34 ISE Magazine | www.iise.org/ISEmagazine
The enormous increase in quantity and diversity of
waste materials generated by humans and the poten-
tially harmful effects on the general environment
and public health have led to increased awareness
and the urgent need for safe disposal of waste.
Waste generation rates are affected by climate,
socioeconomic development and degree of industrialization.
Greater economic prosperity and urban migration results in
greater quantities of solid waste. Reduction in the mass and
volume of solid waste is a crucial issue, especially in the light
of limited access to disposal sites in many parts of the world.
At the same time, the demand for energy is growing rap-
idly, driven by the huge emerging middle class in develop-
ing nations and the power-driven gadgets used by consumers
in these economies. This energy demand is met primarily by
cheap fossil fuels like coal and petroleum and has caused global
concerns about the associated greenhouse effect and global cli-
mate change. This has driven the need to innovate and em-
ploy alternate or unconventional energy sources to ensure the
future well-being of the planet, minimize waste from human
activities and meet high pollution control standards.
Recent studies indicate that if we had the capacity to divert
all of the solid waste that was landlled in 2015 to waste-to-
energy facilities, we could generate enough electricity to sup-
ply about 13.8 million households, 12% of the United States.
Further, if just the nonrecycled plastics in solid waste were
to be source-separated and converted through today’s tech-
nologies into fuel oil, they could produce 135 million barrels
of oil per year; that’s 5.7 billion gallons of gasoline, enough to
fuel 8.9 million cars.
If we could convert our nonrecycled waste to alternative
energy instead of landlling it, we could preserve more than
Converting waste leads
to a sustainable future
New processes create alternative energy, building materials
while keeping refuse from landfills
By Gurram Gopal and Shruthi Suresh
July 2019 | ISE Magazine 35
6,000 acres of open space every year otherwise used to store
Waste-to-energy (WtE) is the generation of energy in the
form of heat or electricity from waste. The process is also
called energy-from-waste (EfW) and includes a variety of
proven and emerging methods aimed to compress and dispose
waste while generating energy from them. Energy-from-
waste is not just about waste management but it is a valuable
domestic energy source contributing to energy security with
an added advantage that it is nonintermittent and can comple-
ment other renewable energy sources such as wind or solar.
As a partially renewable energy source, it can also contribute
to renewable energy targets aimed at decarbonizing energy
generation. This article analyzes current WtE methods and
discusses the potential uses for fly ash, a key waste product of
coal-red power plants.
Types of waste and appropriate WtE methods
Agricultural residues. Large quantities of harvest residues
are produced annually worldwide and are excessively unde-
rutilized. A common agricultural residue is the rice husk,
which makes up 25% of rice by mass. Other residues include
sugar cane fiber (known as bagasse), groundnut shells, cereal
straw, coconut husks and shells. Current farming practices
are to constantly plow these residues back into the soil, burn
them or use them for cattle grazing. These residues could be
processed into liquid fuels or combusted/gasied to generate
electricity and heat.
Animal waste. A wide range of animal wastes can be used
as sources of biomass energy, the most common being animal
and poultry manures. In the past, this waste was recovered and
sold as a fertilizer or simply spread onto agricultural land. But
the introduction of tighter environmental controls on odor
and water pollution means some form of waste management is
now required, which provides further incentives for waste-to-
energy conversion. The most attractive method of converting
these waste materials to useful form is anaerobic digestion,
which gives off biogas used as fuel for internal combustion en-
gines, to generate electricity from small gas turbines, burned
directly for cooking or for space and water heating. Food pro-
cessing and slaughterhouse wastes are also a potential anaero-
bic digestion feedstock.
Sugar industry wastes. The sugar cane industry produces
large volumes of bagasse each year. Bagasse is a major source
of biomass energy as it can be used as boiler feedstock to gen-
erate steam for process heat and electricity production. Most
sugar cane mills utilize bagasse to produce electricity for their
own needs but some are able to export a substantial amount of
electricity to the grid.
Forestry residues. Forestry residues are generated by op-
erations such as thinning of plantations, clearing for logging
roads, extracting stem-wood for pulp and timber and natural
attrition. Wood processing also generates significant volumes
of residues usually in the form of sawdust, off-cuts, bark and
woodchip rejects. This waste material is often left to rot on-
site. However, it can be collected and used in a biomass gasifier
to produce hot gases for generating steam.
Industrial wastes. The food industry produces a large
number of residues and byproducts that can be used as bio-
mass energy sources. These waste materials are generated from
all sectors, meat production to confectionery. Solid waste in-
cludes peelings and scraps from fruit and vegetables, food that
does not meet quality control standards, pulp and fiber from
sugar and starch extraction, filter sludge and coffee grounds.
These wastes are usually disposed of in landll dumps.
Liquid wastes are generated by washing meat, fruit and
vegetables, blanching fruit and vegetables, pre-cooking meats,
poultry and fish, cleaning and processing operations and wine
making. These waste waters contain sugars, starches and other
36 ISE Magazine | www.iise.org/ISEmagazine
Converting waste leads to a sustainable future
dissolved and solid organic
matter. The potential exists
for these industrial wastes to
be anaerobically digested to
produce biogas or fermented
to produce ethanol, and sev-
eral commercial examples of
waste-to-energy conversion
already exist.
Municipal solid waste
(MSW). Millions of tons
of household waste are col-
lected each year with the vast
majority ending up in land-
ll dumps. In 2015, United
States generated more than
262 million tons of MSW,
with 52% of it ending up
in landlls. The percent of
MSW that is recycled or composted grew from less than 10%
in 1970 to nearly 35% by 2011 but it has remained at that level
subsequently (see Figure 1).
The biomass resource in this waste includes the putresci-
ble, paper and plastic, and averages 80% of the total MSW
collected. Municipal solid waste can be converted into en-
ergy by direct combustion or by natural anaerobic digestion
in the landll. At the landll sites, the gas produced by the
natural decomposition of MSW (which is approximately 50%
methane and 50% carbon dioxide) is collected, scrubbed and
cleaned before being fed into internal combustion engines or
gas turbines to generate heat and power. The organic frac-
tion of the waste can be anaerobically stabilized in a high-rate
digester to obtain biogas for electricity or steam generation.
Sewage. Sewage is a source of biomass energy similar to
the other animal wastes. Energy can be extracted from sewage
using anaerobic digestion to produce biogas. The sludge that
remains can be incinerated or undergo pyrolysis (decomposi-
tion under high temperatures) to produce more biogas.
Black liquor. Considered to be one of the most highly pol-
luting industries, pulp and paper consumes large amounts of
energy and water in various unit operations. The wastewater
discharged by this industry is highly heterogeneous as it con-
tains compounds from wood or other raw materials, processed
chemicals as well as a compound formed during processing.
Black liquor can be judiciously used for production of biogas
using anaerobic technology.
Creating a waste hierarchy,
pathway to energy conversion
In an ideal world all waste would be avoided. In reality, this
does not occur for a variety of social, financial and practical
reasons. As waste exists, it is usually best to reuse if feasible.
What cant be reused could either go toward energy recovery,
and if all else fails, ends up being returned to the earth, usually
in a landll. This general approach to waste is referred to as
resource waste hierarchy, shown in Figure 2 (Enright, 2017).
A host of technologies are available for realizing the poten-
tial of waste as an energy source, ranging from very simple
systems for disposing of dry waste to more complex tech-
nologies capable of dealing with large amounts of industrial
waste. Common WtE projects include thermochemical and
biochemical processes.
Combustion of waste has been used for many years as a way
of reducing waste volume and neutralizing many of its poten-
tially harmful elements. Combustion can only be used as an
energy source when heat recovery is included. Heat recovered
from the combustion process with thermochemical conversion
can then be used to either power turbines for electricity gen-
eration or provide direct space and water heating. Some waste
streams are also suitable for fueling a combined heat and power
system, although quality and reliability of supply are important
considerations. Thermochemical conversion pathways include
incineration, pyrolysis and gasification, are characterized by
higher temperature and conversion rates, and are best suited
for lower moisture feedstock. Incineration is the controlled
combustion of waste with the recovery of heat to produce
steam, which in turn produces power through steam turbines.
Pyrolysis and gasification represent rened thermal treat-
ment methods as alternatives to incineration and are character-
ized by the transformation of the waste into a gas as an energy
carrier for later combustion, typically in a boiler or a gas en-
gine. The most common way to generate energy is to use hot
gases from the thermal step to boil water to create steam. This
is then fed into a steam turbine to generate electricity or used
for heating. This is the only route for incineration.
City trash breakdown
Municipal solid waste generation, recycling and composting, combusted and landfilled
(derived from EPA data).
July 2019 | ISE Magazine 37
Advanced thermal treatments create a mixture of products
from the thermal step that still have a lot of chemical energy
stored in them, such as gases and oils. These can be burned
and used to raise steam as above. However, they also have the
potential to be cleaned and burned directly in gas engines or
gas turbines, or converted to transport fuels or synthetic nat-
ural gas. The latter routes have the potential to convert the
energy from the waste more efficiently than through steam
generation, which makes them attractive. However, they are
technically difficult, relatively unproven at a commercial scale
and some of the generated energy is used to power the process,
reducing the overall benefits.
Biochemical conversion relies on biochemical transforma-
tion processes, including anaerobic saturation and fermenta-
tion. It is recommended for wastes having high percentage of
biodegradable matter and valuable moisture content. Anaero-
bic consumption is a reliable technology for the treatment of
wet biological waste.
Organic waste from various sources is composted in highly
controlled oxygen-free conditions, resulting in the production
of biogas that can be used to produce both electricity and heat.
Anaerobic digestion also results in a residue called digestate,
which can be used as a soil conditioner. Alcohol fermentation
is the transformation of organic fraction of biomass to ethanol
by a series of biochemical reactions using specialized micro-
Factors effecting energy recovery from waste
The two main factors that determine the potential for recov-
ery of energy from waste are the quantity and quality (physio-
chemical characteristics) of the waste. Some of these important
parameters include size of constituents,
density, moisture content, quantity
of volatile solids and organic matter,
xed carbon, total inserts and calo-
rific value. In the event of anaerobic
assimilation, important considerations
are the C/N ratio, a measure of supple-
ment focus accessible for bacterial de-
velopment, and the poisonous quality,
representing the presence of hazardous
materials which hinder bacterial de-
Critics of WtE worry that promot-
ing energy from waste discourages
reducing, recycling and other higher
levels in the waste hierarchy. However,
evidence in Europe shows that cases
where energy is recovered from waste
also have high recycling rates. Waste
infrastructure has a long lifetime and
care needs to be taken at the start to
ensure systems can adapt to potential long-term change and
drive waste up the hierarchy, not constrain it.
While modern waste-to-energy plants are sometimes con-
fused with incinerators of the past, the environmental perfor-
mance of the industry has improved significantly. Studies have
shown that communities employing waste-to-energy technol-
ogy have higher recycling rates than communities that do not.
The recovery of ferrous and nonferrous metals from waste-to-
energy plants for recycling is strong and growing each year.
In addition, numerous studies have determined that waste-to-
energy plants actually reduce the amount of greenhouse gases
entering the atmosphere.
Today’s waste-to-energy plants based on combustion tech-
nologies are highly efcient power plants that utilize mu-
nicipal solid waste as fuel rather than coal, oil or natural gas.
Waste-to-energy plants recover the thermal energy contained
in trash using highly efficient boilers to generate steam that
can be sold directly to industrial customers or used on-site to
drive turbines for electricity.
Some WtE plants are highly efficient in harnessing the un-
tapped energy potential of organic waste by converting the
biodegradable fraction of the waste into high caloric value
gases like methane. The digested portion of the waste is highly
rich in nutrients and is widely used as biofertilizer in many
parts of the world.
Major environmental concerns of WtE systems
Incinerators produce a variety of toxic discharges to the air,
water and ground that are significant sources of many power-
ful pollutants, including dioxin and other chlorinated organic
compounds known for their toxic impacts on human health
Resource waste hierarchy
The resource efficiency hierarchy by Niall Enright, at Sustain Success:
38 ISE Magazine | www.iise.org/ISEmagazine
Converting waste leads to a sustainable future
and the environment. Many of these toxins enter the food sup-
ply and concentrate up through the food chain.
In addition to air and water emissions, incinerators create
toxic ash or slag that must then be landlled. This ash contains
heavy metals, dioxins and other pollutants, making it too toxic
to reuse, although industry often tries to do so.
Incinerators emit significant quantities of direct greenhouse
gases, including carbon dioxide and nitrous oxide that con-
tribute to climate change. They are also large sources of indi-
rect greenhouse gases including carbon monoxide, nitrogen
oxide, nonmethane volatile organic compounds and sulfur
dioxide. In fact, incinerators emit more CO2 per megawatt-
hour than does any fossil fuel-based power source, including
coal-red power plants.
But the overall contribution to greenhouse gas emissions is
reduced as the WtE is using inputs that otherwise might end
up in landlls, and also reduces the use of fossil fuels for energy
Converting coal fly ash residue to concrete
Coal fly ash, a residue of burning pulverized coal and lignite in
thermal power stations, is the largest type of waste generated
in the United States and in many other countries, with over
100 million tons produced in the USA every year. Chemically,
y ash is classified as a “pozzolan,” a material when mixed
with water and lime reacts to form cementitious compounds.
It contains a toxic stew of chemicals including lead, arsenic,
mercury and radioactive uranium.
Coal fly ashes are lightweight particles captured in exhaust
gas by electrostatic precipitators and bag houses of coal-red
power plants. Fly ash is very fine with cement-like proper-
ties and has long been used as an additive in cement, though
not without some controversy. Fly ash has two key advantages
as an ingredient in concrete and other building materials: it
improves the quality of the finished products and it creates
significant environmental benefits. Fly ash has both mechani-
cal and chemical properties that makes it a valuable ingredient
in concrete and concrete-based products. Its spherical shape
makes concrete easier to work with during mixing and plac-
ing; fly ash acts like tiny ball bearings moving the aggregates
and other components into voids.
When concrete hardens, the chemical properties of fly ash
provide greater strength, reduced permeability and improved
resistance to several types of chemical attack. The result is a
concrete product that lasts longer, a key sustainability consid-
eration. Fly ash can be used to replace up to 40% of the cement
in concrete, depending on mix requirements.
Conserving landll space by utilizing fly ash is a significant
environmental benefit. In addition, using recovered fly ash
conserves natural resources by eliminating the need to mine
new raw materials. Furthermore, concrete can be produced
using much less water when fly ash is in the mix. Fly ash use
can also significantly decrease greenhouse gas emissions be-
cause it reduces the need for cement production, an extremely
A worldwide standard for sustainable buildings
The U.S. Green Building Council (USGBC), founded in 1993, is a private 501(c)3, nonprofit organization based in Washington, D.C.,
that promotes sustainability in building design, construction and operation. It offers the Leadership in Energy and Environmental
Design (LEED) green building rating system recognized worldwide as an industry standard for sustainable construction.
The USGBC defines “green building” as the practice of designing, constructing and operating buildings to maximize health and
productivity, use fewer resources, reduce waste and negative environmental impacts and decrease life cycle costs.
Its LEED certification is provided for all building types and projects to measure sustainable design in various phases of
construction, including:
• Building design and construction, including new construction, core and shell, schools, retail, hospitality, data centers,
warehouses and distribution centers, and healthcare.
Interior design and construction, complete interior fit-out projects including commercial interiors, retail and hospitality.
Building operations and maintenance, for existing buildings undergoing improvement work and little to no construction.
Neighborhood development, for new land development projects or redevelopment projects containing residential uses,
nonresidential uses or a mix. Projects can be at any stage from conceptual planning to construction.
Homes, including single-family homes, low-rise multifamily (one to three stories) or midrise multifamily (four to six stories).
Cities and communities, for entire cities and sub-sections of a city to measure and manage water consumption, energy use,
waste, transportation and human experience.
LEED also offers recertification for projects that have previously achieved certification under LEED, and LEED Zero for projects
with net zero goals in carbon and/or resources. To learn more about LEED, visit https://new.usgbc.org/leed.
July 2019 | ISE Magazine 39
energy-intensive practice that increases greenhouse gas pro-
Experts estimate that cement production accounts for more
than 5% of carbon dioxide emissions from human sources.
Reducing cement production decreases greenhouse gas emis-
sions on almost a ton-for-ton basis. By gradually doubling or
tripling the design life of concrete with fly ash, natural re-
sources are preserved and the environmental footprint is dra-
matically reduced.
Building with concrete that contains fly ash can contrib-
ute to earning points in the U.S. Green Building Councils
Leadership in Energy and Environmental Design (LEED)
program, which recognizes sustainable use of materials, land,
water and energy, as well as ergonomics and innovative design
(see related article on page 38). Fly ash, in combination with
other qualifying building materials, can contribute to points
earned for recycled content, using regional materials and/or
innovative design. The key to maximizing points is for the
project team (owner, architect, engineer, contractor and con-
crete supplier) to work together early in the construction pro-
Traditional masonry takes significant volumes of energy to
produce, and concrete and brick making are some of the big-
gest sources of greenhouse gasses. Clay bricks are produced
in a kiln and fired at 2,000 Fahrenheit for three to five days.
The kilns are generally left running continuously even when
no bricks are being produced due to the difculty in getting
the temperatures up to optimum levels. According to the Na-
tional Institute of Standards and Technology, the carbon foot-
print for a cubic yard of fired clay brick is 991 pounds and 572
pounds for concrete brick.
The leachability of toxins from fly ash is a critical issue in
determining whether it can be put to beneficial use. It is well
established that fly ash on its own is highly toxic. It is also well
established that those toxic chemicals can be safely contained
in a crystalline matrix when the fly ash is subjected to thermal
or chemical treatments. When used to replace Portland ce-
ment, fly ash reacts with lime to produce a glassy matrix that
inhibits leaching. Firing of fly ash bricks will also produce the
requisite glassy matrix, rendering them inert to leaching.
Vitrification is a thermochemical process that occurs at high
temperatures around 1,500 Celsius that melt the ash and turn
it into slag, a glasslike substance similar in appearance to ob-
sidian. Vitrified slag has been subjected extensively to TCLP
analysis (toxicity characteristic leaching procedure) and found
to be very stable and reliable at containing all toxins in the
glass crystalline matrix. Vitrified slag has been approved for
use as a construction aggregate and filling material.
The downside of this process is the amount of energy re-
quired to melt the ash. High temperature gasifiers, such as
plasma gasifiers, will produce slag instead of ash and is more
efficient than treating the ash in a separate process.
Growing waste demands innovative solutions
Environmentally sound and economically viable methods to
treat biodegradable waste are urgently needed in the world
today. A transition from conventional energy systems to one
based on renewable resources is necessary to serve the ever-
increasing demand for energy, while managing environmental
concerns and enhancing the overall quality of life.
Waste-to-energy plants offer two significant benefits: envi-
ronmentally safe waste management and disposal as well as the
generation of clean electric power. WtE systems have already
reduced environmental impacts of municipal solid waste man-
agement, including emissions of greenhouse gases, and will
play a significant role in sustainable waste management in the
During the past year new WtE systems have opened or have
been commissioned in a number of countries including Can-
ada, Australia, Denmark, Pakistan and Saudi Arabia. Some of
these systems predominantly use combustion while others use
biochemical conversion.
As an example, Bore Hill Farm Biodigester is a biothermal
plant in Warminster, Wiltshire, England, that processes food
waste and creates renewable electricity to power 2,500 houses,
and biofertilizer as well. It diverts waste that would have been
sent to landlls. Waste Management World magazine reported it
is the first English anaerobic digestion facility to be certied
for good operational, environmental, and health and safety
Both business managers and civic leaders need to engage
with WtE specialists and pursue solutions that improve the sus-
tainability of the planet with economically feasible solutions.
Gurram Gopal is an industry professor in industrial technology and
management at Illinois Institute of Technology with an interest in
industrial engineering applications. He has published more than 50
papers and articles and has presented extensively at academic confer-
ences. He received a 2011-2012 Fulbright Scholar Award to teach
and conduct research at Galway Mayo Institute of Technology in
Ireland and has been a Fulbright specialist candidate since 2013.
Gopal developed marketing strategies for some of the world’s largest
pharmaceutical companies as a strategy consultant and manager for
ZS Associates and worked in strategic marketing, supply chain man-
agement and strategic quality at Tellabs Inc. Along with certicates
in ISO, CMM and applied statistics and forecasting, Gopal holds a
bachelors degree in chemical engineering from the Indian Institute of
Technology Madras, and an masters degree and a Ph.D. in indus-
trial engineering from Northwestern University.
Shruthi Suresh is a founder of Zrila Designs, an arts and crafts firm
in India. She obtained a masters degree in industrial technology and
operations from Illinois Institute of Technology, Chicago, specializing
in supply chain management and industrial sustainability.