Ammonia and nitrate measurement in municipal wastewater applications - Case Study
Advantages of the Advanced Sensor Technology real-time continuous online measurement systems for ammonia and nitrate
- Low Initial Procurement Cost compared to competing technologies and offerings
- No need for chemical feed and rebuilding of sensors
- Lower Cost of Ownership due to very long sensor lifetime, low maintenance & cleaning requirements for inline, immersion & submersion ion selective sensors
- Intuitive and easy to use Rosemount 1056 and 1057 analyzer programmed and configured for ammonia and nitrate ion selective analysis
- All new systems come preconfigured, precalibrated and fully qualified to install, plug-in and immediately place into service especially for compliance purposes
- Only periodic 1-point grab sample offsetting is required – this “Standardize” calibration can be performed WITHOUT removing sensor from service!
A number of municipal wastewater plants are now using a variety of nitirification and denitrification type wastewater treatment systems to comply with various environmental and discharge limits (such as NPDES permits). The nitrification and denitrification process used can start with a number of nitrogen sources, but the simplified inorganic oxidation and reduction reaction sequence is commonly written as Ammonia (NH3-N)> Nitrite (NO2 -) > Nitrate (NO3 -) > Nitrogen N2 (off gas). Until now, the technologies to measure the two nitrogen species of greatest interest in such plants (starting ammonia and the oxidized product nitrate) have been only possible with expensive sampling analyzers that required reagents, and significant operator involvement to ensure nominal operation. The result has been the operator and time intensive method of periodic grab sampling to ensure that both the nitrification and denitrification systems are working within the appropriate limits continues to be used at most facilities. Process upsets due to the nitrogen load changing from the incoming wastewater or due to equipment malfunction can result in significant problems such as high nitrate discharges that can lead to problems with permits and other environmental issues. This issue can be particularly problematic with smaller and remote facilities that have less frequent operator presence due to personnel limitations. Often plants will have existing pH, ORP and DO measurement online to control the level of ammonia conversion to nitrate, and to define the period of anaerobic conversion of the nitrate to nitrogen off-gas. Although these processes are pH dependent, often this is not a critical control parameter due to the process pH stability (particular for larger systems). In some cases the ORP (oxidation reduction potential) can act a proxy for the ammonia/nitrate ratios in solution. Although ORP can sometimes serve as a good first order approximation for this ratio, it is highly dependent upon other components in the water and is very sensitive to pH and temperature. Lastly, since ORP is a ratio only measurement it does not provide any information about the ABSOLUTE concentration of neither ammonia nor nitrate.