Microfiltration: How Does it Compare ?

About the author:

Thomas Muilenberg is an applications engineer specializing in the area of membrane systems with General Filter Company, a manufacturer of water and wastewater process equipment for municipal and industrial applications.

Increased focus on technologies that meet tighter regulatory requirements and increased public pressure has motivated municipalities to take a serious look at microfiltration (MF) membranes as a viable treatment option. This article is intended to familiarize you with the basics and discuss how it compares to conventional alternatives.

What is Microfiltration?

Microfiltration is one of a number of membrane processes. Raw water is filtered by passing through a plastic or polymeric material which contains millions of small pores (see Figure 1). Filtering occurs because the membrane pores are large enough to allow water to pass though, yet small enough to restrict the passage of undesirable materials such as particulate matter and pathogenic organisms.

Because this filtration is one in a "family" of membranes, it is useful to compare it to other, perhaps more familiar, membrane technologies including reverse osmosis (RO), nanofiltration (NF) and ultrafiltration (UF). The primary difference between the types of membranes is the size of the pores in the membrane material: the smaller the holes, the smaller the materials the membrane removes. Each membrane has a particular range of applications for which it is best suited (see Table 1).

This membrane has a typical pore size of 0.2µm. It is best suited for the removal of particulate, turbidity, suspended solids, and pathogens such as Cryptosporidium and Giardia. A typical Cryptosporidium oocyst is approximately 3p;5µm in size, which is 15p;25 times larger than the pores. This membrane operates at low pressures of approximately 3p;15 TMP.

How Microfiltration Works

Membrane configuration can vary between manufacturers, but the "hollow fiber" type is the most commonly used. Membranes in the hollow fiber type are cast into small diameter tubes or straws, nominally one meter in length. Thousands of these straws are bundled together and the ends are bonded into an epoxy bulkhead or "potting." The ends of potting are cut off to allow access to the inside of the fibers from the end of the potting. The bundles are then sealed into a housing which is usually PVC or stainless steel. The sealed potting creates a separate, sealed space in the module that isolates access to the inside of the fibers from access to the outside. This membrane and housing combination is called a module. It allows water to be forced through the fiber walls without short-circuiting.

System design is done once the desired flow rate and water conditions are known and a pilot has been performed to determine the required number of modules. The modules are then piped together in a manner which will allow water to be forced from one side of the fibers through the membrane wall and collected from the filtrate side of the modules.

Typically, the water is pumped from the outside of the fibers, and the clean water is collected from the inside of the fibers. This is called "outside-to-inside" flow (see Figure 2). This flow direction is sometimes reversed depending on manufacturer and membrane configuration.

Microfiltration membranes used in potable water applications usually operate in the "dead-end" flow regime. In dead-end flow, all of the water fed to the membrane is filtered through the membrane (see Figure 3). A filter cake that must be periodically backwashed from the membrane surface forms. Recovery rates are normally greater than 90 percent on sources which have fairly high quality, low turbidity feeds.

Backwashing

Periodic backwashing is performed to remove filtered materials from the membrane surface. A water-only backwash backflushes a surge of filtered water through the membrane to lift sediment from the surface and flush it to waste. Some manufacturers use chemical backwashing or high pressure "air-ram" backwashing. However, the goal is the same regardless of method: to remove solids from the membrane by lifting dirt away. Backwashing is performed once every 10p;20 minutes and is normally done on a timed basis in order to prevent severe fouling which might occur if significant pressure were allowed to build up between backwashes.

Chemical Cleaning

Even with backwashing, MF membranes will slowly foul. This is indicated by a gradual increase in operating pressure. In order to maintain system performance over an extended period of time, chemical cleaning is employed. Usually preformed every one to four weeks, it is used to clean and sterilize the membrane. Several chemical cleaning techniques can be employed including chlorinated cleaning (only certain membranes can withstand this method), acid cleaning, caustic cleaning, or a number of proprietary solutions.

Microfiltration vs. Conventional

MF membranes have many features that compare with conventional systems, such as cost competitiveness. Upon first review, it appears that the cost for a membrane package is higher than equipment for conventional filters. However, the MF system is more of a complete package than filters alone. A source water MF plant is essentially complete. There are no chemical pre-feed equipment or feed controls, no flash mixers, no flocculators, and no complicated concrete work such as settling and filter basins. As a result, the total cost of an MF system often compares favorably with its conventional counterpart.

MF systems are easy to operate as filtration, backwashing and cleaning are all performed automatically. In addition, because it filters via a physical straining mechanism, MF usually requires no chemical pre-feed and chemical usage is kept to a minimum. There are no complicated chemical feed systems to monitor and optimize. Furthermore, since almost all bacteria, turbidity and pathogens are removed from the water, the amount of post-chlorination needed may also be reduced. Finally, the absence of chemical pre-feed means no knowledge of chemical mixing and flocculation is required.<

With the straining mechanism, the filtered water quality does not change with spikes in the raw water quality. Since the membrane excludes all particles greater than its pore size, the membrane will consistently remove particles regardless of the amount present. The result is that a spike in the feed turbidity will not show up in the effluent turbidity. Conventional systems, on the other hand, require close monitoring and operation by the plant operator, which is not always possible with smaller systems where the operators may only be on site intermittently.

Figures 4 and 5 depict typical operating data taken from a study performed for a small community. The supply water source is one that experiences relatively high turbidity spikes during storm events. The graphs are for raw and filtered turbidity and particle counts. They illustrate the consistency of filtered water quality over the duration of the study.

The membranes act as a physical barrier to pathogens such as Cryptosporidium and Giardia as well as bacteria. A typical MF pore size is 0.2 µm, and a Cryptosporidium is between 3p;5 µm. As seen in the graphs, even the smallest Cryptosporidium oocyst is 15 times larger than the membrane pore. With increased public concern over pathogen removal in drinking water, this feature is a primary benefit.

Reverse Osmosis / Nanofiltration

RO membranes are capable of the finest separations and are used for softening, chemical recovery, desalination, nitrate and sulphate removal, and radium removal. NF, sometimes called "leaky reverse osmosis," is closely related, capable of some softening and removal of color, THM precursors/organics, pesticides, metals, and viruses. The "tightness" of the membrane is described in terms of Molecular Weight Cut Off (MWCO) and percent rejection of certain ionic substances such as salt. In many cases, chemical pre-feed and proper pretreatment are critical in maintaining plant operation by minimizing fouling. RO membranes operate in the 200p;500 psi trans-membrane pressure (TMP) range for most municipal applications. NF membranes typically operate in the 60p;200 psi TMP range. TMP is the pressure loss across the membrane.

Ultrafiltration

This membrane has a typical pore size of about 0.002p;0.05µm (micron, 10-6 m, 1/25,000 inch). Ultrafiltration is often used for removal of macromolecules, colloids, viruses, and proteins in the biomedical and pharmaceutical industry. Ultrafiltration is sometimes applied to surface or ground water treatment for potable use when the source water is consistently low in turbidity with little chance of spikes. They generally operate in the 20p;50 psi TMP range.