Effluent Treatment plant

Hi Are you talking in terms of Waste Water facilities? if so see the following....

They are used to treat wastewater before it enters another body or is reused. A wastewater treatment system performs several activities: preliminary treatment, primary treatment, secondary treatment, tertiary treatment, and sludge treatment. During preliminary treatment, screens are used to remove large, solid inorganic materials such as paper and plastic. During primary treatment, the wastewater passes through a sedimentation tank to remove smaller, solid organic materials using a gravity settling process. During the secondary treatment stage, an activated sludge process uses submersible mixers to break down any dissolved or suspended organic solids. This is followed by the tertiary treatment stage in which disinfectants, usually chlorine, are mixed into the water to eliminate harmful microbes. Finally, during the sludge treatment stage, the collected sludge is processed for reuse.

Hope this helps.

Brent

Check out this Wikipedia entry for the basics, too.

Assuming you're referring to a WWTP, primary means sedimetation of solids, secondary means biological or chemical, depending on the nature of the waste, and tertiary means final polishing before discharge.