Part 1 SEWAGE
The effect of untreated sewage on rivers
Rivers contain bacteria.
These bacteria use oxygen during aerobic respiration.
Untreated raw sewage contains organic
material ( faeces ,food fragments, soap
Bacteria feed on this organic waste,
breaking it down to release energy.
They use this extra energy to multiply.
The larger numbers of bacteria use up
Oxygen supply is depleted (reduced)
Animals like fish die due to oxygen
The breakdown of sewage by bacteria releases Nitrates
Nitrates and phosphates are used by green algae to grow
fast and cover the surface of the river/lake (algal bloom)
The algae stop light getting to lower aquatic plants, which
then die -- the oxygen they provide during photosynthesis
is no longer available
Organic waste on river bed broken down by bacteria to
release foul smelling gases and rotting materials.
Raw Fish die due to
sewage Nitrates and lack of oxygen
Bacteria feed on little oxygen Smelly gas
Anaerobic bacteria digest
sewage to form methane gas
Water borne diseases
Untreated sewage can
contain micro- organisms
Dysentery, typhoid and
In Britain sewage is
treated in a Sewage
treatment works to make
Natural disasters ;-
earthquakes can fracture
underground pipes and
cause raw sewage to mix
with drinking water.
Treatment of Raw Sewage
This is done at a Sewage Treatment works
Material entering the sewage works is firstly screened to
remove large objects eg. Rags and grit.
Once screened, the sewage is passed to a primary settlement tank
where it stays for 6 hours to separate solid from liquid waste.
Primary settlement tank
Aerobic breakdown of sewage
Breakdown of sewage by anaerobic bacteria will always leave some
sewage material untreated (unsafe).
Aerobic bacteria, however will give complete breakdown, leaving only
carbon dioxide and water (safe).
Sewage + oxygen carbon dioxide + water
Sewage contains a range of materials requiring particular micro-
organisms for their breakdown, therefore a range of micro-
organisms are needed to break it down fully.
Oxygen can be provided in two ways;
A. Biological filtration
The sewage is trickled through filter beds and is broken down by bacteria
coating the stones. Air spaces between the stones provide the oxygen.
B. Activated sludge process
The sewage is mixed with activated sludge
(sewage with sewage – eating bacteria
added to it). Oxygen is then supplied by
bubbling compressed air through the
Primary settlement sludge
Activated sludge tank
The treated sewage is then
passed to final settlement
Here, activated sludge present in
the mixture from either process is
allowed to settle out.
The sludge collected is
reused in the activated
The purified effluent is
released into the river.
Anaerobic breakdown of sewage
Sludge from primary settlement and activated sludge from the final settlement tank
are combined and kept under anaerobic conditions, where bacteria break them down
to produce methane gas (fuel).
Waste from this process is either dumped at sea or treated to kill pathogens, and then
offered to farmers for spraying on fields as fertiliser.
Sewage Treatment -- Summary
Screening / grit removal Grit, stones + rags are removed by a filter
Primary Settlement Solids are separated from liquid waste
Secondary treatment Aerobic breakdown of sewage by a range of micro
Sewage + oxygen carbon dioxide + water
1. Biological filtration – oxygen provided by air
spaces between stones
2. Activated sludge process – oxygen provided
by compressed air
Final Settlement Activated sludge remaining from secondary
treatment settles out before clean water released
Sludge Treatment Anaerobic breakdown of sludge by micro organisms
to produce methane gas
Sludge Disposal Pathogens killed then sludge dumped at sea or
sprayed on fields as fertiliser
A range of micro-organisms are needed to break down sewage because
It contains a range of materials which cannot be broken down by only one type of
Fungal spores and bacteria can
travel in the air and land on
Floors in hospitals ,schools etc are
washed regularly to minimise the
spread of any harmful microbes
We can grow colonies of
microbes using a nutrient gel
or Agar in a petri-dish.
Working with Microbes
• We already know how microbes can help in the
manufacture of food and drink.
• Also that some microbes cause disease if they
enter the bodies of living things.
• It is obviously important when working with
microbes in the lab to adopt “safe practice”.
• This avoids unwanted growth of microbes and
• Contamination is the presence of unwanted,
possible harmful microbes.
• To prevent contamination all equipment must
be clean and sterilised.
• This is done in an autoclave,
where equipment is heated to
very high temperatures by
steam (boiling is not enough).
• The bench is thoroughly cleaned with
disinfectant and hands are washed.
• Sterile (very clean) equipment is collected:
– Petri dishes with agar food jelly
– Plating loop
– Culture of bacteria
– Bunsen burner
•Working close to the hot bunsen flame the loop is
heated till red hot then allowed to cool.
•This kills organisms on the loop
•A sample of bacteria is collected from the culture bottle. The
culture bottle is carefully opened and closed next to the
•Working close to the
flame the loop is now
gently rubbed over the
surface of the agar jelly
in the dish.
•The loop is again heated till red hot and allowed to cool.
•The dish is sealed, then labelled and placed in an incubator.
Label placed on
bottom of dish Tape
Bacterial Culture showing
growth after incubation
Hands washed before and after work Remove bacteria picked up from the
environment / remove any picked up
during lab work
Benches swabbed with disinfectant Remove bacteria and spores from
Lab coat should be worn Prevent contamination of clothes by
Use only safe bacterial cultures Other sources, e.g. air, soil etc. may
contain pathogenic bacteria
Autoclave all equipment Heating in an autoclave kills all foreign
Work beside bunsen, flaming loops and Prevent entry of foreign bacteria and kill
necks of culture bottles cultured bacteria on loops etc.
Incubate bacterial cultures below body This discourages the growth of
temperature pathogens which grow best at 37oC
Autoclave all equipment after use, and Kill all bacteria in case any pathogens
cultures before disposal have appeared
Precautions taken during CREDIT
In most industrial processes involving
microbes, the biggest danger is
foreign bacteria or fungi getting into
the process, rather than the cultured
Some bacteria and fungi make
resistant spores when faced with
adverse conditions e.g. extreme
drought, heat, pH etc.
The thick coat they produce only
disintegrates when conditions become
If these get into the manufacturing process CREDIT
they may cause;
1. A health hazard – they may be pathogenic.
2. Financial loss if a whole batch of product is
To prevent this, all equipment (fermenters, pipework
etc. is frequently steam-cleaned.
The high temperature of the steam can kill the spores.
Bacteria and fungi can
breakdown organic matter
This process is called
Decomposition is a natural
The bacteria or fungus
obtain energy from
breaking down the organic
Decomposers breakdown dead bodies and
waste materials to organic compounds.
The organic compounds contain Mineral salts
that are released into the soil.
Plants use these Mineral salts to grow
Apart from Mineral salts the elements Carbon
and Nitrogen also have to be recycled
Nitrogen Cycle Denitrifying Atmospheric
Nitrogen is used by
animals and plants
to make protein Nitrogen-fixing
bacteria (in root
Soil through roots Plant eaten Animal
nitrates protein protein
Nitrifying bacteria death death
Nitrifying Ammonium Faeces + dead
bacteria compounds (bacteria+fungi)
Plants like clover, peas and beans
have root nodules containing
nitrogen fixing bacteria on the
These bacteria take nitrogen gas
from the air and change it into
nitrates in the roots.
the clover uses the nitrates to
Farmers like clover to grow in a
field to replace the supply of
natural nitrates in the soil
Carbon Cycle CREDIT
There are three main processes;
Photosynthesis - uses carbon dioxide
Respiration – releases carbon dioxide
Combustion – releases carbon dioxide
Carbon Cycle CREDIT
photosynthesis respiration by
Carbon compounds in animals
Carbon compounds in dead combustion
organic matter (humus)
fuels e.g. coal
Food and Fuel from Waste
Many manufacturing processes produce waste products.
These waste products can be upgraded -- fed to
microbes which convert them to products which are
useful to people and other animals.
Advantages of upgrading waste; CREDIT
Unwanted waste products can be converted into
products with a high energy content (fuels), or a high
protein value (foods).
Savings on waste disposal and environmental pollution.
Fuels from Microbes
• Fermentation is an energy releasing process.
• This is carried out by some microbes in the absence
• Alcohol and methane gas are products of
fermentation, they are fermentation fuels.
Fossil fuels are NON RENEWABLE
Fermentation fuels are RENEWABLE
1. Fermentation of Manure by
bacteria To produce methane
feed on fresh
gas ( BIOGAS )
2. Fermentation of sugar into alcohol by Yeast.
Glucose YEAST Alcohol + Carbon dioxide + Energy
The Alcohol can be separated from the water by distillation.
so it can be
used as a fuel
Brazil has lots of sunshine and land for growing crops.
It can therefore grow lots of sugar cane.
It does not have rich supplies of fossil fuels
(COAL,GAS AND OIL ).
These are expensive to import.
Instead, sugar cane is fermented into alcohol which is
then mixed with petrol to make GASOHOL .
This is used instead of petrol in many vehicles.
There are advantages in using fermentation fuels
rather than fossil fuels;
Harmless to the Harmful to the
Easy to obtain Difficult to obtain
Cheap to obtain Expensive to obtain
Protein from Microbes
• Most micro-organisms reproduce themselves
much faster than the cells of plants and
• Given food, water and heat, one bacterium can
reproduce asexually to produce many
thousands of bacteria within a few hours.
The table and graph show bacterial growth under ideal
Most of a bacterial cell is composed of protein.
Industry is able to use fast growing bacteria to
manufacture protein-rich foods.
The bacteria are grown, harvested and dried to form a
protein-rich powder called single-celled protein which is
used as animal feed.
Some fungi produce a protein called mycoprotein, which can
be processed to produce meat substitutes for cooking.