Optimization of Chemical Cleaning of Organic-Fouled Reverse Osmosis Membranes

Secondary effluents from wastewater treatment plants contain effluent organic matter (EfOM), which comprises a range of organic compounds---polysaccharides, proteins, humic acids, and fatty acids. EfOM has been known to contribute significantly to organic fouling of RO membranes in advanced wastewater reclamation plants. The objectives of this dissertation work are: (1) to explore the mechanisms governing the chemical cleaning of organic-fouled RO membranes for a complex mixture of organic foulants and (2) to develop a general methodology for selection of optimal combination of cleaning agents and chemical/physical conditions for efficient cleaning. Alginate, bovine serum albumin (BSA), Suwannee River natural organic matter (SRNOM), and octanoic acid have been selected as model foulants representing, respectively, polysaccharides, proteins, humic acids, and fatty acids in EfOM. The effects of each individual organic foulant and mixtures containing several types of foulants in the absence and presence of Ca2+ on the fouling of RO membranes are systematically investigated. Sodium hydroxide (NaOH), sodium chloride (NaCl), disodium ethylenediaminetetraacetate (EDTA), and sodium dodecyl sulfate (SDS) have been selected as cleaning agents representing an alkaline solution, a salt solution, a metal chelating agent, and an anionic surfactant, respectively. It is proposed that effective membrane cleaning involves the coupling between (1) chemical reaction of the cleaning agent with the foulants in the fouling layer and (2) mass transfer of the cleaning agent from the bulk solution to the fouling layer, and of foulants from the fouling layer to the bulk solution. In attempting to determine optimal procedures for chemical cleaning of organic-fouled membranes, our approach uses atomic force microscopy (AFM) to investigate a cleaning agent's chemical reactivity with the foulants. The cleaning experiments have demonstrated the implication of the order of cleaning agent usage on the overall cleaning efficiency due to specific interaction of each cleaning agent with the foulants within the fouling layer. This dissertation research contributes to a better understanding of the organic fouling of RO membranes by wastewater effluent and the ensuing chemical cleaning of the fouled membranes. The findings have direct implications for mitigating membrane fouling and optimizing chemical cleaning of the fouled membranes, thus minimizing the possible adverse impacts to the permeate quality, operational cost, and the environment.