Study On The Removal Of Organics From Reverse Osmosis Concentrate By The PAC Accumulative Countercurrent Adsorption-MF Processes

Reverse osmosis (RO)-based tertiary or advanced wastewater treatment processesare becoming quite popular in brackish water, seawater desalination and wastewatertreatment and reclamation. With clean water production, RO treatment also produces acertain quantity of reverse osmosis concentrate (ROC, usually ranging between25%and50%of the feed) with a higher concentration of almost all pollutants exsisting inthe influent, including recalcitrant organic compounds, scale precursor ions (e.g., Ca2+,Mg2+, etc). ROC treatment would minimize environmental impacts upon disposal ofthese retentates. For using the ROC produced in a refinery wastewater treatment plantas a secondary source to enhance the overall water reclamation yield of the ROsystem, total harness and organics removal from the ROC, and powdered activatedcarbon (PAC) regeneration were investigated.Above70%removal of total hardness was achieved by accelerated precipitationsoftening with calcite seeding and pH adjustment using sodium hydroxide. Totalhardness decreased to the level of the influent of the RO system.Two hybrid processes，PAC accumulative countercurrent two and four-stageadsorption-microfiltration (namely ACTA-MF and ACFA-MF process, respectively)were investigated for the removal of organics from the ROC. The results showed thatapproximately70%of DOC was removed from the ROC; the effluent met the reverseosmosis (RO) influent quality requirement and could be further reclaimed by an ROsystem to improve the overall recovery rate by more than90%; and more than40%ofthe PAC dose was reduced compared to that in the conventional single-stageadsorption process. A calculation method of the ACTA-MF process for correlating theeffluent quality with the PAC dose was calibrated, and the validity of the method wasconfirmed by a deviation of less than5%. A calculation method of the ACFA-MFprocess was also discussed. The dilution factor (F) affected the PAC dose andmembrane fouling. The PAC dose at F0.3was14.46%lower than that atF=0.5based on the same effluent quality, but the membrane fouling was moresevere in the former case. Fine PAC particles adhering to the membrane surface andblocking the membrane pores dominated the irreversible membrane fouling process atF=0.3. Membrane fouling was mainly reversible at F0.5. Membrane fouling at F0.3was alleviated effectively and organics removal was improved furtherlyby the ACFA-MF process compared to the ACTA-MF process. The ACFA-MFprocess was also feasible for high-salinity ROC treatment. Good correlations betweenthe UV254and DOC of the effluent could facilitate the operation monitoring of theprocesses.The PAC used in the ACTA-MF or ACFA-MF process was regenerated by a novelmethod, i.e., solvent countercurrent multi-stage regeneration. Solvent countercurrentmulti-stage regeneration enhanced regeneration efficiency compared to single-stageregeneration with no extra solvent exhaustion. At the ratio of10g/L of the used PACand methanol, regeneration efficiency was improved from74.34%for single-stageregeneration to79.33%and80.70%for countercurrent two and three-stageregeneration, respectively.