Dissolved Air Flotation in Combination with Ultrafiltration Membrane Modules in Surface Water Treatment

The primary purpose of this research is to evaluate the feasibility of dissolved air flotation (DAF) as pretreatment process for membrane modules in river water treatment. The performances of DAF in combination with an up-flow membrane biofilter (MBF) and a hollow-fiber ultrafiltration membrane (UFM) were investigated respectively. Removal efficiency of ultraviolet absorbance at 254 nm (UV₂₅₄), ammonia nitrogen (NH₃-N), particle counts and microbial index were measured for both systems. Results showed that DAF was a robust pretreatment process for floc particles separation despite of raw water quality fluctuations. For DAF-UFM system, the monitoring data indicated bacteria breeding, nitrite accumulation in membrane vessel and the ammonia concentration of permeate water even exceeded regulatory limits. By contrast, DAF-MBF system was established via the introduction of multilayer filter media between DAF and curtain type ultrafiltration membrane. In MBF, the matured biological activated carbon (BAC) and zeolite particles guaranteed the efficient removal of both organic matters and ammonia. Moreover, the high dissolved oxygen levels in DAF effluent significantly improved bio-adsorption and degradation of pollutants in MBF. The retention of microorganisms by the membrane not only ensured a high concentration of biomass, but also eliminated the risk of microbes leaking out from MBF. In addition, compared with UFM, the curtain membrane exhibited obviously slower fouling development. These results suggested that the DAF-MBF system was more suitable for purifying raw waters seriously polluted by dissolved contaminants.