Efficient Wastewater Treatment System

25 Mar 2015 NUS professors investigated the selective removal of photocatalytic non-degradable fluorosurfactants from reverse osmosis concentrate (ROC).

Recently, photocatalytic treatment of municipal reverse osmosis concentrate has drawn increasing attention due to its relatively high efficiency and low cost. However, photocatalytic reactions by commercially available TiO2 are not able to degrade fluorosurfactants in the ROC sample due to the absence of photoreactive groups in these compounds.

A team led by Prof LI Fong Yau from the Department of Chemistry in NUS has investigated adsorption and coagulation methods and the efficiencies in removing fluorosurfactants. Ferric chloride (FER) coagulation was found to be efficient in removing fluorosurfactants, while powdered activated carbon (PAC) adsorption was inefficient (see Figure).

The technology has wide applications since reverse osmosis (RO) technology is widely used in wastewater recovery and desalination processes. Currently, only 75~85% of feed water can be purified as clean product water by utilising RO membrane for wastewater recovery. The remaining 15~25% of ROC is brackish waste. Without efficient and cost-effective purification technologies, ROC is commonly discharged to water bodies, which poses a potentially serious threat to marine ecosystems due to high organic content. The technology developed will meet the major requirements for ROC reclamation, including high efficiency, low cost, environmentally friendly final products, long catalyst life, and cost-effective regeneration methods.

The analysis and characterisation methods included mass spectrometry, total organic carbon, fluorescence and UV-Visible spectra, scanning electron microscopy, infrared spectroscopy, N2 sorption, zeta potential, and energy dispersive spectroscopy elemental analysis. The FER pre-treatment process performed better than the post-treatment process in removing the fluorosurfactants. FER selectively removed the bulky fluorosurfactants with long branches but not the slim ones with short or no branches. The applicability of Freundlich and Langmuir models for the adsorption processes was also investigated. The team’s results revealed that the PAC removal mechanism was mainly adsorption, whereas the FER coagulation mechanisms involved both entrapment and adsorption.

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Image shows the elucidation of mechanism of selective removal of photocatalytic non-degradable fluorosurfactants from reverse osmosis concentrate by ferric chloride coagulation [Image credit: LIN Xuanhao]

 

References

1. Lin XH, Sriramulu D, Valiyaveettil S, Li SFY. “Selective removal of photocatalytic non-degradable fluorosurfactants from reverse osmosis concentrate.” Water Research 68 (2015) 831.

2. Westerhoff P, Moon H, Minakata D., Crittenden J. “Oxidation of organics in retentates from reverse osmosis wastewater reuse facilities.” Water Research 43 (2009) 3992.