Water turbidity describes the cloudiness or haziness caused by fine particles suspended in the liquid. These particles, which include silt, clay, organic matter, and microscopic organisms, scatter light and reduce water clarity. Reducing turbidity is a fundamental goal in water management because it directly affects the usability and quality of the water source. The presence of these suspended solids requires treatment to ensure the water is suitable for uses ranging from recreational swimming to public consumption. This article explores the established physical and chemical methods used to lower the concentration of these particles.
Understanding Turbidity Measurement and Impact
Water clarity is quantified using the Nephelometric Turbidity Unit (NTU), which measures the amount of light scattered by suspended matter. A specialized instrument called a nephelometer shines a light beam through the water and measures the light scattered at a 90-degree angle. Higher NTU readings indicate greater cloudiness and a higher concentration of particles; regulatory bodies often aim for levels below 0.3 NTU for treated drinking water.
Reducing the NTU level is necessary for functional reasons beyond just improving appearance. High turbidity interferes with disinfection processes, particularly those using chlorine or ultraviolet (UV) light. Suspended particles can shield harmful microorganisms, such as bacteria and viruses, allowing them to pass through treatment untreated. Furthermore, suspended solids can lead to premature wear and clogging in plumbing, pumps, and water treatment equipment like membrane filters.
Physical Methods for Particle Removal
The most straightforward way to remove large suspended solids relies on gravity, a process known as sedimentation. When water is held still in a tank or basin, heavier particles, such as sand and larger silt, naturally settle to the bottom. This technique is typically used as a pretreatment step to reduce the overall particle load. Filtration is the primary physical method for removing smaller, non-settleable particles by passing the water through a porous medium. Filters are selected based on the size of particles to be removed, measured by the micron rating. Typical household sediment filters may remove particles down to 10 microns, effectively trapping fine sand and coarse silt.
Media and Cartridge Filters
Media filters, such as those employing layers of sand and gravel, are common in municipal and larger private well systems. These filters trap particles within the depth of the media bed and are regularly cleaned by reversing the water flow in a process called backwashing. Cartridge filters, often used at the point of entry or point of use in homes, utilize pleated or spun material to strain contaminants, requiring physical replacement once saturated.
Membrane Filtration
For extremely fine particles, specialized options like microfiltration or ultrafiltration use membranes with pore sizes as small as 0.02 to 2.0 microns. This allows them to remove materials like fine clay that would pass through standard filters.
Using Chemical Aids for Clarification
Microscopic particles causing high turbidity, such as fine clay or organic material, often carry a negative electrical charge, causing them to repel each other and remain suspended. Chemical clarification addresses this by neutralizing the charge and encouraging particles to join together.
Coagulation
The process begins with coagulation, where positively charged chemicals are introduced to the water. Common coagulants include aluminum salts (such as alum) and iron salts (such as ferric chloride or ferric sulfate). These chemicals neutralize the surface charge of the fine particles, destabilizing the suspension and allowing the particles to come into contact.
Flocculation
Following coagulation, the water undergoes flocculation, which involves gentle mechanical mixing. This slow movement encourages the neutralized micro-particles to collide and bind together, forming larger, visible clumps called floc. The floc, which can be enhanced with polymer aids, becomes heavy enough to settle out of the water via gravity. If settling is not used, the water is routed to a filter, as the physical media can now easily trap the large floc particles. This two-step chemical process is necessary pretreatment for water sources with high levels of colloidal turbidity.
Applying Turbidity Reduction to Different Water Sources
Drinking Water Systems
For drinking water sources, especially private wells, turbidity reduction is primarily achieved through a combination of physical methods. Sediment filters are commonly installed as a first line of defense to remove grit and large particles, protecting subsequent treatment stages. A low NTU value is important because it ensures that final disinfection steps, such as UV light or chlorination, can effectively inactivate pathogens without interference from shielded particles.
Ponds and Decorative Features
In pond or decorative water features, turbidity is often caused by suspended clay, silt, or excessive algae growth. Chemical clarification is frequently employed using products like alum or gypsum, which cause clay particles to bind together and rapidly settle. Long-term control involves biological and aeration strategies, as increased oxygen levels and nutrient removal limit the growth of microscopic organisms contributing to cloudiness.
Swimming Pools
When dealing with a cloudy pool, the first step involves balancing the water chemistry, particularly the pH, to ensure the sanitizer is working effectively. If cloudiness persists, specialized pool flocculants or clarifiers are used. Flocculants quickly clump fine particles into large masses that are then removed by vacuuming the settled material to waste. Clarifiers work more slowly, binding particles into sizes large enough for the existing filtration system to capture them, often requiring continuous filter operation until the water clears.