The filters will be cased in either concrete or plastic shafts.

Rainwater from the drained area is fed into the inlet (marker 1), which is at the lower end of the shaft. A deflector plate sets up a radial flow.

At place marked 2, sedimentation of particles, especially the sand faction and above, takes place in the hydrodynamic separator. This is due to turbulent secondary flows within a radial laminar flow regime.

The settlable solids are collected at point marked 3 via an opening in the silt trap chamber. This chamber is evacuated periodically, via the by-pass central tube at intervals.

Four filter elements are located within the filter shaft (part marked 4). As waters flow upwards the finer particles are filtered out, whilst the dissolved pollutants are precipitated and absorbed. The filter is easily backwashed, and if completely clogged or exhausted, is easily replaced (often once per year).

At point marked 5, is clean water above the filter elements passing to discharge via an oil trap assembly. In the event of major spill, free floating oils etc are retained here. Normal concentrations of dissolved oils are retained within the filter elements.

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