Dear Ian, I agree with ...

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Dear Ian,

I agree with Alexandros, the fisrt question is: what is the tertiary CW designed for?

If it is for P removal, a P precipitation step added to the activated sludge willprobably be the most efficient/reliable, and cheapest. To my point of view, P adsorption/precipitation on specific substrates, either natural or engineered, should be used mainly on smaller units, where activated sludge are not advisable.

If total N removal is the goal, there again, a proper management of aeration cycle in the AS will do the job.

To my point of view, the best use of CW's as a tertiary treatment after an AS is to securize the outlet in terms of suspended solids, by entrapping any sludge loss. A properly designed vertical flow CW, with relatively fine filtration sand, would be perfect for that purpose.

Best regards,

François