WATER POLLUTION: IMPURITIES, SOURCES, EFFECTS, TREATMENT AND WATER QUALITY STANDARDS

1. Introduction

Water is one of the most important natural resources on Earth. It is essential for drinking, domestic activities, agriculture, industries, transportation, sanitation and the survival of aquatic ecosystems. Although a large portion of the Earth's surface is covered by water, only a limited amount is readily available as fresh water for human use. Increasing population, rapid urbanization, industrialization, agricultural development and improper waste disposal have placed enormous pressure on available water resources.

Water pollution occurs when undesirable physical, chemical or biological substances enter water bodies and alter the quality of water to such an extent that it becomes harmful or unsuitable for its intended uses. Polluted water can affect human health, aquatic organisms, agriculture, industry and the overall environment.

Major sources of water pollution include domestic sewage, industrial wastewater, agricultural runoff, stormwater, solid waste, oil spills and various other human activities. Natural processes can also introduce impurities into water, but human activities have greatly increased the magnitude of pollution in many water bodies.

Water pollution is particularly important in civil and environmental engineering because engineers are involved in water supply, wastewater collection, sewage treatment, industrial wastewater management, drainage systems and environmental protection.

The major topics associated with water pollution include impurities in water, causes and sources of pollution, effects on human health, dissolved oxygen, BOD, COD, prevention of pollution, water-treatment processes, sewage treatment and water-quality standards.

2. Impurities in Water

Natural water is rarely completely pure. As water moves through the atmosphere, soil, rocks, rivers and human settlements, it dissolves and carries various substances. These substances are called impurities.

Water impurities can broadly be classified into:

1. Physical impurities

2. Chemical impurities

3. Biological impurities

4. Radioactive impurities

2.1 Physical Impurities

Physical impurities affect the physical appearance and characteristics of water.

They include:

• Suspended solids

• Sand

• Silt

• Clay

• Organic debris

• Floating matter

• Colour

• Turbidity

• Odour

• Taste

Turbidity

Turbidity is the cloudiness or haziness of water caused by suspended particles. High turbidity reduces the penetration of light into water and can interfere with aquatic ecosystems and water-treatment processes.

Colour

Water may acquire colour from dissolved organic matter, industrial wastes, algae, iron, manganese and other substances.

Odour and Taste

Unpleasant odour and taste may result from organic matter, algae, sewage, industrial chemicals and other contaminants.

3. Chemical Impurities

Chemical impurities can be dissolved or suspended in water.

They include:

• Dissolved salts

• Acids and alkalis

• Chlorides

• Sulphates

• Nitrates

• Fluorides

• Iron

• Manganese

• Heavy metals

• Organic chemicals

• Pesticides

• Detergents

• Oils and grease

Some chemical impurities occur naturally because groundwater comes into contact with rocks and minerals. Others enter water through industrial, agricultural and domestic activities.

Heavy Metals

Metals such as lead, mercury, cadmium and chromium can enter water from industrial activities, mining, metal processing and waste disposal. Depending on the substance and exposure, some heavy metals can be toxic even at relatively low concentrations.

Nutrients

Nitrogen and phosphorus compounds are important nutrients for plants, but excessive quantities in water can promote excessive algal and plant growth.

This process can contribute to eutrophication, which can subsequently reduce water quality and dissolved oxygen.

4. Biological Impurities

Biological impurities include microorganisms and other living organisms present in water.

They may include:

• Bacteria

• Viruses

• Protozoa

• Algae

• Worms and parasites

• Other microorganisms

Pathogenic organisms can enter water through human and animal waste. Drinking water contaminated with disease-causing microorganisms can result in waterborne diseases.

Important water-related diseases include:

• Cholera

• Typhoid

• Dysentery

• Hepatitis

• Gastrointestinal infections

Therefore, microbiological safety is one of the most important requirements for drinking water.

5. Radioactive Impurities

Radioactive substances may enter water naturally or through certain human activities.

Natural radioactive materials can originate from rocks and minerals. Human activities associated with mining, nuclear facilities and radioactive materials can also contribute under certain circumstances.

Long-term exposure to excessive levels of certain radioactive substances can present health risks.

6. Causes of Water Pollution

Water pollution can occur when pollutants are discharged directly into water or when contaminants reach water bodies indirectly through runoff, seepage or atmospheric deposition.

The major causes include:

6.1 Domestic Sewage

Domestic sewage is one of the most common causes of water pollution.

It contains:

• Human excreta

• Food waste

• Soap and detergents

• Organic matter

• Nutrients

• Pathogenic microorganisms

• Suspended solids

When untreated sewage is discharged into rivers, lakes or ponds, it can severely deteriorate water quality.

6.2 Industrial Wastewater

Industrial processes generate wastewater containing different pollutants depending on the type of industry.

Industrial wastewater may contain:

• Acids

• Alkalis

• Heavy metals

• Organic chemicals

• Oils

• Suspended solids

• Toxic substances

• High concentrations of dissolved substances

Industries therefore require appropriate wastewater treatment before discharge or reuse.

6.3 Agricultural Runoff

Agriculture can contribute pollutants through runoff from fields.

Fertilizers may introduce nitrogen and phosphorus into water bodies, while pesticides can introduce potentially harmful chemical compounds.

Soil erosion can also increase suspended solids in rivers and reservoirs.

6.4 Solid Waste Disposal

Improper disposal of municipal solid waste near water bodies can cause pollution. Rainwater passing through waste may carry dissolved and suspended contaminants into surface water or groundwater.

6.5 Oil Pollution

Oil and petroleum products can enter water through transportation, storage, industrial activities, leakage and accidental spills.

Oil floating on the surface can interfere with oxygen transfer and harm aquatic organisms.

6.6 Mining Activities

Mining operations can expose minerals and chemicals to water. Mine drainage and erosion can introduce metals, sediments and acidic substances into nearby water bodies.

6.7 Urban Runoff

Rainwater flowing through urban areas can collect:

• Oil

• Grease

• Dust

• Metals

• Litter

• Sediments

• Other pollutants

This polluted runoff can eventually reach rivers, lakes and other water bodies.

7. Sources of Water Pollution

The sources of water pollution can be classified into point sources and non-point sources.

7.1 Point Sources

A point source is a pollution source that can be identified at a specific discharge location.

Examples include:

• Sewage-treatment plant outlets

• Industrial discharge pipes

• Factory drains

• Wastewater outlets

• Certain mining discharges

Point sources are generally easier to identify and monitor because their discharge occurs from a defined location.

7.2 Non-Point Sources

Non-point pollution originates from widespread areas rather than a single identifiable discharge point.

Examples include:

• Agricultural runoff

• Urban stormwater runoff

• Soil erosion

• Atmospheric deposition

• Runoff from roads

• Diffuse contamination from land surfaces

Non-point pollution can be more difficult to control because the pollutants originate from many locations.

8. Effects of Water Pollution on Human Health

Polluted water can affect human health through drinking, food preparation, bathing, recreation and consumption of contaminated aquatic organisms.

8.1 Waterborne Diseases

Pathogenic microorganisms in contaminated water can cause serious diseases.

Sewage-contaminated water can contain bacteria, viruses and parasites capable of causing gastrointestinal infections and other diseases.

8.2 Chemical Toxicity

Certain chemical pollutants can cause harmful effects when exposure is sufficiently high or prolonged.

Examples include some heavy metals, pesticides and industrial chemicals.

8.3 Heavy Metal Effects

Long-term exposure to certain heavy metals can affect organs and physiological systems.

For example, excessive lead exposure can affect the nervous system, while mercury exposure can have neurological effects.

8.4 Nitrate Contamination

High nitrate concentrations in drinking water can present particular risks to infants and may interfere with the blood's ability to transport oxygen.

8.5 Fluoride

Fluoride is naturally present in some groundwater. Appropriate concentrations can be beneficial for dental health, but excessive levels can cause dental and skeletal fluorosis.

8.6 Long-Term Health Effects

Some pollutants can accumulate in organisms and enter the human food chain.

Persistent chemical contaminants may therefore create long-term health concerns depending on their concentration, toxicity and duration of exposure.

9. Dissolved Oxygen — Concept and Importance

One of the most important parameters used to assess the quality of water is Dissolved Oxygen (DO).

Dissolved oxygen refers to the amount of oxygen gas dissolved in water.

Aquatic organisms such as fish and many microorganisms depend on dissolved oxygen for survival and biological activity.

9.1 Sources of Dissolved Oxygen

Oxygen enters water primarily through:

• Atmospheric oxygen transfer

• Photosynthesis by aquatic plants and algae

• Mixing and turbulence

The amount of oxygen that water can hold depends on factors such as temperature, pressure and salinity.

Generally, cold water can hold more dissolved oxygen than warm water.

9.2 Importance of DO

A healthy aquatic environment generally requires adequate dissolved oxygen.

When large quantities of biodegradable organic matter enter water, microorganisms consume oxygen while decomposing that material.

Consequently, DO can decrease.

If oxygen levels become very low, aquatic organisms may experience stress or die.

Therefore, DO is an important indicator of the condition of a water body.

10. Biochemical Oxygen Demand — BOD

BOD stands for Biochemical Oxygen Demand.

It represents the amount of dissolved oxygen required by microorganisms to biologically decompose biodegradable organic matter present in water under specified test conditions.

BOD is commonly expressed in mg/L of oxygen.

10.1 Importance of BOD

BOD is widely used to assess the organic pollution strength of wastewater and surface water.

A high BOD generally indicates a high concentration of biodegradable organic matter.

For example, untreated domestic sewage generally contains substantial biodegradable organic matter. When discharged into a river, microorganisms begin decomposing this material and consume dissolved oxygen.

Therefore:

Higher biodegradable organic matter → Higher BOD → Greater oxygen consumption

If oxygen consumption becomes excessive, the DO concentration of the receiving water can decrease significantly.

10.2 BOD and Wastewater Treatment

BOD is an important parameter for designing and evaluating biological wastewater-treatment systems.

Treatment processes can reduce biodegradable organic matter and consequently reduce the BOD of treated wastewater.

11. Chemical Oxygen Demand — COD

COD stands for Chemical Oxygen Demand.

It represents the amount of oxygen required to chemically oxidize oxidizable substances present in water under specified test conditions.

COD is also generally expressed in mg/L of oxygen.

COD is widely used because the test can provide an indication of the pollution strength of wastewater relatively quickly compared with a standard BOD test.

11.1 Difference Between BOD and COD

The basic distinction is:

BOD measures oxygen demand associated with biological decomposition under specified conditions.

COD measures oxygen demand associated with chemical oxidation of oxidizable substances under specified conditions.

In many wastewaters, COD is higher than BOD because chemical oxidation can account for substances that are not readily biodegradable.

Simple Comparison

ParameterBODCODFull formBiochemical Oxygen DemandChemical Oxygen DemandPrincipleBiological oxidationChemical oxidationIndicatesBiodegradable organic pollutionChemically oxidizable pollutionTest timeRelatively longerRelatively shorterMain applicationBiological treatment assessmentRapid wastewater pollution assessment

12. Relationship Between DO, BOD and COD

These three parameters are closely connected to water quality.

Consider a river receiving untreated sewage.

The sewage contains biodegradable organic matter.

Microorganisms begin decomposing the organic matter.

This process consumes dissolved oxygen.

Therefore:

Organic pollution ↑ → BOD ↑ → Oxygen consumption ↑ → DO ↓

COD provides another indication of the quantity of oxidizable material in the wastewater.

A major objective of wastewater treatment is therefore to reduce the pollutant load entering receiving water bodies and prevent excessive depletion of dissolved oxygen.

13. Prevention of Water Pollution

Prevention is generally more effective than attempting to restore a severely polluted water body.

Important preventive measures include:

13.1 Proper Sewage Treatment

Domestic sewage should be collected and appropriately treated before discharge.

13.2 Industrial Wastewater Treatment

Industries should treat wastewater according to the characteristics of their effluent before discharge or reuse.

13.3 Control of Agricultural Runoff

Efficient fertilizer application, erosion control, appropriate pesticide management and vegetation buffers can help reduce pollutant runoff.

13.4 Proper Solid Waste Management

Waste should not be dumped into rivers, lakes, ponds or drains.

Segregation, recycling, recovery and scientifically managed disposal help prevent contamination.

13.5 Prevention of Oil Pollution

Proper storage, handling and transportation of petroleum products can reduce spills and leakage.

13.6 Protection of Water Sources

Catchments around reservoirs, rivers and groundwater sources should be protected from uncontrolled waste disposal and other contaminating activities.

13.7 Public Awareness

Public participation is essential.

People should understand the importance of preventing sewage discharge, littering, waste dumping and unnecessary contamination of water resources.

14. Water Treatment Processes

Water treatment is the process of removing undesirable physical, chemical and biological contaminants so that water becomes suitable for its intended use.

A conventional drinking-water treatment system may include several stages.

14.1 Screening

Screens remove relatively large floating and suspended materials such as:

• Leaves

• Plastic

• Sticks

• Rags

• Large debris

Screening protects subsequent treatment units from damage or blockage.

14.2 Aeration

Aeration brings water into contact with air.

It can help remove certain dissolved gases and volatile substances and can assist in oxidizing certain dissolved substances such as iron and manganese under appropriate conditions.

14.3 Coagulation

Very fine particles may remain suspended in water because they do not settle easily.

During coagulation, suitable chemicals are added to destabilize these particles.

14.4 Flocculation

After coagulation, gentle mixing encourages small particles to combine and form larger particles called flocs.

14.5 Sedimentation

The flocs are allowed to settle under gravity.

The settled material forms sludge, which is removed from the treatment unit.

14.6 Filtration

Water passes through a filter medium that removes remaining suspended particles and other impurities.

Sand filtration is a commonly used process in conventional water-treatment systems.

14.7 Disinfection

Disinfection is used to destroy or inactivate disease-causing microorganisms.

Common methods include:

• Chlorination

• Ozonation

• Ultraviolet treatment

The selected method depends on the water quality and treatment objectives.

14.8 Storage and Distribution

After treatment, water may be stored in reservoirs or tanks and then distributed through an appropriate water-supply system.

15. Sewage Treatment

Sewage treatment is the process of removing pollutants from domestic sewage so that the treated effluent can be safely discharged, reused where appropriate or further treated as required.

A conventional sewage-treatment system generally involves several stages.

15.1 Preliminary Treatment

Preliminary treatment removes large and heavy materials that could damage downstream equipment.

Screening

Screens remove large solids.

Grit Removal

Grit chambers remove heavier inorganic particles such as sand and grit.

These processes protect pumps and other treatment units.

16. Primary Treatment

Primary treatment mainly removes settleable suspended solids through physical processes.

Sewage is held in a primary sedimentation tank, where heavier solids settle at the bottom.

Floating materials such as oil and grease may be removed from the surface.

The settled solids form primary sludge.

Primary treatment reduces a portion of the suspended solids and organic load before biological treatment.

17. Secondary Treatment

Secondary treatment primarily uses biological processes to remove biodegradable organic matter.

Microorganisms consume and transform organic pollutants.

Common biological treatment systems include:

• Activated sludge process

• Trickling filters

• Rotating biological contactors

• Other attached-growth and suspended-growth systems

Activated Sludge Process

In the activated sludge process, wastewater is mixed with microorganisms in an aeration tank.

Air or oxygen is supplied to maintain suitable conditions for biological activity.

Microorganisms use biodegradable organic matter as a source of food and convert it into simpler products and additional biological solids.

The mixture then enters a secondary settling tank.

The biological solids settle, and a portion of the settled sludge can be returned to the aeration tank.

This process can substantially reduce BOD.

18. Tertiary or Advanced Treatment

When higher-quality treated water is required, additional treatment may be provided after secondary treatment.

Advanced treatment can target:

• Nutrients

• Fine suspended solids

• Dissolved substances

• Specific contaminants

• Pathogens

Processes may include nutrient removal, advanced filtration, adsorption, membrane processes and additional disinfection, depending on the treatment objectives.

19. Sludge Treatment and Disposal

Sewage treatment produces sludge that must also be properly managed.

Typical sludge-management processes may include:

1. Thickening

2. Stabilization

3. Dewatering

4. Appropriate reuse or disposal

Anaerobic digestion is one method used for sludge stabilization and can also produce biogas.

Proper sludge management is essential because untreated sludge may contain pathogens and other contaminants.

20. Water Quality Standards

Water-quality standards establish acceptable limits or requirements for various physical, chemical and biological characteristics of water.

Standards depend on the intended use of water.

For example, requirements for drinking water are different from those for industrial water, irrigation water or recreational water.

Important water-quality parameters include:

Physical Parameters

• Colour

• Turbidity

• Taste

• Odour

• Temperature

• Total dissolved solids

Chemical Parameters

• pH

• Dissolved oxygen

• BOD

• COD

• Chloride

• Sulphate

• Nitrate

• Fluoride

• Iron

• Heavy metals

• Other chemical contaminants

Biological and Microbiological Parameters

• Total coliforms

• Faecal indicator organisms

• Pathogenic microorganisms

Water-quality standards are important because they provide measurable criteria for determining whether water is suitable for a particular purpose.

21. Importance of Water Quality Monitoring

Water quality cannot be protected simply by establishing standards. Continuous or periodic monitoring is necessary to determine actual water conditions.

Monitoring can help:

• Identify pollution sources

• Detect changes in water quality

• Assess treatment performance

• Protect drinking-water supplies

• Evaluate wastewater discharges

• Protect aquatic ecosystems

• Support environmental regulation

Civil and environmental engineers use water-quality data when designing treatment systems and evaluating environmental impacts.

22. Role of Civil Engineers in Water Pollution Control

Civil engineers have a major role in protecting water resources.

Their responsibilities may include:

• Designing water-treatment plants

• Designing sewage-treatment plants

• Designing sewerage networks

• Planning stormwater systems

• Designing industrial wastewater-treatment systems

• Developing solid-waste management facilities

• Monitoring water quality

• Planning sustainable urban infrastructure

• Protecting groundwater and surface-water resources

A civil engineer must consider not only the technical performance of a treatment system but also its reliability, maintenance requirements, environmental impact and economic feasibility.

23. Integrated Approach to Water Pollution Control

Water pollution cannot be solved by one treatment process or one organization.

An integrated approach is required.

This includes:

Source reduction ↓

Proper collection

↓

Treatment

↓

Safe discharge or reuse

↓

Monitoring

↓

Environmental protection

Industries should minimize wastewater generation wherever possible.

Cities should provide adequate sewerage and sewage-treatment infrastructure.

Agricultural activities should be managed to reduce nutrient and pesticide runoff.

Individuals should avoid dumping waste into water bodies.

Government agencies should establish and enforce appropriate water-quality and discharge requirements.

24. Conclusion

Water pollution is a serious environmental problem that threatens human health, aquatic ecosystems, agriculture, industry and the availability of clean water.

Water contains various impurities that may be physical, chemical, biological or radioactive in nature. These impurities can enter water through natural processes as well as human activities.

The major sources of water pollution include domestic sewage, industrial wastewater, agricultural runoff, solid waste, urban runoff, mining activities and oil pollution. Pollution can originate from identifiable point sources or widespread non-point sources.

The effects of polluted water on human health range from waterborne diseases to harmful effects associated with certain chemical contaminants. Water pollution can also damage aquatic ecosystems and reduce the quality and usability of water resources.

Dissolved oxygen, BOD and COD are particularly important concepts in water and wastewater engineering. Dissolved oxygen indicates the oxygen available in water for aquatic organisms and biological processes. BOD indicates the oxygen demand associated with biodegradable organic matter, while COD indicates the oxygen demand associated with chemically oxidizable substances under specified test conditions.

Prevention of water pollution should begin at the source. Proper sewage collection, industrial wastewater treatment, agricultural management, solid-waste management and protection of water sources are essential.

Water-treatment processes such as screening, coagulation, flocculation, sedimentation, filtration and disinfection can make raw water suitable for its intended use. Sewage treatment generally involves preliminary treatment, primary treatment, secondary biological treatment and, where required, tertiary or advanced treatment.

Finally, water-quality standards and regular monitoring provide the basis for assessing whether water is suitable for drinking, agriculture, industry, recreation or environmental purposes.

The ultimate objective is not merely to treat polluted water, but to prevent pollution, conserve water resources and ensure that clean and safe water remains available for present and future generations.