MICROBIAL ELECTROCHEMICAL SENSORS FOR FRESHWATER AND WASTEWATER MONITORING

Freshwater is essential for socioeconomic development, health of the population and equilibrium of the ecosystem. However, pollution generated by human activities often degrade the quality of this vital resource. In order to address this problem with effective management strategies, continuous assessment of freshwater and wastewater status is necessary. Microbial electrochemical systems (MESs) are attracting increasing attention as sensing tools, due to their low-cost, sustainability, portability, generation of continuous on-line signal and low involvement of operators. In this dissertation, the implementation of MESs as sensors in two different environmental contexts is discussed: wastewater treatment monitoring and agriculture-related pollutants detection. Different prototypes were designed according to their specific application and their detection capabilities were investigated. Regarding the monitoring of wastewater treatment process, floating Microbial Fuel Cells (MFCs) were tested as sensors for organic matter, a parameter that is currently measured with the labour-intensive and time-consuming BOD5 test. The first prototype consisted of a floating frame holding two carbon cloth electrodes separated by a polypropylene felt and a clay layer. The biosensor was tested in the lab using real wastewater as both electrolyte and inoculum, and a calibration curve was obtained. However, when the device was tested in the plant of Carimate (CO), a strong influence of light irradiation on the signal was noticed. During a month of operations, a correspondence between the peaks of voltage and of the peaks of organic load was present, but the signal was delayed of about two days. To further understand the influence of different environmental factors on the current signal, a new device was conceived and compared with the previous configuration. This time the experimentation was carried out at the plant of Bresso Niguarda (MI). The new setup consisted in a cylindrical terracotta separator sealed at one end and held perpendicularly to the wastewater surface by a plastic floater. The carbon cloth anode was placed outside the cylinder, while a cathode of the same material was located inside. Light irradiation, temperature, sCOD and nitrates were continuously monitored for about 20 days. An automatic sampler was built to obtain wastewater samples every two hour every day. The correlation between the physical-chemical parameters and the cell voltages was quite poor, especially for the floating type, which was often subjected to the inversion of the electrodes potential. This was probably due to oxygen diffusion in the first layer of wastewater. The microbial analysis of the biofilms confirmed this aspect, as aerobic strains (e.g. Nocardicaceae) were found on the anodes, and electrogenic bacteria usually present on anodes were found on the cathodes (i.e. Geobacteriaceae). To monitor agriculture-related pollutants (i.e. herbicides), amperometric biosensors based on the inhibition of cyanobacterial photocurrent were studied. Many herbicide compounds are harmful to humans and the environment, and they are currently measured with classic analytic techniques (e.g. HPLC, GC-MS, etc.), which are expensive and time consuming. To build the first prototype, Anabaena variabilis cells were entrapped on a carbon felt electrodes using an alginate hydrogel, and p-benzoquinone was used as electron shuttle to sustain the electron transfer. With this device it was possible to obtain concentration-current calibration curves for two commonly used herbicides (i.e. diuron and atrazine), and the obtained linear range was suitable for environmental analysis. However, to build a long lasting device, a mediatorless configuration is preferable, as the redox mediator is cytotoxic and can undergo photodegradation. For this reason, a new biosensor was created: a paper-based electrode coated with carbon nanotube paint and a titanium nanolayer was used as substrate for the formation of a Synechocystis wt. biofilm. Whit this configuration, only a presence/absence inhibition signal could be obtained for atrazine and diuron, while the herbicide paraquat temporarily enhanced the electron transfer due to its redox mediator capabilities. Nevertheless, this biosensor was able to maintain its sensitivity even after it was kept in the fridge for 22 days, proving its potential as long-lasting device which can be easily stored or shipped after preparation. These results demonstrated that microbial electrochemical sensors are a promising technology for sensing applications, with a great potential for the creation of a smart, diffuse grid of low-cost sensors for the continuous monitoring of water quality. However, further improvements are needed in order to reduce the response time, improve the sensitivity and discern between the influence of different environmental factors on the signal.