Sand filter

 ( Water Filters ) 

The depth of the sand bed is recommended to be around 0.6–1.8 m (2–6 ft) regardless of the application. This is linked to the maximum throughput discussed below.

Guidance on the design of rapid sand bed filters suggests that they should be operated with a maximum flow rate of 9 m³/m²/hr (220 US gal/ft²/hr).K. J. Ives (1990). "Deep Bed Filtration". Chap. 11 of Solid-Liquid Separation, 3rd ed., L. Svarovsky (ed). Butterworths. Using the required throughput and the maximum flow rate, the required area of the bed can be calculated.

The final key design point is to be sure that the fluid is properly distributed across the bed and that there are no preferred fluid paths where the sand may be washed away and the filter be compromised.

Rapid pressure sand bed filters are typically operated with a feed pressure of 2 to 5 bar(a) (28 to 70 psi(a)). The pressure drop across a clean sand bed is usually very low. It builds as particulate solids are captured on the bed. Particulate solids are not captured uniformly with depth, more are captured higher up with bed with the concentration gradient decaying exponentially.

This filter type will capture particles down to very small sizes, and does not have a true cut off size below which particles will always pass. The shape of the filter particle size-efficiency curve is a U-shape with high rates of particle capture for the smallest and largest particles with a dip in between for mid-sized particles.

The build-up of particulate solids causes an increase in the pressure lost across the bed for a given flow rate. For a gravity fed bed when the pressure available is constant, the flow rate will fall. When the pressure loss or flow is unacceptable and the filter is not working effectively any longer, the bed is remove the accumulated particles. For a pressurized rapid sand bed filter this occurs when the pressure drop is around 0.5 bar. The backwash fluid is pumped backwards through the bed until it is fluidized and has expanded by up to about 30% (the sand grains start to mix and as they rub together they drive off the particulate solids). The smaller particulate solids are washed away with the backwash fluid and captured usually in a settling tank. The fluid flow required to fluidize the bed is typically 3 to 10 m³/m²/hr but not run for long (a few minutes). Small amounts of sand can be lost in the backwashing process and the bed may need to be topped up periodically.

• Anthracite: A hard, low-volatile coal, often used as the top layer due to its light density.
• GAC: Functions both as a filter and an adsorbent, capturing dissolved organic compounds.
• Garnet: A dense, mineral-rich medium with a reddish hue.
• Ilmenite: A heavy oxide of iron and titanium, used for its high density.
• Sand: A traditional and widely used filtration medium.

These materials can be used alone or in combination. In multimedia filters, the media layers are always arranged by density: heavier materials like garnet and ilmenite settle at the bottom, while lighter ones like anthracite are placed on top. This configuration creates varying porosity throughout the filter bed, resulting in more effective filtration and different levels of pressure drop.

A typical multimedia filter setup includes anthracite on top, followed by sand, and then garnet at the bottom, all supported by a gravel base. The depth for such filters ranges from 0.6 to 1 meter each layer. Depths above 1 meter significantly increase pressure drop, while shallower beds compromise layer thickness and reduce efficiency.

When the pressure drop exceeds 10 psi, a backwash operation is necessary. During backwashing, water flow is reversed (upward) to lift the filter media and remove accumulated particles, which are then discharged with the backwash water. Common for the backwash is around 3 times the normal filtering flux (must be high enough to lift the filtering media to remove the particles trapped in it).

Filters classification:

• Monomedia Filters: Consist of a single layer, typically sand; these are largely obsolete.
• Deep-Bed Monomedia: Use a single layer of either anthracite or GAC, ideal for consistent water quality and longer operating cycles.
• Dual Media Filters: Comprise two layers, typically sand at the bottom and anthracite or GAC on top.
• Trimedia Filters: Use three layers, (commonly garnet or ilmenite (bottom), sand (middle), and anthracite (top)), offering a balance of filtration efficiency and capacity.

Mixed bed filters parameters:

Passing flocculation water through a rapid gravity sand filter strains out the flocculation and the particles trapped within it, reducing numbers of bacteria and removing most of the solids. The medium of the filter is sand of varying grades. Where taste and odor may be a problem (organoleptic impacts), the sand filter may include a layer of activated carbon to remove such taste and odor.

Sand filters become clogged with floc or bioclogging after a period in use. Slow sand filters are then scraped (see above) while rapid sand filters are backwashed or pressure washed to remove the floc. This backwash water is run into settling tanks so that the floc can settle out and it is then disposed of as waste material. The supernatant water is then run back into the treatment process or disposed of as a waste-water stream. In some countries, the sludge may be used as a soil conditioner. Inadequate filter maintenance has been the cause of occasional drinking water contamination.

Sand filters are occasionally used in the sewage treatment as a final polishing stage. In these filters the sand traps residual suspended material and bacteria and provides a physical matrix for bacterial decomposition of nitrogenous material, including ammonia and , into nitrogen gas.

Sand filters are one of the most useful treatment processes as the filtering process (especially with slow sand filtration) combines within itself many of the purification functions.