Laboratory investigation of inorganic fouling of low pressure UV disinfection lamps

Iron(III) (mainly dosed as FeCl3) is one of the main inorganic constituents associated with the fouling of UV lamps during disinfection of wastewater. A low pressure mercury lamp UV system was operated under laboratory conditions to assess the effects of heat and UV light on fouling. Iron(III) was dosed at different concentrations and fouling was monitored by measuring the UV irradiance. The potential for ferric phosphate fouling and the effects of organics were also studied. Results showed that fouling did not occur in recirculating systems over 12 days of observation. However, in flow-through systems, the extent and rate of fouling depended on the solution Fe concentration. Furthermore, addition of phosphorus increased the fouling rate. Bulk precipitation appeared to be significant at iron(III) dosing over 3 mg/L. Theoretical predictions of an equilibrium model (MINEQL+) showed similar trends with measured concentrations of soluble Fe and P from batch tests. However, model predictions of temperature effects alone could not account for the observed solids deposition on the quartz sleeve. Bulk precipitation, followed by sedimentation, was considered to be the dominant mechanism at high iron (III) concentrations