Urban wastewater, commonly referred to as sewage,
is generally a mixture of domestic waste from baths, sinks,
washing machines and toilets, as described by the
UK’s Department for Environment, Food & Rural
The United Nations estimates that the amount of
wastewater produced annually is around 1,500km³, six times
more water than exists in all the rivers of the world. Every
day, 2m tonnes of sewage, industrial and agricultural waste is
discharged into the world’s waterways. Official
statistics also show that 80% of sewage in developing countries
is discharged untreated into waters, and a total of 70% of
industrial wastes are disposed of in rivers.
In the UK, around 347,000km of sewers collect
more than 11bn litres of waste and this is treated at about
9,000 sewage treatment works before the effluent is discharged
to inland waters, rivers and the sea, according to Defra. In
contrast, in the US, at least 1bn gallons (4.55bn litres) per
day of treated wastewater is reclaimed to meet non-drinking
water needs, such as irrigation of golf courses and public
parks, according to the Center for Sustainable Systems at the
University of Michigan.
If left untreated, the wastewater householders
and businesses produce every day would damage the aquatic
ecosystems and create public health problems. Untreated
wastewater contains organic matter, bacteria and chemicals.
According to Defra, if there were large or continuous untreated
discharges of urban wastewater, the result could be too little
oxygen for fish to survive in UK waterways. The purpose of
wastewater treatment is to remove organic substances to protect
the environment from these issues. This is where industrial
minerals step in.
There are a variety of industrial minerals that
are used in wastewater treatment – each yielding
specific advantages. Minerals such as bentonite purify the
water, while lime products have historically been used to
neutralise potentially hazardous wastewater and, in recent
years, magnesia has been used to neutrliase waste
The US uses around 566,000
tonnes of lime for its wastewater treatment systems.
The above wastewater treatment plant is based in
(Source: Michael Layefsky)
Lime products have been traditionally used to
treat wastewater at municipal treatment plants to control the
pH and alkalinity, precipitate nutrients and remove
To manufacture these products, quarried limestone
is crushed and screened before being burnt in kilns to produce
calcium oxide (CaO), also known as quicklime. Water can be
added to quicklime to produce hydrated lime. Hydrated lime is
added to sewage sludge at some municipal wastewater sites to
kill pathogens, making the sludge suitable for recycling for
use in agriculture. According to lime producer Lhoist, sewage
water can be treated close to where it is created or collected
and transported via a network of pipes and pump stations to a
According to the US Geological Survey (USGS), the
US used a total of 1.52m tonnes of lime in water treatment in
2013. Out of this figure, a total of 566,000 tonnes was used in
wastewater treatments. A further 868,000 tonnes was used in
drinking water and 87,000 tonnes used in acid-mine
The lime products used for these various
applications differ from each other chemically and affect the
treatment process in different ways.
Calcium hydroxide (Ca(OH)2),
also known as hydrated lime, is used as a flocculent in the
purification of wastewater, isolating and removing oils, heavy
metals and suspended metals. It can remove metals from
According to Finnish lime producer Nordkalk,
hydrated lime is strongly caustic. With these products, the pH
level of wastewater can easily be regulated to the desirable
level. In contrast, calcium carbonate (caCO3) is slightly
caustic and dissolves slowly. The product works best when
ground finely and when the pH of the wastewater is less than
6.5, Nordalk says.
According to the USGS, lime also aids in
clarifying and in destroying harmful bacteria. In its 2013
Minerals Yearbook, it states: "The leading use in sewage
treatment is to stabilise the resulting sewage sludge. Sewage
sludge stabilisation, also called biosolids stabilisation,
reduces odours, pathogens and putrescbility of the solids."
Lime stabilisation involves mixing quicklime with the sludge to
raise the temperature and pH of the sludge to minimum levels
for a specified period of time.
Another benefit of using limestone in the
wastewater treatment process is that supplies are globally
abundant, according to Derek Thompson, product manager at lime
producer Lhoist Northern Europe. The company’s UK
arm mines lime in Derbyshire and converts it into high-end
Speaking to IM, Thompson said
wastewater treatment companies have easy access to lime, as it
does not have to be transported over long distances. He also
said that lime products normally come in a "dry, dusty powder
form" and can be used in large wastewater treatment
Quicklime is similar to calcium carbonate, but is
strongly caustic. It is predominately used in potable
(drinking) water systems to either soften the water, remove
hardness or adjust the pH. The World Health Organization (WHO)
states that the pH of most drinking water lies between the
US lime specialist Mississippi Lime
Company’s sales director Robert Rasche says that
demand for quicklime is growing. "Quicklime [is] historically,
and continues to be, a very cost-effective method of potable
water treatment," Rasche explains.
This is a steady market, according to Rasche.
"Growth rates in the use of quicklime in potable water are
modest at 2-4% per year."
Trends supporting lime consumption in the water
treatment industry include stricter environmental regulation in
Europe and the US to protect both the water environment and
drinking water. For instance, the European Union (EU)
established the Urban Waste Water Treatment Directive in 1991
and the US implemented its Clean Water Act in 1972.
Thompson says that new markets are also opening
up in the UK because some London wastewater treatment companies
are having to remove heavy sulphates from sewage systems. Lime
can be used to remove the sulphate and neutralise the water
acid. As a consequence, demand for lime products is starting to
Thompson says that he is noticing a new trend in
the use of lime to treat the by-product of the biogas process
– digestate sludge. Lime is used to sanitise the
digestate and get rid of bacteria, such as salmonella, so that
it can safely be used as fertiliser.
He explains that lime is still by far the most
commonly used mineral to treat wastewater globally and its
application in this industry can be dated back to Roman
However, using lime does come with challenges.
Lime can be abrasive, hazardous and can generate a lot of
sludge. There can be a functional disadvantage. As the material
comes in a dry, dusty powder form, significant amounts can
block water processing equipment. Furthermore, because workers
have to work in a dusty atmosphere, they have to use
respiratory masks to be able to breathe. In contrast, minerals
that come in liquid form can be dosed through a dosing pump and
used in small wastewater treatment plants.
Lime is also used in numerous processes to treat
mining-related discharges in active or abandoned mines,
according to the USGS. The organisation says that these
processes include the treatment of acid-mine drainage from
operating and abandoned mines, specialised treatment processes
such as catalysed cementation of arsenic and other heavy
metals, and treatment of mine tailings that result from the
recovery of precious metals to recover cyanides.
The USGS also maintains that lime is used,
generally in conjunction with soda ash (Na₂CO₃),
for softening municipal and plant process water. This
precipitation process removes soluble calcium and magnesium
cations that contribute to the hardness of water, according to
the organisation. This process also reduces carbonate
alkalinity and total dissolved solids.
In the US, lime consumption for drinking water
treatment decreased in 2013 to 868,000 tonnes, down by about 6%
compared with 2012’s usage figure of 920,000
Moorefield, West Virgina,
US, wastewater treatment plant.
Chesapeake Bay Programme
Magnesia products for wastewater treatment have
gained prominence over the last three-to-four decades. Like
lime and caustic soda, they neutralise wastewater.
Magnesium hydroxide (Mg(OH)₂ or MDH) slurry
is one of the key magnesia products used in wastewater
treatment and is produced primarily by either calcining
magnesite ore (MgCO₃) at temperatures of 700-1,000°C,
which is then slaked to form MDH slurry. It is normally trucked
from the site of production to the place where it will be used
for water treatment. It typically comes in a solid form in a
Magnesia experts concur that the mineral has two
main advantages. First, it is easy and safe to handle. Second,
it is insoluble. According to the USGS, in 2012, environmental
applications (water treatment and stack-gas scrubbing) remained
the largest tonnage end use for domestically produced caustic
calcined magnesia (CCM), accounting for 43% of the total
Israel-based Dead Sea Periclase, a division of
the Israel Chemicals Ltd Industrial Products (ICL-IP) group, is
a well-known producer of high-quality magnesia. Roland Murenik,
vice president of business development and research at Dead Sea
Periclase, told IM that MDH is insoluble and
used as a suspension. It almost works like the medicine, Milk
of Magnesia, that is used to cure indigestion, says
Murenik. "It is like neutralising the acid in your
Murenik says that, unlike caustic soda, which is
very easy to overdose when neutralising wastewater treatments,
you "cannot overshoot with the pH" with MDH.
"This is because if you overdose with MDH, you
will meet the maximum pH, which is 10/10.5. In relation to
caustic soda, it is easy to overdose to enormously high pHs of
13/14 – if there is an overdose for any reason."
"If the cost of caustic soda is around
$400/tonne, there is an economic advantage to using MDH
generally," he continues. "MDH for this application will
typically sell for $500/tonne, but you use far less of this
than you would with caustic soda."
Currently, caustic soda is priced around
$400/tonne, but it varies by region and geographical market.
Most countries have their own production of caustic soda.
According to industrial chemicals specialist AkzoNobel, caustic
soda is produced from the electrolysis of salt. "A concentrated
solution of purified salt (NaCl) in demineralised water (i.e.,
brine) is decomposed in an electrolytic cell by the passage of
an electrical current (DC)," the Dutch company outlines.
"The demand for caustic soda is not solely
determined by the demand of caustic soda itself. Caustic soda
is produced as a by-product of chlorine," Murenik says. "If
there is vast demand for chlorine, then there will tend to be
an excess supply of caustic soda in the market and price of
caustic soda will drop."
Dave Johnson of US-based Premier Magnesia concurs
with Murenik that MDH has many benefits: "It produces less
sludge at the back end of the wastewater treatment process.
Therefore, you pay less in disposal costs. Certainly, MDH is
safer than caustic soda," he says. "If you put your hand in
caustic soda it will burn it. MDH is non-hazardous and safe to
As a result, MDH provides a safer environment for
wastewater treatment plants. "We [Premier Magnesia] have a
high-quality MDH," Johnson told IM. "You can
get low-quality ones, but they can end up clogging pipes and
underperforming. They may work for a little while, but they
will not work in the long term."
Industry experts agree that magnesia is a bit of
a new kid on the block in terms of using it to treat
wastewater, compared to limestone which has been used for
centuries. The US and Japan are two of the main users of
magnesia globally, according to Murenik.
Japan-based Ube Material Industries is a magnesia
producer. Ube Material’s director, Akio Ishida,
told IM that environmental regulation is
driving more companies to use magnesia for water treatment
He says Japan revised an environmental standard
in 1999, which affected the industry. The standard maintained
that boron content in hot spring wastewater needed to be
"tentatively under 500ppm and finally under 10ppm in the
future". According to Ishida, boron is a naturally-derived
ingredient in some hot spring waters and it is "quite difficult
to control its content by using existing facilities".
Therefore, he says that new wastewater treatment
systems have needed to be set up to conform to this
environmental standard. This is where magnesia comes in. "We
suppose that a chemical process utilising highly reactive
magnesia for boron removal will be competitive against other
systems like ion exchange membrane/resin, or others like
activated carbon process – for example, for economic
and handling reasons."
In Japan, mine wastewater often contains
sulphuric acid since there are many volcanic stratums, Ishida
says. He explains: "In many cases, before discharged into a
public water area, [the acid] must be neutralised not to exceed
the pH standard of wastewater."
Usually, cheaper limestone is used for
neutralisation. Recently, however, in order to reduce insoluble
by-products and to avoid secondary damages like sedimentation
in dam lakes, partial use of MDH has been considered, Ishida
Nevertheless, even though using magnesia in water
treatment yields a number of benefits over its competitor
materials, there are still some problems preventing it from
being used more widely.
Murenik says that there are only a few MDH
facilities in the world and supplies of the mineral are short
in Europe and the UK, where usage is small. Murenik
points out another difficulty with using MDH. He says that it
"requires a little bit more skill in installing the
"Once (…) installed, it is the same as if
one is using caustic soda or lime – it needs to be
installed by professionals."
Salt and polonite
Wessex Water, a UK-based water and sewage
treatment company, does not use magnesia in its water treatment
operations. However, it does use minerals such as hydrated lime
Wessex Water uses salt in drinking water
treatment in a number of ways. "The most familiar use will be
the regeneration of ion exchange media using brine," a
spokesperson for the company told
"Ion exchange media are used to provide softened
water for chemical solution make up and in three nitrate
removal plants in the Wessex Water region [south west England,
The company also has an on-site electrolytic
chlorination plant. This creates sodium hyprochlorite from
salt, which can then be used for oxidation or disinfection
processes. Wessex Water is also looking to potentially use
other minerals in the near future for its sewage treatment
The spokesperson told IM that
the company is looking to trial a new technology next year
called Biomag, which employs magnetite (Fe₃O₄) to
enhance phosphorus removal. He explains that Biomag uses
ballast technology to increase the specific gravity (SG) of the
biological floc in aeration plants [a facility which brings air
and water into intimate contact, according to water specialist,
Magnetite is a heavy material. When used in this
process, it bonds with chemical pollutants and weighs them
down. Magnetite has an SG of 5.2 and it also has a strong
affinity to biological solids.
Wessex Water’s spokesperson
explained that the addition of magnetite can increase the
settling rate of the biological floc in aeration plants, and
this means that the mixed liquor concentration can be
increased, and can also result in improved solids, phosphorus
and nitrogen levels or even a smaller settlement tank. Comag is
a similar process using magnetite to enhance settlement after
chemical flocculation, again potentially resulting in lower
levels of phosphorus and metal residuals in the final
Despite their widespread use, minerals do not
have the water treatment market cornered. New research shows
that the extracts from seeds of the Moringa Oleifera tree can
be used for water purification. In a new study, researchers
from Sweden-based Uppsala University claim that the Moringa
seeds can also be used to separate minerals in the mining
Lead researcher on the study, Adrian Rennie, told
IM that the seeds are used as a flocculating
agent. The seeds contain a high fraction of protein. The
protein binds to itself and binds to the surface of a lot of
different materials (including impurities in the water).
Rennie says the seeds act like an "effective glue to stick
particles together". This makes it easier for the human eye to
see them and enhance removal.
He said that there had been interest in the new
technology in developing countries, partly because the seeds
are readily available, but also because there is "very little
risk associated with using too much of it".
"You may have heard of people adding synthetic
flocculants to drinking water, where too much aluminium
sulphate has been added to the drinking water supply by
accident. Subsequently, they have to tell the public to stop
drinking it for a while. In contrast, the seeds are edible. If
somebody adds too much it will not be a real health
The protein from the Moringa Oleifera seeds can
also help companies extract valuable minerals for the mining
process, according to Rennie. He said that his team has
recently been conducting work on using the protein as a way of
separating minerals during the flotation process. "One of the
big issues in mining has been that you typically end up with
low yields of ores (1%) of what you are interested in," he
Rennie adds that the protein can bind to certain
materials and not to others, in order to effectively separate
valuable minerals. Nevertheless, he said that this process will
not "necessarily be replacing all of the processing materials"
that companies use, because "typically, they might be using a
range of materials to process your water depending on what you
want to achieve".
All in all, there are numerous benefits to using
minerals in water and wastewater treatment. Lime, magnesia and
caustic soda products all have their specific advantages. With
the world’s population set to increase to 9.6bn by
2050, it is inevitable that more pressure will be put on the
world’s waterways. Put simply, there will be more
waste and an increasing need to provide clean drinking water.
Minerals can help give the water of life.
Minerals and principal
applications for water treatment
Principal application in water treatment
To purify water.
Coagulants are used in the water cycle as they
neutralise the negatively charged particles in
any raw water with positively charged ions,
which can be either alumina or iron, while in
wastewater, the major application of coagulants
is phosphorous removal.
According to Russia-based ZXR, salts are used
in the treatment of raw and waste water and
include three major products: aluminium
sulphate, aluminium chloride and sodium
The choice of product depends on a complex
number of parameters such as water hardness or
pH, but the choice of feedstock is just as
often based on feedstock availability.
Average feedstock usage is one quarter iron and
three quarters alumina in Europe, however these
figures are skewed from country to country. The
largest markets are identifiable as East Asia,
China, North America and Western Europe.
Due to its ion exchange, flocculation and
sedimentation properties, bentonite is used in
environmental protection for water
clarification and sludge treatment.
Bentonite clay compounds can be engineered to
absorb a wide variety of contaminants and will
encapsulate suspended solids, many organic
compounds and toxicants.
According to the USGS, sales of bentonite for
water treatment and filtration in the US have
been in the 50,000-75,000 metric tonne range
for the past five or six years.
Bentonite is also a key component of barrier
linings: compacted clay/textile composite
linings used to form an impermeable barrier
over wastes such as landfills, farm/sewage
slurries, and wastewater lagoons.
To disinfect water.
Widely used as an alternative to chlorine to
purify/disinfect water in swimming pools and
hot tubs, particularly in North America.
Also used to control algae and bacterial growth
in industrial processes (biocides).
To neutralise wastewater treatments and control
It also helps with the precipitation of
Caustic soda can be highly corrosive. It can
cause damage to the skin and eyes if it comes
into contact with them.
This mineral is used as a flocculent in the
purification of wastewater, isolating and
removing oils, heavy metals and suspended
Hydrated lime is added to sewage sludge at some
municipal wastewater sites to kill pathogens,
making the sludge suitable for recycling for
use in agriculture.
To purify water. It is capable of rapidly
separating liquids, such as oil and water,
which do not normally mix easily together. A
European Commission-backed project, called
Genuis, is currently using the material to
separate oil from seawater.
To control the pH and alkalinity.
Synthetic magnesium oxide will treat water with
a pH below 6. Magnesium hydroxide also helps
with the precipitation of metals and probably
most importantly, phosphorus removal from
Quicklime (calcium carbonate)
To control the pH and alkalinity.
Calcium carbonate treats water with a pH
greater than 6.
To purify water.
It is absorbent and electroconductive.
Contains fullerenes – a global hollow
molecule consisting of several dozens of carbon
atoms. The molecule was discovered by
scientists Robert Curl, Harold
Kroto and Richard Smalley in 1985.
They were awarded the 1996 Novel Prize for
chemistry in 1996 for the roles in the
discovery of the molecule.
A spokesperson from Russia-based shugnite
specialist Shugnite Life claims that customers
tend to buy "shugnite chips" in order to purify
To soften water.
Lime-soda ash treatment for the reduction of
hardness of water involves the addition of
slaked lime (Ca(OH)2) to hard water supply to
remove the carbonate hardness by precipitation,
with the precipitation being removed by
filtration. Non-carbonate hardness is in turn
reduced by the addition of soda ash (Na2C03) to
form insoluble precipitate, which is also
removed by filtration.
According to the USGS, a total of 75,000 tonnes
of soda ash (includes soda liquors and purge
liquors) was used in wastewater treatments in
2013. In contrast, in 2014 only 64,000 tonnes