Filtration is a physical separation process used in municipal and large-scale water treatment to remove suspended solid matter from the liquid. The process involves passing water through a porous medium, which acts as a barrier to trap particles and clarify the water supply. This step is a fundamental component of the multi-stage system that cleans water sourced from rivers, lakes, or groundwater. Filtration ensures the water meets stringent quality standards by physically removing contaminants that cannot be eliminated by earlier treatment phases.
The Role of Filtration in Water Treatment
Filtration serves a primary function in water treatment by significantly reducing the water’s turbidity, which is the measure of its cloudiness or haziness caused by suspended solids. These suspended particles, such as silt, clay, organic matter, and microorganisms, are often too small to be completely removed by sedimentation alone. By intercepting this particulate matter, filtration directly improves the aesthetic quality of the water, making it visibly clearer.
The removal of suspended solids is a necessary pretreatment step that directly supports subsequent processes, particularly disinfection. Many pathogens, including bacteria and viruses, can attach to or be shielded by larger particles in the water. If these particles are not removed, they can interfere with the effectiveness of disinfectants like chlorine, which may not be able to penetrate the solids to neutralize the microbes within.
By providing a cleaner water stream, filtration lowers the overall demand for chemicals, making the entire treatment train more efficient. A consistent, low-turbidity effluent from the filtration stage allows the final disinfection step to work reliably against any remaining microscopic organisms. This ensures a robust defense against waterborne diseases and maintains the integrity of the water supply system.
Fundamental Mechanisms of Particle Removal
Filtration achieves particle removal through two main physical processes: mechanical straining and attachment. Mechanical straining is the most straightforward mechanism, where particles larger than the open spaces or pores in the filter medium are simply blocked from passing through. This principle is dominant in filters with very small, uniform pores, like membrane filters, where the physical size difference dictates separation.
The second primary mechanism, attachment, involves smaller particles adhering to the surface of the filter media grains. These particles, which are much smaller than the spaces between the media, are transported close to the grain surface by water flow. Once in proximity, weak attractive forces, such as Van der Waals forces, along with chemical and electrostatic forces, cause the particle to stick or adsorb onto the stationary filter material.
In granular media filters, a combination of these processes occurs throughout the filter bed’s depth. As trapped particles accumulate, they begin to form a secondary filtering layer, making the pore spaces even smaller and enhancing contaminant capture. Over time, this buildup increases the resistance to water flow, signaling the need for the filter to be cleaned.
Major Filtration Technologies
Granular/Media Filtration
Granular media filtration involves passing water through a bed of materials like sand, gravel, and sometimes crushed anthracite coal. The two most common forms are Rapid Sand Filters and Slow Sand Filters, which differ significantly in operation. Rapid Sand Filters (RSF) are widely used today because they can treat high volumes of water quickly, operating at flow rates between 4 and 21 meters per hour. These filters typically use relatively coarse sand and require extensive chemical pretreatment, such as coagulation and flocculation, to clump small contaminants into larger particles before they enter the filter bed.
The primary removal mechanism in RSFs involves depth filtration, where particles penetrate deep into the bed before being trapped. When the filter becomes clogged, it is cleaned by backwashing, which involves reversing the flow of clean water to fluidize the bed and flush the accumulated solids out to waste.
Slow Sand Filters (SSF) represent the older technology, operating at a much lower rate, typically between 0.1 and 0.4 meters per hour. SSFs use a finer sand and rely on the biological layer that develops on the surface for effective purification. This layer, often called the schmutzdecke, is a complex, living layer of algae, bacteria, and protozoa that biologically consumes and traps contaminants. Since filtration is concentrated at the surface, cleaning is performed by physically scraping off the top one to two centimeters of the clogged sand layer.
Membrane Filtration
Membrane filtration technologies use synthetic barriers with precisely engineered pore sizes to physically exclude contaminants. These systems are categorized by the size of the particles they are designed to remove.
Types of Membrane Filtration
- Microfiltration (MF) membranes have the largest pores (0.1 to 10 micrometers) and are effective at removing suspended solids and most bacteria.
- Ultrafiltration (UF) membranes have smaller pores (0.001 to 0.1 micrometers) and can reject larger molecules, colloidal particles, and viruses. This level of filtration is often used as a pretreatment step.
- Nanofiltration (NF) operates with pores as small as 0.001 to 0.01 micrometers, allowing it to remove organic molecules, divalent ions that cause water hardness, and some monovalent ions.
Membrane systems operate under pressure to force the water through the microscopic pores, providing a reliable barrier to contaminants. Since separation is based purely on size exclusion, these technologies produce high-quality water. The choice among membrane types depends on the size of the contaminants present and the desired purity level of the finished water.