Optical fiber sensors for low cost water quality monitoring

Ensure availability and sustainable management of water and sanitation for all is one of the Sustainable Development Goals settled by the United Nations for the 2030.?? However, today, billions of people in the world still live without safe drinking water and millions die every year due to waterborne diseases. People are dying due to contaminated and polluted water. For this reason, a new, real-time, effective, and low-cost device to detect various pollution could help improving the quality of life of a huge part of humanity, not to mention the reduction in deaths. Nowadays the water quality monitoring is performed without a real global "water quality standard": most water quality is evaluated at a local or national level. The World Health Organization (WHO) has released guidelines for the drinking water quality, but it is not mandatory to follow guidelines and it is under the responsibility of the local authority to ensure safe water. This thesis investigates the possibility of applying optical fiber-based sensors for real time, continuous and on field monitoring of water for the early detection of the quality variation. The focus of the thesis is the development of a very low-cost device able to help in particular the developing countries, in which the water quality problem is the worst. The choice of working with optical fibers is due to their advantages, like small dimensions, robustness to environment, electromagnetic immunity, and the possibility to implement real time and remote sensing. Thanks to those advantages, optical fibers are widely used both in scientific and industrial applications. Water quality monitoring can take advantage of these qualities too, overcoming the limitations of traditional measurement approaches, which require in-situ sampling, transportation to labs, and finally the quantification through complex instruments. And this requires hours to days and specialized technicians able to perform tests and read the results. The goal is to build a low cost and small device, that allows the real-time and ubiquitous monitoring of the water quality, and without the need of trained personnel for the reading of the results. This is done implementing an optical fiber-based refractometer that exploits the surface plasmon resonance (SPR) to measure the variation of the refractive index, instantly warning any change occurred from the normal situation. ?? The surface plasmon is an electromagnetic surface wave that propagates in a direction parallel to the interface between the metal and the outer dielectric, being very sensitive to any change that can occur at the external medium (water in this application).?? SPR sensors are known for their very high sensitivity, which allows the detection of part per million (ppm) contamination in the water. The SPR sensor is complemented by an all-fiber temperature sensor based on fiber Bragg gratings to compensate for the temperature dependence. The results shows that an accurate refractive index sensing can be done also with low-cost devices such as a LED and a photodiode connected to a microcontroller. This keeps also very compact the device and allows also to build a solution usable at the end side of the water distribution system, ideally on every tap. The actual device is a prototype implemented for preliminary validations of the results.?? Further studies must be done to improve the LED’s driving circuit to integrate it removing the external breadboard and to design a proper shape of the device to mount it on taps or pipes.