Sand filtration is a widely used process in both municipal and residential water treatment, serving as a physical barrier to remove impurities. This method relies on water passing through a bed of granular sand media, which captures solid particles and clarifies the water. Understanding what a sand filter is designed to remove, and what it cannot remove, is necessary to evaluate water quality. This article clarifies the mechanics of how sand filters work and outlines the specific contaminants they target, as well as their limitations.
The Physical Mechanics of Filtration
Sand filtration is often simplified to the concept of a sieve, but its effectiveness relies on a combination of physical processes beyond simple straining. The most direct mechanism is straining, where suspended particles are physically larger than the spaces between the sand grains and are trapped near the surface of the filter bed. However, the majority of removed particles are actually smaller than these pore spaces, especially in a clean filter bed.
A more significant removal mechanism is adsorption, where smaller particles stick to the surface of the sand grains. This occurs due to physical forces or electrostatic attraction between the particle and the filter media, causing the contaminant to adhere to the jagged edges of the sand. As water flows through the filter bed, heavier particles may also settle out of the flow path onto the sand surfaces, a process known as sedimentation.
Over time, the trapped solids and organic matter form a sticky layer near the top of the sand bed, especially in slow sand filters, which is sometimes called the “schmutzdecke”. This layer acts as an increasingly finer filter, trapping smaller particles and significantly enhancing the filter’s overall efficiency. The sand itself provides a substrate for this biological layer, which also helps to break down organic impurities through microbial action.
Contaminants Effectively Removed
Sand filters are highly effective at removing solid matter suspended in water, which is the primary source of cloudiness. The process is successful at reducing turbidity, the measure of water clarity caused by fine particles like silt, clay, and microscopic debris. A typical sand filter is rated to remove particles in the range of 20 to 40 microns and larger, depending on the sand media used.
This filtration capability makes them proficient at capturing various suspended solids, including dirt, rust flakes, and pollen. When chemicals called coagulants are added before filtration, they cause microscopic particles to clump together into larger masses known as floc. These larger particles are then easily captured by the sand filter, leading to higher removal efficiency for finer contaminants.
Sand filters are also valuable for removing certain types of large biological matter. They are effective at removing the cysts and oocysts of large waterborne protozoa, such as Giardia lamblia and Cryptosporidium. These pathogens, which cause gastrointestinal illness, are large enough (typically 3 to 15 microns) to be strained or adsorbed by a properly operating sand filter. While they are not sterilizing agents, sand filters significantly reduce the concentration of these parasites.
Substances Sand Filters Cannot Target
Despite their effectiveness against suspended solids, sand filters are limited in their ability to address contaminants that are not particulate. Dissolved solids, including inorganic minerals, salts, and chemical compounds, pass right through the sand media because they are integrated into the water at a molecular level. Simple sand filtration does not change the chemical composition of the water, so it cannot remove hard water minerals or dissolved metals.
The filter’s physical nature renders it ineffective against most chemical contaminants. Dissolved organic compounds, pesticides, herbicides, and pharmaceuticals are not adsorbed or strained by the sand grains. Removing these requires specialized media, such as activated carbon, which uses a different process to chemically attract and hold the molecules.
Sand filters are not reliable for removing the smallest waterborne pathogens. Bacteria (often smaller than 2 microns) and viruses (as small as 0.02 microns) are too minute to be consistently trapped by the sand grains. While a mature slow sand filter can remove a high percentage of bacteria, simple or rapid sand filtration requires a dedicated disinfection step, such as chlorination or ultraviolet light treatment, to ensure the water is safe.