Investigation of the nature of adhesion of organic foulants to membrane surfaces and strategies for cleaning
Principal Researchers: Erin Devitt
The adsorption of naturally occurring organic materials to membrane surfaces is frequently cited as the primary cause of chronic fouling of membranes used for both desalting and as pretreatment for membrane desalting in water treatment and wastewater recovery. However, the nature of fouling is poorly understood and stands as a key impediment to developing improved methods for membrane cleaning. This effort directly addresses the nature of interactions between natural organic matter, ionic components of the feedwater matrix, and the membranes in the fouling process.
The characteristics of organic materials that determine their relative propensity to foul membranes appear to include their affinity for the membrane, molecular weight, functionality, conformation, and membrane characteristics. For example, hydrophobic membranes have been observed to be more prone to fouling by natural organic material than are hydrophilic membranes. Usually, greater charge density on the membrane surface is associated with greater membrane hydrophilicity. Polysulfone, cellulose acetate, ceramic, and thin film composite membranes used for water treatment and wastewater recovery typically carry some degree of negative surface charge. Conditions that render dissolved organic materials more hydrophobic should augment adsorptive fouling of membranes. Calcium ions and protons may associate with functional groups on NOM molecules rendering them more hydrophobic. While solution chemistry may affect the hydrophobicity of natural organic matter, the formation of salt bridges and direct precipitation of NOM may also reduce its stability in water. These trends have been observed in the fouling of membranes by proteins. These laboratory observations support field experience where hard surface water has been observed to exhibit a greater tendency to foul RO membranes than soft surface water despite the fact that both waters produced the same SDI.
In addition to their direct affect on membrane fouling, the organic matter in natural waters has been shown to play a determinant role in the cohesion of colloids deposited on the membrane. Analysis of the organic foulants in natural waters and their relative concentrations in the deposited cake suggests that polyphenolic compounds, proteins, and polysaccharides bind together colloids that deposit on the membrane and may cement the cake to the membrane surface. Adsorptive fouling and stabilization of the cake (or formation of a gel layer) by organic matter in water, impairs the efficiency of purely hydraulic cleaning methods such as backflushing, fast pulsing, or crossflow reversal. As a consequence, reagents used for chemical cleaning of the membrane must efficiently deteriorate or redissolve organic compounds. This likely explains the efficacy in many cases of solutions of caustics and enzymes used to chemically wash membranes.
In this effort we seek to identify the nature of organic foulant/membrane interactions as a basis for selecting improved cleaning strategies and thereby improving membrane performance. Questions being addressed in this work include, Why do PSs and PHAs, adhere to the membrane to a much greater extent that proteins? Does NOM fouling occur in the membrane matrix or on the surface? How do changes in the solution chemistry such as hardness, ionic strength and pH affect NOM/membrane interactions? How do NOM and inorganic colloids interact in the fouling process? What type of cleaning strategies optimally correspond to specific fouling mechanisms? How do current cleaning strategies affect the presence, structure, and chemical bonding of organic foulants?
A battery of analytical methods selected to elucidate the physical condition and surface chemistry of the membrane surfaces is applied to fouled, cleaned and new membranes. These methods include: Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) to examine functional groups on membranes, location of foulants with depth into the membrane, the nature of association between foulant and membrane and the quantity of foulant; Atomic Force Microscopy (AFM) to observe the physical structure of surfaces in fouled and unfouled and cleaned conditions at the nanometer scale; determination of BET isotherms to obtain estimates of membrane surface area and pore size distribution, measurements of electrokinetic properties of membrane surfaces by streaming potential and in some cases, electrophoretic mobility of membrane fragments. Membrane surface chemistry will also be evaluated by titrations and contact angle measurements. Foulant adsorbability will be characterized by generating isotherms and observing reversibility under various conditions of solution chemistry.