Functional behaviour and microscopic analysis of ammonium sensors subject to fouling in activated sludge processes
Fouling is an issue associated with all sensor instrumentation deployed in wastewater that causes a loss in sensitivity and reproducibility of the sensor elements, thus requiring frequent re-calibration. This paper presents a comprehensive analysis of the fouling development in activated sludge process with a case study on ammonium sensors and Ion-Selective Electrodes technology. The response time of the electrodes is found to be the most impacted by fouling. By analysing step-change experiments with a diffusion model, after one week of fouling the response time is demonstrated to increase exponentially with time. The performance of the sensor is also affected in terms of measurement accuracy, showing a negative drift of the fouled sensor (-0.11 mg NH4+ l-1d-1). Scanning electron microscope analysis and energy dispersive x-ray spectroscopy elemental mapping were performed over new and used sensor membranes to study the irreversible fouling composition and morphology. Fouling appears as thick coating with different agglomerates and crevasses, which reveal damages on the PTFE protective layer of the membrane. Fe, P, Ca, Mn, S, K and Cl were the main elements detected, in decreasing order. The high content of Fe in the fouling layer originates from the addition of ferric salts to the primary treatment of the plant, which becomes a major contributor to the inorganic fouling of the sensor. The study also quantifies the increase in Total Suspended Solids (TSS), Volatile Suspended Solids (VSS), and total Fe in the reversible fouling layer over time as described by a saturation model. However, the relative composition remains stable: 84% of VSS/TSS and 20% of Fe/iSS, on average.