Real-time in Situ Monitoring of Water Systems Using Electrochemical Sensors

With the increasing population, rapid urbanization, ecological environment deterioration, and serious water pollution, the fresh water quality and quantity becomes crucial to human beings’ health and wellbeing in determining the health of individuals and whole communities. Wastewater quality is a critical factor to the design and operation of the wastewater treatment plants (WWTPs). Short-term and long-term shocks (e.g. organic compounds and heavy metals) in wastewater disturb the stability of WWTPs. Real-time wastewater shock sensors are critical to provide effective precaution strategies and minimize shock impacts. Microbial fuel cell sensors have been developed for biological oxygen demand (BOD), chemical oxygen demand (COD), organic substances, and toxins in wastewater. However, existing MFC sensors still utilize traditional MFC configurations (e.g. tubular bioreactor, single chamber, and cube shape which poses difficulties for real-time shock monitoring. These MFC sensors have the volume of 20–150 mL with inlets and outlets, which are the operational systems by themselves, and make it difficult for direct installation on wastewater facilities. In addition, the voltage output of MFC sensors is closely associated with open circuit potential (OCP) and inner resistance (Rin), and keeping stable Rin is critical for reducing the possibility of fault signals of MFC sensors. But large volume (normally 20–150 mL) of wastewater contained the existing MFC sensors may cause unstable Rin, and increase the possibility of fault signals. Moreover, these MFC sensors need at least 1–2 weeks to acclimate electrogenic bacteria, meaning that they must be inoculated long time before the occurrence of shocks, which is unrealistic for monitoring unexpected wastewater shocks. Therefore, one novel small-sized MFC with good sensitivity to the shocks should be developed. The mm-scale flat microliter membrane-based microbial fuel cell (in Chapter 2) and paper-based multi-anode microbial fuel cell (PMMFC) (in Chapter 3) exhibited short acclimation time (2-3 hour), good sensitivity to the shocks, and high mechanic strength by reducing the internal resistance and improving the output voltage respectively. MFCs have a good ability to provide effective precaution strategies and minimize shock impacts for the wastewater. However, it is difficult for the MFCs to detect specific water quality parameters, such as temperature, conductivity, DO, pH, Cl- etc. Especially, MFCs cannot be utilized as on-line in-situ sensors to the drinking water, which would cause further contaminate to the water. Therefore, there is an emerging need to develop low-cost easily-deployable and durable small-sized water quality sensors with high sensitivity. Micro-scale glass pipette electrodes and micro-scale electrical sensors fabricated by photolithography with chemical vapor deposition (termed as PCVD) are main technologies to monitor water quality parameters. However, the fragile glass pipette structure, time-consuming fabrication, and the need for bulky micromanipulator to position micro-electrodes posed severe problems for field applications. And the PCVD technology possess high fabrication cost, complicated fabrication process, strict fabrication condition, which is difficult for direct deployment in water/wastewater systems. Novel micro-electrode array (MEA) fabricated using inkjet printing technology (IPT) was developed as a water quality sensing technology in the PhD research. By printing multiple mm-sized electrodes on a single flexible film, the MEA possessed distinct advantages over traditional “single-point” probes: small sensor size, compact structure, multiple-parameter measurement in a sampling, easy fabrication and deployment, longterm stability, and ultralow low cost (in Chapter 4). Water quantity in the soil monitoring is another issue we consider in my PhD research. Limited freshwater resources and deteriorating environmental quality have raised global awareness for sustainable irrigation technology and spatially distributed drought monitoring. Soil moisture (water quantity in the soil) can be ex-situ measured in labs, including removing a soil sample from the field, drying it for 24 h in an oven at a temperature of 105°C and weighing it before and after drying. However, this traditional measurement has major issues of soil sample disruption during sample transport and storage, being a non-continuous measurement of soil moisture, and being nonrepresentative of genuine soil moisture in field condition. Remote sensing has also been used for monitoring surface soil moisture. Remote sensing utilizes the coincident measurements of the surface emission and backscatter to estimate the soil moisture. However, the detection depth has been only limited to top soil (depth 5 cm) and the frequent and costly maintenance has been a severe obstacle for accurate soil moisture monitoring along soil depth. A mm-sized flat SMS using gold CD etching approach was developed in this Chapter (Chapter 5). MSMS possesses distinct advantages including easy fabrication, mm-sized electrode for enhanced accuracy, durable antiscratch sensor surface for long-term monitoring and low cost (<$1/sensor).