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11. Conservation of

Resources: Recycling Water by

The Treatment of Sewage

OBJECTIVES
> To recall that pathogens cause disease only when they gain access to body tissues
> To recognise that food and water are possible means of entry to body tissues
> To understand the processes involved in the provision of a safe water supply

Limiting the spread of diseases The smell can be extremely unpleasant, 282

Pathogenic microbes can only cause disease if especially in tropical areas.
they are able to enter the body and invade the The sewage can overflow, especially during
tissues. The body has defences against disease, rainy periods. This allows faeces, perhaps
but there are a number of weaknesses. For contaminated with human pathogens, to reach
example, food and water may contain pathogenic water supplies and agricultural land. The
microbes. Salmonella and Escherichia coli (E. sewage (and pathogens) can be washed down
coli), which cause food poisoning, enter the body through the soil and contaminate nearby water
in this way, and so do Cholera vibrio and Amoeba supplies.
which are waterborne organisms causing cholera Where a good water supply is available, a flush
and dysentery, respectively. We try to protect WC is connected to the water carriage system and

ourselves from these diseases by making sure our a flow of water carries the waste away.
drinking water, milk and foods are pathogen-free. The waste is then treated at a sewage treatment
Safe water: sanitation and water plant so that the valuable water can be recycled.
The page opposite shows one type of sewage
treatment
treatment plant called the activated sludge
Living organisms need water for a number of
system.
reasons. Water can be lost very rapidly from the
The treatment of the sewage has two functions:
human body and we need access to a supply of
to destroy or eliminate potential pathogens -
drinking water. Water supplies are kept potable
either by the high temperature in the
(pathogen-free and drinkable) by both sanitation
anaerobic digestion tank, or by chlorination of
and by sewage treatment.
the water before it is discharged
Sanitation is the removal of faeces from waste
to remove organic compounds (mainly in
water so that any pathogens they contain cannot
faeces and urine). These might otherwise
infect drinking water.
contribute to the biological oxygen demand
Pit Latrine
(BOD) of the water into which the treated
In camps and in isolated houses sewage can be
sewage is discharged. Organic compounds
disposed of in a hole. The hole is dug deep
are digested by fungi and bacteria.
enough to accept a large quantity of sewage
In the activated sludge chamber, powerful jets of
(ideally several metres deep), and then filled in
air keep the sludge aerated so that the processes
with soil to keep away flies and rats. This type of
of decomposition and nitrification can be
disposal system - called a 'long drop' or pit latrine
completed in 8-12 hours. This means that large
- is cheap to produce and does not depend on
quantities of sewage can be processed very
running water, but has several disadvantages:
quickly.
Learning Module Organisms and Their Environment | Class 11 36