On-Line Nitrate/Nitrite Probes for BNR Process Control

Real time nitrogen monitoring plays an increasingly important role for the successful biological nutrient removal at wastewater treatment plants. Though traditionally dissolved oxygen has been the main process control parameter a greater understanding of the biological mechanisms and process conditions like nitrite/nitrate concentrations to achieve lower effluent values becomes necessary, specifically in terms of nutrient reduction. If the process is incorrectly monitored, nitrite may build up to toxic concentrations. When the facility's influent ammonia loading fluctuates depending on the origination of wastewater, manual control of aeration is not only difficult, but can create more problems. If too much air is delivered, complete the nitrification process can be achieved, but reduced denitrification results in high nitrates in the effluent. When reliable NO₃ ⁻ and NO₂ ⁻ readings are captured on-line, correct process control would lead not only to reduced aeration blower power but also reduced staff response and adjustment time of aeration equipment. This will also reduced chlorine consumption at the back end of the plant.

In this paper, the reliability of the online nitrite/nitrate monitors and the applicability of their readings in automatic control systems are discussed with regards to operational cost savings and technical feasibility. Great advances in sensors and analyzer technologies have been made over the past decade. An overview of UV sensors and techniques for monitoring of NO₃ and NO₂ will be discussed. Other non-UV type analyzers that could be used to monitor NO₃ and NO₂ are also presented.