Efficient Flotation Device For Waste Water Treatment Experiment And Theoretical Study

This paper mainly studied the characteristics of a high-efficient dissolved-airflotation (DAF) device for the removal of colloid matter. The main factors studiedinclude the recycle flow ratio (R), dissolved-air pressure (P), coagulant dosages andsurfactant dosages. The influence of these parameters on the removal efficiency wasanalyzed. Experiments were also conducted on the application of the DAF for thetreatments of wastewater from a paper mill.The results of experiment using clean water show that as the upflow water velocityincreases, the rising velocity of air bubbles in the contact area (V_c) increases, but that inthe separatation area (V_s) decreases. However, the flow rate of the dissolved air-watermixture has almost no effect on V_c and V_s.The study shows that mixing after the pump gains much better treatment result thanmixing before the pump. For a humic acid suspension with initial concentration ofUV₂₅₄ = 0.426cm^-1, TOC=9.63mg/L, the mean removal efficiency is 81.7% for UV₂₅₄, and54.6% for TOC under the condition of mixing after the pump.The coagulant dosages, recycle flow ratio (R) and dissolved-air pressure (P) haveeffects on the flotation performance. By orthogonal test, it is found that the effect on theremoval efficiency is coagulant dosages> R > P. The optimal condition is coagulantdosages=70mg/L, R=30% and P=0.275⁰.35MPa for the treatment of dissolved colloidsolution.Coagulation pretreatment is required for good flotation performance for the removal ofdissolved colloid matter. Under the condition of addition of dodecyl sodium sulfonate assurfactant without coagulant, the zeta potential of the colloids is -18mV and the turbidityremoval is 20%. The zeta potential becomes 10mV and the turbidity removal is more than85% after coagulant dosage of 85mg/L. Adequate addition of surfactant is beneficial tobetter flotation performance, but overdosing may deteriorate the removal efficiencyunder the condition of certain coagulation dosage addition.At pH=4.0, the COD_Cr removal efficiency for the treatment of wastewater from apaper mill is 20% without coagulation, while the removal efficiency is 47% withcoagulation. This shows that compared with the conventional treatment of coagulation-flocculation followed by gravity of settling (CGS), DAF can achieve thesimilar degree removal efficiency in much shorter time.