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Gravity Filtration is a scientific way to filter out the impurities in the solutions. In the chemistry laboratory, it always used to filter out the solids from the Precipitation reaction, it always has three different filtrations( standard, vacuum and pressure). The basic logic of filtration is due to the direction of gravity is pointing to the center of the Earth. Many high school or universities has already teach the knowledge about gravity filtration to students. Lots of industry used this filtration to filter out their Industrial raw materials as initial step.

''This article is belonging to the Analytical chemistry. It is a way to filter out the different chemical compounds or to filter out the impurity and then the examples in industry, laboratory and real world examples. Also it will list some operations and precautions in experiments.''

Method and Principle of Filtration
The removal of suspended matter through the media is generally considered about two steps: transport and attachment. The way of transporting is to move particles into another place by through the filter paper.

Gravity filtration in chemical lab is an easy way to remove solid impurities or the precipitation from an organic liquid. The impurity can will remain at the filter paper. Gravity filtration can be used to collect any insoluble solid and their derivative filtration: vacuum filtration have higher efficiency  of filtration. The Gravity of Filtration is based on the Gravity，it will make the insoluble impurities still remain at the filter paper and the solutions will permeate to the erlenmeyer. For different densities and viscosity of solutions can affect the efficiency of gravity filtration, the higher viscosity of solutions will have a lower efficiency of filtrations.

History of Gravity filtration
In the early human era, people get clear water from the muddy river or lake by plunging the sand hole on the bank to the depth below the river or tank level. The sands will be filtered and then clear water will drop through the hole, they used this way to reform their city and purify their urban water.

In the farming place, people used the gravity filtration theory to let water is the high place through the low place and put the filter on each ramps, in this case, the sand and little stones will be filter out and left the clear water through the field.

In asian region, people pump the water from well and put them into a jar with a small hole at the bottom, the jar will fill with some little stone and the hole will cover with several layers of gauze in order to filter out the sand in the water by gravity.

Standard gravity filtration:
This filtration is used in the laboratory and has a higher efficiency to filter out the solutions which contains the solids or the insoluble precipitation. Filter paper should be fold to make them like a cone and then put it on the top of bottle of the Glass Funnel. Then put solutions and let them through the filter paper, the solution which has been filtered will be held by a conical flask. The rest materials on the filter paper are the solids or the flocs. If you need to filter out large volume of solutions, you have to change the filter paper in order to keep the efficiency of filtration.

The Vacuum Filtration:
The vacuum filtration is an another way to filter out impurities in the solutions, it has a higher efficiency and need more equipments to support this filtration.

The Vacuum filtration have more functions, it can collect the crystals after crystallization and precipitation when all water in the insoluble chemicals were absorbed into the flask. But this filtration can not filter out the large solids in the solutions. The first step is that we need to set up the Buchner Funnel  and put the filter paper inside to filter out the large size of insoluble chemicals and then use vacuum pump to take the air out of funnel. In this case, the pressure at top of bottle is the standard atmosphere which is higher than the pressure in the funnel, it will push the water into the funnel and remain the insoluble chemicals, the long time of vacuum filtration, the more dry chemicals or solids and precipitation can get.

This filtration has a higher strict requirements than others because it needs a better Air tightness.

Pressure filtration:
This kind of gravity filtration is used the pressure to remove the water from the materials contains liquid and remain the chemicals without liquid to form a non-liquid materials.

These models of the filtertions were usually called plate filters and frame filters which usually used in industry, because they combined of varieties of frames and plates on which filter media.

Experimental error
In many laboratories, the gravity filtration is used to filter out the insoluble solids to determine the yield of reaction, so in order to minimize the range of error of calculation, there are several experimental errors which we need to take care.

The solids will be remained at filter paper or the fummel and not put all of them on the top balance, in this case, the real yield of product will have a large gap between the ideal yield of product.

The lower air tightness will due to a lower efficiency of filtration and many water on the top surface of impurities will not drop down to the conical flask or the final impurities are still moist. in this way, the real yield of precipitation may be larger than the ideal yield of precipitation.

The incorrect way to fold or choose the filter paper may influence the filtration, damage the filter paper will allow the small size of solids drop down to the conical flask by water or solutions.

Gravity filtration in seawater.
It is a variety of filtration operations were tested with seawater for dissolving some high concentrations of dimethylsulfoniopropionate.

It used in the seawater to reveal the Low dissolved DMSP. The gravity filtration can be aimed to large volume of solution by the glass fiber. To avoid the DMSPd released, they changed the smaller volume gravity filtration and list the basic operation of how to collect samples. It also limits the equipment size and operating time. They propose the gravity need to in the hydrostatic pressure to filter. In this case, they found that the biggest concentration of DMSPd are far smaller than the previous report’s normal concentration, this change in the measurement can raise the accuracy of data. This article was published as an academic report, so, it has a high notable value and can clearly elaborate the current application in the world.

Conventional Gravity Filters
These Filters contain three stages which are Flocculation, Clarification and Filtration.

Some typical rapid gravity filters contain filter tanks which are made by coated steel, stainless steel or the aluminum. This filtration used the theory of Gravity Filtration, the operations is that the influent flows will go down through the filter and then they will be captured by the underdrain. The important part is that the Filter Media, it aimed to remove the particles from the water, it usually has 3 layer: Anthracite Coal, Silica Sand, Gravel.

These filtrations have a highly effective for removing the impurities and used less time to clean so it will have a low-cost and save more water. The simple operation is also their one of advantages.

Gravity filtration to remove the microorganisms
This equipment was to remove the parasite and other contaminant such as lead. In this report, the equipment contain the multi-hole filters and their diameters can allow the water flow by the gravity, in the flow way, it will take the parasite out of the equipment. This equipment used the gravity filtration and got the patent in American.

Gravity Media Filters
These filters is used for a wide variety of the manufacturer and industry applications. The gravity theory is a way to let the fluid stream to pass through the granular media to remains or filter out the impurities. It can support the large volume of solutions.

Some Gravity Filter System in Chemistry Industry by this machine can remove chlorine and other organics or remove the iron and some heavy sediment or sand.

Gravity Filter（liquid）
The liquid will remove from the gas stream by Coalescers in a single stage of elements, the vessel which below the elements will be entered by the flow and the passes through the distributor. This is a primary separation device which can remove the coarse by gravity and then coalesce the cartridges in an inside-to-out direction. In this case, the liquids will through the structure of the element and then filter by gravity and be drained from vessel finally.

Model SK gravity filter
This gravity filter is an open sand filter system and use for the water treatment in the extremely economical environment. It can suit in the variety of pressure-controlled backwashing. This filter is an automatic gravity filter which use the different pressure and backwashing the system with the injector. The system has no controls, the filters do not need the moving parts and it has no pumps, because it required the backwashing water in the tank which is below the filter.