Spectrophotometric Determination of Standard Parameters in Wastewater

More efficient spectrophotometric analysis of pollutant parameters in wastewater

For many years, UV/Vis spectroscopy has been an established standard method with standardized procedural guidelines for the qualitative and quantitative analysis of a variety of parameters of wastewater such as nitrate (NO₃), nitrite (NO₂), ammonium (NH₄), phosphate ions (PO₄), and total phosphorus (P_tot) content. This article shows how you can make these standard analyses even more efficient while retaining full compliance with standards and regulations.

As supplies of drinking water and process water become increasingly scarce, a comprehensive qualitative and quantitative analysis of wastewater is essential for increasing quality of life and for improving public health. There are numerous national and international standards that, among other things, govern treatment processes or define limits regarding the content of certain harmful substances. There are also strict regulations pertaining to the analysis of individual parameters. These standardized measurements include detection of total phosphorus as well as phosphate, ammonium, nitrate, and nitrite ions in industrial and household wastewater. The first step in the purification of wastewater in treatment plants is the removal of solids using physical methods. In the biological stage that follows, bacteria and yeast control oxidation processes. These processes entail oxidation of the primarily organically bound nitrogen (in the form of proteins, nucleic acids, and urea) to NO₃ and NO₂. The third stage – the chemical treatment process – causes the precipitation of PO₄.

The spectrophotometric analysis of these compounds (known as UV/Vis spectroscopy) is governed by the standard German process for water, wastewater, and sludge analysis (DIN 38405) as well as a variety of other standards. These provide detailed instructions for analysis, also with relation to sample preparation, interfering ions, and detection limits. There are similar guidelines and standards in many countries.

Although these measurements have been part of the standard repertoire of wastewater analysis methods for many years, there is still considerable potential for optimization in the analysis process. New challenges such as the growing number of samples or stricter requirements regarding analysis reliability necessitate greater efficiency in analysis solutions. Through the use of modern, highly sensitive spectrophotometers, software that relieves users of laborious tasks, and automation solutions for high sample throughputs, laboratories today are capable of saving much time and effort in the analysis process. The example analysis provided below, of total phosphorus, phosphate, ammonium, nitrate, and nitrite ions in wastewater, demonstrates the specific form this can take.

Fast measurements, reliable results

The samples were prepared in accordance with relevant standards and measured within the following times:

• NH₄: 1–3 h
• NO₃: 1 h
• NO₂: 20 min
• PO₄ / P_tot: 10–30 min

The baseline, blank, and sample values were measured one after the other. A baseline measurement was taken against air. For the blank sample, distilled water was used and treated identically to the wastewater samples. The absorbance of the blank sample was automatically subtracted from the subsequent samples. To determine the concentration of NH₄, NO₃, NO₂, PO₄, and P_tot, the first step was to plot the calibration curve based on the referenced standards, for which ten reference solutions of each reference substance were prepared with different concentrations according to the literature. Table 2 shows the different production conditions of the calibration standards and the preparation of the calibration curve for each parameter according to the referenced standards.

Table 2: Conditions for the creation of calibration curves according to standards

The calibration curve for NO₃ is shown as an example in Figure 1. The absorbance of the blank measurement using distilled water is automatically deducted from the absorbance values of the measured calibration standards by the ASpect UV software. The calculated absorbance values of the calibration standard solutions are specified as a function (y-axis) of the concentrations of the standard solutions (x-axis). The latter is expressed in milligrams per liter (mg/L). The relationship between absorbance and concentration proves to be linear in the specified concentration range. The ASpect UV software automatically calculates the slope and R² value of the calibration curve. The concentration of the unknown samples is calculated automatically in relation to the measured absorbance. The concentration is also automatically calculated if information about the dilution and weight of the sample are provided.  This eliminates the risk of calculation errors and allows the user to save valuable laboratory time.

When analyzing wastewater samples, it is important to be aware that the effect of other substances on absorbance must be determined before the measurements are performed, as interfering ions (NO₃ and NO₂: chloride; PO₄: arsenate) can affect the test results significantly. If it can be demonstrated that interfering ions are not present, the wastewater samples are treated with the detection reagents described in the standards. The absorbance of the wastewater samples is then measured and the concentration is determined using the calibration curve. If the selected parameters are measured frequently, the use of the Quickstart function is recommended in order to store the methods and calibration curves (see Figure 2) for later use. This enables rapid and precise measurement of additional samples. Once the software has been opened, the method is launched and the results are automatically recorded and analyzed in just four clicks (the sample sequence, including the number of samples and sample names, can also be modified).

Five water samples were collected from the treatment plant after purification. The results are shown in Table 3. Continuous monitoring of the concentration of various parameters makes it possible to study the wastewater purification process in treatment plants. The measurement data are collected and saved so that public authorities can verify that the limits have been adhered to. It is also possible to continuously improve and optimize the purification process.

Table 3: Concentration of the selected parameters exemplified for five wastewater samples from a sewage treatment plant

Ammonium may be present in different species in wastewater samples, depending on the pH value of the solvent (as ammonium ions (NH₄⁺), ammonia (NH₃), or ammonium hydroxide (NH₄OH)). To detect all possible ammonium species and thus obtain reliable results regardless of the origin and pH value of the wastewater, heavily alkaline conditions are necessary. Conversely, to detect NO₃ and NO₂ ions, highly acidic solutions are needed to catalyze the formation of the colored target compounds, which can be detected at their characteristic wavelengths. Given the extreme pH values of the solvents, handling the samples is a complicated process. Also, the use of large quantities of solvents should be avoided to limit the amount of waste that requires special disposal measures. This is why measurements should ideally be performed in 10 mm cuvettes, though the standards also permit the use of 40 mm or 50 mm cuvettes. Another advantage of using 10 mm cuvettes rather than 40 mm or 50 mm cuvettes is the increased capacity for sample automation. While 40 mm or 50 mm cuvettes only fit individually or can only be positioned in a 6-fold cuvette changer, the 10 mm cuvettes can also be used in a cuvette carousel with 15 positions or an 8-fold cuvette changer. Additional steps can be automated using the sipper system and APG Autosampler, which is also in Analytik Jena’s accessory portfolio. The SPECORD PLUS series UV/Vis spectrophotometers are especially suitable for the analysis of wastewater samples as they feature a special cuvette position for turbid samples. The cuvette can be placed closely to the detector, which reduces the amount of stray light. Standardized test kits can be used for water analysis parameters such as SAC, turbidity, COD, BOD, sulfate, and many more with the round cuvette holder. Low-concentration samples can be analyzed using the 100 mm cell holder to magnify layer thicknesses with ease.