Types of Membrane Filtrations

Please watch this American Water Course excepts that review the types of membrane filtration and there corresponding micron filtrations sizes. Please take notes as you watch, since it will be useful for this weeks quiz.

Membranes contain very fine pore openings that allow water to pass through and block the passage of any contaminant larger than the pore diameter. Membranes used in water treatment are classified by their pore diameter. Classifications, from the largest pore diameter to the smallest, are microfiltration, ultrafiltration, nanofiltration, and reverse osmosis.

The type of membrane used depends on the constituents to be removed from the water being treated. During water treatment, water is typically pumped against the surface of the membrane; however, water can be pulled through the membrane by a vacuum. The water pressure forces water through the membrane and the constituents that do not pass through form a waste stream that may require treatment and proper disposal.

Membrane Filtration Theory and Practices

Let's begin this unit by watching the overview by Susan Masten, titled "ENE 483: Membrane Filtration Theory and Practice." As you watch the video, please turn on the closed caption and take notes, which will be useful for this weeks discussion.

Pressure Vessel or Submerged Flow

Typical water treatment membrane filtration units are installed in pressure vessels or submerged in tanks. The membranes are hollow fibers or threads with an outside diameter ranging from 0.5 to 2 mm and a wall thickness of 0.07 to 0.6 mm. In the pressure vessel installation, thousands of membrane fibers or threads are arranged in racks or skids, with each pressure vessel ranging from 4 to 12-inches in diameter and 3 to 18-feet long. Filtration flow may be from outside to inside or from inside to outside of the hollow fiber membrane.

Submerged or immersed membrane filtration systems are membrane modules suspended in basins containing the water to be treated. The treated water may then be pulled through the membrane by vacuum. Some treatment plants have removed sand from the sand filters and installed submerged membrane modules in the old filter basin.

Membrane Flow Types

Two different types of membrane filtration feed water flows exist:

Cross-flow filtration systems
Dead-end filtration systems

figure 10.1.GIF

In cross-flow filtration systems, the flow is from the inside of the membrane, through the membrane, and the filtered water flows out of the system. The flow inside the membrane flows along the inside surface of the membrane, becomes concentrated, and flows out the end of the membrane fiber as a waste stream. In dead-end filtration systems, the water being filtered may flow from the outside into the hollow fiber or from the inside to the outside; however, in this system, no waste stream is produced. All solids accumulate on the membrane during filtration and are removed during backwash.

figure 10.2.GIF

Membrane Fouling

Membrane fouling can be described by whether the cause of the fouling can be removed (reversible or irreversible, by the material causing the fouling (biological, organic, particulate, or dissolved), and by the means of fouling (cake formation or membrane pore blockage).

Whether the cause of fouling can be removed depends on the type of membrane used and the constituents in the source water. During continued operation, the flow through the membrane may decrease; however, the flow may be recovered by backwashing and cleaning.

The constituents in the water being filtered may cause fouling. During membrane filtration, microorganisms are transported to the membrane surface where biofouling may occur. These microorganisms may not be removed by backwashing; however, they may be controlled by using chlorine. Some membrane materials such as cellulose acetate or polypropylene can be damaged using chlorine. Membrane manufacturers are tending to use materials that are not damaged by chlorine.

Dissolved organic matter may cause membrane fouling. The extent of the fouling problem depends on the characteristics of the dissolved organic matter, the membrane material, and the characteristics of the water being filtered.

During membrane filtration, particulate matter from the water being filtered collects on the membrane surface in a porous mat called a filter cake. Particulate fouling is usually reversible during periodic backwashing.

Natural organic matter can be the most common form of membrane fouling. Dissolved organic matter includes wastes and portions of aquatic plants and animals as well as organic matter washed into surface water from land. Sources of dissolved organic matter include organic chemicals found in biological systems and dissolved organic chemicals from industrial and commercial wastes. Fouling depends on the characteristics of the dissolved organic matter, the membrane material, and the source water.