Pollutant Removal Efficiency And Operation Stability Enhancment On Dissolved Air Flotation

Water treatment process with dissolved air flotation is concerned because of its better treatment effectiveness on low temperature and low turbidity water, high algae-laden water and high-colour water. In recent years, the water pollution was critical while the water quality and quantity were worse in China. The treatment of micro-contaminated source water becomes an urgent problem. However, the ability of air flotation process to remove particulate matter was mainly concerned in the past, the research work on the enhanced treatment of organic matter was limited. In order to apply dissolved air flotation process in the treatment of micro-contaminated source water, the study on the pollutant removal efficiency and the enhancement of operation stability is of great practical significance.The treatment efficiency and influencing factors of micro-polluted water by air flotation process were studied. It was found that appropriate increase in coagulant dosage, shorter flocculation time, stronger flocculation intensity, neutral initial pH value, addition of Ca2+ ions and small amount of kaolin added could be used as the means for enhanced coagulation of organic matter by air flotation process. The average humic acid removal by coagulation and air flotation was about 80%. The effectiveness of nitrobenzene removal by coagulation and air flotation process was decreased with the increase of initial pH value and TOC value, increased with the increase of Ca2+ ion concentration, firstly increased and then decreased with the increase of turbidity while the maximum removal rate was 50%. The occurrence of this phenomenon is a consolidated results,which depend on the variations of humic acid microstructure under different water conditions and the effectiveness of nitrobenzene adsorption and removal by air flotation process at coagulation stage was studied.The mechanism of organic matter treatment by air flotation process was studied. The three-dimensional fluorescence detection results showed that the humic acid molecule fluorescence intensity changed with the increase of initial pH value, the complexation reaction of humic acid with aluminum ion was weakened. It inferred that more aluminum salt decomposed into high polymer, a large number of humic acid formed conducive flocs for air flotation process with capture mechanism. With the increase of pH value, the coverage of humic acid on the mica surface decreased. Under acidic conditions, the observed molecular radius being smaller than neutral and alkaline conditions while aggregate structure was looser under alkaline condition. Ca2+ ion played an important role in humic acid molecular aggregation. A small amount of Ca2+ ions addition would result in great aggregation phenomenon to improve humic acid adsorption. Scanning electron microscopy results further showed that the phenomenon of floc clusters was obvious as the initial pH value was increased. The flocs were more easily adhered with micro-bubbles to form foam-floc combination, which could be removed by buoyant force because the density of combination was less than 1. It proved the conclusions in three-dimensional fluorescence scanning above.The effects of internal structure of optimized separation zone on air flotation process stability were studied. It was found that both the water collection system of air flotation tank and vertical plates in separation zone influenced the operation effectiveness and the stability of air flotation tank. By using perforated plates collection system and adding vertical plates with small space length in the bottom of separation zone, the stability of air flotation tank was enhanced. The effects of effluent water were basically constant with the variation of hydraulic loading for vertical plates air flotation tank which could operate at hydraulic loading of 18m/h. Under the same hydraulic loading conditions, the effect of the vertical plates air flotation tank was better than the traditional air flotation tank for both turbidity removal and organic matter removal. The vertical plates air flotation tank had a high adaptability with the variation of parameters such as recycled ratio and was easy for operation management. As well, the vertical plates air flotation tank also had a great adaptability with changes of water quality, operation under the conditions of low temperature low turbidity, low temperature medium turbidity and low temperature high turbidity achieved good results on the removal of turbidity and organic matter at hydraulic loading of 18m/h. The flow field feature of separation zone in vertical plates air flotation tank changed with the flow rate and recycled rate. As the hydraulic loading and recycled ratio increased, the bubble layer thickness increased. The three-dimensional vortex appeared in separation zone influenced the operation effect when the hydraulic loading was too large or recycled ratio was too small.Full scale experiment on the combined sedimentation with dissolved air flotation was performed. The experimental results showed that the effects of combined treatment technology on the removal of organic matter, algae, ammonia nitrogen and odor were excellent. Since the solution of problem in the difficulty of flocs settlement and the block of filter after the flocculation of high algae bearing water, the backwash cycle of filters extended from a minimum of four hours to more than ten hours during serious outbreak of algae, the water plant resumed normal production of water, greatly reduced the water consumption. It was found that the flotation part added the increase in power consumption, but the production water consumption and the coagulant dosage reduced, so that the cost of water production reduced. By using high adaptable technology combined sedimentation with dissolved air flotation, the sedimentation tank reformation had the advantages of low cost, low investment, high qualified water and high water production. This process could solve the water treatment difficulty in traditional high algae-laden seasons. The second phase project by adding vertical plates in separation zone of air flotation tank was prepared for the actual situation of further improvements in water production.