Nitrate/Nitrite
Nitrification Process Control
Statement of the Problem:
Nitrification is a biological wastewater treatment process for the conversion of ammonia to an oxidized form of nitrogen. The process is mediated by two specialized microorganisms. The first microorganism, nitrosamonas, converts ammonia into nitrite (NO2). The second microorganism, nitrobacter, converts nitrite into nitrate (NO3). Complete nitrification is achieved when substantially all of the ammonia present is fully converted into nitrate, leaving little or no remaining ammonia or intermediate nitrite in the effluent.
Assuming that continuous, real-time measurements are available, one of the first signs of a decrease in the nitrification rate is a corresponding decrease in the NO3-N concentration, accompanied by an increase in the NH3-N and NO2-N concentrations. Failure to promptly detect and correct nitrification problems can result in a buildup of nitrite to toxic concentration and/or a drop in dissolved oxygen to the point where further nitrification is inhibited.
Control Strategy:
Frequent automatic analysis of ammonia, nitrite and nitrate from sample points within the aeration basin can be used to monitor nitrification process and to observe the results of operational changes such as increased or decreased aeration rates, RAS rates and retention times.
When the process is achieving full nitrification under variable influent ammonia conditions, aeration rates may be able to be reduced without sacrificing nitrification efficiency, thus saving substantial energy costs.
Some operational strategies may allow a small concentration of ammonia to bleed through in order to provide a source of ammonia for chloramine formation during disinfection. This strategy requires careful and continuous analysis of ammonia, nitrite and nitrate, especially under variable influent ammonia loading conditions.
High nitrite concentrations can result in excessive consumption of chlorine during disinfection. Continuous monitoring can assure that the process is fully nitrifying at all times.
Apparatus:
One ChemScan® Process Analyzer can detect ammonia, nitrite and nitrate as individual parameters from multiple process sample points or basins. A complete system can monitor up to eight sample points.
Sample filtration using a ChemScan® on-line ultrafilter is required for samples from aeration basins. Complete, fully integrated sample handling, analysis and data communication systems are available.
Denitrification Process Control
Statement of the Problem:
Denitrification is a term applied to a biological wastewater treatment process used to convert nitrate (NO3) into nitrogen gas. The process employs a special class of bacteria that metabolize carbon and oxygen. Under anoxic (low/no oxygen) conditions in nitrified wastewater, these bacteria are forced to strip an atom of needed oxygen from a nitrate or nitrite molecule.
Carbon is typically obtained by the bacteria from a carbon source such as methanol which is added to the treatment process as a food source. The availability of nitrate and methanol are therefore the limiting factors that sustain a given population of denitrifiers.
Underfeed of methanol will limit the reduction of nitrate in the process, while overfeed of methanol will result in a higher BOD value in the final effluent. The ideal is a methanol feed rate which is sufficient to obtain a desired amount of nitrogen removal given an overall process demand, which is a function of nitrate concentration and wastewater flow rate variables for a given set of process conditions.
Process Control Strategy:
Methanol feed can be controlled through the use of on-line analysis of nitrate concentration before and after the denitrification process, as shown in Figure 1.
Prior to the process, nitrate concentration can be monitored and combined with flow and other factors to produce a calculation of methanol feed for the process. A measurement of nitrate concentration after the process can be used to trim the methanol feed setting.
Apparatus:
On-line analysis of nitrate can be performed at frequent intervals from multiple sample points using a ChemScan® Process Analyzer. The ChemScan® system measures light absorbance at numerous wavelengths in the ultraviolet and visible wavelength range. This information is used to calculate the concentration of nitrate in the sample while compensating for the presence of background chemicals and turbidity variations.
If desired, ChemScan® can also detect nitrite, which is an intermediate step in the denitrification process, as another indicator of process efficiency. Detection of total oxidized nitrogen (nitrate plus nitrite) is also possible.