text: * Incomplete denitrification is often observed in the B-stage if the influent C/N ratio is low. Direct by-pass of a part of A-stage influent with high organic matter content to the B-stage is used to increase the C/N ratio.
  * High sludge production in the A-stage is a drawback for WWTPs that are not equipped with anaerobic digestion of sludge because it increases sludge treatment costs.
  * Sludge from A-stage has poor settling properties.
  * High retention times cause an increased need for additional reactors to maintain throughput increasing equipment costs