Hygiene and Environmental Health Module: 17.  Water Pollution and its Control

Study Session 17  Water Pollution and its Control

Introduction

In the previous study sessions you have learnt about the importance of protecting water from contamination and the techniques for undertaking a sanitary survey of drinking water. In this study session, you will learn about the sources and types of water pollution, the public health impacts and indicators of water pollution and approaches to the control of pollution.

Learning Outcomes for Study Session 17

When you have studied this session, you should be able to:

17.1  Define and use correctly all of the key words printed in bold. (SAQ 17.1)

17.2  Describe the sources and types of water pollution. (SAQ 17.2)

17.3  Explain some techniques for the control of water pollution. (SAQ 17.3)

17.4  Describe techniques for taking samples from drinking water sources.(SAQ 17.4)

17.1  Sources of water pollution

Water is a good solvent. This is the reason why many different chemical substances are found dissolved in water. Gases in the atmosphere will dissolve in rainwater as it passes through the air. By the time water reaches a stream or river, it will contain a variety of chemical compounds dissolved within it from the air and from the rocks and soil through which it has percolated. These compounds may be completely harmless, naturally occurring substances, but they may also include pollutants.

Pollution can be defined as the introduction into the natural environment (air, water or land) of substances (pollutants) that are liable to cause harm to human health or to animals, plants and the wider environment. Water pollution occurs when a river, lake or other body of water is adversely affected due to the addition of pollutants (Figure 17.1).

Water quality can be affected by pollution from point sources and non-point sources. Point sources are identifiable points or places, such as a pipe or channel, which discharge directly into a body of water. This might be from wastewater treatment plants, factories and industrial plants, latrines, septic tanks or piped discharge from barnyards and other places where livestock are confined. Non-point sources are those where pollution arises over a wider area and it is often difficult to locate the exact place of origin. For example, fertiliser or pesticide washed from a field by rain may seep into a river or stream at many places both on the surface and through the soil. Pollution from non-point sources, also known as diffuse pollution, contributes most of the contaminants in rivers and lakes. Other non-point sources are pollution from construction sites and other land disturbances. The problems in identifying the exact point of origin make non-point sources much more difficult to control.

Figure 17.1  Washing lorries and cars in rivers is a source of water pollution. (Photo: Nicholas Watson)

17.2  Types of water pollutants

17.2.1  Sediments and suspended solids

Sediments consist of fine particles of mostly inorganic material such as mud and silt washed into a stream as a result of land cultivation and construction. They may also arise from demolition and mining operations where these activities take place. The presence of solid particulate material suspended in the flowing water is the reason why many rivers look brown in colour, especially in the rainy season. The particles are called suspended solids while they are carried (suspended) in flowing water. When they settle to the bottom, they are called sediments.

Large quantities of inorganic matter, in the form of suspended solids, may reduce light penetration into the water which can affect the growth of plants. Sediments may even suffocate organisms on the river bed. River water may also contain organic matter, such as human and animal wastes, which can deplete (reduce) the oxygen in the water if the river is slow-flowing (see Box 17.1). This can lead to anaerobic conditions which may create unsightly conditions and cause unpleasant odours.

17.2.2  Nutrients

Phosphorus and nitrogen are common pollutants generated from residential areas and agricultural runoff, and are usually associated with human and animal wastes or fertiliser. Nitrogen and phosphorus are plant nutrients required by plants to grow. They are spread on farmland in the form of fertilisers. Rain washes these nutrients into rivers, streams and lakes. If the nutrients are present in large quantities, they can encourage excess plant growth in the water causing the phenomenon known as an algal bloom, which means a sudden increase in the population of microscopic algae. If a water body has high nutrient levels it is said to be eutrophic; the process is called eutrophication. The main problem of eutrophication is that the suddenly increased population of aquatic plants can die off equally quickly. The decay of the plant material by bacteria can cause deoxygenation of the water.

17.2.3  Biological pollutants

Biological pollutants are microorganisms (bacteria, viruses, protozoa and helminths) that are harmful to humans and other forms of life. Infectious diseases caused by biological pollutants, such as typhoid and cholera, are the most common and widespread public health risks associated with drinking water.

Microorganisms may get into water with dust from the air as rain falls, and when water passes through soil which is polluted with human and animal wastes. The contamination of water supplies with raw sewage (human and domestic wastes generated from residential areas) is the most common route for biological pollutants to enter water.

When contaminated river water moves downstream it is possible that any pollutant will be diluted as more water flows in and so increases the total volume of water in the river. This dilution may be enough to reduce the contaminants sufficiently to minimise the possible health effects but this process may not work for all pathogens.

17.2.4  Chemical pollutants

17.2.5  Types of pollutant defined by their source

Pollutants from certain sources may be a mixture of the types described above and therefore need a separate category because they combine several possible impacts. Municipal wastewater and agricultural wastes are in this category.

Municipal wastewater is generated from residential areas and often contains high concentrations of organic matter, phosphorus and nitrogen, pesticides, toxic chemicals, salts, inorganic solids such as silt, as well as pathogenic bacteria and viruses.

Agricultural wastes are generated from livestock and poultry farming and from growing crops. They can be the source of many organic and inorganic pollutants in surface waters and groundwater. Agricultural wastes include sediment from erosion of cropland, and phosphorus and nitrogen compounds that originate in animal wastes and commercial fertilisers. Animal wastes require oxygen to be broken down in water bodies and can also harbour pathogenic organisms. The extensive use of fertilisers and pesticides in agricultural regions means that both surface and groundwater are affected by these pollutants.

17.3  Public health impacts of water pollution

Waterborne infectious diseases are transmitted primarily through contamination of the water sources with excreta of humans and animals who are either active cases or carriers of disease. Carriers do not show any signs of disease although they have disease-causing agents in their body that can be transferred to others; active cases are people who are displaying visible signs of disease. Use of contaminated water for drinking or cooking, or contact with contaminated water during washing or bathing, may result in infection.

The dose or amount ingested that is necessary to cause illness depends on the type of pathogen. Exposure to a single pathogenic organism does not always result in infection and disease. Sometimes many pathogens, perhaps several hundred, must be ingested to cause infection. The minimum infectious dose also varies with the age, health, nutritional and immunological status of the exposed individual. Infants and young children, people who are debilitated, people who are living in unsanitary conditions, people who are sick and the elderly are at greatest risk of waterborne diseases.

17.4  Indicators of water pollution

The physical, chemical and biological characteristics of water are changed when the water is contaminated with different pollutants. Water is colourless, odourless and tasteless, as you know, but when it is polluted with physical and chemical pollutants the water may have colour, odour and taste.

To know whether water is polluted with specific bacterial contaminants, samples should be taken and sent to a laboratory for analysis. As noted in Study Session 16, E.coli is the standard indicator organism for faecal contamination of water and for the possible presence of faecal pathogens. For water intended for drinking, the World Health Organization (WHO) recommends that E.coli must not be detectable in any 100 ml sample. In most developing countries like Ethiopia the priority is to get from ‘bad’ quality (more than 1,000 faecal coliforms per 100 ml) to ‘moderate’ quality (less than 10 faecal coliforms per 100 ml). ‘Good’ quality is classed as zero faecal coliforms per 100ml.

17.5  Effects of pollution on water sources

17.5.1  Streams and rivers

Streams and rivers are not only potential water sources for humans, they are also important aquatic habitats for many plants and animals. Pollution can have a damaging effect on aquatic ecosystems (see Box 17.1) as well as potentially on human health.

Figure 17.2  Lakes and rivers are important aquatic ecosystems supporting many forms of life in and around the water. (Photos: Pam Furniss)

The effect of pollution on streams and rivers depends on the type of pollutant. Some substances are acutely toxic to aquatic plants and animals and will cause dead zones downstream from the pollutant source in which no living organism is found. Other pollutants are health concerns to humans, but have little impact on aquatic communities.

One of the most common types of freshwater pollutant is biodegradable organic material. When a high concentration of organic material such as raw sewage (human excreta) is discharged into a stream, the levels of dissolved oxygen in the water may fall so low that the water is completely deoxygenated.

17.5.2  Lakes

The effect of pollution on lakes differs in several respects from the effect on rivers. Water movement in lakes is slower than in rivers so re-aeration is slower. The reduction in flow rate as a stream enters a lake also causes sediments to settle out of the water and slowly accumulate on the bottom of the lake. Some pollutants are bound to the solid particles and will therefore also accumulate in the sediment.

17.5.3  Groundwater

Water that moves through the soil will, to some extent, be purified naturally. However, this is not always true because soil cannot remove all pollutants. Many soils have the ability to remove certain types of pollutants, including phosphorus, heavy metals, bacteria and suspended solids. However, pollutants that dissolve in water, like nitrate and ammonia from fertilisers and animal wastes, can pass through soils into the groundwater. This may cause high concentrations of pollutants in local drinking water wells. Leaking from underground storage tanks, solid waste landfills, improperly stored hazardous waste, careless disposal of solvents and hazardous chemicals on ground surfaces are other potential sources of groundwater pollution.

17.6  Problems of using polluted water

The impacts of using contaminated water for drinking have been discussed in other study sessions. However, we use water for other purposes that can also be affected by water pollution such as irrigation, for livestock and for recreation.

Contaminants in irrigation water (Figure 17.3) may accumulate in the soil and, after a period of years, render the soil unfit for agriculture. Even when the presence of pesticides or pathogenic organisms in irrigation water does not directly affect plant growth, it may potentially affect the acceptability of the agricultural product for sale or consumption.

Figure 17.3  An irrigation channel carries water from the river to the fields. (Photo: Pam Furniss)

Poor quality water can affect livestock by causing death, sickness or impaired growth. Some substances, or their degradation products, present in water used for livestock may occasionally be transmitted to humans. The purpose of good quality water used for livestock watering is, therefore, to help protect both the livestock and the consumer.

Contaminated water also has health problems for those who swim in it. They may become ill if the water is contaminated with faecal material or with microorganisms that could cause gastrointestinal illness or ear, eye or skin infections. For example, schistosomiasis is contracted simply by swimming or standing in water that is contaminated with the Schistosoma worm.

17.7  Control of water pollution

The control of pollution should ideally take place at the point of generation, or, in other words, it should be prevented at source. As you have learned from the sanitary survey, you should look out for possible sources of pollutants in your locality.

The control of excess nutrients is an important issue both from a public health perspective and to keep natural waters free from eutrophication. An increasing proportion of water pollution originates from diffuse (non-point) sources, such as agricultural use of fertilisers. Farmers may need guidance on good agricultural practices that will help reduce water pollution from agriculture. For example, the amount of fertiliser used and the timing of its application can make a significant difference.

Pollution prevention is best achieved by ensuring that each potential point source is properly sited, designed, constructed and managed; the aim being to contain the pollutants and prevent their uncontrolled release to the environment. Sources of pollution should be sited as far from watercourses as possible (at least 15 m away) and below any water sources on the site. Appropriate use of excreta disposal, solid waste disposal and animal waste disposal will help prevent contamination of both surface and groundwater. (You will learn more about this in the waste management study sessions that follow.)

Springs usually become contaminated when latrines, animal yards, sewers, septic tanks, cesspools or other sources of pollution are located on higher land nearby. In areas with limestone rocks, contaminated material can enter the water-bearing channels in the rock and descend through cracks and holes or other large openings and may be carried along with groundwater for long distances. Other rock types can have a similar effect so it is important to have knowledge of the local geology to assess the probability of groundwater contamination.

Monitoring of the quality of spring water and other sources would be done by you and environmental health experts.

For rainwater harvesting, pollution control means proper maintenance of the roof and gutters and careful cleaning at the beginning of every wet season. Some form of mesh should be placed between the guttering and the pipe that leads to the storage tank to prevent the entry of coarse debris; it then becomes important to clean the screen regularly to prevent blockage. The worst fouling of roofs occurs when they are situated under trees in which birds roost. A rainwater storage tank should be completely covered and well maintained.

The catchment area of the water source is the total area of surrounding land that slopes towards the source. Water can become polluted from sources in the catchment even though they may be some distance away. Ideally, the whole catchment area should be protected to avoid pollution and erosion. Preserving the vegetation in the surrounding area can help protect the spring from pollution and from siltation caused by soil erosion.

17.8  Sampling methods for bacteriological testing

During the course of an investigation into a disease outbreak or as part of routine monitoring, you may be required to take water samples to be sent for microbiological or chemical analysis. It is important that samples are taken carefully and correctly to ensure they can be used for an accurate assessment of the condition of the source. When water samples are collected for analysis, you should take care to ensure that there is no external contamination of the samples. Glass bottles, rather than plastic, are best used for sampling. Both bottles and stoppers (caps) must be sterilised. Bottles should be clearly labelled with the place where the sample was taken and the date. You should be able to obtain sample bottles from your regional public health microbiology laboratory or your local environmental health office.

You may need to take water samples from a tap, river, lake, water tank or dug well, and each has a slightly different procedure to follow.

17.8.1  Sampling procedure from a tap or pump outlet

To obtain a representative sample of water, you should carefully follow the sampling procedure described below and illustrated in Figure 17.4. The steps are:

• Clean the tap/outlet using a clean cloth to remove any dirt.
• Open the tap and turn on at maximum flow and let the water run for 1 to 2 minutes; then turn it off.
• Sterilise the tap for a minute with the flame from a cigarette lighter, or an ignited alcohol-soaked cotton-wool swab.
• Open the tap again and allow the water to flow for 1 to 2 minutes at a medium flow rate.
• Open a sterilised bottle by carefully unscrewing the cap or pulling out the stopper.
• Immediately hold the bottle under the water jet and fill.
• While filling the bottle, hold the cap face downwards to prevent entry of dust, which may contaminate the sample.
• Place the stopper in the bottle or screw on the cap. A small air space should be left to make shaking before analysis easier.

Figure 17.4  Procedures for sampling water from a tap. (Source: WHO, 1997, Guidelines for drinking water quality, Volume 3)

17.8.2  Sampling procedure from a watercourse or reservoir

The steps are:

• Open the sterilised bottle as described above.
• Fill the bottle by holding it by the lower part and submerging it to a depth of about 20 cm, with the mouth facing slightly upwards. If there is a current, the bottle mouth should face towards the current (Figure 17.5).
• The bottle should then be capped or stoppered.

Figure 17.5  Sampling of water from surface water (rivers, ponds etc.). (Source: as Figure 17.4)

17.8.3  Sampling procedure from dug wells and similar sources

The steps are:

• Prepare the bottle with a piece of string and attach a clean weight to the sampling bottle. (Figure 17.6)
• Take a 20 m length of clean string rolled around a stick and tie it to the bottle string.
• Open the bottle as described above.
• Lower the bottle, weighed down by the weight, into the well, unwinding the string slowly. Do not allow the bottle to touch the sides of the well.
• Immerse the bottle completely in the water and lower it well below the surface but without hitting the bottom or disturbing any sediment.
• Raise the bottle when it is judged to be filled, rewind the string on the stick to bring up the bottle. If the bottle is completely full, discard some water to provide an air space.
• Stopper or cap the bottle as described previously.

Figure 17.6  Procedure for sampling water from a well. (Source: as Figure 17.4)

Summary of Study Session 17

In Study Session 17, you have learned that:

1. Water pollution is any contamination of water with substances that are detrimental to human, plant or animal health.
2. Water pollutants can be of point or non-point source depending on whether substances are discharged directly into a body of water or indirectly from diffuse sources.
3. There are various types of water pollution including organic and inorganic sediments, nutrients, biological and chemical pollutants.
4. Biological pollutants include bacteria, viruses, protozoa and helminths. They enter the water from human faeces from infected people and are the cause of many water-related diseases.
5. The effects of water pollution on an aquatic ecosystem depend on the type of pollutant and the type of water body.
6. Ideally pollution control should take place at the point of origin, i.e. pollution should be prevented at source.
7. There are specified procedures to follow when taking a water sample for analysis to ensure the sample is representative.

Self-Assessment Questions (SAQs) for Study Session 17

Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering the following questions. Write your answers in your Study Diary and discuss them with your Tutor at the next Study Support Meeting. You can check your answers with the Notes on the Self-Assessment Questions at the end of this Module.