Treatment of Sewage Raw sewage consists of water containing organic wastes faeces toilet paper food scraps detergents
Dec 18, 2015
Treatment of Sewage
Raw sewage consists of water containing organic wastes
faeces toilet paper
food scraps detergents
The raw sewage containing the mixture of materials arrives at a sewage treatment works
A typical sewage treatment process
Preliminary Treatment Sewage will:
be screened
have grit removed
be left for several hours to let solid particles settle in settlement tanks as sludge
Sludge from the settlement tank goes to the sludge digester for further treatment
Sludge digesters(each digester is 140 ft tall and holds 3 million gallons of sludge)
End product – pellets for fertiliser
Methane gas produced
The liquid passing out of a
container is called an effluent
Effluent
Secondary Treatment
oxygen must be provided for the decay micro-organisms to do their work
products will be harmless to the environment
organic compounds broken down by micro-organisms
Secondary Treatment (providing oxygen) – two types
effluent pumped up central pipe through rotating arms
sprinkled onto a bed of stones (air spaces between stones)
stones coated with micro-organisms which receive oxygen Micro-organisms feed on organic
material and break it down
purified effluent leaves
Biological Filtration Aeration
effluent mixed with activated sludge (rich in bacteria)
aerated by blowing compressed air through it
stirred with paddlesMicro-organisms receive oxygen Micro-organisms feed on organic
material and break it down
purified effluent leaves
‘Purified’ sewage passes to final settlement tank
Remaining suspended matter settles out as activated sludge
Effluent now harmless and can be safely released into nearest waterway
Effluent to river
Activated sludge used to produce methane gas
Methane gas fermentation from activated sludge
Microbes feed on sewage sludge
Anaerobic respiration (fermentation)
Methane gas produced
Methane can be used as a fuel to drive some of the other processes at the
sewage works
Methane from sewage is used in developing countries for cooking and
heating
Methane fermenters
Sewage Sludge Incineration
Agriculture
Sludge is disposed of in many ways:
Dumped at sea
Sludge pellets ready for market as fertilizer
Surfers against sewage publicises health risks from polluted water
This is dried toilet paper on the beach in Troon, Scotland
Aerobic respiration – complete breakdown of sewage
organicmaterial
+ oxygen energy + carbon dioxide
+ water
Anaerobic respiration – incomplete breakdown of sewage
organicmaterial
a littleenergy
+ methane gas
Fermentation and micro-organisms
Methane fermentationMicro-organisms release energy from fresh manure in the absence of oxygen (anaerobically) to produce methane gas
Chemical oxygen demand (COD)
Chemical oxygen demand (COD)
If the products of anaerobic respiration were released into the waterway
They would be used by further micro-organisms in the river
These micro-organisms would use up the river’s oxygen supply
Semi-treated sewage, therefore, is said to have a ‘Chemical Oxygen Demand’
The aim of a sewage treatment plant is to reduce the COD of the
effluent to the lowest possible level before releasing it into the
waterway
Need for a range of micro-organisms
mixture of many different complex organic materials
Sewage
fats carbohydrates
proteins
each species of micro-organism is only able to break down a few substances
many different species are needed to ensure complete breakdown of all sewage materials to carbon dioxide, water and simple inorganic substances
In high concentration, however, some of them could later pollute the waterway by causing an algal bloom
Bacteria
Protozoa
Fungi
Most of the inorganic substances are harmless
Algal bloom