Removal of oil droplets from oil-in-water mixtures by dissolved air flotation (DAF).

Dissolved air flotation (DAF) is a separation technique, often used after a primary gravity separator to enhance the quality of the wastewater, so it can be released to streams, rivers, and the sea in a manner not to violate the environment. DAF works by removing oil droplets from oil-in-water mixtures by air bubbles of an average diameter of 50 μm with a standard deviation of 5.5 μm. The air bubbles used in these experiments were generated as a result of rapid pressure reduction of water saturated with air when it released from the bottom of the DAF tank. The main aim of the DAF experiments reported here was to measure the removal efficiency of oil droplet mostly in a diameter range between 15 and 80 μm that were created using a static mixer. The DAF tank located at the University of Surrey was a scale model of existing DAF unit used by Thames Water plc. The effects of seven operating parameters that are believed to affect the performance of DAF were investigated. The operating parameters consist of inlet oil concentration, air saturator pressure, temperature, the salinity of continuous phase, type of oil, flow rate of the mixture and coagulant dosage. Two independent analysis methods were used to estimate the removal efficiency of oil droplet. They are a droplet counting and an oil-in-water measuring methods. The droplet counting method used a Coulter Counter that provided numbers of oil droplet passed through the aperture based on the selected size range. The oil-in-water measuring method used an ultraviolet and visible spectroscopy (UV-Vis), which the removal efficiencies were estimated from the absorbance values that were measured at the optimum wavelength of 400 nm. The analyses done with these two methods found that the inlet oil concentration and flow rate of the mixture into DAF tank were inversely proportional to the oil droplet removal efficiency. The other parameters such as saturator pressure, temperature, water salinity and alum dosage were directly proportional to the oil droplet removal efficiency. Vegetable oil, which has larger spreading coefficient than lamp oil obtained a better oil droplet removal efficiency. Coulter Counter showed that a better removal efficiency for vegetable oil obtained at larger oil droplet ranges size, 50μm and above. This was because the oil droplets were removed by gravity and enhanced by air bubbles. Contrary to lamp oil, which the worst removal efficiency was obtained at larger size ranges due to the coalescence of oil droplets. Results from these experiments were used to obtain a correlation that can predict removal efficiency. This was done by performing dimensional analysis. It was carried out using Buckingham Pi and scaling methods. It involved with the identification of two non-dimension and nine dimensional parameters. The dimensional analysis concluded that the removal efficiency is a function of eight other dimensionless groups, which are ratio of inlet oil and mixture flow rat