics to global environmental
affairs. The environment like all global systems is complex and in order to approach issues of
great complexity it is important to maintain a non biased ear by weighing all the options
presented globally to combat environmental degradation. Fuel is an ongoing source of global
environmental concern both because of high dependence levels and resulting negative
environmental impacts. Some impacts include air and water pollution as well as increases in
global temperature. U.S. society and industry rely on oil reserves to carry on life as we know it.
Following the events of September 11th the U.S. has put a halt on a majority of oil
imports from the Middle East. Because of this the U.S., as well as many other countries who
strive for independence in terms of energy, is turning to a greener option: biofuel. Typical food
crop biofuels are produced from the starchy portion of energy crops. One energy crop that
recently entered the biofuel market on a major scale is cassava. Many characteristics make it an
ideal energy crop however it also has a toxic chemical defense that classifies it an
environmental nuisance. If global goals include slowing environmental degradation and
reducing the population’s carbon imprint, investments in “greener” fuels are essential.
Cellulosic and food crop biofuels are a greener option than petroleum based fuels. The
political, biological, and chemical aspects of biofuels and an in depth exploration of cassava as
an energy crop will now be further discussed.
Literature Review
The United States alone consumes close to 20 million barrels of petroleum on average
per day. About 75% of this is used for transportation. Globally petroleum is consumed at a rate
of over 85 million barrels per day (Energy, 2011). Statistical evidence shows that the United
States, which in 2010 was home to only 4% of the global population, was responsible for 25%
of the total daily global petroleum consumption (Bureau, 2010). Based on statistical data there
is no denying that the United State’s dependence on petroleum fuel is substantially higher
than a majority of the world. The exponentially growing global human population
accompanied with development of underdeveloped nations means oil demands will continue
to soar; that is unless we adapt to alternative energy options or consume in a more sustainable
fashion.
There are two fundamental political viewpoints regarding this challenge to meet global
future fuel needs. The first is that of the “survivalists”. Survivalists like Lester Brown preach
that if we do not learn to use our non renewable resources sustainably (petroleum) while
strongly pursuing fuel alternatives, it is not only fuel shortages that we will be facing but also
major consequences resulting from harmful CO₂ emissions (global climate change) which may
lead to an increase in severe weather as well as global water shortages. Survivalists believe
that earth has a limited amount of resources and that humans need to reduce their
environmental imprint substantially in order to prevent population crash and further
deterioration of the globe (Dryzek, 2005).
On the opposite side of the spectrum are the “prometheans” who believe in the power
of technology to always overcome environmental challenges. In the face of “fuel shortages”,
they do not believe there is a need to worry because “fuel is only sought after when it is
needed, therefore we have no idea how much of it we have. We have always discovered more
fuel in the past, and even if we do run out, technology will adapt to create new fuels” (Dryzek,
2005).
Though their viewpoints differ greatly both discourses agree on the need for alternative
fuel sources (though the level of necessity may vary). Over the last couple of decades there has
been an enormous push toward biofuel development. Biofuels are fuels produced from
biological sources, like plant biomass, that are renewable. Biofuels can be produced from any
starchy plant material. Most biofuels are currently produced from food crops; however current
methods leave the majority of plant biomass behind as byproducts. An alternative is cellulosic
biofuel which converts cellulose into ethanol. This method can use any plant biomass as raw
products for ethanol production.
Why the push for alternatives
Though the prometheans believe that planet earth has no limits there are a great
number of individuals who see recent events as a sign that we are depleting resources at such
a highly unsustainable rate that if we do not change our ways we will suffer in a world lacking
it’s essential fuel source (Dryzek, 2005). Though through technology we have created great
amounts of renewable energy (solar, wind, hydro, and biofuels), none are currently enough to
completely stop using fossil fuel based energy. We must loosen our grip on oil and do so
before it is too late.
In America, the middle and lower classes have taken the hardest hit to their wallets. On
average, the price of gas in the United States has almost doubled since 2006 (Historical Price
Charts, 2011). Though prices tend to fluctuate some areas have seen prices soar to over $5.00 a
gallon. An assessment of U.S. average gas prices over the last 6 years show that since the
significant spike and decline in 2008, gas prices have been on an overall incline (Figure 1).
Figure 1: Average gas prices in the United States since 2006 to the present.
The 2008 spike was a result of natural disasters and political events that
indirectly caused an increase in the global oil market. As a result of such
events inflation took its course leading to price increases that peaked mid
2008. Following this peak, prices rather than global events became the fuel
selling determinant. High prices led to a recession that resulted in low
confidence in consumers and a decrease in demand, triggering the huge
drop off in gas prices seen in the figure.
Aside from price, there are other concerns that are common to many U.S. citizens and
other individuals worldwide. Are oil reserves really diminishing? Scientists can only
hypothesize but many agree that the global consumption of fossil fuels is far more demanding
than the earth can sustain for future generations. Since the BP Gulf oil spill the public has
become more aware of the depths we are going to in order to keep our national oil reserves
high. This accidental spill caused by high risk extraction has left hundreds of miles of beaches
disrupted, and has caused over 2,000 square miles of Louisiana’s wetlands to disappear (Golf
of Mexico Oil Spill (2010), 2011).
Repercussions like those of the BP oil spill have not had a significant enough impact for
oil companies to avoid similar high risk
projects. One currently of great political
conflict is the expansion of the Keystone XL
Pipeline. This venture will extend the current
pipeline an additional 1,700 miles through
U.S. soils (Figure 2). The XL pipeline will put
water and soil at a high risk for damaging
and undetectable potential leaks. Though the
pipeline’s design team has changed its
original coarse proposal to avoid some
environmentally sensitive areas, there is no
stopping this expansion project (Frosch,
2011). Our dependence on oil may prevail,
until the oil preserves run dry or prices soar to unaffordable heights.
Figure 2 (above): Original proposed rout of the Keystone XL Pipeline expansion.
Restructuring our nation
Though future oil projects are underway and more will continue to be proposed, there
is hope for a more sustainable future. The environmental discourse, “ecological
modernization” can be applied to support the production of biofuels. This discourse views
environmental problems as structural, and believes that the solving of such issues requires
economic reorganization of nations (Dryzek, 2005). Money is a key player both for
consumption and production of fuel. If governments can make greener fuels more cost
beneficial for business, then biofuels will be more embraced.
Thailand in particular has conducted multiple economic analyses regarding the
implementation of biofuel production into their culture. A study by David Bell (2011) found
that though pursuing alternative fuels would initially be more costly than importing
petroleum as they currently do, the Thai economy would benefit greatly because self
production would keep all fuel profits within their border (Bell, 2011). Economic assessments
are complex because they require views from all angles that are often difficult to see. When
considering biofuels versus fossil fuels it is not only production costs and sales that matter;
pollution, waste management, maintenance costs, and environmental impacts are all
important. As ecological modernization would suggest, “It pays to be green” (Dryzek, 2005).
Pollution cleanup, water treatment, and soil remediation are far more costly than pursuing
cleaner, greener practices.
Why Biofuels?
Besides being renewable, biofuels are more environmentally friendly than their fossil
fuel based counterparts because they reduce carbon dioxide emissions that lead to global
climate change. Climate change is a major global concern. This claim is not exclusively
supported, however when facing issues of such magnitude it is wise to implement
precautionary practices rather than continue to accelerate when possibly approaching a brick
wall (Dryzek, 2005). Scientists have found that the burning of fossil fuels results in major
carbon dioxide emissions that, due to their immense sum are slowly deteriorating parts of
earth’s ozone layer. Biofuels offer the advantage of being carbon dioxide neutral over their
production lifecycle; therefore biofuel consumption in combination with or in replacement of
fossil fuels would potentially reduce environmental atmospheric pollution (Wu, 2010).
Biofuel production would decrease fossil fuel imports while supporting the economy of
producing nations. Especially in developing nations, an increase in production would also
increase employment and rural development. Unfortunately, like all political issues, biofuel
implementation is complex; for every positive argument there is a counterargument.
Why not?
In kitchen cabinets throughout America (and much of the world) almost every food
label includes the ingredient corn (in one form or another). Whether aware of it or not, corn
products are a significant portion of our diet. Corn is a source of energy as well as food for
both humans and livestock. Statistics show that in America, four out of every ten ears of corn
are put towards ethanol production. That is 40% of our total corn production. Another 40% is
allocated to feed livestock while the remaining 20% is food for the human population (Rattner,
2011). This is the basis of the food versus fuel controversy. Our exponentially growing
population (Figure 3) is putting increasing pressure on agriculture to meet food demands for
both humans and livestock. As demands for food, fuel, exports, and livestock needs continue
to increase, pressures on farmers will also continue to rise. (Rattner, 2011). Due to the limited
amount of crop land available; any land used to grow energy crops is land that cannot be used
for food production. Ethanol production has and will continue to increase as the push for
alternative fuels becomes more intense (Figure 4).
Figure 3: This figure shows the global human population growth curve.
The human population is growing at an exponential rate and currently
totals over 7 billion people.
Figure 4: This figure shows the amount of ethanol the U.S. has produced in
millions of gallons from 1980 to 2010 (RFA, 2011).
Another matter of significant concern is water. Both food and energy crops alike require
large amounts of water for successful growth. For areas like the Midwest in the United States
water is not a concern because precipitation provides necessary irrigation. However, the
American west has reason to fear. The Colorado River is being used to irrigate over 4 million
acres of agricultural fields in Arizona and California. In 2006 about $1 billion dollars was spent
irrigating those western states (Pearce, 2006). The intense irrigation of dessert regions is
causing the depletion of underground aquifers as well as rivers to run dry. For example, the
once mighty Colorado no longer reaches the sea. Though not dry, the river is substantially
lower than in the past (Pearce, 2006). Prometheans may believe there isn’t a need to panic, but
if we do not pump the brakes on water use in the near future, that brick wall may be closer
than we thought (Dryzek, 2005).
0
2000
4000
6000
8000
10000
12000
14000
1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010
Eth
ano
l Pro
du
ctio
n in
Mill
ion
s o
f G
allo
ns
Time in Years
Historic U.S. Fuel Ethanol Production.
Globally, an increase in food crop ethanol production means that the following could be
affected: an increase in crop land, which will increase water runoff due to the lack of natural
vegetation, an increase in irrigation which will negatively impact the water cycle, an increase
in food prices, an increase in GMOs (genetically modified organisms-crops) which leads to an
increase in weed and pest resistance, and a variety of waste products that must be treated or
managed. Ideally, we would identify a biofuel crop that maximizes energy output while
minimizing the environmental effects. A thorough understanding of the typical biofuel
production processes must be understood in order to maximize energy output while
minimizing the harmful waste products.
Typical ethanol production processes
Cassava Starch Production
Cassava starch is produced from the root of the cassava plant. One production process
of cassava starch is known as “wet milling” (Figure 5) (Cassava Starch Production, 2005).
There are 5 main steps involved in the extraction of starch from the tuberous root: preparation,
starch washing, dewatering/drying, and milling/packaging. The preparation step includes
obtaining cassava roots from the field and then washing and peeling them. Roots must be
obtained from outside sources within 48 hours of harvest because of cassava’s highly
perishable traits (Cassava Starch Production, 2005). The clean tubers are then grated into
smaller pieces, mixed with water, passed through a number of filtering screens, and allowed to
settle. This starch washing process is essential to obtain high starch recovery. The resulting
starch/water mixture is then dewatered and dried. The dried cassava is processed in a mill to
produce flour from the cassava mash. The cassava starch production process results in the
formation of flour (Cassava Starch Production, 2005).
Because of its highly perishable character and its success rate of growing in tropical and
subtropical regions, countries like China purchase cassava starch (in the form of flour or dried
chips) rather than obtaining fresh tubers from self growing them or importing them. For this
reason (as well as others) cassava ethanol production is typically a two stem process: 1. Starch
production, 2. Ethanol production. The production of ethanol from cassava starch involves 5
main steps (Figure 6): liquefaction, saccharification, cooling, fermentation, and distillation.
The cassava starch is combined with process water, its pH is adjusted by the addition of an
enzyme, and the resulting solution is heated to 190°F. The liquefaction increases the viscosity
of the mixture. This mixture is then cooked and cooled multiple times. During this time the
enzymes are breaking the starches down into short chain dextrins (Brown, 2007). For
fermentation to occur an additional enzyme is added to the mixture and it’s pH and
temperature are adjusted. In the fermentation tank this enzyme breaks down the dextrins into
sugars (Brown, 2007). The ethanol/water mixture is then distilled by heating mixture to
remove water. Finally a molecular sieve is used to completely dehydrate the ethanol. The
completed fuel is then stored until transported elsewhere for consumption (Brown, 2007).
What makes a good energy crop candidate?
Though any plant could potentially be used to produce fuel, there are particular
characteristics that denote a good energy crop. Because ethanol production relies on the
conversion of starches to simple sugars and eventually to alcohol, ideal energy crops should be
high in starch content (Adelekan, 2010). Due to the current depressed state of the economy,
cost is a significant interest for all types of production. Therefore ideal energy crops are those
Figure 6: (Right) (Cassava starch-
based) Ethanol production flow
chart: A majority of cassava based
ethanol is produced from cassava
flour or starch, therefore starch
production and ethanol
production are carried out
separately (Cassava Starch
Production, 2005).
Figure 5: (Left) Cassava starch
production flow chart: Starch
production from cassava is most
common in Nigeria and Thailand.
In 2010 98% of Thailand’s cassava
starch production was sold to
China to produce biofuel (Cassava
Starch Production, 2005).
capable of being grown at a low cost (Adelekan, 2010). This includes purchase of water,
fertilizers, seeds, and maintenance. These crops vary from region to region. The Midwest soil
and climate is ideal for growing corn while the tropics are ideal from growing cassava and
sweet sorghum. Plants that are tolerant to drought, flood, high/low sun exposure and a variety
of temperatures are preferred (depending on conditions of growing region). Also favored are
crops that have high yield varieties available; however these GMO’s must be purchased
yearly, are more expensive and can be unrealistic in poor regions of the world. One of the top
candidates in the future world market for biofuel production is the cassava plant due to its
high starch content.
Introduction to Cassava (history):
Cassava is a potato-like plant whose starchy portion lies below the soil. It is considered
a perennial shrub that tends to grow in bundles. Cassava is native to South America, however
it can be found in many tropical and sub-tropical countries including Brazil, Congo,
Democratic Republic, Thailand, and Indonesia (Adelekan, 2010). China has also begun to
invest in cassava ethanol production. Because of its high starch content, cheap cost, and
resistance to harsh conditions the poor have come to depend upon it as a staple food source,
and thus this crop is often referred to as “the third-world crop.” Cassava is one of the most
important carbohydrates in the tropics, ranking forth, and close behind sugar cane, corn and
rice. It is cassava’s chemical makeup that makes it both one of the best and worst candidates
for an energy crop.
Cassava- characteristics and chemistry
Familiarity with all of the cassavas phytochemistry is important to assess the relative
importance of the plant when compared to other food and energy crops. Therefore, though
very specific, the following traits are key characteristics that cassava possesses. Cassava has an
average starch content of 32% which compares to that of corn which averages 40%
(Blagbrough, 2010). Though cassava does not have the highest % starch composition or energy
crops it does have one of the very highest crop yields per hectare (table 1) (Wu, 2010). On
average cassava produces 6,000kg of ethanol per hectare annually, while corn produces only
2,050 kg per hectare annually (Adelekan, 2010).
Crops Yield (tons/ha/yr) Conversion rate to
ethanol (L/ton)
Ethanol yield
(kg/ha/yr)
Sugar cane 70 70 4,900
Cassava 40 150 6,000
Carrot 45 100 4,500
Sweet Sorghum 35 80 2,800
Maize 5 410 2,050
Wheat 4 390 1,560
Rice 5 450 2,250
Table 1: Comparison of the ethanol yields of a variety of energy crops (Adelekan, 2010).
The main chemical components in cassavas leaves and roots are cyanogenic glycosides,
hydroxycoumarins, terpenoids, flavan-3-ols, fatty acids and esters (Blagbrough, 2010).
Cyanogenic glycosides are found in all cassava tissues. Their primary function is to protect the
plant from herbivory. Potentially toxic levels of cyanogenic glycosides (between 6 and
370mg/kg) have been found primarily in cassava’s roots (Blagbrough, 2010). When cells are
damaged, the cyanogenic glycoside called linamarin is contacted with the enzyme linamarase.
This contact produces acetone cyanohydrins which readily decompose in to HCN (hydrogen
cyanide acid) and other chemicals. If consumed prior to reducing the cyanogenic glycoside
content, irreversible health effects may occur. Hydroxycoumarins are secondary metabolites
that help the plant combat phytopathogens and abiotic stresses. They have also been found to
regulate plant growth, stress and hormone levels (Blagbrough, 2010). Particular
hydroxycoumarins in the roots have been found to accumulate within 48 hours after harvest.
These metabolites cause the physiological break down of the roots. Due to this cassava has a
very short shelf life.
Terpenoids in cassava contribute to its scent, flavor and color. Roots have varying
concentrations of β-carotene which gives some roots a bright yellow to orange color. Past
studies have shown that roots possessing a higher β-Carotene concentration had a slower rate
of post-harvest physiological deterioration (Blagbrough, 2010). Flavan-3-ols can only be
identified in healthy roots following harvest and accumulate in tissues from one to seven days
and then begin to decline at a rapid pace. Flavanols also aid in root breakdown.
Due to its rapid deterioration rate it must either be eaten quickly following harvest in
order to avoid potential sickness or processed before starches begin to break down and are no
longer useable. Though this trait puts extra pressure on farmers to transport cassava to
processing plants quickly, typically cassava’s most unfavorable characteristic is thought to be
its high concentration of cyanogenic glycosides.
Cyanogenic glycosides:
There are procedures to reduce the cyanogenic glycoside content in cassava roots prior
to consumption/production. However, if these procedures are not performed adequately,
consumption can cause Konzo or other illnesses. Konzo occurs more commonly in nutrient
deprived children and causes individual’s legs to be permanently paralyzed (Blagbrough,
2010). There have been two procedures previously studied to address the problem by reducing
the cyanogenic glycosides in cassava roots.
A study conducted at Ohio State University (Siritunga & Sayre, 2004)found that the
concentration of cyanogens in the roots are almost entirely synthesized and transported from
the leaves. Following this discovery the Ohio state research team generated transgenic cassava
where the synthesis of cyanogenic glycosides is inhibited in the leaves by the expression of
particular gene fragments (Siritunga & Sayre, 2004).This procedure was found to reduce the
concentration of cyananogens in roots by more than 99%. The alternative solution studied by
the same team was to increase the rate of cyanogenesis and volatilization during food
processing. This is done the generation of transgenic cassava that over-expresses HNL
(hydroxynitrile lyase-an enzyme that catalyzes the breakdown of acetone cyanohydrine into
cyanide) in its leaves and roots (Siritunga & Sayre, 2004). This acceleration in cyanogenesis
significantly reduces the accumulation of acetone cyanohydrins during processing to avoid
releases of cyanide. The advantage of the second alternative is that the plant still maintains its
cyanogenic properties throughout its growth to avoid herbivory, and the additional HNL
encourages the rapid breakdown of the toxins so processing is quicker and easier (Siritunga &
Sayre, 2004).
The previous examples are the most recent research in cassava toxicity reduction;
however they are not used on a large scale due to their price and complexity. The current
popular method of processing cassava to remove toxins before use is to pound and boil leaves
for about 30 minutes. The problem with this is that boiling removes essentially all of the
protein content from the cassava as well as the cyano-toxins. Methods to remove cyanogens
from cassava leaves while preserving protein content is described in a paper by Howard
Bradbury and Ian Denton. The first method involves pounding leaves, followed by washing at
30 degrees Celsius while the second method avoids pounding leaves but simply performs
washes of varying temperatures of water (Bradbury & Denton, 2011).The pounding method
was found to remove more of the cyanide concentration while preserving essentially all of the
protein composition. However, this procedure consumes large amounts of water. Because the
washes contain sums of cyanide they are considered a waste product, and unless properly
disposed of can cause damage to the soil or other aspects of the ecosystem.
Assessment of Cassava Production factory effluents:
A study published in the journal “Chemistry and Ecology” was analyzed to gather
specific raw data. This research explored the effects that cassava effluents were having on two
fish species in the Nile River in Africa. Researchers obtained cassava roots from a farm in
Nigeria and processed them using factory procedures. The cyanide concentration of the
resulting effluent was found to be 190.62mg/L which by far exceeds the WHO (world health
organization) for wastewater effluents. Characteristics of tilapia and mud catfish in the area
were compared to others located away from factory effluent releases. Results of the study
showed that the toxicity of the water significantly impacted the fish body weight and
hematological parameters (dealing with diseases of the blood) (Adekuncl, Arowolo, Omoniyi,
& Olubambi, 2007).
Evaluating cassava as a biofuel crop:
A slurry of negative and positive effects could be discussed indefinitely making it
difficult to weigh the pros and cons of producing biofuel from cassava. I hypothesize that
cassava production on a large scale for the purpose of producing biofuels will have a negative
effect on both human and environmental health with regard to water, soil, and the
atmosphere. The following assessment will attempt to weigh some of the positive and the
negative outcomes associated with ethanol production by way of cassava. This evaluation will
focus on the following aspects: crop yield, ethanol conversion rate, water use, and water, soil
and atmospheric contamination. Both current and futuristic projections will be incorporated.
____________________
Methods
The results for this evaluation were obtained by accumulating information from
published scientific journals and some online sources. The first section of the results highlights
the positive aspects of producing cassava as a biofuel crop. Data from the introduction section
was compiled to create figures 7, 8, and 9 which display cassava as a good energy crop with
regard to crop yield, ethanol conversion rate, and ethanol yield in comparison with corn. The
next section of the results is split into sub sections and focuses on the negative results of using
cassava for ethanol production. Data from the Austrilian Journal of Crop Science was compiled to
develop figure 2 which is a graphical depiction comparing the concentration of cyanogenic
glycosides in the leaf and root tissues of cassava (Ubalua, 2010). Table (2) was created using
data from referenced articles to establish general effluent values from current cassava starch
production factories. These values were used to construct graphs that generally estimates the
mass of cyanide released in effluent for the amount of cassava starch produced (figure 11).
Formula 1 was developed to calculate the data presented in figure 11.
Average amount of cassava starch produced daily 150 tons
Waste effluent discharge per ton of starch
produced
4,000-6,000
Liters
Maximum Cyanide Content of waste water effluent 200mg/L
Table 2: Production statistics from Nigeria cassava starch production factory (Adekuncl,
Arowolo, Omoniyi, & Olubambi, 2007).
Formula 1:
Formula 1: Since the study reported that between 4000 and 6000 L of effluent were
discharged for every ton of cassava starch produced both and upper and lower limit are
displayed in the formula and on the constructed graph. In the formula CN is the
chemical formula for cyanide.
Assessment of Cassava Production factory effluents:
The values found in table 2 (Department, 2006) were used to construct one simple
graph. This graph was constructed to show the amount of cyanide being discharged by way of
factory effluent (Formula 1) per tons of cassava produced. A logarithmic scale for effluent was
used to produce a more readable figure. The values of tons of starch on the x-axis were used
for the following reasons: 1, 10, 50, and 100 were used to show the effluent discharges on a
smaller scale, modern mechanized factories produce about 150 tons of cassava starch a day,
using the daily value the monthly (4500) and yearly averages (54000) were also included.
Table 4 was then constructed as a reference to compare acceptable concentrations of CN
in water to the amount being discharged from cassava production factories. The safe drinking
water standard provided by the EPA is 2ppm (or .000002mg/L) (EPA, 2011). Next the toxicity
limit for waste water set by the WHO (World Health Organization) was presented to be .03-.05
mg/L (Adekuncl, Arowolo, Omoniyi, & Olubambi, 2007). Table 4 is presented below.
Safe Drinking Water Standards (EPA, 2011) 2ppm (.000002 mg/L)
WHO Toxicity limit in oxygenated surface water
(Adekuncl, Arowolo, Omoniyi, & Olubambi, 2007)
.03-.05 mg CN/L
WHO Cyanide content in starch (Somboonchai,
Nopharatana, & Songkasiri, 2008)
10 mg CN/kg starch
Table 4: Water standards as determined by the WHO (World Health Organization) and the EPA (U.S.
Environmental Protection Agency) for drinking water, surface water and cassava starch production.
Investigation of HCN (from cassava) impact on the soil
Table 3 was constructed containing some of the physical and chemical characteristics of
hydrogen cyanide. This information was obtained in combination from articles from the Plant
Science journal and the Australian Journal of Crop Science. The interaction with water
(miscible), volatilization and harmful concentration of HCN for microbial populations were
used in assessing HCN’s level of impact on the soil as well as ground water. Figures 13, 14,
and 15 are shaded maps of Nigeria/ Africa that were obtained from online sources in an
attempt to compare cassava growth/ production with rainfall, soil type and soil degradation.
Investigation of HCN (from cassava) impact on the atmosphere
Figure 12 was obtained from a published scientific journal by the author Li. This figure shows
the concentration of HCN gas in the surface air around the globe. Li’s study focused on the
contribution that coal burning power plants have on surface HCN levels. HCN was determined to be a
toxic gas that can cause health problems at certain concentrations. This figure was used to make
assumptions about the impact industrial waste from cassava production and possibly ethanol
production may be having on total HCN in the atmosphere (Li, Daniel, Robert, & Colette, 2003).
____________________
Results
Highlighting positive characteristics of cassava as a biofuel crop
In a comparison with corn which accounts for the highest percentage of global annual
ethanol production, cassava yields are much larger than that of corn (figure 7); Cassava on
average produces 8 times more ethanol per hectare per year than corn even though corn has a
much higher ethanol conversion rate (L/ton) than cassava (figure 8). As a result of its high crop
yield per hectare cassava produces a higher average ethanol yield (kg/ha/yr) than corn does
(figure 9).
Figure 7 (on left) and Figure 9 (on right): These figures display a comparison of cassava and
corn based on crop yield (left) and ethanol conversion rate (right).
40
5
0
10
20
30
40
50
Cassava Corn
Cro
p Y
ield
in t
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s/h
a/yr
Energy Crop
Comparison of Cassava and Corn
based on Crop Yield
150410
0
200
400
600
Cassava Corn
Eth
ano
l Co
nve
rsio
nra
te in
L/t
on
Energy Crop
Comparison of Cassava and Corn based on Ethanol Conversion
rate
Figure 9: Display of a comparison of the energy crops cassava and corn based on ethanol yield
in kg/ha/yr.
Highlighting negative characteristics of cassava as a biofuel
All tissues of cassava contain cyanogenic glycosides; the most prevalent of these is
linamarin (Blagbrough, 2010). Depending on tissue type, there is high variation in the amount
of cyanogenic glycosides present in tissues (figure 10) (Ubalua, 2010). The primary concern of
focus in this study is the environmental impacts associated with cassava starch production and
conversion to ethanol. A cassava starch production factory in Nigeria was assessed focusing
on starch production, effluent discharge and the concentration of –CN in effluent (Table 2)
(Adekuncl, Arowolo, Omoniyi, & Olubambi, 2007). During cassava starch production
processing water is often recycled (Somboonchai, Nopharatana, & Songkasiri, 2008). The HCN
concentration of this recycled water is between 10 and 50 mg CN/L. This causes an
accumulation of HCN in the starch throughout processing as well as in the final product
6000
2050
0
1000
2000
3000
4000
5000
6000
7000
Cassava Corn
Eth
ano
l Yie
ld in
kg/
ha/
yr
Energy Crop
Comparison of Cassava and Corn based on Ethanol Yield
(Somboonchai, Nopharatana, & Songkasiri, 2008). HCN concentration of cassava flour across
Nigeria ranged from 18.6 to 94.9 mg HCN/kg (dry weight) (Services, 2006). The amount of
HCN released into the environment as a result of discharging untreated cassava starch factory
waste water is shown in figure 11.
Figure 10: Concentration of cyanogenic glycosides in leaf and root tissues of cassava. There is a
low and high limit for cyanogenic glycosides concentration in root tissues shown on the graph.
The concentration in roots varies from 100 to 500mg per kg of fresh weight (Ubalua, 2010).
0
1000
2000
3000
4000
5000
6000
Leaves Roots
Co
nce
ntr
atio
n o
f cy
ano
gen
ic
glyc
osi
de
s (m
g/kg
)
Cassava plant tissue
Concentrations of cyanogenic glycosides in leaf and root tissues of cassava
High Conc.
Low Conc.
Figure 11: The upper and lower limit for the concentration of –CN in effluent that is
discharged from cassava starch factories per the number of tons of cassava starch being
produced. The y-axis is displayed using a logarithmic (base 10) scale. The following values on
the x-axis, 150, 4500, and 54000, are the average amount (tons) of cassava produced daily,
monthly, and annually (consecutively) (Adekuncl, Arowolo, Omoniyi, & Olubambi, 2007).
Industrial waste is the most significant source of hydrogen cyanide (HCN) in the
environment (Ubalua, 2010). HCN’s physical characteristics are the driving factors behind this
weak acid’s harmful effects on the environment (Table 3).
0.10
1.00
10.00
100.00
1000.00
10000.00
100000.00
1 10 50 100 150 4500 54000
Cya
nid
e in
Eff
lue
nt
(in
hu
nd
red
th
ou
san
d m
g)
Tons of Cassava Starch
Mg of Cyanide released (in effluent) pertons of cassava starch produced
mg of cyanide in lower limit of wastewater production
Boiling Point 25.9±.3 °C (equivalent to 78.6°F)
Visual Clear, Colorless liquid
Odor Described as bitter almonds
Interaction with water Miscible with water above tempertues of -
23.3°C
Level of volatilization High
Concentration in soil at which
microbial populations are adversely
affected
.3mg HCN/ kg of soil
Table 3: Physical characteristics of HCN (Elias, Nambisan, & Sudhakaran, 1997) (Ubalua, 2010)
It has been determined that oceans are the dominant sink for HCN gas. The major
sources include emissions of burning fossil fuels, coal power plants, and industrial waste.
Once in the atmosphere HCN gas has a tropospheric lifetime of about 5.3 months (Li, Daniel,
Robert, & Colette, 2003). A global projection of the amount of surface level HCN gas in the
troposphere was constructed from samples from aircraft and projected in the following
figure(Figure 12) (Li, Daniel, Robert, & Colette, 2003).
Figure 12: A global depiction of the concentration of HCN gas in the surface air of the
troposphere in pptv (parts per trillion * volume) (Li, Daniel, Robert, & Colette, 2003).
HCN is a threat to soil microorganisms as well as groundwater sources. In an attempt to
visually quantify the effect that cassava growth and production has on soils, the amount of
rainfall received (Figure 13), the soil type (Figure 14), and the amount of soil degradation
(Figure 15) are graphically displayed. The high amount of cassava produced in Nigeria is
likely to have an effect on the severity of soil degradation (Table 5).
Figure 13 (left) and Figure 14 (right): The average annual rainfall and the soil type in Nigeria
are displayed on the shaded sketches above. The less rainfall an area receives, the higher the
concentration of cyanogenic glycosides in cassava grown there will typically be (without
inputs of irrigation). Soil type is important to assess the risk HCN has on groundwater
(Aregheore, 2009).
Figure 15 (left): This map displays the
severity of the soil degradation of the
regions of Africa (Aregheore, 2009).
2007 annual cassava production 34,410,000 tons
Average cassava yield 88,800 tons/ha
Table 5: 2007 total cassava production and average crop yield in Nigeria
____________________
Discussion:
The results support the hypothesis that cassava production and processing for biofuel use are more so
negatively impacting environmental health. Using current production and manufacturing practices it
was determined that the negative outcomes of such production outweigh the positive aspects of pursuing
this energy crop in particular. Other alternatives should be pursued if possible.
Highlighting the positive characteristics of cassava as a biofuel crop
Since corn is the leading ethanol producing energy crop today a comparison with
cassava, an increasingly popular energy crop, was conducted. This comparison assessed these
crops’s yield, ethanol conversion rate, and ethanol yield. Even though cassava has a
substantially lower ethanol conversion rate than corn (figure 8) (Corn produces an average of
410 L of ethanol per ton of raw material, while cassava only produced 150L/ton), cassava
produces more ethanol per hectare per year (figure 9) due to its high yield (figure 7). Cassava’s
yield is 8 times that of corn per hectare annually. Cassava’s high ethanol production per unit
area makes it a good energy crop candidate (Adelekan, 2010).
Surface waters
Cyanogenic glycosides are present in all of cassava’s tissues at potentially harmful
concentrations. Leaves contain substantially higher concentrations than roots (figure 10). There
are two separate processing procedures involved in the production of ethanol. First is the
production of cassava starch from raw tubers and second is the production of ethanol from
this resulting starch. The amount of cyanide remaining in cassava starch it typically much
lower than concentrations of the raw material, therefore cyanide waste products are less of a
concern for cassava based ethanol production plants than for starch production plants.
However some starch production factories recycle their process water causing the buildup of
cyanide in starches overtime including the final product (Services, 2006). If eaten, flour with
such high concentrations of HCN can cause sickness especially in women and children. Also if
this starch is sold for the production of ethanol, there will be high levels of HCN in the waste
effluent that if not properly treated, could cause damage to the surrounding environment.
The concentration of cyanide within wastewater effluent from the largest cassava starch
factory in Nigeria was studied. On average the effluent contained 200mg of HCN/ L. Daily
releases of cyanide from effluent discharge totaled about 10 million milligrams (Figure 11)
(Adekuncl, Arowolo, Omoniyi, & Olubambi, 2007). This study assessed the negative effects the
high concentrations of cyanide were having on fish populations in the area receiving factory
discharge. Fish were smaller than typical and cyanide was found in their tissues. People living
in the area were reported as complaining of sickness following consumption of fish from this
area (Adekuncl, Arowolo, Omoniyi, & Olubambi, 2007). These results show that mass
production of cassava as an energy crop (or food crop) has a significant negative impact on the
environment and humans through water systems.
Potential threat to soils and groundwater
Typically if industrial effluent containing hydrogen cyanide (or other cyanide
compounds) infiltrates soils, microorganisms will biodegrade the toxin into non toxic
constituents or use them in their own defense. When soil concentrations of HCN become
higher that 0.3 mg/kg, microbial populations are adversely affected. When concentrations are
even higher the soil can become toxic, killing off microbial life (Ubalua, 2010). Soil health and
plant success rates in soil are heavily dependent upon the presence of microorganisms. Toxic
concentrations of effluent entering soil will increase the degradation/erosion of soils. Figure 15
shows the severity of soil degradation throughout Africa (Aregheore, 2009). Though this is a
result of an accumulation of factors, Nigeria shows relatively severe soil degradation that may
be a result of intense cassava grown and production facilities in some areas.
HCN is also a threat to groundwater. The level of threat varies depending upon the soil
type, amount of rain, and concentration of –CN the soil is exposed to. The mobility of cyanide
through the soil is increased in soils with a low pH, high negative soil charge, and low clay
content (Ubalua, 2010). Figure 14 shows the location of different types of soil in Nigeria. The
most northern area is primarily composed of a sandy soil. Sandy soils are extremely
permeable, and would increase the mobility of the toxin cyanide. Therefore groundwater in
the northern region of Nigeria is at a higher risk of contamination with -CN effluent discharge
(Ubalua, 2010). As previously described high concentrations of -CN can kill off populations of
microorganisms in the soil. This occurrence also causes an increase in risk for groundwater
since the -CN is unlikely to be broken down.
Currently a majority of cassava is picked by hand and therefore less of its tissues are
punctured in the removal process. However as pressure on cassava production continues to
increase due to China’s interests in pursuing cassava for the production of ethanol, these
methods are likely to switch to mechanical rather than human driven techniques. Cyanide
concentrations in factory effluent and direct discharge from cassava plants has the potential to
significantly impact soil and groundwater in areas involved with cassava production. Research
is needed to avoid mass poisoning by way of groundwater if cassava production continues to
increase.
Potential atmospheric contamination
HCN is highly volatile and has a low boiling point of 25.9°C. Both of these
characteristics contribute to the rapid evaporation of this acidic compound (Elias, Nambisan, &
Sudhakaran, 1997). The most prevalent ways that HCN enters the atmosphere are by way of
coal power plants, vehicle emissions (using fossil base fuel), and industrial emissions and
discharge. Figure 12 shows the levels of hydrogen cyanide present in surface air around the
globe in 2001. The areas with the highest concentration of HCN are those near the equator.
This is in part due to the direction of wind cells (Li, Daniel, Robert, & Colette, 2003). This study
by Li strongly emphasized the contribution coal power plants and vehicle emissions have on
surface HCN concentrations, however if they are such a substantial contributor then I believe
there should be a higher concentration of HCN over North America. The U.S. highly regulates
industrial emissions which may be why the levels are lower there however industrial effluent
from factories like those previously discussed that produce cassava starch in mass quantities
may have a bigger influence on atmospheric HCN than previously thought. Mid-Africa, where
Nigeria is located has relatively high levels of HCN. Studies should be conducted to assess the
amount that HCN in effluent from cassava starch factories is contributing to high atmospheric
HCN conditions.
____________________
Conclusion
In conclusion the results of this evaluation indicate that if current production and
processing methods are continued as scale increases, the negative environmental impacts out-
weigh the positive characteristics cassava has as a biofuel production crop. Cassava
production poses a significant threat to surface water, soils and groundwater where waste
containing HCN from factories is being discharged. Though there is no evidence of this, since
HCN from cassava readily evaporates and is a toxic gas, there is a potential risk for the
atmosphere as well. There are potential options for avoiding some of these environmental
threats however. If proper waste water treatment systems were implemented at all production
factories, harmful effluents into the environment could be avoided. Also various versions of
transgenic cassava exist that inhibit the production of linamarin in the roots, reducing the
HCN concentration by 98% (discussed in introduction). These options are more costly than
current methods however they would significantly reduce HCN contamination. If steps are
taken to reduce the amount of CN released to the environment, cassava has the potential to be
a good energy crop.
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