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Microbiologically treated wastewater for irrigation
By: M.R.
Suseela
Population
explosion and rapid industrialization have resulted in deterioration of natural
water bodies. Despite the abundance of natural water bodies, water has become
scarce. Wastewater, the major contaminant includes, domestic sewage,
agricultural wastes and industrial effluent. This wastewater is released into
the rivers, lakes and ponds. It may also be disposed, directly in open land,
where part of it evaporates and part of it percolates. All such direct
disposals involve serious risks to health and sanitation, besides, valuable
resources of manure and water are wasted. As a potential carrier of pathogenic
microorganisms, water can endanger health and life of biological systems. It
may also lead to the evolution of bad odour and spread of enteric diseases
(cholera, typhoid fever, dysentery), and fish mortality in the receiving water.
It is, therefore, necessary to employ treatment facilities that purify
wastewater prior to its disposal into water bodies. Wastewater treatment
procedures, both natural and artificial, are largely dependent upon microbial
activity.
Pollutants
in wastewater may be chemical and microbial. Sewage consists of approximately
99.9 percent of water, 0.02 to 0.03 percent suspended solids, and other soluble
organic and inorganic substances. On a percentage basis, the solids appear
small. However, the tremendous volume of material handled (several hundred
million gallons), contains solids to the tune of hundreds of tonnes. The
chemical constituents are extremely important and are subject to variations,
between communities, as well as within a community, even from hour to hour.
Inorganic chemicals initially present in the water-supply will likewise be
present in the sewage. Organic compounds are contributed through human excreta
and other domestic wastes. However, both organic and inorganic compounds are
added by industrial wastes. For example, sugar factories, paper mills,
tanneries, distilleries add organic substances, while mines and metal
industries contribute acids and salts of metal and other inorganic wastes.
Since the
composition of wastewater varies, it is to be expected that the types and
number of microbes will fluctuate. Fungi, protozoa, algae, bacteria and viruses
are the main groups of microorganisms. Raw sewage may contain millions of
bacteria per ml including the coliforms, streptococci, anaerobic spore-forming
bacilli, the proteus group and other types originating in the intestinal tract
of humans. Sewage is also a potential source of pathogenic protozoa, bacteria
and viruses. The causative agents of dysentery, cholera and typhoid may occur
in sewage. The poliomyelitis virus and infectious hepatitis virus
are excreted
in the faeces of infected hosts and thus may appear in sewage.
Pollution of
water is measured in terms of biochemical oxygen demand (BOD). One of the
primary reasons for treating wastewater prior to its being returned to the
water resource is to reduce the drain on dissolved oxygen supply of the
receiving body of water. The magnitude of the BOD is related to the amount of
organic material in the wastewater i.e., the more oxidizable organic material,
the higher the BOD.
The
composition of the micro-flora and microfauna of a stream is a good indicator
of pollution. Presence of
Sphaerotilus
natans
indicates
considerable organic pollution and the smell of hydrogen sulfide is evidence of
anaerobic sulfate reduction and should serve as an alerting signal.
Underground
sources such as wells and springs provide most of the water for individual
homes in rural areas. Surface water, however, should not be used for drinking
purposes unless it is subjected to purification, since there is constant danger
of contamination and consequent transmission of disease. As water penetrates
through the layers of soil, water from wells and springs undergoes filtration,
which removes suspended particles, including microorganisms. It is of prime
importance that the supply of groundwater selected be located at safe distance
from possible sources of contamination, e.g., septic tanks, cesspool.
The main
operations involved in a large-scale water purification plant to produce water
of a quality safe for human consumption are sedimentation, filtration and dis-infection.
Sedimentation occurs in large reservoirs, where the water remains for a holding
period. Large particulate matter settles to the bottom. Sedimentation is
enhanced by the addition of alum (aluminum sulfate) at the treatment plant,
which produces a sticky flocculent precipitate. Many microorganisms, as well as
finely suspended matter, are removed as this precipitate descends through the
water in the settling basins. The water is next passed through sand filter bed,
which removes 99 percent of the bacteria. The majority of largescale water
treatment facilities employ chlorination for dis-infection.
Wastewater
treatment is necessary before it can be disposed off without producing
significant undesirable or even harmful effects. However, some communities
still dispose off inadequately treated wastewater into natural bodies of water
either because they are indifferent to consequences or because it is assumed
that the body of water is sufficiently large and so located that dilution
prevents hazards. Communities can no longer rely on disposal of wastewater by
dilutions. There is an increasing demand for domestic and industrial water
necessitating more reuse of water that receives wastewater.
Dr. (Ms.) M.
R. Suseela is a scientist at the National Botanical Research Institute, Lucknow
- 226001 (India). |
