Now that we've spent some time discussing different types of water contamination, let's move
on to how societies in ancient and modern Pakistan have tried to address this contamination. In
other words, what are some ways that we can remove these contaminants?
As always, let's begin by looking at how ancient societies in Pakistan dealt with this issue. As
discussed previously, for the most part, people in ancient Pakistan accessed drinking water from
a direct, natural source - whether it be from well water or from river water. While people were
quite aware of the various potential impurities in water, lack of the same scientific knowledge we
have today prevented people from really knowing exact specifics of contaminants, so potential
treatment methods were often broad and experimental - sometimes working, and sometimes
not. Ancient South Asian texts have uncovered that sand and gravel filtration was one method
sometimes used for filtering water. Although the specifics of exactly how these ancient people
used these materials to filter water is not completely understood, sand and gravel are still quite
common items used for different water filters even today, providing us insights into how these
materials might have helped remove water impurities in ancient South Asia. Sand and gravel,
arranged in layers, trap and strain particles suspended in water. This type of filtration method
focuses around physical contaminants in water. Sand and gravel can sometimes help to trap
small bugs or organisms, suspended dirt particles, and larger materials or sediment. Historians
have also made some insights from Mughal Era texts. Abu'l-Fazal, the grand vizier of the
Mughal emperor Akbar, wrote that "His Most Gracious Highness uses a mix of local water and
holy waters. First is water from the Ravi which is filtered using saltpetre, then is water from
Hardwar or Sorun, and to this is added a few drops of Aab-e-Zamzam." Saltpetre is another
name for the chemical compound potassium nitrate, and was found naturally in some regions of
the Mughal Empire. However, interestingly, there is thus far no well established evidence
behind saltpetre as a water treatment method, so it is unclear what purpose or impact filtering
this water with saltpetre would have had for the Mughal Emperor, and thus this compound may
have been added for a variety of other reasons. Going back further into ancient Pakistan,
through extensive scholarly research on the Indus Valley sites, it has become clear that the
people of the Indus Valley Civilization were aware of the dangers of drinking contaminated
water. With only limited knowledge of biological sciences and mechanisms to remove or treat
bacteria, viruses, and parasites in water, in order to keep their water clean they instead
practiced methods of preventing contamination in the first place. Specifically, by examining the
various water transportation infrastructure used by these ancient peoples in Pakistan, we get
some clues on how the went about protecting themselves from biological contamination.
So at Mohenjodaro you had basically a channels goings through the streets, and they would basically
pick up water not only from bathing platforms and maybe also from waste areas
where humans dumped waste, but also they may have also picked up water from all different
sources, different times of year would be different water would be included in these channels.
And it's mostly not for obtaining water so much as it was for draining water, those ditches and channels.
The Indus people in Pakistan were one of the first societies in the world to use a covered
drainage system. This system included many mechanisms of ensuring that sewage water
(which could have contained many of the dangerous biological contaminants discussed) did not
seep into uncontaminated water. Tar, for instance, was used to line the drainage tunnels
holding sewage, helping to prevent leaks and seepages. Of course, while such preventative
methods were taken to avoid water-borne diseases and biological contamination, ancient
societies in South Asia also experimented with ways of purifying potentially contaminated water.
For example, one method of removing bacterial contaminants in ancient societies such as those
in South Asia was through the usage of crushed seeds from the Moringa oleifera tree. Recent
studies have asserted that this method of adding the crushed seeds to water helps to kill
bacteria. The seeds also have the effect of making the water less cloudy. Another very
interesting way ancient South Asians may have helped prevent the spread of waterborne
diseases is by simply storing water in metal containers. In ancient South Asia, brass, an alloy of
copper and zinc and sometimes with other metals, was used to store water. Modern scientific
research has found that an interesting property of brass containers is that the copper in such
containers can disrupt biological systems. The element acts by interfering with the membranes
and enzymes of cells; for bacteria, this can mean death. Pots made of brass, an alloy of copper
and zinc, shed copper particles into the water they contain. Finally, one of the most basic
techniques of water purification practiced in ancient South Asia was the process of boiling water.
Boiling water kills or inactivates viruses, bacteria, protozoa and other pathogens by using heat
to damage structural components and disrupt essential life processes. Ancient South Asian
texts indicate that boiling was also used as one of the means of water purification. Let's return
to the the present day and learn about some of the water treatment technologies used by
contemporary Pakistan. Now, as you can probably imagine, water treatment techniques today
have developed quite substantially in Pakistan. Given the vast amount of different methods used
today to treat water, we will be primarily focusing on 3 methods of water treatment: reverse
osmosis, chlorination, and distillation. We will then end by learning a little bit about desalination,
and the different water treatment methods used in this process. Let's begin with reverse osmosis.
So one of the examples of these technologies that have been used is called reverse osmosis, and this can be used to take saline water, either ocean water or brackish
water, and convert it into clean, what we define as potable or drinking water.
Reverse osmosis works by pushing water under pressure through a semipermeable membrane.
This process removes ions, molecules, and larger particles from drinking water.
So you have to have some important components to this, one you have to have the appropriate
membrane, it's very tight, it's a very small pore size, so essentially the contaminants are
retained, are prevented from being pushed into the membrane, and one of the challenges of
this is that not all the water can go through the membrane, so when you do this you push water
through, it takes energy, so you actually pressurize these systems
where you push water through very tight membranes
Think of reverse osmosis like a screen door during the summers: we want the fresh air from
outside in, but at the same time, we want to keep the pesky mosquitos and other bugs out. In
most reverse osmosis treatment centers in Pakistan and around the world, there are two
materials that make up most of these semi-permeable membranes: cellulose acetate, and
aromatic polyamide. In very simple terms, feed water is pumped into a reverse osmosis system
and you end up with two types of water coming out of the reverse osmosis system: permeate, and waste
water The permeate, or treated water, that comes out of a reverse osmosis system has the majority of
contaminants removed. The wastewater is the water that contains all of the contaminants that
were unable to pass through the reverse osmosis membrane. Reverse osmosis is capable of removing up
to 99% of dissolved salts, particles, and organics from feed water. This filtration process can also
remove a significant amount of bacteria and viruses, although it should still not be relied on for a
100% removal of these. Given its effectiveness in removing ions and salts, reverse osmosis is
also used as a method of desalinating water for drinking water in Pakistan and around the world
as well. Now that we've gone over some of the theory of reverse osmosis, let's look at some
specific examples in Pakistan where it is being used. In 2015, it was reported that the Sindh
provincial government has invested 5.4 billion rupees for the installation of 750 solar-powered
reverse osmosis water purification plants across the sprawling desert district, to help get safe
drinking water to the region's over 1.5 million people. This project not only takes advantage of
the potential of reverse osmosis, but the use of solar energy to help power the pumps needed
for the system show the rise of sustainable thinking in meeting some of Pakistan's energy and
water goals. From a less macro scale, the companies Aquaguard and ROplant have also been
leading distributors of reverse osmosis systems for industries and domestic use across Pakistan
since 1997. Reverse osmosis is an essential part in the filtration process of water from major
bottled water distributors in Pakistan, such as Nestle. However, one of the key drawbacks of
reverse osmosis is that it requires a lot of energy.
The problem there is that we need a lot of energy because it's typically, we need membranes that hold back the salts basically so that what comes out is basically
distilled water, which you have then actually enriched with other salts, so that it's drinkable.
So when building reverse osmosis plants, governments have to be aware of the potential
energy burden it may put may put on a community. Choosing an appropriate water treatment
method can often be a difficult process. Let's move on to a very different form of water
treatment: chlorination. So far, we have discussed treatment methods which seek to remove
contaminants from water. Our next treatment method does something different, it uses
additives. Sometimes, substances that are either naturally found in some sources of drinking
water or artificially added into our drinking water are done to either provide extra health benefits
for consuming the water, or protect against other more dangerous contaminants of water. An
example of such a substance is chlorine. The process of adding chlorine into water is known as
chlorination. Let's learn a little bit about the history of chlorination.
The history the use of chlorine to decontaminate water supplies is very interesting, it's actually closely tied up
with my own university. There was a fellow named Abel Wolman, A-B-E-L, and Abel
Wolman was a public health officer for the state of Maryland who was recruited by our university
to start the sanitary engineering department, and he was famous because although he didn't
invent chlorination, he was the one who standardized the amount of chlorine that should be
added to water supplies to ensure that there was no danger of disease. He standardized this,
and then he pioneered its use around the world saving millions of lives, and we still use his
methods today. His department of sanitary engineering is now my department of
environmental health and engineering, so 80 years on his legacy continues.
In Pakistan, chlorine is a significant additive in drinking water, although reports have shown that
Pakistan's water supply could use greater amounts of chlorination. Chlorine is a highly efficient
disinfectant, and is added to public water supplies to kill disease-causing pathogens, such as
bacteria, viruses, and parasites, that commonly grow in water supply reservoirs, on the walls of
water pipes, and in storage tanks. Chlorine inactivates a microorganism by damaging its cell
membrane. Once the cell membrane is weakened, the chlorine can enter the cell and disrupt
cell respiration and DNA activity (two processes that are necessary for cell survival). In other
words, chlorine helps to kill many dangerous biological contaminants in water, preventing them
from harming us through waterborne diseases.
Another thing that you do also is that you have to make sure that there is enough chlorine in
the water, antibacterial, such that no matter where in the pipe system you are, that there'll be
sufficient residual chlorine there to kill anything that might harm, any bacterial contaminants
that might harm people, and you have to take into account the fact that in many parts of the
system, the flows may be relatively low, there may not be much demand, and so the water can
actually be somewhat stagnant, and that's when there is the danger of infiltration. So, at the
treatment, the water supply treatment plant, you're injecting chlorine, and at fairly high levels,
hoping that there's enough left such that in every nook and cranny of the distribution system,
there's sufficient chlorine to make sure that there's no danger of bacterial contamination or disease.
The chlorination process involves adding chlorine to water, but the chlorinating product does not
necessarily have to be pure chlorine. Chlorination can also be carried out using
chlorine-containing substances. The three most common types of chlorine used in water
treatment are: chlorine gas, sodium hypochlorite, and calcium hypochlorite. From a
macro-level, in Pakistan, chlorination is becoming more and more common in areas of high
population density, including cities. Cities such as Karachi have taken greater measures to
ensure that drinking water undergoes chlorination. However, many argue that more efforts need
to be made in Pakistan to chlorinate water on a larger scale. From a micro-level, chlorine
tablets have been developed for individual water purification use across Pakistan. For instance,
following an outbreak of the brain-eating Naegleria fowleri amoeba in Karachi, many residents
of the city bought chlorination tablets to help purify their drinking water, especially since at the
time it was reported that over 45% of the drinking water in the city was not being adequately
chlorinated by the government. However, as with every water treatment method, there are
some important considerations and dangers that have to be considered.
So chlorination has some very interesting issues associated with it. Because chlorine can
interact with organic chemicals and produce more complicated molecules involving chlorine as
well as hydrocarbons, that can be harmful to human health. So this has motivated water
systems to be careful on how much chlorine they use so they don't overuse it, but also, to try to
choose sources of water that do not have these hydrocarbon sources of contamination,
pollution, that could then interact with the chlorine.
Next, let's discuss distillation. Now, distillation may seem like a fancy word, but in reality, it is a
process I'm sure many of you are familiar with. Distillation is the technique of heating a liquid to
create vapor which is collected when cooled separate from the original liquid. In other words,
distillation uses the process of evaporation to purify water. Distillation is one of the more simple
methods of water purification, but it is also a relatively effective one. Operated properly,
distillation can remove up to 99.5% of impurities from water, including bacteria, metals,
nitrate, and dissolved solids. This is because distillation does two things. First,
during the evaporation process, inorganic compounds and large non-volatile organic molecules
do not evaporate with the water and are left behind. Second, the boiling process also kills
microorganisms such as bacteria and some viruses that cannot survive in the intense heat.
However, one of the key reasons why distillation is becoming less common around the world is
that it is an extremely energy intensive method of water treatment. An immense amount of
energy is needed to evaporate water, and then further energy is needed to help in the cooling of
water. There is also a lot of energy lost in this process, including energy to the surrounding
environment during the water heating process. Nonetheless, the simplicity of this approach of
water treatment allows one to explore other sources of energy, including solar energy. Solar
distillation is the use of solar energy to evaporate water and collect its condensate within the
same closed system. Solar distillation has an enormous potential in a country such as Pakistan,
which receives relatively high amounts of sunlight in certain regions.
In a place like Lahore, very sunny, you can use solar distillation, it's a slow process, and it takes a
lot of land, but it has the advantage of not using so much electricity or fossil fuels to run that.
So, a quick recap. We have discussed 3 important methods of treating water and removing or
purifying contaminants: reverse osmosis, chlorination, and water distillation. We will end today's
lesson with a brief discussion on desalination, and how the water treatment methods we have
discussed thus far tie into desalination processes. Simply put, desalination is the processes of
taking salt out of sources of water that would normally be undrinkable, such as sea water, and
making it safe for human consumption. Now, the question is, how do we take salt out of water?
Let's investigate.
So other ways that people have done this is use different approaches to take salt water and
change that into potable or drinking water, and again this can be through either advanced
treatment say reverse osmosis or distillation type of thing, even solar.
If you live near the ocean, you can use a process called reverse osmosis, so you can characterize
that as sense, it's a very very fine filter whose pores are so small that even salt molecules can't
get through, or sodium chloride ions cannot get through, that only pure water can go through.
In order to force the water through from salt, from the salty side to the pure side, you need
immense pressures on one side because water tends to want to travel from the less salty side
to the salty side, there's actually, so you have to fight against that natural tendency, that
desalination process through reverse osmosis to pressurize the water and force it through the
filter takes a lot of electric energy. Another desalination process is called flash distilization, so
basically you can boil the water, make steam, and then condense the steam, and as you can
imagine, that takes a lot of energy as well because you're heating up the water, and then you're
cooling it. Efficient distillation processes will take the waste heat, when you cool the water and
condense it, you can harvest some of that heat and use it to preheat then the salt water,
but still the process is very inefficient.
As you can see, distillation and reverse osmosis are both important methods used in
desalination processes. So I'm sure you might be wondering, if there is clearly so much salty
ocean water in the world, and we have all these modern methods of desalination, why don't we
simply just desalinate ocean water to meet all our water needs? Well, there are a lot of
potential downsides to desalination that we cannot ignore. First, we have the problem of the
waste produced through desalination processes.
The technologies for those exist, the technologies for desalination exist, the waste that is produced in the rejection of the materials from those
has 2 possibilities. One it is highly toxic, so you have to deal with that, or there might be a lot of materials
that can be extracted from it. Unfortunately extracting those materials from the waste products also
requires a fair amount of energy and is expensive.
But the larger problem is energy usage. Desalination requires a lot of energy,
and getting this energy can be very expensive.
When you're using seawater, it's a very expensive process, often costing 10 or even 100 times
as much as traditional ground or surface water sources, and so it's something that cities are
turning to when they have no alternative, they've maximizing their reuse, there's no other safe
surface or groundwater sources to use and so, they turn to the ocean.
You can do it but it takes incredible amounts of energy and adequate technology and
appropriate maintenance and control of that.
Desalination plants in Pakistan have grown quite significantly in recent years. Recently, in the
port city of Gwardar, a desalination plant was built which is said to be providing 16.6 million liters
of water per day, with capacity to increase to 34 million liters per day. Researchers and
government officials have also identified the high potential for desalination facilities in provinces
such as Sindh and Balochistan. Let's review what've covered in today's lesson. We began by
discussing water treatment technologies of ancient and medieval Pakistan, looking at the
various ways people living in the land that is today Pakistan treated their water to address the
various water contaminants they faced. We then examined 3 examples of modern water
treatment methods: reverse osmosis, chlorination, and distillation. We went through the key
principles behind how each method treats water, and how each is being used in Pakistan. We
then learned about desalination, how it is conducted using the various water treatment methods
discussed, and its role in Pakistan. Now that we've learned a little bit about water treatment, our
next lesson will build on this content and focus on a specific category of water treatment:
wastewater treatment. See you in the next lesson.
Không có nhận xét nào:
Đăng nhận xét