Wow, thank you all for your ...

Wow, thank you all for your responses so far - that is very helpful.  To respond to some of the issues and questions: the water supply is sourced from a dolomitic spring (karst) in South Africa so is quite a hard water.  There is provision made for dosing ferric chloride, however the local authority has a very low revenue base and a poor reputation on O&M, so we are not confident that they will keep stock of the ferric, and secondly mechanical breakdowns on the works may take some months to repair.  The effluent receiving stream is fed by the same dolomitic spring and is pristine.  The purpose of the wetland is then  to reduce P and remaining N, but also as a buffer when there are failures on the activated sludge works. 

We have considered floating wetlands as they have been recently piloted in another municipality.  Will definitely re-visit this option.  We also have waste products generated by mining operations (e.g. ferric sludge from acid mine water treatment) which we could consider. I will also follow up on the other leads.  Many thanks