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In case of separated sewage system (black waters separated from the grey ones) it's necessary to use a septic tank (Imhoff or three-chamber type) in order to treat wastewater from toilets and a grease trap to treat kitchen and soapy waters as shown in the following schemes.

 

• with Imhoff tank: grey waters join the black ones after the pretreatment stage

 

 

• with three-chamber septic tank: grey waters are conveyed into the third chamber

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If black and grey waters are not separated it's necessary to use exclusively an Imhoff tank, without using a grease trap.