Online Total Ammonia Nitrogen Measurement in Wastewater Using the Salicylic Acid method with Novel Fluidics Design
Originally published December 2023: Influents Magazine
Effluent water quality monitoring is crucial for safeguarding aquatic ecosystems
Effective effluent water quality monitoring is crucial for safeguarding aquatic ecosystems and human health. One of the key parameters in assessing water pollution is ammonia. Ammonia can be in ionized (NH4+) or unionized (NH3) forms. Total Ammonia Nitrogen (TAN) measures nitrogen in both forms which is vital for ensuring safe effluent release. Recently, advancements in fluidics and controller design have greatly improved the ease of maintenance and operation of “wet chemistry” analyzers, making them capable of up to 100 days of unattended operation and hence practical for use in wastewater treatment plants. This article explores the salicylic acid colorimetric method, its accuracy, and its specific deployment in the Thermo Scientific Orion 8010cX online ammonia analyzer for TAN detection.
The salicylic acid method is a well-established technique used for the detection of TAN in water samples
The salicylic acid method is a well-established technique used for the detection of TAN in water samples. It is based on the reaction between ammonium, salicylate and hypochlorite under alkaline conditions that forms a blue-green complex that can be measured colourimetrically. This method is widely recognized and, in the case of the subject analyzer, executed in accordance with Standard Method SM 4500-NH3 G making it a suitable choice for TAN measurement. Versus the automated phenate method in SM4500-NH3 G, the salicylic acid method uses salicylate instead of phenol to form the salicylic acid analog of indophenol blue that results in the blue-green colour of the reacted sample. The salicylate substitution also reduces the toxicity of the reaction by-products.
Comparing salicylic method with other online TAN detection methods such as gas sensing electrodes (GSEs) and Nessler’s method
When comparing salicylic method with other online TAN detection methods such as gas sensing electrodes (GSEs) and Nessler’s method, the salicylic acid method has a lower limit of ammonia detection and has been shown to have greater accuracy . The salicylic acid method also avoids toxic chemicals such as mercury used in Nessler’s method. Also, since the measurement involves reacting the sample directly with reagents and measuring light absorbance through the sample, it does not require the use of a gas permeable membrane to separate the sample from an internal electrolyte. Gas membranes are known to be a regular maintenance item due to fouling or wear.
Both the GSE method and salicylic acid method are suitable for use in wastewater samples and can tolerate a range of background matrices. Using a bypass-style pre-filter ahead of the analyzer can further reduce interferences and decrease maintenance by reducing contaminants in the sample. Pre-filtering is unlikely though to substantially reduce GSE membrane replacement frequency. Table 1 illustrates sample parameter threshold guidelines that can often be exceeded with an analyzer using the salicylic acid method equipped with a 200 µm bypass prefilter.
Table 1 – Threshold Sample Parameters with Salicylic Method and Pre-filter
| Parameter | Range/ Thresholds |
| Total Dissolved Solids | ≤850 mg/ L |
| Total Suspended Solids | ≤250 mg/ L |
| pH | 4 to 9 |
| Color (platinum cobaltic method) | < 180 |
| Ca2+, Mg2+, PO42-, NO3–* | 137.5 mg/ L |
| Cl–* | 27,500 mg/ L |
| SO42-* | 5,500 mg/ L |
| *for measurement range 0.02 to 2 mg/ L; thresholds increase at higher ranges | |
Syringe Pump versus Peristaltic Pump
Accurate dosing of reagents is important for measurement accuracy. Typically, in wet chemistry analyzers, multiple pumps are employed to deliver reagents, sample, standards or waste at various stages of analysis. Peristaltic pumps are common in analytical instrumentation and have the advantage that the chemicals being dispensed do not contact pump internals. This is since the pump acts directly on flexible tubing thereby eliminating exposure to chemicals. Because a series of rollers alternate to compress the tubing, a pulsating effect on the pump output results. Syringe pumps are another option for use in analytical instruments. Although more limited in terms of flowrate, the syringe pump’s pumping action is especially smooth, and they are generally considered more accurate than peristaltic pumps. Since the fluid to be pumped is drawn inside the pump, special care must be taken with respect to chemical exposure.
The subject analyzer uses a single syringe pump in conjunction with an Electronic Rotary Valve (ERV) to improve dosing accuracy and reduce overall maintenance. A syringe pump ensures precise and consistent fluid movement while the ERV cooperates with the pump to connect it to a required port. This allows the multiple different fluids in the analyzer to be drawn from their source and then delivered to a specific port all with a single pump (see Figure 1). The result is a simplified fluidics arrangement that reduces overall maintenance time and cost when compared to a multiple pump configuration. In addition, this fluidics design expands the range of the salicylic acid method using dilution; as a result, measurements from 0.02 to 500 mg/ L TAN concentration are possible. In field testing, the online analyzer has demonstrated strong correlation with both the well-known Nessler lab method and the salicylate lab method with R2 values of 0.9922 and 0.9992 respectively.
The syringe pump also facilitates a pull-in/ push-back actuation that reduces reagent consumption and helps keep the system clean. Based on a 2-hour sampling frequency, this results in only 5 liters of waste generation per month and allows the analyzer to run for 100 days on a single set of reagents. To prevent internal contamination of the syringe pump, a buffer ring is used that maintains a volume of deionized (DI) water downstream of the pump. This buffer ensures that chemicals in the sample or in the reagents do not enter the pump. A simple two-way valve between the pump and a DI water source allows the buffer to be replenished.
Figure 1 – Fluidics Circuit with Syringe Pump and Electronic Rotary Valve
Reduce Operator Burden and Extend Maintenance Intervals
To further reduce operator burden and extend maintenance intervals, advances in analyzer electronics and software improve automation and control capabilities. Greater processing power and memory in the controls allows manufacturers to improve analyzers’ ability to run unattended for extended periods. For example, self-cleaning routines can now be used to substantially reduce manual cleaning. In addition, automatic calibration that puts the analyzer back on-line after completion is available to reduce manual intervention and keep the analyzer running reliably. Expanded controls capability also allows analyzers to use additional sensing to monitor status more thoroughly – flow sensors, for example, can be added to ensure the presence of a sample in analyzer tubing. Feeding information such as this back to the controller makes it possible to take corrective steps or to inform the operator of a problem. Richer user interfaces are also made possible with more computing power. Now with industrial touchscreens, user navigation can be provided in a familiar manner to that one would see on a mobile device or computer. They also improve multiple language support – see Figure 2.
Figure 2 – Industrial Touchscreen Interface
Safer Effluent Release – reducing the detrimental impact on aquatic life
The salicylic acid colourimetric method incorporated in an analyzer with precise fluidics and advanced controls improves the practicality of “wet chemistry” analyzers for use in wastewater facilities. Furthermore, this online method provides a robust and accurate TAN measurement for regulatory compliance monitoring. By taking a fresh look at analyzer fluidics design, and utilizing greater computing capacity, analyzers can operate unattended for up to 100 days. Moreover, these enhancements ensure safer effluent release, reducing the detrimental impact on aquatic life.
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