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"Everyday Engineering"

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Water Pollution and Its Control

Bi`nh Anson, Ph.D.

I. WHAT IS WATER POLLUTION?

Water pollution is commonly defined as any physical, chemical or biological change in water quality which adversely impacts on living organisms in the environment or which makes a water resource unsuitable for one or more of its beneficial uses. Some of the major categories of beneficial uses of water resources include: public water supply, irrigation, recreation, industrial production and nature conservation.

Occasionally, pollution may derive from natural processes such as weathering and soil erosion. In the vast majority of cases, however, impairment of water quality is either directly or indirectly the result of human activities.

Virtually all categories of water use contribute to pollution. Every time water is used, it acquires one or more contaminants and its quality declines. Whenever any resource is processed or consumed, some of it becomes waste and is disposed of in the environment. In a large number of cases the waste materials are or become water borne and contribute to water pollution.

Both the nature of a pollutant and the quantity of it are important considerations in determining its environmental significance. Generally, readily degradable substances are quickly broken down in the environment and are of great concern only when they are disposed of in sufficiently large quantities that a significant burden is placed on the natural purification processes.

On the other hand, we also produce and use a multitude of synthetic substances, a great many of which are non-biodegradable or degrade extremely slowly. Such recalcitrant substances persist in the environment for prolonged periods of time and may therefore become progressively more concentrated. Many of these substances are toxic or carcinogenic and may accumulate in the tissues of organisms. Such pollutants are particularly worrisome as they tend to build up in successive trophic levels of a food web.

When characterising pollution and for formulating control and management strategies, it is useful to distinguish between "point" and "non-point" sources.

POINT SOURCES are discrete and readily identifiable and, as a result, they are relatively easy to monitor and regulate. Most sewage (wastewater of mainly domestic origin, containing among others, human excreta) from urban areas and industrial wastewaters are discharged from point sources.

NON-POINT SOURCES, on the other hand, are distributed in a diffused manner. The location and origin of non-point sources are sometimes difficult to establish and they are therefore less amenable to control. Runoff from large urban or agricultural catchments (*), carrying loads of sediments and nutrients, are examples of non-point sources of water pollution.

II. MAJOR TYPES OF WATER POLLUTANTS AND THEIR EFFECTS

Human activities give rise to water pollution by introducing various categories of substances or waste heat into a water body. The more common types of polluting substances include pathogenic organisms, oxygen demanding organic substances, plant nutrients which stimulate algal blooms, inorganic and organic toxic substances and oil.

2.1 Pathogenic Organisms

Many serious human diseases such as cholera, typhoid, bacterial and amoebic dysentery, enteritis, polio and infectious hepatitis are caused by water-borne pathogens. In addition, malaria, yellow fever and filariasis are transmitted by insects that have aquatic larvae.

Faecal pollution of water resources by untreated or improperly treated sewage is a major cause for the spread of water-borne diseases. To a lesser extent, disease causing organisms may also be derived from animal rearing operations and food processing factories with inadequate waste water treatment facilities.

In most developed nations, the spread of water-borne infectious diseases has been largely arrested through the introduction of water and sewage treatment facilities and through improved hygiene. But in many developing countries, such diseases are still a major cause of death, especially among the young. A strong correlation exists between the infant mortality rates of various countries and the percentage of the population with access to clean water and sewage disposal facilities.

2.2 Biodegradable Organic Substances

Human and animal wastes as well as effluents from industries processing plant or animal products contain a mixture of complex organic substances such as carbohydrates, proteins and fats as their major pollution load. These substances are readily biodegradable and when introduced into the environment are quickly decomposed through the action of natural microbial populations.

Some of the organic matter is oxidised to carbon dioxide and water while the rest is assimilated and used for the synthesis of new microbial cells. In due course, these organisms will also die and become food for other decomposers. Eventually virtually all of the organic carbon will be oxidised.

When a biodegradable organic waste is discharged into an aquatic ecosystem such as a stream, estuary or lake, oxygen dissolved in the water is consumed due to the respiration of micro-organisms that oxidise the organic matter. The more biodegradable a waste, the more rapid is the rate of its oxidation and the corresponding consumption of oxygen. Because of this relationship and its significance to water quality (dissolved oxygen levels in the water), the organic content of waste waters is usually measured in terms of the amount of oxygen consumed during its oxidation, termed the BIOCHEMICAL OXYGEN DEMAND (BOD).

In an aquatic ecosystem, a greater number of species of organisms are supported when the dissolved oxygen concentration is high. Oxygen depletion due to waste discharge has the effect of increasing the numbers of decomposer organisms at the expense of others.

When oxygen demand of a waste is so high as to eliminate all or most of the dissolved oxygen from a stretch of a water body, organic matter degradation occurs through the activities of anaerobic organisms which do not require oxygen. Not only does the water then become devoid of aerobic organisms, but anaerobic decomposition also results in the formation of a variety of foul smelling volatile organic acids and gases such as hydrogen sulfide and mercaptans (certain organic sulphur compounds). The stench from these can be quite unpleasant and is frequently the main cause of complaints from residents in the vicinity.

2.3 Plant Nutrients

The availability of plant nutrients, particularly nitrogen and phosphorus are important determinants of the biological productivity of aquatic ecosystems. Nutrient deficient aquatic environments are called "oligotrophic" and those rich in nutrients, "eutrophic". Young lakes are generally oligotrophic, but they naturally accumulate nutrients over time, derived from drainage and sediment run off from its catchment. When human activities greatly accelerate nutrient enrichment of water bodies, the process is called "cultural eutrophication".

Sewage, animal wastes and many industrial effluents contain high levels of nitrogen and phosphorus. Another major source is fertiliser run off from urban and agricultural catchments.

While in the long term, cultural eutrophication accelerates the natural successional progress of aquatic ecosystems towards a terrestrial system, in the short term problems arise due to cyclic occurrences of algal blooms and decay. In warm weather, nutrients stimulate rapid growth of algae and floating aquatic weeds. The water often becomes opaque and has unpleasant tastes and odours.

When these organisms die they become food for decomposer bacteria. Depletion of dissolved oxygen leads to anaerobic conditions and a general decline in the ecological and aesthetic qualities of the water body. Algal blooms also reduce light penetration into the water making it impossible for seagrasses and other bottom anchored plants to survive.

2.4 Toxic Inorganic Pollutants

Many inorganic substances are released by natural weathering of rocks and are washed into water courses. However, human activities such as mining and mineral processing as well as wastage have been responsible for far greater quantities of toxic inorganic pollutants entering water supplies and aquatic ecosystems.

Of particular concern among these are arsenic, an ingredient of some pesticides, and heavy metals such as mercury, lead, tin and cadmium as they tend to accumulate in tissues. Mine drainage and leaching of mine tailings as well as metal finishing and inorganic chemical industries are major sources of metal pollution in the water environment.

MERCURY poisoning causes birth defects and permanent brain damage. The worst case of mercury poisoning of a community to date occurred in Minamata in Japan, due to the consumption of contaminated fish and shell fish from the bay which received discharges of chemical industry effluents over a long period of time. Insoluble metallic mercury is converted by bacteria in the marine sediments into water soluble methyl mercury. It is then concentrated through the trophic levels of the food chain due to selective retention in tissues.

LEAD is known to cause damage to the nerve system, and some experts have recommended a tolerance limit of less than 10 parts per billion in drinking water. Prior to the introduction of PVC water pipes, lead piping and cast iron pipes using lead solder were widely used for public water supplies. Some 10 million households in Britain apparently still receive their water supply through lead piping.

The main source of environmental lead is probably automobile exhausts. Lead has long been an additive to petrol to boost its octane rating. Exhaust fumes from cars and other vehicles are eventually washed down by rain and enter watercourses. Some inorganic industrial effluents also contain lead.

Other heavy metals such as CHROMIUM and ZINC are present in effluents from metal finishing industries. It is believed that discharge of waste waters from many small metal finishing operations in Perth has caused ground water pollution. Chromium salts are added to cooling waters of air conditioning systems for corrosion prevention. Significant contribution of this metal therefore comes from this source as well.

Production of alumina from bauxite gives rise to highly caustic effluents which also contain dissolved aluminium and other metals. ALUMINIUM has recently been implicated in the Alzhiemers disease, although there is still uncertainty about the link. CAUSTIC EFFLUENTS can raise the pH of receiving waters to levels unsuitable for many organisms.

ACIDIC EFFLUENTS are produced from mine drainage and by many industries such as ore smelting, metal finishing, leather tanning and petroleum processing. Many lakes in the northern hemisphere have been acidified due to acid precipitation. Increased levels of sulfur and nitrogen oxides in the atmosphere from the combustion of fossil fuels such as coal and petroleum are the principal causes of acid rain.

Acidification lowers the pH of the water, especially when there is little buffering capacity in the form of alkalinity to neutralise it. Fish, amphibians and many insects will be killed by increased acid levels and in severe cases only a few resistant species such as fungi may survive.

2.5 Toxic Organic Chemicals

Many thousands of natural and synthetic organic chemicals are in use today for the manufacture of a variety of products ranging pesticides, pigments, pharmaceuticals and plastics. Several of these are known to cause birth abnormalities, genetic defects and cancer. Some chemicals like DDT and PCB's are concentrated in tissues to dangerous levels. Many are only very slowly biodegradable and persist in the environment for long periods of time.

The major causes of organic chemical pollution are improper disposal of domestic and industrial wastes and herbicide and pesticide run off from farming areas, where they are used in substantial quantities. In addition, large quantities of hazardous wastes have, in the past, been disposed of in landfills with inadequate containment from where they are slowly leached into surface and ground water supplies, eventually finding their way into the food chain.

2.6 Oil Pollution

Petroleum is one of the major energy sources today and huge volumes of oil are transported between points of production and consumption around the globe. All along these major transportation routes oil spills happen regularly and oil slicks are ever present. With serious spills, many marine birds and other animals are choked to death by the oil slick. Even when dispersed, many hydrocarbons in the oil are toxic to aquatic organisms. Some are thought to be carcinogenic. Oil being lighter than water floats on the surface as a thin film which can interfere with the transfer of gases such as oxygen and carbon dioxide, as well as heat, between the water and the atmosphere.

Routine petroleum refining, storage and use also results in pollution by leaking oil, oily waste water and sludge. A significant proportion of underground fuel storage tanks in service are thought to leak oil into the ground water.

2.7 Thermal Pollution

Tremendous quantities of waste heat is produced by power plants and to a lesser extent by a broad spectrum of other industries. Cooling water drawn from the ocean, river, lake or aquifer is passed through heat exchangers where it absorbs the waste heat and is subsequently discharged back into the environment.

Significant rises in water temperature can be caused in the receiving water in the vicinity of cooling water disposal sites. Such increases in temperature can greatly alter the species composition in ecosystems as organisms normally tolerate temperature variations only over a very small range. At higher temperatures, oxygen solubility is reduced, but bacterial respiration rate will increase, making the water more prone to deoxygenation.

Temperature variations will also cause alterations in pH due to changes in the degree of ionisation and increased solubility or precipitation of bottom deposits.

III. CONTROL OF WATER POLLUTION

With increasing urbanisation and expanding agricultural and industrial production, water pollution problems have progressively become more serious and necessitated the adoption of suitable control measures for ameliorating pollution.

For a given body of water, the desired level of quality is usually specified in terms of parameters such as dissolved oxygen concentration, nutrient levels etc. The intended beneficial uses of the water resource are generally the basis on which the required quality criteria are formulated. Sources of pollution should then be regulated so as to achieve and maintain the minimum required water quality. This is usually accomplished through effluent discharge standards which specify the compliance requirements for the disposal of effluents in the environment.

Approaches to controlling sources of water pollution may be grouped into three broad categories: (1) minimisation of waste or pollutant generation, (2) Treatment prior to disposal of waste streams at source, and (3) "in-situ" reduction or elimination of pollution.

3.1 Minimisation of Pollutant Generation

Reduction of the quantity of waste or pollutants generated by an activity is obviously the most desirable approach to pollution control. Since it conserves resources that would otherwise be wasted, and at the same eliminates the cost of removing pollutant after they are produced, it is the cheapest and most effective alternative. For non-point pollution sources, this is perhaps the only practicable method of pollution control. Yet, this approach has not been exploited by society to its fullest extent.

As a general rule, a resource becomes a waste when it can no longer be economically utilised or recovered. It is then disposed of in the environment in the cheapest manner possible. Availability of economical technology for resource processing and usage has been a main determinant of when the resource is discarded as waste.

In the past, decisions concerning resource usage or waste disposal have been governed largely by immediate economic considerations and have not always considered the effects of these actions on the quality of the environment. As accountability for environmental damage gains increased recognition, fostered by a growing desire within society for sustainable development and a cleaner environment, more attention and effort will undoubtedly be devoted to reducing resources going to waste and causing pollution.

Minimising soil erosion by improved agricultural practices (e.g. by minimising surface runoff and leaving crop residues in the ground), more efficient use of nutrients (e.g., though the use of slow release fertilisers) and the development and use of biological pest control techniques in preference to the use of non-biodegradable toxic chemicals are some of the measures for minimising water pollution from agriculture.

Considerable potential also exists in many industries to reduce waste generation. Development and use of non-polluting technology to modify or replace existing manufacturing processes, and recycling or recovering materials that would otherwise be wasted are two approaches which not only reduce pollutant generation, but can sometimes even result in a saving for the industry by minimising or eliminating the need for waste treatment for pollutant removal.

In other cases, it may be more practical to segregate strong and weak waste streams to facilitate materials or energy recovery. Good house keeping practices, such as for example minimising spillage and materials wastage, can also lead to waste reduction and savings in production cost.

3.2 Wastewater Treatment at Source

In nature, a variety of different mechanisms operate to degrade and transform waste materials into stable, harmless end products such as carbon dioxide. This cleansing ability is often referred to as the "self-purification" or "assimilative" capacity. When the quantities of wastes to be disposed of are large, however, the natural purification processes become overloaded and can no longer assimilate the wastes without adversely affecting environmental quality. Man-made treatment systems are then needed to reduce pollutant loads to acceptable levels for discharge. For the most part, these purification systems make use of the same mechanisms as in the natural environment to bringing about waste stabilisation.

The multitude of different wastewater treatment technologies can be classified as physical, chemical and biological processes, depending on the nature of the purification mechanism employed. The character of the pollutants and the form (suspended or dissolved) in which they are present usually determine the most suitable process for their removal. For example, gross suspended solids and floatable materials such as oil and fat are readily removed by physical processes such as sedimentation or flotation respectively.

BIOLOGICAL METHODS are effective and economical when the waste water contains mostly biodegradable pollutants such as organic matter. A key advantage of biological processes is that the microorganisms involved in waste stabilisation are themselves produced in the process.

For dilute wastes - including general domestic wastewaters, "aerobic" biological processes (activated sludge, oxidation ponds and aerobic biofilter) are usually favoured since they are capable of producing an effuent with very low residual pollutant concentrations. These processes, however, require oxygen, in proportion to the pollutant load present. Oxygen is supplied through aeration, which is a significant cost component.

For strong wastes, "anaerobic" biological treatment in enclosed vessels is generally preferred as they proceed in the absence of oxygen, and in addition produce a useful, energy-rich by-product in the form of methane. The effuent from anaerobic processes, however, contain higher levels of residual organic materials and may require further polishing treatment (often in aerobic processes).

CHEMICAL TREATMENT is used when the pollutant of interest is non- biodegradable and is not amenable to removal by simple physical means (e.g. when it occurs in dissolved form). Heavy metals are typically removed by chemical precipitation, while toxic substances such as cyanide may be chemically oxidised. An important disadvantage of chemical treatment methods is that they generally require dosing with a chemical which can prove to be quite expensive. In addition, disposal of the chemical sludge produced in these processes may also pose some problems.

When a community based treatment system is impractical, it is still possible to provide a degree of treatment prior to discharging sewage into the environment. A popular method used for individual homes and small groups of residences is the SEPTIC TANK. It consists of a simple baffled tank which traps most of the solids in the waste water and also affords some decomposition of soluble organic matter. The effluent is disposed of into the ground through a system of leach drains. As solids progressively accumulate in the tank, it is necessary to periodically desludge the system, typically every 3 to 7 years.

As deep sewering in built-up areas is very expensive, other more efficient alternatives to the septic tank are also desirable for on- site use. In recent years, a number of new systems, which are essentially miniature versions of the biological processes used for large-scale plants have become available.

3.3 In-situ Pollution Control

Waste minimisation and treatment help prevent pollution from occurring and should be the principal approaches to water quality maintenance. Occasionally, however, when a water body is already adversely affected, it will be necessary to consider action aimed at helping the ecosystem recover from the impact of pollution. Methods to facilitate this are collectively grouped under in-situ control techniques.

Aeration of lakes and reservoirs, especially when they are thermally stratified (in summer), has been used to prevent anaerobic conditions from occurring. Forced circulation of water in stratified lakes is an alternative method. Dredging nutrient rich superficial sediments from highly eutrophic lakes, while very expensive, has sometimes helped reduce occurrence of severe algal blooms. Addition of aluminium or iron salts to assist the precipitation of phosphorus has also been practiced in some lakes to control dissolved phosphorus levels in the water.

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(*) water catchment (Australian, British) = watershed (US)

oOo


Binh Anson, Ph.D.
anson@saigon.com

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