Crossflow membrane filtration systems, such as reverse osmosis (RO), nanofiltration (NF) or ultrafiltration (UF), are good alternatives to traditional filtration and chemical treatment. However, to get the best performance from them, careful attention must be paid to feedwater conditions.
Pretreating feedwater can lengthen the life of membrane elements, improve the quality of the water produced and reduce the amount of maintenance and cleaning a system requires.
The importance of feedwater conditions is evident when you examine how crossflow membranes work. In simple terms, a crossflow filtration system separates an influent stream into two effluent streams - the permeate and the concentrate.
The permeate is the portion of the fluid that has passed through the semi-permeable membrane. The concentrate stream, on the other hand, contains constituents that have been rejected by the membrane.
An inherent advantage of crossflow filtration is its ability to continuously operate in a self-cleaning mode. It's self-cleaning because suspended solids and rejected solutes are constantly swept away from the membrane surface.
Membrane fouling occurs when materials from the feed stream collect on or near the membrane surface and restrict water permeation. Fouling may occur as layers of deposition on the surface of the membrane (cake fouling), a hardened layer on the membrane surface (scale), particle insertion into the pore channel or entrance (pore blockage), or chemical attachment of particles to the membrane (adsorption).
Most types of fouling can be avoided or minimized with pretreatment and attention to the system's crossflow requirements.
Solubilty Limits of Feedwater Salts
In parts per million at 30º C.
Calcium Carbonate (pH 10)…65
Magnesium Sulfate (pH 10)….9
Barium Carbonate (pH 10).…24
Silica (pH 8)…......................120
There are four major configurations for membrane modules: plate-and-frame, hollow fiber, tubular and spiral-wound. Although there are advantages to each configuration depending on the application, the most popular is the spiral-wound.
Most crossflow systems, including those with a spiral-wound configuration, require turbulent flow. This necessitates a reasonable crossflow velocity across the membrane.
Crossflow requirements for membrane elements depend on the application and element size. Most pure water applications require the concentrate flow from any individual element to be at least 20 to 40 gallons per minute (gpm)(151.4 Lpm) for an 8-inch diameter element, and 4 to 8 gpm (15.1 to 30.3 Lpm) for a 4-inch element.
Within a given system, minimum concentration rates are influenced by element design factors, including whether the outer covering of the membrane element is made of fiberglass or loose mesh. Higher crossflow velocities lead to less fouling and extended membrane life, so it's critical to consider the fouling tendency of the feedwater and the need to maintain adequate crossflow rates when designing the membrane system.
The membrane should be chosen for the specific application objective -- particle removal, total organic carbon removal, hardness reduction or ultrapure water production, for example.
The most widely used membranes for RO are the cellulosic (CA) and polyamide (PA) types, rated at 97 to 99 percent or more NaCl rejection, with a nominal molecular weight cut-off (MWCO) of 150 to 250 Daltons for organic solutes.
Nanofiltration, also available with CA- and PA-type membranes, displays characteristic salt rejections from 95 percent for divalent salts to 40 percent for monovalent salts, and a MWCO rating of approximately 300 Daltons for organic solutes.
The most popular UF membranes are polysulfone (PS), proprietary fluorinated materials (VF) and CA, each generally having a 1,000 to 100,000 MWCO rating.
Consideration must also be given to the chemical compatibility and stability of the membrane at a given feed stream pH. The many membranes available allow a designer to choose among several polymer types to match most pH concerns.
Resistance to oxidizing agents such as chlorine and iodine must also be investigated. CA membranes, for instance, usually exhibit a higher tolerance for chlorine than PA membranes, making CA an ideal choice where the disinfecting benefits of a continuously chlorinated feed stream are required. If PA membrane elements are used in a chlorinated feed stream, chlorine must be removed upstream. This is typically accomplished with an activated carbon filter or by the addition of a reducing agent such as sodium metabisulfite.
The ongoing development of membrane technology continues to expand both the range of chemical compatibilities and physical operating conditions (including pressure, temperature and pH) of cross- flow filtration systems.
The concentration factor of a crossflow membrane filtration system is determined by system recovery, which is the ratio of permeate to feed volume. For example, a system providing 15 gpm (56.8 Lpm) of permeate from a 20 gpm (75.7 Lpm) feed stream would be operating at 75 percent recovery and would increase the concentration of unwanted substances in the reject stream by a factor of four.
For most water purification systems, recovery rates are well defined and predictable. If a system approaches or exceeds the designed recovery, concentrated salts may form a scale on the membrane surface. Solubility limits aren't generally a concern with systems such as UF that pass dissolved salts through the membrane.
The solubility levels of dissolved mineral salts, C02 and silica are greatly affected by pH. Membrane systems rejecting substantial quantities of dissolved constituents must operate at concentration factors safely below any solubility limits.
For RO and NF, steps must also be taken to ensure that solubility limits of scale-forming constituents aren't exceeded. In general, slightly soluble salts such as CaCO3 are more soluble at a lower pH. By lowering the pH, a higher recovery may be achieved.
Commercially available antiscalant chemicals may also be used in certain applications to extend recovery levels slightly beyond the solubility limits of scale-forming constituents.
A more common method of dealing with mineral salts is to use ion exchange softening as pretreatment to the membrane system. The main function of the softener is to remove scale-forming calcium and magnesium ions from hard water. Softening is a simple and cost-effective process, but it's important to regularly monitor the performance of the softening system to ensure hardness is being removed and not breaking through to the membrane.
Iron is another common membrane foulant. Found in most water supplies, it's particularly prevalent in supplies drawn from wells. In its ferrous state (Fe+2), it's soluble. However, when it's oxidized to its ferric state (Fe+3), it's insoluble and forms a precipitate.
Oxidation typically occurs when oxygen or another oxidizing agent such as chlorine is introduced. Iron precipitation typically gives water a brown, rusty appearance. If iron isn't removed in a prefiltration step, it fouls and stains the surface of the membrane, reducing both flux and rejection performance. In many cases, ion exchange softening may help remove soluble ferrous iron.
Multimedia and cartridge filters can remove large particles that would otherwise foul the membrane. Backwashable dual- or multimedia filters remove suspended particles, including precipitated iron greater than 20 microns in size.
A schedule of routine monitoring is the best way to ensure a membrane system is operating under optimal conditions. For small, point-of-use (POU) systems it may be more cost-effective to replace membrane elements rather than to institute a monitoring program. However, it's important to monitor process variables such as inlet pH, hardness levels, turbidity, temperature, iron, chlorine, conductivity, flow rates and operating pressures for larger systems.
Operational data should be recorded frequently, ideally every day or once per shift. This data may be used to spot trends in operating conditions and alert the user of pertinent maintenance issues, such as membrane replacement or cleaning. Feedwater data can also be used to assess the effectiveness of the prefiltration system.
Crossflow membrane filtration, whether combined with an existing treatment system or used alone as the primary treatment method, offers benefits not attainable with conventional filtration. If a process requires ultrapure water, RO systems have a proven track record. Even if a process doesn't require water with the highest degree of purity, membrane technology can offer many advantages. When designed with careful attention to system chemistry, crossflow requirements and proper pretreatment, a membrane system should provide trouble-free performance for many different applications, with little required maintenance.