On-Line Nitrate/Nitrite Probes for BNR Process Control

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ABSTRACT
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.

INTRODUCTION
Three most important criteria for on-line and in-situ instruments are suggested as follows:

  • Good correlation with the reference method
  • Sample preparation not required
  • Quick responseUV Technology

The instrument utilizes no chemicals, no membrane, no pumps, no filters, no wipers, nothing but low power energy. There are no moving parts and is built to run without ever leaving the water, with minimum operating costs.

Recent advances in the development of In Situ Ultraviolet Spectrophotometers now make it possible to rapidly collect UVspectra with wavelength resolution better than 1 nm for extended periods of time. Many dissolved compounds of interest to BNR process, e.g., nitrate, nitrite, and organic material, absorb UV light. Each of these compounds has a unique absorption spectrum, which allows individual components in complex mixtures to be quantified.

Characterization of spatial and temporal variability of dissolved chemicals is greatly enhanced by the ability to measure concentrations directly with no chemical manipulation and with a temporal resolution of approximately 1 second.

MEASURING METODS
Nitrate and Nitrite could be measured by On-line Probes, On-line Sensors or Analyzers

On-Line UV Probes
On-line UV spectrometric probes represent a precise measuring technique with long-term stability, which permit quasi-continuous recording of the selected parameter in the smallest measuring cycles of minutes. The disturbance variables for optical measuring, such as turbidity/suspended solids, are eliminated by spectral recording.

On-Line Sensors
These control engineering demands led to the development of on-line ion-selective measuring techniques, which are capable of directly recording the respective selected parameter nitrate and nitrite during the process both quickly and without sample preparation. In terms of accuracy, practical compromises can be made.

Analyzers
Depending on measurements and applications, analyzer systems require standard and reagent solutions as well as sample preparation. Measuring intervals and automatic cleaning cycles are adjustable. These instruments automatically and recurrently compare measurements against reference standards and deliver high precision measurement values.

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