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To make a very general classification, considering the condition of the substance to be filtered from the water, it can be examined under the particle filtration and membrane filtration classes
With particle filtration, solids are separated from the liquid. Particle filtration is implemented by bag filters, deep filters, and cartridge filters.
Where the high water quality is the demand, membrane filtration is a better option comparing to particle filtration. Commonly used membrane filters are reverse osmosis filters, nanofiltration filters, and microfiltration filters.
Also, filter types can be classified based on particle separation method:
# A filter can separate the particles from the water by holding the particles with the filter media surface.
#Particles can accumulate in the media of a filter.
#A filter media can adsorb particles.
#Biological digestion can occur on the surface of a filter media.
Besides a general classification of the filters, micron and mesh concepts should be talked about before introduce water filtration methods. These simple barriers contain pores, a filter can be a metal or plastic material or even a chemical.
The filtration ability of the filters is expressed in two different units in terms of particle size.
1- Mesh: Mesh unit describes the ranges of the filter screen. If the mesh number of a filter is large, the solid particle size it can hold is small.
2-Micron: 1 millimetre is a thousand times larger than 1 micron. For instance, a person’s hair is 60–80 microns in size.
Pore size has great importance in filter selection. According to the size of the contaminants to be treated from the water via filtration, the buyer decides the pore size of a filter.
Sediment Filters:
Sediment filtration is a process where removing sediments such as silt, rust, minerals, plant residuals, animal residuals, stones, debris, dirt, etc. from the water. Sediments are relatively bigger particles and most of the time they are suspended solids in the water. Sediment filters are widely used in industrial, commercial or domestic water treatment. If the water includes big particles, sediment filters will be helpful for the protection of the following equipment in the system while they help to remove unwanted suspended solids from the raw water.
Sediment filters are capable of removing particles from 1–100-micron size
A sediment filter is at the first filter in a system in general because it helps the protection of the following equipment. Sediment filter blocks the particle on the surface. Contaminants smaller than sediment filter pores pass through the filter.
Cartridge Filters:
They are modular type filters. With the filtration process, they remove particles and sometimes even chemicals. There are many types of cartridge filters. For instance, they can be manufactured with different methods such as melt-blown, string wound, etc.
Bag Filters:
Bag filters are preferred in small applications. As the name suggests, bag filters look like a long bag. Wastewater enters this bag-like filter. Here the particles are captured and separated from the water. If you need a filter for a cost-effective small application, a bag filter will do the job.
Carbon Filters
In carbon filtration, in an activated carbon bed, some contaminants in the water are removed. These contaminants are generally substances such as chlorine, ozone, and other organic material. Activated carbon filtration is generally used in a system where odour and taste of water is an issue.
An activated carbon bed is employed for the adsorption of the substances.
It is not a good practice to remove the thin solids in the water from the water with the activated carbon process because the contaminants trapped in the activated carbon filter bed cannot be removed from the filter by backwashing. The reason for this restriction about removing the thin particles is that the activated carbon bed is composed of highly porous granules and has a sponge-like structure. This structure, which consists of many porous granules, has a very large area for the contaminants trapped in the porous. In water treatment, 1 gram of activated carbon means more than 3000 m2 surface area.
Activated carbon filter allows water to pass through while blocks VOCs, some of the organics and negatively charged particles.
Carbon filtration can be applied by an activated carbon block or granular activated carbon.
Both systems work similarly. Carbon material itself adsorb negatively charged ions.
Bacteria and viruses cannot be removed by carbon filtration.
Upflow Filters:
For an in-depth filter media, the water flow is downward from the filter bed. This changes when the filter is back washed.
While the backwashing process takes place, the filter is cleaned with the flow of water from the bottom of the filter bed to the up of the filter bed.
It is possible to ensure that water flows upwards while the filtration process is taking place. This upstream provides an advantage, but it also raises some problems. By the backwashing of the filter media, the filter media tends to turn into a more dense material. This density takes place in the lower part of the filter bed, which is, in its downward direction. A small amount of density occurs at the top of the filter bed. In this case, during the downward filtration, there is gradually increasing filtration from the wider filter layer. This contrasts with the ideal filtration geometry. As the filtration progresses, it should occur from media with smaller pores. This problem is solved by making the flow of water going upwards.
Reverse Osmosis Filters:
Reverse osmosis is the most advanced level of the filtration processes. These systems are capable of removing salts at the %99 level. Also bacteria, viruses, dye, sugar, and protein can be removed from water via reverse osmosis filters.
These filtration systems are often placed before reverse osmosis systems. Ultra filtration takes place under low pressure. Water passes through very small pores here. The size of these pores ranges from 5 nm to 100 nm. Chemicals can be used for backwashing such filters.
Sand Filters
Slow Sand Filters:
Filter media is often sand in slow sand filters. The sand filter bed can be 1–2 meters deep. Slow sand filters are commonly found on the market in cylindrical form.
The length and width of the filtration tank are determined based on the flow rate required on the filters. Over time, a layer of biological film forms on the top of the slow filters. Water is purified from contaminants by microbiological activities on this layer formed on the surface of the sand.
Rapid Sand Filters:
The filter media of fast sand filters are usually sand. However, in some cases, anthracite is used instead of sand.
The depth, size, and area of the filter media are important for storing the captured particles.
The principle in this system is to catch the contaminants in the water on the filter surface. Rapid sand filters can be cleaned by backwashing.
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