The short answer
Nitrate Analyzers
Nitrate is measured online in a wastewater plant to control denitrification, where the nitrate at the end of an anoxic zone or in a recycle stream tells the controller whether to send more mixed liquor back or add carbon, and to fine-tune aeration, and in a water plant or a wellfield to watch a source against the drinking water limit. Ultraviolet absorbance is the dominant online technology: nitrate absorbs strongly in the ultraviolet, a probe or an analyzer measures the absorbance at that wavelength and corrects for organics and turbidity using other wavelengths, and there are no reagents. Ion-selective electrodes read nitrate directly in the process with a chloride correction and drift like every electrode; wet-chemistry analyzers are accurate and slow and used where the number must match the laboratory. The unit is milligrams per liter as nitrogen almost everywhere, and a reading as nitrate is four and a half times larger for the same water.
Key points
- Ultraviolet absorbance is the main online technology: no reagents, fast, with multi-wavelength compensation for organics and turbidity.
- Ion-selective electrodes read in the process and need chloride compensation and frequent calibration.
- Wet chemistry matches the laboratory and needs reagents, sample conditioning, and time.
- The main uses are denitrification control, aeration and recycle control, and source water monitoring against the drinking water limit.
- Report as nitrogen; the factor to nitrate is about 4.43 and a unit mistake is a large error.
Technologies
| Technology | How it works | Strengths | Weaknesses |
|---|---|---|---|
| Ultraviolet absorbance probe | A lamp and detector across a gap in the process; absorbance near the nitrate peak with correction wavelengths | No reagents, fast, in the process, low maintenance with a wiper | Organics, turbidity, and nitrite interfere; needs a compensation model fit to the water; optical path fouling |
| Ultraviolet spectral analyzer | A full spectrum across the ultraviolet with a chemometric model for nitrate and other parameters | Several parameters from one instrument, good compensation | Model must be calibrated to the water; cost |
| Ion-selective electrode | A nitrate-selective membrane electrode with a chloride electrode for compensation | Low cost, in the process | Drift, chloride and other interferences, membrane life, frequent calibration |
| Wet chemistry | Reduction of nitrate to nitrite and colorimetric reading, automated | Laboratory-equivalent accuracy | Reagents including a reduction column, sample conditioning, slow, waste |
Ultraviolet measurement
Nitrate absorbs ultraviolet light strongly around two hundred nanometers, and the absorbance across a known path length gives the concentration by the same law the laboratory uses. Dissolved organics and suspended solids also absorb and scatter in that region, so the instrument reads additional wavelengths where nitrate does not absorb and subtracts their contribution using a model. The model is fit for the water at commissioning by comparing readings with laboratory results over a range of conditions, and it is checked whenever the water changes character, after a storm, a process change, or a new industrial discharge. A short path length suits high concentrations; a long path length is needed for the low range in a water source. The optical windows foul, and a probe with an automatic wiper or air cleaning holds its reading for weeks; one without needs cleaning on a schedule the fouling rate sets.
Uses
- Denitrification control
- Nitrate at the end of the anoxic zone tells the controller whether denitrification is complete: high nitrate means more internal recycle, more carbon, or more anoxic time; near zero means the recycle can be reduced. The loop saves recycle pumping energy and carbon.
- Aeration and recycle
- Nitrate in the aerated zone and in the recycle indicates nitrification and sets the recycle rate; with ammonia, it gives the whole nitrogen picture.
- Effluent monitoring
- Total nitrogen permits are met by controlling effluent nitrate; the trend gives hours of warning.
- Source water
- A well or a river intake is watched against the drinking water maximum of ten milligrams per liter as nitrogen; a rising trend triggers blending, treatment, or a source change.
Ion-selective electrodes
A nitrate electrode responds to nitrate and, to a lesser degree, to chloride and other anions; a chloride electrode in the same body corrects for the main interference. The electrode is calibrated against laboratory results on grab samples, drifts, fouls, and is replaced when its slope falls out of range. It is the low-cost choice for a trend in the anoxic zone and the wrong choice for a compliance number.
Maintenance and verification
- Clean optical windows and electrodes on the schedule the fouling rate sets, and keep the wiper or air cleaning working.
- Compare with laboratory grab samples taken at the probe on a schedule; adjust the model or the calibration when the difference exceeds the written tolerance.
- Refit the ultraviolet compensation model when the water changes character; a model fit in dry weather may read wrong in wet weather.
- Check nitrite when the ultraviolet reading and the laboratory disagree; nitrite absorbs in the same region and a plant with incomplete nitrification has some.
- Log every cleaning, verification, and calibration; the drift record sets the schedule.
Frequently asked questions
- Ultraviolet probe or electrode for the anoxic zone?
- An ultraviolet probe with a wiper for most plants: no reagents, less drift, and a reading that holds between cleanings. An electrode where the budget is tight and the loop only needs a trend, with weekly calibration accepted.
- The probe reads higher than the laboratory after every rain.
- The organic and solids load rose and the compensation model is not fitting the new water. Refit the model with samples across wet and dry conditions, and check the path length for the range.
- Can one instrument measure nitrate and ammonia?
- Some spectral analyzers estimate several parameters from one spectrum, and combined probes exist with separate electrodes. Each parameter still needs its own verification against the laboratory.
- How do I verify a source water analyzer near the drinking water limit?
- With laboratory samples on a schedule set by the regulator and the trend, using an accredited method, and with a documented action level below the limit. The online analyzer gives the warning; the laboratory gives the compliance number.
Related topics
- Ammonia AnalyzersOnline ammonia measurement in water and wastewater: ion-selective probes, gas-sensing electrodes, and wet-chemistry analyzers, what each is good for, the interferences that decide accuracy, and the maintenance and calibration it demands.
- Dissolved Oxygen MeasurementOptical and membrane DO sensors for aeration control: how each measures, why optical has taken over, air calibration with pressure and salinity corrections, sensor placement in an aeration basin, fouling and cleaning, and the response time that limits the control loop.
- Turbidity MeasurementNephelometric turbidimeters for filter effluent and raw water: how 90-degree scattering is measured, the methods and the NTU, low-range versus high-range instruments, bubbles and the debubbler, formazin calibration and verification standards, and the compliance record the instrument exists to produce.
- Calibration ProceduresHow a utility calibrates its instruments consistently: the as-found and as-left discipline, tolerance from the loop rather than the data sheet, the reference standard and its traceability, the five-point check with hysteresis, adjusting only when needed, intervals set from history, and the procedure written once per instrument type.
- Calibration TroubleshootingWhen an instrument will not calibrate or stay calibrated: wrong or expired standards, slope and offset out of range, compensation errors, procedure mistakes, loop scaling that disagrees with the instrument, and deciding when to adjust, replace.
- EPA Rules and Their Control System ImplicationsThe federal environmental agency rules that shape what a water or wastewater control system must measure, record, and report: the surface water rules, residual and byproduct rules, groundwater and lead rules, discharge permits, and record retention.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.