Long-term experiences with external carbon sources for nitrogen removal

Methanol and ethanol have been used for three years as external carbon sources in a nutrient removal system based on pre-precipitation and post-denitrification in a single sludge activated sludge plant. Based on these long-term experiences it has been shown that the nitrogen standards of 8 mg N/l in the effluent wastewater can be met with both carbon sources. The process entails operational flexibility and the possibility to optimize the nitrogen removal due to seasonal variations in influent wastewater characteristics. Very high specific nitrate utilization rates were measured in the system with the use of external carbon sources. Rates of around 10 mg N/g VSS.h were reached with ethanol and around 3 mg N/g VSS.h with methanol. These rates were much higher than experienced in a pre-denitrification system with the use of the influent organic material as carbon source for denitrification. A start-up with the addition of ethanol led to a direct response of the system while a start-up with methanol resulted in a much longer adaptation period before full effect of the carbon source added was reached.