Sand filter - WikiHQ

thumb|right|Sand filter used for water treatment

Sand filters are used as a step in the water treatment process of water purification.

There are three main types; rapid (gravity) sand filters, upward flow sand filters and slow sand filters. All three methods are used extensively in the water industry throughout the world. The first two require the use of flocculant chemicals to work effectively while slow sand filters can produce very high quality water with pathogens removal from 90% to >99% (depending on the strains), taste and odour without the need for chemical aids. Sand filters can, apart from being used in water treatment plants, be used for water purification in singular households as they use materials which are available for most people.

History

The history of separation techniques reaches far back, as filter materials were already in use during ancient periods. Rushes and genista plants were used to fill sieving vessels that separated solid and liquid materials. The Egyptians also used porous clay vessels to filter drinking water, wine and other liquids.

Sand bed filtration concept

thumb|Sand filters on a tomato farm in [[California]]

A sand bed filter is a kind of depth filter. Broadly, there are two types of filters for separating particulate solids from fluids:

Surface filters, where particulates are captured on a permeable surface
Depth filters, where particulates are captured within a porous body of material.

In addition, there are passive and active devices for causing solid-liquid separation such as settling tanks, self-cleaning screen filters, hydrocyclones and centrifuges. In addition, they are usually used to purify the fluid rather than capture the solids as a valuable material. Therefore they find most of their uses in liquid effluent (wastewater) treatment.

Particulate solids capture mechanisms

Sand bed filters work by providing the particulate solids with many opportunities to be captured on the surface of a sand grain. As fluid flows through the porous sand along a tortuous route, the particulates come close to sand grains. They can be captured by one of several mechanisms:

Direct collision
Van der Waals or London force attraction
Surface charge attraction
Diffusion Regeneration methods allow the reuse of the filter medium. Accumulated solids from the filter bed are removed. 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.

Mixed bed filters

Filters that utilize different types of media arranged in layers are known as mixed bed or multimedia filters. Common filter materials include sand, anthracite, granular activated carbon (GAC), garnet, and ilmenite, each selected for their specific physical properties:

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:

{| class="wikitable"

! Parameters!! Typical Range

|Each layer of material [m]||0.6-1.0

| Flow rate [gpm/ft^2] || 3-7

| Pressure drop [psi] || 3-7

| Backwash trigger || Pressure drop exceeds 10 psi

Uses in water treatment

All of these methods are used extensively in the water industry throughout the world. The first three in the list above require the use of flocculant chemicals to work effectively. Slow sand filters produce high-quality water without the use of chemical aids.

Passing flocculated water through a rapid gravity sand filter strains out the floc 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 bioclogged 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 nitrates, 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.

Advantages and limitations

One of the advantages of sand filters is that they are useful for different applications. Moreover, the different types of operation modes: rapid, slow and Upflow, allow some flexibility to adapt the filtration method to the necessities and requirements of the users. Sand filters allow a high efficiency for color and microorganisms removal, and as they are very simple, operating costs are very low. What is more, its simplicity makes the automation of the processes easier, thus requiring less human intervention.

The main limitations of this technology would be related to the clogging, that is, the obstruction of the filter media, which requires a significant amount of water to carry out the backflush operation and the use of chemicals in the pretreatment. Furthermore, slow sand filters usually require larger land areas compared to the rapid flow, especially if the raw water is highly contaminated. However, despite these limitations, they offer much more capabilities and that is why they are extensively used in the industry.

Challenges in the application process

In the process of water treatment, one should be aware of certain factors that might cause serious problems if not treated properly. Aforementioned processes such as filter ripening and backwashing influence not only the water quality but also the time needed for the full treatment. Backwashing reduces also the volume of the effluent. If a certain amount of water has to be delivered to e.g. a community, this water loss needs to be considered. In addition, backwashing waste needs to be treated or properly discarded. From the chemical perspective, varying raw water qualities and changes in the temperature effect, already at the entrance to the plant, the efficiency of the treatment process.

Considerable uncertainty is involved regarding models used to construct sand filters. This is due to mathematical assumptions that have to be made such as all grains being spherical. The spherical shape affects the interpretation of the size since the diameter is different for spherical and non-spherical grains. The packing of the grains within the bed is also dependent on the shape of the grains. This then affects the porosity and hydraulic flow.

References

nl:Zandfilter