Model based control of external carbon dose rate in a full-scale predenitrification system

In wastewater treatment plants (WWTPs) with biological nitrogen removal it is commonthat large amounts of an easily biodegradable carbon source have to be added for completedenitrification. At the Rya WWTP in G\uf6teborg, which is a predenitrification system with postnitrifyingtrickling filters (NTFs) in the recirculation stream, this occurs during periods of loworganic loads and after nitrate chock loads after flooding and back washing of the NTFs. In thisstudy, this costly control problem is regarded as an uncertain dynamic system. A model based feedforward loop, using biased information about the recirculated nitrate flow is combined with a robustPI feed back controller that applies large stability margins and ensures that these are fulfilled for alllikely operating points of the system. Different feed forward strategies to deal with possible overdosagedue to lack of past information in periods when full denitrification has occurred withoutexternal carbon, are assessed by simulation. After evaluation of the controllers by simulation usinga simplified activated sludge model a controller was implemented at Rya WWTP with good results