Natural streams have a self-purification capacity to break down and remove pollutants. However, as human settlements grew, the amount and types of pollutants entering water bodies exceeded this capacity. Smaller streams were affected first as dissolved oxygen levels dropped, harming aquatic life. The speed and completeness of natural purification in a stream depends on factors like water volume, flow rate, temperature, and sunlight exposure. Dissolved oxygen is particularly important for breaking down biodegradable organic matter and supporting aquatic life.

1. Natural Purification in streams and Rivers
Natural forms of pollutants always present in surface water. In the early civilization, many of the impurities
were washed from the air, eroded from land surface, or leached from the soil ultimately reached to surface
water. The self purification processes were able to remove or render these materials harmless. As civilized
evolved, human activity increased the amount and changed the nature of pollutants entering water bodies.
As settlements grew into villages, villages into towns, cities and megacities, the quantity of waste products
increased until the self purification capacity of water bodies was exceeded. Smaller streams were affected
first, with larger streams and lakes ultimately becoming polluted.
When decomposable organic waste is discharge into a water body. A series of physical, chemical and
biological reactions are initiated. The stream relieved of its pollution burden. This process is known as Self
purification or natural purification process. This process continuously changing water quality characteristics
through out the reach of the stream. The speed and completeness with which these processes occur depend
on many variable like volume, flow rate, turbulence of flow, variation in sunlight and temperature. In natural
waters, these systems variables are set by nature and can seldom be altered. DO concentration in water is
the most important factor that effect the decomposable organic matter in a stream. It is also necessary to
maintained mandated DO in the stream. Therefore the type and nature of wastewater treatment depends
on the condition and best usage of the receiving stream. The same processes that serve to purify natural
water system also work in engineered systems. In water and wastewater treatment plants, the rate and
extent of these processes are managed and controlling the system variable.

2. Natural Purification in streams and Rivers
Flowing water bodies like streams, canals and rivers can recover rapidly from degradable, Oxygen
demanding wastes and excess heat through a combination of dilution and bacterial decay. This natural
recovery process works as long as pollutants do not overload the stream and drought, or water diversion
for agriculture or industry do not reduce their flows. However, these natural dilution and biodegradation
processes do not eliminate slowly degradable and non degradable pollutants.
In a flowing stream, the breakdown of degradable wastes by bacteria depletes dissolved oxygen. This
reduces or eliminates populations of organisms with high oxygen requirements until the stream is cleansed
of wastes. The depth and width of the resulting oxygen sag curve and thus the time and distance needed
for a stream to recover, depend on the volume of incoming degradable wastes and the stream’s volume,
flow rate, temperature and pH level. Similar oxygen sag curve can be plotted for heated water from
industrial and power plants is discharged into streams.
Despite progress in improving stream quality in most of the developed world, large fish kills and drinking
water contamination still occurs. One main reason of these disasters is accidental or deliberate releases of
toxic inorganic and organic chemicals by industries or mines. Similarly, non point runoff of pesticides and
excess plant nutrients from crop land and animal feedlots.
In case of lakes and reservoirs, dilution of pollutants often are less effective than streams. In lakes and
reservoirs flow is usually vertical, and have slow movements. That is the reason they are more vulnerable
to contamination by runoff or discharge of plant nutrients, pesticides and toxic substances.

3. Dissolved Oxygen, Aquatic organisms and BOD

4. Water Quality Management in Rivers
• The objective of water quality management is to control the discharge of pollutants so that water quality
is not degraded to an unacceptable extent below the natural background level. This can be achieved to
control the waste discharges into rivers. The level of pollutants can be by measuring the pollutants, predict
the impact of pollutants on water quality, and decide the levels acceptable for intended use of water.
• The impact of pollution on a river depends both on the nature of the pollutants and the characteristics of
river likes discharge and speed of flowing water,, depth of river, type of bottom, bank and the surrounding
vegetation and types of aquatic life. Some pollutants, like Oxygen Demanding wastes and nutrients are
common and have profound impact on almost all types of rivers .
• Effect of Oxygen Demanding Wastes on Rivers: The oxygen Demanding wastes either organic or inorganic
in a river cause depletion of the dissolved oxygen in water. This pose a threat to fish and other aquatic life
if the concentration of DO falls below a critical point. To predict the extent of oxygen depletion, it is
necessary to know how much waste is discharge and how much oxygen is needed to degrade the wastes.
As oxygen is continuously replenish from atmosphere as well as the photosynthesis process of algae and
other aquatic plants , as well as consumed by organisms, the concentration of oxygen in the river is
determined by the relative rates of these competing processes. Organic oxygen demanding wastes are
commonly measured by calculating the amount of oxygen consumed during degradation in natural water.
The amount of oxygen required to oxidize a substance to carbon dioxide and water is known as theoretical
oxygen demand. Like glucose react with oxygen converted to CO2 and H2O is
• C6H12O6 + 6O2 ↔ 6CO2 + 6H2O.

5. If the oxidation of an organic compound is carried out by microorganisms using the organic matter as a
food source, the oxygen consumed is known as biochemical oxygen demand (BOD). The actual BOD is less
than the ThOD due to incorporation of some of carbon into new bacterial cell.
• DO sag Curve: The concentration of dissolved oxygen in a river is an indicator of water quality. All rivers
have some capacity for self purification. As long as the discharge of ODW is well within the self-purification
capacity, the DO level remain high and a diverse population of plants and animals can be found. As the
amount of waste is increases, the self purification capacity can be exceeded, causing detrimental changes
in plants and animals life. The river losses its ability to cleanse itself and the DO level decreases, causes the
reduction of aquatic life specially fish. If the DO is completely removed, fish and other higher animals are
killed or driven out and extremely noxious condition result. The water become blackish and foul smelling
as the sewage and dead animal life decompose under anaerobic condition.
• In rivers one of the major factor is the ability to access the capability to absorb a waste load. This is done
by determining the profile of DO concentration downstream from a waste discharge. This profile is the DO
sag curve. The DO concentration reduces as ODW are oxidized and then rises again further downstream.
• The DO sag curve is to determine the minimum DO concentration that protect the aquatic life in river. For
a known waste discharge and known set of river characteristics, the DO sag equation can be solved to find
the DO at the critical point. If this value is greater than the standard, the stream can adequately assimilate
the waste. If the DO at the critical point is less than the standard, then additional waste treatment is
required. The source of oxygen in river is the reaeration from atmosphere and photosynthesis of aquatic
life. The Oxygen depletion occurs due to Carbonaceous and Nitrogenous BOD as well as BOD already
present in the river upstream due to wastes discharges.

6. • Other factors affecting DO level in Rivers. The DO sag curve assumes that there is one point source
discharge of waste into a river. In actual case multiple point source as can be handled by dividing the river
into reaches with a point source at the head of each reach. A reach is a length of river specified on the
basis of homogeneity, that is channel shape, bottom composition and slope etc. The Oxygen deficit and
residual BOD can be calculated at the end of each reach. Dividing the river into reaches necessary
whenever flow regime changes, since reaeration coefficient will also be changed. Some rivers contain
large deposits of organic matters in sediments. These can be natural deposits of leaves and dead aquatic
plants or can be sludge deposits from wastewater receiving little or no treatments. In any case, the
decomposition of the organic matter places an additional burden on the stream’s oxygen resources.
• Effect of Nutrients On Water Quality in Rivers. Although ODW are the important pollutants, nutrients can
also contributes to deteriorating water quality in rivers by causing excessive plant growth. They are those
elements required by plants for their growth, including plant tissues, carbon, nitrogen phosphorus and
other variety of trace elements. By preventing any of these elements plant growth is prevented. The
availability of nutrients is not the only requirement for plant growth. In many rivers, the turbidity caused
by eroded soil particles, bacteria, and other factors prevents light from penetrating far into the water,
thereby limiting plant growth in deep water.. Strong water currents also prevent rooted plants growth.