Treatment of Sewage Raw sewage consists of water containing organic wastes faecestoilet paper food scraps detergents.

Treatment of Sewage

Raw sewage consists of water containing organic wastes

faeces toilet paper

food scraps detergents

The water becomes mixed with:

plastic bags

rags, road grit and many other materials

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

raw effluent treated water suitable for discharging to a watercourse

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

Special bacteria may therefore be used to remove excess ammonium compounds, nitrates and phosphates

Ammonium compounds

Nitrates

Phosphate

Bacterium type 1

Bacterium type 2

Bacterium type 3

Harmless nitrogengas released to air

Phosphate stored in granules