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[[File:SE DD Bildergalerie Klaerwerk Kaditz Faulung.jpg|thumb|Large sewage works in Germany]]

[[File:SE DD Bildergalerie Klaerwerk Kaditz Faulung.jpg|thumb|Large sewage works in Germany]]

'''Sewage treatment''' is the process of dealing with [[sewage]] so that is does not cause harm to people or to rivers. Sewage flows in [[sewer]]s from houses and [[factory|factories]]. When it arrives at a sewage treatment works it passes through many stages. Larger works often have more stages than a smaller works.

'''Sewage treatment''' is the process of dealing with [[sewage]] so that it does not cause harm to people or to rivers. Sewage flows in [[sewer]] pipes from houses and [[factory|factories]]. When it arrives at a sewage treatment plant it passes through many stages. Larger treatment plants often have more stages than smaller ones.  

Large objects are removed using [[screen]]s. The sewage then flows through a [[tank]] so [[sand]] can fall out. Large objects and sand might damage [[machine]]s in later stages.  

Large objects and rags are removed using [[screen]]s. The sewage then flows through a [[wikt:tank|tank]] so [[sand]] and grit can fall out. This protects the [[machine]]s in later stages.

[[Fat]]s and solids are removed in the first treatment stage.  Sewage enters a tank where solids fall to the bottom and fat floats to the top.  Fats and solids stay in the tank and the water continues to the next stage. This is called [[Primary Treatment]].

[[Fat]]s and solids are removed in the most important stage.  Sewage stays in a ''first'' tank until solids fall to the bottom. Fat also floats to the top.  Fat and solids stay when the water leaves.

Water leaving the first treatment stage may be cleaned by [[bacteria]] and other very small animals and plants in the [[Secondary Treatment]] stage.  These bacteria use [[oxygen]] from the air to eat pollution and [[chemical]]s from the sewage. Clumps of bacteria grow while eating the pollution from the water.  These clumps of bacteria eventually fall to the bottom of the tank.  The clumps of bacteria stay in the tank and the cleaner water leaves.Some of the bacteria falling to the bottom of the ''secondary'' tank are pumped back to the mixing tank so there are enough bacteria to create a thick brown mixture.

Water leaving the most important stage may be cleaned by [[bacteria]] and other very small animals and plants in the ''second'' important stage.  These bacteria use [[oxygen]] from the air to eat [[chemical]]s from the sewage.

Small treatment plants may use a small pond, called a lagoon, to hold the sewage while bacteria eat the pollution.  Larger treatment plants use machines to help the bacteria process the sewage.  Some treatment works have [[Rock (geology)|rocks]] or pieces of [[plastic]] for the bacteria to grow on.  Water is pumped over the rocks or plastic.

Cleaning bacteria may live in a small pond in simple works with enough land to build a pond.  Larger works use machines to help the bacteria find the chemicals.  Some works have [[Rock (geology)|rocks]] or pieces of [[plastic]] for the bacteria to grow on.  Water is pumped over the rocks or plastic.

Other works don't have rocks and plastics for the bacteria to live on.  These works may blow [[air]] through the water to mix it with clumps of bacteria.   

Clumps of bacteria grow while eating the chemicals from the water.  The cleaned water stays in a ''second'' tank until these clumps of bacteria fall to the bottom.  The clumps of bacteria stay when the cleaned water leaves.

Clean water on the top of the ''secondary'' tank or pond can then flow to a river or the sea but it may also need further treatment to make it cleaner.  Some treatment works kill bacteria that might make people sick.  Some works use chemical [[poison]]s, like bleach, to kill these bacteria.  Other works use [[ultraviolet]] light, the kind of light that causes a [[sunburn]]. This is called [[Tertiary treatment]].  

[[Solid]]s from the first and second treatment  tanks can be used to make [[methane]] gas and fertilizer for farmers fields or may be dried and sent to a landfill.

 

 

[[Solid]]s from the ''first'' and ''second'' tanks can be used to make [[methane]] gas and fertiliser for farmers fields.

{{Commons category}}

{{Commons category}}