Degradation of 4TBP by Advanced Oxidation Process, CFD Modeling and Validation for UV Reactor

Abstract

Advanced Oxidation Processes (AOPs) are innovative, cost-effective, catalyzed chemical oxidation processes for treating pollutants in low or high concentration from contaminated soil, sludge and water. The common used AOPs in drinking water treatment include UV/H2O2 process, UV/Ozone Process, UV/Titanium Dioxide and Fenton’s Reagent. AOPs are ultraviolet driven, which share predominance from photochemical technology, and often, give the clients dual benefit of both environmental contaminant treatment and disinfection. Endocrine Disrupting Chemicals (EDCs) are very disturbing contaminants measured in natural waters. Phenols and their tert-butyl derivatives are important contaminants belonging to EDCs. After a successful workshop for developing alternative drinking water treatments at Shanghai, AOPs with UV/H2O2 technology are chosen to remove 4-tert-butylphenol (4TBP) from Shanghai water. The kinetics of reaction was studied in the first part of this thesis. The results show that UV/H2O2 process can effectively decrease 4TBP concentration than hydrogen peroxide alone. Good free oxidation radical production can be achieved within UV dose range from 0 to 200mJ/cm2 by a low pressure mercury lamp. The 4TBP degradation process fits with pseudo first order equation for UV-dose and H2O2-dose. However, at very high H2O2-doses, the scavenging of hydroxyl-radicals needs to be taken into account. Computational Fluid Dynamics (CFD) modeling of UV reactor and validation of CFD model were studied in the second part of this thesis work in order to provide an applicable UV reactor design for the 4TBP treatment and also give possible reactor improvement suggestions. The CFD model used in this study is a 2-D model developed using software Comsol, V3.3a, based on a current UV reactor design at Kiwa Water Research, the Netherlands. The developed UV dose model includes three parts, a k-? flow model, a UV intensity model and a random walk model. Different feed flow rates and different lamp configurations were studied by the model. The calculation results show that a higher feed flow rate contributes to a relative narrow UV dose distribution than the lower flow rate. With three lamp configurations, position 0 is the best among the three with the highest average UV dose as well as the narrowest dose distribution pattern. Model also predicted low pressure lamps have about 8% higher power output to UV dose efficiency than medium pressure lamps. Validation of the flow model was helped by flow measurements at Delft University of Technology. Experimental studies of velocity measurements by Laser Doppler Velocity Meter were conducted together with salt and dye dose experiments. After comparisons of model predictions and experimental measurements, it was found that the k-? CFD flow model demonstrated generally good qualitative prediction of flow inside the reactor but failed to give correct prediction of recirculation zones behind the quartz tubes. There are dead zones of water at the top and bottom near the inlet of the reactor. Bigger areas exist behind the quartz tubes that have water recirculation than the model predicted, which may result 25% of more UV dose prediction by the model. And differences caused by 2-D model and 3-D measurements may result about 20% less UV dose model prediction. Current UV reactor design at Kiwa Water Research, position 0 and low pressure mercury lamps applied at a feed flow rate of 4.1m3/h appears to be an applicable design for advanced oxidation treatment of 4TBP by UV/H2O2 in Shanghai. High roughness quartz tubes walls and relative smaller ratio of reactor to feed pipe diameters are recommended to improve reactor performance in the recirculation and dead zones with current design. Further investigations of the dose model and UV-sensitive dyed microspheres particle tracking experiments are recommended.