Filtration

Filtration is a method of reducing the amount of suspended particles in a liquid by using porous layers. These particles include mud, plankton, paint, organic matter, and other suspended materials. The first filters used were sand and gravel beds, which were called sand filters. The speed of water passing through these filters is slow (0.2-0.6 m/hr), so they are called slow filters. In comparison to these types of filters, there are other filters called fast filters (4-11 m/hr), which remove suspended particles faster.

Types of filters:

Filters are usually divided into two important groups:

• Surface filters
• Depth filters

Surface filters:

These filters have a small diameter and thickness, and suspended materials are captured on their surface and mostly act as a garbage collector. These filters are usually used in cases where pre-treatment processes such as coagulation and flocculation are not required. In general, surface filters are used when the concentration of suspended materials in the sample is one percent by volume. Filter paper or bag filters are included in this category of filters.

Depth filters:

These filters have a depth between 60-80 cm and are made of layers of sand or anthracite coal with different grain sizes. In these filters, as the liquid passes through the bed, suspended materials and fine particles remain between the pores and pores of the bed and clear water is discharged. As a result of the continuous operation of the filter, suspended matter in the water accumulates between the bed particles, and as a result, the intensity of the water flow through it decreases and the pressure drop inside the filter increases. In this case, it is necessary to wash the filter, which is done by passing water in the reverse direction. For washing, water is passed through the filter path at a high linear speed (15-60 m/hr) in such a way that the space between the bed particles increases by 50%. In this case, the water can circulate well between the particles and separate the mud and suspended matter remaining from the filtering process from the sand and gravel and transfer it to the outside of the filter.

These filters have two widely used types, called gravity filters and pressure filters.

Pressure filters:

The body of these filters is made of steel and is mostly cylindrical. These filters are available in two forms: horizontal and vertical. The depth and size of the filter bed are designed to provide sufficient space for further expansion during backwashing. Vertical filters are superior to horizontal filters because in vertical filters the physical filtration process is carried out along the entire length of the bed. Also, the amount of water required for washing vertical pressure filters is much less than horizontal ones. Therefore, vertical filters are more economical. The pressure drop is lower and the speed is higher. In pressure filters, the thickness of the sand or anthracite layers is also affected by the particle size. Of course, the use of anthracite is more suitable for industrial purification. Because in the first stage, water at pH = 8-9 dissolves some silica. The presence of silica impurities in water is problematic for steam boilers and turbines, and in the second stage, due to the lower density of anthracite compared to sand, a lower flow rate and pressure are required for filter washing.

Gravity filters:

Gravity filters are often cubic or rectangular, although they also exist in circular shapes. These filters operate at ambient pressure and are usually used in plants where large volumes of water must be filtered. These filters are usually cement-based and are 1.5-10m wide, several to several hundred metres long, and 3-5m high. The bed of these filters is similar to vertical pressure filters. Although these filters require a lot of space, they are simple to construct