Design and Test of a Lower-Cost Water-Quality Sensor for Nitrate

Nitrate is a water pollutant with significant environmental and health effects. Current field-deployable nitrate sensors are expensive, limiting monitoring and management efforts. We designed a lower-cost optical nitrate sensor suitable for field use. We built a prototype unit for less than $800 and tested the unit in a laboratory setting in both distilled and river water with different levels of added nitrate. Our calibration shows that the device effectively senses nitrate in the 0 to 100 mg/L range we chose for relevance to real-world applications. We developed a method for temperature correction of the optoelectronics and applied a third-order polynomial fit for nitrate response to reach similar performance in clean water to commercial sensors, with an RMSE of 0.67 mg/L and R² > 0.99 on the fully averaged data set, and a limit of detection of 1.73 mg/L on 20 min averaged data. We identified a need for an improved reading protocol and further calibration work to address challenges with matrix effects in dirty water. We pointed out the key issues for future production of the sensor, including unit-to-unit variability and interference from other compounds. This novel sensor design provides promise as a low-cost but effective nitrate water quality sensor.