Control of external carbon addition in biological nitrogen removal process for the treatment of coke-plant wastewater

An online monitoring and controlling system was developed for controlling external carbon addition in the biological nitrogen removal (BNR) process of a coke-plant wastewater treatment plant Two control regimes, on-off control and feed-forward control, were applied and their performances were compared under normal and abnormal nitrate (NO3--N) input conditions. The water quality of the final effluent was stably controlled under normal nitrate input conditions by using the on-off control algorithm that is connected to the effluent nitrate concentration. However, unexpected nitrite accumulation was observed under abnormal nitrate input conditions (i.e., sudden increase of nitrate concentration), resulting from chemical oxygen demand (COD) limitations during denitrification. On the contrary, the feedforward control algorithm more reliably controlled external carbon addition even under abnormal nitrate input conditions. The input COD/NO3--N was controlled between 3.55 and 3.62 during the operational period when the feed-forward algorithm was applied. Through trial and error, the optimum value of COD/NO3--N for denitrification was determined to be 3.59 in separate experiments.