Removal of Nutrients in Constructed Wetlands

Dear wise friends, we are planning to construct a wetland as a tertiary treatment after an activated sludge treatment unit (0.8 Ml/day domestic sewage).  The activated sludge is not designed to remove phosphates, but reasonable denitrification will occur (in an orbal raceway configuration with aeration and anoxic conditions occuring before re-aeration).  We are planning to use a broken shale gravel bed which according to the literature will absorb excess phosphates, and planting vetiver grass into the bed (horizontal flow). We realise the shale may become saturated after a time, and no longer act as a sink for phosphates.  I would appreciate other's experience and comments.