Optimization of a nutrient-removing wastewater treatment plant using on-line monitors

Marselisborg WWTP is designed for 220,000 population equivalent as an AB process i.e. biosorption followed by a BIO-DENITRO plant. The plant was designed to remove nitrate in summer only. It was designed to remove phosphorus by pre-precipitation in the biosorption process and by simultaneous precipitation in the biological tanks. Introduction of an on-line control system for the nitrogen removal made it possible to remove nitrate all year to effluent values 1-3 mg N/l below the effluent standard of 8 mg N/l. The control system automatically introduced about 50% longer denitrification time in the tanks. During the last 4 years, the amount of filtered COD has through optimization increased from 65% to above 80% in relation to total COD. This in combination with the longer time without aeration in the biological tanks has made it possible to optimize biological phosphorus removal at the plant. It is possible to remove about 400 kg P/d biological out of about 500 kg P/d. Strategies to run the biological P removal simultaneously with the nitrogen removal in the same biological tanks has been tested. It seems possible to control the biological phosphorus removal by modifying the on-line control system for the nitrogen removal.