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The movement of water molecules through a thin membrane. The osmosis process occurs in our bodies and is also one method of desalinating saline water.


The site of discharge of a liquid from a pipe. Applied particularly to the point at which a sewer discharges to a treatment works or receiving water, or the point at which a conduit discharges the effluent from a treatment works into a receiving water.

Oxidation pond

A man-made body of water in which waste is consumed by bacteria.

Oxygen depletion

The reduction of the dissolved oxygen level in a water body.


The addition of ozone, particularly to a raw water or biologically treated sewage to decolorise and disinfect it. Also termed 'ozonization'.


An unstable oxidizing agent, that consists of three oxygen atoms and can be found in the ozone layer in the atmosphere. It is produced by electrical discharge through oxygen or by specifically designed UV-lamps.

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