sewage & fish farming 1

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Please return the Notes after your exam ARIF ULLAH (01817721521)

Sewage

Sewage is the water-borne waste of a community. It has three main constituents-

domestic waste, industrial waste and surface water.

Domestic Waste include

Tea leaves from kitchen sinks

Detergent from washing machines

Soaps in bathwater, faeces and urine

Industrial Waste include

Plant waste such as cellulose from paper mills

Pharmaceuticals

Clay from potteries

Oil from garages and machinery works

Heavy metals such as Zinc, Iron and Lead

Surface water includes rainwater from paved areas, roads and roofs.

Risk of Leaving sewage untreated and exposed to rain and flies:

When raw sewage is discharged into a lake or river, not only does it constitute a

health hazard, but it provides so much for bacteria that they increase in vast

numbers. These are aerobic bacteria which make such a demand on the oxygen

dissolved in the water that fish and other organisms are deprived of oxygen and

die. This is called eutrophication.

Moreover, by urinating and defaecating carelessly about the place, people can make

it easy for vectors such as houseflies, cockroaches and rats to become infected with

pathogens which can spread to other, healthy people.

Carelessness over these functions also results in the cysts of amoebic dysentery,

the eggs of tapeworm and blood fluke, and the larvae of hookworm.


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Description and identification from a drawing, a section through a pit

latrine

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0000100000009a10000066090000250000000c0000000e000080250000000c0000000e000080120000000c00000001000000520000007001000001000000a4ffffff000000000000000000000000900100000000000004400022430061006c0069006200720

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41cb5120014b21200200000004964ae5614b212001cb5120020000000fffffffffc027300d064ae56ffffffffffff0180ffff0180dfff0180ffffffff006b0000000800000008000022b6a

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200000000100000084b2120084b21200e878a45620000000acb21200fc0273006476000800000000250000000c00000001000000250000000c000000010000002500000

00c00000001000000180000000c0000000000000254000000540000000000000000000000350000006f000000010000006c468740c48c87400000000057000000010000004c0000000400000000000000000000009a10000066090000500000002000a8f0

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090000000000004600000014000000080000004744494303000000250000000c000


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140000000400000003010800050000000b0200000000050000000c022101fe01040000002e0118001c000000fb020200010000000000bc020000000001020222537973

74656d003f00003f3f3f3f3f3f3f3f3f3f0800000001003f3f3f3f3f00040000002d010000040000002d01000004000000020101001c000000fb02f5ff00000000000090010000

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1. Concrete slab at the top- It prevents seepage into the surrounding soil and

prevent entry of rats and other vectors

2. A long handle cover over the squatting plate, prevents the escape of smells

and entry of flies.

3. A fly wire over the ventilation keeps out flies while ventilating at the same

time

4. The lower part of the pit must be lined with bricks to prevent the soil from

collapsing inwards.


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The reasons for the careful seating of pit latrines in relation to water

sources

1. Pit latrine is located in areas where the soil is permeable to water

2. There should be no wells within 30m of the pit latrine to prevent

contamination by seepage

3. The latrine should never be sited uphill from a well of water source

4. The pit latrine should be dug as deep as the soil will allow so

Aim of Sewage treatment

1. To lower the BOD sufficiently so that the effluent from the sewage works

can be discharged into a natural watercourse without grossly upsetting its

ecological stability

2. As far as possible to destroy or eliminate pathogens which

may endanger wildlife such as shellfish and birds living in the water.

Humans may also eat shellfish from, or bathe in which treated sewage is

discharged.

Both these aims can be achieved by a combination of physical, chemical and

microbiological processes.


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Large Scale Sewage Treatment Plants

Large scale sewage treatment is the best method of disposal which is essential in

urban areas. A very gentle slope called sewer, carries the sewage to the sewage

work. At a sewage work, two main methods of sewage disposals are used. They

are the activated sludge method and the biological filter method. Both involve

the same initial treatment of sewage.

Screening- First the crude sewage flows through a grid or screen in, which large

materials like paper and sticks are held. These are removed and burned.

Sedimentation- Sewage then slowly passes over open channels where the grid

settles. This is later washed and returned to the land. The sewage now passes

into sedimentation tanks where the solid materials settle at the bottom as

sludge. Ferric sulphate may be added to speed up the settling by flocculating the

particles. The liquid part, called effluent, remains at the top. The sludge is then

pumped into tanks where the bacteria decompose it. It is then dried, either by

spreading it out on the land or by heating. The drying produces gas methane,

which can be burned to supply the source of heat. The dehydrated sludge is

finally used as manure, but not for root crops or others where the manure would

come into direct contact with the part used for food.

Biological Filter

This process uses a biological filter to treat the effluent run off from the

sedimentation tank. This filter consists of concrete tanks some two meters high,

filled with clinker, coke, or small stones. These stones become covered with a

film containing numerous bacteria and protozoa. The clinker also ensures a good

air supply so that these microorganisms are extremely active and feed on

sewage. The effluent is sprinkled on to these filters from rotating pipes above

them. The larva of some flies may also feed on sewage and speed up the

process.


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The effluent from this filter now passes through a humus tank to allow any

remaining solids to settle. The effluent is now pure and is passed into rivers.

Tests are done to ensure the purity of this effluent.

Activated Sludge

Activated sludge method takes up less space and it is a quicker process. After

large objects and grit have been removed the liquid is stirred by paddles, driven

by machines. This breaks up organic matter, chiefly faeces and paper into small

particles. The liquid and particles pass into one or more sedimentation tanks and

then into a large aeration tank where air is bubbled through the liquid. The

oxygen in the air enables aerobic decomposing urea and other organic materials

such as cellulose.

The liquid passes to a further sedimentation tank in which sludge settles out.

This sludge contains both bacteria and ciliates which feed on the remains. The

ciliates also produce effluents, which binds dumps of organic materials, bacteria

and ciliates together. Some of the sludge is returned to the aeration tank, where

it keeps up the supply, of active organisms. Most of the sludge passes into a

large closed digestor. Anaerobic bacteria live in the digestor and these

decompose the sludge to produce methane, a gas which will be collected and

used as a fuel. Sometimes enough methane is produced to provide all the power

needed to drive the machinery at the sewage works. Eventually the sludge which

contains no more pathogens is removed from the digestor dried and used as a

fertilizer.


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Comparison of the relative purity of water from rivers, canals, shallowwells, bore holes, and rainwater collected from a roof as appropriate locally

River or lake water: It is the surface water collected after rain has fallen and

drained off from higher around to lower areas. Generally river water is

contaminated and has to be purified for drinking processes. Large scale water

supplies generally use river water. But this water is treated before it is sent to

individual building.

Well Water

Water which has soaked into the ground after passing through the porous layers

of earth may be trapped when it meets an impervious layer such as clay. This

water is can be brought back to the surface by means of wells. A well is a narrow

channel dug from the surface downwards to where the water has collected. Two

types of wells:

Shallow Wells: This well yield the water which has collected on the first

impervious layer of soil below the surface. The water is often polluted and must be

filtered and sterilized before drinking.

Deep Wells: The water from a deep well has been collected between the first and

second impervious layers. As it is farther away from the surface, it is cleaner and

safer than water in a shallow well. It is also less likely to dry up. However the

water has to be pumped up.

A well should be protected from contamination. Wells should be dug on high

ground and be at least 30m away from any latrines. The inner surface of a well

shaft should be a surrounding wall of about 1-15m high to prevent people from

falling into the well. In addition, the presence of a cemented area or apron around

the wall prevents spilled water from seeping through. This area should have a


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drain running round it. To draw up the water it is more hygienic to use a pump

than to use a bucket with a rope.

Rainwater collected from roofs

Rainwater is collected from the gutters fixed all round the edges of roofs of

buildings. The water is then channeled into a collecting tank.

Rainwater looks fairly clean. However there may be impurities such as germs or

dissolved chemicals substances that may not be visible to the naked eye. For

drinking purposes rainwater must be filtered and boiled for at least ten minutes.

But for washing purposes or watering the garden no such treatment is required.

Water Purification

The supply of water for large towns is provided by the water works. The water

may be obtained from lakes, canals or rivers. Artificial reservoirs or lakes may be

made by constructing dams in some regions. This water may flow to the water

works by gravity or pumps may be used.

Screening: First the water is pumped through grid screens. Suspended solids are

removed by first coarse and then fine grid screens.

Sedimentation: The water is then stored reservoirs to allow some solids to

settle. Sunlight and air help to destroy bacteria at this stage. Within the reservoir

protozoa, which feed on the bacteria are in turn eaten by crustacean and insect

larvae who themselves fall prey to fish. Such food chains help to purify the water.

On leaving the reservoir the water may be passed through further screens called

microstrainers. They have a fine meshwork capable of removing plankton from the

water.

The water now passes to a settling tank where coagulation assists in the

deposition of particles. Colloidal particles are flocculated by the addition of a


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coagulant. This coagulant may be ferric chloride, ferric sulphate or aluminium

sulphate, which form the Hoc of particles. The coagulant is rapidly mixed with

water by mechanical stirrers. The floe formed is then removed as form of sludge,

bv form of sedimentation. Fine bubbles of air may be used to help the rise of

sludge prior to its removal. Such aeration helps aerobic decomposition in heavily

polluted water. The oxygen also displaces carbon dioxide to reduce the activity of

the water and removes iron and manganese as hydroxides. The gelatinous

coagulant waste is formed into a filter cake for disposal.

Filtration: The next process, filtration, is done either by slow sand filters or rapid

gravity filters. The sand particles in the former become coated with a jelly-like

film, secreted by micro-organisms, which traps bacteria.

Rapid gravity filters are made artificially from aluminium oxide which also forms a

jelly-like film. Protozoa on the film feed on the trapped bacteria. Rapid filters work

more quickly and need less land and operating labour. They are less efficient at

removing determents and organic matter. Slow sand I liters clog more easily and

have to be cleared more frequently. Layers of activated carbon are used in some

filters. Coarser sand and stones in the lower layers help the water to flow.

Chlorination: Finally, disinfection by chlorine or Ozones ensures the removal of

all harmful organisms. The amount of chlorine required to destroy all the bacteria

is calculated, taking into account that used up in other chemical reactions. The

Chlorine gas forms hypochlorous acid in the water and this releases oxygen, killing

the bacteria on contact. The water is stored in large storage tanks and then

supplied to factories and houses through pipes.


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Fish farming

Principles of fish farming

Eggs and sperm are collected from captive adult fish, and fertilization occurs in

tanks. The eggs are kept in controlled conditions until they hatch, then the larvae

are provided with suitable high-energy food for rapid growth.

When the larvae have grown enough, they are transferred to outdoor cages.

For freshwater fish, eg. Trout, these cages are usually in lakes. For marine fish,

e.g. salmon, the cages may be in sea lochs, or in the open sea.

The cages keep the fish in one place, and allow them to be fed easily, while

preventing losses from predators. Fish can be kept in the cages for 1-2 years,

before being killed for food.

Steps involved in fish farming by farmers:

a. Restricting movement - larvae are kept in small tanks and adults are kept in

cages, so that they do not use energy unnecessarily.

b. Controlling food intake - for maximum growth rate, fish must be given large

quantities of nutrient-rich food. This normally processed into pellets made

from mollusks and small fish (of varieties human do not choose to eat, e.g.

sand eels), with added vitamins and minerals. Such a diet is very expensive.

c. Controlling temperature- fish are ectodermic i.e. the do not control their

body temperature, so they do not use energy keeping warm. However, they

will grow faster if they are kept in warm conditions. It is easy to control the

temperature of the indoor stages i.e. eggs and larvae, but once the fish

have been transferred to outdoor cages, it is virtually impossible.

d. Control of disease- Large numbers of fish kept in tanks or cages are at risk

of disease, e.g. from parasites such as lice, and from fungi and bacteria.

This is avoided in two ways:

i) Spraying the water in the cage are with pesticides to kill parasites.

This is only effective if the cage is in a relatively closed arc, e.g. small


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lake.

ii) Adding antibiotic and anti-fungal drugs to the fish diet.

e. Farmers try to maximize the amount of product they obtain by using high-

yield fish. Some of these have been developed by selective breeding and

others by genetic engineering.

f. Farmers must balance the demands of maximum food production (and

therefore high profits), against the costs and responsibilities of the

production process. The cost includes energy to heat indoor stages, drugs

and pesticides to prevent disease, and very high food costs.

Responsibilities to the environment / Problems of fish farming

When caged fish are fed on processed, pelleted food, some of it fall through the

bottom of the cage and settle on the lake or seabed. Here the pellets will

disintegrate, releasing their nutrients. This has a damaging effect on the natural

food web in this region.

When the cages are sprayed with pesticides to kill parasites, e.g. sea lice, the

pesticide will spread through the water and affect other invertebrates, disrupting

food webs.

lntraspecific predation: When one organism of one species kills another

organism of the same species is called lntraspecific predation.

Interspecific predation: When one organism of one species kills another

organism of other species is called Interspecific predation.


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sewage & fish farming 1

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