Why And How
Sewage Is Treated


What is sewage?

Sewage is used water of a community.  Since water is used for washing it contains dirt, sweat, food particles, fats, carbohydrates, proteins and soap detergents.  Water is also used for flushing toilets and therefore contains faeces, urine and paper.  Sewage contains large numbers of bacteria, some of which originate from the human digestive tract and contain pathogenic bacteria.


Why treat sewage?

Sewage treatment first started to prevent waterborne diseases e.g. cholera.  If drinking water is contaminated with sewage it will have a high bacteria content.  Some bacteria make humans very sick and may eventually kill them.  Since rivers are used for drinking water supply and recreational purposes it is best to treat sewage to reduce its bacterial and nutrient content before discharge to the environment.

More recently with rising population it was found that sewage entering rivers also damaged the rivers appearance and aquatic life.  Sewage is contaminated primarily with organic material.  This organic material is food for bacteria which are universally present in the environment.  In consuming food the bacteria use oxygen and multiply their numbers until the food supply is exhausted.  Sewage has enough organic matter in it to consume 300 mg/l of oxygen in the presence of bacteria.  Oxygen only dissolves  from the air in clean water to about 9 mg/l.  Hence if sewage enters rivers untreated the bacteria can quickly reduce the oxygen content of the river to zero.

Unfortunately fish require dissolved oxygen to live.  They absorb the dissolved oxygen through their gills.  In a similar way we absorb oxygen from the air through our lungs.  Whereas fish die without dissolved oxygen, many bacteria do not.  They have an ability to change their metabolism to live without free oxygen.  In doing so they generate unpleasant odorous compounds such as hydrogen sulphide (commonly known as rotten egg gas).  Hydrogen sulphide reacts with iron to form black iron sulphide.  Hence, when sewage is entering rivers it can kill fish and make the river odorous.  Black odorous mud can also result.

An important part of sewage treatment is removing sewage's ability to consume oxygen.  This oxygen demand is a measure of the sewage strength and is called the biological oxygen demand (BOD) and is normally measured over 5 days.  Sewage treatment effectiveness is measured in terms of the 5 day BOD (written BOD⁵).


How is sewage treated?

Bacteria will proliferate in sewerage, particularly if there is an ample supply of dissolved oxygen.  Furthermore, bacteria are slightly more dense than water and many bacteria have the ability to clump together when concentrated, into relatively fast settling particles.

Many aerated wastewater treatment systems (AWTS's) use this property of bacteria cultures to purify the water.  The sewage is mixed with a culture (called "activated sludge" or the "bugs") of not only bacteria cells but a large number of larger naturally occurring organisms, and this "mixed liquor" is agitated in the presence of air to dissolve oxygen (this is called "aeration").  The culture grows on the food or organic matter in the sewage until all the organic matter has been removed from the wastewater.  This is called "cell synthesis" and during the process about one half of the organic matter is converted to carbon dioxide and energy for cell life and the other half is converted to more cells.  The mixed liquor is then allowed to settle (this is called "clarification").  The activated sludge settles to the bottom leaving clear clean water in the upper half of the clarifier.  This treated wastewater is then removed from the clarifier for disinfection and distribution to a land application area.

Activated sludge has another interesting property.  If aeration is continued after the food supply has run out, the activated sludge eats itself away.  The bacteria cease multiplying and after a while they die.  The larger organisms eat dead bacteria and eventually die themselves.  This process is called "endogenous decay".

After prolonged aeration all that is left is a small inert residue of stabilised organic matter which cannot be further degraded.

The extended aeration process uses both cell synthesis and endogenous decay in the same tank to remove dissolved and suspended organics from the water and also to greatly reduce the volume of activated sludge.

The synthesis and decay processes are represented diagrammatically below.

CO² + H²O

CO² + H²O

Dead
Cells

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