Sand filter

src: i.ytimg.com

Sand filter is used as a step in the water purification process.

There are three main types; quick sand filters (gravity), sand filters up, and slow sand filters. These three methods are widely used in the water industry worldwide. The first two require the use of flocculant chemicals to work effectively while slow sand filters can produce high quality water free of pathogens, flavors and odors without the need for chemical help. Sand filters can, in addition to use in water treatment plants, are used for single household water purification because they use materials available to most people.

Video Sand filter

Histori

The history of separation techniques reaches far back, because the filter material has been used during the ancient period. Rush and genista plants are used to fill sieves that separate solid and liquid materials. Egyptians also use porous clay vessels to filter drinking water, wine and other liquids.

Maps Sand filter

The Sand Filtration Concept

The sand filter is a kind of depth filter. In general, there are two types of filters to separate solid particles from liquids:

• Surface filter, where particles are taken on a permeable surface
• Filter Depth, where particulates are captured in porous porous material.

In addition, there are passive and active devices for causing solid-liquid separation such as settling tanks, self-cleaning display filters, hydrocyclones and centrifuges.

There are several types of depth filters, some using fibrous materials and others using granular materials. Sand bed filters are an example of a granular loose media depth filter. They are usually used to separate small quantities (& lt; 10 parts per million or <10 g per cubic meter) of fine solids (& lt; 100 micrometers) of aqueous solutions. In addition, they are usually used to purify liquids rather than catching solids as valuable materials. Therefore they found most of its use in the processing of wastewater (wastewater).

Particle Solid Capture Mechanisms

Sand bed filters work by providing solid particles with many opportunities to be captured on the surface of sand grains. When the liquid flows through the porous sand along a twisting route, the particulate approaches the grain of sand. They can be captured by one of several mechanisms:

• Live collision
• Van der Waals or London's tensile strength
• The attractiveness of surface loads
• Diffusion.

In addition, particulate solids can be prevented from being captured by surface load repulsion if the sand surface charge is the same sign (positive or negative) as in solid particles. In addition, it is possible to remove captured particulates even though they may be re-captured at greater depth in the bed. Finally, contaminated sand grains with particulate solids may become more attractive or repel additional particulate solids. This may occur if by following the particulate sand grain loses the surface charge and becomes attractive to the additional particulates or vice versa and the surface charge is maintained holding further particulates from the sand grains.

In some applications it is necessary to pre-treat the waste flowing into the sand bed to ensure that solid particles can be captured. This can be achieved by one of several methods:

• Adjust the surface load on the particles and sand by changing pH
• Coagulation - adding a small cation with a high load (aluminum 3 or calcium 2 is usually used)
• Flocculation - adds a small amount of polymer chain charge that forms a bridge between solid particles (make it bigger) or between solid particles and sand.

Operation Regime

They can be operated either with the liquid flowing up or flowing down the latter fluid to be much more mundane. For the device to flow down, the liquid can flow under pressure or by gravity alone. The pressure sand bed filter tends to be used in industrial applications and is often referred to as a quick sand filter. Gravity feed units are used in water purification especially drinking water and these filters have been widely used in developing countries (slow sand filters).

Overall, there are several categories of sand filters:

1. quick sand filter (gravity)
2. quick sand filter (pressure)
3. sand filter up
4. slow sand filter.

The sketch describes the general structure of a fast-pressed sand filter. Filter sand takes up most of the space. It sits nicely on the nozzle floor or above the drainage system which allows filtered water to come out. Pre-treated raw water enters the filter chamber above, flowing through the filter media and the drain through the drainage system at the bottom. The large process plant also implements systems to distribute raw water evenly to the filter. In addition, a distribution system that controls airflow is usually included. This allows constant air and water distribution and prevents the flow of water that is too high in certain areas. The typical grain distribution comes out due to frequent backwash. Smaller diameter grains are dominant at the top of the sand layer while coarse grains dominate at the bottom.

Two processes affecting filter function are cooking and regeneration At the beginning of running the new filter, the filter efficiency increases with the number of particles captured in the medium. This process is called filter maturation. During the maturation filter, the effluent may not meet the quality criteria and should be reinjected in the previous step at the plant. The regeneration method allows the reuse of filter media. The accumulation of solids from the filter pad is removed. During the backwash, water (and air) is pumped backward through the filter system. Backwash water can be partially injected in front of the filter process and generates waste that needs to be disposed of. The backwashing time is determined by the turbidity value behind the filter, which should not exceed the specified threshold, or by the head loss in the filter media, which must also not exceed a certain value.

Fast Pressure Sand Filter Design

Smaller sand grains provide more surface area and therefore higher inlet water decontamination, but also require more pumping energy to move fluid through the bed. A compromise is that the fastest press sand filter uses granules in the range of 0.6 to 1.2 mm even for special applications, other measures can be specified. Larger feed particles (& gt; 100 micrometers) will tend to block the pores of the bed and turn them into blinding surface filters quickly. Larger sand granules can be used to overcome this problem, but if significant amounts of significant solids are in the feed, they should be released upstream of the sand filter by a process such as precipitation.

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

Guidelines on fast sand filter designs indicate that they should be operated at a maximum flow rate of 9 m ³ /m ² /hr (220 US gal/ft ² /hour). By using the required throughput and maximum flow rate, the required area of ​​the bed can be calculated.

The final design point of the design is to ensure that the fluid is properly distributed throughout the bed and no preferred fluid pathway where sand can be washed away and the filter becomes disturbed.

Rapid sand pressure filters are usually operated with feed pressure of 2 to 5 bar (a) (28 to 70 psi (a)). Decreasing pressure on a clean sand bed is usually very low. It is formed as a solid particle that is caught in bed. Particulate solids are not captured uniformly with depth, more captured higher with beds with exponentially decaying gradient concentrations.

This type of filter will capture particles to a very small size, and has no actual cut-off size under the particles that will always pass. The shape of the particle-size-size efficiency curve is a U-shape with high particle retention rates for the smallest and largest particles with a slope between medium-sized particles.

The accumulation of particulate solids causes an increase in pressure loss in bed for a given flow rate. For gravity-fed beds when pressure is constant, the flow rate will decrease. When the pressure or flow loss is unacceptable and the filter does not work effectively much longer, the bed will be re-destroyed to remove the accumulated particles. For this fast pressurized sand filter this occurs when the pressure drops around 0.5 bar. The backwash fluid is pumped backward through the bed until it is fluidized and has grown to about 30% (sand grains begin to mix and when they rub together they dissipate particulate solids). Smaller particulate solids are cleaned with backwash fluid and captured usually in a settling tank. The fluid flow required to fluid the bed is usually 3 to 10 m ³ /m ² /hour but not long (minutes). A small amount of sand can be lost in the backwashing process and the bed may need to be refilled periodically.

Slow Sand Filter Design

As the title suggests, filtration rates are modified in slow sand filters, however, the largest difference between slow and fast sand filters, is the biologically active layer of sand, since microbial communities are introduced to the system. The recommended depth and usually of the filter is 0.9 to 1.5 meters. The microbial layer is formed within 10-20 days from the beginning of the operation. During the filtration process, raw water can seep through the porous sand medium, stopping and trapping organic materials, bacteria, viruses and cysts like Giardia and Cryptosporidium . The regeneration procedure for slow sand filters is called friction and is used to mechanically remove dry particles on the filter. However, this process can also be done underwater, depending on the individual system. Another limiting factor for treated water is turbidity, which for slow sand filters is defined to be 10 NTU (Nephelometric Turbidity Unit). Slow sand filters are a good choice for limited budget operations because filtering does not use any chemicals and requires little or no mechanical help. However, as the population continues to grow in the community, slow sand filters are replaced for quick sand filters, largely because of the running time.

Characteristics of Fast and Slow Sand Filtering

Mixed bed filters

Filters can be built with different layers, called mixed bed filters. Sand is a common filter material, but anthracite, granular activated carbon (GAC), garnet and ilmenite are also common filter materials. Anthracite is a harder and more volatile material than any other coal. Ilmenite and garnet are heavy compared to sand. Garnet consists of several minerals, causing a red color change. Ilmenite is iron oxide and titanium. GAC can be used in the adsorption and filtration process at the same time. These materials can be used alone, or in combination with other media. Different combinations provide different filter classifications. Monomedia is a single layer filter, generally consisting of sand and now replaced by newer technology. Monomedia deep-bed is also a one layer filter consisting of anthracite or GAC. Bed monomedia filters are used when there is consistent water quality and this gives longer running time. Double media (two layers) often contains a layer of sand at the bottom with an anthracite or GAC layer above. Trimedia or mixed media is a filter with three layers. Trimedia often has garnet or ilmenite in the lower layer, sand in the middle and anthracite at the top.

Use in Water Treatment

All of these methods are widely used in the water industry worldwide. The first three in the list above require the use of flocculant chemicals to work effectively. Slow sand filters produce high quality water without using chemical help.

Water passing flocculated through a gravity sand filter quickly suppresses the floc and the particles trapped within it reduce the amount of bacteria and remove most of the solids. Filter media is sand with various levels. Where taste and smell can be a problem (organoleptic impact), the sand filter may include an activated carbon layer to remove such flavor and odor.

Sand filters become clogged with floc or bioclog after a period of use and they are then washed or pressure washed to remove floc. This backwash water is run into a settling tank so floc can settle and then disposed of as waste material. The supernatant water then returns to the treatment process or is discharged as a waste water stream. In some countries, mud can be used as a soil conditioner. Inadequate filter maintenance has been the cause of contamination of drinking water.

Sand filters are sometimes used in sewage treatment as a final polishing step (see Waste Treatment). In this filter the sand traps of suspended materials and bacteria and provide a physical matrix for the breakdown of bacteria from nitrogen, including ammonia and nitrate, into nitrogen gas.

Sand filters are one of the most useful treatment processes because the filtering process (especially with slow sand filtering) incorporates many purification functions.

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 handled properly. The above-mentioned processes such as filter maturation and the effect of washing back affect not only the water quality but also the time required for full maintenance. Backwash also reduces the effluent volume. If some water should be sent to eg. a community, this water loss needs to be considered. In addition, backwash waste needs to be treated or disposed of properly. From a chemical perspective, a variety of raw water quality and changes in temperature effects, already at the entrance to the plant, the efficiency of the treatment process.

A considerable uncertainty associated with the model used to create sand filters. This is because the mathematical assumption must be made as all grains become round. The spherical shape affects the interpretation of the size due to the different diameters for round and non-spherical grains. The packing of grains in the bed also depends on the shape of the grain. This then affects the hydraulic porosity and flow.

src: www.enlight-inc.com

See also

• The American Water Works Association
• Water treatment
• Water purification
• Jewell water filter

src: images-na.ssl-images-amazon.com

References

Source of the article : Wikipedia