Human excreta and improper excreta disposal pose major public health risks like soil, water, and food contamination which can spread diseases. There are various methods for excreta disposal, including latrines, septic tanks, and sewerage systems, with the goal being proper segregation and treatment of excreta to prevent transmission of diseases. Community sanitation aims to break the disease cycle at key points like excreta disposal, water protection, and fly control.

2.  Human excreta is a source of infection. It is an
important cause of environmental pollution. Every
society has a responsibility for its safe removal and
disposal so that it does not constitute a threat to
public health.
 The HEALTH HAZARDS of improper excreta disposal
are :
 (1) soil pollution,
 (2) water pollution,
 (3) contamination of foods, and
 (4) propagation of flies. The resulting diseases are
typhoid and paratyphoid fever, dysenteries,
diarrhoeas, cholera, hookworm disease,. ascariasis,
viral hepatitis and similar other intestinal infections
and parasitic infestations

6.  Community medicine aims at breaking the
disease cycle at vulnerable points. The
disease cycle may be broken at various levels
: segregation of faeces, protection of water
supplies, protection of foods, personal
hygiene and control of flies. Of these, the
most effective step would be to segregate
the faeces and arrange for its proper disposal
so that the disease agent cannot reach the
new host, directly or indirectly.

10.  There are a number of methods of excreta
disposal. Some are applicable to unsewered
areas, and some to sewered areas.
 A classification and description of the
various methods of excreta disposal is given
below :
 I. Unsewered areas
 1. SERVICE TYPE LATRINES (CONSERVANCY
SYSTEM) Nightsoil is collected from pail or
bucket type of latrines by human agency, and
later disposed of by burying or composting.

11.  2. NON-SERVICE TYPE (SANITARY LATRINES) (a) Bore hole
latrine
 (b) Dug well or pit latrine
 (c) Water-seal type of latrines -
 i) P.R.A.I. type
 (ii) R.C.A. type
 (iii) Sulabh Shauchalaya.
(d) Septic tank
(e) Aqua privy.
 3. LATRINES SUITABLE FOR CAMPS AND TEMPORARY USE :
 (a) Shallow trench latrine
 (b)Deep trench latrine
 C)Pit latrine
d)Bore hole latrine

12.  II. Sewered areas
 1. WATER-CARRIAGE SYSTEM AND SEWAGE
TREATMENT –
 (a) Primary treatment Screening Removal of
grit Plain sedimentation
 (b) Secondary treatment Trickling filters
Activated sludge process
 (c) Other methods - (i) Sea outfall
 (ii) River outfall
 (iii) Sewage farming
 (iv) Oxidation ponds

16.  A sanitary latrine is one which fulfils the
following criteria :
 (1) Excreta should not contaminate the
ground or surface water .
 2) Excreta should not pollute the soil .
 (3) Excreta should not be accessible to flies,
rodents, animals (pigs, dogs, cattle, etc.)
and other vehicles of transmission.
 (4) Excreta should not create a nuisance due
to odour or unsightly appearance.

35.  The solids settle down in the tank, to form
"sludge", while the lighter solids including grease
and fat rise to the surface to form "scum". The
solids are attacked by the anaerobic bacteria
and fungi and are broken down into simpler
chemical compounds. This is the first stage of
purification, called anaerobic digestion. The
sludge is much reduced in volume as a result of
anaerobic digestion, and is rendered stable and
inoffensive. A portion of the solids is transferred
into liquids and gases (principally methane)
which rises to the surface in the form of bubbles

36.  The liquid which passes out of the outlet pipe
from time to time is called the "effluent". It
contains numerous bacteriae, cysts, helminthic
ova and organic matter in solution or fine
suspension. The effluent is allowed to percolate
into the sub-soil. It is dispersed by means of
perforated or open-jointed pipes laid in trenches
90 cm (3 ft.) deep and the trenches are then
covered with soil. The effluent percolates into
the surrounding soil. There are millions of
aerobic bacteria in the upper layers of the soil,
which attack the organic matter present in the
effluent. As a result, the organic matter is
oxidized into stable endproducts, i.e., nitrates,
carbon dioxide and water. This stage of
purification is called aerobic oxidation.

37.  To sum up, two stages are involved in the
purification of sewage. The first stage,
anaerobic digestion takes place in the septic
tank proper, and the second stage, aerobic
oxidation takes place outside the septic
tank, in the sub-soil. Together, these two
stages complete the purification of sewage.

42.  It has very limited use under Indian
conditions. The closet consists of a metal
tank containing a disinfectant fluid. The
active ingredients of the fluid are
formaldehyde and quaternary ammonium
compounds. In addition, a harmless water
dye and a deodorizing substance are usually
incorporated . A seat with a cover is placed
directly over the tank. Nothing except the
toilet paper should be thrown into the
chemical closet.

45.  The water carriage system or sewerage system
implies collecting and transporting of human excreta
and waste water from residential, commercial and
industrial areas, by a net-work of underground pipes,
called sewers to the place of ultimate disposal.
 It is the method of choice for collecting and
transporting sewage from cities and towns where
population density is high.
 There are two types in water carriage system the
combined sewer system and the separate sewer
system. In the combined system, the sewers carry
both the sewage and surface water. In the separate
system, surface water is not admitted into sewers.
The separate system is considered the system of
choice today.

61.  Sewage arriving at a disposal work is first passed
through a metal screen which intercepts large
floating objects such as pieces of wood, rags,
masses of garbage and dead animals. Their
removal is necessary to prevent clogging of the
treatment plant.
 The screen consists of vertical or inclined steel
bars usually set 5 cm (2 in) apart.
 In some plants, the screens are of the fixed type
while in others, the screens are of the moving
type.
 The screenings are removed from time to time
either manually or mechanically, and disposed
off by trenching or burial.

63.  Sewage is then passed through a long narrow
chamber called the grit chamber or detritus
chamber. This chamber is approximately 10 to 20
metres in length; it is so designed as to maintain
a constant velocity of about 1 foot per second,
with a detention period of 30 seconds to 1
minute .
 The function of the grit chamber is to allow the
settlement of heavier solids such as sand and
gravel, while permitting the organic matter to
pass through. The grit which collects at the
bottom of the chamber is removed periodically
or continuously, and disposed off by plain
dumping or trenching.

65.  Sewage is now admitted into a huge tank called the
primary sedimentation tank. It is a very large tank,
holding from 1/ 4 to 1/3 the dry weather flow. There
are various designs in primary sedimentation tank.
 By far the commonest is the rectangular tank.
Sewage is made to flow very slowly across the tank at
a velocity of 1-2 feet per minute. The sewage spends
about 6-8 hours in the tank.
 During this long period of relatively still conditions in
the tank, a very considerable amount of purification
takes place mainly through sedimentation of
suspended matter. Nearly 50-70 per cent of the solids
settle down under the influence of gravity. A
reduction of between 30 to 40 per cent in the
number of coliform organisms is obtained

66.  The organic matter which settles down is called
sludge and is removed by mechanically operated
devices, without disturbing the operation in the tank.
While this is going on, a small amount of biological
action also · takes place in which the microorganisms
present in the sewage attack complex organic solids
and break them down into simpler soluble substances
and ammonia.
 A certain amount of fat and grease rise to the surface
to form scum which is removed from time to time
and disposed of. When the sewage contains organic
trade wastes, it is treated with chemicals such as
lime, aluminium sulphate and ferrous sulphate.
Addition of one of these chemicals precipitates the
animal protein material quickly.

68.  The effluent from the primary sedimentation
tank still contains a proportion of organic
matter in solution or colloidal state, and
numerous living organisms. It has a high
demand for oxygen and can cause pollution
of soil or water. It is subjected to further
treatment, aerobic oxidation, by one of the
following methods : (a} Trickling filter
method
 (b) Activated sludge
process.

72.  Activated sludge process is the modern method
of purifying sewage, in place of the trickling
filter.
 The "heart" of the activated sludge process is
the aeration tank.
 The effluent from the primary sedimentation
tank is mixed with sludge drawn from the final
settling tank (also known as activated sludge or
return sludge; this sludge is a rich culture of
aerobic bacteria), The proportion of activated
sludge to the incoming effluent is of the order of
20 to 30 per cent. The mixture is subjected to
aeration in the aeration chamber for about 6
to 8 hours

80.  Sea coast towns and cities may dispose of
their sewage by discharging it into the sea.
For instance, nearly two-thirds of untreated
sewage of Greater Mumbai is discharged
every day into the Arabian Sea. Purification
takes place by dilution in the large body of
sea water, and the solids get slowly oxidized.
The drawback of this method is that the
offensive solid matter may be washed back
to the shore and create public nuisance. In
order to prevent this, the sewage outfall is
designed to discharge the sewage into deep
water at many points

82.  Raw sewage should never be discharged into
rivers. The present day practice is to purify
the sewage before it is discharged into
rivers. How far the sewage should be purified
depends upon the dilution the river provides
to carry on aeration and self-purification

84.  If sufficient and suitable land (porous soil) is
available, sewage may be applied to the land
after grit removal, screening and a short period
of settlement.
 This type of treatment is practised in some
Indian towns and cities and is known as Sewage
Farming or Broad Irrigation. An acre of land
would be required to treat the sewage of 100-
300 persons.
 The land is first laid into ridges and furrows.
Sewage is fed into the furrows intermittently
and crops are grown on the ridges. The crops
that are found suitable to grow are those which
do not come in contact with sewage and likely to
be eaten raw

85.  . Fruit trees whose fruits are high above the
ground (e.g., plantain) can be grown. But
sugar cane, coriander, cuccumber, tomato,
onion, etc. should not be grown. The farm
should be under the direction of a competent
agricultural expert. During the rainy season,
it may not be possible to operate the sewage
farms. Badly managed farms stink, a
condition described as "sewage sickness"
because of lack of sufficient aeration and
rest pauses to the land. Alternate methods of
disposal may have to be provided during the
rainy season.

86.  A cheap method of sewage treatment is the
oxidation pond which has been referred to by
many different names waste stabilization
pond, redox pond, sewage lagoons, etc. The
term "waste stabilization pond" is more
appropriate. The term 'waste' includes both
sewage and industrial wastes. Although an
old method of purifying sewage, oxidation
pond has attracted the attention of public
health engineers only recently.

87.  The oxidation pond is an open, shallow pool 1 to
1.5 m (3-5 ft.) deep with an inlet and outlet .
 To qualify as an oxidation pond, there must be
the presence of (1) algae (2) certain types of
bacteria which feed on decaying organic matter,
and (3) sun-light. The organic matter contained
in the sewage is oxidized by bacteria (hence
oxidation pond) to simple chemical compounds
such as carbon dioxide, ammonia and water.
 The algae, with the help of sunlight, utilize the
carbon dioxide, water and inorganic minerals.
for their growth, Thus there is a mutually
beneficial biological balance between the algae
and bacteria in oxidation ponds

88.  Oxygen that is needed for oxidation is
derived to a small extent from the
atmosphere but mostly from the algae which
liberates oxygen under the influence of
sunlight. Consequently, sunlight is an
important factor in the proper functioning of
oxidation ponds. Cloudy weather definitely
lowers the efficiency of the process. The
oxidation ponds are predominantly aerobic
during sunshine hours as well as some hours
of the night. In the remaining hours of the
night, the bottom layers are generally
anaerobic.

89.  . Thus the sewage purification in oxidation
ponds is brought about by a combination of
aerobic and anaerobic types of bacteria. The
effluent may be used for growing vegetable
crops (land irrigation) or may be discharged
into a river or other water courses after
appropriate treatment. Mosquito nuisance is
avoided by keeping weed growth in the
neighbourhood of oxidation ponds to a
minimum and the water line free from
marginal vegetation

91.  Other methods recommended are (1)
oxidation ditches and (2) aerated lagoons.
These methods make use of mechanical
rotors for extended aeration. For treatment
of the wastes of a population between 5,000
to 20,000 an oxidation ditch requires an area
of one acre as compared to 22 acres for an
oxidation pond and 2.5 acres for an aerated
lagoon. These are low-cost treatment
methods for the purification of sewage.