WATER

What is the Water Cycle?This is the natural process that occurs and

allows for the transformation in the liquid, solid and gaseous states.

The Importance of the Water CycleIt is essential for the following reasons:

1. Industrial processes2. Drinking water3. Agricultural purposes4. Domestic use5. Replaces water lost6. It regulates the Earth’s temperature

Water Purification MethodsMETHODS

Distillation Ion exchange

Softening

Deionization

Electro deionizatio

n

Filtration

Screen filters

Sand filters

Carbon filters

Membrane Filters

Chemical Treatment

Clarification

Disinfection

UV radiation

Desalination

METHODSH2O is purified to produce water suitable for domestic

consumption.one type of contaminant is removed by one method, hence a

combination of methods is used.

DistillationH2O(l)<---------> H2O(g) (evaporation and condensation)

Disadvantage Volatile impurities with boiling points < 100°c evaporates to

be distilled. It also requires careful maintenance to ensure purity of water.

The process is slow and requires a lot of E . However, it is inexpensive.

Ion ExchangeH2O is percolated through a column of

spherical resin beads and the ions in the H2O are exchanged for the ions fixed to the beads.

Three common ion-exchange methods are:

1.Softening used to remove Calcium and Magnesium ions

from H2O(s). As the H2O percolates through the column, the calcium and magnesium ions are exchanged for the absorbed sodium ions on the resin beads. The soluble sodium ions remain in the softened water.

Ca2+aq

+ 2Na -resin Ca-resin + 2Na+aq

DeionizationThis method removes minerals from water.

This acidic resin exchanges hydrogen ions for cationic contaminants while the alkaline resin exchanges the hydroxyl ions for anionic contaminants in water.

ElectrodeionizationThis is where water is deionized while the ion

exchange resins are continuously regenerated by an electric current in the system. This regeneration of the resins is made possible by the electric current splitting up the water molecules into hydrogen and hydroxyl ions.

Some advantages of using this technology are: The system can operate continuously without

shutting down to regenerate the resins.

The resins do not get exhausted and lose efficiency which allows it 2 produce water of consistent purity.

The electrical conditions within the system inhibit the growth of micro-organisms.

NB: Softening and deionization don’t remove most organic compounds or micro-organisms and thus the resins promote the rapid production of bacteria.

FiltrationThis is where filters are used to remove

suspended material from water. The filtration method implemented depends

upon the types of contaminant that needs to be removed and their particle size.

There are several filtration methods. They are:

1. Screen filterThis is mainly used to remove large debris.

The screen is made of a metal, plastic or synthetic fiber mesh enclosed in a frame which is designed of various shapes and sizes.

2. Sand filtersThese are layers of sand or other suitable granular materials used to reduce suspended solids from water. Sand filters can be used as a pre-filtration method for

membrane systems.

3. Carbon filters use pieces of charcoal to remove contaminants from H2O. It reduces sediments, volatile organic compounds (VOC) and

chlorine from H2O. This method operates like this:- the charcoal is activated with a positive charge which causes

organic contaminants in the water to absorb onto its surface. It also catalyzes the conversion of chlorine to chlorides and

hydrogen sulphide to sulphates which reduces undesirable odours and taste in water.

Despite all of this, it does not remove most inorganic compounds from water. Thus, it is often used as a pre-treatment method.

Membrane filtrationThis method utilizes a semi-permeable

membrane and pressure to filter water. The membrane for each type of filtration is rated

for a particular particle size. The standard size rating is in microns. Contaminants are removed from water

depending on the micron rating of the membrane.Reverse osmosis

An external P forces H2O through a membrane that retains the solute on one side and allows the pure water to pass through to the other side.

The process forces H2O molecules from a region of high solute concentration through a membrane to a region of low solute concentration.

Chemical treatmentThere are several types of chemical treatment of water.

These include:

ClarificationThis works by using chemicals to coagulate and flocculate

small suspended particles together in the water. The larger particles are called flocs. The flocs either sink to the bottom or float to the top of

the water so that they can be removed. However, the clarified water may still contain suspended

particles and thus would require further treatment. DisinfectionThis is usually the last step in water treatment plants. The water is disinfected with chlorine gas, chloramine

sodium hypochlorite , chlorine dioxide or ozone to kill any pathogens which pass through the filters.

Some problems of using certain disinfectants are:

Cyst forming protozoa are resistant to chlorine.There is growing concerns over chlorine’s reaction with

organic compounds to form cholinated organic products which are carcinogenic

However, ozone is effective in destroying cyst forming protozoa unlike chlorine. It also produces fewer harmful by-products leaving no residual disinfectant in the water.

UV radiationKills pathogens and oxidizing organic compounds in

H2O. Viruses are more resistant and so cysts and worms are

unaffected. UV radiation leaves no residual disinfectant in the water.

Desalinationremoval of salt from water. There’re two(2) main methods used.

1. Distillation 2. Reverse osmosis which produces fresh water

along with concentrated brine(NaCl). This method possesses several problems. These include:

Organisms which pass through the screens are killed during the water treatment process.

This results in loss of marine life when organisms collide with screens at the intake positions.

The effluent is about twice as salty as the ambient sea water and when it is discharged into the sea it may create localized areas that are too salty for sea life.

Importance of Dissolved Oxygen to Aquatic Life

Oxygen can dissolve in water directly from the atmosphere

during photosynthesis of aquatic plantsAquatic life needs dissolved oxygen in water to

survive O2 is needed for respiration and decomposition.

Most natural waters require 5-6ppmAquatic life becomes stressed if dissolved oxygen

falls to 3-5ppmAquatic life dies if dissolved oxygen falls below

2ppm.

The level of dissolved O2 is decreased in the following processes:

Respiration-increase in aerobic respiration decrease the amount of dissolved O2

Temperature-as temperature increases the amount of dissolved O2 decreases

Eutrophication –excess nutrients in water cause algae to grow, when these die dissolved O2 is used for decomposition.

Sources of Water PollutionWater pollution occurs when bodies of water are adversely affected due to human activities.

MAJOR SOURCES OF WATER POLLUTION

Industrial

Agricultural

Municipal

MunicipalThis arises from and sewage from homes and

commercial establishments. It is carried along pipes to sewage treatment plants where it is treated and disposed of in the environment safely. In developing countries, an estimated 90% of wastewater is discharged directly into rivers and streams without any treatment.

As a result of this, people contract diseases such as diarrhea, dysentery, cholera and tuberculosis and other ailments spread by microbes. Organic waste in sewage causes eutrophication to occur when it enters the water. As a result, aquatic organisms are starved of their oxygen and they eventually die. Lastly, dumping of garbage items such as plastic and synthetic fibers can entangle, injure and even kill aquatic animals.

AgriculturalThe unsystematic use of fertilizers,

herbicides and pesticides in agriculture indirectly contribute to water pollution. These chemicals used first dissolves into the water as it falls upon the earth (rain) and is then washed downward into the soil. Some of these chemicals may move down as far as a ground water supply whilst the rest may remain in soil.

The contaminants that reaches the groundwater may spread over a large area before degradation occurs. Drinking this water can have a variety of adverse effects on man which may include; diarrhea, vomiting, skin and eye irritation and risk of cancer. This is lethal to aquatic organisms since it was designed to kill plants and animals.

IndustrialWaste from industries often are discharged

directly or indirectly into the water system. Industrial sources of pollution include:

Nuclear waste- nuclear power stations produce radioactive waste.

Oil spills- these may occur during extraction, transportation and refining of the crude oil. Eg. The recent oil spill in the Gulf of Mexico.

Lead- waste from smelting, car batteries and plumbing

Industries. It leaches into the water from underground lead pipes and lead solder used in copper pipes. It is a cumulative poison and is not excreted from the body.

Mercury- this enters the environment as waste from chlor-alkali industry, burning of coal and mining. Fluorescent light bulbs, thermometers and other scientific apparatus that contains mercury eventually contaminate water supplies when thrown away in garbage.

Mining- this is the extraction of naturally occurring minerals from the earth. Rainwater leaches these compounds out of the surface of the earth which run off into natural waterways. Water pollution usually continues long after the mining operations have ceased.

Municipal

Agricultural

Municipal

Municipal

Municipal

Industrial

Industrial

Industrial

Pictures of Water Pollution

SummaryIn conclusion, water is life and we should use it wisely. We should also dispose of our waste in a proper and effective manner to prevent water pollution or any or pollution for that matter from occuring. Thus, you be able to preserve not only the environment but also wildlife for our future generation.

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