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 landﬁlled 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 landﬁlling 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

garbage.

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-ﬁred 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/gasiﬁed 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 landﬁll 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-

ﬁll dumps. In 2015, United

States generated more than

262 million tons of MSW,

with 52% of it ending up

in landﬁlls. 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 landﬁll. At the landﬁll 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 can’t be reused could either go toward energy recovery,

and if all else fails, ends up being returned to the earth, usually

in a landﬁll. 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 reﬁned 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.

FIGURE 1

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-

organisms.

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,

ﬁxed 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-

velopment.

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 efﬁcient 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 caloriﬁc 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

FIGURE 2

Resource waste hierarchy

The resource efﬁciency hierarchy by Niall Enright, at Sustain Success:

www.sustainsuccess.co.uk.

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 landﬁlled. 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-ﬁred power plants.

But the overall contribution to greenhouse gas emissions is

reduced as the WtE is using inputs that otherwise might end

up in landﬁlls, and also reduces the use of fossil fuels for energy

generation.

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,

ﬂy 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-ﬁred

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 landﬁll 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-

duction.

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 Council’s

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-

cess.

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 difﬁculty 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

future.

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 landﬁlls. Waste Management World magazine reported it

is the ﬁrst English anaerobic digestion facility to be certiﬁed

for good operational, environmental, and health and safety

performance.

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 certiﬁcates

in ISO, CMM and applied statistics and forecasting, Gopal holds a

bachelor’s degree in chemical engineering from the Indian Institute of

Technology Madras, and an master’s 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 master’s degree in industrial technology and

operations from Illinois Institute of Technology, Chicago, specializing

in supply chain management and industrial sustainability.