FOULING OF UV LAMPS WITH VARYING INFLUENT WATER QUALITY
Abstract:As part of an on-going study by our group into fouling phenomena of UV lamps, the fouling of UV lamps was examined at the St. Eustache, Que., wastewater treatment facility. This plant provides biological treatment to mainly domestic wastewaters via biofilters. Alum is used in the summer months only for phosphorous removal. A pilot plant study was performed there in order to compare the composition and quantity of foulant material formed with biologically-treated versus physicochemically-treated effluents. The Trojan UV 3000 pilot plant consisted of 3 modules with two lamps each. The modules were separated with baffles enabling each channel to work independently of the others. Only one module irradiated UV light, while the other two served as controls: one was off (“OFF”), and one emitted visible light only (“VIS”). Tracer studies were performed, and the dispersion number was found to be approximately 0.01 in each channel, indicating uniformity of flow and low to moderate dispersion through the three channels. The composition of the foulant formed on the lamps was quantified in terms of organic, inorganic, and biological material. Furthermore, water quality parameters were monitored to assess any correlation found between the influent quality and the foulant composition.
For the first run, which was conducted in the summer (thus alum was being added), the VIS foulant consisted primarily of algae and the OFF lamps had a brown sludge-like accumulation. These lamps always accumulated the highest mass of foulant. Foulant on the UV lamp consisted only of a thin dry film. It was found to be inorganic in composition, and principally composed of Ca, Al and Mg corresponding to 80, 108 and 20 mg/g dry foulant respectively. There were no measurable amounts of VSS, COD or protein in the UV foulant, whereas the control lamps had up to 90% VSS in the foulant material. No change in UV disinfection efficiency was observed after 4 weeks, indicating that fouling was not a significant problem with this biologically-treated water. Similar results were obtained during a run conducted in the winter when there was no addition of alum.
The second phase of the study involved the pumping of ferric chloride into the influent immediately upstream from the UV system in order to simulate the residual iron concentrations which would be present if ferric chloride was being used as a coagulant. Fe concentrations ranged between 0.2 and 1.4 mg/L in one run, and between 2.6 and 4 mg/L in the other. In both cases, within one week the UV system had failed due to the formation of a wet foulant on the UV lamps, and the applied UV dose in the system was essentially zero. In fact, an Fe concentration as low as 0.2–1.6 mg/L in the influent resulted in a decrease of the applied UV dose following only one day of operation. The foulant material was composed of inorganic, organic and biological proteinaceous material. An inner, inorganic foulant layer was composed principally of Ca and Mg.
A wet, reddish foulant formed the outer layer, onto which materials such as Al, organics and protein could bind. The Al concentration here was as high as 40 mg/g dry foulant, and for a representative volume, the fixed solids, volatile solids and protein concentrations were 6,250, 7,500, and 1,035 mg/L respectively. The volatile component of this layer was thus over 50% of the total dry mass, and the protein comprised a rather significant 7.5%. For the controls, the protein content in the foulant was much higher, up to 8,000 mg/L.
The Fe concentration in the UV foulant increased from 64.9 mg/g to 150.6 mg/g as the Fe dose increased. Furthermore, an increase in Fe concentration in the influent also resulted in an increase in concentration of Mg, Al, Ca, Mn and Zn in the UV foulant. Therefore, it was clear that the addition of ferric chloride initiated the foulant formation, and that a combination of organic and inorganic material on the UV lamps reduces the performance of the disinfection process. Al and Fe made up a large percentage of the foulant when they were added to the influent as coagulants. However, it appeared that when alum was added as a coagulant, the Al content of the foulant was then much higher in the UV foulant (∼1.9 mg/g), compared to the controls (∼0.2 mg/g). For the runs where Fe was added, the inorganics content of all foulants was similar. This relationship suggests that Fe in the wastewater may bind to the organic material present on the lamps, or that the Fe may be present in organo-complexes when attachment to the lamp occurs. Al was present in the foulant when its corresponding wastewater concentration was very high. This occurred when alum was being added as a coagulant (0.29 mg Al/L in the influent), or when the foulant contained organics onto which aluminum could bind but alum was no longer being added (only 0.053 mg Al/L in influent).
The results obtained clearly demonstrate how the chemical treatment which occurs upstream from the UV system has a direct effect on the formation and composition of the foulant material. As expected, they also show that the UV light itself affects the quality of the foulant, but in ways not always anticipated or obvious.
Document Type: Research Article
Publication date: 2000-01-01
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