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.
[email protected]
For
discussion on this column, join [email protected]
Copyright
© 1994 - 1997 by VACETS and Binh Anson
