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 processes with a case study on ammonium sensor and ion-selective electrode 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⁻¹ d⁻¹). Scanning electron microscopy 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 a thick coating with different agglomerates and crevasses, which reveal damage 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. This 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 using a saturation model. However, the relative composition remains stable: 84% for VSS/TSS and 20% for Fe/iSS, on average.

中文翻译：

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活性污泥工艺中易结垢的铵传感器的功能行为和微观分析

结垢是与废水中部署的所有传感器仪器相关的问题，会导致传感器元件的灵敏度和可重复性下降，因此需要频繁地重新校准。本文以铵传感器和离子选择电极技术为例，对活性污泥工艺中结垢的发展进行了综合分析。发现电极的响应时间受结垢的影响最大。通过使用扩散模型分析阶跃变化实验，在结垢一周后，响应时间随时间呈指数增长。传感器的性能也会受到测量精度的影响，显示出结垢传感器的负漂移（-0.11 mg NH ₄ ⁺ l ^-1 d^-1）。在新的和使用过的传感器膜上进行了扫描电子显微镜分析和能量色散X射线光谱元素标测，以研究不可逆的结垢成分和形态。结垢表现为带有不同附聚物和裂缝的厚涂层，这表明对膜的PTFE保护层有损坏。Fe，P，Ca，Mn，S，K和Cl是检测到的主要元素，按降序排列。结垢层中高含量的Fe源于在工厂的初级处理中添加铁盐，这成为传感器无机结垢的主要因素。如饱和模型所述，该研究还定量了可逆结垢层中总悬浮固体（TSS），挥发性悬浮固体（VSS）和总Fe随时间的增加。然而，