Study On Removal Of Organic Pollutants And Hardness From Reverse Osmosis Concentrate

In recent years, with the rapid development of membrane technology, reverseosmosis technology took an important place in the field of wastewater reuse as themain wastewater regeneration unit operation. But along with the dramatic increase inthe quantity of reverse osmosis concentrate (ROC), the problems originated fromrefractory organic matter and high salinity in the ROC needed to be solved in thewater industry field. If the concentrate did not properly handled before emission, itwould inevitably have a negative influence on the soil, the surface water, the marineenvironment and so on. Granulation softening method and powdered activatedcarbon(PAC) adsorption-microfiltration(MF) combination process were used toremove the hardness and the organic pollutants in the ROC, witch came from apetrochemical wastewater reuse workshop in Tianjin.Granulation softening method could improve the utilization of chemicals,accelerate the softening process compared with precipitation softening method.Calcium carbonateand sodium hydroxide were respectively used as seed crystal andsoftening agent to remove the hardness of the ROC. The test showed that addition ofseed crystals could increase the removal rate of hardness, and the effluent turbiditywas low as well. It was considered that the seed crystal preparation work was finishedwhen the granulation reached30times, the best softening condition of the test wasthat the dose of sodium hydroxide and calcium carbonate was0.95g/L and2g/Lrespectively, settling of30minutes after mechanical mixing at180r/min for30minutes, the removal rate of the hardness would reach70%in this condition.PAC adsorption-MF combination process could effectively removeorganicpollutants from ROC, the cumulative countercurrent adsorption gave full play to thePAC adsorption capacity. According to the theoretical calculation, the cumulativetwo-stage countercurrent adsorption saved more than40%of the PAC than singlestage adsorption operation under the same removal efficiency of organic matter. Thetheoretical calculation was verified by a lab-scale column. The results showed that theeffluent DOC and COD were close to the target value. This process could remove theorganic pollutants in the ROC efficiently and save the addition of PAC at the sametime, which would improve the cost-effectiveness of the process and is of betterapplication potential. Continuous aeration, intermittent discharge of PAC andincreasing the dilution factor could all effectively reduce the membrane fouling, and then slow down the decline of the specific flux. After the operation, the specific fluxof the membrane could be basically recovered after chemical cleaning.