Performance evaluation of complimentary split-ring resonator (CSRR) in detecting ammoniacal nitrogen (NH3-N) for water quality applications / Muhammad Faris Sapuri

Ammoniacal Nitrogen (NH3-N) is one of the major contents found in Malaysia's polluted water sources. This polluted water does not only affect the human who consumes the water, but it also causes harm to flora and fauna. Furthermore, according to Malaysia’s National Water Quality Standards, the presence of 0.9 mg/L NH3-N and above requires extensive treatment. Therefore, it is very important to have a sensor that can detect the presence of NH3-N, especially at low concentrations. Compared to other types of sensors, microwave sensors are favourable for their low cost, design simplicity, and reusable capabilities. The complementary split-ring resonator (CSRR) has been chosen because it exhibits higher sensitivity compared to other types of microwave sensors. The main objective of this research work is to evaluate the performance of CSRR in detecting NH3-N, especially at low concentrations. For that, five CSRRs were designed, and each of them resonates from 1 GHz up to 5 GHz, respectively. They were designed and simulated on an FR-4 substrate by using the CST Simulation software. The chemical solutions that were used in this work to load the CSRR are ethanol and NH3-N, where the concentration of the solutions varies. The plexiglass container was used as a sample holder to prevent the sample from having direct contact with the metal ring of CSRR. The S-parameters responses of each CSRR unloaded and loaded with the samples were measured. The detection of the sample was based on the shift of the resonant frequency. In order to validate the detection of the NH3-N by using the CSRR, the complex permittivity of the NH3-N needs to be determined from the measured Sparameters. Therefore, a mathematical model of the CSRRs was developed by using the Debye relaxation model, which defines the relationship between the complex permittivity of any liquid sample and the measured resonant frequency and the Q-factor of the CSRRs. Then, the complex permittivity of the NH3-N determined from the model was compared with the measured value obtained from the dielectric probe. Finally, the sensitivity of the CSRRs was analysed. Measurement results show that each CSRR shows a shift in resonant frequency when it is loaded with NH3-N. However, at lower concentrations of NH3-N, the detection performance of CSRR varies. The detectable lowest concentration of NH3-N is 25 mg/l by using 3 GHz CSRR. Furthermore, the 3 GHz CSRR gives the closest value of complex permittivity, which implies that the best working frequency for the detection of NH3-N using CSRR is 3 GHz. The sensitivity of 3 GHz CSRR is 0.26, which is comparable with the other microwave sensors already presented in the literature. Nevertheless, further investigations are required especially in improving the sensitivity of CSRR sensors for NH3-N detection.