For starters, let’s define Filtration and Separation.


In short, filtration and separation can be any mechanical, physical or biological operation used to separate solids from liquids, by causing the latter to pass through the pores of some substance, called a filter. This filter can be paper, cotton-wool, sand or any other porous material.

This processes are used both in nature and in engineered systems. For example, one type of filtration that is useful for all of us is the filtration of the water we are drinking – it’s extremely important to confirm your water has been purified or treated before drinking. This type of separation is relatively easy and water purification can be done by boiling, filtration, distillation or chlorination. While filtration is an important separation technique in engineered systems, it’s also common in everyday life.


Besides filtrating the water we are drinking, there are many more everyday activities where this process can be used:

  • Many aquariums use filters
  • The kidneys are also an example of a biological filter
  • Air conditioners and vacuum cleaners use filter to remove dust
  • Brewing coffee involves passing hot water through the ground coffee and a filter

Typically, filtration is an imperfect and a not-so-easy process when it comes to mechanical or physical operations. For example, the two problems that usually occur are that part of the liquid can somehow stay stuck in the filter or some of the small solid parts can find their way through the filter. That’s why, there are different types of filtration. Which method will be used depends largely on whether the solid is a particulate (suspended) or dissolved in the fluid. The important thing is that they all aim to attain the separation of substances. The selection of the appropriate method or technique is usually determined by the nature of the situation. Usually, there are four methods commonly used for filtration and separation: General, Vacuum, Hot and Cold Filtration.


General Filtration

General Filtration, also known as Gravity Filtration , it is the most commonly used method to remove an insoluble solid material from a solution. It is the most basic form that uses gravity to filter a mixture. This mixture is poured from above onto a filter and gravity pulls the liquid down. It uses a polyethylene or glass funnel with a stem and filter paper. Most of the solid in the mixture should settle before filtering. The solid will stay in the filter, while the liquid will flow below it.


Vacuum Filtration

In this type of filtration, the solution that needs to be filtered is drawn through a filter paper by applying a vacuum to a filter flask with a side arm adaptor. It is usually very fast and efficient way of filtering. Related to this, there is a similar technique that uses a pump to form a pressure difference on both sides of the filter. Lastly, when applying vacuum filtration, it is very important that the correct size of filter paper is used.


Cold Filtration

This method uses an ice bath to rapidly cool down the solution instead of  leaving it out to slowly cool down in room temperature. In general, it is used to quickly cool a solution when the solid is initially dissolved, prompting the formation of small crystals instead of getting large crystals when cooling the solution down at room temperature.


Hot Filtration

Sometimes during a Gravity Filtration, crystals can grow in the filter and stop the process of separation. This is where Hot Filtration comes in handy. By using Hot Filtration, the solution, filter and funnel are heated to minimize the formation of crystals in the filter. It is best carried out using a fluted filter paper and a stemless filter funnel. Due to the absence of stem in the filter the re- crystallization of solid in the funnel is prevented. This is one of the most efficient and commonly used measures used to prevent the formation of crystals.


What are the alternatives to filtration?

As with every technique and method, there are many alternatives and separation methods other than filtration. Although filtration is a very efficient method for separation, it can be much more time consuming. For example, if very small amounts of solution are involved, the filter may soak up too much of the fluid and cause a problem. In other cases, the solid material can stay trapped in the filter. That’s why, two other processes that are usually used rather than filtration are decantation and centrifugation. In short, centrifugation involves spinning a sample, rather than filtering the mixture of solid and liquid particles. It can be extremely useful for solids which don’t filter well. In decantation, the layer closer to the top of the container which is less dense of the two liquids is poured off, leaving the other component or the more dense liquid of the mixture behind. It is also known as incomplete separation. Although there are many other alternatives, Filtration is still one of the most efficient and commonly used techniques.

Filtration and Separation – Techniques, methods and processes

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