Membrane Filtration as an Alternative: Part 1

About the author: Dr. Mohamed Lahlou is the technical assistance specialist for the National Drinking Water Clearinghouse. This article originally appeared as a tech brief for the NDWC.

Dr. Mohamed Lahlou

A semipermeable membrane is a thin layer of material capable of separating substances when a driving force is applied across the membrane. Once considered a viable technology only for desalination, membrane processes are increasingly employed for removal of bacteria and other microorganisms, particulate material and natural organic material that can impart color, tastes and odors to the water. They also can react with disinfectants to form disinfection byproducts (DBP). As advancements are made in membrane production and module design, capital and operating costs continue to decline.

Membrane filtration systems’ capital costs, on a dollar per volume of installed treatment capacity basis, do not escalate rapidly as plant size decreases. This factor makes membranes quite attractive for small systems. In addition, for groundwater sources that do not need pretreatment, membrane technologies are relatively simple to install. The systems require little more than a feed pump, a cleaning pump, the membrane modules and some holding tanks. According to a 1997 report by the National Research Council, most experts foresee that membrane filtration will be used with greater frequency in small systems as the complexity of conventional treatment processes for small systems increases.

Membrane processes have become more attractive for potable water production in recent years due to the increased stringency of drinking water regulations. Membrane processes have excellent separation capabilities and show promise for meeting many of the existing and anticipated drinking water standards. The Surface Water Treatment Rule (SWTR) and the anticipated Groundwater Disinfection Rule have led to the investigation of ultrafiltration (UF) and microfiltration (MF) for turbidity and microbial removal. The new Disinfectants/Disinfection Byproduct (D/DBP) rules have increased interest in NF and UF membranes for DBP precursor removal.

Potable water treatment traditionally has focused on processes for liquid-solid separation rather than on processes for removing dissolved contaminants from water. Thus, the effect of the 1996 Safe Drinking Water Act (SDWA) amendments has been to encourage water treatment professionals to consider unconventional treatment processes, such as membrane technologies, either alone, or in conjunction with liquid-solid separation.

While all types of membranes work well under proper conditions, choosing the most appropriate membrane for a given application still remains crucial. (See Figure 1.) In many cases, selection is complicated by the availability of new types of membranes, applications or site-specific conditions. Bench and pilot tests are powerful tools for situations where process risks and uncertainties exist or the cost impacts from problems are potentially high.

Membrane classification standards vary considerably from one filter supplier to another. What one supplier sells as a UF product, another manufacturer calls a NF system. It is better to look directly at pore size, molecular weight cutoff (MWCO) and applied pressure needed when comparing two membrane systems. MWCO (a measure of membrane pore dimensions) is a specification used by membrane suppliers to describe a membrane’s retention capabilities.

Microfiltration is loosely defined as a membrane separation process using membranes with a pore size of approximately 0.03 to 10 microns, a MWCO of greater than 100,000 daltons and a feedwater operating pressure of approximately 100 to 400 kPa (15 to 60 psi). Representative materials removed by MF include sand, silt, clays, Giardia lamblia and Cryptosporidium cysts, algae and some bacterial species. (See Figure 2.) MF is not an absolute barrier to viruses. However, when used in combination with disinfection, MF appears to control these microorganisms in water.

The primary impetus for the more widespread use of MF has been the increasingly stringent requirements for removing particles and micro-organisms from drinking water supplies. Additionally, there is a growing emphasis on limiting the concentrations and number of chemicals that are applied during water treatment. By physically removing the pathogens, membrane filtration can significantly reduce chemical addition, such as chlorination.

Another application of MF is for the removal of natural or synthetic organic matter to reduce fouling potential. In its normal operation, MF removes little or no organic matter; however, when pretreatment is applied, increased removal of organic material as well as a retardation of membrane fouling can be realized.

Two other applications involve using MF as a pretreatment to RO or NF to reduce fouling potential. Both RO and NF have been used to desalt or remove hardness from groundwater.

MF membranes provide absolute removal of particulate contaminants from a feed stream by separation based on retention of contaminants on a membrane surface. It is the "loosest" of the membrane processes and, as a consequence of its large pore size, it is used primarily for removing particles and microbes and can be operated under ultra-low pressure conditions.

In the simplest designs, the MF process involves prescreening raw water and pumping it under pressure onto a membrane. In comparison to conventional water clarification processes where coagulants and other chemicals are added to the water before filtration, there are few pretreatment requirements for hollow-fiber systems when particles and microorganisms are the target contaminants.

Prefilters are necessary to remove large particles that may plug the inlet to the fibers within the membrane module. More complex pretreatment strategies sometimes are employed either to reduce fouling or enhance the removal of viruses and dissolved organic matter. In such cases, pretreatment by adding coagulants or powdered activated carbon (PAC) has been used. In some cases, the cake layer built up on the membrane during the water production cycle can remove some organic materials.

It may be necessary to adjust the feedwater pH by chemical dosing prior to membrane filtration. This process maintains the pH within the recommended operating range for the membrane material used. It should be noted that pH adjustment is not required for scaling control, since MF membranes do not remove uncomplexed dissolved ions.

MF membranes, under the most conservative conditions, appear to act as an absolute barrier to selected bacteria and protozoan cysts and oocysts. However, unlike UF, MF does not remove appreciable densities of viruses. Therefore, it is necessary to complement MF with a post-membrane disinfection process. Chemical disinfection can be accomplished by applying chlorine, chlorine dioxide or chloramines. It should be noted that long contact times are required to inactivate viruses.

For municipal-scale drinking water applications, the commercially available membrane geometries that are the commonly used are spiral wound, tubular and hollow capillary fiber. However, spiral-wound configurations are not normally employed for MF due to the flat-sheet nature of the membrane, which presents difficulties in keeping the membrane surface clean. Unlike spiral-wound membranes, hollow-fiber and tubular configurations allow the membrane to be backwashed.

Membrane "package" plants are normally utilized for plants treating less than one million gallons per day (mgd). The components of the plant may include prescreens, a feed pump, a cleaning tank, an automatic gas backwash system, an air compressor, a membrane integrity monitor, a backwash water transfer tank, a pressure break reservoir, an air filter for the gas backwash, controls for the programmable logic controller and a coalescer.

In MF, there are two methods for maintaining or re-establishing permeate flux after the membranes are fouled.

• Membrane backwashing: In order to prevent the continuous accumulation of solids on the membrane surface, the membrane is backwashed. Unlike backwashing for conventional media filtration, the backwashing cycle takes only a few minutes. Both liquid and gas backwashing are applied with MF technology. For most systems, backwashing is fully automatic. If backwashing is incapable of restoring the flux, then membranes are chemically cleaned. The variables that should be considered in cleaning MF membranes include frequency and duration of cleaning, chemicals and their concentrations, cleaning and rinse volumes, temperature of cleaning, recovery and reuse of cleaning chemicals, and neutralization and disposal of cleaning chemicals.

• Membrane pretreatment: Feedwater pretreatment can be used to improve the level of removal of various natural water constituents. It also is used to increase or maintain transmembrane flux rates and/or to retard fouling. The two most common types of pretreatment are coagulant and PAC addition.

UF involves the pressure-driven separation of materials from water using a membrane pore size of approximately 0.002 to 0.1 microns, an MWCO of approximately 10,000 to 100,000 daltons and an operating pressure of approximately 200 to 700 kPa (30 to 100 psi). UF will remove all microbiological species removed by MF (partial removal of bacteria) as well as some viruses (but it is not an absolute barrier to viruses) and humic materials. (See Figure 2.) Disinfection can provide a second barrier to contamination and is recommended.

The primary advantages of low-pressure UF membrane processes compared with conventional clarification and disinfection (postchlorination) processes are

• No need for chemicals (coagulants, flocculants, disinfectants, pH adjustment);

• Size-exclusion filtration as opposed to media depth filtration;

• Good and constant quality of the treated water in terms of particle and microbial removal;

• Process and plant compactness; and

• Simple automation.

Fouling is a limiting factor responsible for most difficulties encountered in membrane technology for water treatment. UF is certainly not exempt from this fouling control problem. Therefore, membrane productivity should be thoroughly researched.

UF is a pressure-driven process by which colloids, particulates and high molecular mass soluble species are retained by a process of size exclusion. Therefore, UF provides a means for concentrating, separating into parts or filtering dissolved or suspended species. UF allows most ionic inorganic species to pass through the membrane and retains discrete particulate matter and nonionic and ionic organic species.

UF is a single process that removes many water-soluble organic materials, as well as microbiological contaminants. Since all UF membranes are capable of effectively straining protozoa, bacteria and most viruses from water, the process offers a disinfected filtered product with little load on any post-treatment sterilization method such as UV radiation, ozone treatment or even chlorination.

Unlike RO, the pretreatment requirement for UF normally is quite low. Fortunately, due to the chemical and hydrolytic stability of UF membrane materials, some of the pretreatments essential for RO membranes (e.g., adjustment of pH or chlorine concentration levels) do not apply. However, it may be necessary to adjust the pH to decrease the solubility of a solute in the feed so that it may be filtered out.

UF is designed to remove suspended and dissolved macromolecular solids from fluids. The commercially available modules are designed to accept feedwaters that carry high loads of solids. Because of the many uses for UF membranes, pilot studies normally are conducted to test how suitable a given stream is for direct UF.

Water containing dissolved or chelated iron and manganese ions needs to be treated by an adequate oxidation process in order to precipitate these ions prior to UF membrane filtration, as with all membrane processes. This is recommended to avoid precipitation of iron and manganese in the membrane or, even worse, on the permeate side of the membrane (membrane fouling during the backwash procedure). Preoxidation processes generally used include aeration, pH adjustment to a value greater than eight, or addition of strong oxidants such as chlorine, chlorine dioxide, ozone or potassium permanganate.

Natural Organic Matter (NOM) is important to the potential fouling of the UF membrane and, consequently, in permeate flux that can be used under normal operating conditions. It is an interesting design option to use PAC or coagulants to pretreat the water to remove NOM and, consequently, decrease the surface of membrane needed.

UF membranes can be fabricated in either a tubular or flat-sheet form. Package plants, skid-mounted standard units that allow significant cost savings, usually are employed for plants treating less than 1.5 mgd. The primary skid-mounted system components may include an auto-cleaning prefilter, raw water pump, recirculation pump, backwash pump, chlorine dosing pump for the backwash water, air compressor (valve actuation), chlorine tank, chemical tank (detergent), programmable logic controller with program and security sensor (high pressure, low level, etc.).

The UF membrane plant may be divided into several subcategories.

• Raw water intake and pressure pumps;

• Pretreatment, which includes prescreening, prefiltration and pH adjustment (if required) or any of the needed pretreatments;

• UF units;

• Chemical cleaning station, backwash station (which uses chlorinated product water), chlorine station, conditioner/ preservative station; and

• Line for discharging or treatment of backwash water.

Operation and performance of a UF membrane plant are greatly influenced by raw water quality variations. Turbidity as well as Total Organic Carbon (TOC) of the raw water are water quality parameters that drive the operation mode and membrane flux for all UF plants.