The development and evaluation of a microstill with conductance detection for low level ammonia monitoring in...

Monitoring low levels of ammonia in chloraminated water can be challenging but is important for effectively managing potable water disinfection. The lower the concentration of free ammonia that can be determined, the tighter the control at the disinfection point, which supports better maintenance of chloramine residuals in the distribution system. In this study a micro-distillation technique was used to selectively separate ammonia into a boric acid solution allowing determination by conductance (i.e., micro-distillation and conductance measurement instrument - Micro-DCMI). The optimised technique had a linear calibration range of 0.01-60 mg NH₃ L^-1 with a limit of detection (LOD) of 0.014 mg L^-1 and limit of quantification (LOQ) of 0.045 mg L^-1. With the optimum operational parameters of 80 °C distillation temperature, 0.3 L min^-1 air flow rate, 0.1 mol L^-1 boric acid concentration and 8 min pre-concentration time, a sample throughput of 7 samples per hour was achieved. A laboratory comparison between Micro-DCMI and the standard ammonia selective electrode method was conducted on water samples obtained from a chloraminated drinking water distribution system. The results showed that the ratios of the free ammonia values determined by Micro-DCMI and an ammonia selective electrode were between 0.82 and 1.2 for 11 water samples. A custom-built automated analytical system was evaluated at a water treatment plant in Australia and the results compared favourably with the in plant online ammonia ion selective analyser. This study indicated that the Micro-DCMI method was a simple, robust and low cost online monitoring system suitable for determining low concentration ammonia to manage chloramination.