Backwash (physical phenomenon)
Encyclopedia
In terms of water treatment
, including water purification
and sewage treatment
, backwash refers to the reversal pumping of water, and sometimes intermittent with compressed air
, through the process. The filters
used in water treatment can be of many types, including rapid sand filter
s, pressure filters, granulated activated carbon (GAC) filters
and self-cleaning screen filter
s. However, all filters get dirty, and to keep them functional, they have to be either replaced or cleaned periodically.
The process, called backwashing, involves several steps. First, the filter is taken off line and the water is drained down to the filter bed. Then, the air wash cycle is started which pushes air up through the filter material causing the filter bed to appear to boil
. This breaks up the compacted filter bed and forces the accumulated particles into suspension
. After the air wash cycle stops, the backwash cycle starts with water flowing up through the filter bed. Clean water is passed through the filter bed in order to wash
the material and remove most of the accumulated particles. This cycle continues a fixed time or until the turbidity
of backwash water is below a set value. In some cases, the additional step of air/water wash (simultaneously) is done after air wash cycle and followed by rinse water wash. This uses less water compared to traditional processes and has a higher particle removal efficiency.
Backwashing is a form of preventive maintenance
to prevent further clogging of the filter medium. Backwashing in water treatment plants can be automated process, usually run by programmable logic controller
s (PLCs). The backwash cycle is usually triggered
when the differential pressure
over the filter exceeds a set value.
Water treatment
Water treatment describes those processes used to make water more acceptable for a desired end-use. These can include use as drinking water, industrial processes, medical and many other uses. The goal of all water treatment process is to remove existing contaminants in the water, or reduce the...
, including water purification
Water purification
Water purification is the process of removing undesirable chemicals, materials, and biological contaminants from contaminated water. The goal is to produce water fit for a specific purpose...
and sewage treatment
Sewage treatment
Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants...
, backwash refers to the reversal pumping of water, and sometimes intermittent with compressed air
Compressed air
Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe, 10 percent of all electricity used by industry is used to produce compressed air, amounting to 80 terawatt hours consumption per year....
, through the process. The filters
Filter (water)
A water filter removes impurities from water by means of a fine physical barrier, a chemical process or a biological process. Filters cleanse water to various extents for irrigation, drinking water, aquariums, and swimming pools.-Methods of filtration:...
used in water treatment can be of many types, including rapid sand filter
Rapid sand filter
The rapid sand filter or rapid gravity filter is a type of filter used in water purification and is commonly used in municipal drinking water facilities as part of a multiple-stage treatment system...
s, pressure filters, granulated activated carbon (GAC) filters
Carbon filtering
Carbon filtering is a method of filtering that uses a piece of activated carbon to remove contaminants and impurities, utilizing chemical adsorption....
and self-cleaning screen filter
Screen filter
A screen filter is a type of filter using a rigid or flexible screen to separate sand and other fine particles out of water for irrigation or industrial applications...
s. However, all filters get dirty, and to keep them functional, they have to be either replaced or cleaned periodically.
The process, called backwashing, involves several steps. First, the filter is taken off line and the water is drained down to the filter bed. Then, the air wash cycle is started which pushes air up through the filter material causing the filter bed to appear to boil
Boiling
Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid...
. This breaks up the compacted filter bed and forces the accumulated particles into suspension
Suspension (chemistry)
In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
. After the air wash cycle stops, the backwash cycle starts with water flowing up through the filter bed. Clean water is passed through the filter bed in order to wash
Washing
Washing is one way of cleaning, namely with water and often some kind of soap or detergent. Washing is an essential part of good hygiene and health....
the material and remove most of the accumulated particles. This cycle continues a fixed time or until the turbidity
Turbidity
Turbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....
of backwash water is below a set value. In some cases, the additional step of air/water wash (simultaneously) is done after air wash cycle and followed by rinse water wash. This uses less water compared to traditional processes and has a higher particle removal efficiency.
Backwashing is a form of preventive maintenance
Preventive maintenance
Preventive maintenance has the following meanings:#The care and servicing by personnel for the purpose of maintaining equipment and facilities in satisfactory operating condition by providing for systematic inspection, detection, and correction of incipient failures either before they occur or...
to prevent further clogging of the filter medium. Backwashing in water treatment plants can be automated process, usually run by programmable logic controller
Programmable logic controller
A programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...
s (PLCs). The backwash cycle is usually triggered
Event-driven programming
In computer programming, event-driven programming or event-based programming is a programming paradigm in which the flow of the program is determined by events—i.e., sensor outputs or user actions or messages from other programs or threads.Event-driven programming can also be defined as an...
when the differential pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
over the filter exceeds a set value.