Inorganic fouling at quartz:water interfaces in ultraviolet photoreactors

Fouling of quartz jackets around mercury arc lamps represents a performance limitation in ultraviolet (UV) photochemical reactors. The purpose of the present study was to investigate mechanisms of inorganic fouling at quartz:water interfaces in UV photoreactors. Approaches to the problem include conducting pilot-scale experiments in the laboratory and in the field, numerical simulations of fouling processes, and exploration of physico/chemical treatment to mitigate the quartz fouling problems. Fouling behavior was examined at different observation scales with analytical tools which included UV transmittance measurements, wet chemistry, atomic absorption spectrometry, surface/solid-phase analytical methods, scanning electron microscopy, and microscopic image analyses. Experimental results showed that a broad spectrum of metals that are commonly found in wastewaters was identified in fouling materials. Temporal analyses of fouling materials indicated that fouling was a linear process with time. An increasing trend in longitudinal distributions of fouling material supported the hypothesis that thermally-induced precipitation plays a significant role in inorganic fouling of quartz under conditions of moderate precipitation rate. A second mechanism, impaction of pre-existing particles on quartz surfaces, resulted in heterogeneity in spatial accumulation around quartz jackets. Time-course UV intensity measurements indicated that iron and aluminum-based treatments (e.g., for phosphorous removal) accelerated fouling processes significantly. Fluid shear also played a role in inorganic fouling in that it influenced the agglomeration of coagulated/precipitated particles. Illumination of UV lamps was found to diminish organic accumulation and promote inorganic fouling. A numerical model that accounts mass, heat, and momentum transport characteristics in the immediate vicinity of the quartz:water interface addressed the increasing mixing energy due to velocity gradient inside the momentum boundary layer as approach velocity increases. Thermal effects were illustrated by a temperature field and an increasing saturation excess in the direction of flow. The model also demonstrated that accelerated fouling processes could result in local pH depression (proton production) upon precipitate formation. Altogether, the mathematical model was found to be capable of explaining fouling behavior observed in the pilot-scale experiments qualitatively and providing insights into fouling mechanisms. Several physico/chemical strategies were reviewed for reducing quartz fouling. Methods of UV system treatments are recommended based on the results and findings of this research