Electrochemical nitrite sensing for urine nitrification

Abstract

Sensing nitrite in-situ in wastewater treatment processes could greatly simplify process control, especially during treatment of high-strength nitrogen wastewaters such as digester supernatant or, as in our case, urine. The two technologies available today, i.e. an on-line nitrite analyzer and a spectrophotometric sensor, have strong limitations such as sample preparation, cost of ownership and strong interferences. A promising alternative is the amperometric measurement of nitrite, which we assessed in this study. We investigated the sensor in a urine nitrification reactor and in ex-situ experiments. Based on theoretical calculations as well as a practical approach, we determined that the critical nitrite concentrations for nitrite oxidizing bacteria lie between 12 and 30 mg_N/L at pH 6 to 6.8. Consequently, we decided that the sensor should be able to reliably measure concentrations up to 50 mg_N/L, which is about double the value of the critical nitrite concentration. We found that the influences of various ambient conditions, such as temperature, pH, electric conductivity and aeration rate, in the ranges expected in urine nitrification systems, are negligible. For low nitrite concentrations, as expected in municipal wastewater treatment, the tested amperometric nitrite sensor was not sufficiently sensitive. Nevertheless, the sensor delivered reliable measurements for nitrite concentrations of 5-50 mg_N/L or higher. This means that the amperometric nitrite sensor allows detection of critical nitrite concentrations without difficulty in high-strength nitrogen conversion processes, such as the nitrification of human urine.

Keywords: Amperometric sensor; Continuous measurement; Critical nitrite concentration; Electrochemical measurement; In-situ measurement; Nitrite measurement.

Graphical abstract

Fig. 1

Scheme of the electrochemical nitrite measurement.

Fig. 2

Picture of the large (A) and the small (B) amperometric nitrite sensor.

Fig. 3

The optimal nitrite concentration as a function of pH based on the theoretical and practical approach. The theoretical calculations were done with an average ionic strength of 0.16 mol/L and an average activity coefficient of 0.75 for a temperature of 25 °C.

Fig. 4

Data in the nitrite range of 0 – 50 mg_N/L with the linear fit and the corresponding 95%-confidence interval. (A) Data from the nitrification reactor during April 2018 with the large sensor. (B) Data from the nitrification reactor during October 2018 with the small sensor.

Fig. 5

Residuals of the nitrite concentration of the large (A) and the small (B) sensor for each month.

Fig. 6

Relative prediction residuals of the nitrite concentration in dependency of the nitrite concentration of the large sensor for each month showing the range of (A) 0–50 mg_N/L and (B) 5 to 50 mg_N/L.

Fig. 7

Relative prediction residuals of the nitrite concentration in dependency of the nitrite concentration of the small sensor for each month showing the range of (A) 0–50 mg_N/L and (B) 5 to 50 mg_N/L.

Fig. 8

(A) Linear curve and the corresponding 95%-confidence interval describing the current density in dependency of nitrite, neglecting temperature and pH (Model B, Equation (4)). (B) Comparison of the residuals of the model best describing the correlation (Model A, Equation (3): including nitrite, temperature and pH) and the linear model only depending on nitrite (Model B). The results were obtained with the large sensor.

Fig. 9

Samples of the aeration dependency assessment with the fitted linear curve (excluding the data at ∼0 Nm³/h) with the corresponding 95%-confidence interval. The results were obtained with the large sensor.

Fig. 10

Current density measured at electric conductivities between 0 and 50 mS/cm, for artificial nitrite solutions at 25 mg_N/L and 50 mg_N/L. The results were obtained with the small sensor.

Fig. 11

Results from the ex-situ drift analysis with the large sensor. The data points were collected after the sensor was cleaned.

Fig. 12

Results from the in-situ drift analysis with the large sensor (A) and the small sensor (B) over the nitrite concentration range of 0 – 50 mg_N/L.

Fig. 13

(A) Comparison of the ex-situ and in-situ calibration curves from November 2018. (B) Trend of the sensor signal of an artificial nitrite solution at 25 mg_N/L, the pH and the dissolved oxygen concentration. The data were collected with the small sensor.