At risk of being obvious how about the Common Reed (Phragmites australis)?

Thats the one Fractal. I could remember the 'australis' bit but not that it was Phragmites. This is the reed that is recommended for treatment systems for septic tank effluent.

thanks for ur reply, we have australis in the first stage of the system bt we need a plant that can be cut many times a season because the reed can only be cut once at the end of the summer, wat do you think to comfrey?

I don't think that it will like standing with its feet in water. Why does it need to be cut several times a year? I have not heard of this practice before. How about water cress, but I would use it for compost rather than eating! Is the reed bed system for sewage or for an industrial effluent? PM me if you don't want to discuss this in an open forum.