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 common that large amounts of an easily biodegradable carbon source have to be added for complete denitrification. At the Rya WWTP in Göteborg, which is a predenitrification system with postnitrifying trickling filters (NTFs) in the recirculation stream, this occurs during periods of low organic loads and after nitrate chock loads after flooding and back washing of the NTFs. In this study, this costly control problem is regarded as an uncertain dynamic system. A model based feed forward loop, using biased information about the recirculated nitrate flow is combined with a robust PI feed back controller that applies large stability margins and ensures that these are fulfilled for all likely operating points of the system. Different feed forward strategies to deal with possible overdosage due to lack of past information in periods when full denitrification has occurred without external carbon, are assessed by simulation. After evaluation of the controllers by simulation using a simplified activated sludge model a controller was implemented at Rya WWTP with good results