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Acid solutions are used to remove milk-based calcium fouling and water-related hardness issues.The alkaline cycle is the most complex of the cleaning steps. The solution must be properly buffered to maintain the correct pH, because the concentration of caustic changes due to its reaction with foulants. Straight caustic is seldom recommended for membrane cleaning. A combination of nonionic and anionic surfactants will usually provide the best results. The addition of water-conditioning agents and a buffer to the surfactant cocktail completes the recipe for an effective additive.

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Membrane FoulingWhen a Liqui-Cel® Membrane Contactor system is placed between a clarification unit and an ROunit to remove CO2 from a water stream, watch for changes in water pH. Water may containspecies that are soluble in water in acidic pH ranges but insoluble in alkaline pH ranges. If waterpH increases as the water passes through the contactor (this happens when dissolved CO2 isremoved from water) the insoluble species might precipitate on the membrane surface.Precipitation of solids can also occur when water is treated with flocculation chemicals followed bya change in pH. For example, alum (aluminum sulfate) is often used to remove suspended matterfrom water. The removal takes place by coagulation, flocculation and precipitation in the waterclarifier tank. The precipitation occurs in the form of polymeric aluminum hydroxide at certain pHranges. As CO2 is removed from the feed water, a change in pH may be enough to precipitateexcess aluminum hydroxide or other ionic compounds on the membrane’s surface. The thincoating or deposits will prevent normal gas transfer through the membrane and the contactor’sLiqui-Cel® Membrane Contactors Design & Operating Guidelines27removal efficiency will drop. This process is reversible by cleaning the contactor with an acidsolution such as 3% w/w ortho-phosphoric acid solution (refer to the Liqui-Cel® MembraneContactor Cleaning Guidelines). The acid solution may dissolve surface precipitates on membraneand will restore the contactor degassing performance to original manufacturer's specifications.When the contactors are installed downstream from carbon beds, a 2 micron absolute filter shouldbe installed and the carbon beds should be flushed to drain after the carbon change-out until allcarbon fines are flushed out. Care should also be taken when back flushing carbon beds that aredownstream of membrane contactors. Filtration should be added to keep the contactors free fromcarbon particles, which can be difficult or impossible to clean.When Liqui-Cel® Membrane Contactors are installed upstream of RO membranes, a cleaningcycle is also recommended in order to prevent fouling of the Liqui-Cel® Membrane Contactor.Please refer to the Liqui-Cel® Membrane Contactor Cleaning Guidelines available on-line atwww.liqui-cel.com or from your Membrana representative.

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Type of Membrane Fouling, Water Quality Indicators, and Control MeasuresAccording to the type of fouling materials, four categories of membrane foulingare generally recognized. They are (a) inorganic fouling/scaling, (b) particle/colloidsfouling, (c) microbial fouling, and (d) organic fouling. A brief description on the natureof fouling, relevant water quality as indicators, and control measures are summarizedbelow for each type of membrane fouling.Inorganic Fouling/ScalingInorganic fouling or scaling is caused by the accumulation of inorganicprecipitates such as metal hydroxides, and “scales” on membrane surface or within porestructure. Precipitates are formed when the concentration of chemical species exceedingtheir saturation concentrations. Scaling is a major concern for reverse osmosis (RO) and

nanofiltration (NF). RO and NF membranes reject inorganic species. Those speciesform a concentrated layer in the vicinity of membrane-liquid interface - a phenomenonreferred to “concentration polarization”. For microfiltration (MF) and ultrafiltration(UF), inorganic fouling due to concentration polarization is much less profound, but canexist most likely due to interactions between ions and other fouling materials (i.e.,organic polymers) via chemical bonding. Some pretreatment processes for membrane4filtration such as coagulation and oxidation, if are not designed or operated properly, mayintroduce metal hydroxides on membrane surface or within pore structure. Inorganicfouling/scaling can be a significant problem for make-up water of caustic solutionsprepared for chemical cleaning.Particulate/colloid FoulingAlgae, bacteria, and certain natural organic matters fall into the size range ofparticle and colloids. However, they are different from inert particles and colloids suchas silts and clays. To distinguish the different fouling phenomena, particles and colloidshere are referred to biologically inert particles and colloids that are inorganic in natureand are originated from weathering of rocks.In most cases, particles and colloids do not really foul the membrane because theflux decline caused by their accumulation on the membrane surface is largely reversibleby hydraulic cleaning measures such as backwash and air scrubbing. A rare case ofirreversible fouling by particles and colloids is that they have smaller size relative tomembrane pore size. Therefore, those particles and colloids can enter and be trappedwithin the membrane structure matrix, and not easily be cleaned by hydraulic cleaning.

Microbial/Biological FoulingMicrobial fouling is a result of formation of biofilms on membrane surfaces. Oncebacteria attach to the membrane, they start to multiple and produce extracellularpolymetric substances (EPS) to form a viscous, slimy, hydrated gel. EPS typicallyconsists of heteropolysaccharides and have high negative charge density. This gelstructure protects bacterial cells from hydraulic shearing and from chemical attacks ofbiocides such as chlorine.Severity of microbial fouling is greatly related to the characteristics of the feedwater. Water quality parameters that indicate the potential of microbial fouling areclassified into three categories:(a) Parameters indicating the abundance of microbes,(b) Parameters indicating nutrient availability,(c) Parameters indicating environmental conditions for microbial growth,Organic FoulingOrganic fouling is profound in membrane filtration with source water containingrelatively high natural organic matters (NOM). Surface water (lake, river) typicallycontains higher NOM than ground water, with exceptions. For source water high inNOM, organic fouling is believed to be the most significant factor contributed to fluxdecline (Mallevialle et al., 1989; Lahoussine-Turcaud et al, 1990). Microfilters usuallyremove insignificant amount of organic matter, as measured by dissolve organic carbon(DOC). DOC as an indicator for organic fouling is probably neither proper nor adequate.Efforts to identify the effects of subgroups of NOM on membrane fouling have yet beenable to draw definitive conclusions.

5The effects of various operating strategies against different types of fouling aresummarized in Table 1. As indicated in Table 1, chemical cleaning is an effective controlstrategy for all types of membrane fouling.

Membrane Chemical CleaningThe key issue to understand membrane fouling and cleaning is to understandinteractions a) between fouling materials and membrane, b) between cleaning chemicalsand fouling materials, c) between cleaning chemicals and membrane, and d) among thefouling materials.Interactions Between Fouling Materials and MembraneMembrane fouling is a complicated phenomenon of which causes of fouling aremultiple and poorly understood. The core of the issue is interactions between foulingmaterials and membrane, and between fouling materials themselves. It has beenrecognized that electrostatic interaction and hydrophobic/hydrophilic interaction betweenmembranes and fouling materials have a significant bearing on membrane fouling. Thisis particularly true to more difficult fouling problems caused by adsorption of naturalorganic matters and biopolymers on the membrane. The balance between the forces ofelectrostatic repulsion and hydrophobic adhesion determines the outcomes of membranefouling, as well as the efficiency of chemical cleaningElectrostatic InteractionsSurface charge of membrane media is the results of ionization of particularfunctional groups existed on the membrane surface (e.g., carboxyl and amine). Becauseionization of a functional group depends on pH, surface charge of a particular membraneis also pH-dependent. In pH range of typical natural waters, most membranes appear tohave a neutral to negative net surface charge. On the other hand, colloids, particles, anddissolved organic matters typically carry negative charges at the pH of natural water.Therefore, there is a tendency of electrostatic repulsion between membranes and thoseconstituents.Natural organic matter, a major factor of membrane fouling, typically containsabout 50% humic substances. Aquatic humic substances (AHS) are polyprotic acids(Malcolm, 1985). Major acidic functional groups include carboxylic and phenolicfunctional groups. About 70% - 80% of total acidity is from carboxylic groups, and restis from phenolic hydroxyl groups, as illustrated by a typical titration curve of HAS inFigure 2 (adapted from Thurman, 1985)