To backwash the membranes, the air is introduced at the bottom of the membrane and water and air move upward, causing vibration and friction to the membrane. The backwash water is removed. Backwashing is normally done while the process continues and is usually automated.
Gravity Sand Filters
There are three main types of sand filters, which are used for water purification. These types of sand filters include:
- Rapid (gravity) sand filters
- Upflow sand filters
- Slow sand filters
All three types of sand filters are used extensively in the water industry throughout the world. Rapid and upflow sand filter methods require the use of flocculant chemicals to work effectively while slow sand filters can produce very high-quality water without the need for chemical aids.
A sand filter is a type of depth filter, where particulates are captured within the porous body of the material. They are usually used to separate small amounts of fine solids from aqueous solutions. Also, they are used to purify the fluid rather than capture the solids, which may be a valuable material, and are most commonly used in liquid effluent treatment.
Sand filters can be operated with either upward flowing fluids or more commonly downward flowing fluids. For downward flowing devices, the fluid can flow under pressure or by gravity. Pressure sand bed filters tend to be used in industrial applications and are often referred to as rapid sand bed filters. Gravity-fed units are used in water purification, especially drinking water, and they have found wide use in developing countries.