In-Situ Measurement of Ammonium in Wastewater Using a Tilted Fiber Grating Sensor

Ammonia nitrogen, which consists of ammonium and free ammonia, is one of the key parameters to evaluate the water quality and it is an important analyte in the biotechnology and agriculture. However, ammonia nitrogen exists mainly as ammonium in most waters as the pH is below 9. In this work, we propose a tilted fiber Bragg grating (TFBG) sensor that is surface-functionalized with a thin film of polyvinylidene fluoride bromophenol blue (PVDF-BPB) for ammonium detection in wastewater. The surface structure of the PVDF-BPB film was modified by reacting with ammonium, resulting in a correlative modification of extinction ratio of TFBG cladding modes. The reaction strength between the ammonium and the bromophenol blue was proportional to the concentration of ammonium. By calculating the integral area of all cladding modes versus reaction time, the concentration of ammonium can be precisely quantified. Experimental results showed that the proposed sensor could provide a linear response of ammonium detection over a concentration range of 0.1 - 10 mg/L within 5 minutes. Finally, we demonstrate the feasibility of the proposed sensor for measurement of ammonium in wastewater, using our probe gave results comparable to a commercial ammonium probe.